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Professor of Materials Science

BSc University of Manchester
DPhil University of Oxford

Ceramics and Inorganic Materials

Research in the group, part of the Gordon laboratory, is focused on the micro-mechanics of brittle materials in four areas, much in collaboration with the Rolls-Royce UTC and researchers in Germany, Switzerland and Spain:

  • enhancing plasticity and toughness;
  • the deformation of layered structures;
  • abrasion at high temperatures and speeds;
  • the cracking and dispersion of colloidal diamond.

Enhancing plasticity and toughness

The aim is to understand the factors that control plasticity in harder materials, in particular how the ease of plasticity is related to the electronic structure, as well as how these might be manipulated to increase the toughness to improve reliability and performance of materials. This is being carried out using in-situ microcompression techniques, in collaboration with both RWTH Aachen, Germany and EMPA, Switzerland. Using digital image correlation, methods have been developed for measuring both yield stress and toughness in a wide range of brittle materials such as complex metallic alloys, with very large unit cells, hard coatings, as well as deformation in very hard materials, such as diamond and c-BN.

Deformation of layered structures

The behaviour of a range of layered structures, including both oxides and ternary carbides and nitrides is also being studied. Such materials often show a pronounced hysteresis. It is generally accepted that this is associated with the nucleation, growth and subsequent collapse of kink bands. Using in-situ synchrotron X-ray diffraction, in collaboration with Imperial College and Linköping University in Sweden, it has been shown that this behaviour arises quite naturally in the plasticity of a polycrystalline array.

Abrasion

Work on abrasion and abrasive systems is directed towards dramatically improving the materials available for preventing the flow of gas around turbine blade tips. This includes studying the processes that occur during abrasion at high temperatures and at high speeds, the properties required for abrasives operating under these extreme conditions, and how they can be fixed to the blade tip and a range of alloys capable of withstanding the extreme stresses and temperatures have been developed.

Dispersion and cracking in colloidal diamond films

Although often considered a purely elastic process, it has been shown that most of the work done during cracking is associated with irreversible processes at the crack tip, associated with the movement of colloid particles. Using colloidal diamond as a model system, work is underway to understand how this work is influenced by inter-particle forces.

Micrographs of a magnesium aluminate spinel pillars after deformation at 200 °C. The slip bands can be clearly seen on the surface of the pillar and are consistent with slip on {110}<1-10>

 

  • N.G. Jones, C. Humphrey, L.D. Connor, O. Wilhelmsson, L. Hultman, H.J. Stone, F. Giuliani, W.J. Clegg, "On the relevance of kinking to reversible hysteresis in MAX phases", Acta Materialia69 (2014) 149-161. doi: 10.1016/j.actamat.2014.01.045
  • S. Liu, J.M. Wheeler, P.R. Howie, X.T. Zeng, J. Michler, W.J. Clegg, "Measuring the fracture resistance of hard coatings", Applied Physics Letters, (2013), 102[17] (2013) 171907-4.doi: 10.1063/1.4803928
  • L. Goehring, W.J. Clegg, A.F. Routh, "Plasticity and fracture in drying colloidal films", Physical Review Letters, 110[2] (2013) 024301. doi: 10.1103/PhysRevLett.110.024301
  • S. Korte, W.J. Clegg, "Micropillar compression of ceramics at elevated temperatures", Scripta Materialia60[9] (2009) 807-810. doi: 10.1016/j.scriptamat.2009.01.029