Heterogeneity in Hardmetals and Superhard Materials
For a decade or more the European Hard Materials Group has promoted regular discussion meetings (WINTEREV/SUMMEREVs) to facilitate discussion of topical research items, such as mechanical and microstructural test methods, amongst, primarily, a European community of research institutions and industry. SUMMEREV 18 addressed a broadly defined topic of heterogeneity.
Hardmetals and Superhard Materials are well known for their supreme wear resistance. Improving their current suite of properties to provide industry with quality tools with high reliability is a major challenge for the industrial and academic research community internationally active in this technology. Much of what is known about hard tools’ underpinning microstructure is supported by knowledge of the composite behaviour at a mesoscopic scale; i.e. what might be termed the Representative Volume Element-RVE. Arguably, much less is known about the local and macroscopic variability in properties, for example what we might describe as heterogeneity of the structure – but on many scales. For example: at the nano (dislocations, phase orientation, phase interfaces, precipitates, solid state chemistry), micro (grain and phase size and shape); meso (crack propagation across phases and phase boundaries) and not least at the macro scale (variability in properties between different zones of graded structures and at or near coating/substrate interfaces).
The nature of interfaces is fundamental at all scales. It can be between contingent grains of dissimilar orientation, between different phases, between coatings and substrate or between macro-regions in graded structures. Effective research will need to cover both experimental and modelling approaches. Hard materials are by their nature heterogeneous in structure and at the micro and meso scale the size dependence and the discrete nature of deformation (often under constraint) together with damage and fracture within individual phases controls performance. Capturing microscopic phenomena and bringing them to the macroscale is essential for the correct interpretation of strength, toughness, wear and fatigue degradation phenomena in Hardmetals and Superhard Materials.
Identification and quantitative description of physical mechanisms that control defect and damage development is a major challenge. It is particularly important to apply advanced characterisation techniques such as in situ scanning electron microscopy (SEM) testing, microscale testing, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), focussed ion beam (FIB), and 3-D tomography to provide equivalent information to that yielded by optical and conventional electron microscopy. Extensive and detailed use of these techniques together with appropriate modelling tools helps to characterise the deformation behaviour and operative mechanisms at the relevant microstructural scale for Hardmetals and Superhard Materials.
SUMMEREV 18, held at KTH Royal Institute of Stockholm, Sweden, focused on a range of these topics, with special emphasis on the characterisation of heterogeneity.