Posted on 09/11/2010

The Tenth MML European User Meeting took place on Oct 27th - 28th 2010 at Politecnico di Milano, Milan. The meeting was highly successful, with excellent speakers providing a useful insight into the relevance of nanomechanics in a whole host of diverse application areas. We thank all those to contributed and all the delegates for their attendance and finally we would like to thank Prof Pasquale Vena and his group at Politecnico di Milano for hosting us.

The meeting was opened by Paul Grasske, Managing Director of Micro Materials Ltd. The recent news of MML returning to private ownership was shared as this has brought new investment to the organisation. There was also a short introduction to the excellent benefits of the improved Platform 3 of the NanoTest system and a round up of all the new European NanoTest users.

This was followed by a short welcome talk by Professor Pasquale Vena, Politecnico di Milano who spoke about the diverse research lines at the Department of Structural Engineering (Dipartimento di Ingegneria Strutturale) and in particular the Laboratory of Biological Structure Mechanics (LaBs), established in 2000. The research topics include the area of Micromechanics of Biological Structures and Biomaterials that rely on several experimental facilities to match their research level, including MML's NanoTest.

The opening session of the meeting was chaired by Professor Pasquale Vena of Politecnico di Milano.

Prof Bill Clyne of The Gordon Laboratory, University of Cambridge opened the session with a presentation on high strain rate impact indentation and the use of quasi-static indentation for residual stress measurements. He showed that pendulum based indentation, which the NanoTest employs, provides the flexibility to carry out nanoindentation in impact mode, with some challenges in the modelling.

Prof Clyne concluded that it should be possible to infer constitutive relations, including strain rate dependence, from impact indentation data, using displacement histories & residual indent shapes and although sensitivities are likely to be relatively low at the moment, good progress is being made.

Dr Alessandro Patelli of CIVEN, Venice, Italy presented the use of nanoindentation and microscratch in the optimisation and characterisation of coatings for industrial applications and gave an overview of the different coatings produced at CIVEN and their materials characterisation.

Dr Patelli discussed the many industrial needs for different substrates for specific applications, in particular the plastics, mechanics and production processes involved in optimising nanostructured hard coatings, barrier coatings and antiscratch coatings. After thermal annealing, nanoindentation showed an increase in hardness for all coatings and micro scratch showed a critical load decrease due to internal stresses. In micro scratch tests, high TiB2 content showed that fracture is typically fragile exhibiting buckling and spallation. With patented anti-scratch coatings, Dr Patelli showed how there is a positive correlation between scratch resistance and layer thickness and used the additional NanoTest output of friction results to support this. The effects of hot embossing on the change in mechanical properties of the coatings before the glass transition temperature were illustrated from results extracted from high temperature indentation tests. Overall, Dr Patelli concluded that the nanoindentation and micro scratch tests are fundamental tools for the development of such coatings.

Dr Vineet Bhakhri of Imperial College London presented the mechanical deformation behaviour of ceramic and metallic systems at elevated tempatures. He showed that by performing nanoindentation tests at several temperatures up to (500°C) 773K with a high temperature MML NanoTest indenter, average thermal drifts of 0.02 nm/sec at (200°C) 473K and 0.06 nm/sec at 773K were recorded.

In the metallic system where Gold was used, a Haasen plot activation analysis of the deformation process indicated that more mechanical work must be applied during the constant-loading rate stage of the nanoindentation test, due to the predominance of work hardening, compared to the constant-load stage where considerably more dislocation recovery occurs. Supporting TEM investigations confirmed that dislocation interactions were the deformation rate controlling obstacles at high temperature.

In the ceramic system, where Ti_0.44 Al_10.56N coating was used, both the Elastic modulus and Hardness remained constant over the tested range of temperatures from (27°C) 300K to (350°C) 623K. In this case it was found that lattice resistance glide is the deformation rate controlling mechanism. Dr Bhakhri concluded that the MML high temperature nanoindentation technique used was sufficiently precise in the investigation of the kinetics of plastic deformation in the ceramic and metallic systems.

The second session of the day was chaired by Prof Bill Clyne of the University of Cambridge.

Prof Edoardo Bemporad of Universita degli studi Roma Tre opened the session with a talk on the preparation of milled micropilars in a 3.8μm CAE-PVD TiN coating on a WC-Co substrate, an an innovative combination of focused ion beam (FIB) machining and subsequent nanoindentation testing to study the effect of surface elastic residual stress on their nanomechanical behaviour.

Prof Bemporad mentioned that nanoindentation techniques can represent an alternative in the field of stress measurement at the micro-scale due to its relative speed, cost, automation and ease of coupling with FIB.

The average residual stress was calculated by two different sets of load-depth curves, the first one obtained at the centre of stress relieved pillars and the second one from the undisturbed (residually stressed) surface. In addition, nanoindentation on stress relieved pillars also enabled a more accurate evaluation of Young's modulus and the Hardness of the coating.

High load nanoindentation and application of energy methods for fracture toughness evaluation were used to study the effect of residual stress on crack propagation modes. It was found that compressive residual stress plays a relevant role in determining the fracture behaviour and failure modes of the coating. The microstructural observations by TEM analysis on the cross section of the indentation showed plastic deformation at the nano-scale occurs by formation of shear bands inside the columnar grains, independently of residual stress. Prof Bemporad concluded that the method shows good promise and future work involves the application of this method to metallic and amorphous materials and multiphase materials (for intergrannular stress evaluation).

Dr Sandra Korte of University of Cambridge presented a talk which looked at microcompression testing at elevated temperatures and in particular the effect of size on cracking in ceramics and the results and considerations for conducting microcompression experiments on silicon micropillars at temperatures up to 500°C degrees.

With MML's NanoTest, uniaxial microcompression tests without confining pressure were used to avoid any affect on deformation mechanisms, and preferred for characterization of individual slip sytems since with nanoindentation, confinement of the surrounding material would mean all slip systems would be activated regardless of the critical resolved shear stress and phase transformation under pressure. The suspected transition in dislocation structure was studied through the preparation of TEM specimens.

Dr Korte also showed that testing at high temperatures is necessary due to thermal activation of dislocation motion against the resistance of the crystal lattice in bcc metals and ceramics. The results showed reliable stress-strain data at upto 500°C, 2μm diameter micro pillars that were plastic from 200°C without confining pressure (300 nm diameter at RT) and yield stresses that were in good agreement with literature, particularly at high stresses and again, this was supported by successful TEM characterisation.

Dr Nick Bierwisch of Saxonian Institute of Surface Mechanics (SIO) presented complimentary software developments which correct the time dependent displacement effects in nanoindentation analysis.

Dr Bierwisch stated the importance of such a correction between analysis and real-coating substrate systems especially at high temperatures due to the temperature dependency of mechanical properties, in particular Young's modulus, Yield strength and Hardness.

In the determination of physical parameters by nanoindentation, Dr Bierwisch showed how SIO have extended the standard Oliver & Pharr analysis method which gives only effective Young's modulus and Hardness to include coatings that may be inhomogenous due to layers and gradients and also for time dependent material behaviour due to creep and high temperature. Dr Bierwisch concluded by saying that these methods are simpler and provide new possibilites in the fields of surface testing and material optimisation.

Prof Ben Beake of Micro Materials Ltd concluded the second session by speaking on nanoindentation creep on viscoelastic materials above and below the glass transition temperature.

Prof Beake showed that since accurate modelling of the viscoelastic properties of polymeric materials rely heavily on the quality of experimental data then important tests such as the constant load creep tests carried out on PMMA can help identify the influence of instrumental stability on the accuracy of modelling indentation creep.

The first indication of loss of modulus is from equating the unloading modulus with the storage modulus and tan delta from the strain rate sensitivity calibration chart since for a given indenter geometry there is an approximate correlation between the strain rate sensitivity parameter and tan delta. Tests on uniaxially drawn PET films at elevated temperatures show a peak in the loss modulus in the vicinity of the glass transition. Prof Beake also spoke on how the mechanical properties and creep behaviour of atactic-polypropylene were studied around its glass transition temperature using the sub-ambient temperature capability of the NanoTest. These tests showed an influence of the glass transition on the creep response and indirectly onto Hardness and Young's modulus from the unloading analysis.

The third session of the day was chaired by Dr Marc Masen of the University of Twente.

Prof Mojtaba Ghadiri of the University of Leeds opened the session with a presentation on the fracture behaviour comparison of Aspirin under quasi-static indentation and single particle impact loading and the effect of cleavage planes, an investigation that has been carried out with the use of MML's NanoTest NTX3. Prof Ghadiri spoke about how under both quasi-static testing conditions, using nano-indentation and dynamic impact tests, Aspirin demonstrated a stong anisotropy in its fracture behaviour. During nano-indentation tests on the (100) and (001) faces, cracks propagate in the [010] direction on the (001) and (100) fracture planes, respectively. While the Hardness is found to be similar for both faces, slip occurs more readily on the (100) then (001) plane, suggesting the former to be the preferred slip plane. Furthermore, the fracture toughness of the (001) plane was distinctively lower than that of the (100) face, which indicated that the (001) is the preferred cleavage plane of the material. Observations of the damage morphology of the particles after dynamic impact testing showed that both chipping and fragmentation of Aspirin occur via cleavage planes, which is in good agreement with observed fracture mechanisms during nano-indentation. Prof Ghadiri concluded by saying that the presence of cleavage planes is a dominant factor in the fracture mechanism of Aspirin under quasi-static and impact loading conditions and therefore when nanoindentation is used as a predictive tool for breakage mechanisms under impact loading conditions, the fracture anisotropy of a material should be considered.

Dr Davide Carnelli of Politecnico di Milano detailed the work he has carried out on nanoindentation tests, using MML's NanoTest, on tissue engineered rabbit bone with ASCs (adipose-derived stem cells)that were used for regeneration of full thickness bone defects in proximal epiphysis of tibia of twelve New Zealand rabbits. The defects were implanted with graft material in two scaffold treatment groups: a empty hydroxyapatite disk and then in a hydroxyapatite disk seeded with ASCs and although both showed similar mineral properties after 8 weeks, the result of histological analyses showed that the osteogenic abilities of the scaffold-treated defects were greater than the scaffold free samples (those with untreated defects). Nanoindentation tests (maximum loads 1mN, 5mN and 50mN) were adopted to characterize the mechanical reponse of tissue engineered bone obtained through the two different treatments and out of the two scaffold treated groups, the cell-seeded scaffold construct (hydroxyapatite disk seeded with ASCs) showed significantly higher siffness and hardness at the shallower depths (200-500 nm) but lower at the higher depths (2000 nm) against the empty hydroxyapatite disk defect treatment. When both were compared to the native tissue, the latter was stiffer at all scales except against the cell-seeded tissue which was still stiffer at the shallower depth. Dr Carnelli mentioned that further investigations would include a relationship between maturity and mechanical properties since there is an indication that mechanical properties of tissue engineered bone in the early stages of the healing process are higher due to higher mineral content.

Dr James Dean of the University of Cambridge presented recent findings from sub-ambient temperature indentation. Dr Dean spoke about the operation of the cryostatic indentation stage that has been developed in the Gordon Laboratory, Cambridge, allowing indentation tests to be performed at temperatures as low as -170°C. To prevent condensation and hence a faster cooling system, the indentation tests were conducted inside a vacuum chamber (10-6 hPa). This allowed a study of the nanomechanical properties, from preliminary indentation tests, to be investigated over a range of temperatures but also at high strain rates, and it follows that below the glass transition temperature a much higher load (almost 4 times) was required to reach the same depth as tests carried out at room temperature. Dr Dean also spoke on the use of nanoindentation to measure residual stresses in surface layers, and residual stress generation, which showed matching experimental and curve fit models of creep strain at various loads but in particular at lower loads. The experiments also showed decreasing peak indentation loads and hardness with increasing in plane residual stress and stability against the imposed temperature drops where the experimental values of indentation loads and hardness were lower and then higher respectively in comparison to the predicted models.

The last session of the day was chaired by Prof Ben Beake of Micro Materials Ltd.

Dr Marc Masen of the University of Twente opened the last session with a talk on an investigation into two- and three-body abrasive wear whereby it was stated that a non-statistical approach is required when modelling abrasive wear. This is because many surface models in tribology are based on the assumption that surfaces are composed of a collection of small asperities e.g. the well known Greenwood and Williamson models which assume the asperities are equally sized and spherically shaped with a defined height distribution. Dr Masen presented work that initially focused on the behaviour of a single asperity in scratching contact (scratch test) with a counter surface which employed MML's Micro Test pendulum and the wear scars were measured using an interference microscope to quantify the volumetric wear as a function of indenter shape and normal load. Thus, knowing the behaviour of the unit-event, the total abrasive wear volume on the surface can be more closely achieved. It follows that such an approach is also possible for three-body abrasive wear i.e. when the scratching action is made by a particle that is not an asperity on one of the contact surfaces e.g. when driving through a sand storm. Dr Masen concluded by presenting experimental work into the abrasive properties of sand i.e. the study of the influence of a sand particle properties on abrasive wear. The approach included measuring the properties (hardness, size and geometry) of different sand samples and developing a model for scratching sand particles and then to perform dry sand rubber wheel tests.

Dr Gerard Bell of of Micro Materials Ltd and the University of Birmingham and Dr Jian Chen also of the University of Birmingham were the last speakers of the day and presented the work they did together on the low temperature mechanical properties and nano-scratch study of a graded a-C:H(Ti)/TiCN/TiN/Ti tribological coating. The investigation included the first low temperature (25°C to -30°C) nanoindentation which took advantage of the stability of the NanoTest's recent upgrade to Platform 3, which provides additional stability when working at non-ambient temperatures and has faster calibration and a new soft-balancing system which eliminates the pendulum mass balance adjustments when changing between indenter types. It was found that room temperature scratch tests failed the coating, but at 0°C, a significant increase in Hardness/Young's modulus. Resistance to crack propagation increased consistently with decreasing temperatures down to -30°C. Dr Bell stated that the new sub-ambient capability is one that allows the ability to probe the tribological properties of surface engineered systems on the nano and micro scales. Furthermore, this advanced testing technique can be used in many applications including aerospace, automotive and polymer industries and in conjunction with other types of tests including nano-impact, scratch and wear, which MML's NanoTest is well suited for.

If you would like any further information on any of the talks or would like to be informed of next year's European User Meeting, and for any general queries related to Micro Materials Ltd please contact tahsin@micromaterials.co.uk

 

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