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Hochtemperatur-Kriechverhalten SiOC-basierter Gläser und Glaskeramiken

Fachliche Zuordnung Mechanische Eigenschaften von metallischen Werkstoffen und ihre mikrostrukturellen Ursachen
Förderung Förderung von 2013 bis 2018
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 247069226
 
Erstellungsjahr 2019

Zusammenfassung der Projektergebnisse

The present project was focused on the assessment of the intrinsic (thermo)mechanical and thermal properties of SiOC glasses and glass ceramics. Particular attention was paid to the interplay between composition/microstructure of SiOC and their intrinsic properties. This was achieved by a systematic study of different series of well characterized SiOC materials synthesized at constant conditions. Systematic studies indicated that Young’s modulus, indentation hardness, creep resistance and viscosity of silicon carbides increase with increasing amount of Si-C bonds/β-SiC nanoparticles. In SiOC glasses, this is related to the increased network connectivity due to the substitution of two-fold coordinated oxygen by four-fold coordinated carbon atoms. For SiOC glass ceramics, Young’s modulus, indentation hardness, creep resistance and viscosity can be treated as additive functions of the volume fractions, shape and distribution of the constituting phases. Despite SiOC glass ceramics comprise a vitreous silica matrix (with lower performance in comparison to SiOC glasses), they are greatly benefiting from the good mechanical properties of the well dispersed β-SiC nanoparticles and are able to reach values comparable to SiOC glasses. In the case of creep resistance and viscosity, this is enabled by the presence of a strong interface between β-SiC nanoparticles and the vitreous silica matrix. However, SiOC glass ceramics cannot simply be interpreted as physical (additive) mixtures between SiC/C/SiO 2 giving linear dependencies between volume fraction and properties of the respective phases. Although many properties can be regarded as additive function of the constituting phases like in regular nanocomposites, creep resistance proves that the continuous partitioning of β-SiC nanoparticles during phase separation leads to a unique interface between β-SiC and vitreous silica matrix, giving rise to an outstanding creep behavior at high temperatures. For SiOC glasses, the thermal transport is reduced upon the incorporation of carbon atoms in comparison to vitreous silica. In contrast, in SiOC glass ceramics thermal transport is increased, when the volume fraction of β-SiC is high enough to participate in a thermally conducting percolating path. The segregated carbon phase is present in both SiOC glasses and glass ceramics, and contributes to the thermal transport as well as to the thermal expansion of SiOC glass ceramics. Moreover, the segregated carbon phase is responsible for the enhanced anelastic recovery of SiOC glass ceramics. The segregated carbon phase has only a moderate influence on Young’s modulus, creep resistance and viscosity. Young’s modulus is decreased upon incorporation of segregated carbon, whereas creep resistance and viscosity are increased. With the systematic knowledge of the intrinsic (thermo)mechanical and thermal properties of SiOC glasses and glass ceramics, future work can be focused on the fabrication of protective coatings based on silicon oxycarbides. The low thermal transport in SiOC materials is the basic requirement for efficiently creating a temperature gradient in this kind of coatings that reduce the temperature transfer to underlying layers. Such protective coatings consist of several layers taking over different functions and an evolving oxidic layer. Therefore, the thermal expansion plays a key role when working at high temperatures, where mismatches in CTE in the different layers lead to thermal stresses in the materials. The elastic properties of the individual materials determine the amount of stress the material can withstand without cracking. Finally, hardness and a high creep resistance are key factors for long-lasting protective coatings that can withstand external forces and stresses. The original goal of the modeling part of the project was to develop reliable bond-order potentials based on electronic-structure calculations and employ these for large-scale simulations to obtain atomistic insights into high-temperature creep behavior of SiOC ceramic glasses. However, our efforts to generate amorphous model systems that can be treated within the framework of electronic-structure calculations were plagued by inconsistencies between calculated and experimentally reported formation energies, unless finite clusters with additional hydrogen were considered. We therefore adjusted our work plan and used Cu-Zr glasses with additional crystalline secondary phases as generic models.

Projektbezogene Publikationen (Auswahl)

 
 

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