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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

emoving the droplet

emoving the droplet of the IM-AF couple from the substrate and examining a cross-section using SEM and EDS, as shown in Figure 4.9. (a) M I Al (b) Spot - I Z C R Figure 4.9 (a) Interface of AF-IM after droplet pull-off. M: alloy, I: Fe-rich interfacial phase, R: resin which fills the gap between ceramic substrate (C) with inclusions of zirconia (Z). (b) EDS spectrum of the spot analysis of phase I. Bright areas in the SEM backscatter image indicate phases with higher atomic weights. These were distributed in the substrate (C) and adjacent to the metal (M) and designated as an interfacial phase (I) and phase Z in Figure 4.9. The phase I was found to contain mainly Al with Fe and Mn and Si. The Fe originated from the commercial AlMg9 alloy (IM) where it was added in order to prevent sticking to the tool steel die walls during high pressure die casting. It accumulated on the substrate by gavity during the relatively long holding time. The second bright phase, which was distributed in the substrate (C) and is marked with Z in Figure 4.9, was ZrO2 which originated from the powder preparation step, where the powders were milled in a ZrO2 container with ZrO2 balls. The abrasive properties of Al2O3 appear to be the cause of this contamination. The comparatively lower hardness of TiO2 and MgO prevented abrasion of ZrO2 during powder processing and therefore no zirconia debris was found in the TF and MO substrates. 99 Fe Si Mn Fe Fe

As shown in Figure 4.9, apart from the ZrO2 debris in the ceramic phase, there was a particle embedded in the iron-rich phase which was covered with a small layer of the substrate AF. As the metal droplet was pulled off prior to metallographic preparation, the weakest link in the metal-ceramic couple appears to be the first few microns of the ceramic substrate. 4.3. Ceramic powders 4.3.1 Powder particle size The particle size measurements of all powders were carried out in the as-purchased state. In addition, AO was sized after a milling process used during the preform processing. The particle size distributions of TO and MO were not specified in the data sheets of the supplier. Cumulative volume fraction / % (%) 100 80 60 40 20 0 10 -1 AO_as purchased AO_milled TO MO 10 0 100 10 1 Particle size /µm (µm) Figure 4.10 Cumulative particle size distributions of the initial ceramic powders AO, TO and MO in the as-purchased state and AO after a milling step. The characteristic median particle size d50 shown in Figure 4.10 was measured to be 1.3 µm and 1.5 µm for TO and MO, respectively. The distribution of TO was significantly broader than that of MO. The unmilled AO and MO powders both showed an S-shaped curve on the log-normal plot in Figure 4.10. The unmilled TO powder was bimodal as indicated by the 10 2

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