2-D reinforcement structure for fracture strength and fracture toughness enhancement for alumina

Despite the attractive properties of advanced ceramics, they are not popular for structural applications even though they possess high strength. Their low fracture toughness and brittle fracture mode are unwelcome for high integrity structure. Moreover, they fail at loads far below their theoretical strengths due to their inherent flaws. These have led to the development of reinforcing strategies to help enhance the fracture resistance of ceramics. However, reported strengths value are still far below theoretical strength, most reinforced ceramics suffer trade-off between strength and toughness, and most present reinforcement methods are material specific.;In this work, a generic method for reinforcing ceramic materials for the enhancement of fracture resistance is described. Continuous ductile ligaments oriented in two orthogonal directions and forming 2-D network grid reinforcement structure was employed. The method involves two major stages: fabrication of regular channels in 2-D in ceramic matrix to form the preform and infiltration of the preform with the required reinforcement. Two different materials: carbon fibers and soft metal alloys were used as sacrificial materials for fabricating the aligned 2-D regular channels in alumina matrix. After the porous preforms were formed, molten aluminum alloys were infiltrated into the channels by the application of mechanical pressure, and this completes the composite fabrication process. Mechanical tests show that some porous preforms having area fraction of 2.7 % exhibited 27 % higher flexural strength than the solid specimens despite the porosity contained and this has been attributed to the ability of the channels to reduce the population and distribution of cracks in the porous material. The reinforced composites were also subjected to mechanical tests which revealed 217.6 % enhancement in flexural strength for the 7.79 % Al 7075 alloy reinforced composite. This magnitude of property enhancement was achieved due to the confinement of the matrix in the loop of the reinforcement and the beneficial residual compressive stress generated as a result of the difference in coefficient of thermal expansion (CTE) of the alumina and aluminum. The residual compressive stress delays crack initiation and crack propagation in the alumina matrix. It also reduces the stress concentration factor in the matrix, leading to higher failure stress and higher fracture toughness.