Mechanism of Mild to Severe Wear Transition in Alpha-Alumina

Friction and wear experiments were conducted on high purity alpha-alumina sliding against a similar material in air under different contact loads and at temperatures ranging from 23°C to 900°C. Experimental results indicate that tribochemical reactions between water vapor and alpha-alumina at room temperature produce aluminum hydroxide which results in relatively low coefficients of friction and low wear rates. Both the coefficient of friction and the wear rate of alumina were low at intermediate temperatures (200°C to 800°C), if the contact stress was below a threshold value. Above this load, wear occurred by fracture, the wear coefficient exceeded a value of 10⁻⁴, and the coefficient of friction increased to 0.90. At 900°C, the coefficient of friction was 0.40 and the wear coefficient was reduced to a value less than 10⁻⁶, because of the formation of a silicon-rich layer on the wear track. A contact mechanics model based on linear elastic fracture mechanics indicated that propagation of cracks from pre-existing flaws controls the onset of catastrophic wear in the intermediate temperature range.