Preparation Of Coating On The Surface Of Porous Silicon Nitride Based Ceramic

To improve the strength of porous silicon nitride based ceramic materials, the coating was prepared on the surface of porous silicon nitride based substrate. Alumina sol silicon powder composite slurry was prepared, and alumina sol was chosen as the dispersed medium and silicon powder was reactive substance. The stability of slurry was improved by adding surfactants and the sintering activity was improved by adding Y2O3 and MgO. The coating was prepared on pretreated substrate by dipping method for many times and coated substrate was heated at low temperature and sintered at high temperature in nitrogen atmosphere. The stability mechanism, the phase change of coating at different temperatures and the combination property between coating and substrate were analyzed.The stability of slurry added 1.5 wt% PEG-400 as surfactant was the best. The viscosity of 10 wt% slurry added 1.5 wt% PEG-400 was 5 mPa·s, and the slurry was Newtonian flow behavior with good stability and suitable for the slurry coated on porous substrate. In this slurry system, alumina colloidal particle was adsorbed to the surface of silicon powder by connecting -OH groups with Si-O groups. When 1.5 wt% PEG-400 was added, the wettability of powder was the best. The one side of PEG-400 was adsorbed to the surface of powder by -OH groups, and the other side was connected with -OH groups of alumina sol. These crosslinked groups were arranged around the particle, and the stability of powder was improved by steric mechanism. When the substrate was coated, alumina sol firstly penetrated and filled the pores in the substrate because of the liquidity, and in the end the alumina sol layer with certain thickness was achieved. The powder in the slurry was deposited on the sol layer.As the substrate was coated by the slurry without sintering aids and sintered at 1420℃in nitrogen atmosphere, the phases on the prepared material wereβ-Sialon and Al₂O₃. The coating was prepared with dense layer nearby the substrate and loose layer in outer layer. The combination between coating and substrate was poor. While the substrate was coated by the slurry added with sintering aid and sintered at 1420℃in nitrogen atmosphere, the phases on the prepared material wereβ-Sialon, Al₂O₃ and small amounts of O’-Sialon. The preparing coating had the structure with dense layer nearby the substrate and dense layer in the outer layer. The combination between coating and substrate was perfect. Alumina sol had excellent reactive activity, and improved the liquid sintering by increasing the amount of liquid phase, and then the dense structure was produced. The Sialon with different solid solubility was synthesized in different areas in coating.The biggest bending strength of coated porous substrate was 91.882 MPa increased by 28.952%. The quality of resulting material was good and the coating was relatively dense.