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 glassceramic 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 crackpinning 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 glassceramic 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 glassceramics 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.
Data are presented for the thermal diffusivitylconductivity of hot-pressed mixtures of S i c and AIN. Results indicate that hot-pressing at higher temperatures, which permits solid-solution formation, results in significantly lower thermal diflusivityl conductivity than is obtained by hot-pressing at lower temperatures, at which discrete phases exist.
An analysis is presented of the role of the physical properties which affect the thermal buckling behavior of ceramic materials for various geometric and thermal conditions. For a column with slight initial curvature or a curvature as the result of a transverse temperature gradient, expressions are derived for the maximum increase in temperature, AT, , , , or transverse temperature gradient, or heat flux to which the column can be subjected to avoid failure under the influence of the bending stresses. From the solutions obtained several "thermal buckling resistance parameters" are defined for comparing the relative thermal buckling resistance of brittle materials. Numerical examples indicate that thermal buckling of brittle materials of the proper geometry can occur relatively easily.
Thermal shock behavior was compared for a cordierite glass and glass-ceramics subjected to a water quench. Crystallization significantly increases the critical quenching temperature difference (AT,); this effect was attributed to enhanced strength and thermal conductivity offset by a corresponding increase in Young's modulus. The strength retained following thermal shock also increased significantly on crystallization for AT > AT,, probably as the result of stepwise increases in fracture toughness with increasing crack size of the glass-ceramic.
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