Characterisation of NiCr wire coatings on Ni- and Fe-based superalloys by the HVOF process

Abstract

Microstructural uniformity has long been considered necessary to improve the thermal, chemical and mechanical properties of materials. Metallurgists are continuously searching various processes with the intent to produce a uniform microstructure throughout the material. This study reveals the successful high-velocity oxy-fuel (HVOF) deposition of near-uniform Ni–20% Cr wire coatings, approximately 200–250 μm thick. The coatings were deposited onto Ni- and Fe- based superalloys for hot corrosion applications. The microstructural characteristics of the deposits have been studied using the combined techniques of X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX). The coatings exhibited characteristic splat-like, layered morphologies due to the deposition and re-solidification of successive molten or semi-molten droplets. Under the employed HVOF spray conditions, a high proportion of the feedstock wire appeared to have been fully melted prior to impact on the substrate. The developed coatings have a dense and nearly uniform microstructure, with almost uniform hardness values in the range of 600–630 Hv and porosity less than 1%. XRD and EDAX results revealed that the main constituent outstanding phase of Ni–20% Cr coating is Ni. A very small fraction of Cr 2O 3 oxide phase has formed in the structure of the coating in the form of either intersplat lamellae or globules. Some partially melted particles are also observed in the structure of the coatings.