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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

where νi is the

where νi is the stoichiometric coefficient and μi the chemical potential of a given system. Chemical equilibrium calculations can be made for a system which has been uniquely defined with respect to temperature, pressure and composition. The FACT database was used to provide data for the calculations. The software was used to calculate the concentrations of chemical species when specified elements or compounds react or partially react to reach a state of chemical equilibrium. 3.2. Contact angle 3.2.1. Sessile drop substrates To obtain dense substrates, fine grained powders of different oxide ceramics were used. The properties of the initial Al2O3, TiO2, MgO powders, the sintering conditions and the theoretical densities of the fully dense material are listed in Table 3.1. To ensure full dispersion and de-agglomeration, the powders were milled with water using a Fritsch Pulverisette ball mill with 12 mm diameter balls for 5 min. The container and the balls were made of ZrO2. Subsequently 2 wt.% of the binder Mowiol 8-88, a partially hydrolysed polyvinyl alcohol, was added to enable uniaxial pressing of the final powders. Then the slurry was poured into aluminium alloy dishes and placed in a freezer adjusted to -24°C and stored there for 24 h. The dishes were placed in a Christ Alpha 1-4 freeze-drying unit and the slurry dried at 256 Pa for a minimum of 8 h. The dried powders were then passed through a 250 µm mesh sieve to produce a granulated powder. To make the powder pressable, the binder was activated with 2 wt.% of water dispersed into the powder. This was performed by spreading 20g of powder on a sheet of A4-size paper and atomising water above this bed until the mass of the bed increased by the required value. It then was stored for 24 h and subsequently pressed uniaxially to a pressure of 100 MPa to produce green compacts with a diameter of 30 mm and a height of 4 mm. The organic agents in the green compacts were pyrolysed with a low heating rate of 20°C/h up to 500°C and subsequently heated at a rate of 300°C/h to be 63

sintered at 1550°C, which represents the maximum temperature of the tube furnace used. In order to achieve near full densification, the Al2O3 (AF) and MgO (MO) specimens were sintered again in a custom-made industrial furnace at AAflow in Aalen, Germany, at 1730°C for 4 h. In all cases, the sintering resulted in less than 5% of residual porosity. Table 3.1 Sessile drop substrate fabrication: powder materials, sintering conditions and resulting density. Data of purity, and grain size were provided by the suppliers. Other parameters were evaluated in this study. Designation AF TF MO Ceramic type Al 2O 3 corundum TiO 2 rutile MgO Powder supplier /type Sasol HPA0.5 Kronos 1001 Magnifin AM B21 Purity % 99.95 ≥99 99.5 Average grain size d 50 µm 0.47 0.35 0.8 Sintering temperature °C 1730 1550 1730 Sintering time h 4 2 4 Theoretical density kg/m³ 3950 4260 3580 Relative density % td 97.2 ± 1.2 95.1 ± 1.5 98.1 ± 1.3 For surface finishing, the ceramic discs were mounted in a polymer resin. To obtain a smooth surface, the samples were ground and polished using metallographic equipment and consumables. The final diamond polishing step was continued until the surface roughness parameter Ra was less than 0.5 µm. The polished discs were removed from the polymer and cleaned of organics by heating at a rate of 20°C/h to 1000°C in air. Subsequently the discs were cooled to room temperature at 20 C/h. After the treatment, the surface of the samples were examined visually and found to be smooth without thermal etching effects. 3.2.2. Metal alloys Two commercial aluminium high pressure die casting alloys to BS EN 1706 (148) were used for the contact angle measurements and the preform infiltrations. One was a near-eutectic Al- 64

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