Electron microscopic studies of plasma-sprayed coatings

This work characterizes the coating surface and profile (cross-section perpendicular to the substrate surface) of plasma-sprayed deposits. Metal (mild steel and a composite of Ni-Al) and ceramic (alumina and zirconia based materials) deposits were produced and examined by scanning electron microscopy. The ceramic coatings were also examined by transmission electron microscopy. Structural observations from the controlled fracture of coatings are also included. The ceramic coatings exhibited similar morphological features to the metal coatings in the form of flattened particles, dendritic artefacts on the particle surface, a columnar crystal structure and porosity of various forms. The columnar structure that was observed in some flattened particles may be explained by assuming efficient heat extraction through the underlying structure (substrate or coating). In a similar manner the dendritic artefacts would be expected to preferentially nucleate on the surfaces of large particles