Viscoelastic toughening of aluminosilicate refractory ceramics

This paper presents the results of a combined experimental and theoretical study of the effects of Na₂O addition on the microstructure, viscoelastic toughening and thermal shock resistance of aluminosilicate refractory ceramics. The Na₂O was added to change the viscosity-temperature characteristics of the glassy phase and to promote viscoelastic toughening. It was observed that doping of an aluminosilicate with 4-6 wt.% Na₂O significantly alters the microstructure, from mullite within a glassy matrix to aluminum oxide laths within a glassy matrix. The glassy matrix is shown to form viscoelastic ligaments. These bridge cracks form and grow during cold shock from elevated temperatures (1100-1400 °C). The viscoelastic bridges have the net effect of shielding the crack tips from transient thermal stresses due to thermal shock. They therefore improve the cold shock resistance of the ceramic. The insights developed from the experiments were used to guide the development of a fracture mechanics model for the prediction of viscoelastic toughening due to crack bridging.