Fabrication and characterization of fibre reinforced UHTC composites

The thermal, chemical and mechanical environments typical of aero-propulsion applications, such as those characteristic of combustion chambers of high performance propulsion engines, introduce many problems from the point of view of materials. Next generation propellants have become more energetic, resulting in higher temperatures and very hostile, erosive environments. Current materials such as graphite for boost throat applications have repeatedly shown unacceptable ablation/erosion rates, leading to a loss in performance due to throat widening. At the same time, external thermal protection systems (TPS) of civil space vehicles flying at Mach 7 must be able to withstand high temperature (beyond 2000°C), high heat flux (up to 15 MWm-2) and the mechanical stresses associated with vibrations at launch and re-entry into Earth’s atmosphere. Therefore, new thermal protection materials endowed with good oxidation, thermal shock and ablation resistance above 2000°C are required. Ultra-high temperature ceramics (UHTCs) have been gaining increasing interest among the scientific community as potential candidates for the application in extreme environments owing to their high melting points (> 3000°C) and high temperature strength but they have low fracture toughness and thermal shock resistance. In order to increase the damage tolerance of UHTCs, carbon fibres were considered as reinforcements. This thesis deals with the development and characterization of long fibre reinforced ultra-high temperature ceramics, with the objective of providing an understanding of the high temperature behaviour. The PhD activity was divided into three parts: - The first part was focused on the development and characterization of fibre reinforced ZrB2/SiC composites for the fabrication of thermal protection systems. A baseline material containing ZrB2 + 10 vol% SiC was produced by slurry infiltration of carbon fibre preforms with an aqueous suspension containing the UHTC powders and hot pressed. The microstructure was analysed on the polished and fracture surface of the sample with SEM, EDX and XRD techniques. Then the flexural strength and fracture toughness were studied and correlated to the microstructure features. Following the mechanical characterization, the kinetics of oxidation were investigated via TGA in order to find the critical temperatures for the oxidation process. On the results obtained during this part of the work, the influence of SiC content on the mechanical properties and oxidation resistance was studied. SiC was varied in amounts ranging between 5 – 20 % and the oxidation behaviour was studied up to 1650°C in air, showing how high amount of SiC lead to the full coverage of the specimens with a protective borosilicate layer. - The second part of the PhD activity was focused on the thermo-mechanical characterization from room temperature up to 2100°C of fibre reinforced composites with higher refractoriness, based on ZrC, TaC and HfC, for the application in nozzle inserts. In collaboration with Missouri University of Science and Technology (MS&T), under the supervision of professor Bill Fahrenholtz and Greg Hilmas, high temperature mechanical tests up to 2100°C were carried out on carbide composites produced at ISTEC. The determination of the yield strength at high temperature is of great importance for the design of engineering components that must withstand severe stresses in hostile environments. The studied composites retained high strength even above 2000°C, displaying plastic behaviour only at 2100°C and rupturing only at high strains. - The third part of the PhD activity dealt with a novel joint processing route based on slurry infiltration followed by reactive melt infiltration to produce carbon fibre reinforced ZrB2/ZrC composites. In collaboration with the German Aerospace Centre (DLR) in Stuttgart, under the supervision of professor Dietmar Koch, carbon fibre preforms were infiltrated with B/ZrB2 powders via slurry infiltration and then with liquid Zr2Cu via reactive melt infiltration. The boron present in the starting powders led to the formation of fine ZrB2 particles in the metal matrix, while the fibres reacted partially with the zirconium alloy to produce core rims of ZrC around the fibres. From mechanical tests, these specimens possess higher strength than the ZrB2/SiC studied during the first section of this thesis but are also stiffer and more brittle, and therefore better suited for applications where high ultimate strains are not mandatory.