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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

monolayer

monolayer of the most stable reaction product on the substrate and the liquid melt was assumed. Naidich (71) proposed that extensive chemical reactions between the constituents to be the predominant mechanism to improve wettability. According to Aksay et al. (87) the major contribution of the enthalpy of formation on wetting improvement starts at the beginning of spreading. From the initial wetting angle θ0 it decreases towards an intermediate minimum θmin, and then increases again to reach the equilibrium wetting angle θeq, as shown schematically in Figure 2.10. Thus a time-dependent driving force of spreading is subdivided into different regimes as shown in Figure 2.10. As proposed by Laurent et al. (88) the difference between θ0, θmin and θeq is calculated using Equation 13: Δγ ΔG r r cos θmin = cosθ 0 − − Equation 13 γ lv γ lv where Δγr is the contribution of reduced interfacial tension due to reaction, ΔGr the heat of reaction due to formation of an interfacial phase and γlv the surface tension of the metal melt. θ 0 θ eq θ min θ M θ0 S M P 33 θ min S P M ⎛ Δγ r ⎞ Ar cos ⎜ ⎟ ⎝ γ lv ⎠ ⎛ ΔG ⎞ r Ar cos ⎜ ⎟ ⎝ γ lv ⎠ Figure 2.10 Schematic of the conditions in reactive wetting of a substrate (S) by a liquid metal melt (M). A reaction product (P) is formed along the interface (87) . θ eq S t

The heat of reaction ΔGr may be estimated using the Ellingham-Richardson-Jeffes diagram (89) as presented in Figure 2.11, where the free energy of formation ΔG0 is given as a function of temperature. Metals with a high heat of formation are found at the lower end and those with low heat of formation are at the upper end of the diagram. As shown in Figure 2.11, when Ca and Mg oxidizes in the temperature range from 0 to 1000°C, the free heat of formation is larger (i.e. more negative) than that of Al2O3. In contrast, the oxidation of titanium and silicon emits a lower amount of heat. The heat to form ZrO2 is close to that required to form Al2O3. Landry et al. (90) investigated reactive metal alloy-ceramic systems such as a Cu-Si alloy in contact with vitreous carbon . The wetting angle initially decreased with increasing time. In contrast to the predictions of Aksay et al. (87) , no minimum wetting angle and no inflection point were observed. This behaviour was attributed to the increase in surface roughness as a result of the formation of reaction products on the substrate. Thus it was proposed that the triple line was hindered from moving through pinning on the rough surface. In contrast to Aksay et al. (87) and Naidich et al. (71) , some other investigators (91,92) proposed that the influence of the enthalpy of formation in systems of low or moderate reactivity is negligible. They suggested that the decisive factors in reactive wetting were the changes in interfacial structure due to adsorption processes and the formation of new phases at the interface. Espié et al. (93) investigated the wetting behaviour of CuPdTi alloys on mullite, quartz and alumina surfaces. The spreading on all three substrates was nearly identical which was attributed to the formation of Ti2O3. The differences in the heat of formation of the three systems, characterised by deviations in the thickness of the reaction layer, exhibited no contribution to the spreading behaviour. 34

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