Wear resistant low friction coatings for engine components

Abstract: Engine development today is driven by cost, performance and government regulations. Customers want cars and trucks to consume less fuel, last longer, pollute less and be safer. Several of the requirements have tribological associations. For example, product longevity can be improved by lowering friction and using more wear-resistant components. In recent decades, the use of new coating application procedures and techniques has produced remarkably advances in relation to cutting tools. The process temperature at which coatings are applied has been lowered to below 200 oC. Thus it is now possible to coat low-alloy temper-sensitive steels, which are widely used in the automotive industry in machine elements such as gears, bearings and cam followers.The aim of this work has been to investigate the possibility of using sputtered amorphous carbon coatings to reduce friction and prevent wear in engine components, and specifically in valve train components. Test equipment simulating near-normal running conditions for the valve mechanism has been developed and used to test standard and coated valve components. The mechanism has also been analysed and simulated numerically. The results show a low velocity difference between the injector cam lobe and the roller, except for a short interval at the top dead centre of the rocker arm. In that region the slip increases significantly at higher speeds due to inertial forces.A three-dimensional finite element parameter study of the coating thickness, elastic modulus, asperity contact size and wavelength has shown that tensional stresses at the coating surface increase significantly when asperity contacts approach and interact. Testing of different thicknesses in rolling contact, together with finite element stress analysis, showed that a higher tensional stress level through the coating thickness increases the possibility of cracks propagating down to the interlayer and causing delamination of the coating. Tests with a rapid load increase on two carbon coatings show no transition from mild to more severe wear. Instead the contact is dimensioned by the plastic deformation of the underlying substrate.