Parameter studies on high-velocity oxy-fuel spraying of MCrAlY coatings

Abstract

Todays thermally sprayed MCrAlY coatings are commonly manufactured by the vacuum plasma spraying (VPS) process. This technique provides dense and oxide-free coatings. However, mainly due to the vacuum procedures this production is cost intensive and time consuming. The third generation of high-velocity oxy-fuel (HVOF) systems nowadays offer processing of materials that are sensitive to oxidation even in atmosphere. This is mainly due to the achievement of higher kinetic energy of the particulates and lower melting degrees which enables particle flattening in a plastic state. The work presented here focuses the influences of process parameters of a gas-driven HVOF system on the microstructure and oxygen content of MCrAlY coatings. The major parameters were subjected to DOE investigation to estimate both single and interacting effects. It was found that spray distance, fuel/oxygen ratio and powder feed rate exert a major influence on microstructure and oxygen content, whereas powder feed gas rate is not significant. Further parameters of significance are substrate temperature, shroud-gas type, fuel-gas type and powder size fraction. The coatings with the lowest oxygen contents were subjected to heat-treatment in vacuum and oxidation in air. Subsequently they were metallographically investigated. A comparison with VPS coatings showed that HVOF-sprayed MCrAlY coatings show similar oxidation behavior.