Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

Abstract

In this study, thick Cr2AlC coatings were first synthesized via plasma spraying of Cr3C2–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr2AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr2AlC, Cr7C3, Cr23C6, and (Cr, Al)Cx solid solutions. The short residence time during spraying led to incomplete reactions in the Cr3C2@Al–Cr agglomerates, resulting in the formation of (Cr, Al)Cx. Post annealing provided sufficient energy for the transition of (Cr, Al)Cx → Cr2AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)Cx phase led to a slight increase in the Cr2AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr2AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)Cx and high temperature annealing were the primary factors to promoting the formation of the Cr2AlC phase in sprayed coatings. This study indicates that the Cr3C2@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr2AlC coatings.