In the development and application process of engineering ceramic products, ceramic ball bearings are a typical example of the widespread application of engineering ceramics in the industrial field. Ceramic ball bearings have excellent comprehensive performance such as long service life (2-5 times that of steel bearings), high speed, good overall accuracy and stiffness, good thermal stability, and no magnetism. They have a very broad application prospect in working conditions such as high temperature, high speed, high precision, acid alkali corrosion, electrical corrosion, strong magnetic field, no lubrication or medium lubrication. In high-speed precision ceramic ball bearings, the most commonly used is hybrid ceramic ball bearings, where the ball is made of ceramic balls and the bearing ring is still made of steel. This type of bearing has a relatively high degree of standardization and will not make significant changes to the machine tool structure, making it easy to maintain and especially suitable for high-speed operation. The assembled high-speed electric spindle has advantages such as high speed, high stiffness, high power, and long service life.
Compared with traditional bearing steel, precision ceramic balls have excellent comprehensive properties such as low density, high hardness, high elastic modulus (stiffness), wear resistance, low coefficient of thermal expansion, good thermal and chemical stability, insulation, and no magnetism. Silicon nitride is considered the best material for manufacturing bearing rolling elements and has achieved great success in the application of ceramic ball bearings. Ceramic ball bearings can operate without adding any grease, avoiding the occurrence of premature bearing damage caused by grease drying in ordinary bearings. At present, ceramic balls have been widely used in various fields such as aerospace, military, petroleum, chemical, and high-speed precision machinery.
1. Common materials and characteristics
The ceramic balls used in the market mainly include silicon nitride ceramic balls (Si3N4), zirconia ceramic balls (ZrO2), silicon carbide ceramic balls (SiC), and high-purity alumina ceramic balls (Al2O3 ). Si3N4 has become the most widely used variety due to its superior comprehensive performance. The reason why precision ceramic balls can replace steel balls is that they have characteristics such as low density, medium elastic modulus, low thermal expansion coefficient, and excellent internal chemical properties. The most important feature is that their failure mode, like bearing steel, occurs in a pre existing peeling mode, while ZrO2 and Al2O3 both occur in a destructive failure mode such as fragmentation. Therefore, ZrO2 and Al2O3 are relatively less applied. The following table provides a brief comparison of the main properties of the four materials.
Table 1- Performance Comparison of Four Materials
A. Silicon Nitride Ceramic Balls
Silicon nitride ceramic material has light weight, fine surface, high moisture content, wear resistance, high toughness, high temperature resistance of 1400 ℃, and is not easily deformed. Compared to ZrO2 material, Si3N4 all ceramic bearings are suitable for higher speeds and load capacities, as well as for higher ambient temperatures. The thermal expansion coefficient of silicon nitride ceramics is only 1/4 of that of bearing steel, reducing the sensitivity of bearings to temperature changes and helping to prevent jamming. At the same time, it can be used as a precision ceramic bearing for high-speed, high-precision, and rigid spindles, with a maximum manufacturing accuracy of P4 to UP levels.
B. Zirconia Ceramic Balls
Zirconia ceramics are not oxidized, not easily corroded, non magnetic, resistant to high temperatures of 1000 ℃, not easily deformed, and have a thermal expansion coefficient similar to that of metals in the industrial environment, but have weak resistance to strong acid and alkali corrosion. The density per cubic centimeter can reach as high as 5.95-6.05g/cm3. Among the four commonly used materials for making ceramic spheres (Si3N4, SiC, Al2O3, ZrO2), zirconia ceramics have a higher toughness, reaching over 10MPa · m1/2. The thermal expansion coefficient is close to that of metals, which can meet the needs of good bonding with metals. Zirconia ceramics have self-lubricating properties, which can solve problems such as pollution caused by lubricating media and inconvenience in addition; Good corrosion resistance, can also be used in medium acid, medium alkali, seawater and other media; High temperature resistance, zirconia ceramics have almost no change in strength and hardness at 600 ℃; Non magnetic, insulating, and can also be used in magnetic fields. However, dimensional stability varies greatly with temperature, and the form of rolling fatigue contact failure is destructive fragmentation, which is not as stable as silicon nitride materials in some critical situations.
C. Silicon Carbide Ceramic Balls
Silicon carbide ceramics have the highest usage limit temperature, high strength at high temperatures, highest thermal conductivity, best thermal shock resistance, highest elastic modulus, and lowest density among the four types, and have the best corrosion resistance. They can withstand a mixture of concentrated hydrofluoric acid and heated strong acids, and can be used in extremely strong corrosion resistant environments.
D. Alumina Ceramic Balls
The main component of alumina ceramic balls is high-quality alumina, with a bending strength of up to 250MPa. Hot pressed products have a bending strength of up to 500MPa, and have excellent wear resistance. They are widely used in the manufacturing of grinding wheels, ceramic nails, bearings, etc.
2. Precision level of ceramic balls
The accuracy of ceramic balls has been graded in the market, and Table 3 and Table 4 respectively list the explanations of professional evaluation standard terms and international general grade standards. Ceramic balls usually have an accuracy of G100 or higher for bearings, and between G3-G20 for high-precision bearings.
Table 2 Important Indicators of Precision Level
Table 3 International Standard (ISO3290-1:2014) (Unit: μ M)
