Properties

5.7.6.2 Protection against abrasion

Ceramic materials are employed in highly stressed tribological systems because of their resistance to abrasion. As well as the property of great hardness, this calls for high stiffness (modulus of elasticity), high pressure resistance and high fracture resistance. In the case of ceramics, however, this is at the expense of fracture toughness. Other positive properties offered by ceramics for use in tribological systems are their low-density, low thermal expansion, and their resistance to temperature and to corrosion.

Whereas in the case of adhesion and tribo-chemical reactions, the wear behaviour is largely dependent on the chemical and thermo-chemical properties of the wear-protection material, in the case of abrasion taking place through the Mechanisms of adhesion and surface spalling, the abrasion resistance is determined by the physical properties of hardness and fracture toughness. This is illustrated schematically in Figure 102.

Figure 102: Dependency of the abrasion resistance, RV, on hardness and fracture toughness
determined in a defined model system

The maximum resistance to abrasion is therefore achieved when the wear protection material has an optimum combination of hardness and fracture toughness.

Figure 103: Curve showing abrasion minima and maxima

When abrasive wear is important, the relationship between the minimum and maxima of abrasion can be helpful when selecting the main body. It is based on the familiar Mohs' hardness scale: a harder material abrades a softer material.

Figure 104: The effect of hardness on abrasion resistance, R_V, with reference
to the abrasion resistance of aluminium oxide, R_{V AlO}

Because of their high hardness levels, abrasion is of subsidiary importance in the wear of ceramic materials.
Ceramic materials, furthermore, have little tendency to cold welding that occurs in metals, due to the high lattice bonding forces (ionic and covalent bonds). Adhesion as an abrasion mechanism can largely be neglected.
Equally, tribo-chemical reactions occur relatively rarely when ceramics are used (they always occur, but are relatively small). In closed tribological systems they cannot be ruled out, in particular in the presence of solid body and mixed friction.
In contrast, surface spalling is the main wear mechanism in brittle ceramic materials in both closed and open tribological systems.
It is triggered by high stresses from normal forces, or by high kinetic energy. For example, inclined jet abrasion largely follows the function WV = ½ mv² · sin½ where = angle of incidence. In conveyor systems such as pipe bends, pipe splits, pipe contractions and nozzles with angles of up to 30°, aluminium oxide has therefore been found to be very effective, whereas rubber is optimum at larger angles.

Figure 105: The effect of the angle of incidence on the abrasion resistance, R_V, in
relation to the maximum abrasion resistance