Metallographic and SEM investigation of the microstructure of thermally sprayed coatings on steel substrates

Using optical microscopy and SEM microprobe analysis, an investigation of the microstructure of thermally sprayed alloy coatings on steel substrates has been carried out. Particular attention has been paid to the development and application of stereometric metallography for the estimation of volume parameters of the spherical grains observed in the structure of the thermally sprayed coatings. The spherical grains were formed from initial powder particles that reached the coated substrate in a semi-molten state, and thus are a kind of inhomogeneity, which could act as a concentrator of internal stresses in the coating. The results suggest that for flame spraying, the spherical grains result mainly from initial powder particles with higher than the average dimensions, whilst for the plasma spraying, the spherical particles are those with significant concentration deviations. Some important parameters (mean diameter, size distribution curves, volume fractions) of the spherical grains as estimated by the stereometric method of Scheil–Schwartz–Saltikow are given in the paper. The results give some additional information about the process of structure formation of thermally sprayed coatings as a function of the initial powder parameters and the type of the thermal spraying procedure.