HVAF spraying of NiTi coatings: Microstructure, phase transformation and shape memory behavior

Deposition of various coatings on surface of engineering components with the aim to improve their performance concerning wear, corrosion, friction, and thermal protection is already a standard practice. In special cases, depositing metallic NiTi shape memory alloy coatings may be a viable alternative for hard ceramic coatings. NiTi coatings offer additional benefits originating from their unique functional thermomechanical properties. However, fabrication of thick NiTi coatings turned out to be difficult. Standard electroplating and laser cladding methods are not suitable for NiTi, the most widely used plasma spray methods tend to produce chemically inhomogeneous coatings that do not transform martensitically, cold sprayed NiTi coatings suffer from poor adhesion to the substrates. In this work, we report on first ever successful fabrication of thick NiTi coatings (100–300 μm) that display functional thermomechanical properties and simultaneously show very good adherence to the substrate. We used high velocity air fuel thermal spray method to fabricate NiTi coatings deposited on mild steel using four different sets of fabrication parameters. Chemical composition, porosity, microstructure, phase transformation and functional thermomechanical properties of the NiTi coatings were evaluated. Although the coatings contain inhomogeneous microstructure, voids, oxide particles, high density of dislocation defects and internal stress, they undergo martensitic transformation upon cooling and/or mechanical loading. As sprayed NiTi coatings need to be annealed to display functional thermomechanical properties. Despite their limited tensile strength, the coatings displayed thermal actuation in 3-point bending tests and shape memory effects in nanoindentation and scratch tests.