Deformation of Nanostructures and Small Volumes

Lead Research Organisation: University of Cambridge
Department Name: Materials Science & Metallurgy

Abstract

Nanotechnology has been identified world-wide as a crucial area for the advancement of scientific understanding with a clear route to improving quality. of life and creating wealth. However, the deformation behaviour of small volumes of material is not well understood, in particular the effect of the measured hardness increasing as the size of the indentation (and hence of the volume of materials deformed) decreases. Our interdisciplinary approach to this problem is unique internationally and uses techniques and concepts developed in semiconductor technology to solve fundamental problems in materials science and metallurgy.The key aims are to unify the very different theories that are used to explain the mechanical strength of nanostructures in different contexts. Critical thickness theory is highly developed as a way to understand the strained layers used in semiconductor technology. Strain gradient theory has been developed to explain the size effect in which small stressed volumes appear to be stronger. The work at QMUL has introduced two new concepts. The first is that the initiation of plasticity starts throughout a finite minimum volume. The second is that there is a minimum rate of relief of elastic strain energy required to initiate plasticity. These four ideas are undoubtedly different expressions of a single underlying principle, and our central aim in this proposal is to identify that principle through experiment and theoretical development.At Queen Mary, we will design structures to be grown at the Central Facility in Sheffield. We will carry out mechanical tests - nanoindentation, bending, at room temperature and high temperature - and we will use the results to guide theory. Through collaborators at Cambridge, we have access to unique facilities for looking at, for example, the material under a one-micron indent. Through collaborators in industry, we have access to the latest Xray techniques, for analysing, for example, the strains in a bent beam specimen.The total cost of the programme will be about 600000. This is worthwhile and timely since Nanoscale applications need to be underpinned by fundamental research in materials, where EPSRC observe that there are rich new areas for uncovering novel materials behaviour. This proposal falls under the themes of Nanostructured materials, and Materials phenomena and properties, in EPSRC's research priorities.

Publications

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Howie P (2011) Fracture modes in micropillar compression of brittle crystals in Journal of Materials Research

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Korte S (2008) Elastic and plastic properties of In x Ga 1- x As in Journal of Physics D: Applied Physics

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Korte S (2008) Discontinuous yield in InGaAs thin films in Surface and Coatings Technology

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Korte S (2011) Deformation of silicon - Insights from microcompression testing at 25-500°C in International Journal of Plasticity

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Korte S (2010) Onset of plasticity in InxGa1-xAs multilayers in Acta Materialia

 
Description The initial aim of the work at Cambridge was to understand the deformation of the coherently strained InxGa1-xAs multilayers, which had given rise elsewhere to the ideas on a minimum volume being required for deformation. While there must inevitably be a scale at which this happens, for instance as the sample size approaches the activation volume, the sample sizes here tended to be larger than this. An assumption of the initial analysis was that the yield stress did not vary with the In concentration in the layers. To investigate this, films of InGaAs were made with varying In concentrations. Using spherical indentation to determine the onset of yield, it was shown that both the elastic modulus and yield stress were influenced by the In concentration and that this assumption was incorrect.



The deformation of the multilayers was studied using spherical indentation. Similar results were obtained to those at QMUL. An essential point of the initial analysis was that the onset of deformation should occur in a volume extending further than a single layer thickness. However this proved impossible to see by examining the sample in the electron microscope after it had been deformed. Micropillar compression and nanoindentation experiments in the transmission electron microscope were therefore carried out at the National Centre for Electron Microscopy at L.B.N.L. in Berkeley, California. These showed that deformation was more prevalent in the weaker layer. Based on these results, a straightforward analysis to be developed, using the information gathered on the effect of In on the yield stress, where the yield pressure of the multilayer is related to the onset of flow in the weaker layer determined by both its intrinsic yield pressure and coherency strains. This gave good agreement both with our observations and with the previous experimental observations obtained elsewhere, as well as being consistent with the observations of the effects of internal stresses in films in the literature.



The observation that the micropillars could be plastically deformed without cracking led to the micropillar compression being investigated as a technique for studying plasticity in brittle materials without the requirement for high confining pressures. Work, in collaboration with the group of J. Michler at EMPA, CH, showed that such behaviour occurred below a critical pillar size and that this could be quantitatively explained by a decrease in the crack driving force in small pillars, consistent with observations in a range of systems. It was also shown, for the first time, that in single crystals individual slip systems could be interrogated, whereas in indentation multiple slip systems are required, that tests could be carried out over a range of temperatures, again for the first time, and that size effects in hard materials are much more limited at the length scales investigated than they are in soft metals.
Exploitation Route The techniques developed here enable the study of plastic flow in brittle materials. Normally such materials would break rather than plastically deform. However, the plastic flow behaviour can be of great importance both at low and high temperatures in structural applications, in hard coatings for cutting tools and even in electronic materials. This work is now being developed in a range of other projects, some with industrial funding and others with academics in the UK, Europe and the US.

The development of small-scale test techniques has, more recently, been used to study deformation in diamond, in collaboration with Element 6, whose main research centre moved from South Africa to Harwell in 2013. There has also been work studying the deformation of hard coatings in collaboration with the Singapore Institute for Manufacturing Technology (SIMTech). This has led to coatings being made, where recent work has shown that plastic flow requires dislocations to be nucleated within each 10 nm grain, giving rise to great improvements in wear resistance.
Sectors Aerospace, Defence and Marine,Energy

 
Description The observation that the micropillars could be plastically deformed without cracking led to the micropillar compression being investigated as a technique for studying plasticity in brittle materials without the requirement for high confining pressures. Work, in collaboration with the group of J. Michler at EMPA, CH, showed that such behaviour occurred below a critical pillar size and that this could be quantitatively explained by a decrease in the crack driving force in small pillars, consistent with observations in a range of systems. It was also shown, for the first time, that in single crystals individual slip systems could be interrogated, whereas in indentation multiple slip systems are required, that tests could be carried out over a range of temperatures, again for the first time, and that size effects in hard materials are much more limited at the length scales investigated than they are in soft metals.
Sector Aerospace, Defence and Marine
Impact Types Economic

 
Description Accelerated Metallurgy (ACCMET)
Amount € 700,000 (EUR)
Funding ID 263206 
Organisation European Commission 
Department Seventh Framework Programme (FP7)
Sector Public
Country European Union (EU)
Start 03/2011 
End 02/2016
 
Description Building New Capability in Structural Ceramics
Amount £77,009 (GBP)
Funding ID EP/F033605/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 07/2008 
End 06/2013
 
Description Doctoral Training Partnership(DTP) in Structural Metallic Systems for Gas Turbine Applications-Universities of Cambridge,Swansea and Birmingham
Amount £9,469,808 (GBP)
Funding ID EP/H022309/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 10/2009 
End 09/2014
 
Description HiTempProp
Amount £109,605 (GBP)
Funding ID 252520 
Organisation European Commission 
Department Seventh Framework Programme (FP7)
Sector Public
Country European Union (EU)
Start 10/2011 
End 03/2013
 
Description Structural Metallic Systems For Advanced Gas Turbine Applications
Amount £2,740,824 (GBP)
Funding ID EP/H500375/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 10/2009 
End 09/2014