Preparation And Properties Of Laminated Al₂O₃ Ceramic Matrix Composites

Ceramic matrix composites are a type of materials having important applications. They can make full use of the favorable features of ceramics better and overcome the shortages of brittleness, low fracture toughness and sensitive to defects. In this study, sintered alumina ceramic slices were used as the matrix material, Al slurry, Al foil and resin were respectively used as the binding materials. Alternative stacking, screen-printing and brushing were applied to form billets with laminated Al₂O₃/Al or Al₂O₃/resin alternation structure. The Al₂O₃/Al and Al₂O₃/resin laminated ceramic matrix composites were prepared from the billets with the processes of heat treating in vacuum, heat treating in air and hot pressing.In order to understand the effect of binding strength on laminated ceramic composites, some tests were designed to measure the binding strength of different binding materials. The result shows that binding strength of Al slurry heat treated in vacuum and treated at 650℃in air are lower than that heat treated over 700℃in air . Among the organic binding materials we tested the binding strength of ?nanbao'brand epoxy resin is the highest.Al₂O₃/Al laminated ceramic matrix composites have been prepared by heat treated in vacuum or hot pressed in vacuum using the billets stacking Al₂O₃ slices and Al slices alternatively. The bending strength, fracture toughness and fracture work of the samples heat treated in vacuum all are relatively low because of there exist considerable microcracks in the binding layers. The samples hot pressed in vacuum have tightly binding interfaces and thin binding layers. The bending strength of the composite reaches about 387MPa which is higher than that of the original Al₂O₃ slices, that means the composite is a typical strong interface laminated composite. The fracture toughness and fracture work of the composite are 10.5 MPa·m^1/2and 1714 J/m² respectively.Al₂O₃/Al laminated ceramic composites can be also prepared by heat treated in vacuum or in air using the stacks of Al₂O₃ slices with Al slurry layers which were coated by screen-printing method. Heat treatment process has great influence on the microstructure and binding strength of the interfaces. The samples heat treated at 650℃in vacuum or in air have typical characters of weak interfaces. Its displacement-load curve has a sawtooth shape. However, samples heat treated in air at 700℃, 750℃and 800℃have stronger interfaces. The properties of these samples are more similar to strong interface laminated composites. The bending strength of the laminated ceramic matrix composite decreases and the fracture toughness and fracture work increase when its layer number increases although the distribution of the fracture work values are relatively dispersive. Simulation to stress formed in laminated composites showed that the compressive stress in Al₂O₃ layer tends to increase if the thickness of Al layer increases while the thickness of Al₂O₃ slice remains a certain value. Likewise, the tensile stress in Al layer tends to increase if thickness of Al₂O₃ layer increases while the thickness of Al keeps a constant.Six different type of resins were used as binding materials to prepare Al₂O₃/resin laminated composites. The results of mechanical performance tests show that nanbao expoy resin is the best one among the all resins. The bending strength decreases and the fracture toughness , fracture work increase as the layer quantity and resin thickness increase. The fracture process of laminated ceramic matrix composite prepared using the resin with lower binding strength and good plastic nature has following features that the ceramic layers break first, then the binding layers peel from the ceramic slice surfaces, a bigger strain would occur before the whole sample break down. On the other hand, when the composite is made using the resin with higher binding strength and lower plastic nature, the fracture crack propagation mode is mainly determined by the mode that crack propagates in the ceramic.Finally, we compared the four types of laminated ceramic matrix composites mentioned above. The hot pressing Al₂O₃/Al foil samples have the highest binding strength interfaces, Al₂O₃/resin samples rank second, the samples prepared by heat treated Al₂O₃/Al slurry coating in air are worse and the samples prepared by heat treated Al₂O₃/Al slurry coating in vacuum are the worst. The main factors influencing bending strength are the sort of binding layer and its binding strength. The factors influencing fracture work are relatively complicated, including the sort of the binding materials, thickness of binding layer, binding strength, stress situation and manufacture process, etc. The fracture toughness is mainly affected by modulus and fracture work.