Mechanical behaviour of mullite composites reinforced with mullite fibres

Mullite matrix composites uniaxially reinforced with uncoated and BN (0.3 or 1 mu m thick) coated mullite fibres were consolidated to 82 to 98% of TD (theoretical density) at 1300 degrees C by using a colloidal processing route to make mullite matrix. The mechanical properties were characterized by flexural testing and chevron-notch tests for fracture toughness measurement. The samples with uncoated and 0.3 mu m BN-coated fibres failed in a brittle manner without any toughening effect of the fibres. In contrast, the sample with 1 mu m BN-coated fibres showed toughened composite behaviour (i.e. load-carrying capacity following maximum load) with a significant enhancement in toughness (e.g. about 12 MPa m(1/2) in the as-fabricated state), despite a relatively low density of about 90% TD. The room temperature ultimate strength (320 MPa) and fracture toughness of this composite decreased a little by heat treatment at 1320 degrees C for inducing complete transformation to mullite of the matrix, probably due to the degradation of fibre strength.