Work of Adhesion in Al/SiC Composites with Alloying Element Addition
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DUCTION
METAL
matrix composites (MMCs) have been drawing sustained attention due to the unique balance of physical and mechanical properties. Particularly outstanding are the SiC-reinforced Al and Al alloy composites (Al/SiC composites), which exhibit properties such as high modulus, high specific stiffness, high-temperature strength, low coefficient of thermal expansion, and good wear resistance. These excellent comprehensive properties make Al/SiC composites prime candidates for use in the aerospace and automobile industry.[1] When Al/SiC composites are fabricated by liquid state processing, such as stir casting and liquid metal infiltration, the adhesion behavior between molten Al and SiC is a major concern. The extent of adhesion is important in determining the wetting of SiC by molten Al, the bonding strength of the interface, as well as the mechanical properties of the resultant composites. Therefore, it is of great significance to investigate the adhesion behavior at the interface. From the viewpoint of energy, the adhesion behavior between a solid and a liquid can be characterized by the work of adhesion Wa. Wa is defined as the work per unit area necessary to separate the solid–liquid interface and is related to the characteristic interfacial energy rij of the solid (S)-liquid (L)-vapor(V) system by the Dupre´ equation: Wa ¼ rsv þ rlv rsl
½1
XIN FANG, Postgraduate Student, and TONGXIANG FAN and DI ZHANG, Professors, are with the State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 17, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
The work of adhesion has been extensively studied from both experimental and theoretical perspectives. Experimental determination of Wa is generally based on the wetting experiment of liquid metals on ceramics.[2] The wettability can be measured by many methods, such as sessile drop, maximum bubble pressure, and tensiometric immersion-emersion technique.[3,4] With measured contact angle h and surface tension rlv , Wa can be obtained by the Young-Dupre´ equation: Wa ¼ rlv ð1 þ cos hÞ
½2
However, the experimental determination of Wa presents two major disadvantages. First, the contact angle h is highly sensitive to the test conditions such as furnace atmosphere, substrate surface roughness, and surface heterogeneity,[5] which results in a large scatter of contact angle for a given system. Second, it would be both expensive and time consuming to experimentally determine the relationship between Wa and the composition of liquid alloy. Therefore, theoretical prediction stands out as being more convenient and useful. In the past few decades, many theoretical models have been proposed to predict the work of adhesion in liquid metal/ceramic systems. McDonald and Eberhart[6] made the first quantitative estimation of the work of adhesion in Al2O3-transition metals (Me) systems. They assumed that chemical bonds between Me atoms and O2- oxygen ions were set up at specific
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