Microwave Sintering Process And Toughenging Mechanism Of La₂O₃-Nb₂O₅-Al₂O₃Composite Ceramics

Al₂O₃ceramic is one of the most structural ceramics widely used. However, itslow fracture toughness influenced the work reliability and security of ceramiccomponents. So the core of the research is to improve the toughness.The La₂O₃-Nb₂O₅-Al₂O₃composite ceramic materials were prepared bymicrowave sintering technology by adding two different ratio of La₂O₃-Nb₂O₅intoα-Al₂O₃. The effect on the microstructure and the mechanical properties of differentmicrowave sintering processes was investigated. The microwave sintering behavior ofAl₂O₃based composite ceramic was analyzed by comparing the results ofmicrowave sintering and conventional sintering. And both of the principle ofalumina crystal structure and the toughening mechanism of Al₂O₃based compositeceramic were investigated by X-Ray Diffraction （XRD）, Scanning Electron Microscope（SEM） and Energy Dispersive X-ray Detector （EDS）.The results show that the phases of7.5La₂O₃-5Nb₂O₅-87.5Al₂O₃compositeceramics were α-Al₂O₃,LaNbO₄and LaAl₁₁O₁₈by microwave sintering. Sinteringtemperature has a great influence on the formation of columnar crystals of7.5La₂O₃-5Nb₂O₅-87.5Al₂O₃composite ceramic. Under the time of heat preservationfor15min, there are no columnar crystals at1450°C, but with small amount ofcolumnar crystals at1475°C, and Completely generated at1525°C. Themicrohardness of7.5La₂O₃-5Nb₂O₅-87.5Al₂O₃composite ceramic was12.3GPa, thefracture toughness was6.49MPa·m1/2, the bending strength was356.4MPa. Thephases existed in7.5La₂O₃-10Nb₂O₅-82.5Al₂O₃composite ceramic were α-Al₂O₃and LaNbO₄after microwave sintering. LaNbO₄become liquid phase in the processof microwave sintering, which reduced the sintering temperature of the compositeceramics, and can make it densification at （1475°C,15min）. The microhardness of7.5La₂O₃-10Nb₂O₅-82.5Al₂O₃composite ceramics was12.6GPa, the fracturetoughness was6.26MPa·m1/2, the bending strength was301.3MPa.The Al₂O₃composite ceramic sintering temperature were significantly decreased bymicrowave sintering compared to conventional sintering, the liquid phase is formedat lower temperatures, to promote the interface reaction, meanwhile, appeared an abnormal growth model in the lower temperature. the continue growth of pure Al₂O₃grain arranged closer and cross-growth in the process of microwave sintering,prohibited the grain growth in an abnormal growth model, and final formed equiaxedcrystal. The tabular crystal of Al₂O₃grew into laminar crystal, and finally intocolumnar crystal in the process of preparing7.5La₂O₃-5Nb₂O₅-87.5Al₂O₃and7.5La₂O₃-10Nb₂O₅-82.5Al₂O₃composite ceramics by microwave sintering. It wasanalyzed that many kinds of toughening mechanisms played roles in the Al₂O₃composite ceramics, the columnar crystals toughen with crack deflection and particlepullout and so on. LaNbO₄which has multiple domain structure can reduce theextension at crack tip to realize the toughness of composite ceramic by absorbingpart of the crack propagation energy.