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Abstract
Recent observations of three classes of nanometer-thick, disordered, interfacial films in multicomponent inorganic materials are reviewed and critically assessed. The three classes of films are equilibrium-thickness intergranular films (IGFs) in ceramics, their free-surface counterparts, that is, surficial amorphous films (SAFs), and their metallic counterparts. Also briefly reviewed are several related wetting and adsorption phenomena in simpler systems, including premelting in unary systems, prewetting in binary liquids or vapor adsorption on inert walls, and frustrated-complete wetting. Analogous diffuse-interface and force-balance models are discussed with the goal of exploring a unifying thermodynamic framework. In general, the stability of these nanometer-thick interfacial films does not follow bulk phase diagrams. Stabilization of quasi-liquid interfacial films at subeutectic or undersaturation conditions in multicomponent materials can be understood from coupled interfacial premelting and prewetting transitions. More realistic models should include additional interfacial interactions, for example, dispersion and electrostatic forces, and consider the possibility for metastable equilibration. It is suggested that quasi-liquid grain boundary films in binary metallic systems can be used to validate a basic thermodynamic model. These nanoscale interfacial films are technologically important. For example, the short-circuit diffusion that occurs in interface-stabilized, subeutectic, quasi-liquid films explains the long-standing mystery of the solid-state activated sintering mechanism in ceramics, refractory metals, and ice.
ACKNOWLEDGMENTS
The author acknowledges the support from a National Science Foundation CAREER award (DMR-0448879; Ceramics Program; program manager: Dr. L. D. Madsen) and an Air Force Office of Scientific Research Young Investigator award (Metallic Materials Program; program manager: Capt. B. P. Conner, Ph.D.). The author is indebted to Prof. Y.-M. Chiang, M. Tang, Prof. W. C. Carter, and Dr. C. M. Bishop for insightful discussions. The author also thanks X. Shi for assistance in inputting the references and proofreading part of the manuscript and H. Qian for providing . The author acknowledges two anonymous reviewers for their valuable suggestions.
The author dedicates this article to Dr. R. M. Cannon (1943–2006) for his consistent support and inspiration of this research and for his mentorship and friendship. Dr. Cannon has made many seminal contributions to this fascinating subject. This critical review was largely motivated by the numerous stimulating discussions with him during 2003–2006.