Mechanical behaviour of SiC (Hi-Nicalon) fibre-reinforced Si{sub}3N{sub}4 matrix composites under monotonic and fatigue tensile loading

摘要

Fully dense Hi-Nicalon fibre reinforced Si{sub}3N{sub}4 composites were fabricated by slurry infiltration followed by hot-pressing. The deposition of a pyrolytic carbon coating on the fibres prior to fabrication resulted in a non brittlemechanical behaviour and a great increase in toughness by matrix microcracking and pull-out of the broken fibres. Two types of pyrolytic carbon coatings deposited under different CVD conditions were used in the present study and resulted in clearlydifferent interfacial properties thus mechanical behaviour. Composite "A" exhibited a higher average tensile stress (700 MPa) along with a higher matrix crack density (9 cracks/mm) and shorter pullout length (～100μm) than composite 'B" (630 MPa, 6cracks/mm and 200μm respectively). Mechanical properties were established through hysteresis loop meddling which revealed that interfacial bonding strength and sliding stress were considerably reduced by the use of carbon coating "B". Fatigue propertiesalso appeared to vary according to the fibre-matrix interfacial characteristics. while composite "B" showed a fatigue limit of less than 460 MPa, for composite .A" delayed failure always never occurred for a maximum stress of less than 550 MPa. Thisdifference was attributed to the rapid progression of interface debonding thus fibre overloading and fracture when composites exhibited lower interfacial contribution.