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.