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At least 289 records · Page 16

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NASA NTRS1988

Adhesion, friction, and wear of plasma-deposited thin silicon nitride films at temperatures to 700 C

The adhesion, friction, and wear behavior of silicon nitride films deposited by low- and high-frequency plasmas (30 kHz and 13.56 MHz) at various temperatures to 700 C in vacuum were examined. The results of the investigation indicated that the Si/N ratios were much greater for the films deposited at 13.56 MHz than for those deposited at 30 kHz. Amorphous silicon was present in both low- and high-frequency plasma-deposited silicon nitride films. However, more amorphous silicon occurred in the films deposited at 13.56 MHz than in those deposited at 30 kHz. Temperature significantly influenced adhesion, friction, and wear of the silicon nitride films. Wear occurred in the contact area at high temperature. The wear correlated with the increase in adhesion and friction for the low- and high-frequency plasma-deposited films above 600 and 500 C, respectively. The low- and high-frequency plasma-deposited thin silicon nitride films exhibited a capability for lubrication (low adhesion and friction) in vacuum at temperatures to 500 and 400 C, respectively.

Miyoshi, K.↗

NASA NTRS1989

Adhesion, friction, and wear of plasma-deposited thin silicon nitride films at temperatures to 700 C

Miyoshi, K.↗

NASA NTRS1989

Friction and wear of oxide-ceramic sliding against IN-718 nickel base alloy at 25 to 800 C in atmospheric air

The friction and wear of oxide-ceramics sliding against the nickel base alloy IN-718 at 25 to 800 C were measured. The oxide materials tested were mullite (3Al2O3.2SiO2); lithium aluminum silicate (LiAlSi(x)O(y)); polycrystalline monolithic alpha alumina (alpha-Al2O3); single crystal alpha-Al2O3 (sapphire); zirconia (ZrO2); and silicon carbide (SiC) whisker-reinforced Al2O3 composites. At 25 C the mullite and zirconia had the lowest friction and the polycrystalline monolithic alumina had the lowest wear. At 800 C the Al2O3-8 vol/percent SiC whisker composite had the lowest friction and the Al2O3-25 vol/percent SiC composite had the lowest wear. The friction of the Al2O3-SiC whisker composites increased with increased whisker content while the wear decreased. In general, the wear-resistance of the ceramics improve with their hardness.

Sliney, Harold E.↗

NASA NTRS1991

Servo Reduces Friction In Flexure Bearing

Proposed servocontrol device reduces such resistive torques as stiction, friction, ripple, and cogging in flexure bearing described in LAR-14348, "Flexure Bearing Reduces Startup Friction". Reduces frictional "bump" torque encountered when bearing ball runs into buildup of grease on bearing race. Also used as cable follower to reduce torque caused by cable and hoses when they bend because of motion of bearing. New device includes torquer across ball race. Torquer controlled by servo striving to keep flexure at null, removing torque to outer ring. In effect, device is inner control loop reducing friction, but does not control platforms or any outer-control-loop functions.

Clingman, W. Dean↗

NASA NTRS1991

Friction damping of two-dimensional motion and its application in vibration control

This paper presents an approximate method for analyzing the two-dimensional friction contact problem so as to compute the dynamic response of a structure constrained by friction interfaces. The friction force at the joint is formulated based on the Coulomb model. The single-term harmonic balance scheme, together with the receptance approach of decoupling the effect of the friction force on the structure from those of the external forces has been utilized to obtain the steady state response. The computational efficiency and accuracy of the method are demonstrated by comparing the results with long-term time solutions.

Menq, C.-H.↗

NASA NTRS1992

Special Course on Skin Friction Drag Reduction

Lecture notes for the AGARD Fluid Dynamics Panel (FDP) Special Course on 'Skin Friction Drag Reduction' have been assembled in this report. The aim and scope of this course was to provide an overview of the state of the art of current technology programs focused on reducing aircraft drag caused by skin friction. This report provides a review of recent progress in the field of skin friction drag reduction for transport aircraft. It provides a general introduction to the technology and then focuses on two specific aspects of drag reduction: maintaining laminar flow by controlling transition and manipulating the turbulence structure in boundary layers to help reduce skin friction. Technology associated with the use of riblets, LEBU, polymers, additives, etc., is also reviewed.

Advisory Group for Aerospace Research and Developm↗

NASA NTRS1990

Apparatus for measurement of coefficient of friction

An apparatus designed to measure the coefficient of friction in certain controlled atmospheres is described. The coefficient of friction observed during high-load tests was nearly constant, with an average value of 0.56. This value is in general agreement with that found in the literature and also with the initial friction coefficient value of 0.67 measured during self-mated friction of 440C steel in an oxygen environment.

Slifka, A. J.↗

NASA NTRS1991

Friction and wear of plasma-deposited amorphous hydrogenated films on silicon nitride

An investigation was conducted to examine the friction and wear behavior of amorphous hydrogenated carbon (a-C:H) films in sliding contact with silicon nitride pins in both dry nitrogen and humid air environments. Amorphous hydrogenated carbon films approximately 0.06 micron thick were deposited on silicon nitride flat substrates by using the 30 kHz ac glow discharge of a planar plasma reactor. The results indicate that an increase in plasma deposition power gives an increase in film density and hardness. The high-density a-C:H films deposited behaved tribologically much like bulk diamond. In the dry nitrogen environment, a tribochemical reaction produced a substance, probably a hydrocarbon-rich layer, that decreased the coefficient of friction. In the humid air environment, tribochemical interactions drastically reduced the wear life of a-C:H films and water vapor greatly increased the friction. Even in humid air, effective lubrication is possible with vacuum-annealed a-C:H films. The vacuum-annealed high-density a-C:H film formed an outermost superficial graphitic layer, which behaved like graphite, on the bulk a-C:H film. Like graphite, the annealed a-C:H film with the superficial graphitic layer showed low friction when adsorbed water vapor was present.

Miyoshi, Kazuhisa↗

NASA NTRS1952

Investigation of Wear and Friction Properties Under Sliding Conditions of Some Materials Suitable for Cages of Rolling-Contact Bearings

An investigation of wear and friction properties of a number of materials sliding against SAE 52100 steel was conducted. These materials included brass, bronze, beryllium copper, monel, nichrome v, 24s-t aluminum, nodular iron, and gray cast iron. The metals investigated may be useful as possible cage (separator or retainer) materials for rolling-contact bearings of high-speed turbine engines. The ability of materials to form surface films that prevent welding is a most important factor in both dry friction and boundary lubrication. On the basis of wear and resistance to welding only, the cast irons were the most promising materials investigated; they showed the least wear and the least tendency to surface failure when run dry, and when boundary lubricated they showed the highest load capacity. On the basis of mechanical properties, nodular iron is superior to gray cast iron. Bronze had the lowest friction coefficient under dry sliding conditions. The results with brass, beryllium copper, and aluminum were poor and these materials do not appear, with regard to friction and wear, to be suitable for cages.

Johnson, Robert L↗

NASA NTRS1993

Boundary-layer transition and global skin friction measurement with an oil-fringe imaging technique

A new oil-fringe imaging system skin friction (FISF) technique to measure skin friction on wind tunnel models is presented. In the method used to demonstrate the technique, lines of oil are applied on surfaces that connect the intended sets of measurement points, and then a wind tunnel is run so that the oil thins and forms interference fringes that are spaced in proportion to local skin friction. After a run the fringe spacings are imaged with a CCD-array digital camera and measured on a computer. Skin friction and transition measurements on a two-dimensional wing are presented and compared with computational predictions.

Monson, Daryl J.↗

NASA NTRS1993

Boundary-layer transition and global skin friction measurement with an oil-fringe imaging technique

A new oil-fringe imaging fkin friction (FISF) technique to measure skin friction on wind tunnel models is presented. In the method used to demonstrate the technique, lines of oil are applied on surfaces that connect the intended sets of measurement points, and then a wind tunnel is run so that the oil thins and forms interference fringes that are spaced proportional to local skin friction. After a run the fringe spacings are imaged with a CCD-array digital camera and measured on a computer. Skin friction and transition measurements on a two-dimensional wing are presented and compared with computational predictions.

Monson, Daryl J.↗

NASA NTRS1995

Large-scale landslide simulations: Global deformation, velocities and basal friction

The cause of the apparent small friction exhibited by long runout landslides has long been speculated upon. In an attempt to provide some insight into the matter, this paper describes results obtained from a discrete particle computer simulation of landslides composed of up to 1,000,000 two-dimensional discs. While simplified, the results show many of the characteristics of field data (the volumetric effect on runout, preserved strata, etc.) and with allowances made for the two-dimensional nature of the simulation, the runouts compare well with those of actual landslides. The results challenge the current view that landslides travel as a nearly solid block riding atop a low friction basal layer. Instead, they show that the mass is completely shearing and indicate that the apparent friction coefficient is an increasing function of shear rate. The volumetric effect can then be understood. With all other conditions being equal, different size slides appear to travel with nearly the same average velocity; however, as the larger landslides are thicker, they experience smaller shear rates and correspondingly smaller frictional resistance.

Campbell, Charles S.↗

NASA NTRS1996

Computational Methods for Frictional Contact With Applications to the Space Shuttle Orbiter Nose-Gear Tire

A computational procedure is presented for the solution of frictional contact problems for aircraft tires. A Space Shuttle nose-gear tire is modeled using a two-dimensional laminated anisotropic shell theory which includes the effects of variations in material and geometric parameters, transverse-shear deformation, and geometric nonlinearities. Contact conditions are incorporated into the formulation by using a perturbed Lagrangian approach with the fundamental unknowns consisting of the stress resultants, the generalized displacements, and the Lagrange multipliers associated with both contact and friction conditions. The contact-friction algorithm is based on a modified Coulomb friction law. A modified two-field, mixed-variational principle is used to obtain elemental arrays. This modification consists of augmenting the functional of that principle by two terms: the Lagrange multiplier vector associated with normal and tangential node contact-load intensities and a regularization term that is quadratic in the Lagrange multiplier vector. These capabilities and computational features are incorporated into an in-house computer code. Experimental measurements were taken to define the response of the Space Shuttle nose-gear tire to inflation-pressure loads and to inflation-pressure loads combined with normal static loads against a rigid flat plate. These experimental results describe the meridional growth of the tire cross section caused by inflation loading, the static load-deflection characteristics of the tire, the geometry of the tire footprint under static loading conditions, and the normal and tangential load-intensity distributions in the tire footprint for the various static vertical loading conditions. Numerical results were obtained for the Space Shuttle nose-gear tire subjected to inflation pressure loads and combined inflation pressure and contact loads against a rigid flat plate. The experimental measurements and the numerical results are compared.

Tanner, John A.↗

NASA NTRS1996

Computational methods for frictional contact with applications to the Space Shuttle orbiter nose-gear tire: Comparisons of experimental measurements and analytical predictions

A computational procedure is presented for the solution of frictional contact problems for aircraft tires. A Space Shuttle nose-gear tire is modeled using a two-dimensional laminated anisotropic shell theory which includes the effects of variations in material and geometric parameters, transverse-shear deformation, and geometric nonlinearities. Contact conditions are incorporated into the formulation by using a perturbed Lagrangian approach with the fundamental unknowns consisting of the stress resultants, the generalized displacements, and the Lagrange multipliers associated with both contact and friction conditions. The contact-friction algorithm is based on a modified Coulomb friction law. A modified two-field, mixed-variational principle is used to obtain elemental arrays. This modification consists of augmenting the functional of that principle by two terms: the Lagrange multiplier vector associated with normal and tangential node contact-load intensities and a regularization term that is quadratic in the Lagrange multiplier vector. These capabilities and computational features are incorporated into an in-house computer code. Experimental measurements were taken to define the response of the Space Shuttle nose-gear tire to inflation-pressure loads and to inflation-pressure loads combined with normal static loads against a rigid flat plate. These experimental results describe the meridional growth of the tire cross section caused by inflation loading, the static load-deflection characteristics of the tire, the geometry of the tire footprint under static loading conditions, and the normal and tangential load-intensity distributions in the tire footprint for the various static vertical-loading conditions. Numerical results were obtained for the Space Shuttle nose-gear tire subjected to inflation pressure loads and combined inflation pressure and contact loads against a rigid flat plate. The experimental measurements and the numerical results are compared.

Tanner, John A.↗

NASA NTRS1996

Measurement of Gear Tooth Dynamic Friction

Measurements of dynamic friction forces at the gear tooth contact were undertaken using strain gages at the root fillets of two successive teeth. Results are presented from two gear sets over a range of speeds and loads. The results demonstrate that the friction coefficient does not appear to be significantly influenced by the sliding reversal at the pitch point, and that the friction coefficient values found are in accord with those in general use. The friction coefficient was found to increase at low sliding speeds. This agrees with the results of disc machine testing.

Rebbechi, Brian↗

NASA NTRS1994

Skin-Friction Measurements in a 3-D, Supersonic Shock-Wave/Boundary-Layer Interaction

The experimental documentation of a three-dimensional shock-wave/boundary-layer interaction in a nominal Mach 3 cylinder, aligned with the free-stream flow, and 20 deg. half-angle conical flare offset 1.27 cm from the cylinder centerline. Surface oil flow, laser light sheet illumination, and schlieren were used to document the flow topology. The data includes surface-pressure and skin-friction measurements. A laser interferometric skin friction data. Included in the skin-friction data are measurements within separated regions and three-dimensional measurements in highly-swept regions. The skin-friction data will be particularly valuable in turbulence modeling and computational fluid dynamics validation.

Wideman, J. K.↗

NASA NTRS1996

Friction and Wear of Ion-Beam-Deposited Diamondlike Carbon on Chemical-Vapor-Deposited, Fine-Grain Diamond

Friction and wear behavior of ion-beam-deposited diamondlike carbon (DLC) films coated on chemical-vapor-deposited (CVD), fine-grain diamond coatings were examined in ultrahigh vacuum, dry nitrogen, and humid air environments. The DLC films were produced by the direct impact of an ion beam (composed of a 3:17 mixture of Ar and CH4) at ion energies of 1500 and 700 eV and an RF power of 99 W. Sliding friction experiments were conducted with hemispherical CVD diamond pins sliding on four different carbon-base coating systems: DLC films on CVD diamond; DLC films on silicon; as-deposited, fine-grain CVD diamond; and carbon-ion-implanted, fine-grain CVD diamond on silicon. Results indicate that in ultrahigh vacuum the ion-beam-deposited DLC films on fine-grain CVD diamond (similar to the ion-implanted CVD diamond) greatly decrease both the friction and wear of fine-grain CVD diamond films and provide solid lubrication. In dry nitrogen and in humid air, ion-beam-deposited DLC films on fine-grain CVD diamond films also had a low steady-state coefficient of friction and a low wear rate. These tribological performance benefits, coupled with a wider range of coating thicknesses, led to longer endurance life and improved wear resistance for the DLC deposited on fine-grain CVD diamond in comparison to the ion-implanted diamond films. Thus, DLC deposited on fine-grain CVD diamond films can be an effective wear-resistant, lubricating coating regardless of environment.

Miyoshi, Kazuhisa↗

NASA NTRS1981

Skin Friction Measurements by a Dual-Laser-Beam Interferometer Technique

A portable dual-laser-beam interferometer that nonintrusively measures skin friction by monitoring the thickness change of an oil film subject to shear stress is described. The method is an advance over past versions in that the troublesome and error-introducing need to measure the distance to the oil leading edge and the starting time for the oil flow has been eliminated. The validity of the method was verified by measuring oil viscosity in the laboratory, and then using those results to measure skin friction beneath the turbulent boundary layer in a low speed wind tunnel. The dual-laser-beam skin friction measurements are compared with Preston tube measurements, with mean velocity profile data in a "law-of-the-well" coordinate system, and with computations based on turbulent boundary-layer theory. Excellent agreement is found in all cases. (This validation and the aforementioned improvements appear to make the present form of the instrument usable to measure skin friction reliably and nonintrusively in a wide range of flow situations in which previous methods are not practical.)

Monson, D. J.↗