COMBUSTION-SYNTHESIZED BETA'-SIALON REINFORCED WITH SIC MONOFILAMENTS

Dense composites of combustion-synthesized beta'-Si3Al3O3N5 (beta'-SiAlON) reinforced with 20 vol.% SIC monofilaments (AVCO SCS-6) were hot pressed at a temperature of 1600 degrees C for 2 h under a pressure of 34 MPa. The mechanical properties of as-fabricated composites were investigated in the three-point flexure mode. The composites exhibited significant improvement in the work of fracture as well as in the ultimate strength, in comparison with monolithic beta'-SiAlON. Ultimate flexure strength values between 682 and 793 MPa for the composite and between 398 and 528 MPa for the monolithic beta'-SiAlON were obtained. A work of fracture of 13-20 kJ m(-2) was obtained for the composite, compared with 1.7-3.1 kT m(-2) for the monolithic material. Optical microscopy and scanning electron microscopy (SEM) examinations of the fractured specimens showed the usual composite toughening mechanisms of microcracking, interfacial debonding, filament bridging and pull-out. The interfacial shear strength as well as frictional stress were also investigated with a fiber push-out technique. The push-out load-deflection curves revealed a moderate interfacial bonding strength of 26 MPa and a frictional sliding stress of 24 MPa. Transmission electron microscopy interfacial characterization was correlated with SEM observation of the interfacial debonding site. It revealed the presence of definite but weak physical bonding between the outermost carbon-rich layer of the SiC filament and the matrix. It appeared that the filament and the matrix were compatible with each other both physically and chemically, despite the fact that the matrix contained 20 wt.% Al2O3 as a secondary phase.