ICMCTF2007 Session E2-3: Mechanical Properties and Adhesion

Thursday, April 26, 2007 8:00 AM in Room California

Thursday Morning

Time Period ThM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2007 Schedule

Start Invited? Item
8:00 AM E2-3-1 Exploiting Quantitative In-Situ Nanoindentation to Investigate the Mechanisms of Plastic Deformation in Thin Films
E.A. Stach (Purdue University); A.M. Minor (Lawrence Berkeley National Laboratory); O. Warren, S.A. Syed-Asif, Z. Shan (Hysitron, Inc.); M. Jin, J.W. Morris (University of California, Berkeley)

Nanoindentation is widely accepted as the preferred technique to study localized mechanical deformation phenomena in materials. However, the mechanisms of deformation can only be inferred from the load-displacement data obtained during a typical instrumented nanoindentation test. In order to elucidate the underlying physics of these process, we have developed and exploited a new technique, that of in-situ nanoindentation in a transmission electron microscope (TEM). In this technique, a voltage-actuated piezoceramic tube is used to position a sharp diamond in-plane with the edge of an electron transparent sample. The tip is then driven into the material using a three-plate capacitive transducer which allows direct measurement of load and displacement data to be recorded concomitant with real time images of the indentation response of the material. In this paper we will briefly review the details of our experimental technique, as well as summarize our result from aluminum thin metal films. We have found that the onset of plasticity in these materials does not occur via sub-surface nucleation of dislocations at the point of ultimate strength, but is rather induced by surface forces.1 Detailed studies of grain boundary motion and grain size effects have shown that indentation loading conditions result in stress-induced coalescence, and thus grain growth.2,3

1A.M. Minor, A.M. Minor, S.A. Syed Asif, Z.W. Shan, E.A. Stach, E. Cyrankowski, T. Wyrobek and O. Warren, Nat. Mat., 5, 697, 2006.

2A.M. Minor, E.T. Lilleodden, E.A. Stach and J.W. Morris, Jr., J. Elect. Mat., 31 (10), 958, 2002.

3M. Jin, A.M. Minor, E.A. Stach and J.W. Morris, Jr., Acta Mat. 52(18), 5381, 2004.

8:40 AM E2-3-3 Recent Advances in Nanomechanical/Nanotribological Testing of Ultra-Thin Films for Stiction Control in MEMS Devices
B.D. Beake (Micro Materials Ltd, United Kingdom); G. Wilson, B. Shi, J. Sullivan (Aston University, United Kingdom); K.E. Cooke (Teer Coatings, United Kingdom); S.R. Goodes (Micro Materials Ltd, United Kingdom)

Reliability of MEMS devices can be limited by stiction forces that develop in use. It is desirable to alter the mechanical and interfacial behaviour of the silicon surfaces and this can be done by the application of thin (10-100 nm), low surface energy and low stress coatings. The closed field unbalanced magnetron sputtering deposition process (CFUBMS) is capable of producing dense, low stress coatings. Optimisation of the performance of sub-100 nm coatings by CFUBMS is made possible by recent advances in nanotribological (nano-scratch and nano-wear) and nanomechanical (nanoindentation) test methods in a commercial instrument. The CFUBMS thin films did not undergo stress-related delamination that can occur behind the moving probe during nano-scratching of thicker, more stressed films. Nanowear results correlate with nano-scratch critical loads. High resolution nanoindentation can be used to probe thin film mechanical properties at depths ~ 5 nm. A novel nanoscale reciprocating wear technique (Nano-fretting[TM]) was found to have potential for evaluating local fatigue performance of ultra-thin coatings. The deposition of <100 nm films on Si appears to be a promising strategy for improving reliability of Si-based MEMS devices [1].

[1] Funding from the UK Department of Trade and Industry - BTIA Programme - Project CHBL/C/019/00024 is acknowledged.

9:00 AM E2-3-4 True Hardness Measurement by Nanoindentation using a Single Sharp Indenter
J.-Y. Kim, S.-K Kang (Seoul National University, Korea); J.-I. Jang (Hanyang University, Korea); D. Kwon (Seoul National University, Korea)
The Oliver-Pharr method has been used in nanoindentation testing to determine contact depth by subtracting the elastic deflection depth from the maximum indentation depth. When severe pile-up/sink-in is generated around the indenter, however, the Oliver-Pharr contact depth leads to significant errors. The relation of pile-up/sink-in to material properties and indentation parameters was investigated by extensive FEM simulation. Two parameters were used to describe the pile-up/sink-in for a sharp indenter. The first is the extent of elastic deformation at some representative strain, such as (greek oy or H)/E (ratio of yield strength or hardness to Young's modulus), hf/hmax (ratio of final indentation depth to maximum indentation depth), We/Wt (ratio of work recovered during elastic unloading to total work input during loading), and the like; the second is the parameter describing the extent of plastic flow, i.e. the work-hardening exponent n. While the first parameter is represented satisfactorily by indentation parameters hf/hmax or We/Wt, the second is not easy to measure in a nanoindentation test using a single sharp indenter. We have now found a relation between the work-hardening exponent n and the characteristic length for the indentation size effect h* that can be measured in a nanoindentation test using a single sharp indenter. We thus propose a novel method to measure true hardness, taking into consideration pile-up/sink-in, with a nanoindentation test using a single sharp indenter.
9:20 AM E2-3-5 Atomic Force Microscope Investigation on Static Versus Dynamic Nanotribological Evaluation of Metal-ZrN and ZrN Thin Films
D.M. Mihute (University of Nebraska - Lincoln); S.M. Aouadi (Southern Illinois University Carbondale); J.A. Turner, S.L. Rohde (University Nebraska - Linclon)
The present study aims to provide a better understanding of the correlation between static nanomechanical properties (nanohardness (H), elastic modulus (E), H/E and H3/E2 ratio) and dynamic properties (resulting from nanoscratch measurements) for three groups of Metal-ZrN thin films (Inconel-ZrN, Cr-ZrN and Nb-ZrN) and ZrN thin films. Metal-ZrN thin films have a great potential for industrial applications as they can combine high hardness with good elasticity and toughness making them effective for wear resistant application. Nanomechanical and nanotribological properties for Metal-ZrN and ZrN thin films deposited by DC unbalanced magnetron sputtering were investigated using atomic force microscope interfaced with a Hysitron Triboscope. The elastic recovery of thin films under a normal load applied during nanoindentation was evaluated and correlated with elastic recovery of thin films under dynamic loading during nanoscratch measurements in order to assess which film compositions could provide superior wear resistance. It is demonstrated that dynamic elastic recovery measurements are at least as predictive as those derived from static indentation tests. Because the nanoscratch tester combines both normal and tangential loading, it may even be a better predictor of wear-resistance than more conventional static nanoindentation testing.
9:40 AM E2-3-7 Limits of Measurement and Analysing Techniques for the Determination of Mechanical Coating Parameters - a) classical indentation and b) indenters of the next generation
N. Schwarzer (Saxonian Institute of Surface Mechanics SIO, Germany)

The analysis of nanoindentation data always requires physical models allowing the extraction of mechanical coating parameters. Only the model chosen appropriate to the experimental set up assures high quality and correct parameter identification. Thus, the paper will treat the following topics:

- Limits of the Hertzian and the extended Hertzian theory for blunt indenters with respect to the mechanical and geometrical parameters of layered materials.

- Limits of the concept of the effectively shaped indenter in connection with the extended Hertzian theory.

- How to extract mechanical coating parameters from the effective half space values classical procedures like Oliver and Pharr do provide?

- Normal and lateral minima for the sample size in dependence on the Young's moduli of the layered structure.

- Normal, lateral, rotating and tilting loads for the next generation of nanoindenters.

10:00 AM E2-3-8 FEM Simulation of the Effect of Surface Roughness on Nanoindentation of Thin Films With a Spherical Indenter
C. Walter, T. Antretter, R. Daniel (University of Leoben, Austria); C. Mitterer (Montanuniversität Leoben, Austria)

The effect of the surface roughness on nanoindentation results was investigated instancing a series of CrN thin films deposited by unbalanced magnetron sputtering. The arithmetic roughness (Ra) of the films ranged between 2 and 10 nm and was measured by atomic force microscopy (AFM). The measured surface topography was incorporated into a finite element model, which allowed to simulate the indentation of an axisymmetric sample by a rigid spherical indenter. At the applied load of 15 mN it was found that plastic deformation could be neglected and thus purely elastic material behaviour was assumed. For roughness values of Ra = 2, 5, and 10 nm a number of 100 indents each were simulated. Subsequently, the software Elastica was used to evaluate Young's modulus of the CrN thin films from the simulated load-displacement curves.

Under the applied conditions, which should be representative for a wide range of magnetron sputtered thin films, the increasing roughness causes a reduction of the contact area and leads to an underestimation of Young's modulus. The mean Young's modulus of all simulated indents on the rough surfaces lies 8-14 % below the Young's modulus determined for a perfectly smooth surface. This deviation seems to be independent of Ra, although the data scatter increases significantly with increasing roughness. Additionally an influence of the lateral extension of the surface texture on the data scatter was observed which is not accounted for in roughness measures such as Ra.

10:20 AM E2-3-9 On the Problem of Properly Designed FE-Models for Mechanical Contact Problems on Layered Materials
M. Herrmann, R. Unger, A. Meyer, F. Molnar, F. Richter (Chemnitz University of Technology, Germany); N. Schwarzer (Saxonian Institute of Surface Mechanics, Germany)

In this paper, limiting factors for contact modelling of layered materials using finite element solutions are presented. Finite element calculations are frequently used as an established and trusted method for mechanical contact analyses. Additionally, several commercial contact models are available. One has to note that a fundamental problem is attributed to the normal and lateral domain boundaries of the finite element model. On the one hand, it is necessary to reduce the CPU time by a properly chosen mesh size. On the other hand, a decrease of the model size results in a systematic deviation due to the boundaries.

The underlying parameters, which influence the finite element results of the elastic stress field, are identified based on comparison to the complete analytical solution of layered materials. Therefore, layered systems have been studied having different elastic moduli and film thicknesses. Secondly, the influence of the normal and lateral dimensions of the finite element model on the contact solution has been considered. It was found, e.g. that the elastic modulus configuration of layer and substrate leads to a dramatic deviation of the resulting finite element contact force from the analytical solution under certain conditions. These deviations are enhanced or extenuated by the varying geometry parameters of the film and model, respectively. Based on these results, empirical relations for a properly sized finite element model will be given. This should help to improve mechanical contact solution using finite models for samples, which in reality can be rather considered as infinite half spaces.

10:40 AM E2-3-10 Understanding Automotive Coatings Behaviour using Nanoindentation
R. Rastegar Tohid, S.J. Bull (Newcastle University, United Kingdom)
There are a wide range of coatings used in the automotive industry for corrosion or tribological protection or for functional surfaces such as tinted windows. For automotive body components it is the appearance of these coating which is critical. A major contributor to this is the mechanical properties of the coatings which contribute to scratch and damage resistance. The mechanical data for such coatings is often difficult to obtain, since they are relatively soft and show time-dependent behaviour compared to traditional hard coatings. The coatings also have different microstructure to bulk materials so bulk material data can not be used in successful analysis. There is thus a considerable need to understand and characterize coatings, especially when coating properties vary from one point to another or when coated substrates are used in forming operations. Since soft materials have a high creep rate or show viscoelastic behaviour, it is important to have a sound understanding of their instrumented indentation behaviour since this will differ considerably from that of the harder, stiffer materials for which the techniques were originally developed. It is well known that the Oliver and Pharr method does not produce accurate results when it is applied to materials with high creep rate. In this study Hysitron Triboindenter fitted with a blunt Berkovich indenter, in situ AFM, and offline SEM have been used in conjunction with a design of experiments (DOE) approach to characterize mechanical properties of different coatings such as galvanized sheet, Durasteel, and the polymeric lacquers as the main auto body coatings. In this presentation time dependent behaviour, microstructure effects, the pile up patterns of these coatings, and their relationship to peak load, strain rate and control strategy will be discussed. Key words: Nanoindentation, Galvanizing, Durasteel, Time-dependent behaviour, Microstructure.
11:00 AM E2-3-11 Deviations in Determining Coatings' and Other Materials' Mechanical Properties, when Considering Different Indenter Tip Geometries and Calibration Procedures
K.-D. Bouzakis, N. Michailidis, G. Skordaris (Aristoteles University of Thessaloniki, Greece)

Recently, a developed FEM-supported method enabled the continuous simulation of nanoindentation and based on that a stepwise determination of materials' stress-strain laws. Furthermore, an accurate description of the contact geometry between the material and the indenter during its penetration into the tested specimen, as well as of the remaining impression were possible.

Various materials such as coatings, Silicon (100), glass BK7, fused silica, tungsten, steels, etc. were tested by the previous methods and the results were used to establish the materials' stress-strain characteristics and the indenter area functions. Additionally, load versus displacement diagrams were calculated by appropriately developed FEM-based algorithms considering spherical indenter tip geometries along with various radii.

In ISO 14577, various indenter tip calibration procedures are proposed for determining the indenter area functions, based on either the Young's modulus or the hardness of reference materials. The indenter calibration procedure, based on the hardness of a reference material, led almost to the same indenter surface area versus the contact depth, as the one determined by the developed FEM-supported algorithm. The indenter surface area functions established by the indenter calibration procedure based on the Young's modulus of a reference material, are deviating from the FEM calculated ones. On the other hand, taking into account spherical indenter tip geometries did not lead to a convergence with the experimental results.

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