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SiHf(B)CN-based ultra-high temperature ceramic nanocomposites: Single-source precursor synthesis and behavior in hostile environments

Yuan, Jia (2015)
SiHf(B)CN-based ultra-high temperature ceramic nanocomposites: Single-source precursor synthesis and behavior in hostile environments.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Hf-containing ultra-high-temperature ceramics (UHTCs) are being pursued for Thermal Protection Systems (TPSs) for high-temperature applications (i.e., future hypersonic vehicles) in harsh environments. Most of these ceramic composites have been prepared using traditional powder techniques; however, the grain sizes of the resulting composites are limited to the micrometer range. Furthermore, nano-sized Hf-containing materials have proven to exhibit tremendously improved structural/functional properties, even at elevated temperatures, compared with microcomposite ceramics. Single-source precursors (SSPs) have yielded promising results in the processing of ceramic nanocomposites; moreover, these composites exhibit unique properties, e.g., high-temperature stability and high-temperature oxidation and corrosion. The objective of this work was to synthesize new Hf-containing ultra-high-temperature ceramic nanocomposites (UHTC-NCs) using SSP-based methods and to investigate their behavior in harsh environments. In the research presented in this PhD thesis, focus was first placed on the synthesis of novel Hf-containing SiHfCN and SiHfBCN amorphous UHTC-NCs derived from polysilazane. Amorphous SiHfCN and SiHfBCN ceramics were prepared from commercial polysilazane (HTT1800, AZ-EM), which was modified through reactions with Hf(NEt2)4 and BH3·SMe2 and subsequently cross-linked and pyrolyzed. The prepared materials were investigated with respect to their chemical and phase compositions using spectroscopic techniques (FTIR, Raman, and MAS NMR spectroscopy) and via X-ray diffraction (XRD) and transmission electron microscopy (TEM). Annealing experiments on SiHfCN and SiHfBCN samples in inert gas atmospheres (Ar and N2) at temperatures ranging from 1300 to 1700 °C revealed the conversion of the amorphous materials into nano-structured UHTC-NCs, whose high-temperature decomposition and crystallization were also investigated. It was found that β-SiC/HfCxN1-x nanocomposites were obtained from SiHfCN upon annealing at 1500 °C. Depending on the annealing atmosphere, HfCxN1-x/HfB2/SiC (annealing in argon) and HfNxC1-x/Si3N4/SiBCN/C (annealing in nitrogen) nanocomposites were obtained from SiHfBCN annealed at 1700 °C. The results demonstrate that the conversion of single-phase SiHf(B)CN into UHTC-NCs is thermodynamically controlled and thus offer insight toward the development of nano-structured ultra-high-temperature stable materials with tunable compositions. The second focus of the present study was the development of dense Hf-containing ceramic monoliths via pressureless sintering (PLS) or spark plasma sintering (SPS) and the development of ceramic matrix composites (CMCs) via polymer infiltration and pyrolysis (PIP) methods. Dense amorphous ceramic monoliths were prepared upon annealing pyrolytic ceramics in nitrogen at 1300 °C. Dense SiHfCN- and SiHfBCN-based UHTC-NCs were successfully prepared via SPS at 1850-1950 °C using high heating rates (~450 °C/min.) and high pressures (≥ 100 MPa). The obtained UHTC-NCs were investigated via spectroscopic analyses (XRD and Raman spectroscopy) and electron microscopy (SEM and TEM) with regard to their phase evolution and microstructure. Despite the very high sintering temperatures, the microstructures of the prepared dense UHTC-NCs remained rather fine, with grain sizes varying from 165 nm down to a few tens of nm. The hardness and elastic modulus of the dense SiHfCN were found to be 26.8 and 367 GPa, respectively, whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa (measured by nanoindentation). Additionally, Cf/SiCN and Cf/SiHfBCN CMCs were fabricated via a simple and low-cost PIP route. Cf/SiC-SiCN and Cf/SiC-SiHfBCN materials with pyrolytic carbon coatings were synthesized using hybrid techniques (CVI and PIP). The bending strength of the prepared CMCs resulted in the observation of brittle fracture surfaces only in the Cf/SiHfBCN material, indicating strong interfacial bonding between the fibers and the matrix; the much higher values of bending strength observed for Cf/SiC-SiCN and Cf/SiC-SiHfBCN resulted from the fact that weak interfaces (pyrolytic carbon) lead to transfer loading. This finding of the present work suggests that a single-source precursor route is suitable for the preparation of a variety of (ultra)-high-temperature ceramics, such as amorphous ceramics, UHTC-NC monoliths, and CMCs. Moreover, we explored the behavior of the prepared materials in harsh environments, e.g., their high-temperature stability with respect to decomposition and crystallization and their oxidation, corrosion and ablation behavior. High-temperature annealing experiments revealed that the SiHfCN and SiHfBCN materials exhibited improved high-temperature stability with respect to decomposition compared with non-modified SiCN. The oxidation behavior of the SiCN, SiHfCN and SiHfBCN ceramic powders was studied via thermogravimetric analysis (TGA) in air at 1200-1400 °C, revealing that the modified SiHfCN and SiHfBCN ceramics exhibited poorer oxidation resistance than that of SiCN. However, parabolic oxidation kinetics of SiHfCN and SiHfBCN were observed, wherein the parabolic rate (Kp) that was obtained from the equation K_p=〖 (∆m/(S_BET×m))〗^2×t^(-1) indicated that the amorphous SiHfBCN ceramic powder exhibited enhanced oxidation resistance compared with that of the SiHfCN. Furthermore, the oxidation behavior of SiHfBCN ceramic monoliths was investigated in a tube furnace (stagnant air, up to 100-200 h). The microstructure and phase composition of the monoliths’ oxide scale was investigated via XRD and microscopy (SEM, BSE and EPMA). The results revealed that the oxidation of the SiHfBCN ceramic monoliths followed typical parabolic kinetics, indicating that the oxidation diffusion was controlled by a passive oxide layer. However, the microstructure and composition of the oxide scale were strongly dependent on temperature. A continuous oxide layer, consisting of cristobalite and hafnia (m- and t- HfO2), was observed at 1200 °C; however, at 1400 °C, it became a discontinuous oxide layer and its composition changed to cristobalite, HfO2 and HfSiO4. Thus, the wide range of Ea values (174 and 140 KJ mol-1, depending on the Hf content) obtained from the apparent or corrected oxidation kinetics indicate the complex nature of their oxidation process, which might be the result of a wide variety of oxygen-controlling mechanisms in both the inward oxygen transport into the oxide scale (borosilicate or silica, hafnia, or hafnium silicate) and the outward transport of gas produced by oxidation reactions. Additionally, an investigation of the oxidation of the prepared dense UHTC-NCs at high temperature revealed that both samples exhibited parabolic behavior. Interestingly, the parabolic oxidation rates of the SiHfCN were comparable to those of other UHTCs (e.g., HfC-20 vol% SiC), whereas the parabolic oxidation rates of the SiHfBCN were 3 to 4 orders of magnitude lower. The results obtained in this study indicate that amorphous Hf-containing Si(Hf)BCN ceramics nanocomposites and nanoscale Hf-containing UHTC-NCs are promising candidates for high-temperature applications in harsh environments. The behavior of Cf/SiCN and Cf/SiHfBCN under subcritical hydrothermal conditions was also investigated at temperatures of 150-250 °C for exposure times of 48, 96 and 240 h. The effect of the ratio between the surface area of the sample and the volume of water used (S/V ratio) on the corrosion behavior of the prepared CMCs was analyzed. For S/V ratios greater than 0.18, the exposure of the CMCs to hydrothermal conditions led to a gain in mass, whereas at lower S/V ratios, a mass loss of the samples was recorded. Because the behavior of the studied samples was representative and reliable at small S/V ratios, both investigated CMC samples were concluded to exhibit active corrosion behavior in a subcritical hydrothermal corrosive environment. Based on the corrosion experiments performed at an S/V ratio of 0.075, the data for the mass loss as a function of the corrosion time and temperature were used to rationalize the corrosion kinetics of the Cf/SiCN and Cf/SiHfBCN samples. Both materials were shown to exhibit excellent stability under subcritical hydrothermal conditions. The corrosion rate of Cf/SiHfBCN was found to be lower than that of Cf/SiCN; furthermore, an SEM investigation indicated that spallation occurred in the Cf/SiCN samples, whereas the ceramic matrix remained attached to the individual carbon fibers in Cf/SiHfBCN. The results of the present study indicate that the incorporation of Hf and B into the SiCN matrix leads to significant improvement in its hydrothermal corrosion performance. Finally, the ablation mechanism of the Cf/SiHfBCN ceramic composites after treatment in a laser ablation environment was investigated. The microstructure and ablation behavior of this composite were studied using SEM combined with EDS. The formation of porous HfO2, molten HfO2 and SixOyHfz yielded fibers with good protection from oxidation and the laser beam. Three regions with different ablation behaviors are proposed based on the temperature distribution. The ablation center exhibited bubble-like structures, corresponding to the melting of HfO2 and SiHfxOy layers that covered the ends of the carbon fibers, and moreover, eroded carbon fibers that retained their original shape were also observed. In the transition region, carbon sheets and oxidation-product particles (HfCxOy and SiO2) peeled off from the eroded fibers and the matrix because of the high vapor pressure. Additionally, the growth of SiC grains and glass with bubble structure, corresponding to SiO2 with inclusions of B2O3 and SiO gas, was observed.

Typ des Eintrags: Dissertation
Erschienen: 2015
Autor(en): Yuan, Jia
Art des Eintrags: Erstveröffentlichung
Titel: SiHf(B)CN-based ultra-high temperature ceramic nanocomposites: Single-source precursor synthesis and behavior in hostile environments
Sprache: Englisch
Referenten: Riedel, Professor Ralf ; Yu, Professor Zhaoju
Publikationsjahr: 13 Oktober 2015
Ort: Darmstadt
Datum der mündlichen Prüfung: 12 Oktober 2015
URL / URN: http://tuprints.ulb.tu-darmstadt.de/5007
Kurzbeschreibung (Abstract):

Hf-containing ultra-high-temperature ceramics (UHTCs) are being pursued for Thermal Protection Systems (TPSs) for high-temperature applications (i.e., future hypersonic vehicles) in harsh environments. Most of these ceramic composites have been prepared using traditional powder techniques; however, the grain sizes of the resulting composites are limited to the micrometer range. Furthermore, nano-sized Hf-containing materials have proven to exhibit tremendously improved structural/functional properties, even at elevated temperatures, compared with microcomposite ceramics. Single-source precursors (SSPs) have yielded promising results in the processing of ceramic nanocomposites; moreover, these composites exhibit unique properties, e.g., high-temperature stability and high-temperature oxidation and corrosion. The objective of this work was to synthesize new Hf-containing ultra-high-temperature ceramic nanocomposites (UHTC-NCs) using SSP-based methods and to investigate their behavior in harsh environments. In the research presented in this PhD thesis, focus was first placed on the synthesis of novel Hf-containing SiHfCN and SiHfBCN amorphous UHTC-NCs derived from polysilazane. Amorphous SiHfCN and SiHfBCN ceramics were prepared from commercial polysilazane (HTT1800, AZ-EM), which was modified through reactions with Hf(NEt2)4 and BH3·SMe2 and subsequently cross-linked and pyrolyzed. The prepared materials were investigated with respect to their chemical and phase compositions using spectroscopic techniques (FTIR, Raman, and MAS NMR spectroscopy) and via X-ray diffraction (XRD) and transmission electron microscopy (TEM). Annealing experiments on SiHfCN and SiHfBCN samples in inert gas atmospheres (Ar and N2) at temperatures ranging from 1300 to 1700 °C revealed the conversion of the amorphous materials into nano-structured UHTC-NCs, whose high-temperature decomposition and crystallization were also investigated. It was found that β-SiC/HfCxN1-x nanocomposites were obtained from SiHfCN upon annealing at 1500 °C. Depending on the annealing atmosphere, HfCxN1-x/HfB2/SiC (annealing in argon) and HfNxC1-x/Si3N4/SiBCN/C (annealing in nitrogen) nanocomposites were obtained from SiHfBCN annealed at 1700 °C. The results demonstrate that the conversion of single-phase SiHf(B)CN into UHTC-NCs is thermodynamically controlled and thus offer insight toward the development of nano-structured ultra-high-temperature stable materials with tunable compositions. The second focus of the present study was the development of dense Hf-containing ceramic monoliths via pressureless sintering (PLS) or spark plasma sintering (SPS) and the development of ceramic matrix composites (CMCs) via polymer infiltration and pyrolysis (PIP) methods. Dense amorphous ceramic monoliths were prepared upon annealing pyrolytic ceramics in nitrogen at 1300 °C. Dense SiHfCN- and SiHfBCN-based UHTC-NCs were successfully prepared via SPS at 1850-1950 °C using high heating rates (~450 °C/min.) and high pressures (≥ 100 MPa). The obtained UHTC-NCs were investigated via spectroscopic analyses (XRD and Raman spectroscopy) and electron microscopy (SEM and TEM) with regard to their phase evolution and microstructure. Despite the very high sintering temperatures, the microstructures of the prepared dense UHTC-NCs remained rather fine, with grain sizes varying from 165 nm down to a few tens of nm. The hardness and elastic modulus of the dense SiHfCN were found to be 26.8 and 367 GPa, respectively, whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa (measured by nanoindentation). Additionally, Cf/SiCN and Cf/SiHfBCN CMCs were fabricated via a simple and low-cost PIP route. Cf/SiC-SiCN and Cf/SiC-SiHfBCN materials with pyrolytic carbon coatings were synthesized using hybrid techniques (CVI and PIP). The bending strength of the prepared CMCs resulted in the observation of brittle fracture surfaces only in the Cf/SiHfBCN material, indicating strong interfacial bonding between the fibers and the matrix; the much higher values of bending strength observed for Cf/SiC-SiCN and Cf/SiC-SiHfBCN resulted from the fact that weak interfaces (pyrolytic carbon) lead to transfer loading. This finding of the present work suggests that a single-source precursor route is suitable for the preparation of a variety of (ultra)-high-temperature ceramics, such as amorphous ceramics, UHTC-NC monoliths, and CMCs. Moreover, we explored the behavior of the prepared materials in harsh environments, e.g., their high-temperature stability with respect to decomposition and crystallization and their oxidation, corrosion and ablation behavior. High-temperature annealing experiments revealed that the SiHfCN and SiHfBCN materials exhibited improved high-temperature stability with respect to decomposition compared with non-modified SiCN. The oxidation behavior of the SiCN, SiHfCN and SiHfBCN ceramic powders was studied via thermogravimetric analysis (TGA) in air at 1200-1400 °C, revealing that the modified SiHfCN and SiHfBCN ceramics exhibited poorer oxidation resistance than that of SiCN. However, parabolic oxidation kinetics of SiHfCN and SiHfBCN were observed, wherein the parabolic rate (Kp) that was obtained from the equation K_p=〖 (∆m/(S_BET×m))〗^2×t^(-1) indicated that the amorphous SiHfBCN ceramic powder exhibited enhanced oxidation resistance compared with that of the SiHfCN. Furthermore, the oxidation behavior of SiHfBCN ceramic monoliths was investigated in a tube furnace (stagnant air, up to 100-200 h). The microstructure and phase composition of the monoliths’ oxide scale was investigated via XRD and microscopy (SEM, BSE and EPMA). The results revealed that the oxidation of the SiHfBCN ceramic monoliths followed typical parabolic kinetics, indicating that the oxidation diffusion was controlled by a passive oxide layer. However, the microstructure and composition of the oxide scale were strongly dependent on temperature. A continuous oxide layer, consisting of cristobalite and hafnia (m- and t- HfO2), was observed at 1200 °C; however, at 1400 °C, it became a discontinuous oxide layer and its composition changed to cristobalite, HfO2 and HfSiO4. Thus, the wide range of Ea values (174 and 140 KJ mol-1, depending on the Hf content) obtained from the apparent or corrected oxidation kinetics indicate the complex nature of their oxidation process, which might be the result of a wide variety of oxygen-controlling mechanisms in both the inward oxygen transport into the oxide scale (borosilicate or silica, hafnia, or hafnium silicate) and the outward transport of gas produced by oxidation reactions. Additionally, an investigation of the oxidation of the prepared dense UHTC-NCs at high temperature revealed that both samples exhibited parabolic behavior. Interestingly, the parabolic oxidation rates of the SiHfCN were comparable to those of other UHTCs (e.g., HfC-20 vol% SiC), whereas the parabolic oxidation rates of the SiHfBCN were 3 to 4 orders of magnitude lower. The results obtained in this study indicate that amorphous Hf-containing Si(Hf)BCN ceramics nanocomposites and nanoscale Hf-containing UHTC-NCs are promising candidates for high-temperature applications in harsh environments. The behavior of Cf/SiCN and Cf/SiHfBCN under subcritical hydrothermal conditions was also investigated at temperatures of 150-250 °C for exposure times of 48, 96 and 240 h. The effect of the ratio between the surface area of the sample and the volume of water used (S/V ratio) on the corrosion behavior of the prepared CMCs was analyzed. For S/V ratios greater than 0.18, the exposure of the CMCs to hydrothermal conditions led to a gain in mass, whereas at lower S/V ratios, a mass loss of the samples was recorded. Because the behavior of the studied samples was representative and reliable at small S/V ratios, both investigated CMC samples were concluded to exhibit active corrosion behavior in a subcritical hydrothermal corrosive environment. Based on the corrosion experiments performed at an S/V ratio of 0.075, the data for the mass loss as a function of the corrosion time and temperature were used to rationalize the corrosion kinetics of the Cf/SiCN and Cf/SiHfBCN samples. Both materials were shown to exhibit excellent stability under subcritical hydrothermal conditions. The corrosion rate of Cf/SiHfBCN was found to be lower than that of Cf/SiCN; furthermore, an SEM investigation indicated that spallation occurred in the Cf/SiCN samples, whereas the ceramic matrix remained attached to the individual carbon fibers in Cf/SiHfBCN. The results of the present study indicate that the incorporation of Hf and B into the SiCN matrix leads to significant improvement in its hydrothermal corrosion performance. Finally, the ablation mechanism of the Cf/SiHfBCN ceramic composites after treatment in a laser ablation environment was investigated. The microstructure and ablation behavior of this composite were studied using SEM combined with EDS. The formation of porous HfO2, molten HfO2 and SixOyHfz yielded fibers with good protection from oxidation and the laser beam. Three regions with different ablation behaviors are proposed based on the temperature distribution. The ablation center exhibited bubble-like structures, corresponding to the melting of HfO2 and SiHfxOy layers that covered the ends of the carbon fibers, and moreover, eroded carbon fibers that retained their original shape were also observed. In the transition region, carbon sheets and oxidation-product particles (HfCxOy and SiO2) peeled off from the eroded fibers and the matrix because of the high vapor pressure. Additionally, the growth of SiC grains and glass with bubble structure, corresponding to SiO2 with inclusions of B2O3 and SiO gas, was observed.
Alternatives oder übersetztes Abstract:
Alternatives AbstractSprache

Hf-enthaltende Ultra-Hochtemperatur-Keramiken (ultra-high-temperature ceramics, UHTCs) sind von Interesse für Temperaturschutzssysteme (thermal protection systems, TPSs) zur Anwendung unter hohen Temperaturen (z.B. zukünftige Überschall-Flugzeuge) in harsch Umgebungen. Die meisten solcher Keramik-Verbundwerkstoffe werden mit traditionelle Pulver-Verfahren hergestellt; allerdings beschränken sich die Korngrößen der entstehenden Verbundwerkstoffe auf den Mikrometer-Bereich. Darüber hinaus nanoskalige Hf-enthaltende Materialien weitreichend verbesserte strukturelle/funktionelle Eigenschaften auf, selbst bei erhöhten Temperaturen, im Vergleich zu den mikroskalige Verbundwerkstoffe. Einkomponentenvorstufen (single-source precursor, SSPs) lieferten vielversprechende Ergebnisse bei der Herstellung von Keramik-Nano-Verbundverkstoffen; jenseits dessen weisen diese komposite einzigartige Eigenschaften auf, z.B. Stabilität bei hohen Temperaturen sowie Oxidation und Korrosion bei hohen Temperaturen. Das Ziel dieser Arbeit war es, neue Hf-enthaltende Ultra-Hochtemperatur-Keramik-Nano-Verbundwerkstoffe (UHTC-NCs) mittels SSP-basierter Methoden herzustellen und ihr Verhalten in harsch Umgebungen zu untersuchen. Der Schwerpunkt der Forschung, welche in dieser Dissertation vorgestellt wird, wurde zunächst auf die Synthese neuartiger Hf-enthaltender amorpher UHTC-NCs basieren (auf) SiHfCN und SiHfBCN gelegt, welche aus einem Polysilazan abgeleitet wurden. Amorphe SiHfCN- und SiHfBCN-Keramiken wurden aus kommerziellem Polysilazan (HTT1800, AZ-EM) synthetischer, welches mit Hf(NEt2)4 und BH3·SMe2 modifiziert und anschließend vernetzt und pyrolysiert wurde. Die hergestellten Materialien wurden auf ihre chemische und Phasenzusammensetzung mittels Spektroskopische verfahren (FTIR, Raman- und MAS-NMR-Spektroskopie) als auch mittels Röntgenbeugung (X-ray diffraction, XRD) und Transmissionselektronenmikroskopie (TEM) untersucht. Auslagerung Experimente der SiHfCN- und SiHfBCN-Proben in einer Inertgasatmosphäre (Ar und N2) bei Temperaturen im Bereich von 1300 bis 1700 °C zeigten die Umwandlung der amorphen Materialien in nanostrukturierte UHTC-NCs, deren Zersetzung und Kristallisation bei hohen Temperaturen ebenfalls untersucht wurde. Es wurde festgestellt, dass β-SiC/HfC (N) Nanoverbundwerkstoffe beim Auslagerung bei 1500° C aus SiHfCN erhalt. Abhängig von der jeweiligen atmosphäre wurden, HfCxN1-x/HfB2/SiC (in Argon) und HfNxC1-x/Si3N4/SiBCN/C (in Stickstoff), Nanoverbundwerkstoffe aus SiHfBCN durch Auslagerung bei 1700 °C erzeugt. Die Ergebnisse zeigen, dass die Umwandlung von einphasigem SiHf(B)CN in UHTC-NCs thermodynamisch kontrolliert abläuft und somit Erkenntnisse im Hinblick auf die Entwicklung nano-strukturierter, bei hoher Temperatur stabiler Materialien mit einstellbaren Zusammensetzungen liefert. Der zweite Schwerpunkt der vorliegenden Studie war die Entwicklung sowohl von dichten Hf-enthaltenden Keramik-Monolithen durch druckloses Sintern (pressureless sintering, PLS) oder Spark Plasma Sintering (SPS) als auch Keramikmatrix-Verbundwerkstoffe (ceramic matrix composites, CMC) durch Polymer-Infiltration und Pyrolyse (PIP). Dichte amorphe Keramik-Monolithe wurden durch Auslagerung pyrolytischer Keramiken in Stickstoff bei 1300 °C erhalten. Dichte SiHfCN- und SiHfBCN-basierte UHTC-NCs wurden durch SPS bei 1850-1950 °C, hohen Heizraten (~450 °C/Min.) und hohen Drücken (≥ 100 MPa) erhalten. Die erzeugten UHTC-NCs wurden mittel Röntgenbeugung diffraktometrie und Raman-Spektroskopie und Elektronen-Mikroskopie (SEM und TEM) im Hinblick auf ihre Phasenentwicklung und Mikrostruktur untersucht. Trotz der sehr hohen Sintertemperaturen blieb die Mikrostruktur der dichten UHTC-NCs feinkörnig, mit Korngrößen von 165 nm bis zum niedrigen zweistelligen nm-Bereich. Die Härte-und Elastizitätsmodul des dichten SiHfCN lagen jeweils bei 26.8 und 367 GPa, während die SiHfBCN-Proben eine Härte von 24.6 GPa und einen Elastizitätsmodul von 284 GPa aufwiesen (durch Nanoindentierung gemessen). Weiterhin wurden Cf/SiCN- und Cf/SiHfBCN-CMCs über einen einfachen und kostengünstigen PIP-Weg erzeugt. Cf/SiC-SiCN- und Cf/SiC-SiHfBCN-Materialien mit pyrolytischer Kohlenstoffbeschichtung wurden mittels hybrider Methoden (CVI und PIP) hergestellt. Die Biegefestigkeit der hergestellten CMCs ergab, dass spröde Bruchoberflächen nur in dem Cf/SiHfBCN-Material festgestellt wurden, was auf eine starke Adhäsion zwischen den Fasern und der Matrix hinweist; die deutlich höheren Biegefestigkeitswerte für Cf/SiC-SiCN und Cf/SiC-SiHfBCN ergaben sich daraus, dass schwache Schnittstelle Faser-Matrix-Bindungen (pyrolytischer Kohlenstoff) zu einem Last-transfer führen. Die Erkenntnisse der vorliegende Arbeit legen nahe, dass sich die Route über Einkomponentenvorstufen für die Herstellung einer Bandbreite von (Ultra-)Hochtemperaturkeramiken eignet, wie z.B. amorphe Keramiken, UHTC-NC-Monolithe und CMCs. Darüber hinaus erforschten wir das Verhalten der SiCN- basieren Materialien in harschen Umgebungen, z.B. ihre Stabilität bei hoher Temperatur bezüglich Zersetzung und Kristallisation als auch bezüglich Oxidations-, Korrosions- und Ablationsverhaltens. Hochtemperatur-Glüh-Experimente ergaben, dass SiHfCN- und SiHfBCN-Materialien gegenüber nicht-modifiziertem SiCN verbesserte Hochtemperatur-Stabilität bei der Zersetzung aufwiesen. Das Oxidations-Verhalten der SiCN-, SiHfCN- und SiHfBCN-Keramikpulver wurde durch thermogravimetrische Analyse (TGA) in Luft bei 1200-1400 °C untersucht, wobei es sich erwies, dass die modifizierten SiHfCN- und SiHfBCN-Keramiken schlechteren Oxidationswiderstand als SiCN aufwase. Allerdings wurde auch die parabolische Oxidationskinetik von SiHfCN und SiHfBCN beobachtet, wobei eine parabolische Rate (Kp), welche aus der Gleichung K_p=〖 (∆m/(S_BET×m))〗^2×t^(-1) abgeleitet wurde, anzeigt, dass das amorphe SiHfBCN-Keramikpulver einen erhöhten Oxidationswiderstand im Vergleich zu SiHfCN aufwies. Weiterhin wurde das Oxidationsverhalten von SiHfBCN-Keramik-Monolithen in einem Röhrenofen (stehende Luft, bis zu 100-200 h) untersucht. Die Mikrostruktur und Phasen-Zusammensetzung der Oxidationsschicht der Monolithe wurde durch XRD und Mikroskopie (SEM, BSE und EPMA) untersuchen. Die Ergebnisse besagen, dass die Oxidation der SiHfBCN-Keramik-Monolithe der typischen parabolischen Kinetik folgt, was darauf hindeutet, dass die Diffusion der Oxidation von einer passivieren Oxidschicht kontrolliert wurde. Allerdings waren die Mikrostruktur und Zusammensetzung der Oxidschicht stark von der Temperatur abhängig. Eine durchgehende Oxidschicht, aus Cristobalit und Hafnien (m- und t-HfO2), wurde bei 1200 °C analysiert; jedoch entstand bei 1400 °C eine brüchige Oxidschicht, und ihre Zusammensetzung wechselte zu Cristobalit, HfO2 und HfSiO4. Entsprechend kann die große Bandbreite an Ea-Werten (174 und 140 KJ mol-1, abhängig von dem Hf-Anteil), auf komplexe Eigenschaften das Oxidationsprozesses hinweisen. Diese können sich aus einer großen Vielfalt sauerstoff-kontrollierender Mechanismen ergeben, sowohl bei dem Sauerstofftransport in die Oxidschicht hinein (Borosilicate oder Silica, Hafnia oder Hafnium-Silicate) als auch bei dem Transport von Gasen, die durch Oxidationsreaktionen entstehen, nach außen. Zusätzlich hat eine Untersuchung der Oxidation der vorbereiteten dichten UHTC-NCs bei hohen Temperaturen gezeigt, dass die Proben parabolisches Verhalten aufweisen. Interessanterweise waren die parabolischen Oxidationsraten von SiHfCN mit denen anderer UHTCs (z.B. HfC-20 vol% SiC) vergleichbar, während die parabolischen Oxidationsraten von SiHfBCN 3 bis 4 Größenordnungen niedriger lagen. Die in dieser Studie gewonnenen Ergebnisse deuten darauf hin, dass amorphe Hf-enthaltende Si(Hf)BCN-Keramiken und nanogroße Hf-enthaltende UHTC-NCs vielversprechende Kandidaten für Hochtemperaturanwendungen in harschen Umgebungen sind. Das Verhalten von Cf/SiCN und Cf/SiHfBCN unter subkritischen hydrothermalen Bedingungen wurde bei Temperaturen von 150-250 °C mit Expositionszeiten von 48, 96 und 240 h untersucht. Die Auswirkung des Verhältnisses zwischen der Oberfläche der Probe und des verwendeten Wasservolumens (S/V-Verhältnis) auf das Korrosionsverhalten der CMCs wurde analysiert. Bei S/V-Verhältnissen oberhalb von 0.18 führte die Exposition der CMCs bei hydrothermalen Bedingungen zu einer Zunahme der Masse, während ein Masseverlust der Proben bei niedrigeren S/V-Verhältnissen festgestellt wurde. Da das Verhalten der untersuchten Proben bei niedrigen S/V-Verhältnissen repräsentativ und verlässlich ist, wurde bei beiden untersuchten CMC-Proben darauf geschlossen, dass sie ein aktives Korrosionsverhalten in subkritischen hydrothermalen Korrosionsumgebungen aufwiesen. Auf Grundlage der Korrosionsexperimente, die bei einem S/V von 0.075 durchgeführt wurden, wurden die Daten des Masseverlusts als eine Funktion der Korrosionszeit und -Temperatur verwendet, um die Korrosionskinetik der Cf/SiCN- und Cf/SiHfBCN-Proben zu rationalisieren. Beide Materialien wiesen exzellente Stabilität bei subkritischen hydrothermalen Bedingungen auf. Die Korrosionsrate von Cf/SiHfBCN erwies sich als niedriger als die von Cf/SiCN; weiterhin legte eine SEM-Untersuchung nahe, dass es bei den Cf/SiCN-Proben zu einer Aufsplitterung kam, während die Keramikmatrix in Cf/SiHfBCN weiter mit den einzelnen Kohlenstofffasern verbunden blieb. Die Ergebnisse der vorliegenden Studie deuten darauf hin, dass die Einbindung von Hf und B in die SiCN-Matrix zu einer deutlichen Verbesserung ihres hydrothermalen Korrosionsverhalten führt. Schließlich wurde der Ablationsmechanismus von Cf/SiHfBCN-Keramikverbundwerkstoffen in einer Laser-Ablations-Umgebung untersucht. Die Mikrostrukturen und das Ablationsverhalten dieser Verbundwerkstoffe wurden mittels SEM in Kombination mit EDS untersucht. Die Bildung von porösem HfO2, geschmolzenen HfO2 und SixOyHfz gab den Fasern einen besseren Schutz vor oxidation und dem Laserstrahl. Drei Regionen mit verschiedenem Ablationsverhalten werden gemäß der Temperaturverteilung an der Probe vorgeschlagen. Das Ablationszentrum wies blasenähnliche Strukturen auf, die sowohl geschmolzenem HfO2 entsprechen als auch den SiHfxOy-Schichten auf den Enden der Kohlenstofffasern; des Weiteren wurden erodierte Kohlenstofffasern gefunden, die ihre ursprüngliche Form beibehalten halten. In der Übergangsregion lösten sich Kohlenstoffschichten und die Partikel der Oxidierungsprodukte (HfCxOy und SiO2) aufgrund des hohen Dampfdrucks von den erodierten Fasern und der Matrix. Weiterhin konnte das Wachstum von SiC-Körnern und Glas mit Blasenstruktur gefunden, welches SiO2 mit Einschlüssen von B2O3- und SiO-Gas entspricht.

Deutsch
URN: urn:nbn:de:tuda-tuprints-50074
Sachgruppe der Dewey Dezimalklassifikatin (DDC): 600 Technik, Medizin, angewandte Wissenschaften > 600 Technik
Fachbereich(e)/-gebiet(e): 11 Fachbereich Material- und Geowissenschaften
11 Fachbereich Material- und Geowissenschaften > Materialwissenschaft
11 Fachbereich Material- und Geowissenschaften > Materialwissenschaft > Fachgebiet Disperse Feststoffe
Hinterlegungsdatum: 18 Okt 2015 19:55
Letzte Änderung: 18 Okt 2015 19:55
PPN:
Referenten: Riedel, Professor Ralf ; Yu, Professor Zhaoju
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: 12 Oktober 2015
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