Microstructural characterization of small-particle plasma spray coatings

The microstructure of a small-particle plasma spray (SPPS) aluminum oxide coating sprayed onto a prepared mild steel substrate has been characterized using a variety of microscopic techniques as part of a process optimization study. The coating was highly conforming to the substrate as evidenced in high-resolution transmission electron microscopy, although some voids were present at the interface. The layered splat microstructure, characteristic of conventional plasma-sprayed coatings but smaller in size, was discerned in partially thinned samples in a focused-ion- beam scanning electron microscope (FIB-SEM). Microcracks and microporosity, generally less than 0.5 μm in size, was also seen between splats using transmission electron microscopy. Cubic alumina of the crystallographic form γ-Al_2O_3 was identified by electron diffraction.