A Discussion on the Limits to Coating Reproducibility Based on Heat Transfer Instabilities

During the past several decades, plasma spraying has emerged as an important rapid solidification technology, aimed to depositing protective and functional coating on a variety of substrates in sectors as diverse as aerospace, manufacturing, automotive, consumer, and biomedical industries. As these applications demand high-quality coatings with reproducible and reliable properties, stringent process control is mandatory, supported by sophisticated diagnostic equipment. However, owing to the highly nonlinear nature of the plasma spray process, non-deterministic coating property changes make these requirements difficult if not impossible to achieve. In this contribution, possible reasons for nonlinearity are discussed based on a stability theory formalism as described by a cusp catastrophe of codimension 2 (Riemann–Hugoniot catastrophe).