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

Using optical microscopy and SEM microprobe analysis, an investigation of t he microstructure of thermally sprayed alloy coatings on steel substrates h as 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 thermal ly sprayed coatings. The spherical grains were formed from initial powder p articles 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 th an the average dimensions, whilst for the plasma spraying, the spherical pa rticles 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-Schw artz-Saltikow are given in the paper. The results give some additional info rmation about the process of structure formation of thermally sprayed coati ngs as a function of the initial powder parameters and the type of the ther mal spraying procedure. (C) 2001 Elsevier Science B.V. All rights reserved.