Silicon nitride composites with magnesia and alumina additives: Toughening mechanisms and mechanical properties

This work was aimed to investigate the effect of magnesia (MgO) and aluminia (Al2O3) additives on the microstructure and mechanical properties of silicon nitride (Si3N4) ceramic composites, focusing on the salient toughening mechanisms. The application of MgO additives led to a denser and more uniform microstructure consisting of distinctive rod-like Si3N4 grains with higher aspect ratios, and thus a high flexural strength of 1039 +/- 78 MPa and fracture toughness of similar to 11.6 MPa m(1/2), in comparison with the composite with Al2O3 additives, where inhomogeneous structure and abnormal grain growth occurred. Underlying toughening mechanisms in the Si3N4 ceramic composites, including crack deflection, crack bridging and rod-like grain pull-out were observed, leading to an enhanced crack propagation resistance. With increasing aspect ratio of Si3N4 grains, the crack deflection angles increased, corroborating the strong toughening mechanisms present in the Si3N4 ceramic composites. The rod-like grain pull-out mechanism was also analyzed.