Papers

ICMCTF2011 Session E2-2: Mechanical Properties and Adhesion

Thursday, May 5, 2011 1:30 PM in Room California

Thursday Afternoon

Start	Invited?	Item
1:30 PM		E2-2-1 Fatigue Damage in Ultra Thin Cu Films Cynthia Volkert, C. Trinks (Institute for Materials Physics, University of Göttingen, Germany) It has recently been observed that thin metal films exhibit completely different fatigue behavior than thicker films and large grained bulk samples. The thin films show much higher fatigue resistance and the typical fatigue damage that is observed in bulk materials, such as extrusions and complex dislocation structures, is replaced by interface cracks and isolated dislocations. These observations indicate reduced plasticity in the thin films and is likely a manifestation of their increased strength. Up until now, studies have focused on Cu films on polyimide with film thicknesses down to 50 nm. Here, a new method for fatigue testing is introduced based on free resonance of a film-coated cantilever in an AFM, which allows fatigue testing of high quality films with thicknesses down to 20 nm and to cycle numbers as high as 10¹¹. Using this method, the high cycle fatigue behavior of very thin films has been systematically investigated in the regime where dislocations are hindered and fatigue damage is dominated by interfaces. The evolution of fatigue damage as a function of film thickness and strain amplitude will be reported as well as the effect of cyclic loading on grain growth.
2:10 PM		E2-2-3 Strain-Rate Sensitivity of Strength in Macro-to-Micro-to-Nano Crystalline Nickel Ryan Humphrey, Alan Jankowski (Texas Tech University) The strain-rate sensitivity of strength is a key parameter to evaluate the deformation of crystalline materials. It is widely reported that many metals strengthen with increasing strain rate, wherein an increase in the strain-rate sensitivity exponent occurs as the grain size decreases. The strain-rate sensitivity exponent as evaluated from a power-law relationship between yield strength and strain rate is thought to increase when the deformation mechanisms change. As an increase in the strain-rate occurs, strengthening is attributable to alloy content, then to dislocation activity, and finally to an increase in effective mass – also known as the phonon drag regime. We will evaluate the behavior of nickel over eight-orders of magnitude change in strain rate to determine if the change in strain-rate exponent is affected by the scale of the grain size from the macro- to micro- to nano- scale range as the strain rate increases. In this study, tensile testing is used to measure the strain-rate dependence of the tensile strength on the grain size in crystalline nickel foils. Similarly, micro-scratch testing is used to determine the strain-rate dependence of the scratch hardness variation with scratch velocity. Results for these two test methods are compiled for strain rates that range from 10^-5 to 10³ sec^-1. It is found that these mechanical test results can be directly compared, and the increase in strain rate sensitivity exponent with increasing strain rate is slower for nanocrystalline than for microcrystalline nickel. This work was supported by the J.W. Wright Endowment for Mechanical Engineering at Texas Tech University.
2:30 PM		E2-2-4 Strain Rate Effects on Coated Surfaces' Response and their Film Fatigue Fracture: An Investigation by a Novel Impact Tester with Modulated Repetitive Force Konstantinos-Dionysios Bouzakis, George Maliaris, Stylianos Makrimallakis (Aristoteles University of Thessaloniki, Greece, CERTH, Greece & IPT, Germany) For investigating the effect of strain rate on the surface response and film fatigue fracture of coated specimens subjected to cyclic impact loads, an impact tester was developed, facilitating the impact force modulation concerning its signal pattern. This device consists of a high rigidity base, a linear drive supported by appropriate electronic equipment and a piezoelectric actuator, for generating the impact force. In this way, impact loads with sinusoidal, triangular or trapezoidal patterns, at adjustable frequencies and impact durations can be applied. By the developed impact tester, experimental results were obtained, revealing the effect of the strain rate on film fatigue fracture induced by the coated specimens dynamic surface response. The surface response depends among others, on the substrate and coating strain, strain rate properties. These properties were quantified via an analytical FEM-supported procedure for describing the coated specimen deformation during the impact test. Based on these data, the critical strain, strain rate combinations leading to film fatigue fracture developed at various impact force signal patterns were determined.
2:50 PM		E2-2-5 Microstructural Analysis of the Failure Mechanisms of Amorphous Carbon Coating Systems in Load-Scanning Tests Harald Hetzner, Jens Schaufler, Stephan Tremmel, Karsten Durst, Sandro Wartzack (University Erlangen-Nuremberg, Germany) Combining a low friction coefficient and good wear resistance, amorphous carbon coatings offer very attractive tribological properties. However, their load-carrying capacity and overload capability are more limited compared to other hard coatings and this still prevents their use in higher loaded applications like rolling bearing raceways or forming tools. The mechanical failure of amorphous carbon coating systems is most often attributed to the high residual stresses and low cracking resistance of the functional carbon layer. But the stress states and fracture behavior of metallic and ceramic adhesion and interlayers as well as the interfaces may also play an important role. Thus, it is necessary to understand the mechanical behavior of the whole coating-substrate system under application-oriented conditions in order to locate and eliminate the weak spots. In the present study the failure mechanisms of different amorphous carbon coating systems deposited on cold work tool steel in dry sliding contact against uncoated cold work tool steel were investigated. The coating-substrate systems were subjected to a tribological model test on a load-scanning test rig under medium to high loading conditions. The test setup consists of two crossed cylindrical specimens that are forced to slide reciprocally against each other while the normal load is gradually increased. The test kinematics results in a straight contact path along the cylinder where each point corresponds to a specific loading condition. In addition to the normal load, the number of load cycles and the surface roughness of the uncoated counter body were varied in the tests. The contact paths of the tested specimens were investigated with atomic force microscopy and scanning electron microscopy in terms of wear and damage of the surface. Furthermore, focused ion beam techniques were used for in-situ material removal in order to prepare micrometer scale cross sections of surface areas showing damage indications. This allowed for the directed study of subsurface failure mechanisms like crack propagation and decohesion of coating interfaces. In addition to the physical tests finite element simulations of the loaded coating-substrate systems were performed in order to determine the stress conditions that are responsible for the initiation of the observed failure mechanisms.
3:10 PM		E2-2-6 A Route to Avoid Thermo-Mechanical Fatigue Damage in Al Thin Films Walther Heinz, Gerhard Dehm (Montanuniversität Leoben, Austria) Cyclic compressive and tensile stresses occur in metallic films and interconnects applied in sensors and microelectronic devices when exposed to temperature changes. The stresses are induced by differences in the thermal expansion coefficients of the adjacent materials. Repeated cycling leads to damage evolution and, eventually, to failure. In this study we report on a successful strategy how to avoid thermal stress induced fatigue damage. We analysed the deformation structures of 0.2 to 2 µm thick Al films subjected to thermal cycling between 100°C and 450°C up to 10000 times. The investigations reveal that a reduction in film thickness or controlling the Al texture and the Al/substrate interface structure can be used to prevent thermo-mechanical fatigue damage. The findings are explained by orientation dependent plasticity and differences in dislocation mechanisms for different interface structures, and less accumulated plastic strain for thinner films. The approach is expected to apply in general for metallic films on substrates.
3:30 PM		E2-2-7 Evaluation of Mechanical Properties in Cu Thin Films Under Various Substrate Conditions by Molecular Dynamics Simulation Jen-Ching Huang, Yi-Chia Liao (Tungnan University, Taiwan) For thin films deposition, surface roughness and substrate temperature control many important physical and chemical properties. Additionally, more and more researches in the thin films growth greatly emphasize the nanoscale characterization with simulation. This paper mainly aim to study the nanoscale mechanical properties of Cu thin films deposited on monocrystalline silicon substrates with various surface roughness at elevated temperatures by molecular dynamics (MD). To investigate the adhesion under different interface roughness, the single-crystal Cu thin films deposited on silicon substrates were tested in nano-tensile, and the strength of Cu thin films was also discussed. To sum up, the surface roughness of substrate could directly affect the thin films morphology and the adhesion between coatings and substrates. Moreover, the substrate temperatures and thickness of single-crystal Cu films may cause significant variation in adhesion.
3:50 PM		E2-2-8 Comparison Titanium and Zirconia Dental Implants’ Stress Analysis Using Finite Element Method Ruhi Yesildal, Filiz Karabudak, Melikepinar Yildirim, Funda Bayindir (Ataturk University, Turkey) The purpose of this study was to use three-dimensional finite element analysis (FEM) to analyze stress distributions patterns in implant restorations made of titanium and zirconia. Two three dimensional (3-D) FEM models of a mandibular incisor implants were modelled surrounded by cortical and cancellous bone. For first model; Ti-6Al-4V for implant fixture and abutment , yttrium tetragonal zirkonium polycrystal (Y-TZP) for zirconium framework, feldsphatic porcelain for superstructure material; for second model; Y-TZP for implant fixture, abutment and zirconium framework, feldsphatic porcelain for superstructure material are used. Two implants and their superstructures were modeled using CAD software Pro/Engineer and the mandibular is modelled using MIMICS software. Two solid models of mandibular incisors transferred to mesh model in FEM (ANSYS) to analyze. No important difference is observed in resolved stresses between implant–abutment-crown combinations, thus, zirconia implant may be viable alternative for esthetic region.
4:10 PM		E2-2-9 Effect of Nitrogen Flow Ratio on Microstructure and Property of Ta-Ti-N Thin Film by Reactive Sputtering of Ta-Ti Target Chen-Kuei Chung, Nei-Wen Chang, Tai-Sheng Chen (National Cheng Kung University, Taiwan) Binary transition-metal nitride coatings of tantalum nitride (TaN) and titanium nitride (TiN) have been extensively studied for the mechanical and microelectronics application due to high hardness and low resistivity. It is of thus interest to study the combination of both TaN and TiN properties for the promising application. In this paper, the effect of nitrogen flow ratio on the evolution of microstructure, hardness and resistivity of the promising Ta-Ti-N thin films has been investigated using reactive magnetron sputtering of the Ta-Ti alloy target. The Ta-Ti-N thin films were deposited on Si(100) substrates at differ ent nitrogen flow ratios (FN₂% = FN₂/(FAr+FN₂) x 100%) of 0-20%. The microstructure, morphology, composition, bonding, hardness and resistivity of films were measured by grazing incident angle X-ray diffraction (GIXRD), atomic force microscopy, energy dispersive spectrometry, X-ray photoelectron spectroscopy, nanoindentation and four-point probe, respectively . GIXRD results showed that the Ti-Ta alloys formed at 0 FN₂% were polycrystalline with the merged bcc α-Ta and bcc β-Ti crystal structure. Increasing FN₂% resulted in the increased nitrogen content and Ta/Ti ratio in Ta-Ti-N films as well as nitrogen-doped α-Ta (α-Ta(N)) and NaCl-type (Ta,Ti)N (c-(Ta,Ti)N) structure. The hardness of Ta-Ti-N films initial increased with FN₂% then decreased at high FN₂% while the resistivity continuously increased with FN₂%. A appropriate FN₂% is good for the formation of Ta-Ti-N film with much enhanced hardness and low resistivity for the novel coating application. The relationship between the process, microstructure and property was further discussed and established.
4:30 PM		E2-2-10 Mechanical Properties of Vapor Deposited Polyimide Robert Chow, Mark Schmidt (Lawrence Livermore National Laboratory) The mechanical properties of vapor-deposited polyimide (VDP) coatings were determined as a function of the cure temperature. Monomers of PMDA and ODA were evaporated from separate sources to coat various metalized and alumina surfaces. A stoichiometric coating was defined by the absence of compositional defects induced by excessive monomer concentrations. The coatings were cured for 2 hrs at temperatures from 150°C to 400°C. The hardness and derived modulus of elasticity were determined from nano-indentation measurements. The coating adhesion was measured [1] from surfaces with and without an application of an adhesion promoter. The adhesion of the VDP was compared to wet-cast Kapton [2] and Parylene-C [3] coatings deposited on the same surfaces. This work (LLNL-ABS-430771) performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. References ASTM standard 5179, Standard Test Method for Measuring Adhesion of Organic Coatings to Plastic Substrates by Direct Tensile Testing. S.C. Lee, F.C. Tai, C.H. Wei, and J.I Yu, ATR-FTIR and Nanoindentation Measurements of PMDA-ODA Polyimide Film under Different Curing Temperature, Materials Transactions, v 48, n 6 (2007), 1554-1557 Xylene Polymers, Encyclopedia of Polymer Science and Engineering, v 17, 2^nd edition (1989) John Wiley & Sons, Inc.
4:50 PM		E2-2-11 Elastic Properties of Metastable Mo_1-xSi_x Alloys: A Brillouin Light Scattering Study Philippe Djemia (Université Paris, France); A. Fillon, Gregory Abadias, Anny Michel, Christiane Jaouen (University of Poitiers, France) Elastic properties of MoSi alloy films have been investigated by the Brillouin light scattering technique thanks to the analysis of the surface acoustic waves in the film on substrate. A transition from crystalline to amorphous state is observed for a Si content, x = 0.20. This transition is accompanied by different modifications of the elastic constants, namely, C₁₁, C₃₃, C₁₂, C₁₃ and C₄₄. A more pronounced softening of the shear elastic C₄₄ constant from 110 Gpa to 60 Gpa is observed from pure molydenum to this critical threshold. The longitudinal constants C₁₁ or C₃₃ have experienced a softening from 420 Gpa to 300 Gpa whereas C₁₂ or C₁₃ constants have not shown modification in this range of Si concentration. An intrinsic consequence of the high supersaturation of MoSi alloys is the development of an important lattice instability. In the amorphous state, the evolution of the elastic properties exhibits two distinct behaviors depending on the electronic properties of the amorphous alloys.
5:10 PM		E2-2-12 The Effect of Film Thickness Variations in Periodic Cracking: Shear Lag Analysis and Experiments Aidan Taylor (Erich Schmid Institute, Austria); Viktoria Edlmayr (Montanuniversität Leoben, Austria); Rishi Raj (University of Colorado-Boulder); Gerhard Dehm (Montanuniversität Leoben, Austria) Periodic cracking experiments are frequently used in the assessment of interface quality in brittle film/compliant substrate systems. Through these techniques it is possible to extract a quantitative measure of interface shear strength and therefore assess the mechanical suitability of these systems for application. The influence of film thickness inhomogeneities on the crack spacing is assessed in this study. While film thickness inhomogeneities are always present in thin film systems, only nominal thickness values have been considered up to this point. By defining two separate regimes of film thickness variation, roughness and unevenness, defined in relation to the crack spacing, the influence of such variation on the data is analysed. The results of this analysis are then considered in reference to a model system of an amorphous alumina film on a copper substrate (Al_xO_y/Cu). A further means of analysing the data produced by such periodic cracking experiments is presented and its application to the Al_xO_y/Cu system is demonstrated. This crack neighbour ratio analysis validates the application of a shear lag approximation in determining interface properties for such systems.
5:30 PM		E2-2-13 Optimized Adhesion Strength of TiSiN Films Deposited by a Combination of DC and RF Sputtering A.R. Bushroa, H.H. Masjuki, M.R. Muhamad (University of Malaya, Malaysia); Ben Beake (Micro Materials Ltd, UK) This paper outlines the experimental studies of adhesion strength of TiSiN films deposited using a combination of direct current (DC) and radio frequency (RF) PVD magnetron sputtering (DR-PVD) on high speed steel (HSS) substrates. An L9 Taguchi orthogonal array was used to conduct the design of experiment for finding the optimum process parameters. Four process parameters, namely, RF power, DC power, Nitrogen to Argon (N2/Ar) gas ratio and the deposition time have been considered. Pareto analysis of variance on the micro scratch of the coating has shown that amongst the variables, RF power has the significant influence on the adhesion strength of the TiSiN films. The surface structure, morphology and composition of films of selected samples have been studied under scanning electron microscopy (SEM), energy dispersed x-ray techniques (EDX) and x-ray diffraction (XRD). The results indicate that higher adhesion strength is achieved using RF power of 100 W, DC power of 500 W, N₂/Ar ratio of 1:2.5 and a deposition time of 6 hours. Subsequent optimization has resulted in the increase of the adhesion strength value from 177 mN, signifying a tremendous improvement of 747.46%.