Novel Measurement of Grain Boundary Fracture Toughness in Nuclear Ceramics

Ceramics are brittle materials with thermal and oxidation resistance properties that are critical to high temperature engineering. Their structural integrity depends on their intrinsic defects, such as pores, and their microstructure's resistance to crack propagation between and through the grains (i.e. intergranular and transgranular toughness, respectively). Intergranular fracture is strongly affected by grain boundary structure, chemical segregation and phase precipitation, and careful use of weak grain boundaries can actually improve the strength of ceramics through crack deflection.

The strength of a polycrystalline ceramic may be predicted from first principles via multi-scale modelling from the atomistic level up the microstructure (https://doi.org/10.1016/j.actamat.2019.03.021). However, there are almost no actual measurements of the properties of grain boundaries to test model fidelity, so such models are not yet effective for the design of novel tough materials for advanced high temperature applications.

We have applied direct measurement of elastic strain fields by high resolution electron-backscatter diffraction (https://doi.org/10.1016/j.jmps.2022.105173) to develop a novel nano-indentation method that evaluates the critical mixed-mode stress intensity factor for crack arrest in a residual stress field. This is a direct measurement of the fracture toughness that avoids the limitations of other micromechanical test methods.

This approach will be developed further and applied to investigate the relationships between grain boundary structure and strength in ceramics. The focus is on materials relevant to nuclear energy, particularly SiC and high entropy ceramics, where we will also examine the effects of ion irradiation that has been shown to affect the grain boundary chemistry (https://doi.org/10.1038/s41563-020-0683-y). This is relevant to the potential safety and performance of high temperature nuclear fuels. 

The project is suitable for graduates with an engineering, mathematical or physical sciences background.