title: Thick ceramic barrier coatings derived from transition metal modified polyorganosilazanes for high temperature applications
Creators: name: Kredel, Samuel Aeneas
Publisher: name: UNSPECIFIED
Year: 2025

description: As part of the DFG-funded research training group MatCom-ComMat, this project focuses on developing novel ceramic coatings for use as environmental and thermal barriers on eutectic Mo-20Si-52.8Ti or similar substrates. This new material system aims to enable the operation of combustion engines, such as aerospace and power plant gas turbines, at temperatures exceeding 1200 °C to increase their efficiency.
To achieve this, the preceramic polymer Durazane 1800 was chemically modified with varying amounts of the transition metals hafnium and tantalum as well as boron because these tailored precursors yield Si_x(Hf_aTa_{1-a})(B_b)C_yN_z ceramics with desirable properties, including low thermal conductivity, high oxidation resistance and relatively high coefficients of thermal expansion for polymer-derived ceramics. The precursors demonstrated excellent processability for coating applications, allowing for the preparation of X-ray amorphous ceramic coatings on various substrates via spin coating followed by thermal treatment. The resulting coatings exhibited good adhesion, homogeneity, evenness, and thermal shock resistance up to a critical ceramic layer thickness above 500 nm. To increase the total coating thickness, a multilayer approach was followed, which, however, has been time and energy intensive. To overcome these drawbacks, a new method for the general fabrication of wet-chemically prepared multilayer coatings was developed and registered as a patent. Using this method, filler- and crack-free polymer-derived ceramic coatings with a thickness of more than 10 μm were prepared for the first time.
At 1200 °C, these ceramic multilayer coatings were able to grant excellent protection against oxidation for both silicon and Mo-20Si-52.8Ti substrates up to at least 50 h. However, for all substrate materials except silicon, severe damage to the coatings occurred due to interdiffusion, an increasing mismatch in thermal expansion coefficients or oxidation of the substrate leading to its expansion or volatilization. Thus, for application as a turbine material as described above, i) a change in the composition of the ceramic coating, ii) the implementation of an interjacent diffusion barrier, or iii) an adjustment in the composition of the substrate is considered to be required in order to transfer the good oxidation properties of the individual materials to the materials compound. Since the developed fast process is not limited to a single precursor, it is expected that a ceramic coating with a changed composition can be similarly prepared. Thereby, the described failure mechanisms may be evaded or inhibited. Furthermore, the prepared ceramic coatings are attractive candidates for environmental barrier coatings on various substrates. 
The implementation of fillers was explored as well, explicitly targeting the use case of a thermal barrier coating. Therein, mostly crack-free and adherent Si_x(Hf_1)(B_{0.5})C_yN_z coatings up to thicknesses above 100 μm on a bond coat on silicon substrate could be obtained. These coatings showed strong oxidation when exposed to air at 1200 °C for 100 hours. Still, they remained adherent and mostly crack-free. Exploring this system with other passive and active fillers, such as Hf₆Ta₂O₁₇, holds promise for advancing these findings.