Mechanistic Understanding of the Damage and Fracture in Ceramic-Matrix Composites under Extreme Conditions

Ceramic-matrix composites (CMCs) have the qualities of being strong, tough, lightweight and stable at high temperatures; they are considered as a serious material candidate to replace superalloys for many applications, such as in the core of gas-turbine engines with the aim of increasing operating temperatures, reduce the need for air cooling, and thus enable superior fuel efficiency to reduce harmful emissions. Over the last ~20 years, CMCs have been used in the augmentor sections of large military engines. Followed major investment from numerous companies and R&D organisations, mainly in the US, EU and Japan, new carbide technologies have been developed to aid the transition of CMCs to commercial gas-turbine engines, not to mention future applications in hypersonics. Despite the fast-growing CMC market, there is not yet an established CMC materials supply chain in the UK. Internationally, the design and processing of the CMCs also are still a challenge due to their complex microstructure (fibre, matrix and porosity). Therefore, there is an important opportunity for the UK to participate and ultimately lead, or at least share in, the global effort in CMC development. These are definitely the prime structural materials of the immediate future.

As a structural material, the mechanical performance of CMCs at elevated temperatures has been a critical factor for consideration in materials validation and adoption. To achieve an optimised design for a particular application, a sound understanding of the evolution of damage and failure mechanisms in CMCs, and how they relate to the intrinsic processing-microstructure-property relationships under extreme conditions, is undoubtedly the key. This sets the imperative and the horizon of the proposed work programme.

In this project, a unique and step-changing, real-time, 3D imaging method will be used to capture the deformation and fracture of CMCs at ultrahigh temperatures (~1000C to 1800C) representative of potential service conditions. By combining with techniques such as diffraction, micro-scale mechanical and multi-scale modelling methodologies, the underlying mechanics controlling the damage evolution in these materials at unprecedented temperatures can be understood and related to processing for improved material design.

The materials studied in this project will be processed or designed in the UK with the aim of enhancing UK-based industrial expertise in CMCs, but also access to international materials that are available. The primary materials of interest are two CMC types that are of high demand in aerospace, automotive and energy applications: continuous fibre reinforced SiC-SiC and alumina-alumina CMCs with the former being most important as a game-changer for advanced, lightweight, super-efficient propulsion units. However, compared to conventional superalloys, these materials are new; what has been lacking from a scientific perspective has been their characterisation in terms of two key aspects: (i) the local properties of the individual constituents, fibre/matrix interfacial strength and residual stresses in the fibre/matrix as a function of process parameters, and (ii) the real time imaging of their damage accumulation leading to crack initiation, in relation to their 3D microstructures, at realistic service conditions, i.e., ultrahigh temperatures, to simulate the working environment of these CMCs. This project will target at both material types with the support from materials processing partners (e.g., Birmingham Univ.) and end-users (e.g., Rolls-Royce plc, Cross-Manufacturing and Westinghouse).

Last but not the least, this project will work closely with modelling experts (e.g., Oxford Univ., Delft Univ. of Technology, and Institute Eduardo Torroja of Construction Sciences) by providing experimental results over multiple length-scales to develop a framework of a microstructure-based mechanistic model for the evaluation of the damage tolerance of CMCs.

The global ceramic-matrix composites (CMCs) market was valued at about $1.80 billion in 2015, among which aerospace applications generated a value of $773.6 million; it is anticipated it will reach $7.51 billion by 2026. SiC/SiC and oxide/oxide composites will remain preferred among end-user, which are also the primary materials of interest for this project. The lack of a supply chain, currently limits the UK involvement in the CMC international market.

In this project, the team will work closely with national and international end-users, such as Rolls-Royce plc, Cross-Manufacturing and Westinghouse, to ensure a direct impact of knowledge gained on the design of CMCs and its adoption in a wider range of components in both aerospace and clean energy sectors (e.g., nuclear and solar energy). As such, this project will bring benefit for the UK to continue building its supply chain and to secure an appreciable share in the global CMC market.

Since the research outcomes from this project are of interest to researchers from a much broader area than only CMCs including fracture mechanics, packaging of electronic devices, multi-scale modelling and so on, by publishing extensively in peer reviewed journals of the highest quality alongside our participation in international and UK conferences, this will maximise knowledge-based impact on academia.

As a class of light weight and fuel-efficient material, CMCs are the key to enable advanced technologies to reduce the UK carbon footprint in automotive, aerospace, nuclear, renewables and defence areas. By understanding how CMCs fail in service, the outcome of the project will directly support the safe adoption of this material to aid the UK achieving its CO2 reduction targets. The quality of living and health of the general public will benefit significantly from such research that fights against global warming.

The project will contribute to delivering highly skilled researchers with experimental, modelling and in particular CMCs expertise into the workforce by providing excellent multidisciplinary training for the PDRA and PhD student. Education and outreach activities are planned to communicate the research to general public to raise their awareness of CMC technology and composites in general. These will be achieved by organising hands-on activities at regular 'University Open Days', local Festivals, Soapbox Science to promote woman in Science and Engineering and so on. The PhD student and PDRA in the project will be encouraged to contribute to public engagement activities.