ICMCTF1998 Session F1/E4-2: Mechanical Characterization - Mechanical Properties and Adhesion (2)

Tuesday, April 28, 1998 1:30 PM in Room San Diego

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		F1/E4-2-1 Tribo-Contacts On Compliant Substrates: Hard Coat Design Incorporating Frictional Dissipation Z. Shi, S. Ramalingam (University of Minnesota) When thin hard coats are used to confer wear resistance to light alloys, the differences in film and substrate elastic properties can result in large film flexure stresses as well as high film-substrate interfacial shear and lift-off stresses, all of which can lead to debonding and coating failure under some conditions. Methods have already been developed and presented to determine these stresses which allow hard coats to be designed for any particular tribological contact geometry and loading without the risk of coating failure. However, power dissipation of varying severity accompanies relative motion in loaded tribo-contacts. The differences in film-substrate thermal properties (thermal diffusivities and coefficients of thermal expansion) can then give rise to significant thermo-elastic stresses. This problem is addressed in this work. Analytical method developed to calculate the thermo-elastic stresses due to an arbitrary heat-flux at the tribocontact is presented. By invoking the superposition principle one may then calculate the total stresses due to contact loading and those due to frictional power dissipation. Calcualted results for a range of tribo-contact conditions are presented and discussed. It is noted that the solution obtained may permit the development of a new, quantitative film-substrate adhesion test free from computational and measurement uncertainties accompanying film and substrate plastic flow as in scratch testing. The proposed adhesion test method is outlined and its relative merits for the quantitative evaluation of film-substrate 'bond strength' are presented and discussed.
1:50 PM		F1/E4-2-2 Comparison of Mechanical Properties of TiN Thin Films Using Nanoindentation and Bulge Test O.R. Shojaei, A. Karimi (EPFL, SWITZERLAND) To characterize mechanical properties and functional reliability of thin film coatings, several experimental techniques have been designed. In this paper mechanical properties of TiN thin films were determined using both bulge test and nanoindentation techniques and the results were compared. Thin layers of TiN were deposited by means of a radio frequency magnetron sputtering facility on Si(100) and amorphous SiN substrates, and coatings are less than 1 micron thick. Bulge test experiments were performed on free standing membranes to determine relationships between deposition parameters and mechanical properties, including Young's modulus and residual stresses. Parallel to bulge test, depth sensing nanoindentation measurements were conducted on the film supporting by their substrates. For both types of experiments, results show increasing values of Young's modulus and hardness with increasing negative bias voltage and nitrogen to titanium (N/Ti) ratio measured by electron probe micro-analysis. The values of bulge test were found to be close to those of nanoindentation for less heterogeneous films. In contrast, for films with highly developed columnar morphology, the values seem to be significantly scattered. Microstructural investigations using scanning electron microscopy and transmission electron microscopy show important morphological changes in the crystalline growth of the film as a function of deposition parameters. In this paper, bulge test and nanoindentation analysis methods were described and the comparison of results were emphasized.
2:10 PM		F1/E4-2-3 The Mechanical Properties of Polycrystalline Silicon Carbide Films using Bulk-Micromachined Diaphragms S. Roy, A.J. Fleischman, C.A. Zorman, M. Mehregany (Case Western Reserve University) This paper reports the measurement of the mechanical properties of polycrystalline silicon carbide (poly-SiC) thin films using bulk-micromachined diaphragms as test structures. SiC is a high-temperature semiconductor with excellent wear resistanc Silicon bulk-micromachining was used to fabricate the poly-SiC diaphragms. First, 3 um-thick polysilicon films were deposited on 1000Å-thick silicon dioxide layers on 4-inch silicon wafers. Next, 2 um-thick poly-SiC films were grown on the polysilic An interferometrical technique was used to measure the deflection of each diaphragm for a range of applied pressures. Deflections were large, but kept below the elastic limit, which was verified by measuring the flatness of the diaphragms after repeate The extended paper will: (1) describe the test structure fabrication process; (2) detail the acquisition and analysis of the load-deflection data; and (3) evaluate poly-SiC as a structural material for MEMS. ¹ S. Roy, C. Zorman, C. Wu, A. Fleischman, and M. Mehregany, Mater, Res. Soc. Proc. 1997, 444, 81.
2:30 PM		F1/E4-2-4 Mechanism for Stress Relaxation in Si-rich Silicon Nitride Thin Films S. Habermehl (Sandia National Laboratories) Low residual stress silicon nitride thin films represent critical layers in advanced surface micromachined components such as micro-motors, gear drives and pressure sensors. Si-rich silicon nitride thin films have been deposited by Low Pressure Chemical Vapor Deposition, LPCVD, at 850°C from mixtures of dichlorosilane and ammonia. The films' elastic properties have been studied as a function of film composition. It is observed that the residual stress decreases with increasing Si content. Fourier transform infrared spectroscopy, FTIR, and ellipsometric data indicate that the local atomic strain is a strong function of the calculated volume fraction of Si contained in the films. A relationship is observed that shows the strain to be inversely proportional to the cube root of the Si volume fraction. A model that accounts for distortion in Si-Si_4-nN_n tetrahedra (n=0-4), upon substitution of silicon for nitrogen in the film, is applied to the data. The model is shown to be consistent with measurements of intrinsic film stress across a compositional range from stoichiometric silicon nitride, Si₃N₄, to nitrogen-free amorphous silicon, a-Si. The conclusion is reached that stress relaxation in the films is a manifestation of strain relief at the molecular level as the Si content increases.
2:50 PM		F1/E4-2-5 Mechanical Behaviour of Submicron Multilayers Submitted to Microtensile Experiments M. Ignat (INPGrenoble, France) Any further development or utilisation of a multilayer in an interconnect system needs a precise identification of it's mechanical limits. Results obtained by micromechanical tensile tests, performed in a SEM, on different sorts of multilayers, consisting of thin films of Ti, TiN, Al vapour deposited on ductile Ti substrates, will be presented. The evolution of the damage (cracking and debonding) comes with or without the observation of the crack deviation at weak interfaces, which caused catastrophic debonding in some of the systems. The results of the experiments, in terms of critical parameters (strains and stresses, cracking distances, material properties) are compared to the prediction of analytical models, which currently assume the elastic response of the layers and perfect bonding. Residual stresses, determined by a x-ray diffraction technique, are considered in the analysis of stress distributions during the cracking of the layers. Taken together: the in-situ observations, residual stress determinations and the chemical analysis in the cracked zones, permit the identification of weak interfaces in the multilayers.
3:10 PM		F1/E4-2-6 The Effects of Particle Pollution on the Mechanical Behaviour of Multilayered Systems M. Poulingue (Leti CENG, France); M. Ignat (INPGrenoble, France); J. Dijon (Leti CENG, France) The mechanical stability of multilayered structures depends on their intrinsic as extrinsic properties. Among them, the thermoelastic properties of the materials consisting the layers, as the deposition conditions, which combined may induce internal stresses; also the microstructural imperfections, which offer sources of damage activation. For example, contineous or sequential laser irradiations on multilayered mirrors, have shown that the observed damage (cracks, holes with or without debonded layers) starts from localized zones, which show growing imperfections. With the aim to precise the mechanism which initiates the damage in the structure, we analysed the response to an external stress of multilayered samples, with and without "polluted" interfaces. The "non polluted" samples correspond to mirror like polished substrates on which were deposited the layers. The "polluted" ones were obtained by the dispersion of fine diamond particles on the polished substrates, before the deposition. The mechanically damage was induced by pulling the samples in tension in a SEM. Critical parameters, characterizing the behaviour and describing the damage mechanisms are compared and discussed.
3:30 PM	Invited	F1/E4-2-7 Mesomechanical Studies of Coated Materials V.E. Panin (Russian Academy of Science, Russia) A new approach to the investigation of coated materials under loading is developed on the basis of physical mesomechanics of materials. It is shown theoretically that the interface between coating and substrate under loading is the origin of very dangerous oscillating stress concentrators. They cause the appearance of a set of quasiperiodically distributed microcracks within coating, localized mesobands within the substrate, and the breaking of bonds between coating and substrate. The experimental investigations of the mechanisms of deformation and failure at the mesoscale level of a wide range of coated materials were performed using a special television-optical system. The observed regularities reinforce the predictions of the theoretical calculations. The definite recommendations are formulated for design of interfaces between coatings and substrates in coated materials.
4:10 PM		F1/E4-2-9 Substrate Preparation and Stud Pull Adhesion of Metal Films on Graphite-Epoxy Composites P. Kraatz (Independent Consultant); W. Mellberg, G. Young (Space Systems/Loral) Sputtered multilayers of chromium and copper are employed as adhesion enhancing layers on surfaces of graphite-epoxy substrates. These sputtered layers provide a basis for electroplating copper and silver. The effect of substrate preparation techniques upon adhesion strength of deposited metal films are investigated. The stud pull adhesion test (Military Standard 883-C-Method 2027) is employed as a tool for quantitative measurement of adhesion strengths of metal films to graphite-epoxy substrates. Preparation techniques studied include dry abrasion, with "Scotch Brite" or 400 grit sandpaper, and wet abrasion, with surfactants, water, and solvents. Highest stud pull strengths are associated with pre-sputter cleaning with aqueous surfactants, DI water, and ethanol, independent of mechanical abrasion. Lowest adhesion strengths are associated with dry abrasion. Effects of electroplating with silver upon adhesion vary with surface preparation before sputtering. Wet chemical etching of sputtered layers before electroplating improves adhesion by removing oxidation damage associated with thermal cycling in air. Auger and ESCA results suggest that formation of a thin carbide phase at the interface between the graphite-epoxy and sputtered chromium correlates with higher adhesion strength, while unusually high oxygen content at the interface correlates with lower adhesion strength.
4:30 PM		F1/E4-2-10 Influence of Nitriding on the Microstructure, Toughness, and Thermal Stability of Modern and Conventional Hot Forging Die Steels M.R. Krishnadev (Laval University, Canada); S.C. Jain (EMTL) This paper discusses the effect of nitriding on the microstrucure and mechanical properties ( toughness and thermal stability) of conventional H13 as well as the new generation of premium grades of microalloyed die steels. Both low blow dynamic instrumented impact testing as well as instrumented Charpy testing have been used to characterize the crack initiation and propagation resistance. SEM and oblique section metallography have been used to charcterize the microstructure and fracture morphology. The relevance of the results to commercial practice are discussed.
4:50 PM		F1/E4-2-11 Examination of General Mechanical Properties and Failure Mechanisms of W, Hard W(C) and W/W(C) Bilayer Coatings E. HARRY, A. ROUZAUD, P. JULIET, J. DANROC (French Atomic Energy Commission, FRANCE) This paper focuses on the correlation of general properties (mechanical and microstructural) of single layers, bilayers and “gradient” layers of W and W© coatings, elaborated by DC reactive magnetron sputtering and their failure mecanisms during scratch test experiments. This study was motivated by the need to understand the elemental mechanical behaviour of single layers to optimise the promising W/W© multilayers. Main characterisations were: (1) morphology and texture investigated by SEM and XRD (2) internal stresses and hardness and (3) adhesion measurements by means of scratch-test experiments. Cristallites size of dense and fibrous W© film and columnar pure W layers are estimated. Compressive stresses of -3 GPa are found both in W and W© films deposited on steel substrate. Coatings hardness is found to be independent of the level of residual stress, but mainly correlated to the grain size and morphology of sputtered films. The behaviour of the coatings is investigated in details in terms of adhesion by extensive scratch test experiments. Main trends are the following: columnar W layer don’t exhibit flaking even at high loads, due to stress accomodation by the columnar structure. Significant chips are observed in the dense W© films resulting from the propagation and coalescence of cracks into the brittle material. The intermediate behaviour is observed in multilayer and gradient coatings. In this last case, the W ductile phase damps the propagation of cracks in the W-C film lowering the flaking of the composite. A correlation between film thickness and this behaviour is also discussed.