Effects of environment and temperature on ceramic tensile strength grain size relations

Overall strength (sigma)-grain size (G), i.e. sigma-G(-1/2), relations retain the same basic two-branched character to at least 1200-1300 degrees C. However, some polycrystalline as well as single crystal strength shifts or deviations are seen relative to each other, and especially relative to Young's moduli versus temperature for poly- and single crystals. The variety and complexity of these deviations are illustrated mainly by Al2O3, BeO, MgO and ZrO2 for which there is considerable data. At similar to 22 degrees C, Al2O3 polycrystals show substantial strength decrease due to H2O while MgO, ZrO2 and BeO polycrystals have limited, variable decreases. Al2O3 single crystals (sapphire) also show substantial strength decreases, but ZrO2 and MgO single crystals show little or none. Sapphire's strength markedly decreases from at least - 196 degrees C to a minimum in the 400-600 degrees C range, then rises to a maximum at greater than or equal to 1000 degrees C, followed by an accelerating decrease with further temperature increase. Polycrystalline Al2O3 shows similar (but less pronounced) strength minima and maxima, or alternatively an approximate strength plateau from similar to 22 to similar to 1000 degrees C interrupting the normally expected strength decreases with increasing temperature at suitably large grain size and absence of defects (e.g. pores) dominating failure. BeO crystals show a linear strength decrease with increasing temperature (T) similar to that of Young's modulus. BeO polycrystals often show a significant strength (apparently grain size and impurity dependent) maximum (at similar to 500-800 degrees C) or plateau (from similar to 22 to similar to 1000 degrees C) interrupting an otherwise continuous decrease. MgO shows similar temperature behaviour to BeO, but more pronounced crystal strength decrease and less pronounced polycrystalline strength maxima. Polycrystalline ZrO2 shows more rapid Young's modulus (E), and especially strength, decreases at similar to 200-500 degrees C than single crystals. More limited data for other materials also shows greater, variable sigma-T versus E-T trends, e.g. MgAl2O4 has a similar, but less pronounced decrease than ZrO2. Collectively these deviations suggest variable impacts on primarily flaw controlled sigma-G(-1/2) behaviour due to factors such as microplasticity, machining stresses, and thermal expansion and elastic anisotropies requiring more comprehensive testing and evaluation to better sort out these effects.