Crack density and tribological performance of hard-chrome coatings

Abstract Despite environmental concerns, the automotive, aerospace and other manufacturing industries still extensively use hard-chrome coatings. This is mainly due to the coatings' ability to combine unmatched aesthetics with high hardness, excellent wear resistance, low friction and good corrosion protection in aggressive environments. However, the performance of hard-chrome coatings is dependent on the micro cracks developed during the deposition process. These cracks alter the surface topography, which has a major influence on the tribological performance of the contact, especially under mixed lubricated conditions. The aim of this study was to investigate the effect of a coating's crack density and a post-polishing process on the friction and wear performance of hard-chrome coatings. Three different crack densities combined with a post-polishing process were tested using the ball-on-disc configuration in a poly-alpha-olefin oil (PAO) lubricated point contact. The results clearly show that for hard-chrome coatings and random, non-uniform crack-type textures the size and density of the crack pattern determine the friction and wear performance of the contact. For the investigated contact conditions the best performance, i.e., low friction, fast running-in and minimum wear, was obtained when combining medium-density cracks of ∼3% and the post-polishing.