Ball crater testing for the measurement of the unlubricated sliding wear of wear-resistant coatings

Abstract This paper describes a simple modification of the conventional ball-cratering test equipment that enables sliding wear and friction experiments to be performed. Tests can be carried out either dry or with lubricants. In the modified test no abrasive is added, and friction is measured by means of strain-gauged flexure elements. Friction and wear are recorded throughout the test. The utility of the test system is illustrated by the results of a number of preliminary experiments on a range of thin, hard coatings, including TiN, diamond-like carbon (DLC), and CrN. The test results are augmented by optical and SEM observations of the wear mechanisms that occurred. The nitride coatings that were tested all showed relatively high values of friction coefficient, in the range 0.8–1.2. This was accompanied by the formation of large quantities of oxide debris, which changed in colour with the composition of the oxide. It was thought that most of the wear had taken place by abrasion from the oxide that had been generated at the interface. In many of the tests, the high contact pressure at the centre of the wear scar prevented ingress of debris at this point and reduced wear, leaving a ribbon of unworn coating across the wear scar. In the tests on DLC coatings no obvious debris was formed and the values of friction coefficients were much lower at 0.2–0.3 for one DLC coating, and 0.5 for the second. Perforation of the coating took place for all the coatings tested at a load of 4.7 N, but the coatings remained intact for most of the coatings tested at 0.47 N.