Microstructure, mechanical and tribological properties of the nc‐CrxCy/a‐C and nc‐CrxCy/a‐C:H nanocomposite coatings on oxygen‐hardened Ti‐6Al‐4V alloy

Abstract

The nanocomposite coatings, nc‐CrxCy/a‐C and nc‐CrxCy/a‐C:H, consisting of chromium carbide nanocrystals embedded in an amorphous hydrogen free carbon (a‐C) matrix or in an amorphous hydrogenated carbon (a‐C:H) matrix deposited on oxygen‐hardened Ti‐6Al‐4V alloy by magnetron sputtering were investigated. Oxygen hardening of titanium alloy was performed by oxidation under plasma glow discharge. For both coatings, the phase identification by high‐resolution transmission electron microscopy revealed the presence of Cr7C3 and Cr23C6 nanocrystals of size 2÷13 nm embedded in an amorphous carbon a‐C and a‐C:H matrix. The microhardness values were found to be 9.7 GPa and 12.1 GPa for the nc‐CrxCy/a‐C and nc‐CrxCy/a‐C:H coatings, respectively, and 10.6 GPa for oxygen‐hardened titanium alloy. Scratch tests indicated good coating‐substrate adhesion for both coatings. It was found that the hydrogenated nc‐CrxCy/a‐C:H coating exhibited lower coefficient of friction ranging from 0.1 to 0.15 (while sliding 1‐mm diameter alumina ball) as compared to friction coefficient of nc‐CrxCy/a‐C coating which showed higher value, up to 0.2. The coated alloy features much better wear resistance than oxygen‐hardened one. Copyright © 2012 John Wiley & Sons, Ltd.