A method for preparing alumina-coated titanium dioxide core-shell micron powder for high-temperature lubricating materials

Abstract

This invention provides a method for preparing alumina-coated titanium dioxide core-shell micron powders for use as high-temperature lubricants. This method utilizes a three-fluid spray drying method to simultaneously feed shell and core material raw materials. Nanoscale TiO₂ powder (core) and Al₂O₃ powder (shell) are then atomized and dried in a single step to produce a micron powder with a core-shell structure. By manipulating the slurry ratio, feed rate differential, and drying parameters, a powder with a narrow particle size distribution, complete coating, and excellent flowability is achieved. The sintered bulk material exhibits a low friction coefficient of 0.25-0.28 at 1100°C. This material combines the high-temperature oxidation resistance of Al₂O₃ with the lubricating properties of TiO₂, making it suitable for ultra-high-temperature sealing components in aerospace applications.

Description

Technical Field

[0001] The invention belongs to the technical field of self-lubricating ceramic-based composite materials and relates to a method for preparing alumina-coated titanium dioxide core-shell micron powder for high-temperature lubricating materials. Background Art

[0002] Sealing components such as the motion mechanisms used to adjust flight modes at the rear end of the power combustion chambers of high-tech equipment such as hypersonic vehicles and reusable spacecraft face the challenge of high-temperature friction and wear under the coupling of multiple extreme factors, including temperatures of 1000°C and above and a strongly oxidizing environment. There is an urgent need to develop high-temperature lubricating materials with strong oxidation resistance and excellent high-temperature tribological properties. Al2O3 ceramics are an ideal matrix for high-temperature lubricating materials due to their excellent comprehensive high-temperature properties, especially their excellent high-temperature oxidation resistance and resistance to degradation of high-temperature mechanical properties. TiO2 ceramics, as a typical Magneli phase, experience a series of ordered oxygen vacancies caused by lattice oxygen loss at high temperatures, which greatly reduces the shear force on the crystal plane and thus has excellent high-temperature solid lubrication properties. The skeleton reinforcement effect and multi-level combination of soft and hard phases of the three-dimensional continuous structure are considered to be one of the most efficient ways to significantly improve the tribological properties of lubricating materials. Since powder is an important prerequisite for the preparation of high-performance composite lubricating ceramics, the present invention aims to lay a solid foundation for the manufacture of composite high-temperature lubricating materials with an anti-oxidation and highly wear-resistant Al2O3 three-dimensional skeleton and a high-temperature lubricating TiO2 core at the micron scale by constructing a composite spherical powder of Al2O3 coated TiO2. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for preparing Al2O3-coated TiO2 core-shell micron powder for high-temperature lubricating materials, to improve the lubricity and wear resistance of ultra-high temperature lubricating sealing materials through the precision-manufactured precursor composite powder, to broaden the two-dimensional friction reduction and wear resistance design approach of the microstructure and composition of lubricating materials, and to provide technical support for the exploration of ultra-high temperature ceramic-based lubricating sealing materials for aerospace.

[0004] 1. Preparation of Al2O3-coated TiO2 core-shell micron powder

[0005] The preparation method of Al2O3-coated TiO2 core-shell micron powder of the present invention comprises the following steps:

[0006] (1) TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol, and deionized water were stirred at room temperature at a speed of 300-350 r / min for 6-8 h to obtain a uniformly mixed TiO2 core material spray granulation slurry; the mass fraction of TiO2 powder in the slurry was 40%-45%, the mass fraction of ammonium polymethacrylate was 0.25-0.5%, the mass fraction of polyvinyl alcohol was 2%, and the mass fraction of deionized water was 52.5%-57.5%;

[0007] Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol, and deionized water were stirred at room temperature at a speed of 300-350 r / min for 6-8 h to obtain a uniformly mixed Al2O3 shell material spray granulation slurry; the mass fraction of Al2O3 powder in the slurry was 40%-45%, the mass fraction of ammonium polymethacrylate was 0.25-0.5%, the mass fraction of polyvinyl alcohol was 2%, and the mass fraction of deionized water was 52.5%-57.5%.

[0008] The purity of the TiO2 powder and Al2O3 powder is ≥99.5%, and the particle size is 60-150 nm; the average molecular weight of the ammonium polymethacrylate is 15,000, and the average molecular weight of the polyvinyl alcohol is 31,000.

[0009] (2) The TiO2 core material spray granulation slurry and the Al2O3 shell material spray granulation slurry were simultaneously fed into a spray dryer (Buchi B-290) using a three-fluid nozzle for spray drying to obtain Al2O3-coated TiO2 core-shell micron powder;

[0010] The TiO2 core material spray granulation slurry and the Al2O3 shell material spray granulation slurry are simultaneously input into the spray dryer through the two interfaces of the three-fluid nozzle for spray drying. The constant speed feeding of the two slurries is achieved by a micro peristaltic pump. The spray drying parameters have a great influence on the coating effect and sphericity of the composite powder. If the inlet temperature is too low, the water content of the powder is too high, which is not conducive to subsequent processing. If it is too high, it is not conducive to the stable molding of the powder. If the spray flow rate is too small, the size distribution of the composite powder is too large. If the spray flow rate is too large, the sphericity of the composite powder deteriorates. Therefore, the spray drying parameters are set as follows: the inlet temperature is 150~170 ℃, the spray flow rate is 480~500 L / h, and the vacuum fan capacity is 40~42 m 3 Peristaltic pump feed rate: The TiO2 core spray granulation slurry feed rate is 8-10 mL / min, and the Al2O3 shell spray granulation slurry feed rate is 10-12 mL / min. The TiO2 core spray granulation slurry feed rate is slower than the Al2O3 shell spray granulation slurry feed rate to ensure the shell coating effect. The specific rate difference is 2-4 mL / min.

[0011] The preparation mechanism of this invention: Spray drying is a cost-effective, efficient, and environmentally friendly granulation technology. It operates by atomizing a suspension into a large number of droplets. A hot air stream then rapidly evaporates the solution, causing spontaneous particle aggregation and spheroidization of the material. A three-fluid nozzle simultaneously delivers the target shell / core material slurry, followed by spray drying to rapidly coat the core material surface with the shell material. This allows for the rapid and precise preparation of core-shell structured powders, ensuring the subsequent densification and formation of core-shell block structures.

[0012] 2. Microstructural Characterization of Al2O3-Coated TiO2 Core-Shell Micron Powders

[0013] This study successfully constructed a micrometer-scale Al2O3@TiO2 composite powder with a core-shell structure by simultaneously injecting shell and core materials through a three-fluid nozzle using a spray drying method. The shell layer is an Al2O3 ceramic with excellent high-temperature oxidation resistance and mechanical properties, while the core layer is a TiO2 ceramic with high-temperature lubrication. XRD and SEM characterizations are as follows:

[0014] Figure 1 This is the XRD pattern of the Al2O3-coated TiO2 core-shell micron powder prepared in this invention. The XRD pattern shows that the prepared Al2O3-coated TiO2 core-shell micron powder is primarily composed of corundum Al2O3 phase (JCPDS No. 74-1081) and anatase TiO2 phase (JCPDS No. 99-0008). No phase transition or other behavior occurred during the granulation process, demonstrating that spray granulation technology can achieve non-destructive manufacturing of material structures.

[0015] Figure 2 The SEM image (a), EDS energy spectrum analysis (b, c, d) and particle size statistics (e) of the Al2O3 coated TiO2 core-shell micron powder prepared by the present invention. Figure 2 (a) shows that the sphericity of the powder is relatively good, and the micron-sized spherical powder is constructed from the initial nano-sized powder. EDS energy spectrum analysis ( Figure 2 (b), (c), (d)) show that the outer layer of the spherical body is an Al element-rich area, while the Ti element signal is weak, proving that the TiO2 core is successfully coated by the Al2O3 shell. Particle size statistics ( Figure 2 (e) shows that the overall size of the powder is in the range of 15-45 μm. Notably, nearly 90% of the spheres are in the 20-40 μm range, indicating a narrow overall particle size distribution and precise control of the microsphere size.

[0016] 3. Performance evaluation of Al2O3 coated TiO2 core-shell micron powder

[0017] 1. Powder flowability test

[0018] Test method: Powder collected from the spray dryer is placed in a powder comprehensive property analyzer (Dandong Better BT-1001) for testing. The Carr Index is determined from a table of flowability indices corresponding to five parameters: angle of repose (the maximum angle formed by the free surface of the powder layer with the horizontal plane in static equilibrium), plate angle (the average of the angle between the free surface of the powder on the plate and the plate after vertically lifting a plate embedded in the powder), compressibility (the ratio of the difference between the tapped density and the bulk density of the same sample to the tapped density), uniformity (the ratio of D60 to D10 of the particle size distribution), or agglomeration (the mass of agglomerated powder remaining on the sieve after vibrating the powder for a specified period of time using a standard sieve). The Hausner ratio is determined from the ratio of bulk density to tapped density.

[0019] Figure 3 The results of flowability tests on Al2O3-coated TiO2 core-shell micron powders are shown. The calculated Carr index is 71 and the Hausner ratio is 1.15. It is generally believed that a Carr index greater than 60 and a Hausner ratio less than 1.2 indicate excellent powder flowability. Therefore, the prepared Al2O3-coated TiO2 core-shell micron powders exhibit good sphericity and excellent powder flowability.

[0020] 2. High temperature tribological properties of materials after powder sintering

[0021] Testing Method: The sintered samples were first machined into 25 mm × 10 mm × 3 mm cuboids. Prior to testing, the samples were polished to a smooth surface with a roughness of less than 0.3 μm and then ultrasonically cleaned with acetone. A KHT-1400M friction and wear testing machine (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences) was then used to investigate the high-temperature tribological properties of the bulk material under atmospheric conditions. The friction test pair, load, frequency, reciprocating stroke, and temperature were φ 6 mm × 20 mm alumina plug, 28 N (contact pressure approximately 1 MPa), 5 Hz, 5 mm, and 1100°C, respectively. The sliding velocity was the product of frequency and stroke, which was 0.05 m / s. During the high-temperature friction test, the friction coefficient was recorded in real time using data acquisition software.

[0022] Figure 4 A graph showing the time-dependent change in the friction coefficient of a bulk material prepared from Al2O3-coated TiO2 core-shell micron powder at 1100°C shows that, under an 1100°C atmospheric temperature, the friction coefficient of the bulk material gradually stabilizes after a running-in period of approximately 5 minutes. During the stable wear phase, the friction coefficient is lower than 0.3, reaching approximately 0.25, demonstrating self-lubrication of the material under extremely high friction temperatures. This demonstrates the excellent tribological properties of the high-temperature lubricating material prepared from the core-shell structured powder of the present invention.

[0023] In summary, this invention, based on the design principles of core-shell composite powders (ceramic matrix coated with solid lubricant) for high-temperature lubricating materials, has produced micron-scale spherical powders of Al2O3-coated TiO2. This micron-scale core-shell powder exhibits excellent coating quality, good sphericity, a narrow particle size distribution, and excellent flow properties. Furthermore, the bulk lubricating material prepared from this powder exhibits excellent tribological properties. This powder is suitable for the synthesis and manufacture of lubricants for aerospace mechanical components subject to friction and wear under extreme ultra-high temperature conditions.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. Excellent core-shell structure coating effect: A three-fluid nozzle is used to simultaneously input the core material (TiO2) and shell material (Al2O3) slurry. Combined with a precise feed rate difference (2-4 mL / min), this ensures that Al2O3 uniformly coats TiO2, forming a complete shell layer, achieving precise preparation of core-shell structure powders.

[0026] The coating structure can prevent TiO2 from oxidizing and failing at high temperatures, while utilizing the high-temperature oxidation resistance of Al2O3 (stable at 1100°C) to protect the lubricating phase.

[0027] 2. Excellent powder performance

[0028] The high sphericity (≥0.85) and narrow particle size distribution (90% of particles are 20-40 μm) significantly improve the powder flowability (Carr index > 70, Hausner ratio < 1.2), which is beneficial to the subsequent molding process.

[0029] The spray drying process enables precise control of nanopowders (60-150 nm) to micron spheres (20-40 μm), avoiding the component segregation problem of traditional mechanical mixing.

[0030] 3. Outstanding high temperature tribological performance

[0031] The friction coefficient of the sintered bulk material at 1100°C is stable at 0.25~0.28, which is much lower than that of pure Al2O3 ceramics (usually >0.5). This is due to: the high-temperature lubricity of the TiO2 core (lattice oxygen vacancies reduce shear force); the wear-resistant skeleton support of the Al2O3 shell delays lubrication phase wear.

[0032] It is suitable for extreme working conditions such as hypersonic aircraft combustion chamber seals, and solves the problem of high-temperature oxidation and friction coupling.

[0033] 4. Highly efficient and environmentally friendly process

[0034] The spray drying method is a one-step molding method with a shorter process and lower energy consumption compared to multi-layer coating or sol-gel methods. The slurry formula (dispersant + binder) is optimized to avoid organic solvent pollution, which is in line with the trend of green manufacturing.

[0035] 5. Expand the design dimensions of high-temperature lubricating materials

[0036] Microstructure-composition collaborative design: The core-shell structure realizes a gradient material with a "hard shell and soft core", providing high-temperature lubricating materials with the dual functions of a three-dimensional continuous Al2O3 skeleton (wear resistance) and a dispersed TiO2 lubricating phase (friction reduction), breaking through the performance bottleneck of traditional multiphase ceramics.

[0037] Application scenario expansion: Suitable for extreme working conditions (>1000℃) such as spacecraft sealing components (such as the rear end motion mechanism of the combustion chamber) and ultra-high temperature bearings, and solves the problem of oxidation failure of existing lubricating materials (such as graphite and MoS2). BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the XRD spectrum of the Al2O3-coated TiO2 core-shell micron powder in the present invention.

[0039] Figure 2 The SEM image (a), EDS energy spectrum analysis (b, c, d) and particle size statistics (e) of the Al2O3-coated TiO2 core-shell micron powder in the present invention.

[0040] Figure 3 These are the fluidity test results of the Al2O3-coated TiO2 core-shell micron powders in the present invention.

[0041] Figure 4 This is a graph showing the change in friction coefficient over time of the bulk material prepared from Al2O3-coated TiO2 core-shell micron powder in an atmospheric environment at 1100°C. DETAILED DESCRIPTION

[0042] The present invention will be further explained below with reference to specific embodiments.

[0043] Example 1

[0044] (1) TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water were taken as the original slurry components of TiO2 core material. A magnetic stirrer was used to stir at a speed of 300 r / min for 6 h at room temperature to obtain a uniformly mixed TiO2 core material spray granulation slurry. The mass fractions of TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water in the slurry were 40%, 0.5%, 2% and 57.5%, respectively.

[0045] Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water were used as the original slurry components of the Al2O3 shell. A magnetic stirrer was used to stir at 300 r / min for 6 h at room temperature to obtain a uniformly mixed Al2O3 shell material spray granulation slurry. The mass fractions of Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water in the slurry were 40%, 0.25%, 2% and 57.5%, respectively.

[0046] (2) The TiO2 core material spray granulation slurry and the Al2O3 shell material spray granulation slurry were simultaneously fed into the spray dryer through a micro peristaltic pump and a three-fluid nozzle (the TiO2 core material spray granulation slurry was fed at a feeding rate of 8 mL / min and the Al2O3 was fed at a feeding rate of 10 mL / min). The spray dryer drying program was set as follows: the inlet temperature was set to 150°C, the spray flow rate was 500 L / h, and the exhaust capacity was 40 m 3 / h. Then, the core-shell micron powder was taken out from the cyclone separator after cooling in the furnace; XRD spectrum ( Figure 1 ) showed that the prepared Al2O3 coated TiO2 core-shell micron powder was mainly composed of Al2O3 and TiO2 phases, without containing other impurity phases. SEM images, EDS element analysis and particle size statistics ( Figure 2 ) showed that the core-shell powder had good coating effect, good sphericity and narrow particle size distribution;

[0047] (3) Flow properties of Al2O3 coated TiO2 core-shell micron powder and tribological properties of sintered bulk: Carr index is 71 ( Figure 3 ), Hausner ratio is 1.15 ( Figure 3 ), the average friction coefficient of the block at 1100℃ is 0.25 ( Figure 4 ).

[0048] Example 2

[0049] (1) TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water were taken as the original slurry components of TiO2 core material. A magnetic stirrer was used to stir at a speed of 300 r / min for 6 h at room temperature to obtain a uniformly mixed TiO2 core material spray granulation slurry. The mass fractions of TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water in the slurry were 42.5%, 0.25%, 2% and 55%, respectively.

[0050] Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water were used as the original slurry components of the Al2O3 shell. A magnetic stirrer was used to stir at 300 r / min for 6 h at room temperature to obtain a uniformly mixed Al2O3 shell material spray granulation slurry. The mass fractions of Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water in the slurry were 42.5%, 0.5%, 2% and 55%, respectively.

[0051] (2) The TiO2 core material spray granulation slurry and the Al2O3 shell material spray granulation slurry were simultaneously fed into the spray dryer through a micro peristaltic pump and a three-fluid nozzle (the TiO2 core material spray granulation slurry was fed at a feeding rate of 8 mL / min and the Al2O3 was fed at a feeding rate of 10 mL / min). The spray dryer drying program was set as follows: the inlet temperature was set to 160°C, the spray flow rate was 500 L / h, and the exhaust capacity was 40 m 3 / h. The furnace was then cooled, and the core-shell micron powder was removed from the cyclone separator. XRD patterns showed that the prepared Al2O3-coated TiO2 core-shell micron powder was primarily composed of Al2O3 and TiO2 phases, without any other impurity phases. SEM images, EDS elemental analysis, and particle size statistics revealed that the core-shell powder had a good coating effect, good sphericity, and a narrow particle size distribution.

[0052] (3) The flow properties of Al2O3 coated TiO2 core-shell micron powder and the tribological properties of the block after sintering: the Carr index is 72, the Hausner ratio is 1.13, and the average friction coefficient of the block is 0.26 at 1100℃.

[0053] Example 3

[0054] (1) TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water were taken as the original slurry components of TiO2 core material. A magnetic stirrer was used to stir at 300 r / min for 6 h at room temperature to obtain a uniformly mixed TiO2 core material spray granulation slurry. The mass fractions of TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water in the slurry were 45%, 0.25%, 2% and 52.5%, respectively.

[0055] Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water were used as the original slurry components of Al2O3 shell. A magnetic stirrer was used to stir at 300 r / min for 6 h at room temperature to obtain a uniformly mixed Al2O3 shell material spray granulation slurry. The mass fractions of Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol and deionized water in the slurry were 45%, 0.5%, 2% and 52.5%, respectively.

[0056] (2) The TiO2 core material spray granulation slurry and the Al2O3 shell material spray granulation slurry were simultaneously fed into the spray dryer through a micro peristaltic pump and a three-fluid nozzle (the TiO2 core material spray granulation slurry was fed at a feeding rate of 8 mL / min and the Al2O3 was fed at a feeding rate of 10 mL / min). The spray dryer drying program was set as follows: the inlet temperature was set to 170°C, the spray flow rate was 500 L / h, and the exhaust capacity was 40 m 3 / h. The furnace was then cooled, and the core-shell composite powder was removed from the cyclone separator. XRD patterns showed that the prepared Al2O3-coated TiO2 core-shell micron powder was primarily composed of Al2O3 and TiO2 phases, without any other impurity phases. SEM images, EDS elemental analysis, and particle size statistics revealed that the core-shell powder had a good coating effect, good sphericity, and a narrow particle size distribution.

[0057] (3) The flow properties of Al2O3 coated TiO2 core-shell micron powder and the tribological properties of the block after sintering: the Carr index is 70, the Hausner ratio is 1.17, and the average friction coefficient of the block at 1100℃ is 0.28.

[0058] In the above embodiments, the purity of the raw materials TiO2 powder and Al2O3 powder used is ≥99.5%, the raw material particle size is 60~150nm, and the purity of ammonium polymethacrylate (average molecular weight is about 15000) and polyvinyl alcohol (average molecular weight is about 31000) are both analytical grade.

Claims

1. A method for preparing alumina-coated titanium dioxide core-shell micron powder for high-temperature lubricating materials, characterized in that: The following steps are involved: (1) TiO2 powder, ammonium polymethacrylate, polyvinyl alcohol, and deionized water were mixed and stirred to obtain a TiO2 core material spray granulation slurry; the mass fraction of TiO2 powder in the slurry was 40% to 45%, the mass fraction of ammonium polymethacrylate was 0.25 to 0.5%, the mass fraction of polyvinyl alcohol was 2%, and the mass fraction of deionized water was 52.5% to 57.5%; Al2O3 powder, ammonium polymethacrylate, polyvinyl alcohol, and deionized water are mixed and stirred uniformly to obtain a spray granulation slurry for Al2O3 shell material; the mass fraction of Al2O3 powder in the slurry is 40% to 45%, the mass fraction of ammonium polymethacrylate is 0.25 to 0.5%, the mass fraction of polyvinyl alcohol is 2%, and the mass fraction of deionized water is 52.5% to 57.5%; (2) The TiO2 core material spray granulation slurry and the Al2O3 shell material spray granulation slurry are simultaneously fed into a spray dryer using a three-fluid nozzle for spray drying to obtain Al2O3-coated TiO2 core-shell micron powder; The input rate of the TiO2 core material spray granulation slurry is 8-10 mL / min, and the input rate of the Al2O3 shell material spray granulation slurry is 10-12 mL / min; the difference in input rate between the Al2O3 shell material spray granulation slurry and the TiO2 core material spray granulation slurry is 2-4 mL / min; The spray drying conditions are: inlet temperature 150~170 ℃, spray flow rate 480~500 L / h, and exhaust capacity 40~42 m 3 / h; The powder consists of an Al2O3 shell and a TiO2 core. The core-shell micron powder has a particle size of 15-45 μm, of which 90% of the powder particle size is distributed in the range of 20-40 μm, the Carr index is greater than 70, and the Hausner ratio is less than 1.

2. After powder sintering, the friction coefficient of the bulk material at 1100°C is 0.25~0.28, and it has high-temperature self-lubricating properties.

2. The preparation method according to claim 1, characterized in that The purity of the TiO2 powder and Al2O3 powder is ≥99.5%, and the particle size is 60-150 nm; the average molecular weight of the ammonium polymethacrylate is 15,000, and the average molecular weight of the polyvinyl alcohol is 31,000.

3. A high-temperature lubricating material, characterized in that: The core-shell micron powder obtained by the preparation method according to claim 1 or 2 is prepared by sintering, and has a stable friction coefficient of 0.25-0.28 in an atmospheric environment of 1100°C, and is suitable for ultra-high temperature friction parts in aerospace.

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