Role of oxides in high velocity thermal spray coatings

Thermal spray coatings applied with high velocity techniques such as high velocity air fuel (HVAF), produce coatings with superior quality in comparison to other traditional techniques such as plasma spraying. To date, our knowledge of the bonding processes and the structure of the particles within thermal spray coatings is very subjective. To improve our understanding of these materials, especially of the surface oxide layer, Ni80/Cr20 HVAF thermally sprayed coatings were studied with scanning electron microscope, nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). In particular, NRA and XPS were used to characterise the oxide composition that gives the coatings their excellent oxidation resistance. The surface oxide on the Ni80/Cr20 particles was found to be only 7 nm thick and enriched in SiO2. A surprising finding was that the composition of the Ni80/Cr20 powder remained unchanged during the coating process despite the high velocity application with the HVAF method