ICMCTF1999 Session E4/F1-2: Mechanical Characterization - Micromechanical Testing and Modeling

Wednesday, April 14, 1999 1:30 PM in Room California

Wednesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	E4/F1-2-1 Nanoindentation Characterisation of Coated Systems : (P-δ) Plots, the P-δ² Analysis & P:S² - A New Approach Using the Continuous Stiffness Technique T.F. Page (University of Newcastle, United Kingdom) Continuous stiffness techniques offer another means of assessing the mechanical properties of coated systems using nanoindentation. This paper will critically assess the differing types of sample information available from simple load-displacement (P-δ) curve approaches, established P-δ² analyses and, most recently, appraising P-S² (i.e. load-vs-(contact stiffness)²) data. As necessary background to establishing the P-S² approach, any effects (on the sample load-displacement response) of the small superposed AC signal used to continuously measure contact stiffness have also been explored. Apart from some small statistical variation in the loads at which crack initiation was observed in a SiC-on-Si coated system, TEM and HRSEM studies showed no detectable differences in deformation sub-structures between nominally identical indentations made with and without the AC signal. While the parameter P/S² is independent of detailed tip shape and is a constant with displacement for monolithic systems, it was found to display unexpected variations with displacement for the coated systems. Since P/S² can also be related to the plasticity index (ψ) widely used to describe the balance between the elastic and plastic responses of materials subjected to contact damage, the observations of maxima in plots of P/S² with displacement, for at least some coated systems, suggests that there may be an optimum contact scale for maximising the elastic contribution to the contact response of such systems. The P/S² approach should also work well with the rougher surfaces often associated with real coated systems.
2:10 PM		E4/F1-2-3 Nanoscale Mechanical Characteristics of Plasma-treated Polymer Surfaces and Plasma Deposited Films S. Dahl, L. Martinu, J.E. Klemberg-Sapieha (Ecole Polytechnique de Montreal, Canada); D. Rats, J. von Stebut (CIM - Ecole des Mines, France) Recent advances in Atomic Force Microscopy (AFM) allow one to perform a complex analysis of materials surfaces and of the near- surface regions; this involves surface morphology, nanotribological characteristics, hardness and elasticity of the surface with nanometer scale resolution. However, understanding of the complex AFM analysis of mechanical properties is still in its early stage, and it frequently requires the use of multiple complementary techniques. In the present work AFM measurements are performed in contact mode using different tip-to-sample forces, while monitoring the force-vs-distance curve as well as the torsion of the cantilever. Different materials are studied such as untreated or plasma treated polymers (polycarbonate, polyester), plasma-deposited films (silicon nitride, diamond-like carbon, carbon nitride) and reference substrate materials (crystalline silicon, quartz). Friction coefficient, nanohardness and elastic modulus are derived for each of these surfaces, and they are compared with the values obtained by depth-sensing indentation, pin-on-disc tribometery, classical scratch test and low-load microscratch test. The results are discussed with respect to the limitations (elastic contact pressure or deformation) of the techniques applied and with respect to the preparation conditions of the samples. The experimental results are supported by modelling the stress distribution for an elastic contact using analytical and numerical methods.
2:30 PM		E4/F1-2-4 On the Application of the Work-of-Indentation Approach to Depth-Sensing Indentation Experiments in Coated Systems A.M. Korsunsky, S.J. Bull (University of Newcastle, United Kingdom) The concept of work-of-indentation arises naturally in the study of hardness. Similarly to the way in which toughness was introduced by Griffith to describe the energy expenditure per unit area during fracture, hardness can be understood in terms of energy expenditure per unit volume during indentation. However, a significant difference exists between the two cases. In the case of brittle fracture, the area concerned has a clear physical meaning of the crack surface. In the case of indentation, however, the volume in question IS NOT that displaced by the indenter. Not only is this volume poorly defined, but it is also difficult to measure. Depth sensing indentation offers remarkable opportunities of analysis and interpretation. The detailed and accurate record of the applied load and the tip displacement during both the loading and unloading phases of the experiment allows the energetic characteristics of the process to be computed. It is then possible to choose as a basis for a hardness definition a combination of any two or more from the following parameters: maximum load P and depth d, total work of indentation W, as well as its elastic W_e, and plastic W_p parts. In the present study, a family of possible hardness definitions were systematically applied to the results of nanoindentation in various bulk materials, and coated systems, with a view to identifying the most suitable formulations appropriate to each case and class of materials. A finite element study of the iindentation process, using various tip shapes and material response laws, has also been carried out. The results suggest some improvements that can be achieved in hardness measurement and interpretation, particularly for ultra low loads.
2:50 PM		E4/F1-2-5 Thermally-Actuated Cantilever Beam for Achieving Large In-Plane Mechanical Forces and Deflections E.S. Kolesar, J.T. Howard, P.B. Allen, J.M. Wilken (Texas Christian University) Numerous electrically-driven microactuators have been investigated for positioning individual elements in microelectromechanical systems (MEMS). The most common modes of actuation are electrostatic, magnetostatic, piezoelectric and thermal expansion. This research focuses on the design, modeling and experimental evaluation of a MEMS thermally-actuated beam. The motivation is to present a unified description of the behavior of the thermal beam so that it can be adapted to a variety of applications in the microsensor and microactuator arenas. The MEMS polysilicon thermally-actuated beam uses resistive (Joule) heating to generate thermal expansion and movement. When current is passed through the actuator from anchor-to-anchor, the larger current density in the “hot” arm causes it to heat and expand more than the “cold” arm. Since both arms are joined at their free (released) ends, the actuator tip is forced to move in an arc-like pattern. Removing the current from the device allows it to return to its equilibrium state. To be a useful MEMS device, a thermally-actuated beam will need to produce in-plane tip deflections that span 0-10 microns while generating force magnitudes greater than 10 micro-Newtons. The thermally-actuated beam design was accomplished with the L-Edit software program, and they were fabricated using the Multi-User Microelectromechanical Systems (MEMS) Process (MUMPs) foundry at the Microelectronics Center of North Carolina (MCNC). Finite element modeling was accomplished with the IntelliCAD computer program. When this CAD software is linked with the MCNC fabrication process description file, it can be used to generate a 3-D solid model and verify a MEMS design. The thermal and electromechanical finite element analyses predicted tip deflections and forces consistent with experimental observations. For example, when the thermal beam’s drive current was 3.5 mA, the “hot” arm’s temperature was 600 degrees Centigrade (Joule heating), and the resulting tip deflection was 4.55 microns. For a beam tip force of 14 micro-Newtons, the displacement was calculated to be 12.9 microns. Finally, the resonant frequency, without air damping, was calculated to be 74.7 kHz. The thermal beam’s performance was also experimentally characterized. When the drive current was varied between 0 and 5 mA, tip deflections spanning 0-13 microns were observed (5.3 microns for 3.5 mA). By using an adjacent simple cantilever beam whose tip could be deflected by the thermal beam, force magnitudes of 8 micro-Newtons were measured. The resonant frequency in ambient air was 68.7 kHz.
3:30 PM		E4/F1-2-7 Contact Fatigue Simulation for Rough Coated Surfaces. M.H. Haselkorn, D.Y. Hua (Caterpillar Inc.) Thin film coatings have been demonstrated to significantly increase the contact fatigue life in a variety of laboratory tests. To understand the reason for these observed increases, a contact fatigue computer simulation was developed to predict the perfromance of thin, hard coatings. This simulation uses the three-dimensional surface roughness to calculate the contact pressure and can accommodate a thin, hard coating. It can be used for either dry or mix lubricated contacts. The simulation has been validated with parametric studies and laboratory test results for diamond-like carbon coatings.
3:50 PM		E4/F1-2-8 Oligocyclic Fatigue Strength in Nitrided Steels B. Cruz, D. Hurley, J.E. Oseguera (ITESM-CEM, Mexico) Based on a statistical model, an analysis and a comparative study of the response to the oligocyclic fatigue in 1018 nitrided steel has been undertaken. The analysis includes the effect of the thickness of the compact nitride layer and the surface finish. The folowing hypothesis were considered for the analysis of the variables. 1) The surface finish does not have an effect on olygocyclic fatigue and 2) the thickness of the nitride layer does not have an effect on oligocyclic fatigue. Transforming the data to have a similar dispersion of the residues and performing the variance analysis the hypothesis were tested: the results indicate that the thickness of the nitride layer has a significant effect on the oligocyclic fatigue behavior while the effect of the surface finish was not significant for these type of test, Also, no significant effect from the interaction between the variables was found. Finally, we carried out the sensitivity analysis on the results and the observations indicate that the taste was more sensitive to differences generated by effects of the layer thickness and differences generated by effects of interaction between these variables.
4:10 PM		E4/F1-2-9 Scratch Resistance of Clearcoats : A Comparative Study of Surface Damage by Taber and Microscratch Testing J. von Stebut, D. Chariot (Ecole des Mines, France); E. Muzeau, B. Magny (Cray Valley, France) Scratch resistance of polymer coatings is frequently assessed by means of the Taber Test (ISO 45 86-2 1997 (3)). In this approach the specimen is rotated in a pin/disk contact geometry under a (90° cone angle, 0.09 mm tip radius) diamond stylus. Constant contact loads are progressively increased until the circular scratch/wear track can be distinguished by the naked eye. For a set of well defined photo polymerised clearcoats the ranking criteria of the Taber test are critically analysed. It is shown that low load micro scratching with the same stylus is much better suited for detailed understanding of prevailing surface damage (micro and nano scratches, tensile type cracks and adhesive failure). This approach is coupled with scanning stylus profilometry and good quality reflected light microscopy. Material specific parameters accessible are on-line and post test track depth, critical cracking loads and crack density. We show that on unloading most of the on-line deformation recovers along the loading direction. Tensile crack propagation progresses at the same speed as the indenter sliding on the coating. We show the influence of coating thickness and curing temperature after testing on the residual damage.
4:30 PM		E4/F1-2-10 Thermomechanical Properties of Carbon-Germanium Alloys J. Vilcarromero (Escola Politécnica da USP, Universidade de São Paulo, Brazil); F.C. Marques (Universidade Estadual de Campinas Unicamp, Brazil) The carbon atom ability to make bonds with different hybridization allows the preparation of a large variety of materials with different structure such diamond, graphite, polymeric films, and nanostructures. The investigation of the carbon properties in different network helps us to understand its properties in addition to the development of new materials. In this work we investigated the properties of carbon-germanium alloys prepared by the sputtering technique, which allowed the preparation of thin films in the whole composition range, i.e. from 0 at.% up to 100 at.% carbon content range, using the same deposition conditions and varying the target composition. The elastic modulus, thermal expansion coefficient, stress, and nanohardnes of these films were investigated for the first time. It was observed that the stress of the alloys decreases in relation to the pure films, a-Ge:H and a-C:H, indicating the degradation of the structure of the alloys. Contrarily to what was expected, the hardness, as well as the elastic modulus, of the alloys with small concentration of carbon is smaller than that of pure germanium films. These properties are analyzed in terms of the structural properties of the films determined by infrared and Raman spectroscopies. Supported by brazilian agencies: Fapesp and CNPq.
4:50 PM		E4/F1-2-11 Finite Element Modeling of the Stresses and Fracture During the Indentation of Hard Elastic Films on Elastic-Plastic Aluminum Substrates R.M. Souza, G.G.W. Mustoe, J.J. Moore (Colorado School of Mines) In this work, a three-step finite element analysis was conducted to study the behavior of wear resistant coatings on soft substrates. Initially, a mesh simulating a system with one hard and elastic film on an elastic-plastic aluminum substrate was developed considering the presence of defects in the film. A sequence of loading steps was then applied to simulate the depostion (intrinsic) stresses, thermal (extrinsic) stresses and contact stresses during the indentation with normal forces, respectively. The influence of film thickness and elastic modulus was studied based on the radial and shear stresses that develop at the film surface and at the film/substrate interface. Results were also compared to ones obtained for the aluminum substrate and no film.