Micro-mechanical testing by fibre pushout of the BN interlayer in SiCf/SiC composites for aero-propulsion

Ceramic Matrix Composites (CMC’s) are finding renewed interest in the aerospace community for use as high temperature components in engines due to the potential for cooling air reductions over metallic parts, amongst other benefits such as weight saving and improving the turbine blade clearance. Quasi-brittle SiCf/SiC composites are toughened by the application of a boron nitride interphase coating to the fibre, which allows for cracks to deviate from the matrix. The principal issues faced by SiC-based composites lie in their degradation in corrosive environments (changing the interphase region and embrittling the overall composite) and their current inadequacy to adopt performance life models. Therefore, maintaining the interfacial properties of the composite at high temperatures is crucial. The extraction of these said properties has however proven itself to be a major engineering challenge in materials science. A few meso-scale and macro-scale techniques such as the transverse bend test and the Brazilian disc compression test have shown experimental reproducibility but are unsupported by sufficient modelling. The most accurate method for determining the properties at the micro-scale remains the push-out method on singular fibres. Herein the talk will present current both advances in using the fibre push-out method and some of the challenges to overcome with push-outs in order to accurately measure the interfacial shear stresses, coefficients of friction and residual compressive stresses at the fibre/matrix interface. The push-out method will be contrasted to the fibre push-back and push-in techniques and a novel \u27via\u27 push-out method will be introduced. Finally, suggestions for improving the method to corroborate with ongoing modelling work will be showcased

Micro-mechanical testing of the hex-BN interphase in SiCf/SiC composites for aero-propulsion

This thesis provides an overview of micro-level mechanical testing of the BN interphase in SiCf/BN/SiC composites, in the context of lifing models for aero-engine components. A wide number of variables were tackled in the development of both traditional and novel techniques to assess micro-level properties of the composites. The latter included the interfacial shear strength (varied), modulus (25.9 ± 5.4 GPa), hardness (1.3 ± 0.1 GPa), fracture toughness (6.7 ± 2.4 MPa.m1/2), debonding toughness (13.3 J.m-2), and frictional sliding resistance (4 ± 1 MPa) of the interphase, alongside other composite constituents where applicable. The use of a nanoindentation apparatus was central to this work – both Berkovitch and flat-punch tip geometries were made use of. Established techniques were assessed in their viability for probing the properties of the BN interphase on its own: these included the fibre push-out, push-in and push-back. Due to the sheer volume of data, the significance of results was assessed statistically. Upon completion of method development, material-dependent variables and their effects on the change in micro-mechanical behaviour were subsequently investigated. The parameters tackled included the variability in measurements from tow location at weave-architecture level (inter-tow), tow level (intra-tow), BN interphase thickness, nature of fibre type, cyclic testing, interphase doping levels and neighbouring fibre geometry; where it was found that a need for intra-tow variability was to be stressed over the importance of inter-tow variability in measurements of mechanical properties, although both were found to be statistically relevant. The ceramic-matrix composites were subsequently exposed to high relative humidity and elevated temperatures; and tests repeated. The degradation from intermediate-to-high temperatures showed little to no change in mircromechanical performance (strength retention ratio of 79.1%) when compared to low-temperature high humidity exposure (strength retention ratio of less than 50% after 500 hours exposure). Despite results from these chapters, a further understanding of the dominating failure mechanism needed to be understood. This led to the development of miscellaneous micro-mechanical techniques performed in-situ (both in SEM, micro and nano-XCT), including the novel trench push-out which imaged mixed-mode failure (simultaneous inside and outside debonding) and defective interphases leading to linking of smaller cracks before push-out. In the context of work by Rolls-Royce plc, the body of results yields both quantitative and qualitative information, for incorporation in coupon-level lifing models. The academic merit of this work lies in the investigation of variability in measurements of composite interphase properties, as well as micromechanical-technique development for a clearer understanding of the damage mechanisms at hand

Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells

Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short circuits at high rates of charge, is one of the greatest barriers to realizing high-energy-density all-solid-state lithium-anode batteries. Utilizing in situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li6PS5Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical ‘pothole’-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear; that is, the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode, and therefore before a short circuit occurs