ABSTRACT
Early in the exploration of ceramics for tribological service, it was found that the chemical environment has a major influence on the friction of these materials in lubricated and unlubricated contact and on their wear resistance. An increase in atmospheric humidity can decrease the wear rate of one ceramic by as much as two orders of magnitude or increase the wear of another by similar amounts. The friction coefficient of silicon nitride is very nearly the same in dry and in humid environments. Thus the total mechanical forces acting on the contacting surfaces are the same. Lubricious oxides have recently been formed on silicon nitride by preoxidation. In order to achieve low friction, it is necessary to prepare a very smooth and flat surface to avoid friction and wear by ploughing. Oxide ceramics, such as alumina and zirconia, are obviously incapable of undergoing the oxidation reactions that are responsible for the decrease of wear in silicon nitride.
