Measuring and tailoring capillary forces during liquid metal infiltration M. Bahraini a, * , J.M. Molina a , M. Kida a , L. Weber a , J. Narciso b , A. Mortensen a a Laboratory of Mechanical Metallurgy, Institute of Materials, Swiss Federal Institute of Technology in Lausanne (EPFL), CH-1015 Lausanne, Switzerland b Instituto Universitario de Materiales de Alicante (IUMA) and Departamento de Quı ´mica Inorga ´ nica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Abstract A new technique is proposed for the direct measurement of capillary forces in systems of relevance to the infiltration processing of metal matrix composites. Capable of handling melt temperatures up to 1500 K and infiltration pressures up to 20 MPa, the technique is essentially a high-temperature analogue of mercury porosimetry. Its accuracy is demonstrated by comparison with other techniques and its use is illustrated with the infiltration of diamond particle preforms by Al and Al–Si alloys at 973 K. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Wettability; Infiltration; Drainage curve; Diamond; Aluminium–silicon 1. Introduction There are few clearer manifestations of capillarity in engineering than with the processing of composite materi- als. In the majority of composite processes, a finely divided reinforcement, typically micrometric fibres or particles, is combined with a liquid matrix, of polymer, metal, or cera- mic. The interfacial area created or transformed in the making of a macroscopic volume of composite is therefore high, such that capillary energy variations become directly apparent in the process. With metals this is exacerbated, for two reasons. The first is that the contact angle h of molten metals on typical reinforcements tends to be high. This angle h, exhibited by the liquid matrix contacting a flat solid surface of the rein- forcing phase along the line of matrix/reinforcement/vapor triple contact, is determined by the classical Young–Dupre ´ equation: cos h ¼ r SV  r SL r LV ð1Þ where r SV , r SL and r LV are the solid–vapor, solid–liquid and liquid–vapor surface energies, respectively. Wetting is generally defined as h < 90° and non-wetting as h > 90°; with metals on ceramics or carbon, h nearly always exceeds 90° [1–3]. Mechanical or chemical means are then needed to ‘force’ combination of the metal with the reinforcement one aims to combine it with [4–11]. A second reason why capillarity is very visible in metal matrix composite processing is that capillary forces are typ- ically high compared with the other main force encoun- tered in liquid matrix processes, namely viscous friction. This is indicated by the capillary number Ca = lv/r LV , where l is the viscosity and v is the triple line velocity in m/s. With metals, viscosities are low (on the order of 10 3 Pa s) and surface tensions are high (on the order of 1Jm 2 ) [12]. Since v is typically below a meter per second, Ca < 1, indicating a dominance of capillarity. The importance of capillary forces in the infiltration processing of metal matrix composites has motivated a sus- tained research effort aiming to quantify, and hence mea- sure, their value in infiltration processing. One of two techniques is generally used to this end. In the first, the rate of unidirectional infiltration through packed reinforcement preforms is measured, using a front tracking device when 1359-0286/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2006.02.007 * Corresponding author. Current Opinion in Solid State and Materials Science 9 (2005) 196–201