Highly toughened composites by compressive surface layers with self-recovery function

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New toughening and strengthening mechanism due to the oxidation-derived compressive surface layers and the self-recovery function of the surface layers have been proposed and demonstrated. The volume expansion for the oxidation of dispersant NiAl forming NiAl_2O_4 with lower theoretical density provides the compressive surface stress. The preparation process of the starting NiAl/Al_2O_3 composites has initially been established, where (Ni, Al)-hydrazino-benzoate precursors precipitated in a water/methanol solution dispersing Al_2O_3 powder for the matrix are converted to fine and spherical NiAl particles covered by the Al_2O_3 powder at 1280℃ in Ar. This composite powder can be densified by a pressure-less sintering at 1600℃. The NiAl is oxidized to NiAl_2O_4 and NiO above 1200℃ in air. One of the oxidation products, NiO diffuses through the grain boundary of the matrix-alumina, reacts with the boundary Al_2O_3, and generates metamorphic layers consisting of boundary NiAl_2O_4 and AL_" … More O_3 grains inside the thin surface films. The thickness of the metamorphic layers is 20-50μm. The development of the surface film is particularly promoted at 1400℃, while it is limited below 1350℃. The development of the metamorphic layer with the compressive stress attaining to -128MPa has been controlled by the volume fraction of NiAl and the oxidation temperature. The strength and fracture toughness of the 10vol%NiAl/Al_2O_3 composite, 530MPa and 4 MPa・m^<1/2>, have been improved to 660MPa and 8 MPa・m^<1/2> by the oxidation. For the intelligent functions, the reproduction of the compressive surface layers without degradation of the strength and toughness has been verified. In addition, the rapid diffusion of the oxidation products filled the Vickers indents and cracks propagated from the indents to recover the strength by the oxidation treatment. These intelligent functions as well as the strengthening and toughening should contribute to the improvement of the reliability of the ceramic-matrix composites. Less