Residual stresses and R-curve behavior of AlN/Mo composite

AlN matrix was added with different amounts of metal molybdenum particles. R-curves were measured during stable crack propagation and a piezo-spectroscopic technique has been used to assess both, the residual stresses in the AlN matrix and the bridging stresses developed along the crack wake (at the metal/matrix interface) at the critical condition for crack propagation. Thermal residual stresses were compressive for Mo fractions ⩽15%; they were ≈0 when 20 vol.% of Mo was added and tensile for fractions ⩾30 vol.%. These stresses affected the composites toughness in two ways: (i) compressive residual stresses in the AlN matrix slightly enhanced the critical stress intensity factor for crack initiation, KI0, of the composite, (ii) tensile residual stresses weakened the metal/ceramic interface, leading to an enbrittlement of the composite. For Mo⩽20%, metal particles bridged the crack whereas when 30 vol.% of Mo was present the metal/ceramic interface was weakened and became the most favorable site for fracture of the bridging sites. As a result the toughness of the composite was strongly reduced. The theoretical R-curves calculated from the average (microscopic) bridging stress distribution obtained by in situ Raman spectroscopy were in good agreement with the experimental data.