Papers

ICMCTF1998 Session E1: Reduction of Friction Through Coating/Surface Modification

Monday, April 27, 1998 8:30 AM in Room California

Monday Morning

Start	Invited?	Item
8:30 AM		E1-1 Plenary
8:50 AM		E1-2 Plenary
9:30 AM		E1-4 Plenary
10:10 AM		E1-6 Plenary
10:30 AM		E1-7 Surface Modification Methods for Reducing Stiction in Microelectromechanical Systems K. Komvopoulos (University of California at Berkeley) Microelectromechanical systems (MEMS) are miniaturized transducer elements fabricated using the growth, deposition, and selective removal of thin films. Demands for increased micromachine sensitivity and reliability have raised strong interest on how integrated circuit materials behave mechanically. Due to the very small mass, surface forces play a dominant role at MEMS interfaces. Consequently, fundamental understanding of the various surface mechanisms that may lead to excessive adhesion forces (stiction) is of paramount importance to micromachine design. In this publication, the significance of residue deposition at interfaces during the release-drying process, liquid bridging, van der Waals forces, and electrostatic charging will be elucidated in light of both experimental and analytical results. Methods for modifying the micromachine design, interfacial topography, and chemical state to reduce the magnitude of the stiction force will be discussed. Experimental techniques for determining the tribological and micromechanical properties of polysilicon microstructures will be presented, and the critical micromachine stiffness for overcoming stiction will be interpreted for different material systems and surface roughness in light of three-dimensional simulation results for rough elastic-plastic rough surfaces.
11:10 AM		E1-9 Lubrication Mechanisms in Zinc Oxide Coatings J.S. Zabinski, S.V. Prasad, J. Theodosakis (Air Force Research Laboratory) The performance of solid lubricant films such as graphite and MoS₂ is strongly dependent on the environment. Moisture is particularly detrimental to the dichalcogenides, such as MoS₂, but its presence is required for graphite to be lubricious. Mechanisms explaining these phenomena have been presented long ago. Zinc oxide thin films have recently been shown to exhibit low friction and long wear life under sliding conditions in environments ranging from dry nitrogen to 85% relative humidity. In this paper, the mechanisms that govern the friction and wear behavior of zinc oxide are explored. Previously, the role of microstructure has been evaluated. It was determined that the presence of a nanocrytalline phase was necessary for providing good tribological properties. Here, the role of chemistry and gas adsorption have been determined. The friction coefficient of zinc oxide in vacuum with a background of 4x10^-6 Torr H₂O was about 0.15. In high vacuum (e.g., 10^-8 Torr), the friction coefficient decreased to 0.1. The ultimate properties of zinc oxide are determined by a combination of chemistry and microstructure. Films that did not have the correct microstructure exhibited high friction and failed immediately in air. However, some of these films provided low friction in vacuum and after about 50,000 cycles they could be exposed to air where they continued to provide low friction up to 2 million sliding cycles. The roles of gas adsorption/reaction and crystal face dependent surface chemistry in determining tribological properties are discussed. It is especially significant to note that low friction and long wear life are observed in environments spanning UHV to air with extreme humidity.
11:30 AM		E1-10 Influence of Doping and Substrate Texture on the Growth of Lubricious Zinc Oxide Thin Films S.V. Prasad, J.S. Zabinski (Air Force Research Laboratory) This paper describes the growth and characterization of doped zinc oxide (ZnO)films deposited on textured substrates. The films were specifically developed for tribological applications. As reported in our previous study¹, zinc oxide films with nanocrystalline features can provide low friction. However, the majority of ZnO films grown by commercial techniques are typically columnar in nature. In the current study, ZnO thin films with dual-phase microstructures comprised of nanocolumns and randomly oriented nanocrystalline grains were developed using dopants and textured substrates. The nanocrystalline phase is intended to provide lubrication while the nancolumns are designed to impart prolonged wear life to the coating. Films were grown onto steel using magnetron sputtering and pulsed laser deposition. They were characterized by high resolution scanning electron microscopy, scanning auger microscopy, x-ray diffraction, Raman spectroscopy and cross sectional transmission electron microscopy. Friction and wear measurements were made using a ball-on-disk tribometer. The effects of various doping elements and the role of substrate texture on the microstructure and the tribological behavior of ZnO films are discussed. ¹ S. V. Prasad and J. S. Zabinski, Wear 203-204 (1997) 498.
11:50 AM		E1-11 Tribological Evaluation of PVD WC/C Low-Friction Coatings P. Hedenqvist (Balzers Sandvik Coating AB, Sweden); M. Larsson (Uppsala University, Sweden) Four different low-friction PVD WC/C coatings on HSS have been characterised and evaluated for tribological properties. The WC/C coatings differed in thickness and carbon content. A PVD TiN-coated HSS composite was used as reference. All coatings were deposited using DC magnetron sputtering. SEM and surface profilometry showed the coated composites to have surface roughness in the range of 10-70 nm. Vickers microhardness measurements gave a range of 1500-1700 HV. The coating residual stress, measured by beam deflection, was found to vary between -0.5 to -2.5 Gpa. All coatings showed good cohesion and adhesion, i.e., a relatively high critical normal load in the scratch test. Abrasive wear (dimple grinder) tests displayed an equal or somewhat lower wear resistance of the WC/C coatings compared to TiN. In the sliding wear (ball-on-disc) test, all WC/C coatings showed much lower friction and wear rate than TiN. The lowest friction coefficient (0.1) and wear rate, of both uncoated ball and coating, was found for the WC/C coating with the highest residual stress.