Time and humidity dependence of indentation cracking in aluminosilicate glasses

The inherent brittleness and poor crack resistance of oxide glasses have always been among their main limitations for many advanced applications. As the formation of cracks leads to amplification of applied tensile stresses and ultimately catastrophic failure, there is an interest in understanding the compositional and structural dependence of crack initiation and growth. The resistance to cracking can conventionally be measured using instrumented indentation that mimics the real-life damage for certain applications. Wada introduced a method to evaluate the crack resistance by counting the number of initiated cracks as a function of the applied load. Experiments have shown that the environmental humidity and the time period between indentation and crack counting both affect the crack resistance value, but unfortunately these parameters are not always reported in literature studies. Here we perform a systematic study of the time and humidity dependence of crack initiation in calcium aluminosilicate glasses. Depending on the experimental conditions (time and humidity), the crack resistance of an aluminosilicate glass can vary by more than a factor of two. Furthermore, the observed radial/median cracks can initiate several hours after indentation. These results therefore indicate the need for a standardized procedure for determination of crack resistance to allow comparison of data from different research groups. We suggest including a sufficiently long waiting period (such as 24 h) between indentation and crack counting, as the majority of the crack initiation will then have occurred.