Tribological Compatibility Between Piston Ring Coatings And Fully-Formulated Engine Oils

The properties of piston ring coating materials and formulation of engine oils are two main factors that influence the tribological performance of piston ring and cylinder bore system. There exist comprehensive and multiplex interaction between them so as to determine the tribological performance of piston ring and cylinder bore system together. In this thesis, based on investigation of the tribological performance of five kinds of piston materials (Nitrided, CrN, B4C, WS2 and MoNx) when lubricated with two GF-3 category fully-formulated engine oils respectively without/with MoDTC, the influences of tribological compatibility between piston ring material and engine oil on the tribological performance of piston ring and cylinder bore system were studied.The MoNx coatings were deposited with planar magnetron sputtering under different nitrogen partial pressures. The coatings obtained were about 4 μm in thickness. They had dense structures and tightly conglutinate to the substrates. The coatings' phase structures change from pure γ-Mo2N to δ-MoN as the nitrogen partial pressure used in depositing process increases. The nitrogen content and hardness of coatings were determined by their phase structures, i.e. they increase as the content of δ-MoN increases.The tribological performance of five kinds of piston rings when lubricated with GF-3 fully-formulated engine oil was investigated on a SRV high temperature friction and wear tester. Combined with surface analysis results, it can be concluded that the tribological performance is correlated with the element composition, compounds and distributing morphology of tribofilm. More P and Ca contents, more iron sulphide and large area distribution mean better performance.When lubricated with the GF-3 engine oil added with MoDTC, friction coefficients of five kinds piston rings were greatly decreased. They all reached 0.03 when tribotests were ended. But their decrease speeds were different. The decrease speed depends on the properties of piston ring materials. The friction coefficient decreases faster when the material has higher hardness, lower roughness and better adhesion to the substrate.By comparing the influence manner and degree of piston ring material properties and engine oil formulation on tribological performance of piston ring and cylinder bore system, the mechanism of tribological compatibility and its influence on the tribological performance of piston ring and cylinder bore system were discussed.