Views
5 years ago

2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

place which was

place which was attributed to the calcination of Mg(OH)2, which was assumed to be the reaction product when MgO was in contact with water during processing. A residue of 76% was measured at the end of the test. This mass corresponded to the nominal amount of magnesia ceramic particulates in the green as apart from 20% of PFA an organic binder and water, each 2 % by weight, were initially added to powder. 4.5. Preform processing The processing parameters such as the amount of pore forming agent (PFA), green sintering temperature and green pressing pressure were varied in order to reach the target total porosity of 60% to 70% in the final preform. As shown in the literature review, this preform volume fraction has been rarely and incompletely investigated for preforms of particulate ceramics. The challenge was to obtain sufficient preform strength at this relatively low ceramic volume fraction to withstand the metal infiltration process. 4.5.1 Influence of PFA content Figure 4.17 shows the correlations between the relative mass of pore former added to the ceramic particles and the resulting porosity. The sintering temperature, the atmosphere (which consisted of air throughout the whole investigation), the sintering time of 2 h and the green compaction pressure of 100 MPa were kept constant. The sintering temperature Tsint was kept relatively low aiming for residual porosity between the ceramic particles in order to achieve preforms with low solid fractions. The initial sintering temperatures were taken from previous studies, for example that of Staudenecker (155) for Al2O3 preforms with a glassy frit (type AGPC), and Schneele (163) for TiO2 preforms. The sintering temperatures for AO and MO preforms were based on supplier’s recommendations. For preforms comprising pure alumina AO with the pore former PF, the minimum sintering temperature was found to be 1500°C. When the pore former was PC it was only possible to 107

obtain usable products when they were sintered at 1600°C. Below 1600°C, the edge strengths were too low for handling. The sintering temperature of the Al2O3 preform with the glassy frit binder (AG) was 1000°C, as suggested by Staudenecker (155) . The sintering activity of the Al2O3 particles was very low at this temperature and the resulting strength of the preforms has to be attributed to the glassy frit binder which nominally softens at 570°C (155) . Preform porosity Φ (%) Tot Tot / % 80 70 60 50 40 30 0 10 20 30 40 50 Green part part PFA content f f (mass-%) PFA PFA / mass-% 108 AOPC_1600°C AGPC_1000°C AOPF_1500°C TOPC_1100°C MOPC_1300°C target range Figure 4.17 PFA addition and resulting total preform porosity Φtot . Data points in circles indicate parameters leading to insufficient edge strength or cracking of the preform after sintering. From the literature (53) the sintering temperature of TO preforms was initially fixed at 1100°C and that of magnesia MO at 1300°C. The latter was suggested by the supplier to be the minimum temperature to get acceptable volume diffusion in order to get sintering bridges between the particles, a prerequisite condition for obtaining sufficient preform strength for particulate preforms without binder additions. As shown in Figure 4.17, the total porosity Φtot of preforms without pore former addition was in the range of 37% to 50%, i.e. all below the target pore fraction. Pore formers were added to increase the porosity. However, increasing the porosity resulted in lower strength and a

PDF File 2 - University of Alabama at Birmingham
no· l'2-'1t1 - Repository - Texas A&M University
eTheses Repository - University of Birmingham
MODULE 2 - eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
eA - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
Cymbeline - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
Caroline Winstanley - eTheses Repository - University of Birmingham
epinician precepts - eTheses Repository - University of Birmingham
Witch Shoes - eTheses Repository - University of Birmingham
pains of sin - eTheses Repository - University of Birmingham
1 - eTheses Repository - University of Birmingham
5% - eTheses Repository - University of Birmingham
View - eTheses Repository - University of Birmingham
here - eTheses Repository - University of Birmingham
J - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
Table of Contents - eTheses Repository - University of Birmingham
THE GOD–MAN - eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
Life on Wings - eTheses Repository - University of Birmingham
Engineering texts - eTheses Repository - University of Birmingham
ford madox brown - eTheses Repository - University of Birmingham
Rewriting history - eTheses Repository - University of Birmingham
Four Quartets - eTheses Repository - University of Birmingham
part i - eTheses Repository - University of Birmingham
PIERRE BOAISTUAU - eTheses Repository - University of Birmingham