Effect of DLC coatings on wear in automotive applications

Diamond like carbon (DLC) coatings have been investigated for application in the automotive valve train and, in particular, the camshaft drive assembly. The drive assembly can have various force transfer devices, such as an overhead bucket tappet assembly or a cam follower assembly. Force transfer devices operate in severe tribological contact situations with maximum Hertzian stresses approaching 1 GN m(-2) and temperatures in the range 100-150 degrees C. Lubrication is often partially starved and, therefore, the thermochemical properties and surface finish of the components are critical to their successful operation. For motor sport applications, DLC offers improved wear resistance over steel alone; and for passenger vehicles, the high hardness and commensurately lower friction coefficient that DLC gives against steel counterfaces offers improvements in fuel economy and lower wear. This paper presents the results from an investigation of the wear of DLC coated shims against standard production camshafts. DLC offers benefits in terms of reducing shim wear, but increases the wear of the camshaft. The wear mechanism observed for DLC is dependent on the oil type, but generally it is a pitting type mechanism. The cam wear occurs by plastic ratchetting. The consequences of the results are discussed in terms of the implementation of such coatings in standard valve train applications.