ICMCTF2001 Session E3-2: Coatings Resistant to Severe and Unusual Environments

Monday, April 30, 2001 1:30 PM in Room California

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	E3-2-1 LIGA Microtechnology and the Need for Coatings J.M. Hruby, R.P. Janek (Sandia National Laboratories) Body of Abstract: LIGA is a micromachining technique that employs x-ray lithography to create small, precise, high aspect ratio patterns in photoresist that are electroplated to produce a metal-filled pattern. The metal pattern can consist of many individual parts that are separated from the wafer to produce precise microparts or the LIGA pattern can be used as a mold insert for injection molding. Injection molding allows for replication in polymers, ceramics, or other metals. If LIGA is used to produce metal parts, the parts are usually used in a microelectromechanical system that moves. A typical application of LIGA is in a micromotor that creates high torque for its size. In this application, the tribology and wear of the microparts are important issues for performance, reliability, and lifetime understanding. In some cases, there is a need to decrease the friction coefficient or the wear experienced on LIGA parts and therefore a need for conformal coatings. If LIGA is used to create a mold insert, then lifetime and durability of the mold insert is important. Again, such application may require a conformal coating. Conformal coatings of such small, precise parts is a daunting task. In this talk, an overview of LIGA processing and application will be presented. Data on friction coefficients and wear of LIGA parts will be presented, both for baseline materials and also for some initial coatings studies. The challenges for coating techniques will also be discussed.
2:10 PM		E3-2-3 Hydrogenated DLC and CN_x Based Multi-layer Films for LIGA Micromachines: Synthesis and Tribological Behavior S.V. Prasad, T.A. Friedmann (Sandia National Laboratories) Electrodeposited nickel is a widely used structural material in microelectromechanical systems (MEMS) fabricated by LIGA. Tribological issues, e.g. friction, wear, debris particle generation, can influence the performance and reliability of Ni based MEMS. This study describes the synthesis and tribological characterization of two multi-layer films specifically designed for electroformed Ni substrates. Both films had a sputtered Ti adhesion layer and a pulsed laser deposited (PLD) tetrahedral amorphous-carbon (ta-C) intermediate support layer. The top lubricious layer in the first set of multi-layer films consisted of a 100 nm thick hydrogenated DLC grown by PLD, while the one in the second set was a PLD CNx film. Friction and wear measurements were made using a linear wear tester in ball-on-disk configuration against steel and silicon nitride counterfaces. Measurements were made in dry nitrogen and laboratory air. Wear scars and transfer films on the counterface were analyzed by high resolution scanning electron microscopy. Results of the study showed that friction and wear behavior of the multi-layer films was influenced such factors as couterface material, roughness of the Ni substrate, the nature of the transfer film on the counterface, and the environment. The optimum tribological behavior, i.e. friction coefficient < 0.04 with no significant debris generation, was observed whenever a smooth transfer film of the lubricant was formed on the counterface.
2:30 PM		E3-2-4 Properties of Thick Sputtered Ta used for Protective Gun Tube Coatings D.W. Matson, ED McClannahan (Pacific Northwest National Laboratory); S.L. Lee (US Army ARDEC); D. Windover (Benet Laboratories) Thick tantalum coatings were deposited on the bore surfaces of 25 mm I.D. cylindrical gun steel substrates using a high-rate triode sputtering apparatus. Sputtering parameters affecting the tantalum phase and microstructure were investigated. Prior work h as indicated that the sputtering gas species and substrate temperature during deposition affect the characteristics of the tantalum coatings.¹ In the work presented here we report on experimental studies aimed at evaluating additional effects resulting from changes in sputtering gas pressure and substrate bias during the deposit. Tantalum deposits of 75 to 140 µm thickness were evaluated using x-ray diffraction, optical microscopy, and microindentation hardness measurements. Using krypton gas, a 200°C substrate temperature, and a 100 V substrate bias, little or no effect was noted on the tantalum coating properties at gas pressures ranging from 3.5 to 50 mTorr. Tantalum coatings deposited under these conditions were found to be primarily dense bcc phase tantalum. Coatings deposited using krypton gas, a 200°C substrate temperature, and 3.5 mTorr gas pressure also showed little variation when deposited at substrate biases ranging from 50 to 150 V. However, the tantalum coatings produced under similar conditions with an unbiased (floating) substrate were found to be the tetragonal phase of the material and had a columnar microstructure. Implications of microstructural and phase effects of tantalum coatings on gun tube applications will be discussed. ¹Effect of sputtering parameters on Ta coatings for gun bore applications, DW Matson et. al., Surface Coatings and Technology (in press).
2:50 PM		E3-2-5 Tungsten Carbide and Co-Base Alloy Coatings for Replacement of Chromium Plating R.N. Johnson (Pacific Northwest National Laboratory); J.E. Kelley, N.J. Price (Advanced Surfaces and Processes, Inc.) Increasingly stringent OSHA and EPA regulations on the use of hexavalent chromium electroplating have driven the search for alternative coatings and processes. ElectroSpark Deposition can be used for many Cr replacement applications, especially those requiring limited area coatings of large parts, in-place coating and repairs in the field or repair depots, and part geometries that cannot be repaired by other alternative processes, such as plasma spray and HVOF. ElectroSpark Deposition is a micro-welding process that produces coatings that are metallurgically bonded, yet with very low heat input into the substrate. Rapid solidification produces nanostructured coatings with enhanced wear and corrosion resistance. WC and cobalt-based coatings by ESD are being qualified for Cr-replacement applications in repair of DoD components. Properties and structure of the ESD coatings are described.
3:10 PM		E3-2-6 Galling Prevention in Extreme Environments with Engineered Surfaces D.N. Hopkins (TXU Electric); T. Spalvins (NASA - Glen Research Center) Time, money, and materials are unnecessarily wasted when galling prevents equipment from being disassembled or results in excessive friction that causes equipment to perform poorly. Thin, highly adherent engineered surfaces of soft metals (such as gold and silver) significantly reduce the risk of galling under harsh conditions where conventional lubricants are often not effective. These harsh conditions include elevated temperature, heavily loads, and long periods of time where the interface remains stagnant. The thin metallic films create a substantial "contrast" in the chemical composition and physical properties between the two contacting surfaces and were applied using the PlasmaBond process. This metallurgical contrast results in a gall-resistant interface. Typically the temperature of the substrate increases by only a few degrees during deposition so it is possible to deposit adherent metal surfaces on plastic or elastomeric substrates without changing their bulk physical or chemical characteristics. Laboratory tests have been developed to assess the galling resistance of these soft-metal engineered surfaces under simulated service conditions. These tests include disassembly of heavily loaded bolts from steel blocks that had been heated for a month, measuring fastener performance (the conversion of applied torque to fastener preload) on successive assembly cycles, and measuring friction between a valve stem and packing over thousands of valve strokes. For threaded fasteners, the test objectives have focused on assembling leak-free gasketed joints and on trouble-free disassembly of gall-prone joints. Each of these test programs included both high-quality conventional lubricants and soft-metal engineered surfaces. TXU Electric began using components with soft-metal engineered surfaces in its generating stations in 1992. This field experience confirms that soft-metal engineered surfaces are more effective against galling than conventional lubricants in harsh conditions.
3:30 PM		E3-2-7 Oxidation Behavior of Cr-Al-N and CrN/AlN Films Synthesized by Cathodic Arc Plasma Deposition S.S. Kim, J.H. Bin, J.G. Han (Sung Kyun Kwan University, Korea) The oxidation resistance of steel substrates used for mechanical applications may be enhanced not only by plasma diffusion process, but also hard coating deposition. It is well-known that transition metal nitride coating such as TiN, CrN and TiAlN are widely used for mechanical applications. Especially TiN is often deposited on cutting tools such as mills and drills, whereas CrN is rather used for forming applications such as drawing dies and moulds. Cathodic arc deposition process is a powerful and commonly used technology. For such hard coating deposition it offers a wide range of advantages. However, nitride coating, which have good tribological properties, are rarely developed for hot temperature applications and, in most cases, it is difficult to both provide good oxidation resistance of a coating and keep its surface mechanical behavior. In this study the Cr-Al-N and CrN/AlN films were deposited on the working tool steels (AISI H13steel) by cathodic arc plasma deposition (CAPD) process. The Cr-Al-N films were deposited under various Al concentrations of 20% to 60%. Relative chemical compositions of the deposited Cr-Al-N films were evaluated by EDX as well as AES. The microstructure and morphology were studied by XRD and SEM respectively. The microstructures of Cr-Al-N films were strongly depended upon target current ratio of the chromium and aluminum. Moreover, we found that the microhardness of the Cr-Al-N films were higher than that of the CrN film. Nano-scaled super-lattice CrN/AlN films were deposited by jig rotation speed control. Lattice images of CrN/AlN films were observed by HRTEM. Micro-hardness of the super-lattice CrN/AlN coatings(Hk 4000) was greater than that of multi-components Cr-Al-N coatings(Hk2400-3500). Detail results including oxidation behavior will be presented.
3:50 PM		E3-2-8 Nanocomposite Coatings Made of YSZ Nano-Crystals in a Gold Matrix A.A. Voevodin, Jeffrey S. Zabinski (Air Force Research Laboratory) Nanocomposite chameleon coatings have been designed to respond to environmental conditions and loading trough self-adjustment of surface properties [1]. The present work investigates the applicability of this concept to composite coatings made of yttria stabilized zirconia (YSZ) nanocrystals embedded into a gold matrix. One of the practical goals was to investigate coating mechanical response in hardness, scratch, and friction tests. The coatings were produced by a hybrid technology, which combines laser ablation deposition of highly oriented nanocrystalline YSZ with magnetron sputtering of gold. The chemistry and structure of the coatings were investigated by x-ray electron spectroscopy, x-ray diffraction, and transmission electron microscopy. The results were correlated with properties determined from mechanical characterization. Coating hardness was evaluated using nanoindentation, while coating adhesion and toughness was estimated using scratch tests. Friction characteristics of the YSZ/Au nanocomposite coatings were explored using pin-on-disk machine at room temperature and at 500 C. Chemistry, structure, mechanical, and tribological properties of the YSZ/Au nanocomposites are discussed. [1] A.A.Voevodin and J.S.Zabinski, Supertough wear resistant coatings with chameleon surface adaptation, Thin Solid Films 370 (2000) 223.
4:10 PM		E3-2-9 Tribology of Tool-chip Interface and Tool Wear Mechanisms H.O. Gekonde (IonBond Inc. (Formerly Multi-Arc)); S.V. Subramanian (McMaster University, Canada) The tribological phenomenon at the tool-chip interface controls chip formation and tool wear. As metal cutting speed increases from low to moderate speeds, the tribological phenomenon at the tool-chip interface tends to change from sliding to seizure, giving rise progressively to partially segmented chips. Once seizure sets in, thermoplastic shear occurs in the secondary shear zone, raising the local temperature at the tool-chip contact. Thermally activated processes set in. Dissolution of the tool into the chip takes place by a diffusion mechanism, causing tool crater wear. The maximum depth of crater is located at some distance away from the cutting edge of the tool. At higher cutting speeds chip segmentation is caused by thermoplastic shear localisation at the primary shear zone causing high temperature rise and rapid tool wear close to the cutting edge of the tool. The change in tribological phenomenon at the tool-chip interface as a function of cutting speed is discussed. Quantitative data is reported to show the onset seizure in metal cutting and the consequences of seizure in promoting dissolution wear by diffusion mechanism. Under conditions of seizure the role of coatings in suppressing dissolution wear is demonstrated in both ductile iron and low carbon steel using TiN and HfN, which are compounds having the least solubility in the work piece (steel matrix) to coat the tools.