Advanced Polymer-Derived Si(Hf,Ta)(B)C(N) Ceramics for High-Temperature Thermal Barrier Applications

Abstract

Thermal barrier coatings are widely used in high-temperature processes, particularly in gas turbines employed in aviation and power generation. These coatings serve as protective layers to significantly reduce the temperature of metallic substrates during operation. The application of such protective layers enables higher operating temperatures and thus enhances the thermodynamic efficiency of gas turbines. Modern turbine blades typically consist of a load-bearing metallic substrate (nickel-based superalloys) and a ceramic protective coating made of yttria-stabilized zirconia (YSZ). However, as these conventional systems are approaching their upper temperature limits, novel thermal barrier systems are gaining increasing attention. One promising approach involves the development of a system based on a MoSiBX substrate (X = Nb, Fe, Ti, Hf, etc.) combined with a polymer-derived ceramic (PDC) with the composition Si(Hf,Ta)(B)C(N) (with varying Hf:Ta:B ratios), aimed at extending the operating limits in high-temperature applications. The focus of this dissertation is the development of novel, high-temperature-stable Si(Hf,Ta)(B)C(N) ceramics. Specifically, the synthesis of single-source precursors was investigated, in which the polysilazane "Durazane 1800" (SiCN precursor) and polycarbosilane "SMP 10" (SiC precursor) were chemically modified with organometallic compounds of hafnium, tantalum and a complexed borane reagent. The successful chemical modification of the Durazane 1800-based precursors was confirmed through spectroscopic methods (NMR, IR), and the rheological behavior was examined, revealing an increase in viscosity by 10⁶ mPas. Despite this increase in viscosity, the precursor remained highly soluble in aprotic organic solvents and demonstrated strong adhesion to silicon substrates, onto which it was applied via spin coating. Pyrolysis converted the precursor into a ceramic, which showed an amorphous structure in X-ray diffraction analysis. However, nanocrystalline precipitations of Hf,TaC(N) were observed as early as 900 °C, as detected by transmission electron microscopy (TEM). The high-temperature stability of the ceramics was investigated under different atmospheres (argon and nitrogen) up to 1700 °C. In nitrogen, depending on the boron concentration, the samples remained amorphous up to 1500 °C, but at 1700 °C, phase separation into SiC, Si₃N₄ and Hf,TaC(N) occurred. In argon, the samples remained amorphous up to 1300 °C, with crystallization beginning above 1500 °C into the aforementioned phases, along with the formation of tantalum and hafnium silicides. Further experiments demonstrated that the modified precursors could be processed into crack-free coatings on silicon substrates through pyrolysis, with these coatings adhering well even when subjected to thermal cycling with rapid heating rates of approximately 170 °C/s in an argon atmosphere. Another key aspect of this work was the production of amorphous Si(Hf,Ta)(B)CN monoliths by warm pressing of precursor powders, followed by pressure less pyrolysis at 1100 °C. The compounds with the refractory metal ratio of Hf₀.₇Ta₀.₃, proved to be particularly promising, as monoliths with low open porosity and no cracks were obtained. These monoliths exhibited a thermal expansion coefficient (CTE) of 4.5 to 5∙10⁻⁶ K⁻¹, closely matching that of the intended MoSiBX substrate, thereby preventing delamination during thermal cycling. Additionally, a low thermal conductivity of ~0.6 to 1.2 Wm⁻¹K⁻¹ was measured via laser flash analysis (LFA) and calorimetry, which is lower than that of YSZ. The behavior of the amorphous samples under extreme conditions was another key focus. Oxidation tests in the temperature range of 1200 to 1500 °C, over 20 hours, exhibited a parabolic oxidation behavior. Furthermore, it was found that the addition of tantalum improved the oxidation resistance compared to Si(Hf,B)CN compounds, which is likely due to the formation of a stable mixed oxide phase with the composition Hf₆Ta₂O₁₇. In further oxidation experiments, a selected sample was exposed to extreme thermal stress using a plasma flame, simulating material resistance under high temperatures and plasma radiation, as encountered in rocket and jet engines, ultimately resulting in ablation. The depth of the resulting crater, which exhibited blister-like structures, was measured via profilometry to evaluate the time-dependent resistance to extreme heat. After 30 seconds of exposure, a cavity depth of 0.4 mm was measured, and the phase formation mirrored that analyzed in the oxidation experiments. The second focus of this work was the determination of the thermomechanical properties of the crystalline monoliths with low porosity. For this purpose, ceramic Si(Hf,Ta)(B)C powders, obtained by pyrolysis of modified SMP-10 precursors, were densified using Spark Plasma Sintering (SPS). The synthesis of these carbide-based powders has been extensively described in previous studies, so the focus of this dissertation was on their material properties. Through SPS sintering at 2200 °C, nanostructured composites with a composition of SiC/(Hf,Ta)C/(B)C and a low residual open porosity of 0.61 - 2.44 vol.% were produced. It was found that boron had a positive effect on the sintering behavior by promoting a finer phase distribution. Boron-free samples exhibited higher porosity and a microstructure in which remnants of the former powder particles were still visible. The homogeneous phase distribution in the boron-containing sample with the composition of Si(Hf₀.₀₄₁Ta₀.₀₁₄)B₀.₀₁C₁.₁₆N₀.₀₂O₀.₁₅ resulted in significantly lower thermal conductivity of 31.14 Wm⁻¹K⁻¹ and an exceptionally high hardness of ~31 GPa compared to that of the boron-free samples. The thermal expansion coefficient of ~4.5∙10⁻⁶ K⁻¹ was comparable to that of the amorphous samples, though the thermal conductivity was much higher. In 50- and 100-hour oxidation tests at 1200 and 1400 °C, conducted via thermogravimetric analysis (TGA), the SiC/(Hf₀.₇Ta₀.₃)C/C sample demonstrated the most favorable oxidation resistance among the examined series. This superior performance is attributed to the formation of the stable Hf₆Ta₂O₁₇ phase.