Development of ultra-high temperature ceramic composites by sol-gel processing
thesisposted on 08.02.2017 by Ang, Caen
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A sol-gel route synthesized nanocrystalline hafnium carbide (HfC) and zirconium carbide (ZrC) for use in composite materials. The starting materials were zirconium and hafnium alkoxides and carbon was introduced by furfuryl alcohol. A block co-polymer surfactant homogenized the oxide and carbon components. Reduction to HfC and ZrC began at a low temperature of 1250°C and removal of the oxide phase was achieved at 1450°C. The carbide powder was nanocrystalline size less than 100nm. Production of HfC included a synthesis step that limited investigation of the sol-gel process. However, purchased alkoxides for zirconium allowed for detailed understanding of phase changes using X-ray Diffraction of the oxide and Raman Spectroscopy of carbon evolution. Morphology changes were observed using nitrogen gas sorption. Scanning and Transmission Electron Microscopy was used to image the carbide lattice, surface oxides and graphene-like carbons in the microstructure. While HfC synthesis demonstrated that shrinking core models apply, the ZrC results indicated that this type of nanoparticle carbothermal synthesis involved agglomeration and necking as a viable mode of mass transport in completing the carbothermal reduction. Understanding of this process allowed for the modest development of composites by sol-gel coating of powders. The sol-gel coating of ZrB2 was successfully applied to coat ZrC nanoparticles on the powder. Detailed refinement of carbon content in the sol-gel coating was necessary reduce the surface oxide intrinsic to the ZrB2 surface, while providing carbon for the sol-gel oxides. The sol-gel coating created a homogenous mix of ~200nm ZrC on the ZrB2 surface after heat-treatment at 1450°C. Densification of the ZrB2-5%ZrC powder was achieved by spark plasma sintering (SPS) at 1800°C, resulting in submicron sized ZrC particles at grain boundaries and triple points. The amount of carbon added to the sol-gel precursor dictated the porosity and thus some properties of the composites. Flexural strength of ~400MPa was obtained from the composites but no significant improvement of fracture toughness was observed. However, an improvement in hardness of about 20% was observed over monolithic ZrB2. The oxidation performance of the composites was improved by the addition of a sacrificial carbide phase. Sharp leading edge samples were oxidized at 3000°C and were compared to a traditional powder mixed composite. The finer and more homogenous distribution of ZrC caused gradual oxidation while maintaining leading edge stability. However, the powder mixed composite failed under the test. This illustrated the importance not only phase selection but also microstructural control. This indicated advantages in sol-gel processing of ceramic composites with improved densification, controlled grain size and improved properties.