where P is the used indentation force and d is the average indent diagonal length. The CR value corresponds to the load value with a probability of corner cracking of 50% . Specifically, the initiation probability of crack initiation is defined as the ratio between the number of cracked corners and the total number of corners during the test. The measurement of CR was also done using the Vickers tip, but here we used varying peak loads. The number of corners with cracks after unloading at different loads and different loading rates were observed using a microscope.