Mechanical Characterization. Micromechanical Testing and Modelling

Thursday, May 3, 2001 1:30 PM in Room California

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1:30 PM E4/F1-2-1 Steps Towards a Mechanical Modeling of Layered Systems
T. Chudoba, N. Schwarzer, F. Richter (Technical University of Chemnitz, Germany)
Up to now the search for suitable film substrate combinations in respect to mechanical applications is mainly based on an empirical approach using results of hardness, scratch, wear or other tests. It is desirable to cut down this time consuming process by a higher degree of modeling. Steps towards this aim are presented using a novel methodology for the evaluation of the response of coated substrates to mechanical contact. This approach is based on the combination of a recently developed theoretical method and high-accuracy indentation experiments using spherical indenters. For different film substrate combinations it is shown how accurate and reliable values of mechanical film parameters like Young's modulus and yield strength can be obtained, even for films below 100 nm thickness. Then the theoretical method is used for the calculation of critical loads for failure of film-substrate combinations in dependence on the radius of the counterpart. It is shown how the addition of an intermediate layer can improve the whole compound and which properties it should have. Finally the influence of an additional tangential load due to friction in a sliding motion on the stress fields is investigated. The conventional scratch test is discussed in the light of the latter results.
2:10 PM E4/F1-2-3 Contact Modelling in the Vicinity of an Edge
N. Schwarzer, I. Hermann, T. Chudoba, F. Richter (Technische Universität Chemnitz, Germany)
In the paper the problem of a spherical contact in the vicinity of an edge will be considered by analytical modelling. Pushed by the increasing use of nanoindentation as a tool for the investigation of the mechanical property profile of cross-section samples the paper is separated into three parts. First a short introduction will be given into the problem of the theoretical modelling and the mathematical procedures necessary to solve load problems for so called quarter spaces, which deal as a model for rectangular bounded bodies. In a second part an overview of the elastic field resulting from a Hertzian load in the vicinity of an edge is given. Finally the effect of the edge and its distance from the contact centre on the measurable parameters indentation depth and force are discussed with first experimental results obtained using a UMIS-2000 nanoindenter system.
2:30 PM E4/F1-2-4 An Investigation of the Wear Track on DLC (a-C:H) Films by Time-of-Flight Secondary Ion Mass Spectroscopy
H. Fukui, M. Irie, Y. Utsumi, K. Oda, H. Ohara (Sumitomo Electric Industries, Ltd., Japan)
Tribological behaviors between diamond-like carbon (DLC) and steel under ambient air without lubricant were investigated. The DLC (a-C:H) film was prepared on cemented carbide substrate by r.f. plasma assisted chemical vapor deposition using methane (CH@sub 4@) precursor. The hydrogen content in the DLC film was approximately 35 atomic %. Pin-on-disk experiments were conducted on DLC-coated disk at sliding velocities at 52 mm/sec under 10 N loading using steel ball (AISI 5210, 6mm in diameter) as the pin material. After the evaluation of friction coefficient and wear resistance, the wear debris on the wear track on the DLC films were investigated using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). The present results of the TOF-SIMS show the existence of macromolecule of hydrocarbon (C@sub n@H@sub m@) on the wear track. This result suggests that the low friction performance of a-C:H film attributes to a formation of some kind of lubricant material containing polymer (macromolecule of hydrocarbon) which is generated by polymerization of a-C:H film during sliding.
2:50 PM E4/F1-2-5 Mechanical Properties of Very Thin Surface Layers Affected by Material Removal Processes
S. Swaminathan, S. Kompella, S.P. Moylan, S. Chandrasekar (Purdue University)
It is well known that solid surfaces are subjected to local, intense deformation by material removal processes such as machining and grinding. Little is known about the mechanical and physical properties of these highly deformed layers. Here, we report results from a study in which nano-indentation has been combined with taper-sectioning to analyze the mechanical properties of these surface layers. Materials investigated were high-strength steels, copper, brass, and titanium, which had been subjected to machining or grinding. The results have shown that the layers are only a few micrometers in thickness; significant changes in hardness occur within these layers; and these layers are composed of sub-microcrystalline structures. Implications for the results to the tribological performance of components are briefly discussed.
3:10 PM E4/F1-2-6 Mechanical Properties of Amorphous SiCN films
K.J. Ma (Chung-Cheng Institute of Technology, National Defense University, Taiwan); C.T. Wang (Chung-Cheng Institute of Technology, National Defense University, Taiwan, ROC); C.L. Chao (Dankan University); H.C. Lo, K.H. Chen (Institute of Atomic and Molecular Science, Academic Sinica, Taiwan, ROC); L.C. Chen (National Taiwan University, Taiwan, ROC)
Ion beam sputtering from SiC targets assisted with an atomic nitrogen source were employed to deposit amorphous SiCN films. Atomic nitrogen source significantly enhances the incorporation of nitrogen in the SiCN films and promotes the formation of Si-N and C-N bonds, which leads to a higher hardness(H) and elastic modulus(E). High nitrogen content (61%) amorphous SiCN films attain a hardness H(~20GPa) typical of inorganic ceramics but have Young’s modulus values E(~79 Gpa) significantly lower than ceramics. The corresponding H/E ratios (~0.25) are relatively high and their values are more similar to that for hard polymers than to those for ceramics. This indicated that amorphous SiCN films are the promising candidates for tribological application. The measured residual stresses of amorphous SiCN films is compressive and is found to increase with increasing the deposition temperature. Post treatments such as rapid thermal annealing and laser irradiation were carried out to promote the formation of Si3N4 phase and the reduction of residual stress.
3:30 PM E4/F1-2-7 Analysis of Surface and Subsurface of Sliding Electrical Contact XC48 steel / XC48 Steel in Magnetic Field
H. Zaidi, K.J. Chin, J. Frêne (Université de Poitiers, France)
This paper reports the analysis of surface and subsurface after the friction tests for XC 48 steel/XC 48 steel sliding couple. The friction tests were carried out with a pin-on-disc tribometer (pin and disc: XC48 steel) in ambient environment with normal load N = 62.85 N, linear speed v = 0.38 m/s, crossed by a D.C. electric current I in contact and submitted to an A.C. magnetic field H applied perpendicularly to the friction area. Experimental results showed that the presence of electric current and magnetic field around the tribocontact modifies the mechanical properties of this couple. Their simultaneous application changes the friction coefficient m from 0.16 without I and H to 0.23 with I and H in our tests and reduces its variation. We could see the smoother surface wear with magnetic field compared with the condition without it. Scratch test showed the difference of the hardness on the surfaces with magnetic field from that without it. The application of magnetic field induced also the changes of ultrasonic vibration in this ferromagnetic material.@paragraph@The magnetic field modifies the mechanical properties of material in the sliding contact by interaction with residual stresses, cyclic contact stresses and movement of dislocation toward the contact surface. We have characterized this interaction by an increase of the microhardness and the activation of the surface contact oxidation.@paragraph@The modifications induced in contact surface and subsurface of XC 48 steel during friction test were analyzed by Vickers microhardness measurement, Scratch test equipped with acoustic emission detector, Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and by Ultrasonic device. The residual contact stress fields resulted by magnetized and non magnetized sliding contact with and without electric current application were determined by X-ray diffraction.
3:50 PM E4/F1-2-8 Microstructure and Stress Development in Magnetron Sputtered TiAlCr(N) Films
F. Huang, J.A. Barnard, M.L. Weaver (The University of Alabama)
TiAlCr(N) coatings were reactively magnetron-sputtered from a Ti-51Al-12Cr alloy target in this study by changing the nitrogen partial pressure over the range of 0% to 25% of the total pressure. The influence of nitrogen addition on the microstructure and stress development in the TiAlCrN films was investigated. It has been found that, with the increase of nitrogen partial pressure, the film microstructure undergoes a transition from amorphous-like metallic to crystalline nitride films, with the only crystalline phase that can be identified via X-ray diffraction analysis being chromium nitrides. The intrinsic stresses determined through the modified Stoney equation are all compressive and become more so with nitrogen addition over most of the partial pressure range, while the stress-temperature curves during annealing vary significantly among the films. Nitrogen additions were found to increase the film's resistance to yielding in compression, which becomes very significant for >=12% nitrogen addition. The stress-temperature behavior is also related to the nitrogen-induced modification in film oxidation resistance as determined through the X-ray reflectivity technique.
4:10 PM E4/F1-2-9 Influence of Substrate Bias on the Structure and Mechanical Properties of ta-C:W Films Deposited by Filtered Cathodic Vacuum Arc
C, Yuhang, B.K. Tay (Nanyang Technological University, Singapore)
Tungsten contained tetrahedral carbon (ta-C:W) films were deposited by an new off-plane double bend filtered cathodic vacuum arc technique. Raman spectroscopy and atomic force microscopy (AFM) are used to characterize the films structure and surface morphology. Substrate bending methods and nano indenter were used to determine the internal stress and hardness. The influence of substrate bias on the surface morphology, roughness, structure, internal stress, and hardness was systematically studied. All the deposited films were atomically smooth. The internal stress, hardness and Young's modulus for the deposited films increase with increasing substrate bias, reaching the maximum at the substrate bias of –80V, then decrease drastically. At the bias of above –500V, the increase of substrate bias results in the slight decrease of internal stress, hardness and Young’s modulus. The correlation between the structure and the mechanical properties of the deposited films were established.
4:30 PM E4/F1-2-10 A Comprenhensive Nitriding Study by Low Energy Ion Beam Implantation on Stainless Steel
F. Alvarez, P. Hammer (Universidade Estadual de Campinas, Brazil); C.A. Figueroa, R.G. Lacerda, R. Droppa, F.C. Marques, D. Wisnivesky (Instituto de Fisica, Unicamp, Brazil)
Plasma nitriding is an established method for hardening steel surfaces. However, even today, we do not have full understanding of the physical phenomena involved in the non-equilibrium process of ion implantation, nitrogen diffusion, and structural changes induced in the material. In this work, stainless steel 316 was implanted with N+ ions at constant 380 @super o@C using an ion source and studied in situ with spectroscopy of electrons. Studies of the hardness of the material as a function of depth and N content, energy of the implanted ions, scanning electron microscopy, and X-rays structural analysis were performed. The superficial nitrogen content increases for decreasing N+ ion energies. This is an expected result since lower implantation energies gives deeper profile of implanted atoms. However, after long depositions (~8 hr) the amount of nitrogen found deep in the material is larger in samples implanted at higher energies. The non-equilibrium character of the bombardment deposition process results in non-proportionally of the absolute N content and hardness. Indeed, much harder material is obtained in samples containing equal amounts of N but produced in longer depositions, i.e., time is fundamental for the necessary phase transformation associated with hardening. Samples annealed in situ at constant N2 pressure and temperature atlow energy (0.2 KeV) implantation show N profiles equivalents to those samples obtained with higher energy implantation (~1 KeV). The latter samples, however, are much harder. This result shows that the intense ion bombardment also has an important effect on the phase transformation associated with the material hardness. As known, ion bombardment with noble gases produces slightly metal hardening underneath the surface. Unexpectedly, we found that samples containing low N content (~3 %) and moderated hardness (3 GPa) increases to 6-7 GPa with Ar+ bombardment. The structural changes due this effect will be discussed.
4:50 PM E4/F1-2-11 Multi-Mode Scratch Testing- Extension of Operating Modes and Upgrade of Instrumentation - A European Standards, Measurements and Testing Study
J. von Stebut (INPL-Ecole des Mines, France)
We present the final state of the art of an E.C sponsored R&D project involving the two major European scratch test instrument manufacturers. Instrumentation upgrade concerns an interactive video microscope with a 1 million pixel resolution, a reliable diagnostic tool for coating damage assessment. Optical resolution allows for post-synchronised off-line crack identification. In addition, highly enhanced acoustic emission sensitivity is implemented. This way online triggering of data storage related to specific (cohesive and/or adhesive) failure events with bursts of cracking energy release, selective post-synchronisation is achieved. The multi-mode operation software menu implements - in addition to standard modes like constant and progressive load operation - fatigue modes like "multi-indentation" in the same indent and "multi-pass scratching" in the same track (or in the scanning mode), the latter both in reciprocating and in unidirectional sliding. These operation modes allow for bridging of the gap between single cycle, high contact pressure, accelerated testing and more realistic, low cycle macro-elastic fatigue testing closer to industrial friction and wear situations. In this paper we present the results of an inter-comparison exercise among 7 partners practising the major operating modes on two types of reference coatings (i.e.: 1.2µm thick DLC and 3µm thick carbon doped chromium) deposited on HSS substrates.