Fabrication, Characterization, and Investigation into the Wear Behavior of NiTi-based Nanolaminate Coatings
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NiTi is a shape memory alloy possessing a stress-induced reversible martensitic transformation. NiTi alloys are used in a variety of industrial applications, however, is prevalent in the automotive, aerospace, and medical sectors. The main challenge of using NiTi is its poor machinability and formability. Applying NiTi as a surface coating may be an alternate manufacturing method that will require limited machining. The objective of this study is to produce a superelastic NiTi surface coating that still possesses excellent wear and dent resistance while reducing forming and machining processes. Ni and Ti were sputtered deposited in layers and annealed to form the NiTi intermetallic phase. NiTi coating was also fabricated through the sputtered deposition of a superelastic NiTi monolayer. The wear behavior of the coatings was studied through scratch, Hertzian-type indentation, and single particle erosion testing. Residual stress and XRD analysis suggest that superelastic NiTi phase precipitated at elevated temperatures which resulted in the excellent dent, scratch, and erosion resistance compared to as-deposited Ni/Ti nanolaminate coating. The as-sputtered, 650˚C, and 800˚C nanolaminate coatings and the monolayer of superelastic NiTi were further subjected to nanoindentation testing. This showed that the elastic modulus to hardness ratio and wear resistance of the nanolaminate annealed at 650˚C is comparable to the sputtered superelastic NiTi monolayer coating. Furthermore, the annealed nanolaminate coating is found to exhibit higher hardness than the monolayered NiTi and comparable dent resistance. The present findings reveal the superiority of the 650˚C annealed nanolaminate NiTi coating over the monolayer NiTi. This study has shown that superelastic NiTi can be produced by annealing Ni and Ti nanolaminates having excellent protective properties in applications where high wear damage is expected.