Study Of The Processing And Mechanisms Of Microwave Sintering Of Y-ZrO₂Ceramics

As a new fabrication method, microwave sintering with the features of rapidheating up, environmental health, energy-efficient has been received widely attentionall over the world. nY-ZrO2(n=0,3,5,7,9mol%) ceramics were prepared by hybridmicrowave sintering(MS) in a multimode microwave cavity at2450MHz. Thenano-sized raw material powders were synthesised by conventional pyrolysis in abox furnace. The MS temperatures were from1100oC to1550oC with holding timeof30min, which was controlled by adjusting the input and reflected power. Thecontrol groups were set up by conventional sintering method in a box furnace withheating rate of2oC/min, sintering temperature of1100oC～1600oC and the soakingtime of120min. The bulk density, micro-hardness, X-ray diffraction and SEM wereused to characterize the as-prepared ZrO2ceramics.In the process of microwave sintering, the sintering schedule of15oC/min waswell controlled by adjusting the input and reflected power. The selective absorbingphenomenon is found by different absorbers at different temperatures. Heating up isattributed to the SiC rods and zirconia at low temperature and high temperatures,respectively. It is an unique characteristic for microwave sintering. Comparing withpure ZrO2ceramics (ZY0), the higher input power and much longer sintering timeare required during microwave sintering of Y-ZrO2ceramicsDense pure monoclinic ZrO2compacts (ZY0) can be prepared successfully byhybrid microwave sintering. Optimized microwave sintering temperature of m-ZrO2compacts is1450oC for30min with the microhardness of4.0GPa and relativedensity of99.0%. Fine and homogeneous grain size is obtained. Meanwhile,micro-cracks were observed along the edges of the indentation but not at the tips forconventional sintered samples, which indicating good toughness for microwavesintered pure monoclinic ZrO2compacts.Compared with the sample of ZY0, The lower densities were obtained formicrowave sintered Y-ZrO2(ZY3, ZY5, ZY7and ZY9) ceramics. And the density ofas-prepared samples decreased with the increasing of the content of Y2O3. More amount of m-ZrO2was detected in the microwave sintered samples than theconventional samples. During microwave sintering run, the transformation ofm-ZrO2â†'t-ZrO2is restrained in the stage of heating. However, the phase transitionof t-ZrO2â†'m-ZrO2is enhanced in the cooling process and at the same time theadditional stress field is induced by microwave field in the crack-tip. This additionalstress field promotes the expansion of micro-cracks, which causes the increasing ofthe content of m-ZrO2and the decrease of density of Y-ZrO2ceramics. Duringmicrowave sintering, quite a few transformation of t-ZrO2â†'m-ZrO2happened in thesamples of ZY0, which causes its high density.The rapid grain growth for samples of ZY0and ZY3occurred from1500oCduring the microwave sintering and decreased for samples of ZY5, ZY7and ZY9.The grain size decreased with the increasing of the content of Y2O3formicrowave-sintered samples. The above phenomenon is due to that theelectromagnetic field enhanced the growth of m-ZrO2during microwave sintering,but not for t-ZrO2and c-ZrO2. So, the more content of m-ZrO2, the more rapid graingrowth happened during microwave sintering.