Analysis of toughening mechanisms of Z_rO₂/nano-SiC ceramic composites

Three effects of nano-particles on the toughness of nano-composite ceramics, namely, nano-particle clustering, crack pinning, and transgranular fracture, are identified from both the experimental and analytical studies. It is found that crack pinning toughens the nano-composite ceramics because a higher stress intensity factor is needed to cause crack propagation around or pull-out of the nano-particle. The nano-particle along the grain boundary steers the crack into the matrix grain due to the strong cohesion between the nano-particle and the matrix. Transgranular fracture increases with the increase of the volume fraction of nano-particles. Since the fracture resistance of the grain boundary is lower than that of the grain lattice, the higher the probability of transgranular fracture induced by nano-particles, the tougher is the nano-composite. Nano-particle clustering, which increases with increasing volume fraction of nano-particles, leads to the reduction of both the strength and toughness of the nano-composite ceramics. The larger the size of the clustered particle, the more defects it contains and, thus, the easier it is for the crack to pass through the clustered particle. That is, the nano-particle clustering can reduce toughening induced by crack pinning. The theoretical prediction, based on the combination of the three effects of nano-particles, is in agreement with the experimental data.