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

Thesis

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.m^1/2), 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.

Authors: De Meyere, R

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: ceramic matrix composites; materials science; fibre push-out; aerospace; composites; silicon carbide; micromechanics
• Subjects: Aerospace engineering; Micromechanics; Ceramic materials; Materials at high temperatures