C / c composite material ablation-resistant gradient composite coating and preparation method

Abstract

This invention provides a novel ablation-resistant gradient composite coating for C / C composite materials and its preparation method. The gradient composite coating mainly consists of a SiC transition inner coating, a refractory metal carbide intermediate layer, and a lanthanum harzianite (La2Hf2O7) outer coating. The preparation method is as follows: (1) mixing and granulation of hafnium oxide and lanthanum oxide powders; (2) high-temperature sintering reaction and synthesis of lanthanum harzianite spray powder; (3) high-temperature heat treatment before coating the C / C composite material; (4) preparation of the SiC transition inner coating using a high-temperature chemical reaction process; (5) preparation of the refractory metal carbide intermediate layer using a plasma spraying process; and (6) preparation of the lanthanum harzianite outer coating using a plasma spraying process. This invention provides a new technical approach for long-term ultra-high temperature ablation protection of C / C composite materials, enhances the overall bonding strength and high-temperature structural stability of the gradient coating, and further enhances the overall erosion resistance and high-temperature protection performance of the gradient coating.

Description

Technical Field

[0001] This invention relates to the field of ultra-high temperature corrosion protection, specifically to a C / C composite material ablation-resistant gradient composite coating and its preparation method. Background Technology

[0002] C / C composite materials possess a range of excellent high-temperature properties, such as high high-temperature strength retention, thermal shock resistance, ablation resistance, and good high-temperature stability (no melting point, sublimation temperature ≥3800℃). Furthermore, the inherent lightweight nature of carbon materials makes them ideal candidates for lightweight thermal structural materials in aerospace applications. Therefore, they are widely used in rocket throat liners, tail nozzles, brake discs, and nose cones and leading edges of next-generation aircraft, demonstrating significant advantages. In addition, carbon materials exhibit excellent corrosion resistance, thus possessing enormous application potential in high-temperature corrosion protection fields such as machinery, metallurgy, and new energy.

[0003] In a high-temperature, oxygen-rich atmosphere, C / C composites begin to oxidize above 400°C. The oxidation rate accelerates with increasing temperature, leading to a rapid deterioration of their mechanical and thermophysical properties. To improve the high-temperature oxidation resistance of C / C composites, researchers both domestically and internationally have developed two methods: anti-oxidation coatings and matrix modification. While matrix modification improves the oxidation resistance of the carbon matrix to some extent, it does not solve the problem of high-temperature oxidation resistance of carbon fibers, thus exhibiting significant limitations. Anti-oxidation coatings effectively isolate oxygen from direct contact with the carbon matrix, fundamentally solving the problem of high-temperature oxidation protection for both the carbon matrix and carbon fibers. This method is considered the most mature and effective anti-oxidation protection method currently available.

[0004] In recent years, scholars both domestically and internationally have developed a series of coating systems to address the high-temperature oxidation and ablation protection of C / C composites. These systems include silicide, carbide, boride, noble metal, oxide, and glass coatings. Relying on the excellent high-temperature self-healing properties of silicon dioxide or borosilicate glass formed by oxidation, long-term protection of C / C composites below 1600℃ has been largely achieved. However, as the oxidation temperature increases, the silicon-based coating begins to transform from inert oxidation to active oxidation, and its oxidation products change from molten silicon dioxide to gaseous SiO, leading to the destruction and failure of the oxide film structure. To further improve the oxidation and ablation resistance of C / C composites at temperatures above 1800℃, an ultra-high temperature ceramic ablation-resistant coating has been developed. During the ablation process, the self-healing oxide formed on the surface of the ultra-high temperature ceramic coating can adhere to the coating surface, thus providing good protection. However, as the ablation time increases, the liquid phase in the molten oxide phase will be continuously consumed. When the molten liquid phase cannot effectively seal the defects such as cracks and pores caused by the high-melting-point zirconium oxide and hafnium oxide phase transformation in the oxide film, the scouring and intrusion of high-temperature combustion gases at the defects will lead to the peeling and failure of the oxide film, thereby further affecting its high-temperature ablation resistance. Therefore, to further improve the oxidation and ablation resistance of C / C composites above 1800℃, it is necessary to develop new ablation-resistant coatings.

[0005] Lanthanum hafnium oxide possesses advantages such as a high melting point (≥2420℃) and low thermal conductivity (only 1.34 W / mK at 1000℃), making it an effective heat insulation and oxygen barrier as an ablation-resistant coating. Furthermore, its superior high-temperature structural stability is unmatched by other oxide ceramics; it undergoes no phase transformation when heated from room temperature to its melting point. Therefore, it effectively maintains the structural stability and integrity of the coating during ultra-high temperature ablation. Compared to the oxide film formed by the ablation of ultra-high temperature ceramic coatings, it exhibits significant advantages in structural stability and erosion resistance. Therefore, its application in ablation-resistant coatings can further effectively improve the ultra-high temperature ablation protection performance (≥2000℃) of C / C composite materials. However, there are currently few literature reports on the preparation of lanthanum hafnium oxide ablation-resistant coatings.

[0006] Although lanthanum hafnium oxide coatings possess excellent oxygen barrier and thermal insulation properties, as well as good high-temperature structural stability, their coefficient of thermal expansion is too high (8.76 × 10⁻⁶) as an ablation-resistant coating for C / C composite materials. -6 K -1 ), and C / C composite materials (1~2×10 -6 K -1The thermal expansion coefficients of the two materials differ significantly. During the ablation process, the mismatch in thermal expansion coefficients will directly lead to the coating peeling and failure. In addition, under high temperature conditions, lanthanum hafnium oxide is incompatible with carbon. The two will react to form carbides, which will cause the coating interface to fail. Therefore, in order to achieve ultra-high temperature protection of C / C composite materials with lanthanum hafnium oxide coating, it is necessary to solve the problems of thermal matching and chemical and physical compatibility between lanthanum hafnium oxide coating and C / C composite materials. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a C / C composite material ablation-resistant gradient composite coating. The gradient composite coating exhibits a gradually increasing coefficient of linear expansion from the inside out, and primarily consists of a SiC transition inner coating, a refractory metal carbide intermediate layer, and a lanthanum hafnium oxide (La2Hf2O7) outer coating. This invention effectively mitigates the ablation resistance of the lanthanum hafnium oxide outer coating (8.76 × 10⁻⁶) by designing a multi-layered gradient composite coating with a gradually increasing coefficient of linear expansion from the inside out. -6 K -1 ) and C / C matrix (1~2×10 -6 K -1 The three-layer gradient structure addresses the cracking and detachment failure caused by the significant difference in linear expansion coefficients. Furthermore, it offers the following significant advantages: the silicon carbide transition inner coating, prepared using a chemical reaction process, provides excellent adhesion between the coating and the substrate, effectively hindering the diffusion and penetration of carbon from the substrate to the outer layer; the refractory metal carbide intermediate layer further alleviates the mismatch in linear expansion coefficients between the SiC inner coating and the lanthanum hafnium oxide outer coating, further suppressing the diffusion of C from the substrate to the outside and O from the outside to the inside, while also acting as a secondary ablation protection barrier; the lanthanum hafnium oxide coating possesses a high melting point (≥2420℃) and low thermal conductivity (only 1.34 W / mK at 1000℃), serving as an outer layer to act as an oxygen barrier, heat insulation agent, and resistant to combustion gas erosion. In addition, the lanthanum hafnium oxide coating exhibits excellent high-temperature stability, with no phase transition during the temperature rise from room temperature to the melting point, demonstrating excellent high-temperature structural stability and maintaining a complete and continuous coating structure during ablation. This gradient composite coating structure not only solves the failure problem caused by the mismatch in thermal expansion coefficients between the lanthanum hafnium oxide outer coating and the C / C matrix, but also leverages the excellent high-temperature resistance, thermal insulation, oxygen barrier properties, and structural stability of the lanthanum hafnium oxide coating. Furthermore, the construction of a double-layer ablation-resistant coating consisting of an ablation-resistant refractory metal carbide intermediate layer and a lanthanum hafnium oxide outer coating further enhances the long-term ablation protection life of this gradient coating, significantly improving the ablation resistance of traditional ultra-high temperature ceramic coatings. This gradient composite coating design and preparation provides a new technical approach for long-term ablation protection of C / C composite materials.

[0008] A method for preparing a C / C composite ablation-resistant gradient composite coating includes the following steps: (1) Mixing and granulation of hafnium oxide and lanthanum oxide powders: Before mixing, hafnium oxide and lanthanum oxide fine powders were dried, and then mixed at a molar ratio of 2:1. Anhydrous ethanol was then added and the mixture was thoroughly ball-milled. After the mixture was ball-milled and homogeneous, it was granulated and dried. To prevent the granulated powder from absorbing water, it was vacuum-dried for later use.

[0009] (2) High-temperature sintering reaction and synthesis of lanthanum hafnium oxide spray powder: The dried granulated powder in (1) was subjected to a high-temperature sintering reaction to prepare lanthanum hafnium oxide spray powder. The high-temperature sintering atmosphere was air or an inert atmosphere, the sintering reaction temperature was 1300-1500℃, and the sintering reaction time was 4-10h.

[0010] (3) High-temperature heat treatment before coating of C / C composite material: The C / C composite material is placed in a high-temperature furnace for high-temperature pore opening treatment. Then the treated material is machined into a predetermined size, polished with sandpaper, ultrasonically cleaned and dried for later use.

[0011] (4) Preparation of SiC transition inner coating by high-temperature chemical reaction process: The heat-treated and machined C / C composite material is placed in silicon powder and then placed together in a graphite crucible. Subsequently, the graphite crucible containing the C / C sample and silicon powder is placed in a high-temperature furnace and heated to 1700-1900℃ for high-temperature reaction to prepare the SiC transition inner coating. By controlling the number of high-temperature reactions, a SiC transition inner coating with a certain thickness is obtained.

[0012] (5) Plasma spraying process to prepare refractory metal carbide intermediate layer: The SiC coating C / C sample in (3) is ultrasonically cleaned and dried, and the refractory metal carbide intermediate layer is prepared by plasma spraying process. By controlling the spraying time and number of spraying, the thickness of the prepared intermediate layer is kept in the range of 50 to 120 μm.

[0013] (6) Plasma spraying process to prepare lanthanum hafnium oxide outer coating: Using the lanthanum hafnium oxide spraying powder in (2) as raw material, the lanthanum hafnium oxide outer coating is prepared by plasma spraying process. By controlling the spraying time and number of sprayings, the thickness of the prepared outer coating is kept in the range of 120 to 230 μm.

[0014] Furthermore, the C / C composite ablation-resistant gradient composite coating has a total coating thickness of 320–550 μm.

[0015] Furthermore, in step (5), the refractory metal carbide intermediate layer is two or more of HfC, ZrC, TaC, TiC, NbC, and SiC.

[0016] Furthermore, in step (1), the fine powders of hafnium oxide and lanthanum oxide both have a particle size ≤1μm.

[0017] Furthermore, the lanthanum hafnium oxide powder obtained after the sintering reaction in step (2) has a particle size of 20–70 μm.

[0018] Furthermore, the C / C composite material in step (3) has a density of 1.70–1.75 g / cm³. 3 .

[0019] Furthermore, the high-temperature opening treatment temperature in step (3) is 2000~2200℃, and the heat preservation time is 0.5~2h.

[0020] Furthermore, in step (4), when the thickness of the SiC transition inner coating is ≤70μm after the first high-temperature reaction, a second high-temperature reaction is required to increase its thickness. Preferably, the thickness of the SiC transition inner coating is 70-200μm.

[0021] Furthermore, the plasma spraying process in step (5) has the following process parameters: spraying power of 38-46kW, argon flow rate of 35-55L / min, hydrogen flow rate of 5-12L / min, and spraying distance of 80-120mm.

[0022] Furthermore, the plasma spraying process in step (6) has the following process parameters: spraying power of 35-40kW, argon flow rate of 35-55L / min, hydrogen flow rate of 4-8L / min, and spraying distance of 80-120mm.

[0023] The gradient composite coating of this patent invention has the following advantages: (1) To further address the ablation protection problem of C / C composites above 2000℃, a three-layer coating with a gradient distribution of linear expansion coefficient from the inside to the outside was invented. It mainly consists of a SiC transition inner coating, a refractory metal carbide intermediate layer, and a lanthanum hafnium oxide outer coating. The gradient structure effectively solves the failure problem caused by the mismatch of linear expansion coefficient between the lanthanum hafnium oxide coating and the C / C matrix, and fully leverages the excellent oxygen barrier and heat insulation properties and good high-temperature structural stability of the lanthanum hafnium oxide outer coating. After 60s of oxyacetylene ablation at 2400℃, the lanthanum hafnium oxide outer coating remained intact, and the bonding between the coating layers was good, indicating that it has the potential for long-term ablation protection at ultra-high temperatures, providing a new technical approach for long-term ultra-high temperature ablation protection of C / C composites.

[0024] (2) By designing different transition layers, intermediate layers and outer coatings, ablation-resistant multilayer gradient composite coatings with a thickness of 320-550 μm were prepared. Based on the construction of a double-layer carbide transition layer and intermediate layer, the high-temperature chemical and physical compatibility problem between the lanthanum hafnium oxide coating and the C / C substrate was effectively solved. In addition, the SiC thick coating prepared by chemical reaction process effectively solved the bonding problem of gradient ultra-thick coatings, and the overall bonding force and high-temperature structural stability of the gradient coating were enhanced.

[0025] (3) Based on the spraying characteristics of refractory metal carbide coatings and lanthanum hafnium oxide coatings, and using differentiated spraying processes, a refractory metal carbide intermediate layer and a lanthanum hafnium oxide outer layer were prepared. During the spraying of the intermediate layer, the content of the oxide phase in the refractory metal carbide coating was reduced by appropriately increasing the flow rate of the protective hydrogen gas, thereby improving the overall temperature resistance and structural stability of the coating. During the spraying process, lanthanum hafnium oxide does not undergo oxidation, so the structure of the lanthanum hafnium oxide outer layer obtained by spraying is more compact, further enhancing the overall erosion resistance of the gradient coating. In addition, the prepared refractory metal carbide intermediate layer and lanthanum hafnium oxide outer layer provide a double-layer protective barrier for the ultra-high temperature ablation protection of C / C composite materials, further improving their high-temperature protection performance and service life. Attached Figure Description

[0026] Figure 1 The XRD pattern of the outermost lanthanum hafnium oxide coating of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1; Figure 2 The image shows the surface microstructure of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1. Figure 3 The cross-sectional microstructure and elemental surface scan analysis results of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 are shown below. Figure 4 This is a photograph of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 after ablation. Figure 5 The curve shows the surface temperature change of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 during 60s of oxyacetylene flame ablation. Figure 6 The images show the XRD patterns of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 after ablation at different times. Figure 7 The microstructure of the central region of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 after ablation at different times is shown. Figure 8 The results of cross-sectional morphology and elemental surface scanning analysis of the ablation center, transition zone and edge zone of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 are shown. Figure 9 The cross-sectional microstructure and elemental surface scan analysis results of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2 are shown below. Figure 10This is a photograph of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2 after ablation for 30 seconds. Figure 11 The microstructure of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2 after ablation for 30 s. Figure 12 The cross-sectional microstructure and elemental surface scan analysis results of the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3 are shown below. Figure 13 This is a photograph of the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3 after ablation for 30 seconds. Figure 14 The image shows the surface microstructure of the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3 after ablation for 30 seconds. Detailed Implementation

[0027] Example 1: Hafnium oxide and lanthanum oxide fine powders with a particle size of 1 μm were dried and then mixed at a molar ratio of 2:1. Anhydrous ethanol was then added and the mixture was thoroughly ball-milled. After uniform ball milling, the mixture was granulated and dried. To prevent water absorption, the granulated powder was vacuum-dried before use. The dried granulated powder was then subjected to a high-temperature sintering reaction at 1350℃ to prepare lanthanum hafnium oxide spray powder. The high-temperature sintering atmosphere was inert, and the sintering reaction time was 5 hours. After sintering, the obtained lanthanum hafnium oxide spray powder had a particle size of 20–61 μm. A density of 1.70 g / cm³ was selected. 3 The needle-punched C / C composite material was subjected to high-temperature heat treatment at 2000℃ for 2 hours, and then the treated material was machined into φ30×10mm shapes. 3 Circular samples were prepared, polished with sandpaper, ultrasonically cleaned, and dried for later use. The heat-treated and machined C / C composite material was placed in silicon powder and then placed together in a graphite crucible. The graphite crucible containing the C / C sample and silicon powder was then placed in a high-temperature furnace and heated to 1900℃ for a high-temperature reaction to prepare a SiC transition inner coating. The high-temperature reaction time was 1 hour. The prepared SiC coating sample underwent a second high-temperature reaction, and the thickness of the prepared SiC transition inner coating ranged from 70 to 150 μm.

[0028] The prepared SiC-coated C / C samples were ultrasonically cleaned and dried. Using ZrC-SiC composite spraying powder as raw material (mass ratio 5:1), a ZrC-SiC interlayer was prepared using plasma spraying. The plasma spraying process parameters were as follows: spraying power 38–46 kW, argon flow rate 35–55 L / min, hydrogen flow rate 5–12 L / min, and spraying distance 100 mm. By controlling the spraying time and number of sprays, the thickness of the prepared ZrC-SiC interlayer was maintained within the range of 50–100 μm. Using the prepared lanthanum hafnium oxide spraying powder as raw material, a lanthanum hafnium oxide outer coating was prepared on the surface of the ZrC-SiC interlayer using plasma spraying. The plasma spraying process parameters were as follows: spraying power 35–40 kW, argon flow rate 35–55 L / min, hydrogen flow rate 4–8 L / min, and spraying distance 100 mm. By controlling the spraying time and number of spraying cycles, the thickness of the prepared outer coating is kept within the range of 150–200 μm.

[0029] Figure 1 The image shows the XRD pattern of the outermost layer of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1. Phase analysis reveals that only lanthanum hafnium oxide diffraction peaks appear on the coating surface, indicating that the prepared coating phase is pure and free of other impurities. This also demonstrates that the prepared lanthanum hafnium oxide outer coating has a dense structure and a certain thickness, preventing X-rays from penetrating it.

[0030] Figure 2 The image shows the surface microstructure of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1. It can be seen that the outermost lanthanum hafnium oxide coating exhibits a typical plasma-sprayed coating morphology, which is mainly composed of a fully molten phase and particles that were not fully molten during the spraying process.

[0031] Figure 3The cross-sectional microstructure and elemental scanning analysis results of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 are shown. The elemental scanning analysis results indicate that the coating consists of a SiC transition layer, a ZrC-SiC intermediate layer, and a La2Hf2O7 outer coating, with thicknesses of 100–150 μm, 50–100 μm, and 150–210 μm, respectively, from the inside out. It can be seen that the interface features between the three coating layers (SiC, ZrC-SiC, and La2Hf2O7) are quite distinct. Due to the relatively rough surface of the SiC layer formed by the high-temperature chemical reaction, the sprayed ZrC-SiC coating can form a jagged structure with the SiC layer, ensuring good bonding between the coatings. Furthermore, the La2Hf2O7 outer layer and the ZrC-SiC intermediate layer are also tightly bonded, and no obvious cracks, pores, or other defects are formed at their interface. Since the coefficients of linear expansion of SiC, ZrC, and La2Hf2O7 are 4.5, 6.7, and 8.76 × 10⁻⁶, respectively... -6 K -1 Therefore, from the inside out, the coefficient of linear expansion of the multilayer coating exhibits a gradient distribution.

[0032] The La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 was ablated by an oxyacetylene flame for 30 s, and its mass ablation rate and linear ablation rate were -0.53 mg / s and -11.30 μm / s, respectively. After further extending the ablation time to 60 s, its mass ablation rate and linear ablation rate were 1.12 mg / s and 6.79 μm / s, respectively, indicating that it has excellent high-temperature ablation resistance.

[0033] Figure 4 Photographs of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 after oxyacetylene ablation at different times. Figure 4 As shown in the photograph of the actual object after 30 seconds of ablation, the color is darkest at the center of the coating, gradually lightening towards the edges, approaching the bright white color of the original spray coating. Unlike ultra-high temperature coatings, the ablated coating surface is dense, relatively smooth, and flat, without obvious protrusions or peeling marks, maintaining the integrity of the coating structure on a macroscopic level. Figure 4 Photo b shows the actual object after 60 seconds of ablation. The coating surface in the ablation center is molten and glassy, ​​indicating that the surface phase has nearly melted. After ablation, the coating as a whole did not show obvious peeling, protrusions, or depressions, and the surface still maintained a complete structural morphology, further demonstrating that the coating has good high-temperature structural stability and long-term ablation resistance potential.

[0034] Figure 5The image shows the surface temperature change curve of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 during 60 s oxyacetylene flame ablation. It can be observed that the surface temperature rises sharply from around 1300℃ to 2200℃, then rises slowly, reaching a maximum of 2414℃. During the ablation process, the outer layer of lanthanum hafnium oxide maintains its intact structural morphology, indicating that it still possesses good structural stability under ultra-high temperature conditions of 2400℃.

[0035] Figure 6 The XRD patterns of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 after ablation for different times are shown. Phase analysis results show that the phase of the outer coating remains consistent with that during spraying after different ablation times, both being the La2Hf2O7 phase. Even under an oxyacetylene flame at temperatures exceeding 2400℃, the phase of the La2Hf2O7 coating did not undergo significant changes, further demonstrating its excellent high-temperature structural stability.

[0036] Figure 7 The microstructure of the central region of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 after ablation at different times is shown. Figure 7 As can be seen, after 30 seconds of ablation, the surface of the La2Hf2O7 coating remains smooth and dense, basically maintaining its overall morphology before spraying. Figure 7 b shows the surface morphology after 60s of ablation. As the ablation time increases, more molten phase appears on the surface of the La2Hf2O7 coating. Under the long-term scouring of the oxyacetylene flame, the molten phase flows and fills the defects on the surface, forming a denser coating morphology.

[0037] Figure 8 The cross-sectional microstructure and elemental surface scanning analysis results of the ablation center region, transition region, and edge region of the La2Hf2O7 / ZrC-SiC / SiC gradient coating prepared in Example 1 are shown. Figure 8 Figure a shows the cross-sectional morphology of the ablation center region. It can be seen that the overall structure of the La2Hf2O7 outer coating is intact, and the structures of the intermediate and transition inner coatings remain almost unchanged. Furthermore, the interlayer interfaces of the coatings remain tightly bonded, indicating good adhesion. Although significant propagation cracks appeared in the La2Hf2O7 outer coating, they were relatively stable in the transition (…). Figure 8 b) and edge areas ( Figure 8c) The La2Hf2O7 outer coating and the ZrC-SiC intermediate layer interface remain tightly bonded, indicating good structural integrity. Because the La2Hf2O7 outer coating is brittle, external forces during mounting and repeated grinding processes can damage the coating structure, leading to a number of cracks. Furthermore, given the self-healing properties of the lanthanum hafnium oxide molten phase formed during ablation, which can effectively fill cracks and other defects, the cracks in the La2Hf2O7 outer coating are mainly related to the external force damage caused by the mounting and grinding processes, rather than being formed during ablation. Meanwhile, elemental surface scanning analysis also confirmed ( Figure 8 After 60 seconds of ablation, almost no oxygen was found in the ZrC-SiC intermediate layer and the SiC transition inner coating, further demonstrating that the La2Hf2O7 outer coating has excellent oxygen barrier and heat insulation effects, providing effective protection for the intermediate and inner coatings. Since the ZrC-SiC intermediate layer and the SiC transition inner coating also have good ablation resistance, even after the La2Hf2O7 outer coating fails during the later stages of ablation, it can still act as a second ablation protection barrier. Therefore, the novel ablation-resistant coating designed and prepared in this invention has excellent long-term ablation resistance potential, providing a new technical approach for ultra-high temperature long-term ablation protection of C / C composite materials.

[0038] Example 2: Hafnium oxide and lanthanum oxide fine powders with a particle size of 1 μm were dried and then mixed at a molar ratio of 2:1. Anhydrous ethanol was then added and the mixture was thoroughly ball-milled. After uniform ball milling, the mixture was granulated and dried. To prevent the granulated powder from absorbing water, it was vacuum-dried for later use. The dried granulated powder was then subjected to a high-temperature sintering reaction at 1350℃ to prepare lanthanum hafnium oxide spray powder. The high-temperature sintering atmosphere was inert, and the sintering reaction time was 5 hours. After sintering, the obtained lanthanum hafnium oxide spray powder had a particle size of 20–61 μm. A density of 1.70 g / cm³ was selected. 3 The needle-punched C / C composite material was subjected to high-temperature heat treatment at 2000℃ for 2 hours, and then the treated material was machined into φ30×10mm shapes. 3 Circular samples were prepared, polished with sandpaper, ultrasonically cleaned, and dried for later use. The heat-treated and machined C / C composite material was placed in silicon powder and then placed together in a graphite crucible. The graphite crucible containing the C / C sample and silicon powder was then placed in a high-temperature furnace and heated to 1800–1850 °C for a high-temperature reaction to prepare a SiC transition inner coating. The high-temperature reaction time was 1 hour. The prepared SiC coating sample underwent a second high-temperature reaction, and the thickness of the prepared SiC transition inner coating was in the range of 70–100 μm.

[0039] The prepared SiC-coated C / C samples were ultrasonically cleaned and dried. Using ZrC-TaC spraying powder (mass ratio 2:1) as raw material, a ZrC-TaC interlayer was prepared by plasma spraying. The plasma spraying process parameters were as follows: spraying power 38–46 kW, argon flow rate 35–55 L / min, hydrogen flow rate 5–12 L / min, and spraying distance 100 mm. By controlling the spraying time and number of sprays, the thickness of the prepared ZrC-TaC interlayer was maintained within the range of 80–120 μm. Using the prepared lanthanum hafnium oxide spraying powder as raw material, a lanthanum hafnium oxide outer coating was prepared on the surface of the ZrC-TaC interlayer by plasma spraying. The plasma spraying process parameters were as follows: spraying power 35–40 kW, argon flow rate 35–55 L / min, hydrogen flow rate 4–8 L / min, and spraying distance 100 mm. By controlling the spraying time and number of spraying cycles, the thickness of the prepared outer coating is kept within the range of 150–200 μm.

[0040] Figure 9 The figures show the cross-sectional microstructure and elemental scanning analysis results of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2. As shown in the figures, the thicknesses of the prepared SiC transition layer, ZrC-TaC intermediate layer, and La2Hf2O7 outer coating are 70–100 μm, 80–120 μm, and 150–200 μm, respectively. It can be seen that the interfaces between the three coating layers are relatively clear and tightly bonded. The linear expansion coefficients of SiC, ZrC, TaC, and La2Hf2O7 are 4.5, 6.7, 7.08, and 8.76 × 10⁻⁶, respectively. -6 K -1 Therefore, from the inside out, the coefficient of linear expansion of the multilayer coating exhibits a gradient distribution.

[0041] The oxyacetylene ablation resistance of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2 was tested. After 30 s of ablation, the linear ablation rate and mass ablation rate of the coating were -0.41 mg / s and 10.30 μm / s, respectively. The negative mass ablation rate indicates that it has good high-temperature ablation resistance.

[0042] Figure 10 The image shows a photograph of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2 after 30 seconds of oxyacetylene ablation. As can be seen from the image, the coating surface is relatively smooth and dense after ablation, with no obvious protrusions or peeling marks. The macroscopic structure remains intact, indicating that it has good resistance to ablation and erosion by combustion gases.

[0043] Figure 11The surface microstructure of the La2Hf2O7 / ZrC-TaC / SiC gradient coating prepared in Example 2 after 30 s of oxyacetylene ablation. From the ablation center region ( Figure 11 a), Transition Zone ( Figure 11 b) and the edge area ( Figure 11 c) The surface of the La2Hf2O7 coating maintains a similar morphology to that before spraying, with a relatively continuous and dense overall structure, and some differences in grain size in different regions. Since the temperature is highest in the ablation center and lowest in the edge region, the grain sizes in the ablation center and edge region are the largest and smallest, respectively. Under the continuous scouring of the oxyacetylene flame, the La2Hf2O7 grain structure on the surface exhibits an undulating pattern, without obvious ablation pits or porous structures, indicating that the coating has excellent high-temperature structural stability.

[0044] Example 3: Hafnium oxide and lanthanum oxide fine powders with a particle size of 1 μm were dried and then mixed at a molar ratio of 2:1. Anhydrous ethanol was then added and the mixture was thoroughly ball-milled. After uniform ball milling, the mixture was granulated and dried. To prevent water absorption, the granulated powder was vacuum-dried before use. The dried granulated powder was then subjected to a high-temperature sintering reaction at 1350℃ to prepare lanthanum hafnium oxide spray powder. The high-temperature sintering atmosphere was inert, and the sintering reaction time was 5 hours. After sintering, the obtained lanthanum hafnium oxide spray powder had a particle size of 20–61 μm. A density of 1.70 g / cm³ was selected. 3 The needle-punched C / C composite material was subjected to high-temperature heat treatment at 2000℃ for 2 hours, and then the treated material was machined into φ30×10mm shapes. 3 Circular samples were prepared, polished with sandpaper, ultrasonically cleaned, and dried for later use. The heat-treated and machined C / C composite material was placed in silicon powder and then placed together in a graphite crucible. The graphite crucible containing the C / C sample and silicon powder was then placed in a high-temperature furnace and heated to 1900℃ for a high-temperature reaction to prepare a SiC transition inner coating. The high-temperature reaction time was 1 hour. The prepared SiC coating sample underwent a second high-temperature reaction, and the thickness of the prepared SiC transition inner coating was in the range of 80–120 μm.

[0045] The prepared SiC-coated C / C samples were ultrasonically cleaned and dried. Using ZrC-HfC spraying powder as raw material (mass ratio 1:1), a ZrC-HfC intermediate layer was prepared by plasma spraying. The plasma spraying process parameters were as follows: spraying power 38–46 kW, argon flow rate 35–55 L / min, hydrogen flow rate 5–12 L / min, and spraying distance 100 mm. By controlling the spraying time and number of sprays, the thickness of the prepared ZrC-HfC intermediate layer was maintained within the range of 70–100 μm. Using the prepared lanthanum hafnium oxide spraying powder as raw material, a lanthanum hafnium oxide outer coating was prepared on the surface of the ZrC-HfC intermediate layer by plasma spraying. The plasma spraying process parameters were as follows: spraying power 35–40 kW, argon flow rate 35–55 L / min, hydrogen flow rate 4–8 L / min, and spraying distance 100 mm. By controlling the spraying time and number of spraying cycles, the thickness of the prepared outer coating is kept within the range of 120–150 μm.

[0046] Figure 12 The figures show the cross-sectional microstructure and elemental scanning analysis results of the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3. As shown in the figures, the thicknesses of the prepared SiC transition layer, ZrC-HfC intermediate layer, and La2Hf2O7 outer coating are 80–120 μm, 70–100 μm, and 120–150 μm, respectively. It can be seen that the interfaces between the three coating layers are relatively clear and tightly bonded. The linear expansion coefficients of SiC, ZrC-HfC, and La2Hf2O7 are 4.5, 6.7, 7.66, and 8.76 × 10⁻⁶, respectively. -6 K -1 Therefore, from the inside out, the coefficient of linear expansion of the multilayer coating exhibits a gradient distribution.

[0047] The oxyacetylene ablation resistance of the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3 was tested. After 30 s of ablation, the linear ablation rate and mass ablation rate of the coating were 8.21 mg / s and 10.70 μm / s, respectively, indicating that the coating has a relatively low ablation rate.

[0048] Figure 13 The image shows the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3 after 30 seconds of oxyacetylene ablation. It can be seen that after 30 seconds of ablation, the coating remains smooth and dense overall, with no obvious protrusions or peeling marks, indicating that its structure is intact.

[0049] Figure 14 This is a microstructure image of the surface of the La2Hf2O7 / ZrC-HfC / SiC gradient coating prepared in Example 3 after ablation for 30 s. After ablation, the ablation center ( Figure 14 a) Transition zone ( Figure 14 b) and edge areas ( Figure 14 c) The La2Hf2O7 coating still retains the structural morphology at the time of spraying. Although some local grains have grown to a certain extent, the coating is continuous and dense as a whole, and no defects such as cracks and pores have formed on the surface, indicating that it has high high-temperature stability and can play a good role in resisting ablation and gas erosion.

[0050] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A C / C composite material ablation-resistant gradient composite coating, characterized in that, The C / C composite ablation-resistant gradient composite coating consists of a SiC transition inner coating, a refractory metal carbide intermediate layer, and a lanthanum hafnium oxide outer coating, with its coefficient of linear expansion gradually increasing from the inside to the outside. The refractory metal carbide intermediate layer is two or more of HfC, ZrC, TaC, TiC, NbC, and SiC, and the thickness of the refractory metal carbide intermediate layer is maintained in the range of 50 to 120 μm, the thickness of the outer coating is maintained in the range of 120 to 230 μm, the thickness of the SiC transition inner coating is 70 to 200 μm, and the total coating thickness is 320 to 550 μm.

2. A method for preparing a C / C composite material ablation-resistant gradient composite coating, characterized in that, Includes the following steps: (1) Mixing and granulation of hafnium oxide and lanthanum oxide powder: Before mixing, hafnium oxide and lanthanum oxide fine powder are dried, and then the two are mixed at a molar ratio of 2:

1. Then anhydrous ethanol is added and the mixture is thoroughly ball-milled. After the ball-milling is uniform, the mixture is granulated and dried. To prevent the granulated powder from absorbing water, the granulated powder is vacuum dried for later use. (2) High-temperature sintering reaction and synthesis of lanthanum hafnium oxide spray powder: The dried granulated powder in (1) was subjected to a high-temperature sintering reaction to prepare lanthanum hafnium oxide spray powder. The high-temperature sintering atmosphere was air or an inert atmosphere, the sintering reaction temperature was 1300-1500℃, and the sintering reaction time was 4-10h. (3) High-temperature heat treatment before coating of C / C composite material: The C / C composite material is placed in a high-temperature furnace for high-temperature pore opening treatment. Then the treated material is machined into a predetermined size, polished with sandpaper, ultrasonically cleaned and dried for later use. (4) Preparation of SiC transition inner coating by high temperature chemical reaction process: The heat-treated and machined C / C composite material is placed in silicon powder and put into a graphite crucible. Then, the graphite crucible containing the C / C sample and silicon powder is placed in a high temperature furnace and heated to 1700-1900℃ to carry out high temperature reaction to prepare SiC transition inner coating. By controlling the number of high temperature reactions, a SiC transition inner coating with a thickness of 70-200μm is obtained. (5) Plasma spraying process to prepare refractory metal carbide intermediate layer: The SiC coating C / C sample in (3) is ultrasonically cleaned and dried, and the refractory metal carbide intermediate layer is prepared by plasma spraying process. By controlling the spraying time and number of spraying, the thickness of the prepared intermediate layer is kept in the range of 50 to 120 μm; the refractory metal carbide intermediate layer is two or more of HfC, ZrC, TaC, TiC, NbC and SiC; (6) Plasma spraying process to prepare lanthanum hafnium oxide outer coating: using lanthanum hafnium oxide spraying powder in (2) as raw material, plasma spraying process is used to prepare lanthanum hafnium oxide outer coating. By controlling the spraying time and number of spraying, the thickness of the prepared outer coating is kept in the range of 120-230 μm, and the total thickness of the coating is 320-550 μm.

3. The method for preparing a C / C composite ablation-resistant gradient composite coating according to claim 2, characterized in that: The hafnium oxide and lanthanum oxide fine powder mentioned in (1) both have a particle size ≤1μm.

4. The method for preparing a C / C composite ablation-resistant gradient composite coating according to claim 2, characterized in that: (2) The lanthanum hafnium oxide powder obtained after the sintering reaction has a particle size of 20-70 μm.

5. The method for preparing a C / C composite ablation-resistant gradient composite coating according to claim 2, characterized in that: The C / C composite material described in (3) has a density of 1.70–1.75 g / cm³. 3 The high-temperature opening treatment temperature described in (3) is 2000-2200℃, and the heat preservation time is 0.5-2h.

6. The method for preparing a C / C composite ablation-resistant gradient composite coating according to claim 2, characterized in that: (4) The SiC transition inner coating, when the thickness of the coating after the first high-temperature reaction is ≤70μm, needs to undergo a second high-temperature reaction to increase its thickness.

7. The method for preparing a C / C composite ablation-resistant gradient composite coating according to claim 2, characterized in that: The plasma spraying process described in (5) has the following process parameters: spraying power of 38-46KW, argon flow rate of 35-55L / min, hydrogen flow rate of 5-12L / min, and spraying distance of 80-120mm.

8. The method for preparing a C / C composite ablation-resistant gradient composite coating according to claim 2, characterized in that: The plasma spraying process described in (6) has the following process parameters: spraying power of 35-40KW, argon flow rate of 35-55L / min, hydrogen flow rate of 4-8L / min, and spraying distance of 80-120mm.

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