Investigation of PVD Piston Ring Coatings With Different Lubricant Formulations

Chromium nitride (CrN) is the main protecting coating applied to top rings for gasoline and diesel engines, due to its excellent wear resistance, low friction and minor environmental impacts, especially in modern engines operating with low viscosity oils. Recently, diamond like carbon (DLC) coatings reported improved tribological performance, but at a higher cost. Therefore, in the present work, wear and friction of CrN and DLC coated rings were evaluated on reciprocating and floating liner engine tests running on 0W-20 and 0W-16 lubricant formulations, which additives tailored for different markets (Japanese, European and Emerging). DLC outperformed CrN for both friction and wear when running on lubricant formulations without molybdenum additives. On opposite, with high molybdenum content additives, CrN presented synergic effects that significantly reduced friction and wear, whilst DLC did not. Same comparison on floating liner engine tests demonstrated again superior performance of CrN by 9% reduction on friction losses, running on oils containing molybdenum additives, whilst DLC lowered 6%. From that, it can be estimated 0.4% fuel saving at urban conditions by combining Japanese lubricant oil formulation and CrN top rings.