Effect of high-speed steel surface nitriding treatment on adhesion and wear resistance properties of nitrogen-doped diamond-like carbon coatings

Abstract

Based on the principle of hollow cathode, diamond-like carbon (DLC) coatings were deposited on high-speed steel by plasma enhanced chemical vapor deposition (PECVD). The mechanism of the experimental parameters on the adhesion and wear behavior of the multilayer coatings, prepared by plasma nitriding substrate and DLC N-doping, were systematically investigated via Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), micro-Vickers tester, friction and wear tester analysis. It was found that the deposition speed and structure of DLC coatings can be efficiently improved by different contents of N doping. With the increase of nitrogen content, the content of sp3 bond in DLC coating first increased to 60.82 % and then decreased to 58.73 %. Meanwhile, the friction coefficient of DLC coating first decreased and then increased. Combined with plasma nitriding treatment for substrate, the wear resistance of DLC coating was effectively improved by optimizing the hardness gradient between the coating and the substrate. Under the same friction conditions, the wear rate was reduced by about 50 %, while the adhesion of DLC coating was improved from HF4 to HF1 in comparison with non-nitriding and non-N-doping DLC coatings. It was revealed by the mechanistic-based model that the resistance by nitriding layers played a more important role than an increase in coatings thickness due to the reasonable hardness gradient in resisting deformation. Based on the N-doped DLC + DLC structure, the surface hardness of the coating gradually increased to 1970.9 HV. The hard outer layer provided sufficient load-carrying capacity and wear resistance during the friction process, thus decreasing the generation of wear debris and the depletion of coating.