ICMCTF2000 Session F1/E4: Mechanical Properties and Adhesion

Tuesday, April 11, 2000 8:30 AM in Room San Diego

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	F1/E4-1 Analysis of Sub-Micron Indentation Testing with Spherical Indenters A.C. Fischer-Cripps, A. Jamting, T.J. Bell, F.J. Lesha (CSIRO Divison of Telecommunications and Industrial Physics, Australia) Estimations of both elastic modulus and hardness of thin film structures are now routinely available from load vs penetration measurements using instrumented indentation testing machines which usually employ the three-sided Berkovich indenter. There is growing interest in the use of spherical indenters for thin film testing as the transition from elastic to plastic behaviour can be studied. However, the analysis of the data presents some interesting theoretical and practical challenges. This paper addresses these issues by first describing and comparing two theoretical spherical indenter analysis procedures and then detailing the corrections required to account for experimental conditions with a particular emphasis on non-uniformities in the indenter geometry
9:10 AM		F1/E4-3 Determination of Mechanical Film Properties of a Bilayer System due to Elastic Indentation Measurements with a Spherical Indenter T. Chudoba, N. Schwarzer, F. Richter (Technical University of Chemnitz, Germany); U. Beck (Bundesanstalt für Materialforschung, Germany) A recently developed theoretical model [1, 2] represents the generalization of the indentation of a sphere into an infinite homogeneous halfspace (Hertzian indentation) to the indentation into an infinite halfspace covered with one or more films having different elastic properties. The model allows the analytical calculation of the complete elastic deformation and stress field within the films and the substrate. Some results of the model shall be confirmed by nanoindentation experiments with an UMIS-2000 nanoindenter into Si₃N₄/SiO₂ and SiO₂/ Si₃N₄ double layers on BK7 glass. The used materials allow accurate measurements due to their homogeneous, amorphous structure and low surface and interface roughness. After the determination of the instrument compliance and the true, depth dependent indenter radius the measured load-depth data are compared with calculation results. It is shown that measurement results can be correctly interpreted by the model. The onset of plastic deformation is investigated for the same samples by multiple partial unloading experiments with a 4µm diamond sphere. The critical load where a first deviation from a wholly elastic response occurs is used for a stress calculation with the theoretical model. The von Mises comparison stress and the radial stress are used for the interpretation of the mechanical behavior of the different film combinations. The usage of the measured results together with the analytical modeling allow an optimization of the thickness of the single layers to get a maximum mechanical stability. [1] N. Schwarzer, F. Richter, G. Hecht, Surface and Coatings Technology 114 (1999) 292 [2] N. Schwarzer: Analytical Modeling of Mechanical Contact Problems on Multilayer Systems, This conference
9:30 AM		F1/E4-4 Determining the Accuracy of Film Properties Computed with the Finite Element Method P.J. Wolff (MTS Nano Instruments Innovation Center) The finite element method is a promising mathematical technique that can be used to determine thin film mechanical properties from a composite film/substrate measurement. Others have used this technique to compute thin film properties and its use will probably increase due to the wide availability of finite element solvers and due to the robustness of the technique. A critical component for using this technique is understanding the uncertainty in the computed thin film properties. The uncertainty will depend on several factors that include; the experimental parameters being matched, the film thickness, the indentation depth, the film properties, and the substrate properties. This paper will examine the effects of these parameters on computed thin film properties and will provide general guidelines for maximizing the accuracy of these computations.
9:50 AM		F1/E4-5 Nanoindentation of a-C and a-C:N Thin Films Prepared by Shielded Arc Ion Plating O. Takai, N. Tajima, H. Saze, H. Sugimura (Nagoya University, Japan) Amorphous carbon (a-C) and amorphous carbon nitride (a-C:N) thin films show excellent mechanical properties such as low friction and high wear resistance. These films have, therefore, the potential for much wider applications. Hence the correct understanding of the nanomechanical properties of the films is most important. In this paper we report on the nanohardness measurements for the hydrogen-free a-C and a-C:N thin films prepared by shielded arc ion plating. The hardness was measured with a nanoindenter interfaced with an atomic force microscope. A Berkovich-type diamond tip with a radius of less than 100 nm was used for the measurements. The shape function of this diamond tip was determined by indenting a standard fused quartz sample of 9 GPa hardness. At first we investigated the effect of substrate bias voltage on the hardness of the a-C and a-C:N films. The a-C films showed maximum hardness of 35 GPa at the bias voltage of -100 V. On the other hand, the a-C:N films did maximum hardness of 15 GPa at -300 V. Next, we studied the effect of loading rate on the loading curves of the films. At low loading rates the loading curves showed different behaviors. After analysis these behaviors were found to relate to the viscoelasticity of the films.
10:30 AM		F1/E4-7 An Investigation of Small Scale Repetitive Impact Testing of Surfaces and Thin Films. J.F. Smith (Micro Materials Limited, United Kingdom); N.M. Jennett (National Physical Laboratory, U.K.) Impact testing on the micro and sub-micro length scales is now possible using recent developments in depth sensing indentation (DSI). Damage in solids is driven by energetically determined mechanisms. A novel development of a pendulum based DSI instrument to create a small scale impact tester is described where the damping and kinetic energy of the pendulum is varied. This enables both the energy of impact and the momentum of the impact to be selected. Continuous measurement of displacement allows determination of the maximum impact depth, the height of rebound and the residual indent depth of each impact. Automation allows repeated impacts at the same point or at an array of points. This enables the study of strain-hardening, progressive impact damage at a single point or the simulation of erosion in a controlled manner. Data from repeated impacts at different energies is reported. A comparison will be made with partial loading and repeat loading curves (quasi static indentation by DSI) and the possibility of relationship to classical rebound (dynamic) hardness will be explored. Results will be presented from both metallic films and ceramic materials.
10:50 AM		F1/E4-8 Porous Chiral Thin Films: Characterization and Mechanical Properties M.W. Seto, M.J. Brett (University of Alberta, Canada) Unique, microstructured thin films have been fabricated with the Glancing Angle Deposition (GLAD) technique. These porous, columnar thin films can be engineered with a diverse range of microstructures using a variety of materials to produce arrays of posts, helices, chevrons, periodically bent nematics (s-shapes), and many other configurations. The GLAD technique utilizes deposition at highly oblique incident angles onto unheated substrates to take advantage of shadowing and diffusion mechanisms. Due to the large incident angle, the shadowing effect is enhanced as areas of previous film growth physically shadow any subsequent flux, resulting in the evolution of isolated columns of material. And since the substrates are unheated, the diffusion length of the adatoms is limited and they do not fill in the voided areas produced from the shadowing process. The helically structured thin films are of particular interest, as they resemble macroscopic springs and suggest that they might behave in an analogous manner. By optimizing the geometry of the “microsprings”, it is theoretically possible to create films that are capable of being actuated. Initial characterization of these chiral films has been underway, with some of the mechanical properties of these microsprings studied using atomic force microscopy (AFM) and nanoindentation. The study revealed that the spring constant of porous microspring films was twenty times lower than that of standard, dense films¹. Further investigation of these microstructured films is in progress, which is expected to reveal film properties that are unique to those fabricated by the GLAD technique and unlike those of traditional thin films. ¹M.W. Seto, K. Robbie, D. Vick, M.J. Brett, and L. Kuhn, "Mechanical Response of Thin Films with Helical Microstructures", J. Vac. Sci. Technol., B 17(5), 1999, pp. 2172-217.
11:10 AM		F1/E4-9 Dynamic Stiffness and Creep Behavior of Magnetic Tapes X. Li, B. Bhushan (The Ohio State University) Magnetic recording tapes use an ultra-thin polymeric material as a substrate for the deposition of magnetic coatings. The dynamic mechanical properties of magnetic tapes play a critical role in determining the performance and life of tapes. The force modulation method in depth-sensing nanoindentation gives a direct measure of dynamic contact stiffness, and being insensitive to drift, allows an accurate observation of small volume creep to be carried out over long time periods. In this paper, the dynamic stiffness, elastic modulus, hardness, and creep behavior of magnetic tapes were studied using a depth-sensing nanoindenter with a harmonic force. The applied stress during creep was also measured. Continuous stiffness measurement technique can be satisfactorily used for studying the dynamic mechanical behavior of magnetic tapes.
11:30 AM		F1/E4-10 Microstructure-property Relationships in Arc-Evaporated Cr-N Coatings M. Odén, J. Almer (Linkoping University, Sweden); G. Hakansson (Tixon, Brukens Sverige AB) Chromium nitride (Cr-N) coatings have received increased attention for tribological applications due to their favorable properties including wear resistance, toughness and oxidation resistance. These properties, in turn, can be strongly influenced by the coating microstructure and residual stress resulting from deposition and subsequent processing operations. In this study these microstructural-property correlations are investigated in Cr-N coatings grown by arc-evaporation. Previous investigations on these coatings have identified the dependence of the microstructure and residual stress on both incident ion energy during growth and ex-situ thermal annealing. Prominent features during growth include formation of metastable amounts of the (textured) cubic CrN phase, and high levels of compressive residual stress and defect density. During annealing to 550°C the residual stress and defect density decrease substantially, accompanied by a diffusion-based CrN to Cr₂N phase transformation. The effects of these microstructural modifications on the elastic-plastic and fracture properties of the coatings are quantatively evaluated. Significant variations in these properties, as determined from a combination of nanoindentation, surface acoustic wave and scratch techniques, occur after annealing. These changes are then correlated to the Cr-N microstructure in order to better understand coating performance. For example, it is shown that hard and adherent coatings can be attained after annealing through the formation of nano-scale equiaxed grains and a diffusion-bonded interface.
11:50 AM		F1/E4-11 The Importance of Temperature as a Nano-scale Mechanical Testing Parameter J.F. Smith (Micro Materials Limited, United Kingdom) In the past decade, the depth-sensing indentation (DSI) technique has been well developed and has found many applications in the evaluation of mechanical properties of thin film materials. Most of the DSI experiments so far, however, have been conducted only at room temperature, since DSI instruments for slowly measuring nano-scale displacements are generally highly sensitive to thermal expansion. Clearly, elevated temperature measurements are highly desirable because many thin films, e.g., wear-resistant coatings on tool bits, are invariably used at higher temperatures. The too°C hot stage previously reported for a pendulum-type nano-testing instrument has been used to investigate several materials. Single crystal Si (100) was shown in earlier work to experience a significant change in Young's modulus at 200°C, while the hardness remained essentially constant. These results have now been extended to 400°C, allowing a deformation model to be proposed. In addition, alumina and titania coatings on glass have been investigated over a wide temperature range. The temperature dependence of the film hardness, modulus and scratch resistance have been measured and the results related to the coating performance.