Quantification of energy absorption by fibre pull-out during failure of SiCf/BN/SiBCN ceramic matrix composites

The fibre/matrix interface is key to controlling the toughness of ceramic matrix composites (CMCs), with fibre pull-out often cited as the primary toughening mechanism. However, previously it has not been possible to measure the extent of energy absorption by pull-out during failure. Here, time-lapse X-ray computed tomography (XCT) is used to resolve the toughening mechanisms including crack deflection, fibre bridging, frictional sliding and fibre pull-out during straining of a 2D plain-woven SiC fibre reinforced SiBCN ceramic matrix composite having a BN interphase. Then by analysing the extent of fibre pull-out for every fibre in the volume, the energy dissipated by progressive fibre pull-out is quantified based on the interfacial shear strength measured by single fibre push-out testing. As a case study, the efficacy of toughening by fibre pull-out before, and after, exposure of the composite in air to 1200 °C for 30 h has been compared. While the high temperature exposure increased the interfacial strength, reduced the extent of fibre pull-out and reduced the energy absorption by a factor of five, in both cases fibre pull-out represents at least 80 % of the total energy absorbed. This highlights the critical determining role of fibre pull-out in developing tough CMCs and the need to engineer a better high temperature resistant interphase for high temperature applications.