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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

time dependence

time dependence of dynamic angles. At t=0 the droplet makes point contact with the solid which results in a wetting angle of 180°. Thereafter the droplet begins to spread onto the solid and the instantaneous value of θ progressively decreases with time. The progression of the wetting perimeter is often spearheaded by a thin precursor film (85) of liquid, so that the measured θ is an apparent rather than a true value. Figure 2.9 Plots of ln((θ/θeq)-1) versus time of the metal – ceramic systems Al-Al2O3 and Al-SiC and the metal-metal system Al-Cu. The slope of the curve represents the parameter A and B the intercept on the t-axis (86) . According to the aforementioned work of Asthana et al. (84) , liquid metal-ceramic or metal- metal combinations exhibit different spreading behaviours. The non-reactive Al-Al2O3 system shows an initial θ of 100° which decreases within 3000s to reach θ=50°. As shown in a later work of Asthana (86) , the equilibrium wetting angle in the Al-Al2O3 system reaches 45°. It is interesting to note that even though the reactive systems Al-SiC and Al-Cu exhibit a much steeper decrease in wetting angle, the initial and the equilibrium wetting angles θeq are higher than that of the Al-Al2O3. 31

In the Al-Cu system, although the presence of oxide film could be a problem in the development of wettability, good wetting was achieved under carefully controlled conditions. A partly reducible oxide film forms at the wetting front under high vacuum. The activation energy for spreading is comparable to the activation energy for dissolution. However interference from the oxide on Al may lead to wetting being controlled by the oxide film. The dominant mechanism of wetting in a given system may change with test conditions but generally the wetting behaviour in the Al-Al2O3 and Al-SiC systems is controlled by oxide effects whereas that in Cu-Al systems is controlled by chemical reactions (86) . 2.2.2. Reactive metal-ceramic systems Although physical and mechanical properties may often limit constituent selection, it is the chemical reactivity of the reinforcement with the matrix alloy either during service or fabrication which will in most cases decide upon the final reinforcement/matrix combination (41) . The influence of the free energy of formation was first proposed by Aksay et al. (87) and Naidich (71) . They proposed that the enthalpy in the vicinity of the triple line controls the spreading of the liquid on the solid material. Naidich (71) subdivided the work of adhesion in a reactive system into two terms as: W ad = Wad ( equil. ) + W ad( non − equil. ) Equation 11 and thus calculated for the interfacial region: γ sl = γ lv + γ sv −Wad ( equil.) −Wad ( non − equil. ) Equation 12 It was reported that the values obtained by calculation of Wad using Equation 11 are in accordance with experimental ones within a factor of 2. The term Wad(non-equil.) was calculated by integration of the free enthalpy of formation between the starting conditions and final equilibrium conditions along the interface. A rapid reaction and the formation of a 32

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