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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The macroscopic wetting

The macroscopic wetting angle data indicate non-wetting behaviour and therefore spontaneous infiltration into open pores is only possible by applying an external pressure. The metallostatic pressure on the substrate due to the droplet height was calculated as 10 -4 MPa. By applying Washburn´s relation (Equation 21) in combination with the mean θst of 152° and the surface tension γlv of the alloy IM of 0.871 N/m (19,20) , the pressure required to fill a 2 µm diameter pore was calculated to be 0.625 MPa. This is nearly 5 times higher than the calculated metallostatic pressure. Despite this, some pore infiltration was observed and these micro-intrusions must result from the wetting conditions on the micro-scale under the droplet. In conclusion, the intrinsic wetting angle θintr has to be lower than 90° and therefore much smaller than θst measured in the sessile drop test. Sobczak et al. (80) found the same behaviour but at significantly higher process temperatures (>1000°C). Here evaporation of Al2O takes place as a result of the high temperature (described by Equation 9). As a result, local rupture of the oxide film at the microscopic asperities of the interface under the weight of the droplet could facilitate the penetration of surface pores by the metal. This local loss of the oxide could lower θintr at the pore/metal interface, thus allowing the metal to penetrate the pore under conditions where the macroscopic θst based on the drop profile has a large obtuse value. The enhanced wetting at lower temperatures (750°C) in the present experiments was a result of evaporation of Mg which prevented oxidation of the droplet surface and therefore enhanced wetting below the droplet. In contrast to the micro-intrusions, segregation of the Fe-rich phase prevented movement of the triple line. An example of the interface after the test is shown in Figure 4.9. No published literature could be found for commercial casting alloy - ceramic wetting couples. Flores et al. (145) published work on the purification of iron-contaminated Al alloys, stating that the quarternary Al8FeMnSi2 intermetallic phase segregates to the melting furnace walls 215

during extended holding and acts as nuclei for further crystallization of Fe-rich phases, resulting in a decrease of Fe in the liquid phase. Similarly, an Al-Fe-Mn-Si phase was found on the substrate, Figure 4.9, which indicates that this phase solidified prior to the residual droplet. As further indicated by the crack plane passing through the pure substrate, bonding between the iron-rich phase and the substrate was stronger than the substrate´s tensile strength which further supports adhesion forces between the Fe-rich phase and the substrate, resulting in pinning of the droplet. Taking into account the inaccuracy of angle measurement of ± 5°, similar wetting behaviour was found when IM-AO was compared with the reactive systems IM-TO (TiO2) and IM-MO (MgO). This has to be attributed to the segregation effect which prevents direct contact between the substrate and the alloy. In summary, in the sessile drop tests, Fe-rich intermetallics nucleated on Al2O3 and hindered spreading of the droplet. Where the intermetallic layer was interrupted, the metal could penetrate open porosity of the substrate. As a consequence, the measured θst did not represent the wetting angle of an Al alloy melt on a ceramic substrate. 5.4.3. Dynamic wetting Mortensen and Wong (121) stated that, even in optimised experimental conditions, the static wetting angles were not able to describe wetting in dynamic preform infiltration and θst is not equal to the dynamic wetting angle θdyn. Garcia-Cordovilla et al. (103) proposed a relation between P0 and θdyn (Equation 20). In the present study the P0 values were determined from the pressure-saturation curves in DSQC according their procedure and an example is shown in Figure 4.48. In any given system, the bulk density of the ceramic phase ρp , the surface tension of the infiltration alloy γlv, the ceramic phase volume fraction Vp and the specific surface area Si were assumed to be constant during infiltration. Compression, which appeared 216

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