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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

2.2. Static wetting in

2.2. Static wetting in metal-ceramic systems Wetting of ceramic substrates by a molten metal plays an essential role in the fabrication and properties of metal matrix composites. The relation between the wetting angle θ and the surface energies was first proposed by Young (70) and describes the balance of horizontal forces due to surface tension acting upon a liquid drop in contact with a solid as shown schematically in Figure 2.7. It is expressed by: θ γ − γ γ sv sl cos = Equation 6 lv Thus the characteristic contact angle θ depends on the surface energies of the liquid-vapour (γlv), solid-vapour (γsv) and solid-liquid (γsl) interfaces. Good wetting is achieved when the contact angle between metal and substrate is below 90° as shown in Figure 2.7b. (a) (b) Figure 2.7 Metal melt droplet on a ceramic substrate. (a) Non-wetting characterised by wetting angles of θ> 90°. (b) wetting systems with� θ< 90°. The work of adhesion Wad is in direct relation to the surface energies and defined as: W γ + γ − γ ad = Equation 7 sv lv sl 25

The work of adhesion characterises the work which is necessary to separate the solid-liquid interface. Combining Equation 6 and Equation 7 results in the following expression: ( 1+ θ ) W ad = γ lv cos Equation 8 Thus the work of adhesion depends only on the surface tension of the metal melt and the contact angle θ. It is usually measured by the sessile drop technique, which yields both the contact angle and the surface tension of metal through analysis of the drop´s profile (13) . The work of adhesion in metal melt-ceramic systems is subdivided into two different categories. The first one is a result of the van der Waals forces and the second is based on chemical bonding between surface atoms of the two different phases (71) . Non-reactive systems are characterised by positive reaction enthalpies, whereas reactive systems show negative reaction enthalpies. The pure metal-metal oxide systems (Me/MexOy), such as Al- Al2O3, are in general categorized as non-reactive systems. 2.2.1. Non-reactive wetting The interfacial energies of Me/MexOy are characterized by weak van der Waals and electronic interactions. As reported by Gallois (72) , the intrinsic contact angles in these systems are generally larger than 90°. It may be different when there are interactions between the oxide and the metal melt or in an reactive atmosphere. Oxygen dissolved in the metal adsorbs at the liquid metal/vapour and the liquid metal/ceramic interfaces, leading to improved wetting with contact angles as low as 70°, as shown by Diemer et al. (73) in the Cu-Cu2O system. Among the metal-ceramic systems, Al-Al2O3 is the most important one. As the solubility of Al2O3 in the Al melt close to its solidus is rather low (74) , less than 0.006 at-% is dissolved, there is no influence leading to the aforementioned wetting improvement in this system. 26

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