Ceramics have been tested and used in ball, roller and plain bearings (bushings). Their potential to resist fatigue, high temperature and loss of lubrication is better than for metals, making ceramics attractive for ball and roller bearings. There are a number of commercially available ceramic bearing types, grouped into one of two categories: all-ceramic or hybrid.
Hybrid bearings consist of ceramic rolling elements in steel races, whilst all-ceramic bearings have both races and rolling elements made from ceramic. Typically all-ceramic bearings are used in specialist high temperature, high speed applications, where the design requirements are very strict and hence can be costly.
Hybrid bearings are finding applications in a growing range of high-speed machines. These include machine-tool spindles which employ ceramics because of their lower centrifugal forces. Centrifugal effects, which can lead to skidding, are proportional to the mass of the balls, so that replacement of steel balls (density 8 g/cm3) with silicon nitride (density 3.1 g/cm3) greatly reduces the stress on the outer race. Another application for hybrids is the use of single crystals of alumina as 'jewel' bearing materials for watches, clocks, gyroscopes and similar precision equipment. Alumina is also used for circulating pump bearings in central heating systems, washing machines and car cooling systems because of its wear resistance, hardness and mechanical strength.
Ceramic rolling elements offer many advantages over traditional bearing elements. These include improved corrosion resistance, lower density, higher hardness, higher compressive strength and longer fatigue life. Typical ceramics that have been used for bearing materials, along with some of their mechanical properties, are shown in the table below.
| Material | Density
(g/cm 3 ) | Tensile Modulus
(GPa) | Vickers Hardness
(kgf/mm2) | Toughness
(MPa √m) |
|---|
Alumina
(99.9%) |
3.96 |
300-400 |
1500-1650 |
4-6 |
Silicon Carbide
(sintered) |
3.15 |
200-450 |
2400-2800 |
4-5 |
Silicon Nitride
(sintered) |
3.11 |
280-310 |
1700-2200 |
5-8.5 |
| SiAlON* |
3.24 |
280-300 |
1650-1800 |
6-8 |
| Zirconia (TZP) ‡ |
6.05 |
200-210 |
1200-1300 |
8-12 |
* SiAlON is an alloy of Silicon Nitride (Si 3 N 4 ) and Alumina (Al 2 O 3 ).
‡Tetragonal Zirconia Polycrystal. |
Other ceramics are used in specialist applications, but the final choice of ceramic (and grade) will depend on a number of factors, such as the environment that the bearings will be used in, operating temperature and cost.
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