CaO–SiO2–Al2O3–Y2O3 glasses as model grain boundary phases for Si3N4 ceramics

The glasses with compositions derived from the eutectic composition [37.78 (Y3Al5O12)·62.22 (SiO2)] of the quasi-binary glass system (Y3Al5O12)-(SiO2) with addition of up to 20 mol.% CaO were investigated as model grain boundary phases for Si3N4 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 (Y3Al5O12–YAG), no crystalline YAG was detected. Apart from yttrium disilicate (Y2Si2O7), anorthite (CaAl2Si2O8), tricalcium aluminate (Ca3Al2O6), and calcium yttrium oxide silicate (Ca4Y6O(SiO4)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 Si3N4 ceramics sintered with addition of Y2O3–Al2O3 are discussed in terms of the observed compositional dependences of the physical properties of CaO–Y2O3–Al2O3–SiO2 glasses.