ICMCTF2006 Session E3-1: Determining Constitutive Relationships by Instrumented Indentation on Techniques

Wednesday, May 3, 2006 8:30 AM in Room California

Wednesday Morning

Time Period WeM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2006 Schedule

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8:30 AM E3-1-1 Determination of Stress-Strain Curves of Ni and NiCo Coatings Using Nanoindentation Techniques in Combination with Inverse Methods and Microtensile Testing
J. Michler, S. Stauss (EMPA, Switzerland); E. Blank (EPFL, Switzerland)

We present two instrumented indentation methods for determining yield stress and strain hardening coefficient of metalic coatings.

The first method is based on a series of indentations with conical indenters exhibiting different included angles. From parametric finite element simulations we have constructed, for each included angle, a dimensionless function relating the characteristic parameters of the indentation loading curve to the Young's modulus, the yield strength, and the strain hardening exponent.

The second method relies on inverse finite element simulations controlled by an automated fitting algorithm that is used to estimate elastic modulus, yield strength and hardening coefficient from spherical indentation load-displacement curves. It turns out that experimental load-displacement curves can be reproduced within convergence limits of the fitting algorithm without the correct set of material parameters of the constitutive model. The convergence could be improved by using starting values for the fitting procedure based on a Meyersâ?T analysis of the load displacement curves, but in order to obtain unambiguous results post-indentation surface topography were incorporated into the fitting scheme.

Both methods were applied to assess yield stress and strain hardening coefficients of electrodeposited Ni and NiCo coatings for UV-LIGA applications. In particular, the influence of of a post-annealing on the microstructure and on the mechanical properties of the coatings is investigated. Conical nanoindentation tips and spherical nanoindentation tips were used. The accuracy of both methods depending on the precision of the cone angles and on tip defects, respectively, is discussed. A comparison with microtensile tests on the same materials was performed to validate the results.

9:10 AM E3-1-3 Determination of Tensile Properties by Instrumented Indentation Technique: Representative Stress and Strain Approach
J.-Y. Kim, K.-W. Lee, J.-S. Lee, D. Kwon (Seoul National University, Korea)
The instrumented indentation technique, because it is fast, precise, and nondestructive, has been widely used to determine such mechanical properties as flow properties, residual stress, fracture properties, viscoelastic properties and hardness of materials and structural units. In particular, instrumented indentation by a spherical indenter can provide hardness and flow properties such as yield strength, tensile strength, and work-hardening exponent, using the characteristic of spherical indentation, where strain level is increased with indentation depth. This paper describes the derivation of the true-stress-true strain relationship from the indentation load-depth curve measured by the spherical indenter based on representative stress and strain approach. The representative stress and strain values, which are basically equivalent to true flow data, were evaluated through correlation constants such as plastic constraint factor and strain proportional constant, which are generally dependent on the plastic property. In this study, these constants are determined as specific and optimal values using FE simulation and proven to be unaffected by the elastic property and strain level. Through determination of precise contact area and optimal representative stress and strain constants, the best agreement between the true stress-true strain (tensile) data derived from spherical indentations was achieved within 5% error for 20 materials included in several classes of materials. In addition, since the accuracy of tensile data can vary depending on such experimental parameters as number of unloadings, unloading ratio, maximum depth ratio and indenter radius, the optimum parameters are determined from analyzing the effects of the experimental parameters on the accuracy of the indentation tensile data quantitatively using Taguchi method.
9:50 AM E3-1-5 Neutral Networks for Determination of Stress-Strain Behavior from Indentation Tests
N. Huber (Institut für Materialforschung II, Forschungszentrum Karlsruhe, Germany)

The complex deformation in indentation testing requires analysis methods that account for nonlinear material behaviour as well as nonlinearities induced by the contact problem. Depending on the generality of the constitutive model, indenter geometry and the specimen (bulk material or thin film), different suppositions need to be fulfilled to solve the inverese problem uniquely. Finally an inverse algorithm is needed to determine the material parameters under the assumption of a constitutive model.

The presentation gives an overview on recent finite element and neural network based approaches for the identification of material parameters and the determination of stress-strain behaviour for different constitutive models and indentation experiments. The importance of a properly chosen loading history, zero-point correction, and tip shape correction for imperfect spherical tips will be demonstrated.

10:50 AM E3-1-8 Nanoindentation on the C and C/Si Nanocomposites by the Continuous Stiffness Measurement Technique
C.-K. Chung, B.-H. Wu (National Cheng Kung University, Taiwan)
The carbon (C) and C/Si nanocomposites were prepared by ion beam sputtering system (IBS) at room temperature (RT) under ultra high vacuum (UHV). Continuous stiffness measurement (CSM) technique was used in the nanoindentation tests to determine the hardness and elastic modulus of the films at deposite temperature of RT and annealed at 900°C for 0.5-1.5 hour. The microstructure is also characterized by grazing incident angle X-ray diffractometer (GIAXRD) and Raman spectroscopy to correlate the relationship between nanomechanical properties and microstructure. The GIAXRD and Raman results reveal that the carbon films in both structures of single carbon films and two-layer C/Si films are amorphous at RT and varied vacuum annealing conditions. The hardness and elastic modulus of as-deposited single carbon film were measured to be about 28.2 and 219.5 GPa, respectively, that is higher than as-deposited two-layers film of 21.9 and 204.9 GPa at RT. It is due to the nanomechanical properties of the amorphous Si (a-Si) film lower than single carbon film. As both films are annealed in vacuum at 900ºC for 0.5-1.5 hours, the hardness of single C film decreases significantly from 28.2 to 18.8 GPa with the increasing annealing time and its elastic modulus also decrease from 219.5 to 166.9 GPa. It is attributed to part of C film slightly graphitized when annealing time increases. The hardness of annealed two-layer C/Si composite films was measured between 19.1 and 21.9 GPa and elastic modulus were ranged from 168.7 to 204.9 GPa. It does not change obviously in hardness at varied vacuum annealing conditions. They indicate that the addition of a-Si layer to C/Si nanocomposite has a more stable nanomechanical properties than single C layer at high temperature.
11:10 AM E3-1-9 Mechanical Behavior of TiC Single Layers and Ti/TiN/TiC/TiCN Multilayers on AISI 4340 Steel
N.A. Sánchez, H.E. Jaramillo (Universidad Autónoma de Occidente, Colombia); H. Riascos (Universidad Tecnológica de Pereira, Colombia); G. Zambrano, P. Prieto (Universidad del Valle, Colombia)

TiC monolayers and Ti/TiN/TiC/TiCN multilayers were deposited onto AISI 4340 steel substrates at 300°C and 4.0x10-2 mbar nitrogen/methane pressures by using a Pulsed-Laser Deposition technique. A Nd: YAG laser (1064 nm, 500 mJ and 7 ns), with a repetition rate of 10 Hz was used. Here we analyzed the difference in structure and morphology for single layers and multilayers structures deposited on stainless steel substrates. AFM analysis presented different morphologies; showing, that the single layer had an average grain size of 0.087 µm; while the multilayers exhibited grain sizes of 0.045 µm. Coating thicknesses were 1 µm, approximately, and monolayer average roughness was 0.21 µm; while a value of 0.15 µm was measured for multilayers. Under a 60 Kg maximum load applied in tension to evaluate the adhesion to the substrate, no detachment of the films was presented. Multilayers evidenced better impact resistance as compared with TiC single layer; this result is considered, bearing in mind that in multilayers propagation of fissures is slower, because the presence of layer inter-phases, lead to fissures strays in other directions. Slight corrosion specks are present; but mass loss was around 16 mg. in multilayers a value was lower than for the TiC single layer that was near 43 mg. Homogeneity, grain size, fracture resistance, corrosion resistance, and adhesion of the multilayers are suitable for mechanical applications of these types of coatings as shown in mechanical measurements. These results indicate that for engineering applications under corrosive environments, the use of this type of multilayered coatings on AISI 4340 steel, are highly recommended.

This work was supported by COLCIENCIAS under the program Excellence Center for Novel Materials, contract No. 0043-2005.

11:30 AM E3-1-10 Density, Stress, Hardness and Reduced Young's Modulus of Tungsten-DLC Coatings
B.R. Pujada (Netherlands Institute for Metals Research, Netherlands); G.C.A.M. Janssen (TU Delft, Netherlands)
Tungsten -DLC films are widely used as protective coating. We produced such films by reactive sputter deposition from a WC target in an argon-acetylene plasma on silicon substrate held at 160°C. By varying the acetylene flow we varied the density, modulus and hardness of the deposited films. Density, hardness and Young's modulus all decreased on increasing acetylene flow. The density decreased from 14.5 gr/cm3 for zero acetylene flow to 4 gr/cm3 for high acetylene flows of 10 sccm. Comparing our measured densities to the density of WC: 15.6 gr/cm3 and graphite: 2.3 gr/cm3 we note that we span almost the entire range. Hardness was found to decreases from 37 GPa to 15 GPa with increase of acetylene flow. For the studied range in acetylene flows the reduced Young's modulus decreased from 330 GPa to 140 GPa, a much smaller range than the variation in Youmg's modulus one would expect going from WC: 700 GPa to graphite: 10 GPa. Therefore the elastic properties of a W-DLC film are not those of a composite material. Even though the material is amorphous a short-range order is expected. The variation of these parameters with respect to substrate bias is also investigated.
Time Period WeM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2006 Schedule