Tribology of DLC films under slip-rolling conditions

Abstract

Today diamond-like carbon (DLC) coatings are used in many applications such as in medical tools, machine tools, computer devices like hard disks and many more. In the near future, the tribological properties of the coatings will allow replacing some of the common lubricant additives like extreme pressure and anti-wear additives. This would have a beneficial impact on the environment and on fuel consumption. However, an extensive use of DLC coatings and a shift towards mass production volumes of DLC-coated parts submitted to slip-rolling conditions will only be possible, if the coated system is fully characterized. Due to the large variety of production processes and the resulting different properties of coatings, a market screening of today's available coatings has been performed. Layers of several different suppliers were tested under defined conditions on specified and comparable substrates. Investigations were carried out to elucidate underlying reasons for the behaviour of the coatings tested. The results of the tests and the respective investigations are summarized in this chapter. Typically, the coated substrates were made of 52100 steel (60 HRC). They were tested on a twin disc tribometer of the Amsler type under mixed/boundary lubrication conditions. Pure paraffinic oil (ISO 46) and polyglycol (environmentally acceptable lubricants, ISO46) were used for lubrication. As a standard, the coated substrates were tested against a counter disc of the same material as the substrate under high initial average Hertzian contact pressure. Different factors such as layer thickness, counter disc roughness and material, nature of the bond layer were investigated. The lack of adherence and the presence of defects on the DLC coating before testing seem to be the most common failure causes. After the tests, Raman spectroscopy reveals that most of the coatings show a structural transformation, probably a graphitization. This can be due to possible local high stresses induced by the counter disc roughness. © 2008 Springer Science + Business Media, LLC.