Weiru Zhang

Effects of Sintering Additives on the Microstructure and Mechanical Properties of Silicon Nitride Ceramics by GPS

Silicon nitride ceramics were prepared by gas pressure sintering (GPS) with different sintering additives, including La2O3, Sm2O3 and Al2O3. Effect of sintering additives on the phase-transformation, microstructure and mechanical properties of porous silicon nitride ceramics was investigated. The results show that the reaction of sintering additives each other and with SiO2 had key effects on the phase-transformation, grain growing and grain boundaries. With 9MPa N2 atmosphere pressure, holding 1h at 1850°C, adding 10wt% one of the La2O3, Sm2O3, Al2O3, porous silicon nitride was prepared and the relative density was 78%, 72%, 85% respectively. The flexural strength was less than 500MPa, and the fracture toughness was less than 4.8MPam1/2. Dropping compounds sintering additives, such as La2O3+Al2O3, Sm2O3+Al2O3 effectively improves the sintering and mechanical properties. The relative density was 99.2% and 98.7% with 10wt% compounds sintering additives. The grain ratio of length to diameter was up to 1:8. The flexural strength was more than 900MPa, and the fracture toughness was more than 8.9MPam1/2.

Defects Analysis of Large Size Silicon Nitride Balls Sintered by Gas Pressure Sintering

The sections of Φ55mm silicon nitride balls, sintered by gas pressure sintering were analyzed. The results show that temperature gradient during of the sintering process from the surface to the core of large size silicon nitride balls occurred because of the lower thermal conductivity of Si3N4. With the diameter increasing, the temperature gradient was more visible. The impurity of raw materials, such as free Si, free C and other metal oxides, like the SiO2 could produce gas, such as SiO, CO and so on, during the sintering process through the thermodynamic analysis. The producing gas exhausted more difficultly with the diameter of silicon nitride balls increasing. These factors were the most important to the defects of large size silicon nitride balls during the sintering process and made cracks and crescent on the surface of balls.

Effects of Rare Earth Oxides on the Microstructure and the Mechanical Properties of High-Purity Alumina Ceramics Sintered at Low Temperature

High-purity alumina ceramics were prepared by sintering at low temperature, using rare earth oxide Y2O3 and Tm2O3 as sintering aids. Adding the mixture of Y2O3 and Tm2O3 in the alumina powders can effectively improve the sintering ability and mechanical properties. The mixture, Y2O3+Tm2O3, can restrain the grains growth, fine the grain size, make the grain uniform and finally help to form compact structure. The transgranular fracture was the main fracture mode. It was found the relative density of alumina ceramics with 0.3wt% Y2O3 and Tm2O3, sintered at 1620°C for 3hrs, can be over 99.2% and the strength, toughness and microhardness reach 533.1MPa, 4.9MPa⋅m1/2 and 17.2GPa, respectively.