Wear behaviour of NiTi shape memory alloys
Date of Issue2016-06-01
School of Mechanical and Aerospace Engineering
NiTi shape memory alloys have been widely researched for tribology related applications such as biomedical devices and micro-electro-mechanical system (MEMS) devices. Micro-scale wear is a critical issue in such applications, which may trigger an inflammation of the human body, or result in a malfunction of MEMS devices. Many studies have compared the wear resistance of NiTi SMA with conventional wear resistant materials. However, a systematic study on the wear behaviour of NiTi SMA has rarely being reported. This study aims at understanding the wear behaviour of NiTi SMA, with the hope to provide a guideline for its better use as wear resistant materials. Ball-on-disk sliding wear tests and reciprocating wear tests were conducted, and an alumina (Al2O3) ball was used as counter-body. The effect of major influential factors on the wear behaviour of NiTi SMAs were investigated. Those influential factors including the temperature, applied load, deformation mode, wear cycles, and sliding speed. The deformation mechanisms involved in the wear process were revealed in consideration of the surface wear features, coefficients of friction, stress-strain curves, estimated contact stresses, and microstructures of NiTi SMA. The results can provide guidelines for the effective use of NiTi SMAs as wear resistant materials. In particular, when the forged ingot Ni 48.2at% Ti SMA was studied, three surface degradation stages were identified in the martensitic state, namely, a near-zero wear stage, a transition wear stage and an abrasive wear stage. Those degradation stages are related to the stable evolution of the coefficient of friction and surface wear features. A special wear feature, herein referred to as the "crown-like structure", was observed for the first time. The crown-like structure was produced in the near-zero and transition wear stage. For the flat annealed Ni 50.9at% Ti SMA, two wear modes were identified in the austenitic state. Mode I is temperature-sensitive and occurred when Af < T < Md. In this mode, the wear process was dominated by the interplay between the contact stress, critical stress for stress-induced martensitic transformation, and the shape recovery property. Mode II occurs when T > Md. This mode is less temperature-sensitive within the testing range. In this mode, the austenitic NiTi loses its super-elasticity and obeys a conventional deformation sequence. Furthermore, the wear behaviour of the forged ingot Ni 48.2at% Ti SMA was studied in three temperature regimes where different phases were present, that is, fully martensitic phase when T < Mf, fully austenitic phase when Mf < T < Af, and martensitic phase coexisted with austenitic phase when T > Af. When T < Mf, it was observed that the coefficient of friction decreased initially and thereafter stabilized at a lower value with increasing wear cycles. Further decrease was noticed when the temperature approached the Af. When tested above the Af, the coefficient of friction decreased more significantly under higher loads. Different types of wear behaviour originated from different deformation mechanisms involved in the wear process, particularly, the martensite detwinning process, the stress-induced phase transformation process, and the superimposed plastic deformation. This study can provide a guideline in applications of NiTi SMAs as wear-resistant materials. For example, since mechanical properties of NiTi SMAs are close to those of cortical bones, NiTi SMA based implants such as joint replacements, bone plates, and spine fracture fixations were developed. However, complex wear modes on the surface may cause undesired reactions from neighboring tissues, leading to a serious inflammation of the human body. Based on the fundamental understandings of the wear behaviour of NiTi SMAs, the surface degradation process of the implants can be predicted. This has great potential in improving the service life and safety factors of the NiTi SMA based implants/devices.