Preparation of pore gradient silicon nitride ceramics by a high-velocity oxy-fuel spraying technique

Abstract

The preparation of pore gradient silicon nitride (Si3N4) ceramics was investigated by thermally spraying dense Si3N4 coatings onto the porous Si3N4 ceramic substrates via a high-velocity oxyfuel (HVOF) spraying method. Spray powders with excellent processability were developed and produced by spray drying and sintering in a nitrogen atmosphere. After sieving, the powders with average particle size of 45-75 μm were selected. Optimization of spray parameters such as spray distance and hydrogen flow rate was carried out in order to find the most decisive factor necessary for the production of dense and well-adhering coatings and pore gradient stuctures. Zirconium phosphate bonded Si3N4 porous ceramics were used as the substrate and coatings were obtained only when spray distance was 200 mm. A pore gradient structure was obtained at lower H2 flow rate, while poor-adhering coatings were obtained at higher H2 flow rate due to the etching. The hardness is significantly improved after Si3N4 coatings were successfully applied.