Somuri V. Prasad

Solid lubricants: a review

The fundamental mechanisms of solid lubrication are reviewed with examples from well-known solid lubricants like the transition metal dichalcogenides and diamond-like carbon families of coatings. Solid lubricants are applied either as surface coatings or as fillers in self-lubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments.

Materials modification using intense ion beams

Pulsed intense ion beams have been developed for applications including surface modification and alloying, and thin-film and nanopowder synthesis. Rapid thermal processing with ions is quite promising for large-scale commercial use, due to the high specific ion energy deposition (joules per cubic centimeter) without reflection, and to the relative efficiency and low cost of the pulsed power ion-beam drivers compared to other high-kinetic energy alternatives. We discuss in this paper the basis for the use of ions in materials processing and the methods of beam formation and impingement on material to be treated, and give examples of recent and ongoing work in materials processing.

EBSD studies on wear-induced subsurface regions in LIGA nickel

Abstract The application of focused ion beam techniques to prepare cross-sections of wear tracks is presented. Electron backscatter diffraction analysis of wear scars on electroformed Ni revealed the formation of two subsurface zones, each with its own characteristic features. Formation of low-angle grain boundaries and spread in the orientation of pole figures were also observed.

Mechanisms of friction in diamondlike nanocomposite coatings

Diamondlike nanocomposite (DLN) coatings (C:H:Si:O) processed from siloxane precursors by plasma enhanced chemical vapor deposition are well known for their low friction and wear behaviors. In the current study, we have investigated the fundamental mechanisms of friction and interfacial shear strength in DLN coatings and the roles of contact stress and environment on their tribological behavior. Friction and wear measurements were performed from 0.25to0.6GPa contact pressures in three environments: dry (<1% RH) nitrogen, dry (<1% RH) air, and humid (50% RH) air, with precise control of dew point and oxygen content. At 0.3GPa contact stress, the coefficient of friction (COF) in dry nitrogen was extremely low, ∼0.02, whereas in humid air it increased to ∼0.2, with minimal amount of wear in both environments. The coatings also exhibited non-Amontonian friction behavior, with COF decreasing with an increase in Hertzian contact stress. The main mechanism responsible for low friction and wear under varying cont...

Atomic layer deposition of tungsten disulphide solid lubricant thin films

The synthesis and characterization of crystalline tungsten disulphide (WS 2 ) solid lubricant thin films grown by atomic layer deposition (ALD) using WF 6 and H 2 S gas precursors was studied. A new catalytic route was established to promote nucleation and growth of WS 2 films on silicon surfaces with native oxide. Scanning electron microscopy with energy dispersive spectroscopy and Raman spectroscopy were used to determine the film morphology, composition, and crystallinity. The films exhibited solid lubricating behavior with a steady-state friction coefficient of 0.04 in a dry nitrogen environment.

Atomic Layer Deposition of Tungsten Disulphide Solid Lubricant Nanocomposite Coatings on Rolling Element Bearings

Atomic layer deposition (ALD) has the potential to provide highly conformal coatings with precise control of thickness. This article describes the application of ALD nanocomposite containing ZnF2 in WS2 matrix solid lubricant coatings on fully assembled rolling element bearings. The torque behavior of the coated bearings was studied during oscillatory contacts and after exposure to vibration. The coatings exhibited a hexagonal layered structure with predominant preferentially orientated (002) basal planes. These basal planes when sheared imparted very low running torque values of ∼ 0.5 mN· m in dry nitrogen. The outer race, inner race, and ball surfaces showed WS2 transfer film protection on the native coating necessary to achieve low torque in dry nitrogen. Structural (re)ordering of the basal and prismatic planes with multiple random and branched orientations was observed through the thickness of the transfer films. There was no evidence of uniformly aligned c-axis perpendicular-orientated basal planes on the transfer film surface. The unique advantages of ALD to apply solid lubricant coatings on rolling elements of fully assembled miniature bearings are compared with conventional solid lubrication techniques.

Application of Diamond-Like Nanocomposite Tribological Coatings on LIGA Microsystem Parts

The major focus of this study was to examine the feasibility of applying diamond-like nanocomposite (DLN) coatings on the sidewalls of Ni alloy parts fabricated using lithographie, galvanoformung and abformung (LIGA: a German acronym that means lithography, electroforming, and molding) for friction and wear control. Planar test coupons were employed to understand the friction mechanisms in regimes relevant to LIGA microsytems. Friction tests were conducted on planar test coupons as well as between LIGA-fabricated test structures in planar-sidewall and sidewall-sidewall configurations. Measurements were made in dry nitrogen and air with 50% relative humidity by enclosing the friction tester in an environmental chamber. In contrast to bare metal-metal contacts, minimal wear was exhibited for the DLN-coated LIGA NiMn alloy parts and test coupons. The low friction behavior of DLN was attributed to its ability to transfer to the rubbing counterface providing low interfacial shear at the sliding contact. The coating coverage and chemistry on the sidewalls and the substrate-coating interface integrity were examined by transmission electron microscopy, Automated eXpert Spectral Image Analysis, and electron backscatter diffraction on cross sections prepared by focused ion beam microscopy. The role of novel characterization techniques to evaluate the surface coatings for LIGA microsystems technology is highlighted.

LIGA Microsystems: Surface Interactions, Tribology, and Coatings

Abstract Deep X-ray lithography based techniques such as LIGA (German acronym representing Lithographie, Galvanoformung, and Abformung) are being currently used to fabricate net-shape components for microelectromechanical systems (MEMS). Unlike other microfabrication techniques, LIGA lends itself to a broad range of materials, including metals, alloys, polymers, as well as ceramics and composites. Currently, Ni and Ni alloys are the materials of choice for LIGA microsystems. While Ni alloys may meet the structural requirements for MEMS, their tribological (friction and wear) behavior poses great challenges for the reliable operation of LIGA-fabricated MEMS. Typical sidewall morphologies of LIGA-fabricated parts are described, and their role in the tribological behavior of MEMS is discussed. The adaptation of commercial plasma-enhanced chemical vapor deposition to coat the sidewalls of LIGA-fabricated parts with diamond-like nanocompositeis described.

Sidewall morphology of electroformed LIGA parts-implications for friction, adhesion, and wear control

LIGA fabricated parts are finding application in a wide variety of micro-mechanical systems. For these systems to operate reliably, friction between contacting sidewall surfaces must be understood and controlled. The roughness of the as-plated sidewall is an important determinate of friction forces at such contacts. LIGA sidewalls were characterized in order to provide a basis for predicting the friction, adhesion, and wear behavior of LIGA micromachines. A variety of unexpected sidewall morphologies were observed during this investigation. Three morphologies were identified: a fine scale roughness, a linear through thickness feature, and a group of larger high aspect ratio features. Each morphology has been associated with a specific aspect of the LIGA manufacturing process. Potential friction, adhesion, and wear management strategies suggested by these features have been discussed. In addition, the asperity behavior in a LIGA sidewall contact has been predicted based on the finest roughness observed.

Electrical resistivity of Au-ZnO nanocomposite films

The electrical resistivity of electron beam codeposited gold and zinc oxide (Au-ZnO) films was investigated over the full composition range. The electrical resistivity was shown to increase monotonically with increasing ZnO content, with three characteristic regimes of behavior associated primarily with (1) grain boundary electron scattering due to grain refinement at ZnO volume fractions below 0.3, (2) percolation theory for ZnO volume fractions at and above the percolation threshold (fc = 0.85), and (3) a transition region between these where it was proposed that resistivity was influenced by the formation of Au-Zn complexes due to an oxygen deficiency in the deposited ZnO. The electrical resistivity of the composite films remained below 100 μΩ cm for ZnO volume fractions below 0.5. A model combining the general effective media equation and Mayadas-Shatzkes grain boundary electron scattering model was shown to generally describe the composition dependence of electrical resistivity for the investigated o...