Frictional behavior and adhesion of Ag and Au films applied to aluminum oxide by oxygen-ion assisted screen cage ion plating

Abstract

A modified d.c. diode ion plating system, by utilizing a metallic screen cage as a cathode, is introduced for coating non-conductors such as ceramics. Screen cage ion plating (SCIP) is used to apply Ag and Au lubricating films on aluminum oxide surfaces. This process has excellent ability to coat around corners to produce three-dimensional coverage of the substrate. A dramatic increase in adhesion is achieved when plating is performed in a reactive 50% O 2-50% Ar glow discharge compared with the adhesion when plating is performed in 100% Ar. The presence of oxygen ion assistance contributes to the excellent adhesion as measured in a pull-type adhesion tester. The Ag and Au film adhesion is significantly increased (above 70 MPa) and generally exceeds the cohesion of the substrate such that portions of the alumina are pulled out. The deposited Ag and Au lubricating films not only reduce the coefficient of friction during sliding contact but also reduce the surface tensile stresses that contribute to undesirable subsurface cracking and subsequently to severe wear. The strong adhesion of Ag and Au films to alumina imparted by reactive SCIP is of critical importance in achieving low and steady friction coefficients and low wear.