HVAF Spraying of NiTi Coatings: Microstructure, Phase Transformation and Shape Memory Behavior

The abstract discusses coating engineering components to improve wear, corrosion, friction, and thermal protection, and proposes NiTi shape-memory alloy coatings as a functional alternative to hard ceramic coatings. It states that thick NiTi coatings are difficult to fabricate with common methods because many approaches produce chemically inhomogeneous coatings or poor adhesion, and that plasma-spray and cold-spray routes have limitations for martensitic transformation and bonding. The work reports first successful fabrication of thick NiTi coatings (100–300 µm) using high velocity air fuel (HVAF) thermal spraying on mild steel. The coatings were evaluated for composition, porosity, microstructure, phase transformation, and functional thermomechanical properties across four processing parameter sets. Although the coatings contain inhomogeneous microstructure, voids, oxide particles, dislocation defects, and internal stress, they undergo martensitic transformation upon cooling and/or mechanical loading; annealing is needed for functional thermomechanical properties. The coatings show thermal actuation in 3-point bending tests and shape memory effects in nanoindentation and scratch tests.