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

• Published in: Surface & coatings technology 2015-04, Vol.267, p.32-39
• Main Authors: Zábranský, L., Buršíková, V., Daniel, J., Souček, P., Vašina, P., Dugáček, J., Sťahel, P., Caha, O., Buršík, J., Peřina, V.
• Format: Article
• Language: English
• Subjects: Annealing; Carbon; Coatings; Cracking (chemical engineering); Differential hardness; Fracture toughness; Hardness; Indentation; Nanocomposite; Thermal annealing; Thermal stability; Titanium; Titanium carbide
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2015.01.058

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. •Thermal stability of ncTiC/a-C:H films with [Ti]/[C]=77/23 and 32/68 was studied.•The hardness and elastic modulus did not decrease substantially up to 700°C.•The fracture toughness of C-rich film decreased during the annealing up to 1000°C.•In Ti-rich film transformation of TiC into Ti and TiC grains started at 500°C.•Indentation induced defects were studied using differential hardness and TEM.