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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

In the case

In the case of a), the diffusion rates of the reacting components and thus the growth rate of the reaction products are low relative to the flow rate of the liquid drop. The liquid at the periphery of the drop will remain in contact with unreacted solid. In the this case, the equilibrium wetting angle (cosθeq) is defined as: θ γ − γ sv Rl cos eq = Equation 14 γ lv where γRl is the surface energy of the liquid-solid reaction product interface. In the second configuration b), the diffusion rates of the reacting components in the solid are high relative to the flow rate of the liquid drop. After equilibrium is reached, the reaction layer extends beyond the triple line. In this case the equilibrium contact angle is given by: γ − γ Rv Rl cos θeq = Equation 15 γ lv where γRv is the surface energy of the interface reaction product- atmosphere. There is up to now no definite model describing the influence of reactivity on wetting behaviour in metal melt-ceramic systems. The enthalpy of formation and the changes in the interface are the two factors leading to changes in the surface tension. Which of these dominates is controversial and depends on the particular system. In dynamic infiltration systems, where liquid metals are forced under pressure into a porous medium, the non- reactive and the reactive wetting are even more difficult to assess. 2.3. Infiltration of porous media The infiltration of porous preforms consists of forcing the liquid metal to infiltrate the preform by applying pressure. The infiltration consists of an initiation step, where a certain 37

capillary or threshold pressure has to be applied to overcome the capillary backpressure. Once the threshold pressure has been overcome, the molten metal flows into the capillaries of the preform. Both infiltration steps will be discussed in detail below. 2.3.1. Dynamic wetting The critical parameter that governs the wettability of a solid by liquid is the work of immersion Wi which is given by: W γ −γ Equation 16 i = sl sv where γsl and γsv are the solid/liquid and solid/vapour surface energies respectively. If γsv>γsl the metal wets the solid. If γsv

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