Processing-microstructure-property relations in anisotropic thermal sprayed composites

Thermal spray is a significantly advanced but inherently complex deposition process that involves successive impingement of molten droplets on a substrate to form coating with a "brick-wall" layered structure. The anisotropic microstructure of coatings is very sensitive to processing conditions and has significant influence on the properties. This study aims to understand the processing-microstructure-thermal property correlation of thermally sprayed coatings. Thermal transport properties of three coating systems forming composites with pores (yttria stabilized zirconia (YSZ) -Air), a second phase (Mo-Mo₂C) and a graded material (YSZ-NiCrAlY) are interpreted from the point of view of microstructure and chemical composition. In the case of YSZ-Air composite, results indicate that porosity contribution from 20-35% decreases the thermal conductivity by 50-70% of the bulk value. For the intrinsic composite of Mo and Mo₂C, which coexist as stable phases, thermal conductivity increases significantly with 1.75wt% carbon addition since it reduces formation of MoO₂ during processing, but decreases with 3.5wt% carbon addition. This is attributed to larger carbide retention in the latter. For the discrete layered and graded composites of YSZ-NiCrAlY, which are made up of varying fractions of these two constituents, thermal conductivity decreases sharply up to 40wt% YSZ and then more gradually with increasing YSZ content. This paper examines these experimental findings by treating the these complex coatings as multiphase composites.