Oxygen Doping and Crystal–Amorphous Multiphase Coupling Synergistically Improve the Wear Resistance of NiTiCu Alloy

NiTi shape memory alloys (SMAs) show excellent performance in vibration damping and corrosion resistance. However, due to low hardness and lack of self-lubrication, they suffer severe friction and wear during service. Although various strategies including doping and heterostructure design have been proposed, the effects are unsatisfactory. In this work, we present a series of (Ni₄₀Ti₄₀Cu₂₀)_100-xO_x (x = 0.1, 0.5) alloys with B2, B19', and worm-like amorphous phases. These alloys display extremely low wear rates (7.553 × 10⁻⁷ and 6.15667 × 10⁻⁷ mm³ N⁻¹ m⁻¹) under a load of 15 N, which are far lower than those of similar metallic structural materials (10⁻⁴ to 10⁻⁵ mm³ N⁻¹ m⁻¹). It should be noted that for the two alloys, the wear rate decreases with the increase of load, whereas the friction coefficient shows an opposite trend, and the wear rates are all in the range of 10^-6 to 10⁻⁷ mm³ N⁻¹ m⁻¹. The above-mentioned excellent performance can be attributed to the following three aspects: (1) the B19' phase and the amorphous phase provide a very high level of hardness; (2) density functional theory (DFT) calculations prove that, under the action of frictional heat, B19' tends to precipitate metal cations, forming a dense oxide film that also has a certain lubricating effect. X-ray photoelectron spectroscopy (XPS) analysis shows that both alloys contain Ti₂O₃, TiO₂, NiO, CuO, etc.; (3) during the friction process, the phase transformation from B2 to B19' and the HDI heterogeneous hardening effect between the crystalline and amorphous phases can effectively relieve the micro-cutting and micro-plastic deformation of the friction pair. This design provides a new solution for NiTi SMAs to achieve both friction and wear performance and functional characteristics in engineering applications.