Comparative Study of Fretting Wear Behavior of Three Kinds of Bonded Solid Lubricating Coatings

The fretting behaviors of MoS_2-based, graphite-based (G-based), and polytetrafluoroethylene-based (PTFE-based) bonded solid lubricating coatings were comparatively investigated. The critical loads of the three kinds of coatings were determined on a scratch tester and correlated with the fretting behaviors of the coatings. Moreover, the original surfaces and the fretted surfaces and cross-sections of the coatings were analyzed by means of scanning electron microscopy and X-ray diffraction. It was found that all the three coatings were able to form transfer films on the counterpart steel surface in the presence of mild plastic deformation and flowing, which contributed to significantly decreasing the friction and wear during the fretting process. The three kinds of coatings were characterized by tangential preferential orientation during the early-stage fretting. The critical loads of the three kinds of coatings were ranked as MoS_2-based coating>PTFE-based coasting>G-based coating, which was not all the same as that of the friction-reducing and antiwear abilities ranked as PTFE-based coasting>MoS_2-based coating>G-based coating. This indicated that the fretting friction and wear behaviors of the coatings were not only dependent on their critical loads but also on their mechanical properties and microstructures. Moreover, the lubrication failure of the tested bonded solid lubricating coatings was attributed to the fretting-induced plastic deformation and flow, initiation and propagation of cracks, and the superficial peeling off in the presence of reciprocal cyclic fatigue stress.