NANOINDENTATION EXPERIMENTS ON SOME AMORPHOUS HYDROGENATED CARBON (A-CH) THIN-FILMS ON SILICON

Ultra-low-load indentation (nanoindentation) experiments have been used to investigate the load-displacement characteristics of two types of hydrogenated carbon films (a "hard" and a "soft" version approximately 230 and approximately 210 nm thick, respectively) deposited from methane on to (1 0 0) single-crystal silicon wafers by a radio frequency plasma-assisted chemical vapour deposition process. Further, the technique was used to explore the changes in the properties of the films with heat treatment in vacuum at temperatures of up to 650-degrees-C. In all cases, the elastic and plastic properties of the films were compared at indentation loads in the range 0-60 mN, the higher loads causing indentor displacements greater than the thickness of the films. For the harder, stiffer coating, penetration resistance was found to decrease with increasing indenter displacement, reflecting the greater load-carrying role taken by the softer silicon with increasing applied load. However, for the softer coating, penetration resistance generally increased with displacement, perhaps reflecting progressive compaction of the coating in addition to the increasing role of the silicon. In both cases, heat treatment severely degraded the mechanical properties of the films due to thermally induced chemical changes and, in the case of the "hard" coating, relaxation of residual stresses. Scanning electron microscopy of both nanoindentations and low-load microhardness indentations clearly reveals the deformation mechanisms associated with contact stresses to include flow and fracture of the film and interfacial decohesion.