The effects of Si incorporation on the microstructure and nanomechanical properties of DLC thin films

A small amount of silicon incorporation into diamond-like carbon (DLC) films prepared by plasma-enhanced chemical vapour deposition (PECVD) onto Al2O3 :TiC substrates was studied by a combination of surface analysis and nanomechanical measurement techniques, namely XPS, Raman spectroscopy, nanoindentation and nanoscratch methods. Addition of silicon to the DLC films leads to an increase in the fraction of sp(3), as deduced from XPS analysis, and a decrease in the Raman band intensity ratio 1(D)/1(G). Although the coated substrates exhibit better scratch resistance and lubricity, the films as deposited are softer than the Al2O3:TiC substrates. Upon silicon incorporation, the mechanical and tribological properties are degraded. Wear protection of the Al2O3:TiC substrate by DLC coating corresponds to the competition between the reduction in friction coefficient and the softening of the films. It is suggested that, for such a PECVD process, the degradation of the mechanical properties is caused by the increased hydrogen content in the deposits when silicon is incorporated, as is shown by the increased Raman spectral background slope. These tendencies are attributable to the development of polymer-like chains, which can weaken the inter-molecular structure of the films.