To improve the toughness of Al₂O₃ ceramic, xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ (x = 2.5-15.0 vol.%) composite ceramics were synthesized via microwave-sintering. The influences of sintering temperature (T_s) and La₂O₃ content on phase composition and mechanical property of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ composite ceramics were studied. It is shown in the results that the T_s of La₂O₃/Nb₂O₅ doped Al₂O₃ ceramic was 150 ^oC lower than that of Al₂O₃ ceramic. With the increase of x, La_0.33NbO₃, LaNbO₄ and LaAl₁₁O₁₈ were formed successively. LaNbO₄, columnar-Al₂O₃ grains and LaAl₁₁O₁₈ with plate-like shape were generated in-situ during the sintering period. Compared to Al₂O₃ ceramic, the fracture toughness of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics were at least 70% higher when 5.0 ≤ x ≤ 12.5. 7.5La₂O₃/10Nb₂O₅-82.5Al₂O₃ ceramic exhibited superior mechanical property: Hv = 12.0 GPa, K_IC = 6.2 MPa·m^1/2 (1,500 ^oC, 30 min)

Keywords: Al₂O₃ ceramics; Microwave sintering; Phase formation; Mechanical properties

Alumina (Al₂O₃) ceramic ranks among the most important engineering ceramics due to high hardness, high mechanical strength, high temperature insulation resistance, excellent thermal conductivity [1]. However, its potential application in engine is limited by high fragility (K_IC-3.0 MPa·m^1/2) [2]. Some additives or second phases such as AlCr₂ [3], WC [4], TiC [5, 6], SiC [7, 8], ZrO₂ [9, 10], Nb₂O₅ [11, 12], La₂O₃ [13], LaAl₁₁O₁₈ [14], LaNbO₄ [15] were doped into the Al₂O₃ matrix to improve mechanical properties. In Al₂O₃-Nb₂O₅ [12] system, some equiaxed Al₂O₃ grains were induced to grow oriented into columnar grains. As to Al₂O₃-La₂O₃ [13] and Al₂O₃-LaAl₁₁O₁₈ [14] systems, the plate-like LaAl₁₁O₁₈ grains significantly improved the fracture toughness of Al₂O₃. Furthermore, the fracture toughness and bending strength of Al₂O₃ was improved by the domain conversion of LaNbO₄[15, 16].
Generally, ceramics can be sintered by conventional, plasma and microwave heating methods [17-19]. Micro- wave sintering, as an efficient sintering method, leads to the volume heating of the sample through the coupling between microwave radiation and the material [20, 21]. Therefore, the temperature gradients resulted from conventional sintering process is prevented [22, 23]. In this experiment, in order to form LaNbO₄ and plate like LaAl₁₁O₁₈ grains in-situ and improve the mechanical property, Al₂O₃ were doped with La₂O₃/Nb₂O₅ (xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃, x = 2.5-15.0 vol.%) and synthesized via microwave-sintering.

xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ (x = 2.5-15.0 vol.%) composite ceramics were synthesized via microwave-sintering. High purity La₂O₃ (99.99 wt.%), Nb₂O₅ (99.99 wt.%) and Al₂O₃ (99.6 wt.%) were weighed according to the volume ratio of xLa₂O₃/10Nb₂O₅-(90–x) Al₂O₃ and then ball milled for 12 h. After dried, the mixed powders were granulated with 5 wt.% polyvinyl alcohol solution (PVA, 10 wt.%) as binder. Then granulated mixtures were pressed into regular bars by uniaxial (100 MPa) and cold-isostatic pressing (200 MPa) successively. These regular bars were placed in a conventional furnace and calcined at 600 ^oC for 3 h to remove binder. The sintering temperature of the samples in the microwave sintering furnace was 1,450~1,500 ^oC and the holding time was 30 min.
All sintered specimens were polished before the test. Their densities were evaluated by the Archimedes method. The phase analysis was carried out by X-ray diffractometry (XRD, D8ADVANCE Diffractometer, Bruker-AXS). The observation of microstructure were performed on thermal etched specimens by scanning electron microscopy (SEM, Nova Nano SEM450, FEI). The elemental analysis was conducted using energy dispersed spectroscopy (EDS). Vickers hardness (Hv) and fracture toughness (K_IC) was tested by HVS-1000 Vickers tester and single edge precracked beam method (SEPB), respectively.

The relative density (h) of xLa₂O₃/10Nb₂O₅-(90–x) Al₂O₃ specimens is shown in Fig. 1. The h of specimens sintered at 1,450 ^oC increased firstly from 93.6% (x = 2.5) to 98.0% (x = 7.5) and then decreased to 93.4% (x = 15.0). Meanwhile, the h of specimens sintered at 1,500 ^oC exhibited slightly higher relative density with the same variation trend and reached the maximum of 98.8% for x = 7.5. It indicated that the sintering tem- peratures of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ were 150~ 200 ^oC lower than that of Al₂O₃ ceramic (1,650 ^oC) [23]. The improvement of sinterability could be attributed to two reasons. On one hand, the doping of La₂O₃ and Nb₂O₅ might result in the formation of liquid phase during the sintering period. On the other hand, the volumetric heating enables microwave sintering to allow for more rapid and uniform heating than conventional sintering, leading to noticeable decreases in sintering temperature and time [24, 25].
The XRD patterns of the xLa₂O₃/10Nb₂O₅-(90–x) Al₂O₃ ceramics are shown in Fig. 2. When x = 2.5, α-Al₂O₃ (ICDD: 46-1212) and La_0.33NbO₃ (ICDD: 53-1023) were detected. With the increase of the content of La₂O₃, LaNbO₄ (ICDD: 22-1125) and LaAl₁₁O₁₈ were formed successively. No diffraction peaks corres- ponding with La₂O₃ and Nb₂O₅ were observed. It indicated that La_0.33NbO₃, LaNbO₄ and LaAl₁₁O₁₈ were generated successively in the process of sintering when the mole ratio of La₂O₃ to Nb₂O₅ increased from 0.30:1 (x = 2.5) to 1.78:1 (x = 15.0). The phase compositions of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics according to XRD analysis were listed in Table 1, which is similar to previous work from our group [26].
At the same time, when x exceeded 7.5, the intensities of diffraction peaks falling within LaAl₁₁O₁₈ increased, i.e., the content of LaAl₁₁O₁₈ ascended with the increasing La₂O₃ content. Of course, since La₂O₃ reacts with Al₂O₃, the content of Al₂O₃ is continuously decreasing. When x = 15.0, the main phase transformed from Al₂O₃ to LaAl₁₁O₁₈.
Fig. 3 is a presentation of the SEM images of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics. All the specimens exhibited a very small number of pores because the high heating rate in microwave sintering is favorable to boundary diffusion and in turn grain growth [27, 28]. When x = 2.5, the specimen was consisted of equiaxed grains and fewer grain boundary phase, which were determined to be Al₂O₃ and La_0.33NbO₃, respectively, by the EDS analysis (as shown in Fig. 3(a)). LaNbO₄ grains were observed as x exceeded 2.5(as shown in Fig. 3(b)) and the amount of which increased with increasing x. Meanwhile, the Al₂O₃ grains for x > 2.5 were smaller than that for x = 2.5 and homogeneously distributed with LaNbO₄. As to x = 15.0, a large quantity of plate-like grains identified as LaAl₁₁O₁₈ were formed (as shown in Fig. 3(f)), indicating that there were LaNbO₄ and plate-like LaAl₁₁O₁₈ obtained in-situ during the sintering process. This phenomenon resembles what was previously reported by Brito et al. [14] and Zhang et al. [15]. As Fig. 2 demonstrates, the SEM and EDS analysis exhibited the results consistent with those of XRD analysis above.
The Vickers hardness (Hv) of 5Nb₂O₅/xLa₂O₃-(95–x) Al₂O₃ composite ceramics was summarized in Fig. 4. As 2.5 ≤ x ≤ 15.0, Hv of the specimens sintered at 1,500 ^oC was higher, to some extent, than Hv of which sintered at 1,450 ^oC. With the increase of x value, Hv increased firstly (2.5 ≤ x ≤ 7.5) and then decreased gradually (7.5 ≤ x ≤ 15.0), and it reached the maximum value for x = 7.5. All the samples exhibited Hv higher than 10.1 GPa. The highest value of Hv is 12.6 GPa (x = 7.5). The variation trend of Hv was similar to that of the relative density. As shown in Fig. 1, the specimens sintered at 1,500 ^oC were denser than which sintered at 1,450 ^oC. Therefore, when composition was determined, Hv of the former was higher than that of the latter [29].
Like Vickers hardness, as shown in Fig. 5, the fracture toughness (K_IC) of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ com- posite ceramics climbed to the maximum for x = 7.5 and then declined gradually with the increasing x. When 5.0 ≤ x ≤ 12.5, the xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ composite ceramics exhibited fracture toughness exceeding 5.3 MPa·m^1/2, over 70% higher than that of Al₂O₃ ceramic (~3.0 MPa·m^1/2) [2].
The micrographs of cracks propagation of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ (x = 7.5) ceramics, originating from the corners of Vickers indentations, are presented in Fig. 6. LaNbO₄ grains reveal transgranular fracture and absorb energy through domain switch before cracking [28]. This sort of energy absorption performed by LaNbO₄ grains in xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics, in some extent, is similar to ZrO₂ in Al₂O₃ ceramics [10, 30]. Remarkably, few columnar Al₂O₃ grains (marked as C-A with arrows in Fig. 6) with length-diameter ratio of about 3:1 were formed, i.e., some Al₂O₃ grains were induced to grow oriented by the combined addition of La₂O₃ and Nb₂O₅. The existence of columnar Al₂O₃ grains in the path of crack propagation forced the cracks to deflect and diverge, extending the length of cracks. In general, the combined effects of domain switching of LaNbO₄, crack bridging and crack deflection play an important role in the significant improvement of the fracture toughness of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics. As a result, 7.5La₂O₃/10Nb₂O₅-82.5Al₂O₃ ceramic exhibited high fracture toughness: 6.2 MPa·m^1/2(1,500 ^oC, 30 min).

xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics were synthesized via microwave sintering at 1,450~1,500 ^oC for 30 min. The relative densities of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics are higher than 93%. The sintering temperature for preparing the composite ceramic was 150~200 ^oC lower than that of Al₂O₃ ceramic. LaNbO₄, columnar-Al₂O₃ grains and LaAl₁₁O₁₈ with plate-like shape were generated in-situ during the sintering period. The fracture toughness of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics were enhanced by the synergistic effect of columnar-Al₂O₃ grains and domain-switched LaNbO₄ grains. Compared to Al₂O₃ ceramic, the fracture toughness of xLa₂O₃/10Nb₂O₅-(90–x)Al₂O₃ ceramics were at least 70% higher when 5.0 ≤ x ≤ 12.5. The 7.5La₂O₃/10Nb₂O₅-82.5Al₂O₃ ceramic exhibited superior mechanical property: Hv = 12.0 GPa, K_IC = 6.2 MPa·m^1/2(1,500 ^oC, 30 min).

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No.51664043, No. 51802140 and No.51064022) and Natural Science Foundation of Jiangxi Province (20192BAB206007)