The evolution of amorphous grain boundaries during in-situ heating experiments in Lu-Mg doped Si3N4

The presence of 1-2 nm wide intergranular glassy films (IGFs) found in ceramics such as Si3N4 and SiC strongly influences the properties of the material, including its fracture and creep behaviour. It is therefore important to know the evolution of the film as a function of temperature. Experiments, wherein liquid phase sintered Si3N4 samples were quenched from high temperatures, have been carried out before and shown intriguing results. In the current investigation, in-situ heating experiments have been conducted in 120 kV and 400 kV microscopes, in order to separate beam irradiation from temperature effects. Based on the literature, it was expected that no changes would occur to the IGF below 1000 deg. C. However, it was surprising to note that the thickness measured at 950 deg. C was higher than that at room temperature. The correlation in trends observed in both microscopes shows that electron radiation has a minimal contribution to the change in IGF width at 950 deg. C. No change to the thickness was observed when heating up to 650 deg. C. After cooling back to room temperature there is a reduction in the thickness and thus a tendency to regain the original value of the thickness before heating. We conclude from these observations that certain material transport processes could be active at rather low temperatures (for Si3N4). Possible mechanisms and sources of artifacts are also discussed