Apparent fracture toughness of Si3N4-based laminates with residual compressive or tensile stresses in surface layers

The effect of macroscopic residual stresses on the fracture resistance and stable/unstable crack growth in Si3N4/Si3N4-30 wt% TiN layered ceramics has been investigated. The laminates were manufactured using rolling and hot pressing techniques. An apparent fracture toughness Kapp was calculated as a function of the crack length parameter a = Y((alpha))a1/2 for the laminates with residual compressive or tensile stresses in the top layers. The toughness increases in the layers with a compressive stress with increasing crack length, and it decreases in the layers with a tensile stress as the crack continues to grow. An explanation for the experimentally measured and calculated Kapp values is proposed. The existence of the threshold stress and the stable/unstable crack growth conditions is discussed.