Impact wear testing of diamond-like carbon films for engine valve-tappet surfaces

There is a strong drive to reduce the energy losses from automotive engines in order to improve fuel economy and reduce emissions of pollutants. The demands of reducing engine losses are pushing engine manufacturers to consider using technologies such as diamond-like carbon (DLC) coatings to give enhanced wear protection of components and provide low friction operation. Valve-train component interactions are tribologically complex, with contacts commonly experiencing mixed boundary and elasto-hydrodynamic lubricated sliding wear, abrasive wear, impact and fatigue. Whilst there have been many studies of the mechanical properties and wear resistance of diamond-like carbon films, there have been fewer studies of the effect of impact at the contact loads and geometries relevant to the automotive valve-train. Thus, a series of inclined impact tests have been conducted at impact angles from 15° to 45° on hydrogenated and non-hydrogenated diamond-like carbon films with and without lubrication to understand how the films respond to impact under controlled conditions. The results have been compared to the coatings performance in a motored and instrumented cam-tappet testing rig. The results show that impact loading in the valve-train has a significant effect on coating durability. Coating durability is affected by impact angle and lubricant and counter-intuitively the introduction of lubrication decreases the coating durability. This is attributed to the lubricant preventing the formation of beneficial tribolayers and also forming an abrasive slurry where DLC wear particles are retained in the contact and create additional abrasive wear.