The influence of microstructure on tribological properties of WO3 thin films

The influence of the microstructure of WO3 thin films upon friction and wear during sliding contact was studied using two different tribometers. The tungsten trioxide (WO3) thin films of controlled microstructure were grown on R-plane sapphire substrates using electron cyclotron resonance (ECR) oxygen-plasma-assisted electron beam evaporation of WO3 pellets. The WO3 films were fabricated to consist of amorphous, polycrystalline or epitaxial microstructure, depending on the deposition temperature, as indicated by in situ RHEED analysis. Pin-on-disk testing of the WO3 films was performed using 10 mm diameter sapphire and 440C stainless steel sliders and 1 N normal force. Epitaxial crystalline WO3 films showed lowest friction( mu similar to 0.3 for both sapphire and steel spheres) and greatest production of debris, while amorphous WO3 films exhibited higher friction coefficients (up to mu similar to 0.6 for sapphire spheres and mu similar to 1.1 for steel spheres), less debris production, and more slider wear. Epitaxial WO3 films exhibited low friction (mu similar to 0.16) in single pass tests with sapphire and steel sliders (1 mm stroke, 0.1 mm/s, 0.1 N load,), whereas amorphous and polycrystalline WO3 films showed transitions to much higher friction (up to mu similar to 0.7). Lower friction and wear of WO3 films with epitaxial structure compared to amorphous structure appears to be due to a lubricious WO3 layer transferred to the slider from the epitaxial film, (C) 1999 Elsevier Science S.A. All rights reserved.