Fabrication, Characterization, and Evaluation of Monolithic NiTi Nanolaminate Coatings

Abstract

Nickel and titanium are common elements in coatings. When Ni and Ti are combined they offer promising characteristics; specifically, the NiTi intermetallic phase. NiTi is a shape memory alloy possessing a stress-induced reversible martensitic transformation. NiTi alloys are used in a variety of industrial applications and are prevalent in the automotive, aerospace, and medical sectors. The problem with using NiTi is its poor machinability and formability. Applying NiTi as a surface coating will provide 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. A full and comprehensive understanding of the formation of the superelastic NiTi phase during coating development is nonexistent. Fabrication of this intermetallic phase is formed through the annealing of sputter-deposited Ti and Ni layers in a coating. Crystalline phases and residual stresses of the coating were established through X-ray diffraction (XRD). The behavior of the coatings was studied through scratch and Hertzian-type indentation testing. XRD and residual stress analysis suggest that intermetallic Ni and Ti phases precipitated at elevated temperatures, which resulted in excellent dent and scratch resistance compared to as-deposited Ni/Ti nanolaminate coating. This indicates that superelastic NiTi can form while annealing nanolaminates, further suggesting that dent- and wear-resistant coatings have the potential to be produced through annealing layers of Ni and Ti to form superelastic NiTi.