Reaction-bonding of Mullite (RBM) in Presence of 3rd Group Metal Oxides Me₂O₃ (Me=Sc, Y, La)

Abstract

An attractive method for the fabrication of mullite-based ceramic matrix composites or mullite coatings is the reaction bonding of mullite (RBM) with Silicon (Si) and Alumina (alpha-Al₂O₃) powders as starting materials, because sintering-induced shrinkage is compensated by Si oxidation-induced volume expansion. However, since Si-oxidation and mullite formation are very sluggish in highly pure RBM systems, very high processing temperatures and times are required. The solution for this problem is the addition of dopant oxides. The doping should generate low-viscosity liquids during processing, which accelerate Si oxidation and mullite formation due to an enhanced diffusion of species. The influence of the 3rd group metal oxides (Sc₂O₃, Y₂O₃ and La₂O₃) on the reactivity of RBM Systems was studied in the temperature range between 1250 and 1500°C. It turned out that the three dopant oxides behave quite differently. Scandia (Sc₂O₃) doped RBM systems exhibit only a minor improvement of reactivity which is explained by an early formation of stable crystalline scandium pyrosilicate Sc₂Si₂O₇. Yttria (Y₂O₃) doped RBM systems exhibit a drastically improved Si-oxidation and mullite formation. This behavior is explained by a reaction sequence leading to the formation of an intermediate Yttrium-aluminosilicate melt and finally the crystallization of Yttrium pyrosilicate Y₂Si₂O₇. Lanthana (La₂O₃) doped RBM systems behave similar to Yttria-doped systems, i.e. show drastically accelerated Si-oxidation and mullite formation. However the formed Lanthanum-aluminosilicate melt does not re-crystallize under the investigated conditions, therefore glass-free reaction-bonded mullite ceramics were not achieved.