Role of nitrogen on the atomistic structure of the intergranular film in silicon nitride: A molecular dynamics study
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Molecular dynamics simulations of intergranular films (IGFs) containing Si, O, N, and Ca in contact with three different types of surface terminations of Si3N4 were performed using a multi-body interatomic potential. IGFs with the same Ca concentration (12 mol% CaO) but different nitrogen concentrations [N/(N + O) ⳱ 0, 15, 30, and 50%] were studied. In all 12 IGFs, Ca ions did not compete with the first adsorbed layer of Si at the IGF/basal crystal interface, but did so at the IGF/prism crystal interface. The simulations show the epitaxial adsorption of Si, O, and N from the IGF onto the basal and prism crystal surfaces. While more adsorbed N was expected as nitrogen concentration increases, there was a significantly larger amount of N adsorbed to the basal surface than to the prism surface. It was found that Ca ions sit closer to the prism surface than the basal surface but move closer to the crystal at both surfaces with increasing nitrogen concentration, although the effect was more pronounced at the basal interface. With the increase of nitrogen concentration, the percentage of two-coordinated oxygen remained about the same, but there was a change in the type of defect oxygen present. In all the simulations, the central position of the first peak in the Si–O pair distribution fixation (PDF) ranges from 1.63 to 1.65 Å, and that of Si–N PDF ranges from 1.71 to 1.73 Å, both consistent with experimental findings. Furthermore, the first peak of both the Si–O and Si-N PDF shifts to larger values as the nitrogen concentration increases, indicating the elongation of the Si–O and Si–N bond in the IGF with the increase of nitrogen concentration. Elongation of both the Si–N and Si–O bonds could lead to weakening of the IGF as nitrogen concentration increases, although competing changes in bonding of the O complicate the effect of N addition on the strength of the IGF.
I. INTRODUCTION
Silicon nitride (Si3N4)-based materials are important structural ceramics for high-temperature applications due to their attractive mechanical properties, which are strongly influenced by the chemistry and microstructure of the grain boundaries (GB) and the GB phase.1–4 At a two-grain GB in Si3N4 ceramics, there typically exists a thin amorphous intergranular film (IGF) about 1 nm thick, with main constituent elements dramatically different from those in the bulk. Electron energy-loss spectroscopy (EELS) investigations of GB films in Si3N4 ceramics have demonstrated that it is important to analyze nitrogen (the main constituent in the bulk) in the IGF to understand the behavior of the IGF.5–7 It has been
found that nitrogen concentration in the IGF largely exceeds the solubility limit in bulk SiO2 glass.8 In the IGF, the anion percent N/(N + O) is reported to be approximately 30%.7 Due to the glassy nature of the IGF and its thin nanometer-scale thickness, it has been experimentally difficult to gain detailed understanding of the atomistic structure of the IGF. On the other hand, computational techniques have recently offered an important
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