Effects of sliding velocity and ambient temperature on the friction and wear of a boundary-lubricated, multi-layered DLC coating

Reciprocating ball-on-disk tests were performed to investigate the effects of sliding velocity and ambient temperature on the tribological characteristics of a diamond-like carbon (DLC) coating under a boundary lubrication regime. The coating consisted of sequentially deposited gradient Cr/CrN, a-C:H:W, and a-C:H layers; the counter surface was an AISI 52100 steel ball. Average sliding velocities (range: 0.01-0.04 m/s), temperatures (range: 27-160 degrees C), and a normal load of 293 N were selected as the test conditions. One fully formulated motor oil containing a glycerol mono-oleate was used as a lubricant. Wear rates of the coatings increased with increasing temperature. As temperature increased from 27 to 55 degrees C, friction coefficients increased, but wear rates of the ball decreased. With an increase in temperature from 55 to 160 degrees C, friction coefficients decreased until 110 degrees C then slightly increased, and wear rates of the ball increased. Increasing sliding velocity led to a decrease in friction coefficients and wear rates of both friction surfaces. Through surface analyses and observations, it was confirmed that graphitization of the worn DLC coatings, reactive tribofilms on the worn ball specimens, portion of the surface asperities in dry sliding contact, and corrosive wear of the ball specimens affect tribological characteristics. (C) 2014 Elsevier B.V. All rights reserved.