Frictional behavior of a synthetic engine oil, SJ/CF/5W-30, was investigated on a SRV reciprocating tribotester with tribo-materials consisting of Mo-sprayed coating, nitrided coating and Cr-plated coating sliding against cast iron operating under stepwise heating conditions. It was observed that, for the three tribo-materials, as operating temperature rose, there invariably existed a local minimum for frictional coefficient (μ), and the specific minimal μ value depended on the type of tribo-materials. Chemical characterization of worn cast iron surfaces using Raman spectroscopy indicated the existence of carbonaceous substances derived from the applied engine oil. There existed some correlation between microcrystal graphitic planar size (La value) and the local minimal frictional coefficient. Of the three tribo-materials, the maximal La value of the carbonaceous species was founded on the Mo-sprayed coating, and correspondingly, the lowest frictional coefficient occurred, indicating a more obvious anti-friction effect.

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