Effect of microstructure and thickness on the friction and wear behavior of CrN coatings

One of the most commonly used tribological thin-film coatings is chromium nitride (CrN), typically deposited by physical vapor deposition. Examples of current applications of this coating include cutting and forming tools, dies and automotive components, such as injection valves and piston rings for diesel engines. In selecting coatings for different tribological applications, one of the critical parameters is the coating thickness. In the present work, the effects of microstructure and coating thickness on the friction and wear behavior of CrN coatings were determined under unidirectional sliding conditions. Tests were conducted with a dry ball-on-flat contact configuration using ∼1-, 5-, and 10-μm thick coatings deposited on a hardened H-13 steel substrate by plasma enhanced magnetron sputtering. The ball specimen was made of WC. The friction behavior was observed to be strongly dependent on coating thickness and microstructure, especially at relatively low loads (5 N). At higher loads, however, the thinnest coating (1 μm) was quickly worn through, while the thicker ones (5 and 10 μm) remained intact. Wear in both the counterface WC ball material and the coatings also depended on coating thickness and microstructure. In all coatings, there was localized damage but minimal wear. Additional tests were done with Si3N4 and 52100 steel balls, and the results indicated different wear and friction behavior from that for WC balls. The observed effect of coating thickness on tribological behavior is attributed to differences in the microstructure and mechanical behaviors of CrN coatings as a function of thickness.