Comparative analysis of thermal stability of two different nc-TiC/a-C:H coatings

The aim of this work is to compare the thermal stability of two nanocrystalline TiC/a-C:H coatings prepared by magnetron sputtering of titanium target in acetylene containing environment. The coatings exhibited different chemical composition ([Ti]/[C] ratio) and structure. Their thermal stability was studied using thermal desorption spectroscopy in the temperature range from 500 to 1000 °C. The hardness and elastic modulus of both coatings remained almost unchanged up to 700 °C, however, they substantially differed from the point of view of their fracture resistance. Up to 700 °C no indentation induced cracking occurred in the titanium rich coating, however, reaching the annealing temperature of 1000 °C, this coating became porous and spontaneous tensile cracking appeared on its surface. On the other hand, the surface of the carbon rich coating did not show any spontaneous tensile or compressive cracking. It remained almost unchanged in the whole studied temperature range. Its resistance against indentation induced cracking gradually decreased with temperature, which was indicated by an increase of the number of cracks inside the residual indentation imprints. The critical depth where the underlying layer started to influence the nanoindentation measurement was found using differential hardness studies. According to XRD tests, the grain size increased for both films already at the first annealing to 500 °C. The XRD also proved that in the case of annealed titanium rich film a large transformation of sub-stoichiometric TiC grains into pure Ti grains and more stoichiometric TiC developed. Raman spectroscopy studies showed that after annealing of Ti rich coating at 1000 °C there remained no amorphous carbon phase anymore.