In-plane microstructure of plasma-sprayed Mg-Al spinel and 2/1-mullite based protective coatings: An electron microscopy study

Planar specimens prepared from thin plasma-sprayed Mg-Al spinel and 2/1-mullite layers are investigated via conventional and analytical transmission electron microscopy in order to evaluate the constraints of rapid solidification on the in-plane microstructure at different scales of resolution. Despite intrinsic differences in the nature and density of structural defects between the two materials studied, their gross-scale microstructures share a characteristic sequence starting from (i) large, spherical core grains, followed by (ii) a radial chill-zone exhibiting columnar to dendritic grain morphologies, which eventually leads to (iii) an impingement zone formed by the intersection of different chill-zones. Relief of thermomechanical strain from the core grains and chill zones gives rise to an equiaxed subgrain architecture and intensive microcracking in the impingement sone. Moreover, the impingement zone represents the region of highest and most varied lattice defect density.