AVS2001 Session SE-MoM: Nanocomposites, Multilayers, & Nanostructured Materials

Monday, October 29, 2001 9:40 AM in Room 132

Monday Morning

Start	Invited?	Item
9:40 AM		SE-MoM-1 Magnetron Sputtered W/C Films with C₆₀ as Carbon Source J.-P. Palmquist (Uppsala University, Sweden); M. Oden, Zs. Czigany, J. Neidhart, L. Hultman (Linköping University, Sweden); U. Jansson (Uppsala University, Sweden) Thin films in the W-C system have been prepared by magnetron sputtering of W with co-evaporated C₆₀ as carbon source. We have previously demonstrated epitaxial growth of several binary and ternary metal carbides as well as superlattice structures and gradient films at very low deposition temperatures (100-500 oC). In this study, we present the first results of epitaxial deposition of several phases in the W-C system. In addition, nanocrystalline tungsten carbide films can be deposited. At low C₆₀/W ratios, epitaxial growth of α-W with a solid solution of carbon was obtained on MgO(001) and Al₂O₃(0001) at 400 oC. The carbon content in these films (10-20 at%) was at least an order of magnitude higher than maximum equilibrium solubility and gives rise to an extreme hardening effect. Nanoindentation measurements showed that the hardness of these films increased with the carbon content and values as high as 35 GPa were observed. At high C₆₀ /W ratios, films of the cubic β-WC_1-x (x = 0-0.6) phase was deposited. This phase is not thermodynamically stable at T< 2500 oC but is frequently observed in thin film deposition. The microstructure of the β-WC_1-x films was dependent on the deposition conditions. At high deposition rates, nanocrystalline films with a grain size <30 Å were obtained in the temperature range 100-800 oC. The hardness of these films varied from 14 to 24 GPa. Also, for the first time, we have demonstrated epitaxial growth of single-crystalline β-WC_1-x films on MgO(001) at very low deposition rates, ~5 Å/min. Finally, at intermediate C₆₀/W ratios, epitaxial films of hexagonal W₂C was deposited on MgO(111), while polycrystalline phase mixtures was obtained on other substrates.
10:20 AM		SE-MoM-3 Synthesis and Characterization of Thermally Stable TiB₂/TiC Nanolayered Superlattice Coatings for Dry Machining Applications K.W. Lee, Y.H. Chen, Y.-W. Chung, K. Ehmann, L.M. Keer (Northwestern University) It was demonstrated from previous studies that nanolayered superlattice coatings with the correct choice of components and layer thicknesses have enhanced hardness, due to interfaces providing barriers against dislocation motion and multiplication. We expect superlattice coatings made of two immiscible components to be stable against interdiffusion. Therefore, the layer structure and reasonable hardness for such superlattice coatings should be preserved at high temperatures. These thermally stable coatings are desirable for protection of cutting tools in dry machining applications, which may operate at temperatures in the 800 â€“ 1000 C regime or higher. For this reason, TiB₂ and TiC were chosen for this investigation. Nanolayered coatings made of these two immiscible components were synthesized by dc dual-cathode magnetron sputtering. Substrate rotation was used to enhance uniformity of the coating. Substrates included silicon, M2 steel and WC cutting inserts. Superlattice coatings with TiB₂ (001) and TiC (111) preferred orientations on Si (001) were synthesized. Transmission electron microscopy studies showed that the layer structure of the coating was preserved after annealing in vacuum at 1000 C for one hour. Room-temperature hardness of these coatings approaches 50 GPa, far exceeding the rule-of-mixture value. Coatings synthesized using the substrate rotation system have improved surface smoothness and reduced internal stress. Wear and durability tests on coated M2 steel and WC cutting inserts demonstrated the improved tribological performance of these coatings under unlubricated conditions compared with other standard coatings such as TiN.
10:40 AM		SE-MoM-4 Chemical Vapor Deposition and Characterization of TiO₂ Nanoparticles W. Li, S.I. Shah, C.P. Huang (University of Delaware); O.J. Jung (Chosun University, South Korea) Chemical vapor deposition (CVD) was used to deposit TiO₂ nanoparticles with and without metal ion dopants. X-ray photoelectron spectroscopy (XPS) and X-ray energy dispersive spectroscopy (EDS) experiments confirmed the TiO₂ chemical composition. X-ray diffraction (XRD) patterns showed the polycrystalline anatase structure of TiO₂. Transmission electron microscopy (TEM) revealed that these particles are nanosized with an average diameter of approximately 20-30 nm. The nanosized particles can provide a large surface to volume ratio and large number of free surface charge carries which are crucial for the enhancement of photocatalytic activity. In order to improve the photocatalysis efficiency, Pd²⁺, Pt²⁺, Nd³⁺ and Fe³⁺ transition metal ion were also incorporated as dopants. The effects of dopants on photocatalytic kinetics were investigated by studying the degradation of 2-chlorophenol (2CP) with an ultraviolet light source. The results showed that doped TiO₂ nanoparticles have higher photocatalytic efficiency than those without dopants with Nd3+ showing the highest efficiency. Time of 90% destruction of 2CP was reduced by one half with Nd3+ doping when compared with undoped TiO₂.
11:00 AM	Invited	SE-MoM-5 Nanocomposite Tribological Coatings with "Chameleon" Surface Adaptation A.A. Voevodin, J.S. Zabinski (Air Force Research Laboratory) Composite coatings where hard nanocrystalline grains are embedded in an amorphous matrix provide considerable improvement in hardness, toughness, wear resistance, and friction reduction. A review of their design concepts is provided with a focus on: (1) improvement in toughness characteristics; and (2) adaptive tribological behavior. Embedding small 5-20 nm hard nanocrystalline grains in an amorphous matrix helps to arrest crack development and introduces ductility through grain boundary sliding. Matrix materials may be selected to provide adaptation of the surface chemistry and/or microstructure to variations in environment and loading conditions to mantain tribological properties. Such materials have been coined chameleon coatings. A combination of nanocrystalline TiC and WC embedded into an amorphous diamond-like carbon (DLC) matrix enabled the coatings to adjust their mechanical response from hard to ductile and significantly reduced the danger of brittle failure. A similar concept was used to improve toughness of composite coatings made of nanocrystalline yttria-stabilized zirconia (YSZ) embedded in an amorphous YSZ/Au matrix. In another example, a combination of nanocrystalline WC and WS2 in an amorphous DLC matrix exhibited surface chemical and microstructural self-adjustment in sliding contact when test environment was cycled from humid to dry. This coating could repeatedly adjust its surface from hexagonal WS2 for sliding in dry nitrogen or vacuum environments to graphitic carbon for sliding in humid air, maintaining a low friction coefficient in both environments. The YSZ/Au nanocomposite developed a gold rich surface layer during heating at 500 °C in air, which considerably improved YSZ tribology in temperature cycling. This coating was further doped with MoS2 and carbon to obtain an environmental adaptation similar to that in the WC/DLC/WS2 system. Chameleon coating designs and applications for advanced tribological coatings are discussed.
11:40 AM		SE-MoM-7 Nanometer-size Monolayer and Multilayer Molecule Corrals on HOPG: A TOF-SIMS, XPS and STM Study Y.J. Zhu, T.A. Hansen, S. Ammermann, J.D. McBride, T.P. Beebe, Jr. (University of Utah) The surface chemistry of highly oriented pyrolytic graphite (HOPG) bombarded with energetic Cs⁺ ions was studied using the combined surface analysis techniques of TOF-SIMS, (time-of-flight secondary ion mass spectrometry), XPS (x-ray photoelectron spectroscopy) and STM (scanning tunneling microscopy). Controlled surface modification and defect production were achieved by bombardment of HOPG with Cs⁺ ions at various energies and at various dose densities. XPS shows cesium implanted into HOPG exists in an oxidized state. The Cs⁺ bombardment of HOPG enhances oxygen adsorption due to both the dissociative adsorption of oxygen at defect sites produced by Cs⁺ ions, and by the formation of cesium oxide. The surface coverage of cesium on HOPG increases linearly with increasing Cs⁺ dose density at low bombardment energies, and decreases rapidly with increasing Cs⁺ bombardment energy due to cesium implantation below the surface. The thermal stability of cesium in HOPG has a complex behavior at elevated temperatures. Defects created by Cs⁺ ion bombardment in HOPG were subsequently oxidized at 650 °C in air to controllably produce nanometer-size monolayer and multilayer molecule corrals (etch pits). Multilayer pits can be produced using higher energy Cs⁺ ion bombardment, and monolayer pits can be produced using lower energy Cs⁺ ion bombardment. The pit density, pit yield, pit diameter and pit depth can be controlled by varying experimental conditions, and they were studied systematically by STM. The measured depth-resolved growth rates for multilayer pits are in good agreement with the model of the growth rate acceleration by adjacent layers. The results obtained lead to a better understanding of the kinetics and mechanism of the graphite oxidation reaction, and more importantly to the accurate production and control of nanometer-size monolayer and multilayer molecule corrals on HOPG.