Mechanical and Microstructural Assessment of Inhomogeneities in Oxide Ceramic Matrix Composites Detected by Air-Coupled Ultrasound Inspection

Ceramic Matrix Composites (CMC) are promising materials for high-temperature applications where damage tolerant failure behavior is required. Non-destructive testing is essential for process development, monitoring, and quality assessment of CMC parts. Air-coupled ultrasound (ACU) is a fast and cost-efficient tool for non-destructive inspections of large components with respect to the detection of material inhomogeneities. Even though ACU inspection is usually used for visual inspection, the interpretation of C-scan images is often ambiguous with regard to critical defects and their impact on local material properties. This paper reports on a new approach to link the local acoustic damping of an oxide CMC plate obtained from ACU analysis with subsequent destructive mechanical testing and microstructural analyses. Local damping values of bending bars are extracted from ACU maps and compared with the results of subsequent resonant frequency damping analysis and 3-point bending tests. To support data interpretation, the homogeneous and inhomogeneous CMC areas detected in the ACU map are further analyzed by X-ray computed tomography and scanning electron microscopy. The results provide strong evidence that specific material properties such as Young’s modulus are not predictable from ACU damping maps. However, ACU shows a high, beneficial sensitivity for narrow but large area matrix cracks or delaminations, i.e., local damping is significantly correlated with specific properties such as shear moduli and bending strengths

Manufacturing of porous mullite fiber compacts by uniaxial hot pressing of semicrystalline MAFTEC® MLS-2 organic bound mats

Highly porous and fully crystalline mullite fiber compacts were fabricated by uniaxial hot pressing of stacked short-fiber mats without binders or sintering aids. The special feature of this new fabrication method is the use of MAFTEC organic bound mat (OBM) type fiber mats consisting of semicrystalline MLS-2 short fibers. In contrast to conventional polycrystalline mullite fibers, semicrystalline MLS-2 fibers consist of amorphous silica and nanoscale transitional alumina. Due to this special microstructure, MLS-2 fibers are less prone to fiber breakage upon shear load and therefore can well withstand pressure-assisted consolidation. Hot pressing of stacked OBM to highly porous fiber compacts is facilitated by the thermally driven softening of amorphous silica above 900 °C; favorable deformation and consolidation rates are achieved above 1050 °C. Above 1250 °C the mullite is crystallized at the expense of amorphous silica and simultaneously both deformation and consolidation comes to an end. Obtained MAFTEC OBM-derived fiber compacts consist only of crystalline mullites, and therefore mechanical properties are favorably retained at high temperatures

Protective Coatings for Enhanced Performance of Oxide-Oxide Composites

Ceramic matrix composites based on oxide fibers and porous oxide matrices are materials developed for various applications in harsh environments. State-of-the art oxide/oxide CMC are based on alumina or mullite fibers and porous alumina matrices. Thermal-chemical degradation of the fibers and matrices is associated with sintering/grain growth, surface reactions with mineral dusts or molten salts, and hydroxylation/volatilization of Si- or Al-hydroxide species. All mechanisms are typically relevant for applications in combustion atmospheres. Moreover, the relatively low mechanical stability of the porous oxide matrices makes CMC surfaces prone to surface degradation by abrasion. Degradation of CMC can effectively be mitigated by functional and protective oxide coatings which can be deposited e.g. by thermal spraying or in the form of particle dispersions and subsequent consolidation by sintering. Thermal protection as well as improved chemical stability are provided by low-thermal conductivity and environmentally stable coating materials. Additional benefits of ceramic coatings can be improved mechanical stability, reduced gas permeability, or thermo-optical properties for enhanced performance of all-oxide CMC in special applications. On the other hand, typical coating deposition processes are associated with additional thermal loads or surface effects which may significantly alter properties of the CMC substrate. Various types of oxide coatings for oxide/oxide CMC and their specific functionalities are presented. The impact of coatings on the overall performance of coated CMC materials is discussed

High-Temperature Behaviour of Polycrystalline Alumo-Silicate Fibers with Mullite Bulk Composition.

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Thermo-Mechanical Analysis of a Silicon Carbide Honeycomb Component Applied as an Absorber for Concentrated Solar Radiation

The study focuses on a numerical model describing the thermo-mechanical processes in an open volumetric solar absorber. In this application thermal loads of up to 850 kW/m² induce mechanical stresses inside the volume of the component. The objective of the study is to identify critical thermal load cases which may especially occur during start-up or shut down cases as well as during the transition of clouds. The study can be subdivided into three major parts. In a first step, the mechanical strength of the investigated materials has been determined in double-ring, three or four-point bending tests. In a subsequent numerical stress analysis the maximum stresses in the walls of the honeycomb structure at fracture load have been determined. The results of this combined experimental/numerical approach have been compared with the strength properties of dense materials. In a second step various transient and stationary load cases were considered and the corresponding temperature distributions were calculated. Thirdly, the thermo-mechanical model was used to determine the stress distributions which have been induced. Special attention was paid on the influence of the macro geometry and cell-density. The results of the study allow important conclusions for the operation of the receiver