Vito Leisner

In the ongoing development of hypersonic technologies, material advancements play a key role in meeting the ever-increasing thermomechanical demands of these applications. Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) offer a promising solution for components operating under such extreme conditions. Their outstanding thermomechanical properties, including high temperature and thermal shock resistance, excellent thermal conductivity and mechanical strength, position them as ideal candidates for applications in fields like leading edges or inlet ramps for ramjets and scramjets. Due to their remarkable material composition, UHTCMCs are capable of operating in temperature regimes that surpass 1700 °C during their operation times under oxidizing atmospheres. At the German Aerospace Center (DLR), a UHTCMC material based on carbon fibres and a zirconium diboride matrix is being developed utilizing Reactive Melt Infiltration (RMI). With RMI, the orientation of the reinforcement fibres can be tailored, to enable the material to fulfill the demanding load requirements. The reactive melt infiltration process comprises three stages: preform fabrication, pyrolysis, and the actual melt infiltration. The foundation for important material properties of the final ceramic, including the matrix composition, is established in the preform production, which is a crucial step in the process. A boron- and zirconium diboride-based slurry is infiltrated into pitch-based carbon fibre fabric. Subsequently, the preforms are consolidated, pyrolysed, and infiltrated with molten Zr₂Cu to obtain the UHTC matrix by in situ reaction with the preform elements. Scanning Electron Microscopy (SEM) and Energy-dispersive X-Ray Spectroscopy (EDX) enable the examination of the microstructural features, including the arrangement and distribution of zirconium diboride within the matrix. Mechanical evaluation of the UHTCMCs is conducted via 3-point bending tests at both room temperature and at elevated temperature at 900 °C. It has been demonstrated that Ultra-High Temperature Ceramic Matrix Composites can be produced by means of reactive melt infiltration, and that they retain their strength even at elevated temperatures.

The implementation of a structural material into the hot section of a turbine engine requires the protection against oxidation and water vapour. Since Mo-Si-based alloys do not provide this protection on their own over a long period of exposure time, the necessity for protective coatings becomes inevitable. Rare earth silicates, especially Yb-silicates, are known for their great potential to protect silica against water vapour recession and also for their oxidation protective behaviour due to their low silica reactivity. Reactive pulsed DC co-sputtering was investigated and used to produce a two-layered coating of Yb₂Si₂O₇ and Yb₂SiO₅. The layers were implemented into a four-layered environmental barrier coating system combined with a bond coat consisting of a graded Mo-Si interlayer and a thin Si top layer intended to form slow-growing silica. The EBCs were tested up to 100 h at 1200 °C and 800 °C isothermally in air on top of three different Mo-Si-based alloys and show promise for oxidation protection. The pesting phenomenon was completely suppressed.