Wetting and Prewetting on Ceramic Surfaces

The stabilization of nanoscale surficial amorphous films (SAFs) for Bi2O3 on ZnO, VOx on TiO2, SiOx on Si, and several other oxide systems provides evidence for the existence of prewetting phenomena with analogies in water and other simple systems, as well as the stabilization of intergranular amorphous films in ceramics. Experimental results show that in the subeutectic regime, the equilibrium film thickness decreases monotonically with decreasing temperature until it vanishes at a dewetting (prewetting) temperature. With increasing temperatures, nanometer-thick SAFs persist into a solid-liquid coexistence regime, in equilibrium with partial-wetting drops, with a gradual decrease in the macroscopic contact angle upon heating. The presence of an attractive dispersion force can significantly delay or inhibit the (otherwise expected) occurrence of complete wetting at higher temperatures. The equilibrium thickness of SAFs is explained from a balance between several interfacial interactions, including dispersion forces, shortrange forces of structural or chemical origins, volumetric free-energy terms, and electrostatic interactions. In a generalized Cahn critical-point wetting model, these SAFs are alternatively considered to be disordered multilayer adsorbates formed from coupled prewetting and premelting transitions. 227 A nn u. R ev . M at er . R es . 2 00 8. 38 :2 27 -2 49 . D ow nl oa de d fr om a rj ou rn al s. an nu al re vi ew s. or g by C L E M SO N U N IV E R SI T Y o n 07 /0 9/ 08 . F or p er so na l u se o nl y. ANRV347-MR38-10 ARI 2 June 2008 20:43 IGF: intergranular (glassy) film GB: grain boundary HRTEM: highresolution transmission electron microscopy SAF: surficial amorphous film INTRODUCTION Impurity-based, equilibrium-thickness intergranular films (IGFs) have been widely observed at ceramic grain boundaries (GBs) (1–5), ceramic-ceramic heterointerfaces (4, 5), metal-oxide heterointerfaces (6–11), and metallic GBs (5, 12, 13), through the use of high-resolution transmission electron microscopy (HRTEM). Recently, free-surface counterparts to these IGFs have been found for Bi2O3 on ZnO {112̄0} surfaces (14–17), VOx on TiO2 anatase (101) surfaces (18), SiOx on Si (19), and several other oxides (10, 15, 16). We refer to these as surficial amorphous films (SAFs), although partial order generally exists within the films. Like IGFs, these SAFs are understood to be quasi-liquid layers that adopt an equilibrium thickness on the order of 1 nm (5, 14–17). From the viewpoint of wetting, SAFs are considered to be an intermediate regime between Langmuir submonolayer adsorption and complete wetting (2, 3, 5, 16, 17). We (5, 16, 17, 20) have proposed that these SAFs can alternatively be considered as disordered multilayer adsorbates that form from coupled prewetting (21) and premelting (22–24) transitions under the framework of a generalized Cahn critical-point wetting model (21). Unique anisotropic wetting behavior with and without nanometer-thick SAFs has been observed (25). This article reviews the experimental observations of, and models for, SAFs and related wetting phenomena on ceramic surfaces.