502 クラック内在型の窒化ケイ素繊維強化複合材料の開発(セラミック/セラミック基複合材料1)

Joints that exhibited tough fracture behavior were formed in a Si/SiC matrix reinforced with Textron SCS‐6 fibers with either boron nitride or silicon nitride fiber coatings. Lapped joints (joints with overlapping ‘fingers’) were necessary to obtain tough behavior. Geometrical requirements necessary to avoid brittle joint failure were proposed. Joints with a simple overlap geometry (only a few fingers) had to be very long in order to prevent brittle failure. Joints with an optimized stepped sawtooth geometry produced composite‐like failures with the stress/strain curves containing an elastic region followed by a region of rising stress with an increase of strain. Increasing the fiber/matrix interfacial strength, by changing the fiber coating, significantly increased matrix cracking and ultimate strength of the joints. The best joints had matrix cracking stress and ultimate strength of 138 and 240 MPa, respectively. Joint failure was preceded by multiple matrix cracking in the entire composite. The high strength of the joints should permit building of structures containing joints with only a minor reduction of design stresses.