Space exploration depends on materials that can withstand extreme conditions, particularly for rocket thrusters. Ceramic matrix composites (CMCs) like carbon-carbon (C/C) and carbon-silicon carbide (C/SiC) have been widely used for rocket nozzle applications ,due to their thermal and mechanical properties. However, the demand for materials capable of higher temperature tolerance and reusability has shifted focus to ultra-high temperature ceramics (UHTCs) and UHTC matrix composites (UHTCMCs). Among UHTCs, ZrB₂ stands out for its excellent mechanical properties, oxidation resistance, and lower cost compared to HfB₂. To overcome ZrB₂'s inherent brittleness and enhance properties like densification and oxidation resistance, additives such as SiC and carbon fibers are introduced. SiC enhances densification, controls grain growth, and improves oxidation resistance via the formation of a SiO₂ protective layer, while carbon fibers improve mechanical strength, oxidation resistance, and reduce density. Spark Plasma Sintering (SPS) is a preferred fabrication method for its ability to rapidly densify materials while maintaining fine microstructures.
Although ZrB₂-SiC composites are extensively studied for high-temperature applications, the relationships between sintering parameters (e.g., temperature, pressure, and dwell time) and densification, microstructure, and mechanical properties remain underexplored. This study investigates these correlations using SPS and examines the impact of milling methods—high-energy milling with WC balls versus regular milling with ZrO₂ balls—on final material properties. The feasibility of incorporating short carbon fibers into the ZrB₂-SiC matrix is also assessed, focusing on the effects of preparation techniques and fiber length.
Fabrication insights revealed that increased sintering temperature generally improved densification due to enhanced atomic diffusion, grain boundary migration, and mass transport. High-energy WC milling achieved superior densification compared to ZrO₂ milling, with ZSW samples reaching a maximum relative density of 99.2%, versus 96.5% for ZSZ samples under similar conditions. ZSW samples, however, developed a secondary ZrO₂ phase due to more intense abrasion and oxygen diffusion during milling, while ZSZ samples maintained a finer microstructure, with an average grain size of 2.65 μm compared to 2.91 μm for ZSW. Attempts to incorporate 35 vol% short carbon fibers were unsuccessful under current sintering conditions, but improvements in fiber distribution were achieved with a rotary evaporator. Shorter fibers showed better structural integrity by reducing stress concentrations.
Mechanical properties were strongly influenced by sintering temperature. Higher temperatures caused grain coarsening, leading to reductions in hardness, flexural strength, and fracture toughness. For instance, ZSW hardness decreased from 14.33 GPa at 1950°C to 13.92 GPa at 2050°C, flexural strength declined from 407 MPa to 384 MPa, and fracture toughness dropped from 3.71 MPa·m¹/² to 3.58 MPa·m¹/². Milling methods also played a critical role; ZSW samples showed lower hardness and toughness due to the softer ZrO₂ phase and coarser grain sizes, with a maximum fracture toughness of 3.76 MPa·m¹/² compared to 3.97 MPa·m¹/² for ZSZ samples. However, ZSW samples exhibited comparable or higher flexural strength (384–516 MPa) due to ZrO₂’s transformation toughening effect, while ZSZ samples ranged from 317 to 476 MPa.

Due to their innately robust thermal and mechanical qualities, ultra-high temperature ceramics (UHTCs) such as zirconium diboride (ZrB2) have been investigated as viable materials to be used in reusable thermal protection systems (TPS). TPSs are vital to a spacecraft’s heat balance in atmospheric reentry and in space. Here, the thermal and optical properties are especially critical in determining the heat balance. However, radiation exposure in space can degrade such material properties, especially over a prolonged mission duration. The interaction of electron radiation-which can be found in the outer Van Allen belt with ZrB2 has not been studied previously and was, therefore, the main scope of this study. The response of thermo-optical properties of ZrB2 to increasing electron radiation doses simulating 5,10, and 50 years of outer Van Allen belt radiation exposure was investigated. ZrB2 samples were made through spark plasma sintering and exposed to 3 MeV electron irradiation. The ZrB2 samples were characterized by their microstructure, thermal conductivity, coefficient of thermal expansion (CTE), emittance, absorptivity, and surface roughness. It was found that ZrB2’s thermo-optical properties showed high radiation resistance at these dosages, and no apparent microstructural change was observed after irradiation. However, the irradiated samples had, on average, a 29 % lower surface roughness than the unirradiated samples, possibly originating from electron sputtering. Moreover, ZrB2 samples produced at various sintering temperatures did not display a different radiation resistance.

In order to establish a sustained extra-terrestrial presence, habitats need to be built on the Moon and Mars using novel materials made from local resources. Several material processing methods can be applied to transform the locally abundant regolith into materials suitable for structural purposes. One promising method is sintering. This process can be used to create strong material using little to no Earth-imported additives. However, sintering still requires a large amount of energy to heat the material. A possible method to lower the energy requirement is by introducing small amounts of sintering aids. However, little is known about the effect of aids on the properties of sintered regolith. This research aims to investigate the effect of sintering aids on the densification and mechanical properties of sintered Martian regolith simulant.

In this study, the chosen Martian regolith simulant, Martian Global Simulant - 1 (MGS-1), was first investigated using a variety of powder characterisation techniques to assess the similarity with actual Martian material. Using the Spark Plasma Sintering (SPS) technique, disk shaped samples were sintered at temperatures between 700 °C and 1060 °C and pressures of 30 MPa to 50 MPa. Several powder mixtures were used. Two different additives, aluminium and bismuth oxide, were used in two weight percentages, 2.5wt% and 5wt%, and mixed with baseline material. Samples sintered from this enriched material were compared to those made using baseline MGS-1 material. In order to assess the mechanical properties, the Ball-on-Ring (BoR) compression test was used to determine the biaxial flexure strength of the samples. Since the BoR compression test is a semi-standardised method, an effort was made to validate the testing procedure and obtained results using soda-lime glass samples. Additionally, mortar disks were created and tested to provide a reference for terrestrial material properties. After compression testing, some samples were ground back into a powder and examined using X-ray Diffraction (XRD) to assess any bulk composition changes induced by the additive and/or sintering process.

The results from the powder characterisation techniques show that the chemical composition of MGS-1 is close to that of actual Martian material. In order to achieve strengths comparable to terrestrial mortar, a relative density of at least 70 % needs to be achieved. For the baseline material, the sintering temperature needs to exceed 1000 °C in order to obtain these results. This value is 950 °C for 2.5wt% aluminium additive material, and 900 °C for 5wt% aluminium, 2.5wt% and 5wt% bismuth oxide additive material. For equal sintering temperatures, the biaxial strength of enriched powders exceeds that of baseline material. Hence, the sintering temperature can be lowered for enriched materials to achieve similar strengths. Samples made from bismuth oxide enriched material exhibited superior properties compared to aluminium enriched material. For aluminium enriched material, no clear increase in properties is observed with increasing additive fraction. For bismuth oxide enriched material, there appears to be an increase in properties with increasing additive fraction. A material behaviour transition from brittle to tough appears to be linked to biaxial strengths exceeding 12 MPa. Compared to literature on Martian regolith-based materials, the results for sintered enriched MGS-1 perform well in terms of required additive fraction and mechanical properties. Using additives could potentially be a way to lower the energy requirement for regolith sintering on Mars. This work opens up areas of further research into the optimal additive, additive amount and sintering parameters for on-site application.

Zirconia (ZrO2) has emerged as a promising technical ceramic, electrolyte for solid fuel cells, and as a topcoat for thermal barrier coatings at high temperatures, etc. in the past decades. The traditional synthesis of ZrO2 usually necessitates a sintering temperature as high as 1200°C. General interest in lowering the sintering temperature to reduce energy consumption and thermal stresses has led to many research works. In this thesis, a novel route of sintering ceramics at lower sinter temperatures named ‘Cold sintering’ was adopted to sinter Zirconia bulks. Sintering is performed with the Spark Plasma Sintering (SPS) technique through in-situ chemical conversion of Zirconium hydroxide [Zr(OH)4] precursors into Zirconium oxide. The sintering process is accelerated by the water vapor emission from the conversion reaction. For the commercial applications of Zirconia, its density, hardness, and the stabilization of favourable tetragonal phases Is necessary. Many methods were adopted to increase the densification of the powders. The applied mechanical pressure is increased, and sintering aids are used to densify the sintered bulks. Mechanical properties like hardness and thermal conductivity values are measured. The dependence of relative densities, hardness, and thermal conductivity on the sinter conditions such as sintering dwell pressure (range: 50 MPa – 300 MPa) and sintering dwell temperature (range 400 °C – 1200 °C) is mapped out as guidance for further material property design. In addition to the improved sintered density, the phase stabilization of tetragonal ZrO2 phases is also enhanced at sintering dwell temperature of 900°C and dwell pressure of 50 MPa using the aliovalent phase stabilizing compounds Yttrium oxide (Y2O3) and Bismuth oxide (Bi2O3).