The Influence of Matrix Phase Viscosity on the Plasma-Spray Deposition of Silicon-Nitride Composite Coatings

Abstract

Abstract Non-oxide ceramics, such as silicon nitride, have a unique combination of high strength, toughness, wear resistance, thermal and chemical stability. However, the use of these materials as thick protective coatings on engineering components has been severely restricted by their decomposition behavior. Silicon nitride, for instance, does not melt but decomposes at ~1900oC and so thermal spraying of pure silicon nitride powder is impracticable. A limited amount of research has been carried out on depositing silicon nitride in various metallic or ceramic matrix materials but none have produced adequate coating microstructures or coating properties. This paper concerns the design of oxide matrix systems for silicon nitride composite coatings. A quantitative model is developed for the viscous flow of two-phase feedstock particles on impact with the substrate and is applied to the deposition of silicon nitride – ceramic matrix coatings. A number of matrix systems are investigated including a series of yttria-alumina and yttria-alumina -silica compositions. The research shows that the oxide matrices successfully protect the silicon nitride from decomposition but that the matrix composition and particle loading have a critical influence on splat flow and coating quality.