Splat morphology and spreading behavior due to oblique impact of droplets onto substrates in plasma spray coating process

Splats are obtained by atmospheric plasma spraying of yttria-partially-stabilized (8%) zirconia and captured by fast traversing flat and inclined substrates in front of plasma torch. The angles of inclination of the substrates enable the particles to impact on the substrate at different angles of incidence. The effects of the angle of impact on the splat final morphology and spreading behavior have been examined by Scanning Electron Microscopy (SEM) and a Coordinate Measurement Machine (CMM) surface profiler. From SEM, oblique impact produces features such as fingering, ridging, splashing, overlapping and elliptical shaped splats. To quantify spreading behavior, two geometric characteristics, namely, spread factor (ξ) and aspect ratio (ψ) are measured by CMM. The spread factor (ξ) is the ratio of the splat circular equivalent diameter to the droplet diameter, for which the splat equivalent diameter is assumed by considering the elliptical shape to occupy the same area as a circular shape. The aspect ratio (ψ) is the ratio of the major to minor diameters of the ellipse, it being assumed that the elliptical shape of the splat is geometrically perfect. This paper presents the spread factors and aspect ratios of individual splats at different substrate inclinations and their comparison to available literature. Curved fitted quadratic equations for spread factor and aspect ratio are proposed. In addition, the Madejski's Reynolds number based spread factor model is modified to fit the authors' experimental data. These equations are necessary for future work on numerical simulation of deposit profiles on realistic engineering parts having complex curved surfaces.