Characterization of capillary forces during liquid metal infiltration

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.