Research progress of aviation ceramic bearing technology

As a key basic part in the field of industrial machinery, bearings are known as the "joints of equipment". The performance of bearings directly affects and determines key indicators such as accuracy, lifespan, limit speed, load-bearing capacity, temperature resistance, stability, reliability and dynamic performance of high-end equipment. Aerospace technology has an especially urgent need for high-quality bearings.

In aero-engine design, bearing materials and technology always account for more than 90% to 95%. It can be said that bearing technology represents the engine's ultimate speed, temperature resistance and reliability level. Bearing materials mainly refer to bearing materials such as high-chromium anti-fatigue bearing steel, high-temperature resistant tool steel, high-strength gear bearing integrated steel, etc. The development of aviation technology has increasingly placed demands on high-temperature, high-speed, and high-reliability bearings and materials. High, which also promotes the development of bearing material technology.

01 Aerospace ceramic bearing materials

The initial goal of the research and development of ceramic bearings is to improve the ultimate performance of bearings for aerospace and defense equipment. Internationally renowned bearing companies such as Sweden's SKF and Germany's Schaeffler have developed high-level ceramic bearing technologies and products. In particular, hot isostatic pressing sintered silicon nitride ceramic bearings provide core technical support for the development of high-end equipment technology. After more than 50 years of research and accumulation, Si3N4 ceramic bearings have been used in helicopter main transmission devices, aviation APUs, aircraft accessory transmissions, missile engines, rocket engines and aerospace satellites, and have become standard equipment for high-speed and high-power spindles in high-end manufacturing equipment. bearings.

German FAG ceramic bearings

In recent years, with the promotion and application of ceramic bearings in the field of high-end equipment, the impact of materials on bearing quality has received widespread attention. ISO26602:2009 "Fine ceramics (advanced ceramics, advanced technical ceramics)—Silicon nitride materials for rolling bearing balls" has The elastic modulus, Poisson's ratio, thermal expansion coefficient, flexural strength, hardness, fracture toughness, microstructure, material classification and other performance indicators of rolling bearing silicon nitride ceramic materials provide clear measurement methods and ranges, ASTM F 2094/F2094M -08 "Standard Specification for Silicon Nitride Bearing Balls" in addition to clear requirements for material quality standards, also clearly stipulates the chemical composition, surface quality, detection methods, geometric parameters and grade uses of silicon nitride ceramic balls.

02 Design and verification technology of aviation ceramic bearings

In order to give full play to the advantages of new ceramic materials, ceramic bearings often need to break through the extreme performance of steel bearings, such as high-speed performance, high-temperature performance, long life and high reliability. Therefore, unlike the design of general rolling bearings, when designing ceramic bearings, in addition to designing basic structural parameters, it is also necessary to carry out surface tribological performance matching design, material matching design, structural dynamic performance design, and thermal characteristic design based on working conditions. , lubrication performance compatibility design and working performance design, etc., but the design concept and results need further verification.

In terms of the material matching design of aerospace ceramic bearings, the issue of fatigue resistance matching of the bearings is mainly considered. Excessive hardness difference will lead to premature fatigue and asymmetric fatigue of bearing components. FAG research results show that hybrid ceramic bearings composed of Si3N4 ceramics, M50NiL steel and Cronidur 30 steel have the best performance matching. The long-term research results of Harbin Institute of Technology show that through the material formula and component design and the secondary modification treatment technology of the bearing steel surface, the hardness of the ceramic components can be adjusted from high to low, and the bearing steel raceway load-bearing area can be modified and If the surface modification is appropriately improved, its service life will be 1.5 to 2 times longer than that of all-steel bearings.

03 Manufacturing and quality control technology of aviation ceramic bearings

Although Si3N4 ceramics have excellent comprehensive properties among all engineering ceramics, compared with bearing steel, its low toughness, high hardness and moderate bending strength are still the fatal weaknesses of ceramic materials that are sensitive to damage and defects. Therefore, low-defect blanks, Low-damage processing technology and non-destructive testing are the most direct means to ensure the quality consistency and working reliability of ceramic bearings, even for silicon nitride ceramic rolling elements manufactured by hot isostatic pressing. Therefore, high-reliability ceramic bearings need 100% non-destructive testing to guarantee. Ceramic non-destructive testing technology is divided into blank testing technology and finished product testing technology. Among them, acoustic emission resonance technology and laser material evaluation technology have good prospects in high-end applications.

Due to the presence of inclusions, micro-pores and grain boundary glass, the brittleness of early ceramic materials has always been one of the most direct factors affecting the failure mode and impact resistance of long-life bearings. In recent years, through extensive research, hot isostatic pressing sintering technology of ceramic blanks has been developed. According to the editor of China Powder Network, by controlling the purity, particle size and consistency, granulation process and phase composition of ceramic powder, etc. , which greatly improves the comprehensive performance of silicon nitride ceramic bearings, and key indicators such as flexural strength and fracture toughness can meet the requirements. Its relative density can reach more than 99.9%, the 3-point flexural strength is higher than 1000MPa, and the fracture toughness can reach More than 8~9MPa·m1/2, which is much better than the lowest threshold value of rolling bearings of 6MPa·m1/2.

04 Development Trend of Aerospace Ceramic Bearing Technology

The development history of aviation bearing technology shows that from M50 to M50NiL, Cronidur 30, and then to the silicon nitride ceramic materials currently being promoted and used, each improvement in bearing materials promotes the rapid development of aviation bearing technology. With its excellent performance, ceramic bearings can provide important basic technical support for the development of aviation equipment. High-performance ceramic bearings will be one of the effective technical ways to cope with the rapid development of aviation equipment.

The future technological development trends of aviation ceramic bearings include:

High toughness and high strength material technology. According to the editor of China Powder Network, silicon nitride powder with high purity, high sphericity and relatively concentrated particle size distribution is used. After pretreatment such as deoxidation and decarburization, harmful components in the powder are removed, and hot isostatic pressing is performed. In sintering, through process optimization, the uniformity of dispersion of sintering aids and the aspect ratio of ceramic grains can be improved to prepare high-quality ceramic blank materials with uniform microstructure, flexural strength of more than 1000MPa, and fracture toughness of more than 8 to 9.

Operational health monitoring technology of ceramic bearings. The speed, temperature and load of modern aircraft are getting higher and higher, and fluctuations in bearing performance and bearing failure will lead to serious system problems or failures. Intelligent health monitoring systems are the overall development trend of aviation equipment technology monitoring, and the health status of bearings and gears Monitoring is one of its core technologies.

Non-destructive testing and reliability evaluation technology. Ceramic rolling elements are multi-phase sintered materials, and their rolling contact fatigue life is significantly affected by parameters such as raw materials, processes, working conditions, and working history. Therefore, the dispersion of bearing life is large, and there are technical difficulties in reliability evaluation. Develop non-destructive testing technology for surface and sub-surface defects and damage, especially fast and reliable non-destructive testing technology, control the quality of finished ceramic components, quantitatively study damage patterns and patterns, ultimate damage resistance, and establish reliability evaluation models. Key focus technologies for high reliability applications of ceramic bearings.