Shashi K. Sharma

Tribological Behavior of Some Candidate Advanced Space Lubricants

Performance of a variety of space lubricants was compared under boundary and elastohydrodynamic lubrication (EHL). The types of fluids studied were naphthenic mineral oil, paraffinic mineral oil, polyalphaolefin, and silahydrocarbon. The silahydrocarbon and the polyalphaolefin lubricants exhibited lower traction under similar conditions. A specific additive package increased the traction of the polyalphaolefin. Volatility characteristics of some of these fluids were also studied. Presented at the 45th Annual Meeting In Denver, Colorado May 7–10, 1990

Rheology of perfluoropolyalkylether fluids in elastohydrodynamic lubrication

The rheological characteristics of two branched and two linear, commercial perfluoropolyalkylether (PFPAE) fluids were studied under high pressures and temperatures. The effects of branching and carbon-to-oxygen (C :O) ratio on the pressure-viscosity-temperature behavior and on the non-Newtonian behavior of these fluids were studied experimentally under high pressures and temperatures. The branching and the higher C :O ratio seemed to increase the pressure-viscosity coefficients of these fluids. The effects of the viscosity and the pressure-viscosity coefficient on the capabilities of these fluids to generate elastohydrodynamic lubrication (EHL) film thickness were studied and compared with experimental measurements. All of the fluids studied seemed to follow the Roelands viscosity model and classical EHL theory (1). The C :O ratio also influenced the temperature dependence of the limiting-shear-strength proportionality constant. The results show that for similar-viscosity fluids, the linear PFPAE with higher C :O ratio is most desirable for wide temperature use.

Degradation of Cu-Al Coating on Ti-6AI-4V Substrate under Fretting Fatigue Conditions

The degradation process of a Cu-Al coating on Ti-6Al-4V substrate was investigated under fretting and fretting fatigue loading conditions. Wear and coefficient of friction (COF) of the coating were investigated as a function of fretting and fretting fatigue cycles. Damage of the coating was also characterized using scanning electron microscopy, energy dispersive spectrometry, and profilometery. COF decreased due to self-lubrication effects from the debris formation under fretting and fretting fatigue loading conditions. At a given normal load applied to fretting pad, coating wear increased with the increasing number of cycles and applied load to the specimen, with the lowest wear occurring under fretting loading conditions. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Fretting Fatigue Behavior of Cu-Al-Coated Ti-6Al-4V

Fretting fatigue of a Cu-Al coating deposited on an alumina-gritted Ti-6Al-4V substrate was investigated. Two types of tests were conducted: one series involved fretting fatigue tests at different bulk stress amplitudes under a constant contact load and the second series was run at different contact loads under a constant bulk stress amplitude. The coefficient of friction (COF) was lower on the coated substrate than on the bare substrate before fretting. However, COFs of the coated and the bare (i.e., uncoated) substrates were identical after the exposure to fretting fatigue cycles since fretting fatigue caused wear of the coating. The coating thickness as a function of fretting fatigue cycles was monitored. The coating damage increased as the applied bulk stress amplitude increased. At lower bulk stress amplitudes, the coating damage was gradual and it survived over one million cycles. However, the coating delaminated from the gritted surface and/or caused premature specimen failure at higher stress amplitudes. At the lowest contact load used in the present study, the contact condition was gross slip and the life of the coating was the shortest due to fretting wear. On the other hand, the higher contact loads induced a partial slip contact condition that caused less coating damage, and the coating life increased with increasing contact load. *The views expressed in this article are those of the authors and do not reflect the official policy or position of the United State Air Force, Department of Defense, or the U.S. government

Effect of Viscosity Index Improvers on the Elastohydrodynamic Lubrication Characteristics of a Chlorotrifluoroethylene and a Polyalphaolefin Fluid

The film thickness characteristics and traction behavior, two of the more important elastohydrodynamic lubrication (EHL) properties, of two important classes of synthetic lubricants have been studied. The effect of viscosity index (VI) improvers on these properties has been determined for both hydrogenated polyalphaolefin (PAO) base fluids and chlorotrifluoroethylene oligomer (CTFE) base fluids. A polyalkylmethacrylate (PMM) VI improver was studied in PAO and a copolymer of vinylidine fluoride and chlorotrifluoroethylene was studied in CTFE. The VI improvers demonstrated insignificant improvement in the EHL film thickness of both the PAO and CTFE base fluids as determined by comparing the measured film thickness of the VI-improved fluids to the predicted film thickness based on measured pressure-viscosity characteristics and kinematic viscosities determined at low shear rates. Similarly, the traction behavior of VI-improved PAO and CTFE fluids was nearly equivalent to that of the respective base fluids, d...

High speed Civil transport (HSCT) hydraulic fluid development

GE Aircraft Engines required a new hydraulic fluid for engine nozzle actuation for the High-Speed Civil Transport (HSCT). Perfluoropolyalkylethers (PFPAEs), because of their nonflammability and high temperature stability, were selected early in the program. The authors have found two formulated PFPAE fluids for the application. The viscosity-temperature data, pressure-viscosity data, EHL (elastohydrodynamic lubrication) film forming ability, thermal stability data, antirust protection data and wear data were measured and the data used to select the candidates. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

Sensitivity of RF Sensors for Bearing Health Monitoring

Coated bearings in high-performance turbine engines have the potential to provide considerable improvement in engine reliability and service-life extension. One coating performance indicator is the level of vibration generated by rolling elements and their cage separators at high-speed bearing operation. This article introduces a nonintrusive, cage-mounted sensor that detects cage vibration and transmits wirelessly to a nearby receiver. The sensitivity of the cage-mounted sensor was compared to a housing-mounted accelerometer in the detection of seeded faults in coated and uncoated bearings. The smallest spall on an uncoated bearing that the sensor could detect was 28 μm deep × 3.2 mm wide, with a reliability of ∼40%. In contrast, the accelerometer consistently detected all levels of flaws, the smallest being 0.4 μm deep × 3.2 mm wide on the coated bearing, with a reliability of 55%. Despite the sensor having a detection rate less than the accelerometer, this research successfully demonstrated that the sensor could detect inner-race-bearing flaws and established its sensitivity level.

Effect of Humidity on Wear of M-50 Steel with a Branched Perfluoropolyalkylether Lubricant

Humidity is an uncontrolled variable in many tribology tests. This report shows that changes in relative humidity can affect wear for all perfluoropolyalkylethers regardless of their molecular structure. Using a Cameron–Plint tribometer under controlled environmental conditions, wear of M-50 steel with a Krytox (a branched perfluoropolyalkylether) lubricant was studied under boundary lubrication conditions at 150°C in air with relative humidity ranging from 1 to 95%. Both wear and friction decreased sharply as humidity was increased from 1 to 20%, then were constant as the humidity was increased to 95%. Thus, wear is highly dependent on humidity when relative humidity is less than 20%. The similar effect of humidity on wear previously observed for Fomblin Z, a linear perfluoropolyalkylether containing difluoroacetal groups, and Demnum S, a linear perfluoropolyalkylether which does not contain difluoroacetal groups, indicates that this may be a general property of all commercially available perfluoropolyalkylethers, branched as well as linear ones.

Wettability aspects of friction and wear reduction by a fluorinated sulphonamide additive in a chlorotrifluoroethylene-based fluid

Abstract Chlorotrifluoroethylene (CTFE) is a non-flammable synthetic material developed as a hydraulic fluid for Air Force systems. The sulpronamide additive is a low surface tension material with a complex molecular structure including several surface active groups. The addition of the sulphonamide additive to the CTFE oil reduced wear on steel specimens by helping in the formation of chemical reaction films on the surfaces. The physical aspects of how this additive helps in the formation of the wear reducing films are discussed. There is evidence that the sulphonamide additive modifies the solid-liquid interface, preventing the spread and loss of CTFE oil over a steel surface. By keeping the CTFE oil close to the interacting asperities, the sulphonamide additive conserves the share of the load carried by the lubricant and makes the CTFE available for the formation of friction and wear reducing surface films containing iron chloride and iron fluoride.

Finite Element Analysis of Multilayered and Functionally Gradient Tribological Coatings with Measured Material Properties

A model has been developed to study the stress distribution in Ti1 − xCx multilayered functionally gradient (FG) coatings, with a top coating of diamond-like carbon (DLC), on 440C stainless steel substrates. Using the finite element method, these gradient coatings were assumed as a series of perfectly bonded layers with unique material properties and layer thickness. In addition, a matrix of nanoindentation experiments were performed to measure material properties of each Ti1 − xCx layer on separate coating blocks. The yield strength of the coating materials was then determined by coupling the finite element analysis model in connection with the nanoindentation technique. Once developed, this model was used to examine the threshold of plasticity and identify the plastic deformation zone inside the multilayered coatings and substrate. This work shows how the multilayered FG Ti/TiC/DLC coating system improves the coating integrity under heavy loading conditions.