Creep of heat treated silicon nitride with neodymium and yttrium oxides additions

At the present work, samples of silicon nitride with 12 wt% of yttrium/neodymium oxides mixture were formed by gas-pressure sintering. Pos sintering heat treatments in nitrogen with a stepwise temperature variation were performed in some samples. The short term compressive creep tests were undertaken in an argon atmosphere, over a stress range of 50–300 MPa and temperature range of 1200–1400 °C. Values of stress exponents near unity for (i) low temperature testing in all materials and (ii) all temperatures for heat treated samples suggest diffusion accommodation processes, involving ambipolar diffusion of ionic species in the grain boundary phases. Crystallization of the remnant phase during heat treatment in a nitrogen atmosphere gives rise to further formation of new phases in the Nd–Si–O–N system such as Nd4Si3O12 and Nd2Si3O3N4. A consequence of this crystallization is a significant reduction in stress exponents and creep rates for the heat treated samples. The wedge crack observed after creep testing at specimens in its as-sintered condition may be related to the increased probability of cavitation in the second phase glassy layers at triple point junction.

Research highlights▶ In all cases, microstructural examination of crept samples showed that existing phases at grain boundaries were associated to the deformation processes. This highlights the importance of the presence and the amount of grain boundary glass. ▶ Crystallization of the remnant phase during heat treatment in nitrogen atmosphere gives rise to further crystallization of the new phases in the Nd–Si–O–N system such as Nd4Si3O12 and Nd2Si3O3N4. A consequence of this crystallization is a significant reduction in stress exponents and creep rates for the heat treated samples. ▶ Diffusional creep may prevail for lower temperatures, low glass content and stresses. Cavitation would start to operate and become increasingly prevalent with increase in stress, temperature and decrease in crystallinity of the grain boundary phase.