Asama Techno-Polis Development Organization , 1-2-6, Zaimoku-cho, Mechanical properties of dense spinel/mica composites were investigated. The composites were fabricated by sintering powder mixtures containing spinel as a matrix and 5, 10 and 20mass% Fph-glass or Tm-glass. Fph-glass has the composition of fluorophlogopite. Tm-glass has the composition of tetrasilicic mica en riched in K2O and SiO2 components. The microstructures of the composites obtained by sintering powder mixtures containing

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... 5, 10 and 20% Tm-glass (Tm-composites: Tm-5, Tm-10 and Tm-20) were fine and the in tergranular phase among the spinel grains consisted of mica crystallites embedded in glass. The microstruc tures of the sintered composites obtained by sintering powder mixtures containing 5, 10 and 20% Fph-glass (Fph-composites: Fph-5, Fph-10 and Fph-20) were coarser and the intergranular phase among the spinel grains was crystalline. Strength values were in the range of 293-199MPa. In Fph-5 and Tm-5, crack propa gation occurred in a brittle fashion through the spinel-spinel grain boundaries. High strength values were obtained only because Fph-5 and Tm-5 composites had smaller critical defect size. In Tm-10 and Tm-20, cracks also propagated mainly in the intergranular glassy region and brittle fracture occurred. In Fph-10 and Fph-20, deviation and branching of the cracks occurred along large-grown mica and spinel grains and led to toughening of the composites. Furthermore, the results of Hertz indentation tests showed that in Fph-10 and Fph-20, quasi-plastic deformation occurred and the post-indentation strength values of these specimens were higher than those of Tm-composites, which showed cone-like cracking. [