Oxide based ceramic matrix composites for high temperature aerospace sealing applications

This thesis looks at the viability of using oxide based ceramic matrix composites for high temperature gas turbine sealing applications. Two material systems namely Nextel312/SiOC, Nextel 312 fibre with SiOC matrix, and Nextel720/Alumina, Nextel 720 fibre with alumina matrix, were studied in detail and modified in order to produce CMC variants that could perform adequately as seals materials.
Nextel 312/SiCO composites were optimised for sealing applications at service temperatures of up to 900°C, using repeated matrix impregnation and pyrolysis (polymer infiltration processing, PIP) at different temperatures. The microstructures were analysed using SEM, TEM, XRD and X-ray computed tomography in order to quantify the differences arising from the manufacturing process. The materials were tested mechanically to determine their strength, ductility, interfacial shear strength and fracture behaviour. The thermal stability of the composites was investigated using thermo-gravimetric analysis (TGA). The objective was to identify the PIP temperature that could provide the optimum combination of properties of high elastic flexural deflection, thermal stability, low interfacial shear stress for toughening and low mass loss up to the intended service temperature. Processing at 950°C or above gave a combination of high effective elastic modulus, good resistance to mechanical damage from tensile strain and good thermal stability. The temperature capability of this material system, as studied in literature and by carrying out experiments in this piece of work, suggests that this material system would not be able to perform adequately as a seal at temperatures ≥ 900°C for extended periods, hence it was not investigated further in this study.
After identifying the key gaps in literature, an investigation was carried out that looked at the deformation and fracture of a 0/90° Nextel 720 alumina/Mullite fibre with alumina matrix ceramic-matrix composite material by in situ synchrotron X-ray computed micro-tomography at ambient temperature and at 1100°C. A three-point loading configuration was used and samples were loaded monotonically until final fracture. The flexural strength was found to be unaffected by temperature. 3D visualisation of the microstructure showed that fracture occurred by propagation of internal cracks at stresses close to the flexural strength. Shear fractures propagated across fibre bundles that were oriented perpendicular to the flexural stress at both room temperature and 1100°C. At 1100°C however, an additional failure mode of delamination was observed, in the form of needle shaped cracks in between fibre bundles parallel to x axis. This may arise from relaxation of thermal residual stresses or could be a function of the creep strains which could develop within the sample.
Once it was established that Nextel 720 with alumina matrix could perform adequately at the given temperature, 8 different variants, each with different fibre layup, weave pattern and processing temperature, were produced. Three-point bend tests were done at 1100°C, to quantify the effective flexural modulus along with the proportional limit stress, PLS, for each of the samples. The results suggested that out of all 8 samples, EF 20, which had a unidirectional fibre weave architecture, sintered at 1250°C, with an involute layup, had the most optimum mechanical properties.
Creep tests were carried out on beam specimens in three-point bend test configuration at 25MPa stress, at 1100°C. The results indicated that EF 20, due to its unidirectional weave configuration, developed lower primary and secondary creep strains, in comparison to EF 19 which had 50% fibre tows in the warp and the weft directions. The involute layup outperformed the 0/90° layup, due to higher shear strength between the layers. The samples sintered at 1250°C had similar creep performance to the samples sintered at 1200°C. Using the effective modulus values calculated for EF 19-1 and EF 20-1, piston rings with a gauge diameter of 140 mm were designed and subjected to a constant deflection condition with a nominal stress of 25 MPa, at 1100°C. The EF 19 ring performed adequately, only losing 13% of their spring at 1100°C. When the fitted stress was beyond the PLS limit permanent microstructural deformation was observed in the sample, EF 20, which resulted in higher stress relaxation than normal. Hence future ring design must be done keeping in mind the PLS. The manufacturing process should be reviewed, such that the machining of the piston ring does not result in cutting through the fibre layers, lowering the number of defects and consequently resulting in a higher PLS. This work was successful in identifying and modifying an oxide based ceramic matrix composite system for use in gas turbine sealing applications at 1100°C.

Keywords: Oxide-Oxide CMCs
X-ray tomography
Aerospace seals
Creep testing
Synchrotron
Ceramic Matrix Composites, Oxide-Oxide CMCs, Aerospace seals, Gas turbines, Creep, X-ray Tomography, TEM, SEM, Digital Volume Correlation, High Temperature X-ray Tomography, Synchrotron
Digital Image Correlation
Digital Volume Correlation
Elevated temperature X-ray Tomography
Advisor: Marrow, T
Galano, M