Fig xA The cross sectional microstructures of the failure

The starting materials were analytical reagents: cerous oxalate decahydrate (Ce2(C2O4)3·10 H2O), ammonium dihydrogen phosphate (NH4H2PO4) and gadolinium oxide (Gd2O3). They were mixed by a planetary ball mill containing agate ball (revolving speed of 350 r/min) in anhydrous ethanol for 10 h using the mass ratio of Ce2(C2O4)3·10H2O, NH4H2PO4 and Gd2O3 in 71 wt%, 25.05 wt% and 3.95 wt%, respectively. The ethanol was removed in a vacuum drying chamber. The powders were calcined at 600 °C for 2 h in air by a muffle furnace for the release of carbon dioxide (CO2), carbon monoxide (CO), water (H2O) and ammonia (NH3) [1], and mixed the polyvinyl alcohol (PVA) with the concentration of 5–10 wt% for granulation. The granules were selected by sieved between 40 and 100 meshes and pressed in a steel AG 556 with diameter of 20 mm at 40 MPa. Then the extruded articles were pressed by cold isostatic pressing at 200 MPa pressure and heated at 520 °C for 2 h in air by same muffle furnace for the decomposition of PVA. Lastly, the green bodies were sintered by different methods:(1)Hot-press sintering (HPS): the green bodies were sintered in graphite die and a mesentary pressure of 30 MPa (heating rate of 10 °C/min, vacuum pressure of less than 10 Pa). HPS were carried out in the temperature range of 1100, 1150, 1175, 1200 and 1250 °C for 2 h, and sintered at 1150 °C in range holding time of 1, 2, 4 and 8 h.(2)Pressureless sintering (PLS): the green bodies were sintered in a muffle furnace (heating rate of 5 °C/min). This sintering was conducted in the temperature range of 1200–1450 °C with 50 °C temperature intervals, and sintered at 1400 °C in range holding time of 2, 4, 8 and 10 h.