Macroscopic scattering of cracks initiated at single impurity atoms

A single atomic defect is enough to deflect a crack as it travels through a crystal, according to research published in Nature Communications today that overturns the conventional wisdom about how materials fracture. But the research also reveals that, despite these defects, a swift fracture can produce atomically smooth surfaces, potentially useful for improving the efficiency of gem processing and making sharper diamond cutting tools.

Although real materials inevitably contain many defects, atomically smooth fracture surfaces are routinely obtained in manufacturing processes. This has led to the commonly accepted picture that individual atomic impurities are not able to deflect a propagating crack. An international team of researchers, based at King's College London and Imperial College London in the Rio Tinto Centre for Advanced Mineral Recovery and the Technion Institute in Israel, combined quantum-mechanical simulations and high-resolution experimental techniques to overturn this view, finding that a single ‘wrong’ atom in a crystal can in fact knock a moving crack out of its course, leading to microscopic ridges on the surfaces of the broken material. The team resolved the apparent paradox of how smooth surfaces can ever be obtained when breaking real materials by showing that the scattering mechanism switches off for fast-moving cracks.

The findings open the door to improved fracture-dependent manufacturing processes, such as the diamond cutting tools needed for high-precision surface finishes in many industries, where understanding that sufficiently fast cracks can guarantee a smooth surface regardless of any impurities could be extremely valuable. Other possible applications include turning the results on their head to allow impurity concentrations to be measured from controlled fracture testing, and the production of bespoke samples with finely controlled surface roughness, useful for calibrating friction models or to tune the interfacial properties of microelectromechanical systems.

Speaking about the research, TYC members Dr James Kermode and Prof Alessandro De Vita said: “This new finding is interesting in itself because it shows a previously undiscovered link between atomic defects and macroscopic results, but we are also excited about the practical applications in drilling and manufacturing.”

Figure: Cracks propagating through a silicon crystal can be scattered by a single isolated boron impurity (left, coloured orange), leading to submicron surface ridges which grow out of the cleavage plane (right, STM image, size 100 x 100 nm)

Journal link: Macroscopic scattering of cracks initiated at single impurity atoms.  J.R. Kermode, L. Ben-Bashat, F. Atrash, J. J. Cilliers, D. Sherman and A. De Vita.  DOI: 10.1038/ncomms3441

Contact details: For more details contact: Dr James Kermode, King’s College London, Department of Physics +44 (0)20 7848 2064 james.kermode@kcl.ac.uk

Kermode_DeVita_2013.jpg
6yBGvF_web.jpg

Follow @tyc_london for updates from the Thomas Young Centre.