Influence of carbon content on the microstructure, mechanical and tribological properties of CrAlCN coatings deposited by DC unbalanced magnetron sputtering

The influence of carbon content on the microstructure, mechanical and tribological properties of CrAlCN was systematically investigated. For this purpose, five duplex coatings of CrAlCN deposited on AISI H13 steel by the magnetron-sputtering technique were developed. The carbon content of the coatings was varied between 10.26 and 80.56 at% by modifying the percentage of methane in the working gas mixture . The effect of carbon content on the morphology, microstructure, mechanical and tribological properties of the CrAlCN coatings was evaluated. Coatings showed a high crystalline structure at low carbon contents, consisting mainly of chromium nitrides and aluminium-chromium nitrides. With the increase of carbon content in the coatings, chromium carbides formed at the expense of nitrides, presenting a highly amorphous structure with high carbon content. Initially, the hardness and Young's modulus of the coatings increased with carbon content, possibly due to hardening of the interstitial solid solution of carbon within the matrix of CrAlN. Then, they were decreased along with the coefficient of friction and wear rate due to the self-lubricating effect of graphite-like carbon. However, above the 54.74 at% carbon, the wear rate of CrAlCN increased again, probably due to the low hardness and adhesion of the coating. The CrAlCN coating with a carbon content of 21.96 at% presented the best balance between mechanical and tribological properties.