The present study examines the feasibility of fabricating Saffil alumina fibre-reinforced mullite matrix composites via a single-infiltration EFD process, and/or EFD plus pressure filtration. An in-situ EFD cell was developed in order to produce fibrereinforced composites. This cell is able to record the weight gain during EFD, i.e., in real time, thus providing information to establish the exact EFD kinetic model. The critical issues in preparing an aqueous based mullite precursor (suitable for EFD) from nano-size boehmite and fumed silica particles were discussed. It has also been shown that dense, homogeneous green mullite composition and sintered mullite microstructures can be prepared successfully only by controlling the short-range particle-particle interactions of the dissimilar particle species within the suspension. A reaction bonded mullite (RBM) slurry containing alumina and silicon carbide powders was developed in order to obtain zero-shrinkage RBM after sintering. Saffil alumina fibre mats pre-coated with mullite using EFD were infiltrated with RBM slurry using PF. By these means, high green density ( 64.5 % TD) and sintered density (94.6 % TD) were achieved. Room and high temperature 4-point bend behaviour of the monolithic and fibrereinforced CMCs were examined. 30 vol % fibre addition increased both the strength and toughness of the monolithic mullite at room and high temperature. At room temperature, a maximum 4-point bend strength (-500 MPa) was obtained from a fibrereinforced RBM composite having a weak mullite interface between the fibre and the mullite matrix. Both monolithic and fibre-reinforced mullite components produced here were able to keep the similar strength from room temperature up to 1300 °C. A slight decrease was observed at 1400 °C whilst a dramatic strength decrease occurred at 1500 °C, as fibre grain growth occurs at this temperature. Debonding, fibre pull-out, bridging, load transfer and crack deflection mechanisms are found to be responsible for the high strength and toughness. The state-of-damage in Saffil alumina fibre-reinforced RBM CMCs subjected to cyclic fatigue was investigated by means of acoustic emission (AE) monitoring and dynamic forced resonance (FR) techniques. FR measurements showed that as the number of cycles is increased, the Young's modulus decreases, whereas the internal friction increases. The sharp increase in Q" 1 as the number of cycles is increased is evidence for the development of significant microstructural damage, such as matrix cracking and delamination, resulting in the creation of new internal surfaces within the composite.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:633209 |
| Date | January 1999 |
| Creators | Kaya, Cengiz |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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