Thermal stability in oxidative and protective environments of a-C:H cap layer on a functional gradient coating

Abstract

Three types of hydrogenated amorphous carbon (a-C:H) coatings were synthesized on stainless steel substrates by a Plasma Assisted CVD process, containing hydrogen contents in the range from 25 to 29 at.%. The effect of annealing up to 600 °C in two different environments on both the structure and the mechanical properties of the coatings were investigated by means of Differential Scanning Calorimetry/Thermogravimetry (DCS/TG), Raman Spectroscopy and Depth Sensing Indentation. The results indicate that the structural modifications occurred in the coatings in both protective and oxidative atmospheres up to 400 °C were due to a complex atomic rearrangement involving the dehydrogenation reaction. A small weight loss, detected by isothermal TG analysis confirmed the H2 effusion. This dense effect proceeds without a change of hardness which was maintained in the diamond-like regime. The annealing in non-oxidative ambiance at temperatures above 500 °C causes both gaseous products effusion and sp3 to sp2 transformation. Raman parameters and hardness values were, under these conditions, similar to those known for a typical graphite-like regime. While the onset temperature of the graphitization process was found to be almost independent of the H content range investigated, the situation was completely different in relation to the oxidation reaction. The highest oxidation resistance was found for coatings with the lowest H content.