The deformation of transition metal carbides, borides and nitrides
Researcher: Philip Howie
Supervisors: Dr Bill Clegg (University of Cambridge) and Dr Luc Vandeperre (Materials)
Funding: Centre for Advanced Structural Ceramics
Philip Howie, supervised by Bill Clegg (Cambridge) in collaboration with Luc Vandeperre and Finn Giuliani (Imperial), is studying small-scale plastic deformation of brittle ceramics. The primary tool employed is the relatively new technique of micropillar compression (see for example Uchic et al., Science 305, 986-989), which allows plastic flow to be observed using simple uniaxial compression experiments in even the most brittle of materials.
Recent research has focussed on the nature of deformation in pillars. If micropillars are to be used in the study of plasticity, it is necessary to understand the limits of this behaviour. A focused ion beam microscope has been used to produce single crystal micropillars in single crystal samples, which have then been compressed using a nanoindenter and imaged using scanning electron microscopy. A size dependence in failure behaviour has been observed: pillars larger than a critical diameter deform by plastic slip followed by brittle cracking, whereas pillars smaller than this diameter behave entirely plastically. A previously published model (see Östlund et al., Phil. Mag. 91, 1190-1199) to explain this transition using conventional fracture mechanics has been expanded and generalised, demonstrating excellent agreement with experimental results in a range of brittle semiconductors and MgAl2O4.
Following on from this, a number of other fracture mechanisms have been observed and treated using the same principles of fracture mechanics. These include cracks in the side of the slipped portions of pillars and cracks in the top surfaces of pillars – see images. Good agreement between experiment and theory have been demonstrated in both cases. This work forms the basis of a paper recently submitted for publication and will be presented at the 12th Conference of the European Ceramics Society in June 2011.
These models are being used to test using micropillar compression experiments in a number of carbide and boride materials. These materials are technologically interesting in their own right and it is hoped that information on the preferred slip systems and true critical resolved shear stresses of these otherwise difficult-to-test materials will be obtained at a range of temperatures.