Towards reliable Ni-Ti components

Shape memory alloys (SMAs) under compressive loading have attracted interest due to the improved fatigue life compared to tension, offering significant advantages in applications such as elastocaloric cooling, actuators, and dampers. However, compression-induced buckling in SMAs necessitates the use of supporting structures (holders), creating tribological contacts that can lead to wear and premature failure. In this study, we have systematically investigated the wear behaviour of pseudoelastic Ni-Ti tubes in contact with various holder materials, including metals (hardened steel, bronze), ceramics (Al▫$_2$▫O▫$_3$▫, ZrO▫$_2$▫), and polymer (Oilon), under cyclic compressive loading exceeding 1×106 cycles. While all Ni-Ti samples reached run-out, significant surface cracking was observed in most tubes, except when using Oilon supports. Numerical analysis predicted fretting wear due to multi-axial stress distribution at holder edges, while additional tribological tests further evaluated wear behaviour and coefficients of friction. Oilon supports provided the best overall performance, minimizing wear while maintaining buckling integrity of Ni-Ti tubes. In contrast, steel supports caused the highest degree of Ni-Ti wear and were highly fit-dependent. Softer metals, such as brass and bronze, exhibited excessive total wear, while ceramics showed promising characteristics but were limited by their brittleness when interacting with high-strain material like SMA. These results highlight that fretting wear- and possibly fretting fatigue intrinsically occurs during extended operation of pseudoelastic Ni-Ti components in contact with metallic or ceramic materials, while wear is exacerbated by increased relative motion. This work provides insights for optimizing material pairings in practical SMA applications not limited to compressive loading or tubular design.