Structural considerations in plasma spraying of the alumina–zirconia composite

The focus of this study is the amorphous and crystalline phase formation in air plasma sprayed alumina–yttria stabilized zirconia coatings. In this multi-component system at compositions close to its eutectic, amorphous structures can arise by virtue of the high cooling rates of melted particles. Two avenues for amorphous phase formation have been identified: in-flight and upon-impact mixing. While the crystalline structure is largely retained by unmelted or partly melted feed particles embedded in the coating, it can also be created in the solidification process. The formation of a supersaturated crystalline phase is proposed. It was found that the formation of the crystalline phases with supersaturated composition in alumina–YSZ composite is possible, in spite of the high cooling rates during spray process.

Research highlights
► Formation of amorphous and crystalline phases in plasma spray composite coating is investigated.
► Coatings using nano and micron size powders are studied.
► Crystalline phases form in the alumina–zirconia coatings despite the high cooling rate.
► In-flight melting and mixing and upon-impact mixing are the sources of amorphous phase formation.