Investigation Of Shape Memory Alloys' Smart Tribological Characteristics

Shape memory alloys (SMA) possess not only shape memory effects but also good anti-wear characteristic under certain conditions, but these alloys own low hardness, it is incapable to explain this phenomena using traditional wear mechanism. On the premise of sensing the environmental stress, SMA can change their pseudo-Young's modulus and pseudo-elastic strain, which improves the contact status between asperities and the surface of material, so behave the smart tribological characteristic. This paper based on the contact problem, through combination of the elastic-plastic finite element method and the experimentation, researched the smart triblogical characteristic and wear mechanism of SMA, at the same time the design rules of the smart tribological material were put forward.In finite element analyses, adopting the isotropic hardening mechanical model, the normal and sliding contact models of one asperity on surfaces of TiNi alloys and multi-asperities contact models were computed. In experiments, the contact behaviors of TiNi alloys were researched using nanoindenter and the combination of Vickers indenter and atomic force microscope, furthermore, the anti-wear magnitude of the TiNi alloy and austenitic stainless steel was compared under dry sliding wear. Summarizing the whole paper, some results were gotten as follows:1. During tribo-contact process, SMA's smart tribilogical characteristics behaved in follow terms:(1) The critical load of plastic deformation was increased, at the same time, the area of plastic deformation was decreased and the maximum elastic strain was enhanced, moreover, the amount of plastic contact asperities was reduced.(2) With the pseudo-Young's modulus decreasing, the maximum von Mises stress, contact stress and frictional stress all minished, furthermore, the increasing extents of the maximum von Mises stress and contact stress with the increasing load slowed down and this trend was more obvious under high loads.(3) SMA's elastic recovery was large and plastic deformation left was small during a particle or asperity indented on its surface. Under the load 2000μN, the elastic recovery of TiNi alloy of pseudo-elastic strain 0.055 was 1.8 times of 45~# steel, but the plastic deformation was only 60% of 45~# steel. Under load 0.49N and 0.98N, the elastic recovery of this alloy was 1.40 and 1.31 times of45# steel respectively.2. SMA's wear mechanism was described as follows: the increasing critical load of plastic deformation, the decreasing area of plastic deformation and enhancement of elastic recovery could minish the particles' plastic embed and reduce the crack range, then the plowed gauge was shallow and narrow, so SMA exhibited good particle wear resistance. The depressing contact stress and postponing of plastic region moving to the surface could reduce the possibility of surface cracks happening, the increasing critical load of plastic deformation and decreasing plastic area not only lessened cracks happening within the material but also limited the cracks happening range, so SMA behaved good fatigue wear resistance. Generally speaking, owing to the smart tribological characteristic during tribo-contact process, SMA owned good particle and fatigue wear resistance.3. The pseudo-elastic strain played the most important role in three parameters i.e. the pseudo-elastic strain, pseudo-Young's modulus and critical stress of martensitic transformation which effected the SMA's smart tribological characteristic. The material with the combination of low pseudo-Young's modulus and high critical stress of martensitic transformation behaved the best tribological characterics, so the design rules of smart tribological materials were put forward:(1) Increasing the material's pseudo-elastic strain.(2) Decreasing the material's pseudo-Young's modulus.(3) Enhancing the material's critical stress of martensitic transformation.The material filling above three conditions simultaneously would possess the best tribological characteristic.4. Using three-dimension finite element model simulated the tip of nanoindenter indenting process on the surface of pseudoelastic TiNi alloy, considering all influencing factors synthetically, the tip's displacements gotten from finite element computing were close to those of the nanoindentation. It indicated that adopting the isotropic hardening mechanical model to analyze SMA's tribo-contact characteristics during loading process was reasonable.