In Situ Toughened Silicon Carbide with Al‐B‐C Additions

Abstract

“ In situ toughened” silicon carbides, containing Al, B, and C additives, were prepared by hot pressing. Densification, phase transformations, and microstructural development were described. The microstructures, secondary phases, and grain boundaries were characterized using a range of analytical techniques including TEM, SEM, AES, and XRD. The modulus of rupture was determined from fourpoint bend tests, while the fracture toughness was derived either from bend tests of beam‐shaped samples with a controlled surface flaw, or from standard disk‐shaped compact‐tension specimens precracked in cyclic fatigue. The R ‐curve behavior of an in situ toughened SiC was also examined. A steady‐state toughness over 9 MPa·m 1/2 was recorded for the silicon carbide prepared with minimal additives under optimum processing conditions. This increase in fracture toughness, more than a factor of three compared to that of a commercial SiC, was achieved while maintaining a bend strength of 650 MPa. The mechanical properties were found to be related to a microstructure in which platelike grain development had been promoted and where crack bridging by intact grains was a principal source of toughening.