Microstructure characterisation of un-melted particles in a plasma sprayed CoNiCrAlY coating

Un-melted particles in thermally sprayed coatings are usually classified as impurity elements. Such impurities have a strong influence on the coating behaviour due to their different microstructural characteristics. In this work, the microstructural and mechanical characteristics of un-melted particles in a vacuum plasma sprayed CoNiCrAlY coating have been investigated. Microstructural analysis revealed the dendritic structure of the as-received powder and a spherical core inside the powder particle. The as-deposited coating comprised a dual phase structure of γ-Ni and β-NiAl, with coarse grains ranging 1-2 μm and fine grains ranging 500 nm or even less. Un-melted particles were found to exist in the coating after plasma spraying. The grain structure in the un-melted particles has not been fully developed, as characterised via electron backscattered diffraction (EBSD). It is shown that the spherical core in the initial powder was retained as un-melted particles due to insufficient melting during plasma spraying. In addition, a semi-continuous β-phase layer was found at the un-melted particle boundaries. Due to the inherently brittle nature of the β-phase layer and poor bonding at the particle boundaries, cracking along the un-melted particle boundaries occurred under compression.