Mechanical Properties and Adhesion
Wednesday, May 2, 2001 8:30 AM in Room San Diego
F1/E4-1-1 Predictive Modelling of the Hardness of Single and Multilayer Coatings
S.J. Bull (University of Newcastle, United Kingdom)
The use of modelling to extract the hardness of a coating from coating/substrate composite data is well-established and a number of different models have been developed which show some success in analysing real data. However, these are reactive models and are not normally able to predict the performance of a coating system which is substantially different from those that have already been investigated. This makes assessing the potential of new coating systems very difficult without producing coatings. As the complexity of coating designs increases, for instance in the multilayer and superlattice coatings increasingly used in the tooling or glass industry, the need for a predictive coating hardness model becomes more acute. This is because the answers to basic questions like what order the layers should be in and how thick should they be cannot be obtained without extensive experiments. This paper will introduce a predictive modelling approach based on apportioning contributions to the work of indentation from the bulk materials and interfaces which make up the coating which has had some success as a predictive model. The advantages and limitations of the model will be discussed, drawing on results for a range of single and multilayer coating systems.
F1/E4-1-2 A New Systematic Method of Characterization for the Mechanical Strength of Thin Films on Substrates
S. Kamiya, H. Kimura, K. Yamanobe, M. Saka, H. Abé (Tohoku University, Japan)
Evaluation of mechanical strength is an important and difficult problem for the case of thin films prepared on substrates. Until now, for example, scratching method has been commonly used which measures the load required to scratch off the film from the substrate surface. However, these common methods define the strength on the basis of complicated fracture introduced at the same time in the film, interface and substrate. Therefore it is difficult to utilize the results of measurement, from a scientific point of view, for the estimation of real performance of thin film structures or for the improvement of those strength. To be more scientific, the strength of a thin film prepared on a substrate should be recognized as the strength of a materials system which is made of the film and the interface. The macroscopic fracture of this system should be controlled by the strength of a film and an interface, as well as the residual strain and elastic properties in/of the film. These properties should be separately obtained to fully characterize the mechanical strength.@paragraph@We have developed a systematic method of measurement for all these properties. By applying external load to the specimen, where the film is projected a little out of the edge of the substrate, cracks can be introduced exclusively in film or along interface. The strength of film and interface is independently evaluated in terms of toughness on the basis of crack extension resistance. The deformation behavior of the film gives its Young's modulus, and the buckling behavior of film projection is used to estimate the residual strain.@paragraph@As examples of application of our method, those properties mentioned above were evaluated for the case of diamond films prepared on silicon wafers as well as cobalt-cemented tungsten carbide cutting tools. We would demonstrate the applicability and ability of our method of measurement for the strength of thin films as the strength of materials systems.
F1/E4-1-3 Investigation of Threshold Load for Yield Initiation in Coating-Substrate Systems
K.C. Tang, R.D. Arnell (University of Salford, United Kingdom)
Due to the numerous possible combinations of coating/substrate materials and configurations, the accurate computation of the yield initiation and the complex mode of plastic deformation in such systems can only be made possible by means of numerical methods. In this study, the threshold loads corresponding to the initiation of plastic deformation in both the coating and substrate under normal contact with a deformable indenter are investigated using finite element methods. A wide range of practical values for the ratios or coating thickness to indenter radius, coating to substrate elastic modulus, and coating to substrate hardness are analysed. This leads to an operational envelope for the coating and substrate materials subjected to similar indentation loading, within which the materials will deform elastically.
F1/E4-1-4 Mechanical Properties of Fluorocarbon Polymer Thin Films Grown by PECVD
S.A.S. Asif (Univ. of Florida, Gainesville); E.J. Winder, K.K. Gleason (Massachusetts Institute of Technology); K.J. Wahl (Naval Research Laboratory)
Thin polymer films have been of considerable interest recently in applications for electronics packaging, biomedical devices, solid lubrication, MEMS devices, antifouling and adhesives. However, evaluating the mechanical properties of polymer thin films is difficult due to the low elastic moduli and penetration depth limitations (to avoid substrate influence). In this paper, we present an approach for measuring the mechanical properties of thin, compliant polymer films using AC force-modulated nanoindentation. @footnote 1@ The dynamic response of the indenter is monitored during tip-sample approach, enabling sensitive detection of the surface. Adhesive interactions, contact stiffness and damping are monitored during force-displacement measurements, and hardness and modulus evaluated. In this study, we investigate the correlation between polymer thin film deposition conditions and the resulting film chemistries and mechanical properties. The thin polymer films were deposited on Si wafers using pulsed plasma-enhanced chemical vapor deposition (PECVD). Two different precursors were used, HFPO (hexafluoropropylene oxide) and HFC-134 (1,1,2,2, tetrafluoroethane); growth conditions were varied by altering the plasma duty cycle during deposition (plasma on-time/plasma off-time). Film thickness was measured by ellipsometry and atomic force microscopy, and chemistry examined by XPS and FTIR. Film thickness varied between 75 and 400 nm. Hardness of the films varied between 0.02 to 0.4 GPa, and storage modulus between 2 and 10 GPa. The results show that the hardness and modulus of the films were dependent on both precursor chemistry and plasma duty cycle. Good correlation was found between the mechanical properties and chemical composition of the films. @FootnoteText@ @footnote 1@ S.A.S. Asif, K.J. Wahl, and R.J. Colton, Rev. Sci. Instrum. 70 (1999) 2408.
F1/E4-1-5 Quantified Small-Scale Dynamic Mechanical Testing
J.F. Smith (Micro Materials Limited, United Kingdom); N. Jennett (The National Physical Laboratory, United Kingdom)
We recently reported @footnote 1@ a number of new small-scale dynamic testing methods appropriate for the investigation of thin films. At that time, these techniques were qualitative in nature, although the results were clearly indicative of the type of surface under test (e.g., work hardened vs. annealed).@paragraph@Two of these dynamic methods have now been quantified: (1) It is possible to produce repetitive impacts on a surface and to monitor both the surface degradation and the accumulated energy delivered to the contact point. (2) dynamic hardness can be measured from the coefficient of restitution of a surface subjected to a small rebound test. The latter is highly significant for dynamic processes such as microelectronic bonding; static and dynamic hardness results are generally quite different.@paragraph@A number of bulk and thin film materials have been examined using quantified dynamic methods. For dynamic hardness, the energy per unit volume has been determined for indentations produced by impact and these values have been compared with the plastic work peformed in quasi-static indentation. For comparison, dynamic hardness measurements have also been made using the diamond rebound technique. For quantified impact wear, the total energy for failure of a fused quartz surface has been related to the energy of the individual repetitive impacts. This is particularly relevant to erosive wear. @FootnoteText@ @footnote 1@ ICMCTF, San Diego, 2000.
F1/E4-1-7 Comparison of Thin Film Adhesion Techniques
W.W. Gerberich, J. Jungk (University of Minnesota); A.A. Volinsky, J. Vella, I. Adhihetty (Motorola, Inc.)
Decreasingly small sizes of thin films in nanotechnology demand reliable means of assessing toughness. A number of techniques are reviewed including the superlayer, the superlayer indentation, the 4-pt. bend and the double-cleavage, drilled compression (DCDC) tests, with the first three providing most of the current data available. Some of the differences involved in terms of phase angle (Mode II/Mode I) and microstructural differences as dependent on test type will be reviewed. The superlayer tests can evaluate as-deposited films whereas the latter two tests above involve sandwich specimens which offer somewhat different constraint and/or utilize bonding temperatures. In addition the sandwich specimens do not currently address film residual stress issues, also different for free standing films. Because of industrial practice to thermally process or anneal films after deposition, e.g.electrochemical copper, or to want to evaluate interface toughness at a layer buried in a film stack, both of these approaches are valid. Where possible we will attempt to highlight differences and similarities as affected by test type and processing variables.
F1/E4-1-9 Buckling Study of Thin Films on Substrate Under Compressive Stresses by Atomic Force Microscopy
C. Coupeau, F. Cleymand, M. George, J. Colin, J. J. Grilhe (University of Poitiers, France)
Thin films mainly produced by sputtering methods often develop very high residual compressive stress of sometimes few GPa and are then susceptible to delamination, resulting in interesting topographical pattern.@paragraph@The aim of this study is to generate external stresses in well-adherent thin films by uniaxial compression of the substrate, in order to characterize the mechanical behavior of the films and analyze the first stage of bucking patterns.@paragraph@Various shapes of bucking structures such as straight-sided wrinkles and worm-like pattern have been in situ observed by atomic force microscopy and studied under compressive stress. Experimental results are presented and compared to elastic energy models.
F1/E4-1-10 Accurate Determination of Young's Modulus and Poisson's Ratio of Thin Films by a Combination of Acoustic Microscopy and Nanoindentation.
M. Bamber, B. Derby (UMIST, United Kingdom)
Advances in nanoindentation technology have allowed easier and more accurate measurement of surface mechanical properties of thin films and multilayers. The error associated with a poorly defined contact area has been reduced by more sophisticated modelling, and continuous strain measurement now allows the Young's Modulus to be determined throughout the loading cycle (Oliver WC, Pharr GM. J. Mater. Res., Vol.7, No.6, pp1564, 1992). Acoustic microscopy can also be used for the measurement of near surface modulus of thin films. Experimental determination of the Rayleigh Wave velocity is used to calculate Young's Modulus with a high degree of accuracy. However, both techniques are dependent on accurate appraisal of Poisson's ratio in order to calculate Young's Modulus. Calculation of Poisson's ratio by traditional techniques is not suited to the field of thin films and especially multilayers. Here we will describe how by combining acoustic microscopy and nanoindentation we can use two independent measurements of the elastic properties of a surface to determine both Young's Modulus and Poisson's ratio of an isotropic surface layer. Experimental results from the near surface region of bulk materials and thin hard surface coatings will be presented.
F1/E4-1-11 Evaluating Interfaces and Adherence by Laser-Acoustics
D. Schneider (Fraunhofer Institute for Material and Beam Technology IWS Dresden, Germany); H. Ollendorf (White Oak Semiconductor); T. Schuelke (Fraunhofer USA - Center for Surface and Laser Processing); B. Schultrich (Fraunhofer Institute for Materials and Beam Technology, Germany)
The interface has an essential effect on the adhesion of coated materials. Especially, the efficiency of hard coatings depends on perfect bonding between film and substrate materials. Substrate pretreatment and interface layers are used to improve the adhesion of the film. Optimizing the interface structure is often a time-consuming process that depends on the availability of test data for the adhesion behavior. Test methods such as scratch test, for-point-bending test, cavitation, impact or sand blasting test are applied to evaluate the adhesion behavior. In comparison of these conventional techniques, results are presented, demonstrating the potential of the laser-acoustic method for evaluating the defect density and interface weakness, both essentially influence the failure of the coating. This non-destructive test method based on surface acoustic waves has already been shown to yield useful results such as the Young's modulus of thin films. Its ability for studying film adhesion is owing to the used waves penetrating through the film into the substrate. They should be sensitive to interface imperfections. Since the elastic modulus reduces with enhanced defect density, it is expected to indicate the degradation of film and interface strength. The investigations were performed for TiN coated steel and CVD-diamond coated WC-Co cutting tools. The adhesion of the TiN films deposited by ion plating was varied by varying the pre-sputtering time with argon ions. A sample series of different adhesion was available in this way for comparing results of destructive adhesion tests with those of the laser-acoustic technique. Interesting correlations were found for the test parameters deduced from these methods, revealing the dominating effect of micro-cracks. CVD-diamond coatings were deposited by a high current DC-arc and by hot filament deposition on WC-Co. The surface quality of the cemented carbide substrate was varied by different etching to study the effect of the surface morphology on the quality of the diamond-coated tools. Depending on the pretreatment, a distinctly reduced elastic modulus of the film was measured, although a rather perfect diamond structure was revealed by Raman spectroscopy. Enhanced porosity and defect density in the region near the interface was identified by REM studies to be the reason for this modulus reduction.