Dominique Chatain

Wettability of monocrystalline alumina by aluminium between its melting point and 1273 K

The wettability of Al2O3 single crystals by aluminium is investigated in the 933–1273 K temperature range by the sessile drop method under a low total pressure (4 × 10−5Pa) and an oxygen partial pressure of about 10−15 Pa. The variation of contact angle with temperature is shown to describe a linear decrease from 103 ± 6° (933 K) to 86 ± 6° (1273 K). The temperature coefficients of contact angle (dθdT = − 0.05 k−1) and work of adhesion (dWadT = 0.6 mJ m−2K−1) are found to be similar to but somewhat higher than those obtained with non reactive metals (such as Cu and Au) on alumina. Results of several wetting tensiometer experiments performed with sapphire cylinders are in close agreement with the above-described wetting behaviour. It is shown that, with the experimental conditions described, the influence of the oxide layer surrounding the liquid metal on wettability can be eliminated from the melting point of Al. By evaluating the gaseous exchanges between the experimental chamber and the oxide layer, using the kinetic theory of gases for molecular flow, the process whereby aluminium is released from its surrounding oxide layer can be discussed. This process depends on the initial thickness of the oxide layer, the oxygen partial pressure and the temperature.

Wettability of SiC by aluminium and Al-Si alloys

The variations with time of the contact angle formed by molten pure aluminium or Al-Si alloys with single crystalline SiC were measured by the sessile drop method in a vacuum of 10−4 to 10−5 Pa at temperatures ranging from 933 to 1200 K. In the Al/SiC system, a “non-wetting-wetting” transition was observed at a temperature that decreases as time increases. After holding times of about 2 h, contact angles were stabilized to acute angles even at the aluminium melting point. Although additions of silicon to aluminium were in such amounts as to prevent Al4C3 formation at the interface, wettability in both pure Al/SiC and Al-Si alloys/SiC systems was not observed to differ appreciably.

Wettability of SiO2 and oxidized SiC by aluminium

Abstract The silica layer grown naturally or artificially on the surface of SiC fibres or particles used in alumina-based matrix composites is supposed to have two functions: protection of the SiC from aluminium attack and improvement of the wettability of SiC by aluminium which would result from the reaction between aluminium and SiO 2 . The effective role of silica in the wetting of aluminium on SiC was studied using the sessile drop method and the immersion-emersion tensiometric technique. Aluminium contact angles were measured first on amorphous SiO 2 and then on thermally oxidized SiC monocrystals (silica layers of 10–50 nm), between 933 K and 1173 K, and under a dynamic vacuum of 10 −4 −10 −5 Pa. In the two systems it appeared that silica acts as an oxygen source which causes oxidation of liquid aluminium. As a result the wetting kinetics was slowed down and even blocked: the apparent contact angle at 973 K is very high (above 150°). At higher temperatures (above 1073 K) deoxidation of aluminium by evaporation of the alumina layer allowed a real interface to be established between the solid and the liquid. However, as the silica reduction reaction occurred before the wetting, the stationary contact angle of aluminium on SiO 2 was found to be that of aluminium on alumina, and the steady contact angle of aluminium on oxidized SiC was that on alumina (at temperatures less than 1073 K) or on SiC (at temperatures higher than 1173 K). The strong reactivity between aluminium and SiO 2 cannot be used to improve the wetting of this metal on SiC. Consequently, silica layers on SiC cannot help the incorporation of particles or the infiltration of fibres by aluminium.

Anisotropy of Segregation at Grain Boundaries and Surfaces

The purpose of this article is to review analytical models of the anisotropy of segregation to grain boundaries (GBs) and surfaces, and to evaluate their predictions. A summary of Gibbsian interfacial thermodynamics is provided as an introduction to the topic. This is followed by a historical overview of previous analytical models. A recently developed model of the dependence of GB segregation on the five macroscopic parameters of GB orientation is outlined, and illustration of how this formulation reduces to the particular cases of segregation to simpler types of interfaces is provided. In addition, some specific aspects of interfacial segregation, which have either been problematic or have lacked satisfactory explanation, are addressed. These include (a) the relationship between the compositions on the two sides of a given GB; (b) the difficulty of meaningful definitions of segregation-free energy (and related thermodynamic quantities such as enthalpy and entropy); (c) the so-called compensation temperature, at which the anisotropy of interfacial segregation seems to vanish; (d) the relationship between surface and GB segregation; and finally (e) an attempt to determine whether segregation increases or decreases interfacial energy anisotropy, and the consequences thereof on the equilibrium crystal shape of alloys. Where possible, comparisons are made with the results of experiments or computer simulations.

Nanometer-Thick Equilibrium Films: The Interface Between Thermodynamics and Atomistics

Model experiments show that nanometer-thick films at interfaces reduce interface energy and form an equilibrium state. Nanometer-thick films at interfaces and surfaces exist in various materials and can substantially influence their properties. Whether these films are an equilibrium or transient state is debated. To address this question, we equilibrated 1.2-nanometer-thick films at gold-sapphire interfaces in the presence of anorthite glass and measured the solid-solid interface energy. The equilibrated film significantly reduced the interfacial energy and could be described by the Gibbs adsorption isotherm expanded to include structure in addition to chemical excess. Unlike artificially made conventional thin films, these films do not break up during equilibration and offer an alternative design criterion for thin-film technology. These results demonstrate that nanometer-thick films at interfaces and surfaces can be an equilibrium state and included in phase diagrams with dedicated tie-lines.

Experimental evidence for a wetting transition in liquid GaPb alloys

Abstract The surface composition of Ga-rich liquid in equilibrium with Pb-rich solid has been investigated between 508 K and the monotectic temperature (586 K) of the GaPb system using Auger spectroscopy. A Pb-rich layer is shown to thicken up to 9 equivalent monolayers of Pb at the surface of the liquid as the temperature increases. The thickness of the surface layer diverges logarithmically as the composition of the liquid approaches the monotectic composition. This is the signature of the existence of complete wetting in the GaPb system occurring in the metastable region of coexistence of the two liquids.

New software tools for the calculation and display of isolated and attached interfacial-energy minimizing particle shapes

Existing methods to rapidly compute interface-energy minimizing shapes with anisotropy are collected and clarified, and new methods are introduced. A description of freely available, platform-independent software for the computation and display of equilibrium geometries is provided. The software relies on a new computational method to rapidly find equilibrium geometries. It also features a graphical user interface and includes the 32 crystallographic point groups to simplify inputting interfacial energies and their associated orientations. When a particle is completely enclosed within a single interface (isolated), the software computes and provides visualization for Wulff shapes. When a particle is enclosed by two interfaces, such as a particle at a grain boundary, the software minimizes their collective interfacial energy; if one of the interfaces is planar, the computation reproduces the Winterbottom construction. When both interfaces are deformable, the software provides a new tool for calculating the particle shape and the distortions of boundaries that are attached to it, even for highly anisotropic interfaces. The properties of particles bounded by two deformable interfaces are discussed, and applications of the software are illustrated. In some cases, the software can be used as a method to infer values of relative interfacial energies from a microscopic observation.

Oxygen adsorption isotherms at the surfaces of liquid Cu and AuCu alloys and their interfaces with Al2O3 detected by wetting experiments

Abstract The wetting of Cu O and Au Cu O alloys on alumina single-crystals and the surface free energy of these liquids have been determined as a function of oxygen activity. The explored oxygen activity range is from 10 −10 to the value corresponding to the formation of the mixed oxide AlCuO 2 . The oxygen adsorption at the surface of a metallic liquid and its interface with an oxide are shown to mainly depend on the external phases in contact with the liquid (vapour or oxide respectively). The free energies of the surfaces show monotonic decreases with log a o above a critical value of a o . The free energies of the metal-oxide interfaces also show monotonic decreases and stepped decreases at low oxygen activity. These are interpreted as oxygen adsorption transitions corresponding to the formation of a 2D-interfacial phase. The independence of surface and interfacial oxygen adsorptions suggests that the wetting and adhesion of a liquid metal on an oxide are not systematically improved with increasing oxygen activity.

Wetting and interfacial chemistry in liquid metal-ceramic systems

Abstract Wetting data for liquid metal-ceramic systems are reviewed, with emphasis on experimental results obtained on oxides by the sessile drop technique. Examples are given in order to illustrate the different wetting behaviours of reactive and non-reactive pure metal-ceramic pairs. The effect of oxygen on the wettability is presented both when it acts as a dissolved element and when it causes the formation of an oxide film on the liquid metal. The influnece of alloying elements is illustrated by numerous results and discussed using a thermodynamic model. Using some examples it is shown that the general behaviours holding for wettability in metal-oxide systems are also valid for the other families of ceramics.

The effects of prewetting and wetting transitions on the surface energy of liquid binary alloys

Abstract Existing regular-solution-based models of surface segregation, and of interface composition profiles, have been assembled in a self-consistent manner to examine the behavior of surface and interfacial energies in binary liquid alloy systems which exhibit wetting and prewetting transitions. Calculations performed using three parameters fitted to macroscopic quantities are consistent with many of the major characteristics expected of these transitions. As two-phase coexistence is approached from the domain of stability of the wetted phase, at temperatures above the wetting transition temperature, the wetting phase forms at the surface of the wetted phase by a series of layering transitions, and its thickness is found to diverge logarithmically. Under conditions of two-liquid phase equilibrium, the surface energies of the two liquids and the energy of the interface separating the two liquids are found to strictly satisfy the condition of perfect wetting. Predictions of the model are found to be in good agreement with previous measurements of the surface energy of liquid Al-Pb and Zn-Pb alloys as a function of bulk composition.