Influence of ceramic reinforcement characteristics on the microstructure and wear behavior of NiTi matrix composites

Because the important properties of the NiTi alloy, such as the shape memory effect, pseudoelasticity, and wear resistance, originate from a thermoelastic reversible martensitic transformation, these properties can be controlled using internal and external stresses. In this research, to improve the wear resistance and to minimize the mismatch stress in the matrix, YSZ microparticles with a thermal expansion coefficient close to that of an NiTi alloy were used as the reinforcement. In samples consolidated using spark plasma sintering, the role of YSZ reinforcement on microstructure and wear behavior was investigated and compared with NiTi-ZrO2 samples. The microstructural studies showed that using YSZ instead of monoclinic zirconia as the reinforcement can produce a lower mismatch stress and, therefore, a higher fraction of austenitic phase remains in the matrix. The austenite phase possesses strain recovery, which is beneficial for tribological applications. Therefore, NiTi-YSZ samples with a higher percentage of austenite and better mechanical properties demonstrated superior wear resistance. For example, the addition of 5 wt% YSZ to NiTi reduced the wear rate from 5.74 x 10-3 to 1.03 x 10-3 mm3/N. m. Investigation of wear mechanisms showed that abrasive, delamination and adhesive mechanisms mainly govern the wear process.