In this study, a water-based corundum suspension was used to deposit 60 μm alumina coatings onto carbon steel substrates by HVOF spraying. The aim was to develop thin coatings with superior wear properties. Hydrogen was used as a fuel gas and process parameters were varied to determine their effect on microstructure and properties. Coating microstructure was examined by SEM to assess particle melting and morphology and XRD was used to study the phase transformation of the feedstock suspension. At higher combustion flame energy, the coating transformed primarily to gamma alumina, while at lower energy, it was found to be a mixture of alpha and gamma alumina. Nanoindentation tests were used to measure the hardness and elastic modulus of individual phases. Ball-on-plate wear tests helped reveal the relationship between wear performance and the alpha-gamma ratios in the coatings.

This study investigates relationships between in-flight particle characteristics and the microstructural properties of air plasma sprayed YSZ. Particle velocity and temperature were measured at different stand-off distances and contour plots were created. Coating porosity and hardness were measured and plotted over the same area. It is shown that in-flight particle characteristics are strongly affected by particle size distribution. Smaller particles in trajectories above the torch central axis were found to be 10 m/sec slower on average than larger particles in the center and lower section of the plume. The section of the plume with the highest temperatures and velocities is about 2 mm below the central axis and is shown to generate dense, hard layers with less than 7% porosity and hardness values on the order of 700 HV. Reynolds numbers determined from in-flight particle data also provide an indication of microstructural properties; where Reynolds numbers exceed 1000, coating porosity is less than 5%.