The fracture toughness of Al 2 O 3 is considerably increased by the incorporation of fine monoclinic ZrO 2 particles. Hot‐pressed composites containing 15 vol % ZrO 2 yield K lc values of ∼ 10 MN/m 3/2 , twice that of the A1 2 O 3 matrix. It is hypothesized that this increase results from a high density of small matrix microcracks absorbing energy by slow propagation. The microcracks are formed by the expansion of ZrO 2 during the tetragonal → monoclinic transformation. Since extremely high tensile stresses develop in the matrix, very small ZrO2 particles can act as crack formers, thus limiting the critical flaw size to small values.
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Nils Claussen (1976) studied this question.
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