AVS1996 Session TF-MoA: Mechanical Properties

Monday, October 14, 1996 1:30 PM in Room 107B

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	TF-MoA-1 Mechanical Properties of Thin Films and its Interface W. Gerberich (University of Minnesota) Special problems abound when attempting to measure either deformation or fracture response in film/substrate systems where the microstructural scale is commensurate with the film thickness. The present studies emphasize use of nanoindentation techniques to evaluate such systems. For very thin films less than 200 nm thick, two special considerations arise. First, for elastic response, the indentation must be very shallow to avoid deconvolution problems associated with substrate contributions. This suggests nanoindentation depths on the order of 10 to 20 nm where 100 nm diamond radius tips remain spherical on both loading and unloading. For this reason, a special algorithm has been developed which successfully predicts nanoindentation modulus for model materials such as aluminum, tungsten and fused quartz. Second, for plastic response, the hardness or flow stress may be affected by existing mean-free path dislocation considerations, dislocation emission, and dislocation drag or pile-up interactions with the film/substrate interface, all of which can be affected by film thickness. This applies to the "singularity" associated with an indenter stress distribution as well as that of a crack tip such that thin film deadhesion can have a similar set of problems. While most of these problems remain unresolved, some progress in measuring flow and fracture properties of films and interfaces has been made and will be presented concerning W/Si, Au/Si, and Co/GaAs systems.
2:10 PM		TF-MoA-3 Atomistic Simulations of the Nanometer-scale Indentation of Thin Films S. Sinnott (University of Kentucky); C. White, R. Colton (Naval Research Laboratory); D. Brenner (North Carolina State University); J. Harrison (U.S. Naval Academy) The indentation of diamond substrates covered with amorphous carbon thin films using diamond-like tips was investigated with molecular dynamics simulations. These are the first reported simulations where the effects of the film thickness and the tip shape on the results have been investigated in detail. The simulations reveal the atomic-scale mechanisms behind the adhesion of the tip to the film, and show how these mechanisms change as the thickness of the film increases. Two diamond-like tips were used in the investigation, one which is atomically sharp on the end and a flat, hydrogen-terminated diamond-like tip. It is found that the elastic modulus of the tip is strongly dependant on its structure. In each case, however, the diamond-like tip has a modulus that is higher than the primarily graphite-like thin film, and an order of magnitude smaller than bulk diamond. The elastic modulus of the amorphous-carbon film was extracted from the force curve data, and found to be 148 GPa in good agreement with experimentally determined values.
2:30 PM		TF-MoA-4 Correlation of Film Stress and the Nano-Mechanical Properties of Au Thin Films K. Jarausch, J. Houston, S. Casalnuovo (Sandia National Laboratories); P. Russell (North Carolina State University) The interfacial force microscope (IFM) was used to investigate the nano-mechanical properties of Au thin films as a function of residual and imposed film stress. In earlier work, the mechanical response of 200 nm thick polycrystalline Au films grown on various substrates and a single crystal <111> Au surface was consistent for an individual substrate, however the values of effective elastic modulus and shear-stress threshold were found to vary by as much as a factor of three from substrate to substrate. Despite this variation, the ratio of the latter to the former was nearly identical, as was the deformation to the plastic threshold, suggesting that the mechanical response of Au remained very consistent. The data did not identify a dependence of the variation on the films' morphology or adhesion to the substrate. In recent experiments the IFM measurements of effective elastic modulus and shear-stress threshold were found to correlate with the difference in residual stress resulting from the film substrate combination. In the present paper we discuss these results along with those obtained from a diaphragm warping device which mechanically and controllably changes the stress-state of the film. The observed dependence of the mechanical response on stress will be discussed in terms of possible mechanisms, identifying how the stress alters the measurement process and causes the variation of the films' mechanical response. These experiments suggest that the IFM used in nanoindenter mode has the potential for being able to measure residual thin-film stress on a very local level. This work was supported under DOE under Contract No. DE-AC04-94AL85000.
2:50 PM		TF-MoA-5 Nanoindentation of Surfaces: Calculations of the Onset of Near-Surface Plasticity J. Hamilton, C. Kelchner (Sandia National Laboratories) Nanoindenters and Interfacial Force Microscopes allow measurements of force as a function of distance as a probe tip is pressed into a surface thereby providing important information about the mechanical properties of ultra-thin films. For a uniform bulk material, continuum elasticity theory predicts that the force will vary as the 3/2 power of the indentation distance. Experimentally, deviations from this Hertzian force law are observed which are generally attributed to the onset of plastic behavior in the near surface region.We have modeled this behavior using the Embedded Atom Method to calculate the forces on a spherical tip with radius of curvature between 20 and 80 Angstroms. Experiments by Houston and Michalski have used an organic passivating layer on surfaces in order to prevent avalanche bonding of the tip and the surface. In order to model this situation, we replaced the spherical tip by a sphere with a nearly hard wall repulsive interaction with the surface atoms. Force vs. distance profiles for the Au(111) surface were calculated during indentation and retraction of the indenter and compared with experiment. Images of atomic positions and dislocation structure were also obtained. Based on these calculations we will discuss the elastic and initial plastic deformation of this surface. The formation of a dislocation loop across the indentation pit will be demonstrated.
3:10 PM		TF-MoA-6 Mechanical Properties of Epitaxial Mo/NbN and W/NbN Superlattices A. Madan, X. Chu, S. Barnett (Northwestern University); C. Engstrom, H. Ljungcrantz, L. Hultman (Linkoping University, Sweden) Epitaxial Mo/NbN and W/NbN superlattices represent a new class of non-isostructural superlattices in which a bcc metal is combined with a NaCl - structure nitride. Epitaxial Mo/NbN and W/NbN superlattices with modulation wavelengths \Lambda\ ranging from 1.4 nm to 25 nm were grown on MgO(001) substrates by dc reactive magnetron sputtering in Ar/N\sub 2\ mixtures. Epitaxy is achieved because of the small lattice mismatch (\< \ 2.0%) between the (001) planes after a 45\super 0\ rotation about the normal. The superlattices were structurally characterized using high and low angle x-ray diffraction, cross-sectional transmission electron microscopy and Auger electron spectroscopy. Computer simulations of the high and low angle x-ray data yield estimated interface widths to be \<=\0.3 nm. Microhardness measurements were made using a nanoindentor with a Berkovich indenter. Enhanced hardness values as high as 30 GPa are observed at superlattice periods down to 1.4 nm for approximately 50% metal fr action, compared to the rule-of mixtures values of around 10 GPa. The hardness decreases with increasing \Lambda\, to 14 GPa at \Lambda\ = 120 nm. The effect of varying the metal fraction from 15% to 90% keeping the superlattice period roughly 5 nm will also be reported. The results will be compared with model predictions of the expected hardness for superlattices based on the shear-modulus difference between the layers.
3:30 PM		TF-MoA-7 Multilayer Film Deposition of TiN\sub X\/AlN\sub X\ on a Rotating Substrate from Reactive Sputtering of Elemental Targets of Titanium and Aluminum H. Jensen, J. Sobota, G. Sorensen (University of Aarhus, Denmark) (Ti,Al) coatings have previously been deposited by reactive sputtering from (TiAl) targets. The present communication will report on a multilayer approach, where a substrate is rotating succeedingly through reactive sputter deposition zones of elemental targets. In the present study an ALCATEL 650 sputtering system with two pairs of sputtering positions in sandwich configurations was modified to deposit from one side on a rotating substrate holder. In reactive sputtering of Al and Ti the well-known hysteresis curves of total pressure versus reactive gas flow for constant Ar flow are observed. It was thus possible to select nitrogen flow values thus controlling deposition parameters and film stoichiometry and micro-structure. Film deposition was performed on mechanically polished tungsten carbide and silicon for tribological studies and on carbon for compositional and structural studies using the RBS and XRD techniques. The deposited film was also characterized by the acoustic- emission scratch test technique (LSRH Revetest) for adhesion studies, and this was used for studying film/substrate mechanical properties. Critical loads derived from scratch measurements of multilayers will be reported and discussed. The multilayer film system was very cohesive, and there was consistency between the critical load measured by friction and the prompt decrease in acoustic emission. This emphasizes the adhesion values measured by the sudden increase of acoustic emission should be considered critically. Preliminary tribological studies will be reported.
3:50 PM		TF-MoA-8 Formation and Optimization of Multiphase Film Properties of (Ti-Cr)N formed by Cathodic Arc Deposition J. Nainaparampil (Systran Corp.); J. Zabinski, S. Laube (Wright Laboratory); A. Korenyi-Both (Hohman Plating and Mfg. Inc.) It is known that titanium and chromium form two types of nitrides. Values for hardness and friction coefficients of titanium nitride are much higher than those of chromium nitride. This is a study of films formed by a combination of titanium and chromium nitride phases. Deposition of (Ti, Cr)N on M-50 steel substrates is performed in an industrial multi-source steered Cathodic arc deposition system. Ti/Cr ratio and nitrogen content of the films were adjusted by process control to vary the coating chemistry and microstructure. Cr and Ti arc sources deposit thin films (1-2 mm) onto a rotating substrate in a pressure regulated nitrogen background gas. Film properties (e.g. stoichiometry, microstructure friction, wear, hardness and scratch adhesion) are determined by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ball on disk tribotmeter and nanoindentation. This data is then correlated to coating chemistry and microstructure for different arc powers for a constant (optimum) N2 background gas pressure. It is found that films formed by approximate ratio Ti:Cr:N = 37:16:46 shows some very desirable properties. To obtain optimum properties in these films, process control is developed to regulate the relative fraction of Ti/Cr/N in a reliable fashion. Special emphasis is given to film stoichiometry control and Ti/Cr composition ratio to achieve optimum tribological properties. Hardness, friction, and adhesion tests are also correlated to XPS and XRD analysis.
4:10 PM		TF-MoA-9 Preparation and Characterization of Superhard CNx/ZrN Multilayers M. Wu, Y. Chung, M. Wong, W. Sproul (Northwestern University) A dual-cathode, unbalanced dc magnetron sputtering system was used to grow CNx/ZrN multilayers. Nanoindentation results showed that under appropriate conditions, it is possible to get the layered films with high hardness in the 50 GPa regime and modulus of about 410 GPa. X-ray diffraction studies revealed a strong correlation between hardness and the occurrence of (111) texture of ZrN. The surface topography was studied by AFM, and it was found that the surface roughness decreased as the bilayer thickness of the coatings decreased.
4:30 PM		TF-MoA-10 Optical Characterization of Carbon Nitride Thin Films Prepared by rf Sputtering R. Zhao, Y. Li, Z. Zhang, S. Xie (Chinese Academy of Sciences) Carbon nitride thin films have been deposited on Si(100) wafers using rf plasma sputtering graphite target in pure N\sub 2\ or mixture of N\sub 2\/Ar atmosphere. The films containing ~30 to 37 at.% nitrogen were grown at very slow rates, typically 2 3~40\Ao\ /hour. The optical behaviors of CN\sub x\ films were studied by IR, Raman and UV-VIS absorption spectroscopes. The IR spectra show that the double peaks at 2150cm\super -1\ and 2040cm\super -1\ observed for the first time can be attributed to the vibrations of CN triple bonds and -N=C=N- or =C=C=N- cumulated double bonds respectively, whereas the weak band centered at 1250cm\super -1\ is due to C-N single bond which is also Raman active( at ~1210cm\super -1\). The broad IR band around 1500cm\sup er -1\ and the two main peaks, 1357cm\super -1\(D) and 1552 cm\super -1\(G), indicate the presence of graphitelike structures which are characterized by the C=C or C=N double bonds. The optical energy gap was found to be ~0.8eV by fitting the absorption d ata to Tauc's equation. In our films, the various bonds between carbon and nitrogen, i.e., C-N, C=N and CN triple bond, can favor the growth of \beta\-C\sub 3\N\sub 4\, cause the formation of sp\super 2\-cluster and saturated the C dangling bonds by N atoms, respectively. The cumulated double bonds can raise the connectivity of CN networks and improve the mechanical property of the films.