JOURNAL OF MATERIALS SCIENCE 35 (2 0 0 0 ) 2167 – 2173 The role of silicon in wetting and pressureless infiltration of SiC p preforms by aluminum alloys M. I. PECH-CANUL, R. N. KATZ, M. M. MAKHLOUF Worcester Polytechnic Institute, Worcester, Massachusetts, USA S. PICKARD Materials and Electrochemical Research Corporation, Tucson, Arizona, USA Silicon plays an important role in the production of Al/SiC metal matrix composites. As an alloying element in aluminum, silicon retards the kinetics of the chemical reactions that result in the formation of the unwanted intermetallics Al 4 C 3 and Al 4 SiC 4 . As a thin coating on silicon carbide, silicon becomes an active participant in a thermally activated chemical reaction that enhances wetting of silicon carbide by aluminum alloys. Consequently, Al/SiC composites made with siliconized silicon carbide and silicon rich aluminum alloys show mechanical properties that are significantly different from those of similar composites produced with unsiliconized silicon carbide or with aluminum alloys that do not contain silicon. It is shown that a silicon coating on SiC significantly enhances wetting of SiC particles by aluminum alloys, reduces porosity, does not affect the modulus of elasticity, but decreases the modulus of rupture of Al/SiC metal matrix composites. C  2000 Kluwer Academic Publishers 1. Introduction Silicon carbide reinforced aluminum matrix compos- ites have attracted considerable attention in recent years because of their potential to exhibit enhanced mechan- ical and physical properties in comparison to their indi- vidual components [1–3]. However, in order for these composites to achieve their full potential, the SiC/Al interface must be carefully tailored so that adhesion be- tween the SiC and the aluminum alloy matrix is strong. Optimizing the Al/SiC interface requires balancing sev- eral factors. While a strong SiC/Al interface may be created by promoting metallurgical bonding between the SiC and the Al through interdiffusion and chem- ical reactions, extensive interactions may degrade the SiC and reduce the overall properties of the composite. Moreover, because useful aluminum alloys typically contain elements that have a high affinity for silicon and/or carbon, interfacial reactions may occur and re- sult in reaction products with inferior properties. Most of the past effort dedicated to improving wetting of SiC by aluminum has centered on modifying the aluminum with surface-active alloying elements, and/or the SiC by surface coatings [4, 5]. For example, it has been shown that coating SiC particles with K 2 ZrF 6 causes a substantial reduction in the magnitude of the threshold pressure required for infiltrating packed beds of the par- ticles with liquid aluminum [6]. It has also been shown that coating SiC with Cu and with Ni improves the SiC/Al interface as reflected in improved mechanical properties [7]. In addition, the use of inert and/or reac- tive gases during the production of Al/SiC composites has been introduced as a means of controlling chemical reactions and interactions at the Al/SiC interface [5, 8]. In this paper, the effect of silicon additions to alu- minum, and silicon coating on the surface of silicon carbide on the wetting characteristics and pressureless infiltration of SiC preforms with aluminum alloys are investigated. 2. Background Wetting is a term that is commonly used to describe the distribution of liquid on solids. Factors that are im- portant for determining the extent of wetting are easily understood by examining a liquid droplet that is in con- tact with a flat solid surface. In the idealized case where chemical reactions between the solid, liquid, and vapor phases are negligible, and where the liquid droplet is small enough so that the gravitational force can be ig- nored, the liquid droplet assumes an equilibrium con- figuration. This equilibrium configuration is dictated by surface free energy considerations in such a manner that the shape of the liquid droplet is uniquely charac- terized by θ , its contact angle with the solid substrate. The relationship between the contact angle θ and the surface free energies at the liquid-vapor γ lv , solid-vapor γ sv , and solid-liquid γ sl interfaces is shown schemati- cally in Fig. 1, and is represented mathematically by Young’s equation, Equation 1 [9] cos θ = γ sv − γ sl γ lv (1) 0022–2461 C  2000 Kluwer Academic Publishers 2167