Effect of Crystallites on Surface Damage and Fracture Behavior of a Glass‐Ceramic

A study was conducted of the effect of crystallization on the fracture toughness, strength, and resistance to surface damage of glass‐ceramic materials with a range of microstructures obtained by different heat treatments. The hardness indentation method was used as a quantitative tool to simulate mechanical surface damage. In the uncrystallized glass and in the glass‐ceramic heat‐treated to result in a uniform fine‐grained structure, crack size increased monotonically with indentation load. In contrast, in the glass‐ceramics heat‐treated to result in a microstructure consisting of larger crystallites (a few micrometers) contained within a fine‐grained matrix, a discontinuity in the crack size vs load curve presented evidence for crack‐pinning at crack sizes which were a small multiple of the intercrystallite spacing. At the position of crack‐pinning, the fracture toughness showed a discontinuous increase with increasing crack size that was attributed to crack deflection. The strength of the glass and fine‐grained glass‐ceramic measured in biaxial flexure decreased monotonically with indentation load. The strength at low values of indenter load of the glass‐ceramic heat‐treated to yield the coarser crystallites within the fine‐grained matrix was independent of indentation load, indicating stable crack propagation prior to fast fracture. At the higher values of indenter load, the coarse‐grained glass‐ceramics exhibited a monotonic decrease in strength with increasing indentation load. The results of this study indicate that the strengthening observed on crystallization of a glass can be attributed to a combination of a decrease in flaw size achieved at a given mechanical surface treatment, an increase in fracture toughness, and a modification in the mode of crack propagation.