Leaching Characteristics of MgO and Al<sub>2</sub>O<sub>3</sub> from Glass-Matrix as a Simulated Grain Boundary Phase of Ceramics

Abstract

Borosilicate glasses containing MgO and Al2O3 ((1-x) [0.5Na2O⋅B2O3⋅3SiO2]⋅xM; x=0.05, M=MgO, Al2O3) were used in the present study as a simulated grain boundary phase of Si3N4-MgO and Si3N4-Y2O3/Al2O3 ceramics. Leach tests by the bulk glass specimens were carried out under hydrothermal conditions using an autoclave, and the leached layer was observed by EPMA and SEM. A large leaching weight loss and thick leached layer were observed in MgO-containing glasses. Another leach test with the same glass powder based on JIS R 3502 was carried out, and the component leach rate, L (g/cm2·h) was calculated from the results of leached solution analysis by ICP. The leach rate of MgO, LMgO=1.23×10-5g/cm2·h, was larger than that of Al2O3, LAl2O3=0.23×10-5g/cm2·h. Thus, the weight loss and film thickness of leached layer by bulk leach test has been correlated to the component leach rate, that is, leach resistance to water of Al2O3-containing glass was superior to that of MgO-containing glass. Results of bulk glass leach tests corresponded well to our previous results on hydrothermal corrosion of Si3N4-MgO and Si3N4-Y2O3/Al2O3 ceramics prepared under the same conditions except for the sintering aid; the former showed leach resistance lower than the latter. Thus, the characteristics of grain boundary of ceramics dominate the hydrothermal corrosion and the component leach rates of sintering aids such as MgO and Al2O3 play a major role in leaching.