Microstructure And Mechanical Properties Of Rare-earth Zirconate-based Multiphase Ceramics

The rare-earth zirconate-based composite ceramics were prepared bypressureless sintering process in air. Ln₂O₃（Ln=La, Nd, Sm, Gd）, ZrO₂, A_l2₃andMgO powders were used as raw materials. The optimal sintering temperature is1650℃, and hold time is10h. The microstructure of rare-earth zirconate-basedcomposite ceramics was investigated by X-ray diffraction （XRD）, scanning electronmicroscopy （SEM）, transmission electron microscopy （TEM） and energy dispersivespectroscopy （EDS）. Then flexural strength of the materials at room temperatureand1200℃, elastic modulus and fracture toughness were determined using thethree-point bending method, and Vickers hardness and density were measured. Thenmorphology of the fracture and the indentation were observed. Finally, the reasonsof toughening enhanced were analysized. The thermal expansion coefficient ofcomposite ceramics was measured and the heat capacity was calculated.The Ln₂Zr₂O₇（Ln=La, Nd, Sm） ceramics sintered at1650℃for10h have apyrochlore structure and Gd₂Zr₂O₇sintered at1650℃for10h exhibits a fluoritestructure. The prepared composite ceramics contain mainly Ln₂Zr₂O₇, LnAlO3andMgAl₂O₄, in addition to gadolinium zirconate composite ceramics outside the restof the material surface also contains very little hexaaluminate lamellae. Compositeceramics have an improved relative density, but also the grain size of the matrixincreases.The room-temperature flexural strength of Nd₂Zr₂O₇-based compositeceramics increases from64.1MPa to182.6MPa, while fracture toughness achievethe maximal value from0.97MPa·m^1/2to2.31MPa·m^1/2. The room-temperatureflexural strength of Sm₂Zr₂O₇was58.4MPa, and Sm₂Zr₂O₇-based compositeceramics with12mol%MgO composition was202.1MPa. Fracture toughness ofSm2Zr2O7was3.38MPa·m^1/2, and fracture toughness of Sm2Zr2O7-based compositeceramics with12mol%MgO composition was7.65MPa·m^1/2. The room-temperature flexural strength of Gd₂Zr₂O₇was75.0MPa, and Gd₂Zr₂O₇-basedcomposite ceramics with9mol%MgO composition was228.4MPa, and fracturetoughness rose from3.50MPa·m^1/2to7.49MPa·m^1/2. Gd₂Zr₂O₇-based compositeceramics with12mol%MgO composition has the maximal of fracture toughness,9.45MPa·m^1/2, at this time the flexural strength is163.0MPa. The average thermal expansion coefficient of neodymium zirconate compositeceramics with6mol%MgO composition is about10.8×10（-6）K（-1）at the temperaturerange of200to1300℃, and the average thermal expansion coefficient of puresamarium zirconate and gadolinium zirconate composite ceramics with3mol%MgO composition are about11.3×10（-6）K（-1）and10.7×10（-6）K（-1）, respectively.