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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

As a summary, P0 is

As a summary, P0 is shown as a function of SiHg and θdyn in Figure 5.6. The surface in the 3D cartesian coordinate system was calculated using Equation 20 on the assumption of a fixed ceramic volume fraction Vp of 0.35 and fluid surface tension γlv of 0.883 N/m. Figure 5.6 also shows the positions of the experimental preforms and infiltration data presented in Table 5.2. From a technical standpoint it seems favourable to achieve low P0. To do so, it is necessary to target low SiHg and θdyn. As shown in the present investigations, the latter could not be influenced significantly, even though reactivity was observed. However, SiHg could be influenced significantly by varying the preform processing conditions as demonstrated for TOPC10 and TOPC20 by varying the PFA content and sintering temperature, Table 5.2. The conditions were even stronger for MOPC20, where SiHg could be influenced by a factor of 8 by variation of sintering temperature (Figure 4.31). Variations in SiHg influenced ligament sizes of the metal phase. Lowering of SiHg led to coarsening of the ligaments and, as discussed in 5.1, to a lower mechanical performance. Therefore a compromise between low P0 and high mechanical performance has to be found. The assumption of complete non-wetting of non-reactive and reactive preforms (53,114) could not be confirmed and may falsify modelling results. For example, AOPC20 has a θdyn of 115° the cosine of which is equal to -0.44 and therefore 2.4 times lower than would be expected for non-wetting (cos θdyn= -1). The difference in absolute values is significant for the AO preforms. Assuming θdyn is identical to that of AOPC20, at SiHg of 9.96 m 2 /m 3 (Table 4.2), P0 is equal to 4.0 MPa compared to 9.5 MPa for the complete non-wetting assumption. The present investigations showed that even if reaction phases were observed in the resulting MMC, the reaction had no influence on P0 due to the delayed initiation of the reaction. Nevertheless, an interesting phenomenon in TOPC10 infiltration in ISQC has to be attributed to reactions and its effect on reducing P0. Figure 4.75 shows that, for all but TOPC10, the 221

preform compression, cpr , increased with increasing v0. At a v0 of 0.36 m/s the preform TOPC10 showed an intermediate low cpr, which was as low as that found for the slowest infiltration (DSQC). The homogeneity of the resulting MMC was comparable or even better than the MMC resulting from DSQC infiltration, as indicated by low void fractions shown by comparing the CT-images in Figures 4.73 and 4.51. An intermediate minimum of θstat was proposed in reactive systems (87) , shown schematically in Figure 2.10. This minimum is reached when the spreading velocity keeps pace with the formation of the newly-formed interfacial phase. In ISQC_10, the preform TOPC10 was infiltrated dynamically, where the pressurized melt was forced to fill the preform porosity and therefore a θdyn has to be implemented. For TOPC10, SiHg was 6.01·10 6 m 2 /m 3 (Table 5.2). Therefore, with 38% porosity and for the standard preform size of 61 mm x 42 mm x 8 mm, the overall preform inner surface area was 123 m 2 and the overall pore volume 1.27·10 -5 m 3 . For ISQC_10 the latter was filled with a v0 of 2.8·10 -3 m 3 s -1 (Table 3.8) which therefore took 4.5 ms. Assuming coverage of the entire inner surface area, the rate of new surface formation dA/dt was 27.3 m 2 /ms. Taking into account the low porosity and the compression of the preform, this is assumed ideal for easy infiltration in terms of low P0 and the shape factor α. On the other hand, at similar v0 of 0.36 m/s (HPDC_18), the phenomenon of intermediate low compression was not observed in HPDC infiltration of TOPC10. This has to be attributed to the significantly higher turbulence in HPDC_18, indicated by a Weber number We of 2015, compared to 252 for ISQC_10 during die filling. As reported by Campbell (6) , in HPDC significant amounts of oxide films are formed on liquid Al which increase with rising We. In contrast to DSQC, in HPDC a massive amount of oxide film was formed and filtered on the surface of the preform when the melt entered it. This is supported by the fact that, for corresponding v0, the preform compression was higher in HPDC than in ISQC. Future work should focus on the applicability of the model of Aksay et al. (87) on pressure infiltration of 222

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