The tribological properties of uncoated and WC-C-coated alumina when simulating mechanical face seal applications are reported. Silicon carbide was also included in the tests, mainly as a reference material, since it has shown good performance in these applications. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and Auger electron spectroscopy were utilized to characterize the worn surfaces. The results show that coating alumina face seals with WC-C increases the wear resistance by one order of magnitude. Further, when used at low contact pressures (below 0.050 MPa), the WC-C coating also reduces the friction significantly. When used at high contact pressures (above 0.050 MPa), however, there is no significant difference in the friction characteristics. At high contact pressures (above 0.050 MPa) all investigated material combinations generate a tribo-film between the mating surfaces. Uncoated alumina forms a non-homogeneous surface film built up by very fine wear fragments, while the WC-C-coated alumina forms a tribo-film which consists mainly of a (W, Cr) oxide compound. In contrast, the silicon carbide forms a more homogeneous silica-based tribo-film. It is concluded that the mechanical and tribological properties of the tribo-films are decisive for the wear behaviour of ceramics run under dry sliding conditions. Further, if a surface coating is to be used in order to improve the tribological behaviour of journal bearings or face seals, it should have good adhesion to the underlying substrate and a high propensity to form a soft tribo-film with sufficiently good cohesion.
