The 11th annual European Nanomechanical User Group Meeting was held on Tuesday Sept 27th in Downing College, at the University of Cambridge.
The College’s neo-classical buildings and beautiful gardens, together with rare September sunshine, gave the meeting a very relaxed and reflective feel.
Session 1, chaired by Prof Bill Clyne of the University of Cambridge, was opened by Prof Konstantin Bouzakis of the Aristotles University of Thessaloniki. Prof Bouzakis spoke on ‘Innovative procedures for obtaining coating material properties based on nano-indentations and nano-impacts’. The talk featured a combination of experimental results and finite element analysis, covering fundamentals such as the effect of tip shape, right through to the effect of testing at high temperature. Also presented were investigations of brittleness and toughness using nano-impact.
The second talk, entitled ‘Nano-impact testing of EB-PVD TBC’s for prediction of erosion performance’ was presented by Prof Ben Beake of Micro Materials Ltd, who carried out the work in conjunction with researchers at the University of Birmingham and Cranfield University. The work presented showed an excellent correlation between wear predicted by nano-impact, and that previously recorded via erosion testing.
Dr Nigel Jennett of the National Physical Laboratory presented ‘New Methods – New Targets for Standardisation?’. In this comprehensive talk, Dr Jennett gave an update on the history of ISO 14577 so far, which is now in widespread use. He followed this with an overview of where further work was needed, and asked the audience for input regarding what other indentation methods would benefit from standardisation. Techniques such as high temperature nanoindentation, nano-impact, creep testing and property mapping were all considered.
Session 2, chaired by Dr Nigel Jennett, focussed on high temperature and creep studies. The session was opened by Dr Luc Vanderperre of Imperial College London, who spoke on ‘The effect of temperature and strain rate on the hardness of ceramic materials’. Dr Vanderperre presented hardness results from indentation studies performed on ceramic materials at ambient and elevated temperatures, and also discussed a new analysis method which allows the strain rate dependence of the hardness and yield strength to be estimated.
Dr Nicola Everitt of the University of Nottingham gave a very interesting talk on ‘Correcting time-dep
endant displacement effects in nanoindentation analysis’. The conventional method of analysing nanoindentation data, popularised by Oliver and Pharr, determines the Young’s modulus using a power law fit to the unloading data from the load displacement plot, and is based on Sneddon’s contact relations between an indenter and a flat surface. This leads to well documented problems when analysing materials which show time dependent depth changes whilst under load
Dr Everitt presented nanoindentation data from representative materials exhibiting visco-elastic and creep behaviour to examine the extent of the influence of time dependent data on the modulus and power law fit extracted from such data, allowing her to evaluate the success of different “correction” techniques in producing parameters within the expected limits for the different materials.
The techniques compared were conventional Oliver and Pharr analysis, a published “creep compliance” method for correcting time dependent depth changes, and a new analysis written by Norbert Schwarzer of the Saxonian Institute of Surface Mechanics which builds on Pharr’s concept of the effective indenter shape but also considers the change in change of the deformed surface with time as well as load (time dependent effective indenter analysis, TDEI).
All three methods were successful in calculating the modulus of a visco elastic polymer (polyamide 66) for both fast and slower the unloading rates. However the conventional Oliver and Pharr method and the creep compliance method gave the fitting coefficients of about 1.9 which is within the physical range of Sneddon’s contact equation but much higher than the 1.2-1.6 range found for room temperature metallic samples. In contrast the TDEI technique produced fitting coefficients of about 1.4, in line with what would be expected if analysing samples without visco-elastic effects.
Even when using a fast unloading rate, conventional analysis of data on a creeping metal (indentation at 650ºC on a P91 PM steel) leads to fitting coefficients outside the range of values predicted by Sneddon’s equations, and an unrealistically high modulus. TDEI produces realistic moduli values and fitting coefficients within the physical range of Sneddon’s equations.
Dr James Dean of the University of Cambridge presented a method for inferring the uniaxial creep behaviour from indentation data in the final talk before lunch, ‘Measuring creep parameters using nanoindentation and finite element modelling’. Creep dwell curves obtained from indentation experiments into copper were presented for a range of temperatures (-50°C to 200°C). These dwell curves were shown to contain information about the characteristic creep response of the copper being studied. Using iterative finite element simulations employing customised primary creep subroutines, the experimentally- measured creep dwell curves were simulated by carefully controlling the creep parameters defined in the subroutine. Dr Dean went on to comment on the validity of using these data to infer more conventional steady- state creep parameters such as the activation energy and the stress exponent.
Session 3, chaired by Prof Ben Beake of Micro Materials Ltd featured a range of talks on different application areas. The session was opened by Prof Dariusz Bieliński of Technical University of Łódź, who delivered an interesting talk entitled ‘Application of nanoindentation and nanotribology to study polymer materials’. Prof Bieliński gave an overview of the benefits and limitations of a range of polymer materials for application specific functions, and the methods available for improving nanomechanical performance.
He went on to discuss how nanoindentation and nanofriction can be used for studying various exploitation aspects of the modification of polymer materials, including fatigue and ageing, swelling and fuel resistance, curing and friction and wear.
Examples on the modification of windshield wipers, seals, hoses, coatings, drug powders, implants and dental composites were presented.
Prof Mohan Ranganathan of the University of Tours spoke on the ‘Global and local characterisation of lead-free solders’. The study featured work on the macroscopic and microscopic characterization of Sn-Ag-Cu solders used in microelectronic applications.
Results showed that the properties obtained from nanoindentation measurements can be different from those obtained from bulk specimens, and reasons for this were discussed. The main differences between global and local measurement results can be attributed to local micro-structure and, to some extent, on the test methodology and analysis techniques.
Jan Schröder of the University of Hamburg presented work on the ‘Nanomechanical properties of crosslinked gold nanoparticle films’ which were produced using a layer-by-layer spin-coating approach. This is an extremely efficient method to obtain reproducible films.
These films can be used as sensor-system, and it is important to know their mechanical properties.
Different thicknesses of films up to 500 nm were investigated via nanoindentation, as well as films crosslinked by different dithiols. The results showed a dependency on the length and the chemical structure of the crosslinker.
The speaker also showed that a greater distance between the particles results in a lower hardness of the film. The trend was clearly shown to be independent of the film thickness.
Session 4, chaired by Prof Konstantin Bouzakis, was opened by Prof Sarah Hainsworth of the University of Leicester. Prof Hainsworth delivered a talk on ‘Nanoindentation of diamond-like carbon films on steel substrates’. This talk focussed on the key parameters that can be obtained from nanoindentation, which help to understand how the DLC films will perform in automotive applications.
Methods for extracting key mechanical properties from the films were discussed along with microstructural and microchemical analyses of the films structures.
The final talk of the day came from Dr Tomasz Liskiewicz of the University of Leeds, who spoke on ‘Contact damage on Si(100) in nanoindentation, nano-scratch and nano-fretting’. Dr Liskiewicz described the importance of the tribological behaviour of Si-based MEMS devices where contact occurs during operation. Contact damage assessment by in-situ measurements of probe displacement during nano-fretting were supported by post-test SEM imaging and wear scar measurement by confocal microscopy. Pronounced lateral cracking observed in high load nano-scratch and nanoindentation tests was shown to be absent in the nano-fretting tests, which is consistent with the reciprocating wear process minimising the accumulation of strain.
With the exception of Professor Hainsworth, all of the work described was carried out on a NanoTest system from Micro Materials Ltd.
