EFFECTS OF THERMAL CYCLING ON THERMAL-EXPANSION AND MECHANICAL-PROPERTIES OF SIC FIBER-REINFORCED REACTION-BONDED SI3N4 COMPOSITES

Thermal expansion curves for SiC fibre-reinforced reaction-bonded Si3N 4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured fro m 25 to 1400 degrees C in nitrogen and in oxygen. The effects of fibre /matrix bonding and cycling on the thermal expansion curves and room-t emperature tensile properties of unidirectional composites were determ ined. The measured thermal expansion curves were compared with those p redicted from composite theory. Predicted thermal expansion curves par allel to the fibre direction were between the measured curves for the strongly- and weakly-bonded composites, but those normal to the fibre direction for both bonding cases were similar to that of the unreinfor ced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature and no loss in tensi le properties from the as-fabricated condition. In contrast, thermal c ycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mech anical properties of the weakly-bonded SiC/RBSN composites the most, r esulting in loss of strain capability beyond matrix fracture and catas trophic, brittle fracture. Increased bonding between the SiC fibre and RBSN matrix due to oxidation of the carbon-rich fibre surface coating and an altered residual stress pattern in the composite due to intern al oxidation of the matrix are the main reasons for the poor mechanica l performance of these composites.