Tailoring of an interpenetrating network ceramic metal microstructure to improve strength:: Al2O3/Ni3Al and Al2O3/Al

Significantly improved mechanical properties may be obtained by producing ceramic matrix composites with an interpenetrating network microstructure. In this work a ceramic matrix with a metal toughening phase microstructure is presented. The material is processed by infiltrating a porous ceramic preform with molten metal. Significantly improved strength, relative to a pure ceramic, results, Specific examples of Al2O3/Al and Al2O3/Ni3Al will be shown. Due the vastly different thermal expansion coefficients of the ceramic and metal phases high residual stresses are formed during cooling following metal infiltration. In the case of an Al2O3/Al composite residual stresses of 100-250 MPa compression in the ceramic and 400-800 MPa tension in the metal will be presented which have been calculated and measured using neutron diffraction. The metal can withstand such high stresses due to the high mechanical constraint exerted on ligaments of <5 mu m diameter, which is significantly less than a grain diameter. The effect of varying metal ligament size and metal content is shown. Due to the low failure strain of the ceramic, relative to the metal, fracture originates at a microcrack in the ceramic, Compressive stresses in the ceramic phase, however, significantly hinder microcrack initiation and explain improvements in strength. It is shown that strength improvements are in excess of what may be explained in terms of a toughness increase consequent of the metal phase. Specific microstructural features, necessary for improvement of mechanical properties, will be outlined by comparing the AI and Ni3Al materials and also be considering different processing techniques.