Relationship between both thickness and degree of crystallisation of BN interphases and the mechanical behaviour of SiC/SiC composites

SiC/BN/SiC-Si single-ply composites with Hi-Nicalon S™ fibres were synthesised by chemical vapour infiltration followed by the liquid route. These are model composites representing the behaviour of industrial composites intended to be used as structural parts in the hot sections of aeronautical engines. Four interphases with two different degrees of crystallisation and, in each case, two different thicknesses were tested. The interfacial shear stress measured by fibre push-out test is significantly higher when the crystallisation degree of the BN interphase is low. Yet, in this case, it decreases with the thickness of the interphase. This higher interfacial bonding is also associated, albeit less significantly, with better macro-mechanical properties as measured by tensile and shear tests. The cracks observed in the composite with different microscopy techniques are also less long and open. Depending on the interphases, the debonds are more or less pronounced and occur preferentially at different locations in the interfacial system.