Reactions Occurring in Post Heat-Treated α/β Sialons: On the Thermal Stability of α-Sialon

摘要

To a powder mixture of an overall α-sialon composition R_(0.4)Si_(10.2)Al_(1.8)O_(0.6)N_(15.4), with R= Nd, Sm, Dy and Yb, were added extra amounts of powder mixtures, (20%), having Si:Al:R and O:N atomic ratios of 2:1:1 and 3:1, respectively. Series of α-rich mixed α/β-sialon ceramics containing about 20-30 vol% glassy phase were prepared from these powders by pressureless sintering at 1750℃. The as-prepared samples were subsequently heated at 1750℃ and then quenched to room temperature or to 1150, 1300 and 1450℃ (with a cooling rate exceeding 400°/min), and were annealed at these temperatures for various times. The samples quenched to room temperature revealed that α-sialon coexists with β-sialon and a liquid phase at 1750℃. The post heat-treatment at the lowest temperature involved a devitrification of the glassy phase and resulted in mixtures of mainly rare earth oxynitrides like the U-phase R_3Si_3Al_3O_(12)N_2, wollastonite RSiO_2N or the B-phase Dy_2SiAlO_5N. Post heat-treatment at 1450℃ induced a reaction between residual liquid and α-phase in the Nd- and Sm-systems and yielded a mixture of an oxygen- and a nitrogen-rich phase in all systems. Thus the melilite phase, R_2Si_(3-x)Al_xO_(3+x)N_(4-x), is formed with all rare earth elements except Yb, which yields Yb_4Si_2O_7N_2. The oxygen-rich phase in the Nd- and Sm-systems was the aluminate RAlO_3, while in the Dy- and Yb-systems the garnet phase, R_3Al_5O_(12), was formed. Similar results were obtained with samples quenched to 1300℃. These findings suggest that the stability of α-sialon is related to the type of sintering aid used. The phase assemblage found in the as-prepared samples is discussed in view of the findings obtained in the annealing experiments.