Thermal stability of microhardness and internal stress of hard a-C films with predominantly sp2 bonds

Annealing in vacuum at temperatures up to 820 degreesC was used to study the thermal stability of mechanical properties of magnetron-sputtered thick (approx. 1.5 mum) a-C films. A predominance of sp(2) bonds was characteristic for all these films. The microhardness, internal stress, electron diffraction, Raman and optical spectra of three sets of films with different initial microhardness (Happroximate to50, 20 and 10 GPa, respectively) were compared. Annealing of the hardest film up to 500 degreesC led to an increase in microhardness accompanied by a decrease in internal stress. Internal stress did not relax completely for hard films, even after annealing up to 820 degreesC, and the microhardness remained rather high (similar to40 GPa). Both the high internal stress and the specific film nanostructure are responsible for the high microhardness of these sp(2)-bonded films. (C) 2003 Elsevier Science B.V. All rights reserved.