IN-SITU TOUGHENED SILICON-CARBIDE WITH AL-B-C ADDITIONS

''In situ toughened'' silicon carbides, containing Al, B, and C additi ves, were prepared by hot pressing. Densification, phase transformatio ns, and microstructural development were described. The microstructure s, secondary phases, and grain boundaries were characterized using a r ange of analytical techniques including TEM, SEM, AES, and XRD. The mo dulus of rupture was determined from four-point bend tests, while the fracture toughness was derived either from bend tests of beam-shaped s amples with a controlled surface flaw, or from standard disk-shaped co mpact-tension specimens precracked in cyclic fatigue. The R-curve beha vior of an in situ toughened SLC was also examined. A steady-state tou ghness over 9 MPa . m(1/2) was recorded for the silicon carbide prepar ed with minimal additives under optimum processing conditions. This in crease in fracture toughness, more than a factor of three compared to that of a commercial SIG, was achieved while maintaining a bend streng th 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 t oughening.