Solid Lubrication of Ceramic Surfaces by IAD-Silver Coatings for Heat Engine Applications

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Abstract
In this study, we investigated the tribological characteristics of alumina ceramics that were coated with silver by means of ion-assisted deposition (IAD). Tests were performed at temperatures up to 400°C on an oscillating-slider wear test machine as a partial simulation of a ring/cylinder system. The results showed that higher test temperatures caused greater wear damage and increased friction of uncoated alumina. At 400°C, the wear rates of the alumina-alumina test pairs were eight to ten times higher than those of specimens tested at room temperature. The steady-state friction coefficients of the uncoated alumina were also rather high, typically ranging from 0.8 to 1.1. In contrast, the wear rate of I AD-silver -coated flats was negligible and the wear rate of counterface pins sliding against these coated flats was reduced by factors ranging from 4 to 73 depending on test temperature. Friction was also reduced to about one-half to one-third that of uncoated pairs. Surface and structure analytical investigations, together with the hot-hardness testing of the alumina substrate at temperatures up to 600°C, were performed in attempts to elucidate the mechanisms responsible for the friction and wear behavior of both alumina/ alumina and alumina/IAD-silver-coated alumina pairs.