CaO–SiO 2–Al 2O 3–Y 2O 3 glasses as model grain boundary phases for Si 3N 4 ceramics

Abstract

The glasses with compositions derived from the eutectic composition [37.78 (Y 3Al 5O 12)·62.22 (SiO 2)] of the quasi-binary glass system (Y 3Al 5O 12)-(SiO 2) with addition of up to 20 mol.% CaO were investigated as model grain boundary phases for Si 3N 4 ceramics. The influence of CaO as model impurity on the physical properties of the glass (density, thermal expansion) and on the crystallisation behaviour was studied. Although the initial composition of the basic glass was that of yttrium-aluminium garnet (Y 3Al 5O 12–YAG), no crystalline YAG was detected. Apart from yttrium disilicate (Y 2Si 2O 7), anorthite (CaAl 2Si 2O 8), tricalcium aluminate (Ca 3Al 2O 6), and calcium yttrium oxide silicate (Ca 4Y 6O(SiO 4) 6), a new phase was detected, not found in the powder diffraction file (PDF) database. Cavities were formed within the devitrified glass due to the volume contraction after crystallisation. Possible implications for the mechanical properties of Si 3N 4 ceramics sintered with addition of Y 2O 3–Al 2O 3 are discussed in terms of the observed compositional dependences of the physical properties of CaO–Y 2O 3–Al 2O 3–SiO 2 glasses.