Microwave Sintering Behavior And Mechanism Of Alumina Ceramics Toughened By Columnar Grains Grew In-situ And Domain Switching

Alumina ceramic has been used widely due to its excellent comprehensivemechanical properties and low cost. It has a very attractive application prospect.However, the low fracture toughness (only about3MPa·m1/2) of alumina creamicmaterial greatly limits its application. Therefore, the key problem of this ceramicapplication is to improve the fracture toughness.The Nb2O5-7.5La2O3-Al2O3composite ceramics were prepared by microwavesintering technology with adding the different ratio of La2O3-Nb2O5into α-Al2O3.The effects of different compositions and microwave sintering processes oncolumnar microstructure were investigated. The corresponding relation of columnarcrystal and sintering processes was investigated by XRD, SEM, EDS and TEM. Thegrowth kinetics of alumina grains and toughening mechanism of composite ceramicswere analyzed, and the EBSD of alumina based composite ceramic was studied.The results show that the Nb2O5-7.5La2O3-Al2O3composite ceramics werecomposed of α-Al2O3,LaAl11O18and LaNbO4when the molar content of Nb2O5isless than the molar content of La2O3. Their columnar grains contained LaAl11O18phase and Al2O3phase. The phases of Nb2O5-7.5La2O3-Al2O3composite ceramicswere α-Al2O3,LaNbO4and Nb2O5when the molar content of Nb2O5is more than themolar content of La2O3. Their columnar grains were Al2O3phase. On this condition,the grain length and ratio of the Al2O3columnar grains increased with the increase ofthe Nb2O5content. The maximum average length of columnar grain was25μm andthe maximum average ratio of length to diameter was6:1. the grain length and ratioof columnar grains in15Nb2O57.5La2O377.5Al2O3composite ceramics increasedwith the increase of the sintering temperature and holding time, the maximumaverage length of columnar grain was25μm and the maximum average ratio oflength to diameter increased to remain unchanged after5:1. The average graingrowth index and average grain growth activation energy of10Nb2O57.5La2O382.5Al2O3composite ceramics were1.69and221.9kJ/mol,respectively. And the average grain growth index and average grain growth activation energy of15Nb2O57.5La2O377.5Al2O3composite ceramics were2.5and434.22kJ/mol, respectively. The toughening mechanisms of alumina basedcomposite ceramic are crack deflection and domain switching. The LaNbO4phasedistributed on the grain boundary mainly, the crack propagation energy was partabsorbed by domain structure. The atomic occupation and coordinates of LaAl11O18phase and LaNbO4phase were acquired by the method of structure refinement. Thedates were obtained by EBSD, such as calibrated kikuchi patterns of Al2O3,LaAl11O18and LaNbO4, orientation imaging figure, mapping and polar figure. Therewas weak (2.94) texture of Al2O3columnar crystal in the direction of [110]. Thesmall amount(≤10%) of random distribution of Al2O3columnar crystal combinedwith matrix and other phases, which was conducive to densification of ceramicduring the sintering process.