Effect of inclusions on size of surface flaws in glass-crystal composites

Indentation fracture studies were conducted on three sodium borosilicate glasses containing a dispersed phase of alumina inclusions with different degrees of thermal expansion mismatch between the glass matrices and the alumina. The alumina inclusions were found to cause a significant decrease in the size of the indentation cracks compared to those in the glass. This effect was greatest at the higher values of indentation load, which resulted in cracks of dimensions of sufficient size that their propagation was impeded by the tougher alumina dispersions. The fracture toughness for the composite samples calculated from the indentation data showed a significant increase with increasing crack size. For the smallest cracks in these composites, the value for fracture toughness was well below the value obtained in an earlier study by the single-edge notch-beam technique. The fracture toughness for the larger crack sizes which interacted with the alumina dispersions showed excellent agreement with the notch-beam data. The residual stresses due to the thermal expansion mismatch appeared to lead to a slight increase in the mean crack size regardless of the direction of thermal expansion mismatch.