Effect of surface treatment by ceramic conversion on the fretting behavior of NiTi shape memory alloy

Three types of surface-treated NiTi samples, M-1 (700 degrees C/0.5 h), M-2 (650 degrees C/ 1 h) and M-3 (400 degrees C/ 50 h), were prepared by ceramic conversion treatment under different conditions. The effect of the surface treatment on the fretting behavior of NiTi alloy was investigated in the Ringer's solution by using a horizontal servo-hydraulic fretting apparatus. The experimental results indicated that the surface layer of the low temperature (400 degrees C) treated samples M-3 was dominated by a single TiO2 layer, while the high temperature (650 and 700 degrees C) treated samples M-1 and M-2 consisted of surface TiO2 layer followed by a TiNi3 layer. These surface layers were found to have a strong effect on the fretting behavior of the NiTi alloy in terms of changes in the shape of the curves of the tangential force (F-t) versus displacement (d), the fretting regimes and the damage mechanisms involved. The stress-induced reorientation of martensite bands in the NiTi alloy could decrease the slope of the F-t-d curve and thus increase the elastic accommodation ability of the NiTi plate against 1Cr13 steel ball pair. However, since the surface-treated layers could suppress the martensite reorientation in the NiTi substrate and thus decrease the elastic accommodation ability of NiTi, the gross slip started at a smaller displacement amplitude for the surface-treated NiTi samples than for the untreated one. The main wear mechanism of the as-received NiTi alloy in slip regime was adhesion and delamination, while the major damage to the high temperature treated NiTi samples M-1 and M-2 was determined as the spallation of surface-treated layers. Due to the high bonding strength of the surface-treated layer with NiTi substrate, the low temperature treated NiTi samples M-3 showed the best fretting wear resistance in all samples tested.