Microstructure and reaction mechanism of Ti-Al-C based MAX phase coatings synthesized by plasma spraying and post annealing

Abstract

This work explored the feasibility of fabricating thick Ti2AlC and Ti3AlC2 coatings by plasma spraying from Ti/Al/graphite agglomerates and post annealing. The phase transformation and microstructure evolution of the sprayed coatings annealed at 500–900 °C were proposed. The as-deposited coating exhibited a micro-laminated structure with mainly of (Ti, Al)Cx, Ti2AlC, residual Al and minor TixAly. The short reaction time of spraying process led to insufficient reaction in C@Ti-Al agglomerates, thus forming dominate (Ti,Al)Cx and limited amount of Ti2AlC. The phase in the coating changed little when annealing below 600 °C. As increasing temperature to 700 °C, both of Ti3AlC2 and Ti2AlC showed obvious growth but Ti3AlC2 was predominate. Under higher temperature (>700 °C), Ti3AlC2 in the coating decreased while Ti2AlC grew apparently. The residual Al, transition phase (Ti, Al)Cx and high temperature annealing were main factors to promote the formation of Ti2AlC phase in sprayed coatings. The as-annealed coating showed increasing hardness with elevated temperature because of the formation of Ti2AlC, reduced pores and homogenous structure. This study implied that element agglomerates would be an effective alternative of expensive MAX phase powders as feedstock for plasma spraying MAX phase coatings.