Rapid solidification and microstructure development during plasma spray deposition

Plasma spray processing is a well-established method for forming protective coatings and free-standing shapes from a wide range of alloys and ceramics. The process is complex, involving rapid melting and high-velocity impact deposition of powder particles. Due to the rapid solidification nature of the process, deposit evolution also is complex, commonly leading to ultrafine-grained and metastable microstructures. The properties of a plasma-sprayed deposit are directly related to this complex microstructure. This paper examines the solidification dynamics and the resultant microstructures in an effort to establish a processing/microstructure relationship. Existing models in the literature developed for splat cooling have been extended and applied for examining the rapid solidification process during plasma spraying. Microstructural features of the splats that are produced by individual impinging droplets are examined through scanning and transmission electron microscopy. The relation of dimensions and morphologies of these individual splats to the consolidated deposit microstructure is considered. In addition, the distinguishing features in the solidification and microstructural development between air plasma spraying and vacuum plasma spraying are explored, and a unified model is proposed for splat solidification and evolution of the microstructure.