Makurunje, Phylis Sarah2016-04-052016-04-052016-04-05http://hdl.handle.net/10539/20175Submitted in fulfilment of the requirements for the degree of Master of Science in Engineering Materials and Metallurgical Engineering University of the Witwatersrand Johannesburg, South Africa 2015A novel Cf/C-SiC-TiC-TaC composite was successfully fabricated by performing reactive melt infiltration (RMI) by means of pressureless spark plasma sintering (SPS) at 1700◦C to permeate molten 66.7Si-17.9Ti-15.4Ta alloy into a porous Cf/C composite prepared by polymer impregnation and pyrolysis (PIP). The Cf/C composite preform was of density 1.54g/cm3 and open porosity 21%, obtained after 3 cycles of PIP with resole phenolic resin. An amorphous carbon matrix was thus obtained and was confirmed by X-Ray Diffraction (XRD) and Raman spectroscopy. The 66.7Si-17.9Ti-15.4Ta alloy prepared by arc melting from TaSi2 and TiSi2 powders formed a solid solution of (Ta,Ti)Si2 and (Ta,Ti)5Si3. When the alloy was infiltrated into the Cf/C composite preform cubic face centred carbides of Ta, Ti and Si, and the (Ta,Ti)C solid solution were obtained at 1700°C by SPS for 30 minutes. At 1600°C the UHTC matrix was predominantly unreacted silicides; SiC is the only carbide which was convincingly formed, although (Ta,Ti)C was observed by XRD and SEM to have incipiently precipitated at the C/alloy interface. At 1800°C the carbide formation reactions extended to the reinforcing fibres, thus compromising the fibres’ reliability. 1700°C was deemed the excellent trade-off temperature for the formation of the Cf/C-SiC-TiC-TaC composite, wherein an acceptable compromise of the extent of carbide-forming reactions completion and limiting the damage of the melt to the fibres. Upon exposure to an oxyacetylene flame of at least 3000°C, the Cf/C-SiC-TiC-TaC composite showed thermomechanical degradation first, and thermochemical degradation thereafter. After 7.5s of exposure to the oxyacetylene flame (4 MW/m2 heat flux) placed 19mm away, the unprotected Cf/C composite showed a mass ablation rate of 0.0402g/s and a linear ablation rate of 0.377 mm/s. The UHTC showed a low mass ablation rate of 0.00388 g/s and a low linear ablation rate of 0.00216 mm/s owing to the scale of the oxides formed adhering to the surface of the composite. The scale, analysed by scanning electron microscopy, X-ray diffraction, electron dispersive spectroscopy and electron probe micro-analysis, showed that the composite constituents had the ability to form glassy self-healing eutectics based on SiO2 (Ta5O2-SiO2 and TaTiO4-SiO2). The Cf/C-SiC-TiC-TaC composite is a promising candidate for the sharp nose and leading edges of hypersonic vehicles.enThesis