Mechanical and Thermal Properties of SiOC-based Glasses and Glass Ceramics

Polymer-derived silicon oxycarbides (SiOC) exhibit improved mechanical properties in comparison to vitreous silica, a unique crystallization resistance and excellent stability in harsh environments. Consequently, silicon oxycarbides are potential candidates for high- temperature applications, for example in ceramic heaters, high-temperature reactors, combustion engines or as part of thermal protection systems. For these applications, the precise knowledge of the mechanical properties like hardness, elasticity and creep, but also of the thermal properties like thermal conductivity and thermal expansion is of paramount interest. In the present study the intrinsic mechanical and thermal properties of silicon oxycarbides were systematically assessed in order to obtain a fundamental understanding concerning the relationship between their phase composition, microstructure and properties. Therefore, a SiOC glass and a series of SiOC glass ceramics with varying compositions were synthesized and carefully characterized. It is demonstrated that the concept of phase separation (i.e. glass vs. glass ceramic) is important in SiOC materials. It has a large impact on thermal expansion, thermal transport and the activation volume carrying deformation at high temperatures (as expressed by the activation energy for creep). Furthermore, it is shown, that upon the proper choice of composition and microstructure, tailored mechanical and thermal properties can be realized within the SiOC system: (i) Increasing amounts of Si-C bonds in SiOC glasses or β-SiC nanoparticles in SiOC glass ceramics leads to an increase of Young’s modulus, indentation hardness, creep resistance and viscosity due to an increase of the glass network connectivity in SiOC glasses and the homogeneous distribution of β-SiC nanoparticles with good mechanical properties, respectively. On the other hand, the incorporation of Si-C bonds reduces the thermal transport in SiOC glasses as lower mass fractal networks and defects/oxygen vacancies are formed. However, amounts > 20 vol.% β-SiC nanoparticles lead to an increase of the thermal transport in SiOC glass ceramics. (ii) The high aspect ratio segregated carbon phase leads to a significant increase in thermal transport as well as in thermal expansion of SiOC materials already for small amounts. It has a moderate influence on Young’s modulus (decrease), creep resistance and viscosity (increase) in comparison to Si-C bonds/β-SiC nanoparticles, whereas hardness remains unbiased. The segregated carbon phase is responsible for the enhanced anelastic recovery of SiOC glass ceramics.