Gas Diffusion During Containerless Hot Isostatic Pressing of Liquid‐Phase Sintered Ceramics

Diffusion and dissolution of gases are considerably higher in glasses than in most crystalline materials. Thus, materials with a glassy grain‐boundary phase are susceptible to gas permeation when they are containerless hot isostatic pressed. Annealing of sinter‐hot isostatic pressed alumina—magnesium aluminosilicate glass (3 to 10 vol%) composites and pure glass samples at 1200° to 1600°C results in dedensification by matrix bloating and swelling. The degree of dedensification increases with the hot isostatic pressing pressure, temperature, and time and increasing annealing temperature. A theoretical prediction of high‐pressure gas permeation is developed based on a diffusion model. The analysis allows a satisfactory explanation for the gas diffusion effect on hot isostatic press densification. The analysis is also useful for developing design criteria for the hot isostatic press schedule and encapsulation materials. Annealing of hot isostatic pressed samples at 1100°C prior to high‐temperature annealing results in no dedensification as a result of out‐diffusion of the internal gases.