Ultra-high temperature ceramic fibres produced by sol-gel and forcespinning processes

Here the sol-gel technique was employed in the production of ultra-high temperature ceramic fibres, utilising forcespinning as the major fibre forming technique. The ultra-high temperature ceramic fibres produced in this work were titanium carbide, zirconium carbide, titanium diboride and zirconium diboride. These were achieved after a series of heat treatment steps from the fibres produced via forcespinning. Upon the production of ultra-high temperature ceramic fibres these were densified via various means such as elevated heat treatments or infiltration. Finally the production of ceramic matrix composites with fibres originating from sol-gel was attempted. Sol-gel was chosen as the means to produced ultra-high temperature ceramic fibres due to its characteristics of nano-scale mixing and it being a liquid based processing technique. The nano-scale mixing allowed the chemicals employed here, namely metal alkoxides (metal oxide source) and furfuryl alcohol (carbon source), to mix homogenously and subsequently produce metal oxide and carbon fibres with good microstructures upon heat treatments. The formation and variation of these sol-gel solutions to produce titanium carbide and zirconium carbide precursor fibres was a strong focus of the work undertaken here, with influence of acids and acetylacetone examined. With acid being shown to accelerate the hydrolysis and condensation reactions in the sol-gel system and acetylacetone being a suitable means to control this acceleration. In conjunction with this the effects of varying sol-gel variables such as furfuryl alcohol were discussed, with furfuryl alcohol content being a suitable means to control the final carbon content in the fibres produced. With the production of a sol-gel solution capable of being formed into fibres, the examination of techniques to do as such was examined. Techniques examined here were electrospinning and forcespinning, with the forcespinning technique being chosen due to its greater production rate. Therefore the variation of forcespinning variables was examined, with it being found that variables that aided in the production of drier fibres upon collection resulted in superior fibre morphologies. These variables included the choice of spinneret, revolutions per minute of the spinneret and collector distance. Upon forcespinning, the resultant fibres are polymeric in nature, therefore for the production of ultra-high temperature ceramic fibres heat treatments are required. Here the role heat treatments play in the pyrolysis of fibres, carbothermal reduction of the fibres, densification of the fibres and phase changes of the fibres is discussed. This is in conjunction with other variables, such as the carbon content of the fibres undergoing heat treatment and the sintering temperature used. It was discovered that with the correct starting fibres (ones with suitable carbon content) the formation of titanium carbide and zirconium carbide fibres was possible. Furthermore, with elevated sintering temperatures up to 2000°C, dense titanium carbide fibres are able to be produced. However zirconium carbide fibres still contain some porosity even up to 2300°C. Here the use of sintering additives (boron nitride) is also discussed, with it being shown that the use of sintering additives allows the production of titanium diboride and zirconium diboride fibres. The formation of these fibres is investigated with the aid of thermodynamics and the investigation of numerous variables such as sintering temperature and time. Through this investigation it was found that to produce fibres that contain predominately titanium diboride or zirconium diboride phase, reduced sintering temperature and hold times were beneficial. With the production of dense ultra-high temperature ceramic fibres, the final topic investigated was the possibility of using these ultra-high temperature ceramic fibres, originating from sol-gel, in ceramic matrix composites. As such ultra-high temperature ceramic fibres were added into matrix materials of either zirconium diboride or silicon carbide. The ability to add a carbon layer onto fibres was also investigate as a means to improve the efficiency of the fibres, by means of coating with furfuryl alcohol. This method proved problematic due to the general loss of fibre morphology upon the coating process. The ceramic matrix composites were sintered via spark plasma sintering and underwent Vickers hardness testing. The cracks introduced via this testing were subsequently examined via scanning electron microscopy, and possible fibre/crack interactions are shown.