The ideal material for bearings is silicon nitride ceramic.

With the development of the machinery manufacturing industry in the direction of high precision and high speed, the application range of bearings is getting wider and wider, and the performance requirements for bearings are getting higher and higher. Especially in some special working environments, metal bearings cannot meet the needs, or even completely can not work. Ceramic materials have become ideal materials for the high-speed manufacturing of precision bearings due to their low density, high elastic modulus, small linear expansion coefficient, wear resistance, high-temperature resistance, and corrosion resistance.

Advantages and Characteristics of Silicon Nitride Ceramics

While silicon nitride is not the hardest nor the toughest technical ceramic, it is considered to have the best combination of mechanical and physical properties in high-performance bearing applications. Let's take a look at the advantages of silicon nitride compared with other bearing materials.

Performance	Silicon nitride	Zirconia	Aluminum oxide	Bearing steel	Stainless steel
Density(g/cm³）	3.25	6.00	3.95	7.85	7.90
Coefficient of thermal expansion（             ）	3.2	10.5	8.50	11	17
Elastic Modulus（kN/mm²）	320	210	380	208	200
Poisson's ratio	0.26	0.30	0.22	0.30	0.30
Hardness	1500	1250	1800	700	/
Strength（N/mm²）	600	950	300	2400	/
Fracture toughness（MN/m³/²）	6	10.5	5.5	25	/
Perveance（W/m•k）	35	2.50	30.00	30~40	15.00
Electric resistivity(mm²/m)	1018	1015.00	1018.00	0.1~1	0.75

Performance comparison of 5 kinds of bearing materials

Which bearing material is better? There are five commonly used bearing materials, namely silicon nitride, alumina, zirconia, bearing steel, and stainless steel. Different bearing materials have different advantages, so they are also suitable for different fields according to their advantages.

Heat resistance

Generally, when the service temperature of steel bearings exceeds 120°C, the hardness will decrease and the rolling life will also decrease. Silicon nitride has good temperature characteristics and is especially suitable for high-temperature environments.

Centrifugal force

The density of silicon nitride is about 3.24×103kg/m3, while the density of bearing steel is about 7.8×103kg/m3, which is only about 40% of the density of bearing steel. Therefore, when the rolling element uses a bearing, the bearing can rotate at high speed. Suppresses an increase in rolling element load due to centrifugal force.

Linear expansion coefficient

The linear expansion coefficient of silicon nitride is about 1/4 of that of bearing steel, so the dimensional change with temperature changes is small, so it is beneficial to use in environments with large temperature changes.

Hardness, modulus of elasticity, Poisson's ratio

Because the elastic coefficient of silicon nitride is about 1.5 times that of bearing steel, the elastic deformation relative to the load is small, and the rigidity relative to the load is high.

Corrosion-resistant, non-magnetic, insulating

Corrosion is a problem when steel bearings are used for machines used in chemical machinery equipment, food, marine, and other departments. In a strong magnetic environment, using steel bearings, the fine powder worn from the bearing itself is adsorbed between the rolling elements and the rolling surface, which will become the main cause of early peeling damage and increased noise of the bearing.

Preparation of Silicon Nitride Ceramic Bearings

Preparation of Silicon Nitride Ceramic Powder

Silicon nitride powder for bearing parts requires the following important characteristics: high purity; high uniform and fine particles; high α phase content. The method of preparing silicon nitride powder that best meets this requirement is the carbothermal reduction nitriding method, and its reaction formula is 3SiO2+6C+2N2=Si3N4+6CO. The powder obtained by this method contains fewer metal impurities and has high purity. The particles are fine and the content of the α phase is high, meeting the requirements of materials for bearing parts.

Molding of silicon nitride ceramic bearing related parts

There are many methods for preparing silicon nitrides ceramic bearing-related components, such as reaction sintering, hot pressing sintering, pressureless sintering, and secondary reaction sintering. In order to obtain a fully dense silicon nitride material, hot isostatic pressing is ideal.

Silicon nitride ceramic bearing-related parts processing

The processing of related parts of silicon nitride ceramic bearings is basically similar to that of bearing steel parts, and the grinding mechanism is basically the same. However, due to the large difference between the various properties of silicon nitride and the properties of steel, there are considerable differences in grinding tools, processing coefficients, and grinding mixtures in machining. Requirements such as type, shape, quantity, strength, crushing characteristics, and wear characteristics are different. The abrasives currently used mainly include silicon carbide, boron carbide, and diamond powder.

Assembly of Silicon Nitride Ceramic Bearings

Generally, rolling bearings are composed of four main parts, namely outer ring, inner ring, rolling elements, and cage. Since there are ten types of rolling bearings, different types of bearings use different cage forms, so the bearing assembly methods are also different.

As an important mechanical basic part, silicon nitride ceramic bearings are unique in the world of new materials because of their excellent characteristics that other bearings cannot match. In recent years, it needs high-temperature, high-speed, cryogenic, Inflammable, explosive, strong corrosion, vacuum, electrical insulation, non-magnetic, dry friction, and other special working conditions, the indispensable replacement of silicon nitride ceramic bearings is gradually being recognized by people.

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