On the near-tip toughening by crack-face bridging in particulate and platelet-reinforced ceramics

The effectiveness on toughening of a near-tip crack-face bridging mechanism has been evaluated and discussed by both an experimental and theoretical viewpoint in the case of ceramic-ceramic particulate and platelet composites. According to the assumption of a strong bonding at the phase-boundaries, which, in ceramics, is often related to the characteristic of high refractoriness, a Barenblatt-like mechanism of elastic bridging was considered. From the experimental side, several brittle-matrix materials reinforced by SiC single-crystal grains of various size and morphology were fabricated and their microstructural parameters and fracture behaviors evaluated according to image analysis, fracture toughness and acoustic emission measurements. Theoretical and numerical computations of bridging-zone length and fracture toughness were performed with taking care to explicit the dependences of these parameters on the size and morphology of the reinforcement phase as well as the mismatches in elastic and fracture properties between matrix and reinforcement. Three model bridge-stress distribution functions were adopted and compared. They were: (1) the Dugdale's or constant clamping; (2) the distributed bridging spring; and (3) the discrete-pin distribution. It is pointed out that, in the range of intrinsic elastic and fracture properties generally reported for ceramic phases, the maximum toughening effect achievable is limited by stereological and micromechanical reasons. The critical stress intensity factor obtainable by optimizing the reinforcement size and morphology in the composite body is hardly higher than a few times that of the matrix material, an order-of-magnitude increase requiring a mismatch in the inherent elastic and/or fracture properties of matrix and reinforcement which is not easily achievable among known ceramics.