Material selection of ceramic bearings，preparation of silicon nitride ceramic bearing and application characteristics of silicon nitride ceramic bearings.

1. Preface

In recent years, with the social progress and the rapid development of science and technology, the service environment and conditions of bearings are becoming more and more diverse, and the requirements for the structure, material and performance of bearings are also becoming higher and higher. Some high-tech fields and machinery working in some special environments, such as aerospace, nuclear energy, metallurgy, chemical industry, petroleum, instruments, machinery, electronics, textile, pharmaceutical and other industries, need to be in high temperature, high speed, high precision, vacuum, non-magnetic Work in special environment without oil lubrication, strong acid, strong alkali, etc. These new requirements can no longer be met only by improving the structure or lubrication conditions of traditional metal bearings. We must develop new materials and make fundamental breakthroughs and innovations. Studies at home and abroad have found that some ceramic materials have excellent properties, can withstand the harsh working environment that metal materials and polymer materials are not competent for, and also have all the important characteristics required by bearing materials. Therefore, the application of ceramic materials in bearing manufacturing has become a hot spot in the development and Application of high-tech in the world, and has become the second target of the technological revolution of mechanical working materials

2. Material selection of ceramic bearings

Structural ceramics can be divided into carbide, nitride, oxide, silicate and other series according to their chemical composition. Because ceramics are inorganic non-metallic compounds rather than simple substances, their crystal structures are not as simple as metal alloys, but complex and diverse, such as face centered cubic structure, dense hexagonal structure, body centered cubic structure and so on. Their element binding forces are mainly ionic bonds and covalent bonds. In practical ceramic materials, there are few pure ionic bonds or pure broken covalent bonds. The vast majority of ceramic chemical bonds are mixed bonds between ionic bonds and covalent bonds. Therefore, compared with general metals, their crystal structure is complex and their surface energy is small. Their strength, hardness, elastic modulus, wear resistance, corrosion resistance and heat resistance are superior to metals, but the plasticity, machinability Thermal shock resistance is not as good as metal, so not all ceramic materials are suitable for making bearings. In addition to the above special properties, ceramic materials used to manufacture bearings also need to have high compressive strength, rolling contact fatigue resistance and spalling failure mode. At present, the ceramic materials available for selection include Si₃N₄, ZrO₂, Al₂O₃, SiC,TiC, etc. their characteristics are shown in the table below:

Table 1 performance list of ceramic materials and bearing steels

In the application of high-performance rolling bearings, rolling fatigue life is one of the most important characteristics required. It can be seen from the above table that although silicon nitride is not the hardest and the toughness is not the highest among industrial ceramics, the rolling fatigue damage of silicon nitride in ceramics is similar to the surface spalling of bearing steel, and has the advantages of low density, medium modulus of elasticity, low coefficient of thermal expansion and high compressive strength. Therefore, silicon nitride is considered to have the best comprehensive mechanical and physical properties and is the best material for manufacturing rolling bearings at present.

3. Preparation of silicon nitride ceramic bearing

The manufacturing process of silicon nitride bearing related parts is raw materials -- mixing -- molding -- hot pressing -- rough machining -- finishing -- Inspection -- assembly -- finished products

3.1 Preparation method of silicon nitride ceramic powder

Silicon nitride powder for bearing parts is required to have the following important characteristics (1) high purity (2) high uniform and fine particles (3) high phase content. Therefore, although there are many methods for preparing silicon nitride powder, such as direct nitridation, chemical vapor deposition, carbo-thermal reduction nitridation, etc., the more suitable method is carbo-thermal reduction nitridation, that is, high-purity fine silicon dioxide powder is used, mixed with carbon powder as reducing agent, and nitrogen atmosphere is 1350~1480 ℃ for reduction nitridation, The reaction formula is 3SiO₂+6C+2N₂=Si₃N₄+6CO. The silicon nitride powder obtained from the reaction is heat treated at 600~700 ℃ in an oxidizing atmosphere to remove carbon. The obtained powder contains less metal impurities, high purity, fine particles and high phase content, which meets the requirements of bearing parts.

3.2 Molding of silicon nitride ceramic bearing related parts

There are many preparation methods of silicon nitride ceramic bearing related parts, such as reaction sintering, hot pressing sintering, pressureless sintering, secondary reaction sintering, etc. In order to obtain a completely dense silicon nitride material, hot isostatic pressing is ideal. Due to the covalent bond characteristics of silicon nitride, the self-diffusion coefficient is small, and it is difficult to sinter dense. Therefore, when manufacturing bearing parts blank, a-Si₃N₄ powder is mixed with sintering additives Y₂O₃ and Al₂O₃ to make composite powder. This kind of powder is put into the graphite mold for molding according to the required amount, and hot pressed within the temperature range of 1750~1800 ℃. In the molding process, pay attention to the control of the role of the liquid phase and make it densified at the same time. The sintered body prepared by this method has long columnar crystalline particles, high density, porosity close to zero, high strength, high toughness and high reliability, and is the best in terms of quality.

3.3 Processing of silicon nitride ceramic bearing related parts

The machining of silicon nitride ceramic bearing related parts is basically similar to that of bearing steel parts, and the grinding mechanism is basically the same. However, due to the great differences between the various properties of silicon nitride and the properties of steel, there are considerable differences in grinding tools, machining coefficients, grinding mixtures, etc. in machining, and the requirements of each process for abrasive particle size, type, shape, quantity, strength, crushing characteristics, wear characteristics, etc. are different. At present, the abrasives used mainly include silicon carbide, boron carbide, diamond powder, etc. In addition, the grinding fluid used in each process is also different. Only when the proportion of abrasive and grinding fluid is coordinated with the corresponding processing process, can the ideal abrasive liquid film be obtained, and the ideal grinding speed and surface quality be obtained.

3.4 Assembly of silicon nitride ceramic bearings

As we all know, general rolling bearings are composed of four main parts, namely, outer ring, inner ring, rolling element and cage. The assembly of rolling bearings can be basically completed in two steps. The first step is to combine the inner ring, outer ring and a group of rolling elements to ensure a certain matching relationship between them, such as ensuring that the bearing should have a certain radial clearance value or a certain tolerance value of nominal width. The second step is to rivet the cage to ensure that the rolling elements are fixed in the bearing ring to make it rotate flexibly without scattering, In order to facilitate the installation and use of rolling bearings in various machines, there are ten categories of rolling bearings, and different types of bearings adopt different cage forms, so the assembly methods of bearings are also different.

4. Application characteristics of silicon nitride ceramic bearings

Silicon nitride ceramic bearings are mainly used in four aspects (1) high speed bearings (2) high temperature bearings (3) vacuum bearings (4) corrosion bearings. The reason why silicon nitride ceramic bearings can work in special environments and circumstances is entirely due to the performance of silicon nitride ceramics. Silicon nitride ceramics make ceramic bearings have the following advantages.

4.1 High speed

The importance of ceramic materials is only 40% of that of the same steel, and the low density can realize the lightweight and high-speed of the bearing, so that the ceramic bearing can suppress the increase of rolling body load and slip (slip and spin slip caused by gyro torque) caused by centrifugal force when rotating at high speed. The speed of ceramic bearing is 1.3~1.5times of that of steel bearing, its value can reach 3million. For example, angular contact ball bearings have a certain contact angle, and there will be rotational sliding between the rolling element and the raceway surface. When ceramic rolling elements with low density are used, not only the rotational sliding is small, but also it plays a beneficial role in bearing heating and surface damage, which is also very beneficial to aerospace vehicles.

4.2 High rigidity

The elastic modulus of silicon nitride ceramics is much higher than that of metal, which is times that of metal. Therefore, the elastic deformation after stress is small, and the rigidity of relative load is high. It can improve the rigidity about 15%~20%, so as to reduce the vibration of machine tools, and obtain good application value in high-precision systems, such as the spindle of ultra-precision machine tools, high-precision aerospace bearings, etc.

4.3 Long life

Because ceramic materials have few dislocations, low mobility, and high hardness, which is generally more than twice the hardness of metal, it can reduce wear and make ceramic bearings have good wear resistance. In addition, the mechanical strength of silicon nitride ceramics is not low. Its tensile strength and bending strength are equivalent to that of metal, while its compressive strength is very high, about twice that of metal materials. Especially under high temperature conditions, it can still maintain high strength and hardness, and the strength remains basically unchanged even at 1200℃. When there are foreign bodies mixed, ceramic balls rarely produce spalling failure pitting corrosion. Therefore, ceramic bearings with good pressure resistance usually have a longer service life. Generally, it is 3-5 times higher than that of steel bearings.

4.4 Low fever

Because the friction coefficient of silicon nitride ceramic material is small, which is about 30% that of standard bearing steel, the thermal conductivity of silicon nitride ceramic is lower than that of metal material, so the heat generated by ceramic bearing is smaller, which can prolong the service life of grease.

4.5 Low thermal expansion

The thermal expansion coefficient of silicon nitride ceramics is about 20% that of bearing steel, so the size change of ceramic bearings with temperature change is small, and the thermal preload generated is low, thus avoiding excessive heat accumulation or fatigue spalling failure, which is beneficial to use in the environment with large temperature change.

4.6 Corrosion resistance

The inactive chemical properties of ceramic materials make ceramic bearings have certain corrosion resistance. Therefore, ceramic bearings can be used in all applications where steel bearings fail in advance due to lack of chemical resistance, such as chemical machinery and equipment, machinery used in food, marine and other departments, and atomic energy equipment.

4.7 Non-magnetic

In the strong magnetic environment, when using steel bearings, the fine powder worn down from the bearing itself is adsorbed between the rolling element and the raceway surface, which becomes the main reason for the early peeling damage and increased noise of the bearing. Because the ceramic bearings are completely non-magnetic and have normal bearing capacity, they can be used in the occasions where completely non-magnetic bearings are needed.

4.8 Insulation

The resistivity of ceramic materials at room temperature is relatively high, which can be used as a better insulating material to prevent the bearing from arc damage.

5. Conclusion

In short, using ceramic materials to make bearings can greatly expand the application range of rolling bearings in various fields. At present, there are many companies that research, produce and sell ceramic bearings all over the world, but most of them are hybrid bearings, that is, the bearing rolling elements are made of ceramic materials, and the inner and outer rings are made of steel materials. China started late in the research of ceramic bearings. Since the eighth five year plan, the country has included the research and development of ceramic ball bearings in the key scientific and technological projects, and invested a lot of funds. Some scientific research institutions and enterprises have also made useful exploration and achieved gratifying results, but they are still in the stage of experimental research. The main reasons that affect the wide application of ceramic materials in ball bearings are its difficult machinability and high manufacturing costs. Recently, our company has made great progress in the research of ceramic bearing balls, the processing speed is more than twice that of the existing domestic technology, and all technical indicators meet or exceed the requirements of class standards, which can play a certain role in promoting the industrialization and commercialization of ceramic bearings in China. Of course, the research on ceramic bearings is still in its infancy, and its research is far from over. Further research and exploration are needed in the following aspects: first, to study the rolling contact performance of ceramic bearings under the conditions of wider adaptability and worse lubrication conditions; second, to study the structural coordination design of ceramic bearing related components, as well as the reliability and economy of processing; third, the nondestructive testing methods and damage prediction technology of ceramic bearing related components, Fourth, formulate inspection standards for ceramic bearings. It is believed that it is not far away for ceramic bearings to enter the practical stage, and their application prospects are very broad. Even as predicted, ceramic bearings will completely replace steel bearings in the future.

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