Project 3: Microstructural and Chemical Developments at the Interfaces between MoSiTi/Bond Coat/PDC (Polymer Derived Ceramics)

Dr. Nathalie Thor (1^st cohort)
M. Sc. Ingrit Sisilia Rosari Nurak (2^nd cohort)

Supervisors: Prof. Astrid Pundt, TT-Prof. Yolita Eggeler

Electron microscopy offers unique insights into material microstructures by providing structural and chemical information at atomic-scale resolution. It is therefore a powerful tool to investigate the effects of elevated temperatures, mechanical loading, and corrosive environments on newly developed materials intended for future high-temperature applications. Furthermore, it enables the characterization of the complete material system, consisting of metallic/intermetallic alloys as the substrate material, aluminum or chromium as the bond coat, and polymer-derived ceramics (PDCs) as environmental barrier coatings (EBCs) and thermal barrier coatings (TBCs) (see Figure 1), even under application-relevant conditions.

During the first cohort, the primary focus was on the optimization of ceramic bulk materials of the EBC and TBC through detailed microstructural analyses. The microstructure of the PDC based on Si(Hf, Ta)BCN(O) was successfully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), and, for selected regions, by high-resolution transmission electron microscopy (HRTEM), including selected area electron diffraction (SAED) [1,2].

During the second cohort, a coating system consisting of Mo₂₇Si₂₀Ti₅₃ as the substrate material, aluminum or chromium as bond coat and Si(Hf, Ta)BCN(O) as the EBC was investigated. The bond coat was produced by pack cementation. Initially, a suitable sample preparation route for electron microscopy had to be established. The results demonstrate that the coating system provides effective protection of the substrate material. However, the coatings exhibit substantial degradation over relatively short exposure times. A key degradation mechanism is the diffusion of titanium from the substrate into the bond coat and the environmental barrier coating (EBC). This process begins during pack cementation and the subsequent deposition of the EBC, resulting in the formation of secondary phases within the coating layers.

In the third cohort, a coating system consisting of Cr₆₁Mo₃₆Si₃ as the substrate material and PDC of SiAlCN as the EBC is being investigated. In addition, the influence of a SiAlCN TBC on the underlying layers is to be investigated. Compared with Mo₂₇Si₂₀Ti₅₃, Cr₆₁Mo₃₆Si₃ exhibits a higher melting temperature and improved room-temperature ductility, making it an even more promising candidate for the intended application [3]. The primary objective is to achieve a better understanding of the interfacial processes, particularly diffusion and phase formation, occurring between the individual layers. To this end, the bond coat will be deposited by sputter deposition, as the lower processing temperatures are expected to reduce interdiffusion between the deposited layer and the substrate compared to pack cementation. Subsequent heat treatments of the fabricated samples will be employed to deliberately induce diffusion and potential phase formation. These processes will then be analyzed using various electron microscopy techniques. In addition, in situ electron microscopy experiments will be conducted, enabling the direct observation of diffusion and phase formation in real time. To further assess the influence of the substrate microstructure on the performance of the protective coatings, a comparative study between additively manufactured and arc-melted Cr₆₁Mo₃₆Si₃ is planned.

[1] Thor, N., Bernauer, J., Petry, N. C., Ionescu, E., Riedel, R., Pundt, A., & Kleebe, H. J. (2023): Microstructural evolution of Si(Hf_xTa_1-x)(C)N polymer-derived ceramics upon high-temperature anneal. Journal of the European Ceramic Society 43(4), 1417-1431, doi:10.1016/j.jeurceramsoc.2022.11.060.

[2] Thor, N., Winkens, G., Bernauer, J., Petry, N. C., Beck, K., Wang, J., Schwaiger, R., Riedel, R., Kolb, U., Lepple, M., & Pundt, A. (2024): Microstructure Characterization and Mechanical Properties of Polymer‐Derived (Hf_xTa_1‐x)C/SiC Ceramic Prepared upon Field‐Assisted Sintering Technique. Advanced Engineering Materials 26(17), 2301841, doi:10.1002/adem.202301841.

[3] Hinrichs, F., Winkens, G., Kramer, L. K., et al. (2025), A Ductile Chromium–Molybdenum Alloy Resistant to High-Temperature Oxidation, Nature 646, 331-337, doi:10.1038/s41586-025-09516-8.

Fig. 1: Material system and layer structures