Papers

ICMCTF2009 Session E2-3: Mechanical Properties and Adhesion

Tuesday, April 28, 2009 8:00 AM in Room California

Tuesday Morning

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8:00 AM		E2-3-1 Mechanical Properties of Metals Below 100 nm - Effects of Size, Temperature and Geometry R. Spolenak (ETH Zurich, Switzerland) Metals become stronger and stronger, when their external dimensions approach the micrometer scale. When their dimension, however, is reduced by another two orders of magnitude, novel effects appear: the scaling law for yield strength changes, strength depends on the loading mode, metals become more brittle and some metals start to exhibit a strong temperature dependence in their mechanical properties. These observations will be illustrated by studies on nonporous gold in a combinatorial approach, thin single crystal gold films by nanoindentation and in-situ X-ray studies on arrays of gold nanointerconnects. The lecture is rounded off by theoretical considerations of scaling limits and how they can be used to design optimal materials microstructures.
8:40 AM		E2-3-3 Hardness Mapping and Nanoindentation of Cold-Spray Coatings D. Goldbaum, T. Shariff, A. Rezaeian (McGill University, Canada); E. Irissou, J.-G. Legoux (National Research Council Canada (NRC), Canada); R. Chromik, S. Yue (McGill University, Canada) Cold-spray is a relatively new method of coating deposition that is achieved at low temperatures by accelerating powders to supersonic speeds through a converging-diverging nozzle. Upon impact, the particles undergo high shearing stresses with localized adiabatic shear instability regions. The material deformation aids in creating a metallurgical bond to the counterpart surfaces, but also results in inhomogeneous strain within particles and perhaps throughout the coating. In this work, hardness profiles and hardness mapping were used to investigate the effect of the deposition conditions and material deformation mechanisms on the nanomechanical properties of cold-sprayed coatings. Pure Cu, Ni, Ti and Ti alloys powders were deposited on a mild steel substrate with particle velocities measured in situ and ranging from 400-800m/s. Nanoindentation testing was conducted on polished cross-sections of coating/substrate specimens. When averaging all measurements for a given coa ting, a trend for hardness with increasing deposition velocity was found, with hardness increasing for Ti coatings, decreasing for Cu coatings, and remaining constant for Ni coatings. A closer inspection of data showed no consistent trends with indent position with respect to the substrate. Instead, variations in hardness were observed between particles and within them as well. Hardness mapping is currently being compared to determinations of strain from electron backscatter diffraction (EBSD). Differences in coating mechanical properties will be discussed in terms of the particle deformation, microstructure and morphology.
9:00 AM		E2-3-6 Ultra Low Indentation Approach to Stress-Strain Behavior of Thin Films J. Nohava (CSM Instruments SA, Switzerland); G. Favaro (CSM Instruments SA, Swizerland); N. Randall (CSM Instruments Inc.) The elastic-plastic behavior of thin films has always been of a great interest. However, obtaining of a plot similar to uniaxial stress-strain curve has been facing serious methodological and technical obstacles. Recently, nanoindentation methods have been employed for determination of such curves and theories relating indentation stress and strain to uniaxial stress and strain were developed. The new CSM Instruments Ultra Nanoindentation Tester has been used for measuring of elastic-plastic response of several thin films. Bulk materials were also used for comparison. The main advantage of the UNHT instrument is its extremely low thermal drift and frame compliance that are indispensable for long term and cyclic measurements. The measurement method consisted of performing repeated indentation with increasing force on one spot of the tested sample. The indentations were carried out using linear and quadratic load increase. Spherical indenters of 10 µm and 20 µm radii were used for indentation. The load applied on the indenter varied from loads of 0.05 mN to stay in the elastic regime up to high loads of 50 mN to enter well into the plastic regime. The indentation stress as a function of indentation strain was plotted and the curve similar to uniaxial stress-strain curve was obtained. The so-obtained curves were compared with plots of elastic to plastic indentation energy ratio as a function of depth. The onset of plastic deformation was observed and it corresponded to change in the elastic to plastic indentation energy ratio. The results show that though such measurements do not have the same physical meaning as the well known stress-strain curve, they can be used as an efficient and fast method to determine the elastic-plastic response of thin films to mechanical deformation in compression.
9:20 AM		E2-3-5 Mechanical Properties of TiN/Ti Multilayer Coating Deposited by LAFAD Technology Y. Cheng, T. Browne, B. Heckerman (American Eagle Instruments, Inc.); C. Bowman, V.I. Gorokhovsky (Arcomac Surface Engineering, LLC.) Multilayer coatings consisting alternating ceramic and metallic layers have attracted great attentions due to the combination of the high hardness of the ceramic layer and the high toughness of the metallic layer. Our previous results showed that the addition of Ti interlayer into TiN ceramic layer significantly reduce the internal stress in the coatings. It is important to characterize the mechanical properties, i.e. hardness, Young’s modulus, and plasticity, of the TiN/Ti multilayer coatings. A large area filtered arc deposition (LAFAD) technique was used to deposit TiN/Ti multilayer coatings with fixed TiN layer thickness and different Ti layer thickness. Scanning electronic microscopy and nanoindenter were used to observe the surface morphology, and characterize the hardness, Young’s modulus and plasticity of the multilayer coatings, respectively. The dependence of the surface morphology and mechanical properties of the coatings on the Ti interlayer thickness was systematically studied. It was found that all TiN/Ti multilayer coatings deposited by our LAFAD technique are dense and smooth. The increase in the Ti layer thickness results in a increase in the grain size and surface roughness, a decrease in the effective hardness and Young’s modulus, and an increase in the plasticity and toughness. The coatings with Ti layer thickness of below 50 nm possess excellent combination of high effective hardness (>20GPa), high plasticity (>69%), and excellent toughness.
9:40 AM		E2-3-4 Small-Scale Tests for Investigating Plastic Flow in Brittle Crystals S. Korte, W.J. Clegg (University of Cambridge, United Kingdom) Plastic flow in brittle materials is conventionally studied by testing material that is sufficiently constrained to prevent cracking, using, for instance, a Griggs cells or, much more easily, by nanoindentation. Much useful information has been obtained in these ways, however, many brittle crystals have complex structures where multiple slip systems operate. In indentation, accommodating the indenter tip in the sample requires that sufficient slip systems operate, so that the measured mechanical behaviour is dominated by the properties of the hardest slip system operating. An alternative approach is to make the sample sufficiently small that insufficient elastic strain energy is available to drive cracking. This can be done using micropillar compression and has been used extensively in metals to study volume effects on flow. These effects are much less marked in hard materials, so that this approach can be used to study flow. Furthermore, it allows deformation based on a single slip system. Here, microcompression of MgO pillars with subsequent TEM analysis was used to demonstrate this. MgO single crystals possess a hard and a soft slip system which can be targeted individually orienting the crystal such that the Schmid factor is close to 0.5 for one and zero for the other. Micropillars of different crystal orientations were subjected to uniaxial compression and analyzed by transmission electron microscopy to identify the Burgers vectors and slip planes involved in order to correlate these results with the respective mechanical properties in each direction.
10:00 AM		E2-3-7 Mechanical and Electrical Behavior of Oxide-Dispersion Strengthened Au Microcontacts R. Vinci, T. Bannuru, W.L. Brown, T. Humplik, K. Mongkolsuttirat (Lehigh University) Incorporation of hard ceramic particles in the surface layer of a metal component can be an effective method for reducing sliding wear. We have extended this approach to address adhesion and degradation of micrometer-scale thin gold film contacts in MEMS devices. Vanadium pentoxide nanoparticles are formed in a gold matrix during reactive sputter deposition. Control of the deposition parameters determines the extent of vanadium oxidation and the volume fraction of the oxide phase. Nanoindentation has been used to characterize the hardness of the films as a function of composition and oxidation extent. Together with characterization of electrical resistivity via four-point probe measurements, the ideal contact resistance under normal force loading can be predicted. We measure the real contact resistance using a custom ball-on-flat test configuration. This test is also used to evaluate adhesion during separation of the contacts. Film strength, contact resistance, and degree of adhesion can be modeled using approaches borrowed from prior studies of either oxide-dispersion strengthened alloys or metal matrix composites. General conclusions are drawn about the applicability of these measurement techniques and models to nanoparticle-reinforced gold films, and about the potential of this reinforcement approach for enhancing wear resistance in normal-force contacts without unacceptable increases in contact resistance.
10:40 AM		E2-3-9 Factors Affecting the Critical Indenter Penetration for the Measurement of the Hardness and Young's Modulus of Coatings S.J. Bull, J. Chen (Newcastle University, United Kingdom) In many coating applications, it is very important to obtain the mechanical properties (such as hardness, elastic modulus, etc.) of the coating independent of the substrate for instance for use in design calculations. The nanoindentation test is only viable approach to assess the properties of very thin coatings (<1μm) since it can operate at the required scale and provides a fingerprint of the indentation response of the coating/substrate system. If coating properties are to be assessed, the key point is to ensure any measured value is free from the influence of the deformation of substrate (or at least the substrate effect is minimized). To measure the hardness of the coating only it is traditionally assumed that, as a rule-of-thumb, when the relative indentation depth (RID, i.e the penetration divided by the coating thickness) is less than 0.1, the substrate will not affect the measured hardness of the coating. However, it is found that this rule is too strict for some systems and too loose other coated systems. A more rigorous assessment is therefore necessary. For the Young's Modulus a much smaller critical penetration is expected and there is doubt about whether it is possible to measure the elastic properties of the coating independent of the substrate at all. In this paper we present a comprehensive investigation on the critical relative indentation depth (CRID) using finite element simulation. The elastic-plastic properties mismatch between coating and substrate has been highlighted and other factors affecting the CRID which have previously been ignored will be discussed. The condition that indenter penetration must be less than a fraction of the coating thickness has been explored and it has been shown that in a particular type of coated system no rule-of-thumb can apply. Furthermore, it is shown that elastic property mismatch has an important effect on the measured hardness and this means that the Oliver and Pharr method generally used to extract hardness from nanoindentation data may give inaccurate results in coating/substrate systems with significant elastic mismatch.
11:00 AM		E2-3-10 Influence of Intrinsic Stress on the Measurement of the Onset of Yielding for Thin Films Using Depth-Sensing Spherical Indentation M. Herrmann, A. Clausner, F. Richter (Chemnitz University of Technology, Germany) It is often observed that hardness is influenced by intrinsic stress in the sample. With increasing uni- or biaxial tensile stress hardness is reduced while with growing compressive stress it increases. This observation complies with basic considerations about the influence of those stresses on shear stresses and is thus frequently used. However, there were also reports which could not prove this finding. For instance in¹ a variation of hardness with changing stress was shown to be due to varying pile-up. When this was properly considered, a stress-independent hardness was obtained. In contrast to hardness, the determination of yield strength refers to the initial stage of yielding and should therefore be better suited to investigate the influence of intrinsic stress. Such measurements are done by loading-partial unloading experiments with spherical indenters and analysing the stress field for the onset of plastic deformation. Here, the intrinsic stress field is considered in addition to the field due to the loaded indenter. We have investigated amorphous hydrogenated carbon (a-C:H) films deposited on silicon having varying compressive stress. The yield strength of the films was determined using the von Mises yield criterion and their hardness was measured applying the Oliver-Pharr method. The intrinsic film stresses do influence the onset of plastic deformation and, hence, have been considered in the analysis of yield strength. To interpret the obtained hardness data, one should keep in mind that a proper hardness testing is connected to a fully-developed plastic zone, thus including an intensive amount of plastic deformation. This can lead to a dramatic change of the stress fields within the volume of the sample and therewith to the above-mentioned effects. However, for the investigated a-C:H films it was found that the ratio of hardness to yield strength was constant within the accuracy of measurement. ¹T.Y.Tsui et al, J.Mater.Res. 11(1996)752-759.
11:20 AM		E2-3-11 Influence of the Nitriding and TiAlN Coating Thickness on the Mechanical and Tribological Properties of a Duplex Coating System on H13 Die Steel R. Torres (PUCPR, Brazil); L. Suzuki (Neodent Co, Brazil); P. Soares (Pontificia Universidade Católica do Paraná, Brazil); C. Lepienski (Universidade Federal do Paraná, Brazil); J.J. Moore (Colorado School of Mines) H13 die steel substrates were low pressure gas nitrided with three different nitriding cases, A, B, and C. The hardness profile revealed that two surface hardness levels and two thickness depths of the nitrided layer were obtained. In the nitriding case A the surface hardness was around 12 GPa and the nitriding thickness around 40 µm. In the nitriding case B, the hardness was the same as in case A, but the nitriding thickness was around 70 µm. The nitriding thickness in case C was the same as in case B, but the surface hardness was around 14.5 GPa. The XRD results showed that the nitriding case microstructure is composed mainly of a diffusion layer with a small amount of CrN precipitates. These nitrided samples were subsequently coated with TiAlN using cathodic arc evaporation in two thicknesses of 3 and 7 µm. These duplex coating samples were characterized with respect to phase chemistry, adhesion, hardness and scratch tests. The phase chemistry determined through X RD and EDS revealed that the TiAlN coating was mostly Ti_0.7Al_0.3N with some peaks of TiN. The instrumented hardness performed on the coated samples showed that the coating hardness changes with the nitriding case depth for a TiAlN coating thickness of 3 µm. On the other hand, the nitriding characteristics do not influence the coating hardness when the TiAlN coating is 7 µm thick. In addition the 7 µm thick coating is harder than the 3 µm thick coating. In the last part of this work, TiAlN was deposited on the H13 substrate without nitriding; it was found that the hardness in this condition is higher than the duplex nitrided/TiAlN coated samples. The worn area probed by the scratch test was smaller for the samples coated with TiAlN of 7 µm thickness.
11:40 AM		E2-3-12 Deformation Mechanisms in TiN/NbN Multilayer Thin Films K.A. Rzepiejewska-Malyska (Empa, Switzerland); A. Baranska, A. Szerling (Institute of Electron Technology, Poland); S. Korte, W.J. Clegg (University of Cambridge, United Kingdom); J. Michler (Empa, Switzerland) The aim of the study was to compare TiN/NbN multilayered ultrathin film systems regarding deformation mechanisms during indentation. TiN/NbN films with different combinations of single layer thicknesses were deposited in the sputtering process with two different temperatures, ~70 and ~1000°C respectively. Single layer reference coatings of TiN and NbN were synthesized for comparison. Mechanical properties were determined by in situ nanoindentation inside a high resolution Scanning Electron Microscope using a cube corner indenter tip. This allowed for observation of pile-up, sinking-in and crack propagation of the coating during the indentation loading cycle which were taken into account for hardness and reduced modulus calculation. In situ SEM indentation at the microscale allowed for observation of the toughness and crack propagation in this scale. In order to get a complete understanding of the fundamental deformation phenomena occurring within the layers and to anal yze the microstructure, transmission electron microscopy (TEM) observations on the cross-sections of the indented areas were prepared using a focus ion beam (FIB) technique. TEM observations revealed that films deposited in lower temperature were nanocystalline, while those obtained with higher temperature were coherent nanocrystalline coatings. We have observed that the mechanical behavior strongly depends on the microstructure of the film. For polycrystalline multilayer films grain boundary sliding was identified as one of the acting mechanisms, while for monocrystalline samples shear band formation and cracks perpendicular to the substrate dominate. The difference in deformation mechanisms observed will be discussed and linked to the features occurred on the load – displacement curves.