Microstructure and mechanical properties of W-C:H coatings deposited by pulsed reactive magnetron sputtering

Abstract

Reported are results of microstructure, mechanical and tribological properties studies for thin, amorphous hydrogenated carbon based coatings with tungsten content from 4.7at.% up to 10.3at.%. Studied coatings have been deposited by pulsed, reactive magnetron sputtering on substrates under planetary rotation. Resulting coatings, characterized by transmission electron microscopy (TEM) also at high resolution (HREM), show multilayer structure consisting of sub-layers of W-C:H type, with alternately high and low tungsten concentration. Thickness and number of sub-layers depend on rotation speed of planetary substrate holder. An average tungsten concentration decreases with increasing partial pressure of reactive gas (C2H2) during deposition. More insight into the microstructure of coatings provided HREM analysis showing crystalline precipitations of about 1–2nm in size as well as tungsten-rich and tungsten-poor W-C:H sub-layers. Raman spectra confirm presence of amorphous, hydrogenated carbon (a-C:H) phase in the coatings. Microhardness of studied coatings depends on tungsten content and increases from 10.7GPa to 13.7GPa, for 5.1at.% and 10.3at.% of tungsten content, respectively. The highest cracking resistance and best adhesion (Lc2=78N and HF1) has been achieved for coatings containing 4.9at.% of tungsten and a sub-layer thickness of 5nm. Tribological processes occurring in the coating–coating contact zone are dominated by graphitization and oxidation of W-C:H coating. Very low friction coefficient (0.04) and low wear rate seems to be an effect gaseous micro-bearing by tribo-generated carbon oxides and methane as well as hydrogen released from the coating. In the W-C:H-steel contact zone a tribo-layer composed of iron and tungsten oxides mixed with graphite-like products is growing at the surface of steel counterpart. This tribo-layer becomes a barrier restricting direct contact of steel with the coating and thus preventing it from further intense wear.