N. Eustathopoulos

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.

Dynamics of wetting in reactive metal/ ceramic systems

Abstract Dynamics of wetting of metallic drops on smooth ceramic surfaces are discussed for the case of liquid-solid couples leading to the formation of a continuous layer of a reaction product at the interface, by reaction between a component of the liquid and the solid substrate. This paper is focused mainly on the following two questions: (i) the driving force of reactive wetting i.e. the origin of this force and the crucial question of the final contact angle in a reactive system and (ii) the different processes which can be rate-limiting, namely the viscous flow, the diffusion within the drop and chemical kinetics at the triple line. From experimental results on model systems published in the last three years, clear answers to point (i) were given. Current experimental and theoretical results on spreading kinetics are the starting point for a fundamental approach to these complex kinetic phenomena.

Dynamics of wetting in reactive metal/ceramic systems: Linear spreading

Abstract The kinetics of wetting in the reactive pure aluminium/vitreous carbon (Cv) system was investigated by the sessile drop technique in high vacuum. It was found that the curve showing the radius of the metal drop base R as a function of time t consisted of a central part where the radial spreading of the drop is a linear function of time, and two extremal parts where significant deviations from linearity are observed. By characterising the Al/Cv interface at different stages of the process, both far from and close to the Al/Cv/vapour triple line, it is shown that (i) in the linear spreading regime wetting kinetics is controlled by a nearly 2D reaction between Al and Cv at the triple line, forming aluminium carbide; and (ii) linearity is closely associated with steady-state growth of the carbide at the triple line. At the beginning of the wetting process, deoxidation of the Al drop and, thereafter, transient-state growth of carbide, cause considerable deviations from linearity in the R ( t ) curve. Moreover, at the end of spreading, when the contact angle tends towards the steady contact angle of the system, a deviation from linearity is also observed and attributed to the roughness of the reaction product. Finally, examples from the literature are given suggesting that linear spreading may be effective in many reactive metal-on-metal and metal-on-ceramic systems.

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.

The wetting of carbon by aluminium and aluminium alloys

The contact angle made by molten aluminium with vitreous carbon was measured by the sessile drop method in vacuum at temperatures up to 1100° C. The effect on wetting behaviour of the oxide layer on the molten metal was highlighted by using two samples of aluminium in different states of oxidation. The investigation involved the variation of certain parameters affecting the stability of the oxide film, e.g. the temperature, additions of Ti, Si, Cr, Be, Ca and Li to aluminium and the time held at a certain temperature. The state of the molten aluminium surface under various experimental conditions was determined indirectly by surface tension measurements.

Contribution to the study of reactive wetting in the CuTi/Al2O3 system

The wetting (kinetics of spreading and stationary contact angles) of CuTi alloys on monocrystalline alumina under high vacuum, at a temperature of 1373 K, by the sessile drop technique was investigated. The morphological and chemical characteristics of the metal-ceramic interface were determined by scanning electron microscopy and microprobe analysis. When the results are analysed, three distinct effects of the Ti solute on wetting can be identified and evaluated semi-quantitatively: (a) a reduction in the solid-liquid interfacial tension by adsorption into the liquid side of the interface; (b) a reduction in this tension by formation of a TiO metallic-like oxide layer in the solid side of the interface; (c) a contribution to the wetting driving force due to the free energy released at the interface by the reaction between Ti and Al2O3.

Work of adhesion and contact-angle isotherm of binary alloys on ionocovalent oxides

Using the monolayer approximation for metal-vapour and metal-oxide interfaces and Bragg-Williams statistics, a simple thermodynamic model has been constructed to calculate the variation in contact angle and work of adhesion as a function of composition in binary alloyionocovalent oxide systems. This model has been used to classify the curves of contact angle and the work of adhesion as a function of composition into three main types of isotherm. Model predictions and experimental results are compared using data on binary alloy-monocrystalline alumina systems.

The role of intermetallics in wetting in metallic systems

Abstract Formation of intermetallic seems to improve strongly wetting of a solid metal by a liquid one. The aim of this study is to identify the reasons of this improvement. For this purpose, wetting of Fe is determined at the same experimental conditions for two liquid metals, one reactive (Sn), the other non-reactive (Pb).

Tension interfaciale solide-liquide des systémes Al-Sn, Al-In et Al-Sn-In

Abstract Interfacial tensions between liquid Al-Sn, Al-In and Al-Sn-In solutions and solid aluminum are determined at various compositions and temperatures by the dihedral angles method. Using an equation deduced from a thermodynamic model, the results lead to an indirect determination of the solid-liquid interface tension of pure aluminum. The relative importance of temperature and composition on the interfacial tension and on the solidification structures are discussed.