TOUGHNESS ENHANCEMENT BY POLYCARBOSILANE COATING ON SIC WHISKERS INCORPORATED IN SI3N4 MATRIX COMPOSITE

Si3N4 matrix composite was fabricated by hot pressing with 20% SiC whiskers coated with polycarbosilane (PCS). The preceramic polymer on the whiskers was pyrolysed during sintering to form a carbon-rich layer at the whisker/matrix interface. Mechanical properties were measured, and compared to those of the composites with whiskers purified with HCl and HF. Elastic modulus and bending strength of the composite with PCS-coated whiskers were lower than those of the composites with other whiskers. Fracture toughness was measured by single-edge notched beam (SENB) and single-edge precracked beam (SEPB) methods. The toughness, including crack-growth resistance measured by the SEPB method, increased from 7.2 MPa m1/2 to 7.9 MPa m1/2 by PCS-coating on the whisker, while the toughness measured by the SENB method decreased from 6.5 MPa m1/2 to 5.7 MPa m1/2. The layer derived from PCS facilitated debonding at the whisker/matrix interface and activated the wake-toughening. Optical microscopic observation of the crack propagation near the interface confirmed enhancement of interfacial debonding by the PCS-coating.