Characterization of fretting damage in NiTi superelastic wires

Some applications of NiTi alloys are susceptible to damage by fretting which occurs between two contacting bodies due to persistent relative movements of small amplitude (1- 300 µm). In the present work, the fretting damage on NiTi wires (50.8 at.%. Ni, 1.2 mm diameter) in 90º cross configuration was studied using imposed displacement amplitudes of δ = 50 µm for normal loads P = 10, 20 and 50 N and δ = 10 µm for P = 50, 100 and 200 N. For δ = 50 µm and P = 10 N, the fretting regime corresponded to Slip Regime (SR) where most of the displacement results in the relative sliding in the whole contact area, with the surfaces exhibiting wear damage. Debris generated in this regime were characterized by TEM and particles corresponding to TiO₂ (rutile) or NiO with individual sizes between 10-20 nm were identified. For δ = 50 µm and P = 20 N a Mixed Regime (MR) was observed while for δ = 50 µm and P = 50 N Partial Slip Regime (PSR) was reached which is characterized by stick region in the central inner zone of the contact. For δ = 10 µm and P = 50, 100 and 200 N the system behaved in PSR. In both PSR and MR the presence of cracks was detected by the decrease in the tangential contact stiffness.