Quantitative model for the viscous flow and composition of two-phase silicon nitride-based particles in plasma-spray deposition

Silicon nitride does not melt but decomposes at 1900 °C and so thermal spraying of pure silicon nitride powder is impracticable. However, the use of silicon nitride and other non-oxide ceramics as thick, thermally sprayed coatings has considerable engineering potential owing to their unique combination of properties. This research shows that embedding fine silicon nitride particles within an oxide matrix to form composite feedstock particles enables the formation of silicon nitride composite coatings with little decomposition of the silicon nitride. Successful deposition of the coatings depends critically on the flow of the feedstock particles on impact with the substrate. This paper concerns the design of oxide matrix systems for the deposition of silicon nitride composite coatings by thermal spraying. A quantitative model is developed for the viscous flow of two-phase feedstock particles at impact. A number of matrix systems are investigated, including a series of yttria–alumina and yttria–alumina–silica compositions. The research shows that certain oxide matrices can provide the required viscous flow and protect the silicon nitride from decomposition.