Tunnelling cracks in Al2O3/Al2O3–ZrO2 layered composites

Two sizes of symmetrical, five-layered ceramic composites with alumina surface layers and alumina–zirconia inner layers were produced. Each composite had the same sequence and composition of individual layers; however, the measured strength of the large-sized sample was only 76 MPa to 115 MPa. This is due to the presence of tunnelling cracks that appear as a consequence of the tensile thermal residual stresses in the inner layers that develop during the cooling of the samples from the sintering temperature. The small-sized composite, with the slightly smaller layer thicknesses, had a higher strength: 317–378 MPa. The number and the position of the tunnelling cracks in the large-sized composite was not uniform through the depth of the sample, indicating the presence of crack splitting. The main reason for the different strengths of the small- and large-sized samples seems to be the different thicknesses of the innermost layers. Analytical calculations confirmed that tunnelling cracks can occur in the large-sized composite, providing that typical values of the material parameters from the literature are taken into account. According to these calculations, a thinner innermost layer in the large-sized composite would result in a crack-free material.