The role of phase transition in the fretting behavior of NiTi shape memory alloy

Fretting wear tests of a NiTi shape memory alloy plate against a GCr15 steel ball (NiTi/GCr15) were performed on a horizontal servo-hydraulic fretting test machine at various temperatures. As a comparison, fretting wear tests of a GCr15 steel plate against a GCr15 steel ball were also conducted under the same experimental conditions. The fretting behavior of GCr15 and NiTi alloy was studied by analyzing both the frictional logs (variation of the tangential force Ft and displacement D with the number of fretting cycles N) and fretting scar features. The experiments showed that NiTi exhibits much better fretting behavior than GCr15 steel even with a relatively low hardness at room temperature. With an increase in temperature from 22 to 200 °C, while the fretting behavior of GCr15 is almost temperature independent, the range of the partial slip regime of NiTi/GCr15 becomes small and the fretting wear resistance of NiTi is found to anomalously decrease with an increase in nominal hardness. It was demonstrated that the unique temperature dependent phase transition properties of NiTi play a key role in its fretting behavior. The stress-induced phase transition not only greatly improves the elastic accommodation ability of NiTi through the large recoverable deformation, but also increases its fretting wear resistance through the transformation shielding effect. With the increasing temperature, the elastic accommodation ability of NiTi decreases not only because of the increase of austenite elastic modulus and phase transition stress but also the plastic deformation of austenite, which in turn causes a small range of the partial slip regime in the NiTi/GCr15 pair. As the temperature increases, the decrease in fretting wear resistance is attributed to the decrease in the elastic accommodation ability of NiTi and the plastic yield of austenite before phase transition at high temperatures.