Fretting wear behaviors of micro-arc oxidation coating sealed by grease

Ceramic coating was prepared on Al–Si alloy (LD11 alloy) by micro-arc oxidation (MAO) method, and then the porous of the coating was sealed by grease. The X-ray diffraction (XRD) analysis showed that the same phase structure was obtained for the MAO and sealed micro-arc oxidation (SMAO) coatings. The fretting wear tests of the two coatings have been carried out under a normal load of 50 N and displacement amplitudes varied from 2 to 40 μm. Dynamic analyses in combination with microscopic examinations were performed in detail through laser confocal scanning microscope (LCSM), optical microscope (OM) and scanning electric microscope (SEM). The experimental results indicated that there was an obvious difference occurred in the fretting running regimes for the SMAO coating, i.e. compared to the MAO coating, the mixed fretting regime was disappeared and the slip regime was shifted to the direction of smaller displacement amplitudes in running condition fretting map (RCFM). The coefficients of friction (COF) of the SMAO coating under all test parameters were greatly lower than that of the MAO coating. At the same time, there was a longer stage with low COF that can be observed in the friction coefficient curves for all test conditions of the SMAO coating. It was clear that the SMAO coating presented an obvious lubricating action during the fretting wear processes. In partial slip regime, the damage of the two coatings was very slight, and the porous structure was still intact even after 104 cycles. The fretting wear mechanisms of the two coatings in slip regime were main abrasive wear and delamination, but higher proportion of the traces of relative sliding was presented on the scars of the SMAO coating. As a conclusion, the SMAO coating exhibited a better resistance for alleviating fretting wear and lengthening service life than that of the MAO coating. At last, the mechanisms of alleviating fretting wear for the SMAO coating were discussed in detail.