The exhaust valve system of combustion engines experiences a very complex contact situation of frequent impact involving micro sliding, high and varying temperatures, complex exhaust gas chemistry and possible particulates. The wear rate has to be extremely low, and the individual wearing events operate at a scale that is very demanding to detect. The tribological conditions in the exhaust valve system are expected to become even worse for engines that will follow the future emission regulations. The regulations demand reduced amounts of soot and particles, sulfur compounds, etc., which today act beneficial for the seating surfaces. The reductions are expected to increase the metal-to-metal contact.
As a step towards a deeper understanding of the tribological behavior of the valve and to evaluate potential options for future valve systems, this paper presents several different valve material systems (including physical vapor deposited coatings, nitrided and hardfaced valves) that have been evaluated in a simplified valve test rig. The tests have been conducted in a heated and clean atmosphere. Light optical microscope, scanning electron microscope and energy-dispersive x-ray spectroscopy have been used to evaluate the wear mechanisms and behavior.
Most of the tested physical vapor deposition coated valves failed due to gross plastic deformation of the substrate. However, the initial wear results reflect the potential of the different coatings, which in the present test improve the situation compared to the uncoated reference. The nitrided valves showed a higher initial wear but never failed in the same manner as the physical vapor deposited coatings. However, as the nitrided valves have a different substrate, a direct comparison cannot be made. The failure mechanism of the coatings differs and the chromium nitride coating seems to be more adaptable under these harsh conditions.