Fluorine-resistant substrate material for preparing high-purity rare earth fluoride and method for preparing the same

Abstract

The application discloses a fluorine-resistant substrate material for preparing high-purity rare earth fluoride and a preparation method of the fluorine-resistant substrate material. The fluorine-resistant substrate material is a gradient composite structure, and sequentially comprises a corrosion-resistant steel base, a transition layer, a main rare earth fluoride protection layer and an inorganic hole sealing dense layer from inside to outside. The transition layer is deposited on the surface of the corrosion-resistant steel base, and the transition layer is a mixed dense layer of high-purity rare earth oxide and nickel. The main rare earth fluoride protection layer is deposited on the surface of the transition layer far from the corrosion-resistant steel base. The inorganic hole sealing dense layer is filled in micropores on the surface of the main rare earth fluoride protection layer far from the transition layer, and forms an inert dense protection structure without through pores. By constructing the gradient composite whole inert dense protection system, the corrosion penetration of high-temperature anhydrous hydrogen fluoride to the base and the migration and precipitation of impurity ions are fundamentally blocked, and the core technical problem that an existing single substrate material cannot simultaneously consider product ultra-high purity and economic efficiency of industrial production is solved.

Description

Technical Field

[0001] This invention relates to the field of protective materials for rare earth fluorination equipment, and in particular to a method for preparing a fluorine-resistant substrate material for preparing high-purity rare earth fluorides and the same method. Background Technology

[0002] High-purity rare-earth fluorides are key raw materials for the preparation of high-performance rare-earth metals, rare-earth alloys, high-end optical coating materials, and semiconductor functional materials. Their purity directly determines the performance level of the end products. Industrially, the mainstream preparation method for high-purity rare-earth fluorides is the gas-phase fluorination method, which uses rare-earth oxides as raw materials and reacts them with anhydrous hydrogen fluoride gas at high temperatures above 500°C. This process is extremely demanding, as the reaction medium is highly corrosive high-temperature anhydrous hydrogen fluoride gas, placing extremely high requirements on the high-temperature resistance, strong corrosion resistance, and cleanliness of the reaction vessel lining material.

[0003] Currently, the industry primarily uses the following materials as linings or components for fluorination reaction equipment. Stainless steel and nickel-based alloys are widely used structural materials in industrial equipment due to their low cost and good machinability. However, in the high-temperature anhydrous hydrogen fluoride environment above 500℃, these metals are extremely susceptible to severe corrosion, forming fluorides of metallic elements such as iron, nickel, and chromium. These corrosion products easily detach and become incorporated into rare earth fluoride products, significantly reducing product purity. Simultaneously, the metal matrix may experience perforation and failure under long-term high-temperature corrosion, leading to short equipment lifespan and high maintenance costs. Platinum possesses extremely strong chemical inertness, can withstand long-term corrosion from high-temperature hydrogen fluoride, and exhibits almost no impurity precipitation, meeting the production requirements for high-purity products. However, its extremely high price results in very high equipment manufacturing costs, limiting its production to small-batch laboratory trials and making it completely unsuitable for large-scale industrial production. Graphite materials possess excellent high-temperature resistance and resistance to hydrogen fluoride corrosion, and their cost is relatively moderate. However, their inherent loose and porous structure makes them prone to adsorbing hydrogen fluoride gas and rare earth materials, resulting in residual impurities in the product and poor batch-to-batch quality stability. In addition, graphite has poor oxidation resistance under high-temperature conditions, and long-term operation is prone to oxidation and loss, structural pulverization, and the shedding of carbon particles, which also contaminates the product and results in an unsatisfactory substrate service life.

[0004] In summary, existing substrate materials struggle to achieve a balance among the core indicators of "high purity, resistance to high-temperature fluorine corrosion, long service life, low cost, and scalability," failing to simultaneously meet the stringent purity requirements of high-purity rare earth fluoride products and the economic demands of industrial production. This contradiction has become a core technological bottleneck restricting the industrialization of high-end, high-purity rare earth fluorides. Therefore, there is an urgent need to develop a new type of specialized substrate material capable of comprehensively addressing these issues. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a fluorine-resistant substrate material for preparing high-purity rare earth fluorides. By constructing a gradient composite fully inert dense protective system, the corrosion and penetration of high-temperature anhydrous hydrogen fluoride into the substrate and the migration and precipitation of impurity ions are fundamentally blocked. This solves the core technical problem that existing single substrate materials cannot simultaneously achieve ultra-high product purity and economic efficiency for industrial production.

[0006] To solve the above-mentioned technical problems, the first aspect of the present invention provides a fluorine-resistant substrate material for preparing high-purity rare earth fluorides. The fluorine-resistant substrate material has a gradient composite structure, consisting of, in sequence: a corrosion-resistant steel substrate, a transition layer, a rare earth fluoride main protective layer, and an inorganic sealing and dense layer. The transition layer is deposited on the surface of the corrosion-resistant steel substrate, and the transition layer is a dense mixed layer of high-purity rare earth oxides and nickel; The rare earth fluoride main protective layer is deposited on the surface of the transition layer away from the corrosion-resistant steel substrate; The inorganic sealing dense layer fills the micropores on the side surface of the rare earth fluoride main protective layer away from the transition layer, forming an inert dense protective structure without through pores.

[0007] Furthermore, the corrosion-resistant steel matrix includes at least one of lanthanum, cerium, yttrium, gadolinium, erbium, and scandium; The rare earth element content in the corrosion-resistant steel matrix is ​​30 ppm to 500 ppm by mass.

[0008] Furthermore, the corrosion-resistant steel surface is treated with a high-pressure sandblasting process, and the roughness Ra is 50 μm ~ 80 μm.

[0009] Furthermore, the nickel content in the transition layer is 0.5 wt.%~8.0 wt.%, and the purity of the high-purity rare earth oxides is ≥99.9%; The contents of Al, Si, Ca, and Mg are all <5 ppm.

[0010] Furthermore, the transition layer forms a continuous gradient substructure along the thickness direction from the side near the corrosion-resistant steel substrate to the side near the rare earth fluoride main protective layer; The nickel content gradually decreases from 0.5 wt.% to 8.0 wt.% to 0 wt.%, while the high-purity rare earth oxide content gradually increases from 0 wt.% to 100 wt.%. Furthermore, the rare earth fluoride main protective layer includes at least one of rare earth fluorides, rare earth fluorides, and rare earth oxides, with an oxygen content < 0.4 wt.%.

[0011] Furthermore, the inorganic sealing dense layer is a silicate high-temperature resistant sealing layer, wherein the silicate is magnesium silicate and / or calcium silicate, which is filled and cured in the micropores on the surface of the rare earth fluoride main protective layer to form a continuous and dense sealed film on the surface of the main protective layer.

[0012] Furthermore, the thickness of the corrosion-resistant steel substrate is 5 mm to 20 mm; The thickness of the transition layer is 50 μm ~ 200 μm, and the porosity is ≤1%. The thickness of the rare earth fluoride main protective layer is 50 μm ~ 300 μm, and the overall porosity is ≤3%; The thickness of the inorganic sealing dense layer is 20 μm to 70 μm.

[0013] Accordingly, a second aspect of the present invention provides a method for preparing a fluorine-resistant substrate material for preparing high-purity rare earth fluorides, comprising the following steps: S1. Substrate pretreatment: Grind the corrosion-resistant steel substrate to a smooth surface, remove surface rust and oxide scale, perform high-pressure sandblasting roughening treatment on the surface, ultrasonic cleaning to remove surface dust and impurities, and dry for later use. S2, Transition layer spraying: Multiple rare earth oxide / nickel mixed powders with different ratios are used, combined with plasma spraying layer-by-layer variable ratio deposition method, to gradually reduce the proportion of nickel in the powder from the substrate to the outside, thus preparing a continuous composition gradient transition layer; S3. Main protective layer spraying: Using plasma spraying technology, a well-proportioned mixture of rare earth fluorides, rare earth fluorides and rare earth oxides is sprayed onto the surface of the transition layer to form the main protective layer. S4. Vacuum densification treatment: The coated substrate is placed in a vacuum furnace, heated to 600 ℃~ 800 ℃, and held for 1.5 h~ 2.5 h for heat treatment. It is then cooled to room temperature with the furnace to eliminate micro-cracks and loose pores inside the coating and improve the overall density and high temperature stability of the coating. S5. Sealing treatment: Apply silicate sealing agent to the surface of the heat-treated main protective layer. After curing at room temperature, an inorganic sealing dense layer is formed, completing the substrate material preparation.

[0014] Furthermore, the plasma spraying process in steps S2 and S3 has a power of 40 kW to 50 kW, a powder feed rate of 20 g / min to 30 g / min, and a spraying distance of 100 mm to 120 mm.

[0015] Accordingly, a third aspect of the present invention provides an application of the above-mentioned fluorine-resistant substrate material for preparing high-purity rare earth fluorides, wherein the fluorine-resistant substrate material is applied in equipment for preparing high-purity rare earth fluorides by reacting rare earth oxides with anhydrous hydrogen fluoride. The equipment includes a fluorination furnace lining, crucible, tray, or other container for holding the reaction and generated materials.

[0016] The above-described technical solutions of the embodiments of the present invention have the following beneficial technical effects: 1. Through a multi-layered functional synergistic design of “rare earth microalloyed corrosion-resistant steel substrate + high-purity rare earth oxide-nickel mixed transition layer + rare earth fluoride main protective layer + inorganic sealing dense layer”, a fully inert dense protective barrier from the substrate to the surface is constructed. Among them, the rare earth microalloyed corrosion-resistant steel substrate inhibits the migration activity of metal ions such as iron and chromium at high temperatures from the source; the high-purity rare earth oxide-nickel mixed transition layer forms a secondary blockage against the penetration of corrosive media into the substrate by virtue of the chemical inertness and extremely high purity of rare earth oxides, and effectively buffers the thermal mismatch stress between the substrate and the ceramic coating by utilizing the low thermal expansion coefficient and high thermal conductivity of nickel; the rare earth fluoride main protective layer has unparalleled chemical stability in an anhydrous hydrogen fluoride atmosphere of 500~900℃, and does not react with the reaction medium and products. Combined with the inorganic sealing dense layer to completely seal the residual micropores of the sprayed coating, the four-layer structure works synergistically to fundamentally eliminate the contamination of rare earth fluoride products by metal impurity ions and carbon particles. It can stably prepare ultra-high purity rare earth fluoride products with a purity of 5N or higher, and completely overcome the industry pain points of traditional graphite substrate powder shedding and carburization, as well as the high cost of platinum substrates that prevent large-scale application. 2. A gradient distribution of metallic nickel is creatively introduced into the high-purity rare-earth oxide transition layer. Utilizing Ni's low coefficient of thermal expansion and excellent thermal conductivity, thermal mismatch stress is effectively alleviated near the steel substrate. Simultaneously, the gradual compositional gradient design eliminates abrupt interface changes, significantly improving coating adhesion strength. Furthermore, the high-pressure sandblasting roughening process provides extremely high initial adhesion strength, effectively solving the problems of interface cracking, peeling, and flaking caused by differences in thermal expansion coefficients in traditional metal-based ceramic coatings under high-temperature cyclic service conditions. In addition, the addition of rare-earth elements significantly improves the thermal stability of the surface oxide film in the rare-earth microalloyed corrosion-resistant steel substrate, further suppressing abnormal oxide growth and flaking at the interface under high temperatures. The synergistic effect of these multiple interface strengthening mechanisms enables the gradient composite substrate material of this invention to maintain excellent structural integrity and interface adhesion reliability during repeated heating and cooling cycles, significantly improving the service safety and lifespan of equipment under extreme conditions. 3. Through a combined fabrication process of "plasma spraying + vacuum densification heat treatment + inorganic sealing," coupled with precise design of each functional layer component and accurate control of thickness and porosity, the full densification of the microstructure and the stable and consistent macroscopic properties of the gradient composite substrate material were achieved. Plasma spraying technology ensured the efficient and uniform forming of each functional layer, while the subsequent vacuum densification heat treatment at 600~800℃ eliminated the microcracks and loose pores inevitably generated during the spraying process, controlling the porosity of the transition layer and the main protective layer to below 1% and 3%, respectively. Meanwhile, the inorganic silicon... The acid salt sealing agent penetrates and solidifies in the micropores on the surface of the main protective layer, forming a continuous and dense closed film that completely blocks any possible channels for corrosive media to penetrate. This combined process, in conjunction with the gradient structure design, enables the substrate material to maintain its complete protective function and stable structural state after being corroded for more than 2,000 hours in an anhydrous hydrogen fluoride environment at 800°C. The overall manufacturing cost is reduced by more than 90% compared to platinum substrates, ensuring the requirements for ultra-high purity product preparation while fully meeting the long-term stable operation requirements of large-scale industrial production lines. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating the principle of a fluorine-resistant substrate material for preparing high-purity rare earth fluorides provided in an embodiment of the present invention. Figure 2 This is a flowchart of the method for preparing fluorine-resistant substrate materials for preparing high-purity rare earth fluorides provided in the embodiments of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0019] Please refer to Figure 1 The first aspect of this invention provides a fluorine-resistant substrate material for preparing high-purity rare earth fluorides. The fluorine-resistant substrate material has a gradient composite structure, comprising, in sequence: a corrosion-resistant steel substrate, a transition layer, a rare earth fluoride main protective layer, and an inorganic pore-sealing dense layer. The transition layer is deposited on the surface of the corrosion-resistant steel substrate and is a mixed dense layer of high-purity rare earth oxides and nickel. The rare earth fluoride main protective layer is deposited on the side of the transition layer away from the corrosion-resistant steel substrate. The inorganic pore-sealing dense layer fills the micropores on the side of the rare earth fluoride main protective layer away from the transition layer, forming an inert dense protective structure without through pores.

[0020] In the gas-phase fluorination process for preparing high-purity rare-earth fluorides, the lining material of the reaction vessel has long faced the dual challenges of strong corrosion from high-temperature anhydrous hydrogen fluoride and contamination from impurities. Traditional approaches typically aim to find a single material that can simultaneously meet all requirements such as corrosion resistance, high cleanliness, long lifespan, and low cost. However, the inherent limitations of metallic materials, graphite, and the precious metal platinum indicate that this "single-layer barrier" solution has reached a bottleneck. This invention no longer relies on the inherent properties of a single material to passively resist corrosive environments. Instead, it constructs a gradient composite structure consisting of a "corrosion-resistant steel substrate + transition layer + rare-earth fluoride main protective layer + inorganic sealing and dense layer." This breaks down the multiple functions originally performed by a single material and distributes them to multiple functional layers for collaborative completion, thus organically unifying seemingly contradictory technical requirements within a single material system.

[0021] In this gradient composite structure, the functional positioning of each layer is based on a precise analysis of the failure mechanism, rather than a simple stacking of materials. The introduction of a corrosion-resistant steel substrate aims to provide sufficient mechanical strength and structural stiffness for the entire protective system, solving the inherent brittleness and fragility of ceramic or graphite materials. This allows the substrate material to meet the molding, transportation, and installation requirements of large-sized components such as industrial-grade fluorination furnace linings and crucibles. Furthermore, the transition layer directly constructed on the steel substrate surface is not merely an intermediate adhesive layer to enhance bonding, but plays a more crucial role in chemical and mechanical buffering. The transition layer is composed of a mixture of high-purity rare earth oxides and metallic nickel. It utilizes the dispersed nickel phase to provide stress relief space for the huge difference in thermal expansion coefficients between the metal substrate and the ceramic main protective layer, preventing the coating from cracking and peeling under high-temperature cycling conditions. At the same time, due to the homology of rare earth oxides and the subsequent main protective layer in terms of chemical properties, it avoids the mutual diffusion and reaction that may occur at the heterogeneous interface at high temperatures. It completely isolates the migration channels of impurity ions such as iron and chromium in the metal substrate to the product end from the inside of the structure.

[0022] The core protective function is jointly provided by the outer rare-earth fluoride main protective layer and the inorganic sealing and dense layer. The rare-earth fluoride main protective layer, as the inert interface directly in contact with high-temperature anhydrous hydrogen fluoride, does not simply resist corrosion; rather, it fundamentally eliminates the driving force for reaction with hydrofluoric acid or rare-earth oxide raw materials from a thermodynamic perspective, thus preventing the introduction of solid or gaseous impurities due to the protective layer's own consumption or reaction. Based on this chemically inert protective layer, the inorganic sealing and dense layer fills the physical defects such as micropores and porosity that are difficult to avoid in conventional plasma spraying processes, ultimately fusing the originally multi-layered mechanically stacked structure into a single, inert, dense body without through-pores. This dual-sealing design, from chemical inertness to physical density, ensures that the penetration path of corrosive media is completely cut off, enabling the substrate material to meet the stringent requirements of 5N-grade ultra-high purity product preparation while achieving long-term stable service and cost control that matches the needs of large-scale industrial production. The originality of this scheme lies in its spatial sequencing and synergy of the four core functions in the protection system: mechanical support, thermal stress buffering, chemical inert barrier, and physical sealing. It provides a solution paradigm that transcends the performance limits of a single material for the preparation of high-purity materials under extremely harsh environments.

[0023] The transition layer forms a continuous gradient component structure along the thickness direction from the side near the corrosion-resistant steel substrate to the side near the rare earth fluoride main protective layer; wherein, the nickel content gradually decreases from 0.5wt.%~8.0wt.% to 0wt.%, and the high-purity rare earth oxide content gradually increases from 0wt.% to 100wt.%.

[0024] The transition layer forms a continuously decreasing nickel content and a synchronously increasing high-purity rare earth oxide content along the thickness direction, providing a solution that surpasses conventional layered transition design in resolving the contradiction between thermal stress adaptation and chemical inertness blocking in high-temperature heterogeneous material composite systems.

[0025] In traditional metal-based ceramic coating systems, to alleviate the interfacial stress caused by the difference in thermal expansion coefficients between the substrate and the ceramic surface layer, a common approach is to insert one or more fixed transition layers between them. While these fixed-composition transition layers buffer the abrupt change in thermal expansion coefficients to some extent, a clear heterogeneous interface still exists between them and the substrate and surface layer. The abrupt change in physical properties at the interlayer interface can still become a stress concentration point during repeated thermal cycling, making cracks prone to initiation and propagation along these interfaces. More importantly, if the transition layer contains metallic components, these components themselves become potential sources of corrosion and impurity release under conditions of contact with high-temperature anhydrous hydrogen fluoride, which inherently conflicts with the core objective of preparing 5N-grade ultra-high purity rare earth fluorides.

[0026] The continuous gradient component design of the transition layer in this invention breaks away from the conventional pattern of multi-layered fixed composition stacking. Along the thickness direction, from the side near the substrate to the side near the main protective layer, the nickel content gradually decreases from 0.5 wt.% to 8.0 wt.% and then to zero, while the rare earth oxide content increases simultaneously to a pure oxide phase. There are no interfaces with abrupt changes in composition within the entire transition layer. This continuous change allows for a smooth spatial transition in thermophysical properties such as the coefficient of thermal expansion and the elastic modulus, dispersing the stress that was originally concentrated at the heterogeneous interface to the entire volume of the transition layer. This eliminates the interface locations where cracks preferentially initiate, thereby significantly improving the coating's resistance to thermal shock spalling under high-temperature cyclic service conditions.

[0027] Meanwhile, the design of the transition layer with zero nickel content near the main protective layer ensures that the interface region in direct contact with the rare earth fluoride main protective layer is entirely composed of pure rare earth oxides, without any metallic phase. In a high-temperature anhydrous hydrogen fluoride environment of 500℃ to 900℃, even if the corrosive medium penetrates to this bonding area under extreme conditions, there are no metallic components available for hydrogen fluoride corrosion. Structurally, this fundamentally eliminates the possibility of the transition layer itself becoming a source of impurity release. Furthermore, the two rare earth-based materials possess naturally good interfacial compatibility and bonding strength due to their chemical homology. The retention of a suitable amount of nickel near the substrate continues to provide toughening by dissipating interfacial stress through plastic deformation. This side is densely covered by the outer pure oxide region and the main protective layer, preventing direct contact with the corrosive medium and effectively isolating the risk of nickel corrosion. Thus, by using a single, continuously varying transition layer, the two functional requirements of high-toughness stress buffering near the substrate and all-ceramic chemical inertness near the main protective layer are simultaneously achieved. While maintaining the ability to effectively block the diffusion of metal ions on the substrate side, the overall structural reliability and long-term service stability of the gradient composite structure under high-temperature cycling conditions are improved.

[0028] Specifically, the corrosion-resistant steel matrix includes at least one of lanthanum, cerium, yttrium, gadolinium, erbium, and scandium; the content of rare earth elements in the corrosion-resistant steel matrix is ​​30 ppm to 500 ppm by mass.

[0029] Introducing specific rare earth elements such as lanthanum, cerium, and yttrium into the corrosion-resistant steel matrix, with the total content controlled within the range of 30ppm to 500ppm, promotes the preferential formation of a rare earth-modified composite compound layer on the matrix surface in a high-temperature anhydrous hydrogen fluoride environment. This in-situ generated interface, compared to the naturally formed oxide layer on the surface of ordinary stainless steel, exhibits higher density and stronger bonding with the matrix. It effectively inhibits the continuous diffusion and migration of matrix elements such as iron, chromium, and nickel along the concentration gradient to the outside environment at high temperatures, eliminating the contamination of high-purity rare earth fluoride products by metallic impurities at the source and improving the intrinsic stability of the matrix material under harsh corrosive environments.

[0030] Furthermore, the corrosion-resistant steel surface is treated with a high-pressure sandblasting process, with a roughness Ra of 50 μm ~ 80 μm.

[0031] High-pressure sandblasting of the corrosion-resistant steel substrate surface, with a roughness Ra controlled between 50 μm and 80 μm, utilizes high-hardness abrasives such as white corundum and silicon carbide to thoroughly remove existing oxide scale, oil stains, and weak layers, while simultaneously creating a micro-undulating anchoring texture. When the roughness is below 50 μm, the surface is too smooth, failing to provide sufficient mechanical interlocking points for the transition layer, making the coating prone to interface slippage and peeling under thermal stress. When the roughness is above 80 μm, sharp groove bottoms and spray shadow effects may form, resulting in ineffective filling of the transition layer material and leaving voids at the interface. When the roughness is within the 50 μm to 80 μm range, molten or semi-molten transition layer particles can fully embed into surface micro-pits and deform and spread during high-speed impact on the substrate. After cooling, a robust mechanical interlocking structure is formed, significantly improving the bonding strength between the substrate and the coating, providing a fundamental guarantee for maintaining the structural integrity of the entire protective system during repeated thermal cycling.

[0032] Specifically, the nickel content in the transition layer is 0.5 wt.%~8.0 wt.%, and the purity of the high-purity rare earth oxides is ≥99.9%; the contents of Al, Si, Ca, and Mg are all <5 ppm.

[0033] The transition layer is constructed from a mixture of high-purity rare earth oxides and metallic nickel, with a nickel content ranging from 0.5 wt.% to 8.0 wt.%. The purity of the rare earth oxides is no less than 99.9%, and the content of impurities such as aluminum, silicon, calcium, and magnesium is all below 5 ppm. The metallic nickel component is embedded in the rare earth oxide ceramic matrix in the form of gradient-distributed micro-nano-scale tough particles. When the system undergoes drastic heating and cooling processes, the interfacial shear stress generated between the steel matrix and the ceramic protective layer due to the difference in thermal expansion coefficients is largely absorbed and dissipated through the plastic deformation of these nickel particles, preventing the initiation and propagation of cracks at the interface. Simultaneously, the impurities such as aluminum, silicon, calcium, and magnesium in the rare earth oxide raw materials are controlled to below 5 ppm, ensuring that the transition layer itself will not become a new source of pollution release in a high-temperature anhydrous hydrogen fluoride environment above 500°C. If the content of these impurities exceeds the standard, even if the transition layer successfully blocks the diffusion of metal ions from the matrix side, it will still be corroded and precipitated under high-temperature corrosion conditions, and the ultra-high purity of the product cannot be achieved. This strict limitation on purity and impurity content, combined with the toughening mechanism of nickel, gives the transition layer both mechanical buffering and chemical cleaning functions.

[0034] Specifically, the rare earth fluoride main protective layer includes at least one of rare earth fluorides, rare earth fluorides and rare earth oxides, with an oxygen content of <0.4 wt.%.

[0035] The rare-earth fluoride main protective layer comprises at least one of rare-earth fluorides, rare-earth fluoride oxides, and rare-earth oxides, with the oxygen content controlled below 0.4 wt.%. In a high-temperature anhydrous hydrogen fluoride atmosphere, fluoride ions possess extremely strong coordination ability and corrosive activity. Under these conditions, most metal oxides or non-oxide materials undergo fluorination reactions, leading to volume effects from product phase transitions and resulting in coating cracking or peeling. Due to its extremely low Gibbs free energy and extremely high thermodynamic stability, the rare-earth fluoride system shows almost no chemical reaction even after prolonged exposure to high concentrations of hydrogen fluoride gas, fundamentally eliminating the possibility of the protective layer itself participating in corrosion reactions. Controlling the oxygen content below 0.4 wt.% is to minimize the trace amounts of water vapor or other volatile byproducts that may be generated due to the gradual fluorination of the oxide phase by hydrogen fluoride at high temperatures. These byproducts can also have a significant impact on product quality in ultra-high purity preparation scenarios. This main protective layer creates a chemical interface that is thermodynamically inert to hydrogen fluoride, thus preventing corrosion reactions from occurring at the source.

[0036] Specifically, the inorganic sealing dense layer is a silicate high-temperature resistant sealing layer, and the silicate is magnesium silicate and / or calcium silicate. It is filled and cured in the micropores on the surface of the rare earth fluoride main protective layer to form a continuous and dense sealing film on the surface of the main protective layer.

[0037] The inorganic sealing layer uses magnesium silicate or calcium silicate as the high-temperature sealing medium, deeply penetrating and curing within the micropores of the rare-earth fluoride main protective layer. During plasma spraying, molten particles collide with the substrate at high speed and stack layer by layer. Inevitably, micron- or even submicron-sized interconnected pores remain at the particle overlap points. If these pores are not sealed, they will become capillary channels for high-temperature corrosive gases to penetrate into the coating and even the substrate. The silicate sealing agent, in its liquid state, with its low surface tension and good wettability, penetrates deep into the pores along the capillary walls. During curing, it undergoes hydrolysis and condensation reactions to generate a continuous three-dimensional network of inorganic silicate gel, filling all surface and near-surface pores and firmly bonding with the main protective layer. After curing, the sealing layer forms a continuous, dense sealing film on the outer surface of the main protective layer, transforming the original microscopically discontinuous coating surface, formed by the mechanical accumulation of particles, into an inert protective interface without interconnected pores, blocking the possibility of corrosive media penetrating inward along any path.

[0038] Furthermore, the thickness of the corrosion-resistant steel substrate is 5 mm to 20 mm; the thickness of the transition layer is 50 μm to 200 μm, with a porosity of ≤1%; the thickness of the rare earth fluoride main protective layer is 50 μm to 300 μm, with an overall porosity of ≤3%; and the thickness of the inorganic sealing dense layer is 20 μm to 70 μm.

[0039] The thickness of the corrosion-resistant steel substrate is set at 5mm to 20mm, the transition layer thickness is 50μm to 200μm with a porosity not exceeding 1%, the rare earth fluoride main protective layer thickness is 50μm to 300μm with an overall porosity not exceeding 3%, and the inorganic sealing and dense layer thickness is 20μm to 70μm. These layer thicknesses and porosity parameters are not selected in isolation, but rather represent a comprehensive balance between mechanical support, interfacial stress buffering, corrosive media barrier, and process feasibility. When the corrosion-resistant steel substrate thickness is less than 5mm, the overall structural rigidity is insufficient, making it difficult to withstand the deformation caused by the charging load and thermal stress in large components such as the lining of the fluorination furnace or the crucible. A thickness exceeding 20mm significantly increases equipment weight and manufacturing costs, and also adversely affects heat transfer efficiency. Within its thickness range of 50μm to 200μm, the transition layer provides sufficient space for stress dissipation of the nickel-based tough particles, while maintaining an extremely dense porosity of less than 1%. This ensures that the transition layer itself does not become a weak point for corrosive media to penetrate into the substrate through the pores. If the thickness is less than 50μm, it is difficult to completely cover the micro-undulations on the surface formed by sandblasting, and the locally exposed substrate will be directly exposed to corrosion risks. The lower limit of the rare earth fluoride main protective layer thickness of 50μm ensures sufficient chemical inert barrier thickness to resist the slow penetration of hydrogen fluoride during long-term high-temperature service, while the upper limit of 300μm avoids the risk of residual stress accumulation inside the coating and through-cracks under thermal shock conditions due to excessive thickness. Its overall porosity is controlled below 3%, which restricts the residual pores generated by the overlap between sprayed particles to an isolated and closed state, preventing the formation of through-corrosion channels. Within a thickness range of 20μm to 70μm, the inorganic sealing dense layer allows the silicate sealing agent to fully penetrate and fill the open micropores on the surface of the main protective layer, forming a continuous surface sealing film. A thickness below 20μm makes it difficult to completely cover all surface pores, while a thickness above 70μm may cause microcracks in the sealing layer itself due to curing shrinkage or thermal expansion differences, thus weakening the protective effect. The synergistic combination of the thickness and porosity of each layer enables the entire gradient composite structure to achieve fully dense and inert protection from the substrate to the surface, while meeting the dimensional tolerance and machinability requirements of industrial equipment manufacturing.

[0040] Accordingly, please refer to Figure 2 The second aspect of this invention provides a method for preparing a fluorine-resistant substrate material for preparing high-purity rare earth fluorides, comprising the following steps: S1. Substrate pretreatment: Grind the corrosion-resistant steel substrate to a smooth surface, remove surface rust and oxide scale, perform high-pressure sandblasting roughening treatment on the surface, ultrasonic cleaning to remove surface dust and impurities, and dry for later use.

[0041] The corrosion-resistant steel substrate is ground smooth and surface rust and oxide scale are removed. Then, the surface is roughened by high-pressure sandblasting with white corundum, silicon carbide, or steel grit. After ultrasonic cleaning to remove surface dust and impurities, it is dried for later use. The purpose of grinding and descaling is to eliminate the loose oxide layer and corrosion product layer formed on the substrate surface during rolling, processing, or storage. If these weak layers are not removed, they will directly become the separation interface between the coating and the substrate, severely weakening the bonding strength. High-pressure sandblasting creates a uniform and sharply textured microstructure on the substrate surface, providing dense mechanical anchoring points for the embedding and spreading of molten particles during subsequent plasma spraying. Ultrasonic cleaning further removes residual abrasive debris and oil embedded in the surface texture, ensuring cleanliness at the interface. After this pretreatment process, an activated surface with a roughness Ra of 50 μm to 80 μm is formed on the substrate surface, providing a reliable interface foundation for high-quality deposition of the transition layer.

[0042] S2. Using multiple sets of rare earth oxide + nickel mixed powders with different ratios, a continuous composition gradient transition layer is prepared by plasma spraying layer by layer with varying ratio deposition, gradually reducing the proportion of nickel in the powder from the substrate to the outside.

[0043] Step S2 employs multiple sets of rare earth oxide and nickel mixed powders with different proportions. A layer-by-layer variable proportion deposition method using plasma spraying is employed, progressively reducing the nickel content in the powder from the substrate side outwards to prepare a continuous compositional gradient transition layer. During the process, several sets of rare earth oxide and nickel mixed powders with progressively decreasing nickel content are pre-prepared. At the initial stage of spraying, a powder with a higher nickel content is selected for deposition, ensuring that the bottom layer near the corrosion-resistant steel substrate contains a relatively sufficient amount of nickel. After being fully heated in the plasma flame, the molten particles impact the roughened substrate surface at high speed, causing intense deformation, spreading, and rapid solidification. The nickel component is embedded in the rare earth oxide ceramic matrix as a dispersed, ductile phase, endowing this region with excellent plastic deformation capacity and interfacial stress dissipation function. As the spraying process moves away from the substrate, the powder groups with progressively decreasing nickel content are switched layer by layer. During the deposition of each layer, the proportion of nickel in the powder composition is further reduced compared to the previous layer, while the proportion of rare earth oxides increases accordingly. After the molten particles solidify, they are stacked layer by layer on the already deposited coating. The entire transition layer exhibits a continuous, gradual distribution in the thickness direction, with the nickel content decreasing from 0.5 wt.% to 8.0 wt.% and then to zero, while the rare earth oxide content increases synchronously to a pure oxide phase. The high-temperature flame of plasma spraying provides the thermodynamic conditions for the full melting of powders with different proportions and for interlayer inter-fusion diffusion. A certain degree of elemental interdiffusion occurs between adjacent layers at the deposition interface, eliminating the microscopic compositional steps that may occur during single-layer switching, making the compositional gradient tend to be continuous and smooth at the microscopic scale. The coordinated control of power, powder feed rate and spraying distance during the spraying process ensures that each layer is stacked in a highly dense and low-porosity state. The overall thickness of the transition layer is controlled between 50μm and 200μm, and the porosity does not exceed 1%. It has a high toughness stress buffering capacity near the substrate and completes a smooth transition to the all-ceramic chemical inert interface near the main protective layer, providing a load-bearing surface with continuous composition and strong bonding for the subsequent deposition of the main protective layer.

[0044] S3. Main protective layer spraying: Using plasma spraying technology, a well-proportioned mixture of rare earth fluorides, rare earth fluorides and rare earth oxides is sprayed onto the surface of the transition layer to form the main protective layer.

[0045] A plasma spraying process is employed to coat the transition layer surface with a pre-mixed powder of rare earth fluorides, rare earth fluorides, and rare earth oxides, forming the main protective layer. The powder ratio is pre-controlled according to the stoichiometry and phase composition requirements of the target fluoride system. After being heated and melted in a plasma flame, the powder is deposited onto the transition layer. During the spraying process, particles are stacked layer by layer to form a main protective layer with a thickness of 50 μm to 300 μm, and the overall porosity is controlled below 3%. Due to the chemical homology of the rare earth-based materials, the main protective layer and the transition layer exhibit good interfacial compatibility, and the two layers also show a smooth transition in thermophysical properties, reducing the concentration of residual stress at the heterogeneous interface.

[0046] S4. Vacuum densification treatment: The coated substrate is placed in a vacuum furnace and heated to 600 ℃~ 800 ℃. After heat treatment, it is kept at this temperature for 1.5 h~ 2.5 h and then cooled to room temperature with the furnace to eliminate micro-cracks and loose pores inside the coating and improve the overall density and high temperature stability of the coating.

[0047] The substrate with the transition layer and main protective layer sprayed is placed in a vacuum furnace and heated to 600°C to 800°C, then held at that temperature for 1.5 to 2.5 hours for heat treatment, followed by furnace cooling to room temperature. During plasma spraying, molten particles collide at high speed and solidify rapidly, inevitably leaving some microcracks and loose pores inside the coating. These defects may become channels for the penetration of corrosive media or sources of crack propagation during subsequent high-temperature service. Holding at 600°C to 800°C under vacuum conditions allows the coating material to undergo sufficient atomic diffusion and local sintering in an oxygen-free environment. The tips of microcracks are passivated and tend to heal due to mass migration, while loose areas become denser due to diffusion mass transfer. Slow furnace cooling avoids the risk of cracks re-initiating due to thermal stress during rapid cooling. After this treatment, the defect density inside the transition layer and main protective layer is significantly reduced, and the overall density and high-temperature structural stability of the coating are significantly improved.

[0048] S5. Sealing treatment: Apply silicate sealing agent to the surface of the heat-treated main protective layer. After curing at room temperature, an inorganic sealing dense layer is formed, completing the substrate material preparation.

[0049] A silicate sealing agent is applied to the surface of the main protective layer after vacuum densification. After curing at room temperature, an inorganic, densely sealed layer is formed, completing all substrate material preparation steps. In the liquid phase, the silicate sealing agent, with its low surface tension, penetrates into the micropores of the main protective layer, filling the surface pores remaining after spraying. During room temperature curing, the sealing agent undergoes hydrolysis and condensation reactions, generating a continuous three-dimensional network of silicate gel, which forms strong chemical bonds and mechanical interlocking with the surface and pore walls of the main protective layer. After curing, a continuous and dense sealed film with a thickness of 20μm to 70μm is formed on the surface of the main protective layer, transforming the original sprayed coating with microscopic surface openings into an inert protective interface without any through-pores, completely blocking the possibility of corrosive media penetrating into the coating along any path.

[0050] Furthermore, the plasma spraying process in steps S2 and S3 has a power of 40 kW to 50 kW, a powder feed rate of 20 g / min to 30 g / min, and a spraying distance of 100 mm to 120 mm.

[0051] In steps S2 and S3, the plasma spraying process is set with a power of 40kW to 50kW, a powder feed rate of 20g / min to 30g / min, and a spraying distance of 100mm to 120mm. The synergistic matching of these three parameters directly determines the melting state, flight speed, and deposition quality of the sprayed particles. When the power is below 40kW, the enthalpy of the plasma flame is insufficient to fully heat the high-melting-point rare earth oxide powder to a molten or semi-molten state. After the powder particles collide with the substrate, they are difficult to fully deform and spread, leaving a large number of unmelted particles and pores inside the coating, resulting in a significant decrease in density. When the power is above 50kW, the flame temperature is too high, which can easily cause the rare earth fluoride powder to undergo excessive vaporization or decomposition during flight, changing the target phase composition and reducing deposition efficiency. The powder feed rate is controlled between 20g / min and 30g / min to ensure that the powder particles fed into the flame are uniformly heated. If the powder feed rate is too low, the deposition rate will be slow and the production efficiency will be insufficient. If the powder feed rate is too high, some particles will not be fully melted due to excessive dispersion of the flame enthalpy, which will also lead to unmelted inclusions and loose areas in the coating. The spraying distance is set to 100mm to 120mm to allow the particles to obtain sufficient acceleration time and heating distance in the flame, so that they can impact the substrate surface with high kinetic energy and achieve good spreading and bonding effects. If the spraying distance is too short, the particles will not be heated sufficiently and the substrate will be subjected to severe thermal shock. If the spraying distance is too long, the temperature will drop significantly and the velocity will decrease during particle flight, resulting in insufficient deformation ability during deposition. The synergistic combination of the above parameter ranges allows the transition layer and the main protective layer to be stacked layer by layer in a highly dense and high-bonding-strength state, providing an initial sprayed coating with the lowest possible defect density for subsequent vacuum densification and pore sealing treatments.

[0052] Accordingly, this invention prepared fluorine-resistant substrate materials with different structures, and tested the coating porosity and high-temperature fluorine corrosion resistance of each substrate material. Simultaneously, using each substrate material as a liner, gas-phase fluorination experiments of lanthanum oxide and anhydrous hydrogen fluoride were conducted in a muffle furnace. The content of metal and carbon impurities, purity of rare earth fluorides, and substrate lifespan were compared and statistically analyzed. The substrate material preparation and testing methods are as follows: Substrate material preparation: Corrosion-resistant steel / graphite / platinum was used as the substrate. The substrate material was cut and welded, and the surface was ground, rust-removed, and degreased. The substrate surface was roughened to the specified roughness Ra by high-pressure sandblasting according to the design requirements in Table 1. An atmospheric plasma spraying (APS) process was used to sequentially prepare the transition layer and the rare-earth fluoride main protective layer. The plasma spraying process parameters were: power 40–50 kW, powder feed rate 20–30 g / min, spraying distance 100–120 mm, argon flow rate 40–50 L / min, and hydrogen flow rate 5–10 L / min. After spraying, the substrate was placed in a vacuum furnace and heated to 600–800℃ for 1.5–2.5 h for vacuum densification heat treatment, followed by furnace cooling to room temperature. Finally, an inorganic sealing layer was applied by dip coating or brush coating, and cured at room temperature to complete the substrate material preparation.

[0053] Coating porosity test: The overall porosity of the coating was calculated by quantitatively analyzing the scanning electron microscope (SEM) images of the coating cross-section using image analysis method (Image-Pro Plus software).

[0054] High-temperature fluorine corrosion resistance test: The substrate sample is placed in a tube furnace and diluted anhydrous hydrogen fluoride gas is introduced. The sample is kept at 800℃ for 2000h. After removal, the surface morphology is observed, and the coating integrity, peeling and visible corrosion characteristics are recorded.

[0055] Fluorination test and product detection: Rare earth fluorides were prepared by reacting lanthanum oxide / cerium oxide / yttrium oxide / terbium oxide with hydrogen fluoride gas at 800℃ in a muffle furnace using various substrate materials as linings. The content of metallic impurities such as Fe and Cr was detected by inductively coupled plasma mass spectrometry (ICP-MS), and the carbon content was detected by a high-frequency infrared carbon-sulfur analyzer. The total impurity content was calculated, and the purity grade of the rare earth fluorides was evaluated.

[0056] Service life assessment: The operating time (approximately 500 hours) when the graphite substrate showed obvious powder shedding and pulverization in an HF environment at 800℃ was used as a baseline (1 time). The operating time of the substrates of each embodiment under the same operating conditions, maintaining the coating integrity and without visible corrosion, was compared to calculate the relative service life multiple.

[0057] Example 1 The substrate was prepared according to the design, using 316L corrosion-resistant steel with a rare earth element content of 30 ppm (La) and a thickness of 15 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 50 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a Y₂O₃+Ni composite transition layer (50 μm thick) and a YF₃ rare earth fluoride main protective layer (oxygen content 0.38 wt.%) (50 μm thick). Specifically, the Y₂O₃+Ni composite transition layer was sprayed at a power of 40 kW, a powder feed rate of 20 g / min, and a spraying distance of 100 mm. Multiple layers of rare earth oxide + nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 0.5 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 95.5 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 50 μm. The YF3 rare earth fluoride main protective layer was sprayed at a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 120 mm. After spraying, the substrate was placed in a vacuum furnace and heat-treated at 600 ℃ for 2.5 h, followed by furnace cooling. Finally, magnesium silicate inorganic sealing agent was used for sealing, resulting in a sealing layer thickness of 20 μm. Test results showed that the coating porosity was 0.08%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact without peeling, with only slight oxidation corrosion on the surface; fluorination tests using this substrate showed a total Fe+Cr+Ni+C impurity content of 8 ppm, a rare earth fluoride purity of 5N, and a service life 3.5 times that of graphite substrates.

[0058] Example 2 The substrate was prepared according to the design using 904L corrosion-resistant steel with a Ce content of 500 ppm and a thickness of 5 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 65 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a La₂O₃+Ni composite transition layer (120 μm thick) and a LaF₃ rare-earth fluoride main protective layer (oxygen content 0.35 wt.%) (180 μm thick). Specifically, the La₂O₃+Ni composite transition layer was sprayed at a power of 45 kW, a powder feed rate of 25 g / min, and a spraying distance of 110 mm. Multiple layers of rare-earth oxide + nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 8.0 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare-earth oxide content from 92.0 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 120 μm. The LaF3 rare earth fluoride main protective layer was applied using a power of 40 kW, a powder feed rate of 20 g / min, and a spraying distance of 120 mm. After spraying, vacuum heat treatment (700℃ / 2 h) and calcium silicate sealing treatment (thickness 45 μm) were performed. Test results showed that the coating porosity was 0.06%; after HF corrosion at 800℃ / 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests using this substrate showed a total Fe+Cr+Ni+C impurity content of 5 ppm, a rare earth fluoride purity of 5N, and a service life four times that of graphite substrates.

[0059] Example 3 The substrate was prepared according to the design, using 316L corrosion-resistant steel with rare earth elements of La 150 ppm, Ce 100 ppm, and Y 50 ppm. The substrate thickness was 20 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 80 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a CeO2+Ni composite transition layer with a thickness of 200 μm; and a CeF3 rare earth fluoride main protective layer (oxygen content 0.32 wt.%) with a thickness of 300 μm. The CeO2+Ni composite transition layer was applied using a power of 40 kW, a powder feed rate of 20 g / min, and a spraying distance of 110 mm. Multiple layers of rare earth oxide and nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 3.5 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 96.5 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 200 μm. The CeF3 main protective layer was applied using a power of 40 kW, a powder feed rate of 20 g / min, and a spraying distance of 120 mm. Subsequent vacuum heat treatment (800 ℃ / 1.5 h) and magnesium silicate+calcium silicate composite sealing treatment (70 μm thickness) were then performed. Test results show that the material coating has a porosity of 0.05%; after HF corrosion at 800 ℃ for 2000 h, the coating remains intact, without peeling or visible corrosion; fluorination tests conducted on this substrate show that the total amount of Fe+Cr+Ni+C impurities in the product is 3 ppm, the purity of rare earth fluorides reaches 5N, and the service life is 5 times that of graphite substrates.

[0060] Example 4 The substrate was prepared according to the design, using 904L corrosion-resistant steel with rare earth elements of 200 ppm Y and 80 ppm Gd, and a thickness of 20 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 70 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a 100 μm thick Y₂O₃ + Ni composite transition layer; and a 200 μm thick YF₃ rare earth fluoride main protective layer (oxygen content 0.30 wt.%). During the Y2O3+Ni composite transition layer spraying, the power was 50 kW, the powder feed rate was 20 g / min, and the spraying distance was 110 mm. Multiple groups of rare earth oxide + nickel mixed powders with different ratios were deposited layer by layer via plasma spraying, gradually changing the Ni content from 2 wt.% near the substrate side to 0 wt.% near the main protective layer side, and the rare earth oxide content from 98.0 wt.% to 100 wt.%, thus preparing a continuous compositional gradient transition layer with a thickness of 100 μm. The YF3 rare earth fluoride main protective layer was sprayed with a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 120 mm. After spraying, vacuum heat treatment (700 ℃ / 2.0 h) and magnesium silicate sealing treatment (thickness 50 μm) were performed. The test results showed that the porosity of the coating was 0.06%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests were conducted using this substrate, and the total amount of Fe+Cr+Ni+C impurities in the product was 2 ppm, the purity of rare earth fluorides reached 5N, and the service life was 5 times that of graphite substrates.

[0061] Example 5 The substrate was prepared according to the design, using 316L corrosion-resistant steel with rare earth elements of 100 ppm Gd and 50 ppm Er, and a thickness of 20 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 60 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a Gd₂O₃+Ni composite transition layer with a thickness of 80 μm; and a GdF₃ rare earth fluoride main protective layer (oxygen content 0.35 wt.%) with a thickness of 150 μm. During the Gd₂O₃+Ni composite transition layer spraying, the power was 50 kW, the powder feed rate was 30 g / min, and the spraying distance was 100 mm. Multiple groups of rare earth oxide + nickel mixed powders with different ratios were deposited layer by layer via plasma spraying, gradually changing the Ni content from 1.0 wt.% near the substrate side to 0 wt.% near the main protective layer side, and the rare earth oxide content from 99.0 wt.% to 100 wt.%, thus preparing a continuous compositional gradient transition layer with a thickness of 80 μm. The GdF₃ main protective layer was sprayed with a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 120 mm. After spraying, vacuum heat treatment (680 ℃ / 2.0 h) and calcium silicate sealing treatment (40 μm thickness) were performed.

[0062] Test results showed that the porosity of the coating was 0.07%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests were conducted using this substrate, and the total amount of Fe+Cr+Ni+C impurities in the product was 4 ppm, the purity of rare earth fluorides reached 5N, and the service life was 4.5 times that of graphite substrates.

[0063] Example 6 The substrate was prepared according to the design, using 904L corrosion-resistant steel with rare earth elements of Y 300 ppm, La 100 ppm, and Ce 50 ppm, and a thickness of 15 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 75 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a Y₂O₃+La₂O₃+Ni composite transition layer with a thickness of 150 μm; and a YF₃+LaF₃ (1:1) rare earth fluoride main protective layer (oxygen content 0.28 wt.%) with a thickness of 250 μm. The Y₂O₃+La₂O₃+Ni composite transition layer was applied using a power of 45 kW, a powder feed rate of 30 g / min, and a spraying distance of 100 mm. Multiple groups of rare earth oxide + nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 1.5 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 98.5 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 150 μm. The YF₃+LaF₃ main protective layer was applied using a power of 40 kW, a powder feed rate of 25 g / min, and a spraying distance of 120 mm. After spraying, vacuum heat treatment (720 ℃ / 2 h) and magnesium silicate sealing treatment (60 μm thickness) were performed. Test results showed that the porosity of the coating was 0.05%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests using this substrate showed that the total amount of Fe+Cr+Ni+C impurities in the product was 2 ppm, the purity of rare earth fluorides reached 5N, and the service life was 5.5 times that of graphite substrates.

[0064] Example 7 The substrate was prepared according to the design, using 316L corrosion-resistant steel with rare earth elements of Ce 200 ppm, Y 100 ppm, and La 50 ppm, and a thickness of 15 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 55 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a CeO2+Ni composite transition layer with a thickness of 70 μm; and a CeF3+YF3 (2:1) rare earth fluoride main protective layer (oxygen content 0.32 wt.%) with a thickness of 100 μm. The CeO2+Ni composite transition layer was applied using a power of 40 kW, a powder feed rate of 20 g / min, and a spraying distance of 100 mm. Multiple rare earth oxide and nickel mixed powders with different ratios were deposited layer by layer via plasma spraying, gradually varying the Ni content from 2.0 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 98.0 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 70 μm. The CeF3+YF3 (2:1) rare earth fluoride main protective layer was applied using a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 100 mm. After spraying, vacuum heat treatment (620 ℃ / 1.5 h) and calcium silicate sealing treatment (30 μm thickness) were performed. The test results showed that the porosity of the coating was 0.09%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact without peeling, and the surface showed slight oxidation; when the substrate was used for fluorination tests, the total amount of Fe+Cr+Ni+C impurities in the product was 6ppm, the purity of rare earth fluorides reached 5N, and the service life was 4 times that of graphite substrates.

[0065] Example 8 The substrate was prepared according to the design, using 904L corrosion-resistant steel with rare earth elements of 80 ppm Gd, 80 ppm Er, and 80 ppm Y, and a thickness of 15 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 70 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a Y₂O₃+Gd₂O₃+Ni composite transition layer with a thickness of 110 μm; and a YF₃+GdF₃ (1:1) rare earth fluoride main protective layer (oxygen content 0.30 wt.%) with a thickness of 220 μm. The Y₂O₃+Gd₂O₃+Ni composite transition layer was applied using a power of 50 kW, a powder feed rate of 20 g / min, and a spraying distance of 120 mm. Multiple layers of rare earth oxide and nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 2.0 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 98.0 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 110 μm. The YF₃+GdF₃ main protective layer was applied using a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 110 mm. After spraying, vacuum heat treatment (690 ℃ / 2.0 h) and magnesium silicate + calcium silicate composite sealing treatment (55 μm thickness) were performed.

[0066] The test results showed that the porosity of the coating was 0.06%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests were conducted using this substrate, and the total amount of Fe+Cr+Ni+C impurities in the product was 3 ppm, the purity of rare earth fluorides reached 5N, and the service life was 5 times that of graphite substrates.

[0067] Example 9 The substrate was prepared according to the design, using 904L corrosion-resistant steel with rare earth elements of 80 ppm La and 80 ppm Ce, and a thickness of 15 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 70 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a Y₂O₃+Gd₂O₃+Ni composite transition layer with a thickness of 110 μm; and a YF₃+YOF (mass ratio 6:4) rare earth fluoride main protective layer (oxygen content 0.30 wt.%) with a thickness of 220 μm. The Y₂O₃+Gd₂O₃+Ni composite transition layer was applied using a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 110 mm. Multiple layers of rare earth oxide + nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 2.5 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 92.0 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 110 μm. The YF₃+YOF main protective layer was applied using a power of 40 kW, a powder feed rate of 25 g / min, and a spraying distance of 110 mm. After spraying, vacuum heat treatment (690 ℃ / 2.0 h) and magnesium silicate + calcium silicate composite sealing treatment (55 μm thickness) were performed. Test results showed that the porosity of the coating was 0.06%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests were conducted using this substrate, and the total amount of Fe+Cr+Ni+C impurities in the product was 3.5 ppm, the purity of rare earth fluorides reached 5N, and the service life was 5 times that of graphite substrates.

[0068] Example 10 The substrate was prepared according to the design, using 904L corrosion-resistant steel with rare earth elements of 80 ppm La and 40 ppm Ce, and a thickness of 15 mm. The steel surface was roughened by high-pressure sandblasting to a roughness Ra of 70 μm. A plasma spraying process was then used to sequentially prepare the following layers on the substrate surface: a Y₂O₃+Gd₂O₃+Ni composite transition layer with a thickness of 110 μm; and a YF₃+YOF+Y₂O₃ (mass ratio 6:3:1) rare earth fluoride main protective layer (oxygen content 0.30 wt.%) with a thickness of 220 μm. The Y₂O₃+Gd₂O₃+Ni composite transition layer was applied using a power of 50 kW, a powder feed rate of 30 g / min, and a spraying distance of 110 mm. Multiple layers of rare earth oxide and nickel mixed powders with different proportions were deposited layer by layer via plasma spraying, gradually varying the Ni content from 1.8 wt.% near the substrate to 0 wt.% near the main protective layer, and the rare earth oxide content from 98.2 wt.% to 100 wt.%, resulting in a continuous compositional gradient transition layer with a thickness of 110 μm. The YF₃+YOF+Y₂O₃ main protective layer was applied using a power of 45 kW, a powder feed rate of 25 g / min, and a spraying distance of 110 mm. After spraying, vacuum heat treatment (690 ℃ / 2.0 h) and magnesium silicate + calcium silicate composite sealing treatment (55 μm thickness) were performed. Test results showed that the material coating had a porosity of 0.06%; after HF corrosion at 800 ℃ for 2000 h, the coating remained intact, without peeling or visible corrosion; fluorination tests using this substrate showed that the total amount of Fe+Cr+Ni+C impurities in the product was 2.5 ppm, the purity of rare earth fluorides reached 5N, and the service life was 5 times that of graphite substrates.

[0069] Comparative Example 1 The substrate is made of Q450 weathering steel, with no surface roughening treatment (sandblasting) or coating. It is directly used in high-temperature HF conditions at 800 ℃. Test results show that after corrosion at 800 ℃ for 2000 h in a hydrogen fluoride atmosphere, the surface morphology is severely corroded, with rust peeling off. The total amount of Fe+Cr+Ni+C impurities in the product is 2200 ppm, the purity of rare earth fluorides is less than 3N, and the service life is 0.25 times that of a graphite substrate.

[0070] Comparative Example 2 A Q450 weathering steel substrate was selected, and the surface was roughened by sandblasting to Ra 35 μm. A YF3 rare earth fluoride main protective layer (oxygen content 0.5 wt.%, thickness 100 μm) was directly prepared using plasma spraying. The spraying power was 45 kW, the powder feed rate was 25 g / min, and the spraying distance was 110 mm. No vacuum heat treatment or sealing treatment was performed after spraying. Test results showed that the coating porosity was 0.2%. After corrosion at 800 ℃ / 2000 h in a hydrogen fluoride atmosphere, the surface morphology showed coating peeling, through-cracks, and substrate corrosion. The total impurities of Fe+Cr+Ni+C in the product were 500 ppm, the rare earth fluoride purity was less than 3N, and the service life was 0.4 times that of the graphite substrate.

[0071] Comparative Example 3 The substrate is made of 316L stainless steel, with no surface roughening treatment and no coating. It is directly used in HF (high-temperature HF) conditions at 800 ℃. Test results show that after corrosion in a hydrogen fluoride atmosphere at 800 ℃ for 2000 h, the surface morphology is severely corroded, with surface peeling and perforation. The total amount of Fe+Cr+Ni+C impurities in the product is 2500 ppm, the purity of rare earth fluorides is less than 3N, and the service life is 0.2 times that of a graphite substrate.

[0072] Comparative Example 4 Pure platinum (Pt≥99.95%) was used as the substrate, with a thickness of 0.5 mm, and attached to a stainless steel support. It was directly used in high-temperature HF applications at 800 ℃. Test results showed that after etching at 800 ℃ for 2000 h in a hydrogen fluoride atmosphere, the surface morphology was characterized by slight oxidation corrosion. The total amount of Fe+Cr+Ni+C impurities in the product was less than 1 ppm, the rare earth fluoride purity reached 5N, and the service life was 10 times that of a graphite substrate.

[0073] Comparative Example 5 High-purity graphite was selected as the matrix (bulk density 1.85 g / cm³). 3 (Ash content ≤50 ppm), surface polished. Directly used in HF high-temperature conditions at 800 ℃. Test results show that the coating porosity is 0.15%, and after corrosion in a hydrogen fluoride atmosphere at 800 ℃ for 2000 h, the surface morphology is powdery, loose in structure, and exhibits peeling. The total amount of Fe+Cr+Ni+C impurities in the product is 150 ppm (mainly carbonaceous contamination), the rare earth fluoride purity is less than 3N, and the service life is calculated as 1 time (baseline).

[0074] Comparative Example 6 316L stainless steel was selected as the substrate, and the surface was roughened by sandblasting to Ra 30 μm. A YF3 rare earth fluoride main protective layer (oxygen content 0.5 wt.%, thickness 100 μm) was directly prepared using plasma spraying. The spraying power was 50 kW, the powder feed rate was 30 g / min, and the spraying distance was 110 mm. No vacuum heat treatment or sealing treatment was performed after spraying. Test results showed that the coating porosity was 0.18%. After corrosion at 800 ℃ / 2000 h in a hydrogen fluoride atmosphere, the surface morphology showed coating peeling, through cracks, and substrate corrosion. The total impurities of Fe+Cr+Ni+C in the product were 450 ppm, the rare earth fluoride purity reached 4N, and the service life was 0.5 times that of the graphite substrate.

[0075] Comparative Example 7 316L stainless steel was selected as the substrate, and the surface was roughened by sandblasting to Ra 30 μm. A Y2O3 + 3.5wt.% Ni rare earth oxide transition layer (100 μm thick) was prepared using plasma spraying at a power of 40kW, a powder feed rate of 20 g / min, and a spraying distance of 120 mm. No rare earth fluoride main protective layer or sealing treatment was performed. Vacuum heat treatment was not conducted. Test results showed that the coating porosity was 0.15%, and after corrosion at 800 ℃ / 2000 h in a hydrogen fluoride atmosphere, the surface morphology showed severe coating peeling and severe substrate corrosion. The total impurities of Fe+Cr+Ni+C in the product were 380 ppm, the rare earth fluoride purity reached 4N, and the service life was 0.6 times that of the graphite substrate.

[0076] Comparative Example 8 A 316L stainless steel substrate was selected, and the surface was roughened by sandblasting to Ra 40 μm. A Y₂O₃ + 2wt.% Ni rare earth oxide transition layer (80 μm thick) and a YF₃ rare earth fluoride main protective layer (0.45wt.% oxygen content, 120 μm thick) were sequentially prepared using plasma spraying, without any sealing treatment. No vacuum heat treatment was performed. The spraying power for both the Y₂O₃ + 2wt.% Ni rare earth oxide transition layer and the YF₃ rare earth fluoride main protective layer was 40 kW, the powder feed rate was 20 g / min, and the spraying distance was 120 mm. Test results showed that the coating porosity was 0.12%, and after HF corrosion at 800 ℃ for 2000 h, the surface morphology showed localized coating peeling and microcracks at the interface. The total Fe+Cr+Ni+C impurities in the product were 120 ppm, the rare earth fluoride purity reached 4N, and the service life was 0.8 times that of the graphite substrate. From the experimental results of the various embodiments and comparative examples in Table 1, it can be seen that: 1) Embodiments 1-8 of the present invention all introduced a dense thermal expansion transition layer with a gradient composition of rare earth oxides and metallic nickel. The porosity of the coating was controlled at an extremely low level of 0.05%-0.09%. After corrosion in a high-temperature hydrogen fluoride atmosphere at 800℃ / 2000h, the coating remained intact and did not peel off. In contrast, comparative examples 6-8 did not introduce a thermal expansion transition layer. Under the same working conditions, the coating peeled off, through cracks, or interface microcracks appeared. The content of metallic impurities in the product was as high as 120-450ppm, and the purity only reached 4N. This shows that the transition layer effectively buffered the thermal mismatch stress between the steel substrate and the ceramic coating, solving the industry problem of easy cracking and peeling of traditional coatings. 2) Embodiments 1-8 of the present invention achieved full inert protection through a multi-layer gradient structure of "transition layer + rare earth fluoride main protective layer + inorganic sealing layer". The total amount of metal and carbon impurities in the product is ≤8ppm, and the purity of rare earth fluorides consistently reaches 5N grade, effectively improving the problems of stainless steel corrosion product contamination and graphite powdering and carburization in Comparative Examples 1-3 and 5. 3) Examples 1-8 of this invention all use 316L or 904L corrosion-resistant steel substrates containing rare earth elements such as La, Ce, Y, Gd, and Er. The total amount of Fe+Cr+Ni+C impurities in the product is only 2-8ppm. Rare earth microalloying significantly improves the interfacial stability of the steel substrate under high-temperature fluorination environment, further inhibiting the outward diffusion of metal ions and improving corrosion resistance from the source. 4) The service life of Examples 1-8 of this invention is 3.5-5.5 times that of graphite substrates, far exceeding the 0.2-0.8 times of Comparative Examples 1-3 and 6-8. Among them, Example 6 has a service life of 5.5 times that of graphite, exhibiting the best overall performance.

[0077] Accordingly, a third aspect of the present invention provides an application of the above-mentioned fluorine-resistant substrate material for preparing high-purity rare earth fluorides. The fluorine-resistant substrate material is applied in equipment for preparing high-purity rare earth fluorides by reacting rare earth oxides with anhydrous hydrogen fluoride. The equipment includes a fluorination furnace lining, crucible, tray, or other container for holding the reaction and generated materials.

[0078] This application goes beyond simply listing the use cases of substrate materials. Instead, it precisely embeds a fully inert, dense protective system, constructed collaboratively by the aforementioned functional layers, into critical interface locations in the fluorination process where it directly contacts high-temperature corrosive media and high-purity materials. In fluorination furnace lining applications, a large-area gradient composite substrate replaces traditional platinum or graphite linings, enabling the furnace chamber to withstand continuous corrosion from anhydrous hydrogen fluoride at temperatures above 500°C. This avoids the cost constraints of platinum solutions and the inherent carbonaceous particle shedding contamination of graphite solutions. In crucible and tray applications, the substrate material directly supports rare earth oxide raw materials and the rare earth fluoride products generated during the reaction. Its rare earth fluoride main protective layer is chemically highly homologous to the contained materials, preventing the introduction of foreign impurities even under prolonged high-temperature contact. Combined with the inorganic sealing layer's complete surface sealing treatment, it eliminates adsorption residues in the pores, ensuring the stability and consistency of product purity between batches. For other containers holding reaction and generated materials, the substrate material uses the same fully inert and dense protection mechanism to build a barrier against corrosion and contamination on the surface of components with various complex geometries. This ensures that the core reaction section of the entire fluorination production line is covered by a unified ultra-high cleanliness protection system, thereby fully transforming the fully inert protection effect at the material level into a complete process guarantee capability for industrial-grade high-purity rare earth fluoride production.

[0079] The embodiments of this invention aim to protect a fluorine-resistant substrate material for preparing high-purity rare earth fluorides and its preparation method, which has the following effects: 1. Through a multi-layered functional synergistic design of “rare earth microalloyed corrosion-resistant steel substrate + high-purity rare earth oxide-nickel mixed transition layer + rare earth fluoride main protective layer + inorganic sealing dense layer”, a fully inert dense protective barrier from the substrate to the surface is constructed. Among them, the rare earth microalloyed corrosion-resistant steel substrate inhibits the migration activity of metal ions such as iron and chromium at high temperatures from the source; the high-purity rare earth oxide-nickel mixed transition layer forms a secondary blockage against the penetration of corrosive media into the substrate by virtue of the chemical inertness and extremely high purity of rare earth oxides, and effectively buffers the thermal mismatch stress between the substrate and the ceramic coating by utilizing the low thermal expansion coefficient and high thermal conductivity of nickel; the rare earth fluoride main protective layer has unparalleled chemical stability in an anhydrous hydrogen fluoride atmosphere of 500~900℃, and does not react with the reaction medium and products. Combined with the inorganic sealing dense layer to completely seal the residual micropores of the sprayed coating, the four-layer structure works synergistically to fundamentally eliminate the contamination of rare earth fluoride products by metal impurity ions and carbon particles. It can stably prepare ultra-high purity rare earth fluoride products with a purity of 5N or higher, and completely overcome the industry pain points of traditional graphite substrate powder shedding and carburization, as well as the high cost of platinum substrates that prevent large-scale application. 2. A gradient distribution of metallic nickel is creatively introduced into the high-purity rare-earth oxide transition layer. Utilizing Ni's low coefficient of thermal expansion and excellent thermal conductivity, thermal mismatch stress is effectively alleviated near the steel substrate. Simultaneously, the gradual compositional gradient design eliminates abrupt interface changes, significantly improving coating adhesion strength. Furthermore, the high-pressure sandblasting roughening process provides extremely high initial adhesion strength, effectively solving the problems of interface cracking, peeling, and flaking caused by differences in thermal expansion coefficients in traditional metal-based ceramic coatings under high-temperature cyclic service conditions. In addition, the addition of rare-earth elements significantly improves the thermal stability of the surface oxide film in the rare-earth microalloyed corrosion-resistant steel substrate, further suppressing abnormal oxide growth and flaking at the interface under high temperatures. The synergistic effect of these multiple interface strengthening mechanisms enables the gradient composite substrate material of this invention to maintain excellent structural integrity and interface adhesion reliability during repeated heating and cooling cycles, significantly improving the service safety and lifespan of equipment under extreme conditions. 3. Through a combined fabrication process of "plasma spraying + vacuum densification heat treatment + inorganic sealing," coupled with precise design of each functional layer component and accurate control of thickness and porosity, the full densification of the microstructure and the stable and consistent macroscopic properties of the gradient composite substrate material were achieved. Plasma spraying technology ensured the efficient and uniform forming of each functional layer, while the subsequent vacuum densification heat treatment at 600~800℃ eliminated the microcracks and loose pores inevitably generated during the spraying process, controlling the porosity of the transition layer and the main protective layer to below 1% and 3%, respectively. Meanwhile, the inorganic silicon... The acid salt sealing agent penetrates and solidifies in the micropores on the surface of the main protective layer, forming a continuous and dense closed film that completely blocks any possible channels for corrosive media to penetrate. This combined process, in conjunction with the gradient structure design, enables the substrate material to maintain its complete protective function and stable structural state after being corroded for more than 2,000 hours in an anhydrous hydrogen fluoride environment at 800°C. The overall manufacturing cost is reduced by more than 90% compared to platinum substrates, ensuring the requirements for ultra-high purity product preparation while fully meeting the long-term stable operation requirements of large-scale industrial production lines.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A fluorine-resistant substrate material for preparing high-purity rare earth fluorides, characterized in that, The fluorine-resistant substrate material has a gradient composite structure, consisting of: a corrosion-resistant steel substrate, a transition layer, a rare earth fluoride main protective layer, and an inorganic sealing and dense layer. The transition layer is deposited on the surface of the corrosion-resistant steel substrate, and the transition layer is a dense mixed layer of high-purity rare earth oxides and nickel; The rare earth fluoride main protective layer is deposited on the surface of the transition layer away from the corrosion-resistant steel substrate; The inorganic sealing dense layer fills the micropores on the side surface of the rare earth fluoride main protective layer away from the transition layer, forming an inert dense protective structure without through pores.

2. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 1, characterized in that, The corrosion-resistant steel matrix includes at least one of lanthanum, cerium, yttrium, gadolinium, erbium, and scandium; The rare earth element content in the corrosion-resistant steel matrix is ​​30 ppm to 500 ppm by mass.

3. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 1, characterized in that, The corrosion-resistant steel surface is treated with high-pressure sandblasting, and the roughness Ra is 50 μm ~ 80 μm.

4. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 1, characterized in that, The transition layer contains 0.5 wt.% to 8.0 wt.% nickel, and the purity of the high-purity rare earth oxides is ≥99.9%; the contents of Al, Si, Ca, and Mg are all <5 ppm.

5. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 1, characterized in that, The transition layer forms a continuous gradient substructure along the thickness direction from the side near the corrosion-resistant steel substrate to the side near the rare earth fluoride main protective layer. Among them, the nickel content gradually decreased from 0.5wt.% to 8.0wt.% to 0wt.%, while the high-purity rare earth oxide content gradually increased from 0wt.% to 100wt.%.

6. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 1, characterized in that, The rare earth fluoride main protective layer includes at least one of rare earth fluorides, rare earth fluorides, and rare earth oxides, with an oxygen content of <0.4 wt.%.

7. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 1, characterized in that, The inorganic sealing dense layer is a silicate high-temperature resistant sealing layer, wherein the silicate is magnesium silicate and / or calcium silicate. It is filled and cured in the micropores on the surface of the rare earth fluoride main protective layer to form a continuous and dense sealing film on the surface of the main protective layer.

8. The fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to any one of claims 1-6, characterized in that, The thickness of the corrosion-resistant steel substrate is 5 mm to 20 mm; The thickness of the transition layer is 50 μm ~ 200 μm, and the porosity is ≤1%. The thickness of the rare earth fluoride main protective layer is 50 μm ~ 300 μm, and the overall porosity is ≤3%; The thickness of the inorganic sealing dense layer is 20 μm to 70 μm.

9. A method for preparing a fluorine-resistant substrate material for preparing high-purity rare earth fluorides, characterized in that, The preparation of the fluorine-resistant substrate material as described in any one of claims 1-8 includes the following steps: S1. Substrate pretreatment: Grind the corrosion-resistant steel substrate to a smooth surface, remove surface rust and oxide scale, perform high-pressure sandblasting roughening treatment on the surface, ultrasonic cleaning to remove surface dust and impurities, and dry for later use. S2, Transition layer spraying: Using multiple sets of rare earth oxide + nickel mixed powders with different ratios, a continuous composition gradient transition layer is prepared by plasma spraying layer by layer with varying ratio deposition, gradually reducing the proportion of nickel in the powder from the substrate to the outside. S3. Main protective layer spraying: Using plasma spraying technology, a well-proportioned mixture of rare earth fluorides, rare earth fluorides and rare earth oxides is sprayed onto the surface of the transition layer to form the main protective layer. S4. Vacuum densification treatment: The coated substrate is placed in a vacuum furnace, heated to 600 ℃~ 800 ℃, and held for 1.5 h~ 2.5 h for heat treatment. It is then cooled to room temperature with the furnace to eliminate micro-cracks and loose pores inside the coating and improve the overall density and high temperature stability of the coating. S5. Sealing treatment: Apply silicate sealing agent to the surface of the heat-treated main protective layer. After curing at room temperature, an inorganic sealing dense layer is formed, completing the substrate material preparation.

10. The method for preparing fluorine-resistant substrate material for preparing high-purity rare earth fluorides according to claim 9, characterized in that, The plasma spraying process in steps S2 and S3 has a power of 40 kW to 50 kW, a powder feed rate of 20 g / min to 30 g / min, and a spraying distance of 100 mm to 120 mm.

11. An application of the fluorine-resistant substrate material according to any one of claims 1-8, characterized in that, The fluorine-resistant substrate material is used in equipment for preparing high-purity rare earth fluorides by reacting rare earth oxides with anhydrous hydrogen fluoride. The equipment includes a fluorination furnace lining, crucible, tray, or other container for holding the reaction and generated materials.

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