Microstructure and toughening mechanisms in spark plasma-sintered ZrB2 ceramics reinforced by SiC whiskers or SiC-chopped fibers

Starting from a ZrB2 matrix, composites containing 10, 20 vol% of SiC whiskers and 20 vol%of SiC-chopped fibers were sintered by spark plasma sintering at 1500°C. The addition of whiskers allowed both strengthening (740-770 MPa) and toughening (5.1-5.7 MPa .m1/2) compared with the reference material. In the fiber-reinforced composite, the increase in fracture toughness (5.5 MPa .m1/2) was accompanied by a decrease of strength (370 MPa). Toughening mechanisms were explored through the analysis of crack propagation. Crack deflection, crack pinning, and thermal residual stresses were the most important mechanisms identified. The experimental toughness increase was successfully compared with the values predicted by theoretical models. Compared with the baseline material, the reinforced composites showed an increased strength at 1200°C in air. The highest value, 450 MPa, was for the fiber-reinforced composite.