Dynamic wetting in pressure-infiltration of ceramic particle preforms by molten metal

This thesis investigates the interaction between mechanically induced flow of a liquid metal into a porous solid, and kinetic effects at the triple line. The approach is experimental and focuses on metal-ceramic systems for which the literature gives values of wetting and local kinetic parameters (Simga, SimgaLV, and interfacial phenomena near the triple line) at high temperature, gleaned by means of the sessile drop experiment. We investigate the infiltration with Al of Al2O3 preforms at 1000°C, 1050°C and 1100°C, and the infiltration with Cu-x (x=Si, Ti or Cr) of porous CGr between 1050°C and 1200°C. Those three alloys are known to form carbides better wetted by the liquid than is graphite. If measurements are taken at various values of applied metal overpressure, in the presence of thermally activated processes at the triple line, time intervenes in capillarity and the front does not stabilise. We observe that the metal continues to invade slowly the porous medium, gradually ¿creeping¿ into its pores under the action of local phenomena that take place at the triple line. We characterise such dynamic infiltration by measuring rates of steady isobaric infiltration, measuring also the variation of infiltration rates at Pth and Pth+0.05MPa. Saturation rates are then computed by means of a linear regression over 100s of the measured saturation for each pressure step. Data are interpreted using an analogy with high-temperature plasticity. This leads to assume that the applied pressure contributes a proportional reduction in the activation energy of the process that governs the rate of preform infiltration, the constant of proportionality defining an activation volume that characterises the thermally activated process. Cu-46at.pct.Si infiltrating carbon is a mildly reactive system that forms SiC at the interface. Isobaric saturation rates of infiltration and their dependence on variations in the applied pressure were conducted between 1050°C and 1200°C. It is found that an activation volume on the order of ~200nm3 makes the data compatible with the Arrhenius law, with an estimated activation energy value of ~400kJ/mol, which is realistic for a process limited by the rate of SiC growth along the interface. Aluminium infiltrating three different Al2O3 preforms shows similar behaviour as does the mildly reactive Cu-Si/C system if particles in the preform are highly angular, regardless of the presence or absence of Na impurities. Isobaric saturation rates measured between 1000°C and 1100°C give a similar activation volume as the Cu-Si/C system, on the order of ~200nm3, and an activation energy ~300kJ/mol, suggesting that the kinetics are likely limited by solid phase diffusion. It is also shown that sodium impurities in alumina serve to facilitate its infiltration by aluminium, likely because sodium alters the oxide skin that initially covers the melt. Finally, in the infiltration of Cu-1at.pct.Cr or Cu-10at.pct.Ti into CGr, one finds that interfacial reaction does not aid pressure infiltration. In the former, it is because initial reaction between melt and preform depletes the alloy in reactive Cr, leading to similar infiltration as with pure Cu. In the latter, initial carbide formation is so extensive that it blocks ingress of the metal into the preform. For alloying elements forming a better wetted interface to aid pressure infiltration, reaction must thus be sufficiently slow compared to the rate of metal flow.

Characterization of capillary forces during liquid metal infiltration

Maryam Bahraini Hasani

This thesis addresses the link between capillarity parameters, i.e. surface tension and contact angle, and the infiltration of porous preforms with molten metal by pressure infiltration The method used is to conduct pressure infiltration experiments of selected preforms with molten aluminum or copper based alloys, and characterize the level and, where relevant, kinetics of metal ingress into the preforms. Several systems are investigated in turn, comprising chemically inert and reactive systems. Capillarity in the infiltration of alumina particulate preforms with copper are investigated by means of infiltration experiments conducted at 1200°C under argon with a low oxygen partial pressure, characteristic of equilibrium with carbon at that temperature. The experiments were run in a gas pressure driven liquid metal infiltration apparatus capable of melting copper under gas pressures up to 20 MPa. Capillarity parameters are extracted from drainage curves that plot the volume fraction of metal in the porous medium vs. the applied pressure. Mercury porosimetry, which is based on the same principle, is also used on similar preforms for comparison. The effect of volume fraction, particle geometry and capillary parameters on the drainage curve are studied and compared with the expression proposed by Brooks and Corey. The influence of the particle volume fraction and capillary parameters characterizing wetting in the two systems is discussed to derive an effective contact angle for wetting of alumina particles by molten metal. The value thus derived agrees with literature data from sessile drop experiments. A new technique is proposed for the direct measurement of capillary forces during infiltration in systems of relevance to the processing of metal matrix composites. A custom-designed device enables dynamic tracking of the molten metal surface while the infiltration apparatus is pressurized. It comprises an LVDT (Linear Variable Differential Transformer) that is fixed to the top of the infiltration machine and whose core is connected via an alumina tube to a graphite plunger that floats atop the liquid metal. Upon pressurization, the floater movement caused by flow of the metal into the preform is tracked by the LVDT. Capable of handling melt temperatures up to 1250°C and infiltration pressures up to 20 MPa, the method is essentially a high-temperature analogue of mercury porosimetry. Its accuracy is demonstrated by comparison with data obtained using other techniques and its use is illustrated with the infiltration of diamond particle preforms by Al and Al-Si alloys at 700°C. The present method for direct measurement of drainage curves during the infiltration process is then used for the characterization of wetting of SiC particle performs by molten aluminum and Al-12.2at%Si. The ceramic particle diameter is varied from 7 µm to 40 µm and infiltration was characterized at 750°C, varying the applied pressure from 0.1 to 10 MPa. From these data, the relevant work of immersion is calculated by comparison to the work exerted by the external gas pressure over the whole saturation range. The thus determined work of immersion is translated in an apparent contact angle during infiltration, which is compared with the values determined by other techniques reported in literature to find good agreement. Kinetics effects in reactive systems such as SiC/Al and graphite/Cu-Cr were studied by direct measurement of the drainage curves varying the pressurization rate and conducting measurements interrupted at a given pressure. In addition to the analysis of the capillarity parameters already used for the non-reactive systems, the data are analyzed by a simple model based on the Brooks-Corey relation allowing to link the triple-line velocity in liquid metal infiltration with the saturation given one characteristic distance. It is found that alloying elements in systems investigated drive infiltration at fixed pressure, but do not assist pressure infiltration. Quite to the contrary, they may even hamper efficient infiltration by blocking narrow channels via formation of reaction products with the preform material.

EPFL2007

High-temperature wettability and thermal conductivity of aluminium nitride reinforced metal matrix composites

Masahiro Kida

Aluminum nitride (AlN) has been applied to various applications, including electronic substrates and packaging, semiconductor production components, wide band gap semiconductor, light-emitting diodes (LED) and acoustic wave devices; at the same time, it is also attractive as a reinforcement in Metal Matrix Composites (MMCs) for thermal management applications with tailored thermal properties. In this thesis, two main subjects are investigated in terms of both the composite processing and its thermal property, namely (i) the high-temperature wettability between AlN and liquid metals and (ii) the thermal conductivity behavior of AlN reinforced metal matrix composites and the associated question of the interface thermal conductance at the AlN/metal interface. High-temperature wettability of AlN particle preforms with pure Al, Cu, Sn and Pb, was investigated by means of drainage curve measurements during pressure infiltration. The measured drainage curves show good agreement with the semi-empirical model in soil science proposed by Brooks and Corey. The total work of immersion, Wi, can be directly calculated by integration of the drainage curves, in turn giving the apparent contact angle during infiltration at high temperature. For Cu, Sn and Pb, the apparent contact angles deduced by integration of experimental drainage curves are in the range 120-140° and agree well with corresponding sessile drop data from the literature. Data for AlN and α-Al2O3 are furthermore similar for both methods of measurement, which results from the presence of a native oxide layer on the AlN particles, which is not eliminated from the particle surface in these systems. For the wettability between AlN/Pb and α-Al2O3/Pb systems, a slight difference is observed and this behavior is attributed to the transformation of the native oxide layer with temperature and the presence of dissolved oxygen in the melt. On the contrary, the wettability of AlN/Al system gives higher value than the sessile drop data measured in vacuum, corresponding rather to the value measured under He-H2 gas at 1373K. This indicates that in the present study there is no elimination of the native oxide layer on the AlN by the formation of Al2O gas, which normally occurs in high vacuum. Moreover, the results of the present technique gradually deviate to the sessile drop data as the initial contact angle gets close or below 90° especially in the α-Al2O3/Al system, suggesting that there is a minimum value, around 105-110°, below which the present infiltration-based contact-angle measurement technique may lose validity. Thermal conductivity behavior of AlN reinforced metal matrix composites produced by pressure infiltration technique has been characterized at both room and elevated temperature in two different microstructural systems, namely (i) particle reinforced system (PRCs) and (ii) partially sintered particle system, i.e., an interpenetrating composite in which the AlN is also a continuous phase (IPCs). The phase contrast of thermal conductivity between AlN (Κd) and matrix metals (Κm) is varied within a range of 0.4

EPFL2008

Kinetic processes in the high-temperature pressure-infiltration of Al into Al2O3

Marta Fornabaio, Andreas Mortensen, Gionata Schneider, Ludger Weber

We explore the influence (i) of the interaction between aluminium and alumina, and (ii) of sodium impurities present in Bayer alumina, on the pressure infiltration of alumina particle preforms with molten aluminium. At 1000°C or above, although the aluminium/alumina system is non-reactive, capillarity-driven solution-reprecipitation processes cause the liquid-solid interface to become mobile. Data show that this can result in infiltration kinetics that resemble those observed with reaction-driven pressure infiltration, namely a continuously increasing melt saturation under fixed infiltration pressure. Resulting isobaric saturation velocities are measured at 1000°C, 1050°C and 1100°C. The role of alumina particle shape and of Na-containing inclusions is investigated. It is found that the main factors affecting the rate of high-temperature isobaric infiltration in this system is the particle geometry. Measured steady infiltration rates give an activation volume on the order of ≈ 200 nm3 and an activation energy in the range of 300-500 suggesting that isobaric infiltration kinetics are governed by diffusion through the solid alumina. Sodium impurities of Bayer alumina are present within β″-Al2O3. They do not influence steady pressure infiltration but ease initial melt penetration into the preform, possibly because evaporated Na2O alters the oxide skin layer that lines the surface of molten aluminium.