Depletion of Grain‐Boundary Phase and Elastic Creep Deformation Upon Ultra‐Long Flexural Stress Rupture Experiments of NC‐132 Silicon Nitride

Abstract

A unique grain‐boundary structure evolution was observed in two MgO‐doped silicon nitride specimens (Norton, NC‐132) that were tested in ultra‐long flexure stress rupture experiments with an applied stress of 266 MPa and fractured at 14 941 and 17 376 h. Transmission electron microscopy showed that, although the starting material had a secondary glass phase both at multi‐grain junctions and along grain boundaries, the tested specimens contained no residual glass phase. Concurrent with the elimination of the secondary glass phase, a continuous network of cracked grain boundaries was observed after long‐term flexure testing consistent with the concept of elastic creep. It is, therefore, concluded that at ultra‐long annealing times, this material is affected by creep deformation via microcrack nucleation and growth due to the depletion of the amorphous siliceous grain‐boundary phase, which is seen as a truly transient, fugitive secondary phase.

Ionic interdiffusion as interaction mechanism between Al and Si3N4

“…Figure shows that the material is mainly constituted of β‐Si 3 N 4 , while WC, Fe 3 N 1.3 , and Fe 7 C 3 are present as minor phases. This is in good agreement with the previously published data regarding characterization of this material . The contact between the test bars and the metal was enhanced by wet hand‐grinding surface of the bar using progressive SiC papers of 120, 320, 600, and 1200 grit, followed by hand‐polishing using colloidal Al 2 O 3 suspensions.…”

“…Since the Al channels are very narrow (~80 nm thickness) and the specific volumes of the reactant (true density of β‐Si 3 N 4 = 3180 kg/m 3 ,) and product (true density of AlN = 3250 kg/m 3 ,) are close, then Si precipitation in the channels can occur and subsequently cause the blockage, although this was not observed experimentally in the present study. However, if it occurs, the diffusion of Al and N through single‐crystal Si must be considered.…”

“…The small white features in the SEM images are WC, Fe 3 N 1.3 , and Fe 7 C 3 phases, as confirmed by the XRD results. The formation of WC in NC‐132 was previously reported, however the formation of iron carbide and nitride in NC‐132 has not yet been reported. Nonetheless, it has to be considered that, according to the XRD data, the content of these iron phases is very low (<1 wt%).…”

B6O: A Correlation Between Mechanical Properties and Microstructure Evolution upon Al2O3 Addition During Hot Pressing

“…This result is consistent with the observation that the interfaces in the Al 2 O 3 ‐doped B 6 O are at least partially wetted, which can also be seen as a snapshot of a cation outward diffusion process. The proposed outward diffusion of cations during cooling results in (i) clean interfaces (possibly with segregants present along the boundary) and (ii) is expected to eventually lead to the formation of microcracks, as reported for a MgO‐doped Si 3 N 4 system on long‐term annealing 35 …”