Effect of microwave sintering temperature on the microstructure and fracture toughness of La2O3/Nb2O5 doped Al2O3 ceramics

Abstract

Al2O3 ceramics doped with 15%Nb2O5 and 7.5%La2O3 (volume percent) were prepared by microwave sintering at 1450~1550°C for 30min. The effect of sintering temperature on the microstructure, especially Al2O3 columnar grains, and the fracture toughness were investigated. The results show that the phases of sintered samples were α-Al2O3, LaNbO4 and Nb2O5, without La2O3. During the sintering period, the in-situ reacted LaNbO4, and the remained Nb2O5 after reacting, changed to liquid which promoted the growth of Al2O3 columnar grains and sintering densification. With the increase of sintering temperature, more and more columnar Al2O3 grains grew instead of equiaxial grains, and the columnar grains length increased from 8μm to 22μm. The average ratio of length to diameter of columnar grains increased firstly and reached to the maximum of 5:1 at 1500°C, then keep unchanged. The fracture toughness increased with the increase of sintering temperature, from 4.83 to 6.48 MPa·m. Introduction Alumina (Al2O3) ceramic,as a kind of structural ceramics, is widely used in engineering due to its high hardness, high wear resistance and corrosion resistance, excellent high temperature stability, in addition to its low cost. At present the main problems of the application of alumina ceramics are still their high brittleness[1] and high sintering temperature[2]. The studies[3-6] of many types of ceramic materials confirmed that microwave sintering can reduce the sintering temperature and sintering time of alumina ceramic. Moreover, the additives doped to alumina matrix and then changing to liquid in the sintering process, can not only promote the densification of alumina but also make the columnar or sheet alumina grains obtained, which improves the toughness of alumina effectively. In this study, Al2O3 ceramics doped with 15%Nb2O5 and 7.5%La2O3 (volume percent) were prepared by microwave sintering. Furthermore, the effect of sintering temperature on the densification, microstructure and fracture toughness were investigated. Raw materials and experimental methods α-Al2O3 (99.6 wt%, average particle size of 4μm) powder are produced by LeiPu Ceramic Material Co. Ltd., Zibo, Shandong, China; Nb2O5 (99.99 wt%, average particle size of 8μm) and La2O3 (99.99 wt%, average particle size of 2μm) powder are both produced by Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. The uniformly mixed powder was obtained after the ball milling of raw material powder followed by drying. Then the composite powder were prepressed into regular bars under a pressure of 100MPa with suitable amount of 5wt% solution of polyvinyl alcohol (PVA) as the binder, and then cold-isostatically pressed under 200MPa. After calcined in conventional furnace at 600°C for 3h to remove organic binder, the green samples were sintered in a 2.45GHz multimode microwave sintering furnace in air atmosphere. Altogether five kinds of samples with the same component of 15%Nb2O57.5%La2O377.5%Al2O3 were sintered at 1450°C, 1475°C, 1500°C, 1525°C, 1550°C respectively, for the same time of 30min. The densities of the sintered samples were measured by Archimedes method. The phase