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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Thus the wettability

Thus the wettability of a liquid with known surface tension γlv may be evaluated by an accurate determination of the threshold pressure to force the liquid into the porous body with a known Si, the particle surface area per unit volume. The commonly used sessile drop experiment only replicates poorly the wetting conditions encountered in metal matrix composites fabrication (102) . In infiltration with constant gas pressure, the three phase contact line generally moves at a velocity of 10 cm s -1 inside the porous medium. The oxide layer and the influence of chemical interaction at the interface both were found to be different from what is observed in sessile drop experiments (102) . Thus, experimental procedures utilizing particles of ceramic materials to measure wettability have been established (1,103) . The molten metal is forced under pressure into a packed bed of powder held at the same temperature as the metal and the threshold pressure P0 to initiate movement of the metal is recorded as a measure of wettability. The relation between the P0 and the dynamic wetting angle θdyn is given by: P V P 0 = Siρ γlvcos θdyn Equation 20 P 1− VP where ρP is the bulk density of the particles, γlv is the surface tension of the liquid phase and Vp is the volume fraction of particles in the packed particle bed. In the experiments by Asthana and Rohatgi (104) VP ranged between 0.50 and 0.60. To determine the threshold pressure for infiltration, the square of the infiltrated height is plotted against the applied pressure Pappl. The data can be fitted to a straight line in agreement with Darcy´s law (103) and the pressure axis intercept gives the threshold pressure P0. Apart from packed ceramic powder beds, investigations using porous sintered preforms are rather scarce. Mortensen and Michaud (105) reported infiltration of Saffil fibre preforms 39

using constant gas pressure. Infiltration with pure aluminium resulted in apparent contact angles θ in the range of 102° to 111°. The results were much closer to sessile drop wetting angles measured in ultra high vacuum than to those measured in air or in a lower vacuum. Jonas et al. (106) infiltrated preforms consisting of Al2O3 particles with pure Al and Al-2% Mg (wt.%) alloy. Even though Mg in Al was considered to improve wetting behaviour (15) , the θdyn were 106° for pure Al and 105° for the Al-Mg alloy, indicating no significant improvement in dynamic wetting. The small effect of Mg on the wetting was attributed to the absence of reactive wetting at the infiltration speeds used and to partial disruption of the oxide layer on the surface of the liquid metal during infiltration. As reported by Garcia-Cordovilla (103) , two methods for determination of the surface area are used for packed ceramic particulates. The most common is the Braunauer, Emett and Teller (BET) nitrogen adsorption method (105) .. The laser scattering method applied to the particulates before compaction shows inaccurate results for applications where wettability is evaluated (103) . Mercury intrusion porosimetry (MIP) is an alternative method which may be used to determine the surface areas of porous media. Most of the MIP measurements in MMC-fabrication (53,107) have been performed solely to determine the pore size distribution of particulate preforms with ceramic volume fraction of more than 0.50. In some cases, the ceramic contents were much lower: Cardinal et al. (108) investigated ceramic volume fraction of 0.35 using hybrid preforms of Al2O3 platelets and Al2O3 short fibres. The volume of intruded mercury at a specific pressure is a result of penetration into cavities larger than a particular size. In MIP measurements, Washburn’s equation is applied: D Hg 4γ Hg cosθ Hg = − Equation 21 P 40

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