Atomistic Structure of Calcium Silicate Intergranular Films Between Prism and Basal Planes in Silicon Nitride: A Molecul
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Molecular dynamics simulations of approximately 15 Å thick intergranular films (IGFs) containing SiO2 and CaO in contact with two surface terminations of the prism (101¯0) and basal planes (0001) of Si3N4 were performed using a multibody interatomic potential. Samples with the same composition (1.5 mol% CaO) and number of atoms but different crystal planes (i.e., the prism and basal planes of Si3N4) were studied. In both the prism and basal cases, the IGF in the final configuration is well-ordered in the interface region. A small number of N ions from the crystal moved into the IGF near the interface, and O ions moved into the N sites in the crystal, indicating the formation of a Si–O–N interface. In addition, Ca ions do not segregate to the IGF–crystal interface. The bonding characteristics of the O ions at the interface with neighbor Si ions are different in the prism and basal cases. Such difference may be explained by the difference in the two crystal Si3N4 surfaces. The Si–O bond length of the IGF has a range from 1.62 Å to 1.64 Å, consistent with recent experimental findings.
I. INTRODUCTION
Silicon nitride (Si3N4)-based materials are considered promising structural ceramics for high-temperature applications. In the intergranular regions between Si3N4 grains, there are typically secondary amorphous phases, which form glass pockets at triple-grain junctions and thin amorphous intergranular films (IGFs) at grain boundaries. Such intergranular films were observed in the late 1970s.1–3 The properties of the intergranular phases determine the overall diffusion-related behavior of the material, such as creep, oxidation, and hightemperature strength of Si3N4.4–6 Kleebe and coworkers showed that the thickness of the intergranular films strongly depends on film chemistry and is constant within ±1 Å.7–9 Clarke et al.10,11 developed a model in which equilibrium film thickness is determined by the balance between the attractive van der Waals force acting across the IGF and a repulsive term due to the steric force and the electrical double-layer force in the intergranular films. Tanaka and coworkers12 characterized both the intergranular silicate film thickness and its local composition in a series of high-purity Si3N4 ceramics doped with 0–450 at. ppm Ca. They found that the thickness of the intergranular films at two-grain junctions depends sensitively on Ca content. In undoped material, the thickness was 10 ± 1 Å. When Ca content increased, the thickness decreased in dilute region (less than 80 ppm Ca), but then increased. Assuming that all Ca and O are 752
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J. Mater. Res., Vol. 19, No. 3, Mar 2004 Downloaded: 17 Mar 2015
localized in the intergranular phase, the calcium concentration at the two-grain junctions in the 80-ppm-doped material was found to be 1.5 ± 0.3 mol% CaO/(CaO + SiO2).12 In this paper, we focus on IGFs with calcium concentration of 1.5 mol%.
II. COMPUTATIONAL PROCEDURE
Molecular dynamics simulations have been performed to simulate the interfaces. To describe the in
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