Nano-scratch, nanoindentation and fretting tests of 5-80nm ta-C films on Si(100)

Wear and stiction forces limit the reliability of Silicon-based micro-systems when mechanical contact occurs. Ultra-thin filtered cathodic vacuum arc (FCVA) ta-C films are being considered as protective overcoats for Si-based MEMS devices. Fretting, nano-scratch and nanoindentation of different thickness (5, 20 and 80. nm) ta-C films deposited on Si(1. 0. 0) have been performed using spherical indenters to investigate the role of film thickness, tangential loading, contact pressure and deformation mechanism in the different contact situations. The influence of the mechanical properties and phase transformation behaviour of the silicon substrate in determining the tribological performance (critical loads, damage mechanism) of the ta-C film coated samples has been evaluated by comparison with previously published data on uncoated Silicon. The small scale fretting wear occurs at significantly lower contact pressure than is required for plastic deformation and phase transformation in nanoindentation and nano-scratch testing. There is a clear correlation between the fretting and nano-scratch test results despite the differences in contact pressure and failure mechanism in the two tests. In both cases increasing film thickness provides more load support and protection of the Si substrate. Thinner films offer significantly less protection, failing at lower load in the scratch test and more rapidly and/or at lower load in the fretting test.