Suspension HVOF spraying of TiO2 using a liquid-fueled torch

Abstract

TiO2 coatings were deposited by suspension spraying using, for the first time, a liquid (kerosene)-fueled torch. A ball-milled TiO2 powder with d50 = 5.67 μm was dispersed in isopropanol and, using a peristaltic pump, it was radially delivered in between the combustion chamber and the conical expansion barrel of the HVOF torch, replacing the conventional dry powder ports with adapted liquid injection nozzles. Coatings are made up of highly flattened splats, whose diameter varies reflecting the size distribution of the feedstock powder, and <2% porosity is obtained when the torch is operated using adequate kerosene and oxygen flow rates and low standoff distances. Substantial amounts of anatase were produced even though the feedstock consisted solely of rutile, due to the particularly rapid cooling of molten droplets impacting on the underlying surface at velocities that could be as high as 1000 m/s. Ball-on-disk dry sliding wear tests reveal relatively low wear rates that decrease from ≈10−4 mm3/(N·m) to <10−6 mm3/(N·m) when the temperature increases from ≈25 °C up to 450 °C. The denseness of the coatings allows them to produce micro-scale plastic deformation with limited brittle fracture.