DLC-W coatings tested in combustion engine - Frictional and wear analysis

Hydrogenated diamond-like carbon coatings doped with tungsten were deposited by reactive magnetron sputtering from graphite and composite graphite/tungsten target in an Ar/methane atmosphere. Polished steel samples for laboratory testing and commercial valve tappets were used as substrates. The microstructure of the coating was studied by transmission electron microscopy (TEM), Raman spectroscopy, and electron energy loss spectroscopy, adhesion by scratchtest method and mechanical properties by depth sensing nanoindentation. Coatings were tribologically tested in lubricated contact in laboratory (pin-on-disc) and field (engine) conditions. The worn surfaces were analysed by Raman and time-of-flight secondary ion mass spectroscopies and observed by TEM. The coatings showed excellent wear resistance with negligible wear; the worn surface was covered by a very thin solid tribolayer formed from oil additives.