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RESEARCH PAPERS

J. Tribol. 1995;117(4):569-574. doi:10.1115/1.2831515.

The friction coefficient and adhesion force between steel balls and flat test pieces were measured during friction under low normal load in order to examine the tribological characteristics. First, the friction coefficients were measured under a constant normal load of 0.8 to 2350 μN, and the adhesion forces were measured before and after each friction. The result showed that the friction coefficient was highest at low normal loads, while the friction force divided by the sum of the normal load and the mean adhesion force was almost constant over the whole range of loads. Second, when the normal load was reduced gradually during friction, friction still acted when the normal load became negative and a pulling off force was applied to the surface. Thus an adhesion force acts during friction and this adhesion force affects the friction force in the same way as the normal load.

Topics: Force , Friction , Stress , Steel , Tribology
Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):575-579. doi:10.1115/1.2831518.

A numerical simulation of slider-disk contact in a magnetic hard disk drive is studied using the Hertzian contact model. The slider-disk contact is caused by flying height fluctuation due to disk runout for very low flying sliders. The rough disk topography is generated numerically by combining a sinusoidal waviness and a Gaussian roughness. For each asperity contact, the radius of curvature is calculated from the disk topography, and the radius is used to calculate the contact force using the Hertzian contact model. The slider’s response to a single asperity calculated using the Hertzian contact model agrees well with the result obtained using the impulse-momentum based contact model. The simulation results of slider-disk contact including suspension dynamics are calculated with and without friction for a “nano-slider.”

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):580-587. doi:10.1115/1.2831519.

Optical interferometry and contact mode atomic force microscopy are used to investigate the various factors that influence the measurement of pole tip recession. These factors include: (a) effect of dissimilar materials on optical interferometry results, (b) size and location of the air bearing surface used as the reference area, (c) the effect of slider crown, (d) the magnification of the objective used in optical interferometry and (e) the dependence of the AFM measurements on slider materials and scan direction of the AFM tip. The possibility of determining a local value of pole tip recession is examined wherein the sputtered alumina, rather than the Al2 O3 -TiC surface, is used as the reference surface. Finally, the effect of contact start/stop (CSS) and constant speed drag testing on pole tip recession is investigated by measuring the change in pole tip recession as a function of the number of start/stop and constant speed drag cycles.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):588-593. doi:10.1115/1.2831520.

In this paper, an optimization technique is utilized to find an optimum configuration of the taper-flat slider positioned by a rotary actuator for enhanced static air-bearing characteristics. The aim of optimization consists in simultaneously minimizing the variation in flying height from a target value, maximizing the smallest pitch angle, and minimizing the largest roll angle, over the entire magnetic recording band. As the design variables, the leading edge taper angle and rail width of a taper-flat slider, and the skew angle at the inside track are chosen since they seem to be the most influential parameters on air-bearing characteristics. The optimum design variables are automatically obtained by using the augmented Lagrange multiplier method, and the static characteristics of the optimally designed sliders are found to be superior to those of the taper-flat sliders of typical configuration over the entire recording band. Results obtained for three taper-flat slider models are reported, showing the effectiveness of the proposed design scheme.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):594-601. doi:10.1115/1.2831521.

An experimental investigation of the surface topography, nanoindentation hardness, and nanowear characteristics of carbon thin films was conducted using atomic force and point contact microscopy. Hydrogenated carbon films of thickness 5, 10, and 25 nm were synthesized using a sputtering technique. Atomic force microscopy images obtained with silicon nitride tips of nominal radius less than 20 nm demonstrated that the carbon films possessed very similar surface topographies and root-mean-square roughness values in the range of 0.7–1.1 nm. Nanoindentation and nanowear experiments performed with diamond tips of radius equal to about 20 nm revealed a significant enhancement of the hardness and wear resistance with increasing film thickness. High-resolution surface imaging indicated that plastic flow was the dominant deformation process in the nanoindentation experiments. The carbon wear behavior was strongly influenced by variations in the film thickness, normal load, and number of scanning cycles. For a given film thickness, increasing the load caused the transition from an atomic-scale wear process, characterized by asperity deformation and fracture, to severe wear consisting of plowing and cutting of the carbon films. Both the critical load and scanning time for severe wear increased with film thickness. Below the critical load, the wear rate decreased with further scanning and the amount of material worn off was negligibly small, while above the critical load the wear rate increased significantly resulting in the rapid removal of carbon. The observed behavior and trends are in good qualitative agreement with the results of other experimental and contact mechanics studies.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):602-606. doi:10.1115/1.2831522.

In order to estimate the contact stress and deformation at the instant of head-disk impact during dynamic loading, Hertz’s impact theory is implemented in a dynamic loading numerical simulator. By using a finite difference model for the air bearing, a finite element model for the slider-flexure-suspension system, and Hertz’s contact model of head-disk impact, we obtain estimates of contact stress, area and time through simulation of the dynamic loading process. With our choice of the material properties and geometric configurations, the duration of sliding contact is about 7 μs, the maximum width of contact is about 10 μm, and the maximum contact stress is about 620 MPa. This is below the yield stresses of the chosen media.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):607-611. doi:10.1115/1.2831523.

Theoretical models describing the contact of rough surfaces have been developed for at least three decades. Experimental verification, however, has not been straightforward up till now, since current measurements suffer from large inaccuracies, often 100 percent or worse. In this paper a new method, based on autofocus techniques, is applied with an accuracy better than 15 percent. Measurements are presented for the contact of a rough polyurethane specimen in contact with a smooth glass plate. It was not yet possible to conclude whether the real area of contact is proportional to the contact load, but the results show the forming of contact agglomerations, which becomes significant at 10 percent real area of contact. Asperity interaction appears to be important, even at low loads. However, one or both of these facts are often not considered in present contact models, questioning their reliability.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):612-616. doi:10.1115/1.2831524.

A scanning probe microscope with a 80 nm radius diamond tip was used to investigate the wear resistance of single-crystal silicon and N+ -implanted silicon. The N+ implantation conditions were 35 to 150 keV and 5 × 1016 ions/cm2 . The N+ concentration depth profile was analyzed by using secondary ion mass spectrometry, and the chemical structure of N+ -implanted silicon was also analyzed by using x-ray photoelectron spectroscopy. The following results were obtained. The maximum N+ concentration on the ion-implanted silicon shifted further below the surface and the thickness of the high ion concentration region increased with the implantation energy. The high N+ concentration region using multiple energies of 35–150 keV during the same ion implantation process was wider than that for the N+ -implanted silicon using a single energy. The wear resistance of ion-implanted silicon was higher than that of single-crystal silicon. The N+ -implanted silicon using multiple energies during the same ion implantation process showed higher wear durability than that of the N+ -implanted silicon using a single energy. The Si2p spectrum of the high N+ concentration region implied a structure similar to a Si3 N4 film, which resulted in higher wear resistance.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):617-621. doi:10.1115/1.2831525.

A correlation is demonstrated between the low-cycle fatigue properties and the scuff resisting properties of 4340 steel. Low cycle fatigue properties were estimated from measured tensile properties for specimens with a wide range of hardness. Scuff resistance was measured in sliding tests using white mineral oil as the lubricant.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):622-628. doi:10.1115/1.2831526.

The worn surface profile of a composite structure was experimentally and numerically investigated focusing on the effects of sliding conditions. Wear tests on composites made of an oxide ceramic and an amorphous metal against a tetragonal zirconia polycrystals-alumina were carried out under various mean contact pressures, P, and sliding velocities, V. The test results showed that the worn surface profiles of the composites changed with the PV value. A new numerical method for simulating the worn surface profile of a composite structure has been developed. The present method is based upon the assumption that the profile of a worn surface is changed by thermal distortion of the sliding bodies due to frictional heating and by elastic deformation due to normal pressure and friction traction. The calculated results were compared with the test results, and the comparison showed that the elastic deformation plays an important role in forming the worn surface profile and that the effect of thermal distortion becomes remarkable with an increase in PV values. The numerical results clarified the contribution of the thermal distortion to the change in the worn surface profile of the composite.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):629-633. doi:10.1115/1.2831527.

Cubic boron nitride (c-BN) film was deposited onto a silicon substrate by means of a magnetically enhanced plasma ion plating method utilizing a hot cathode plasma discharge in parallel magnetic field. In this study, the friction and wear behaviors of the c-BN film, particularly when it came into sliding contact with diamond, were investigated using a reciprocating tribometer in an applied normal load range of 0.1 ~ 4.9 N. The results showed that the friction coefficient of the c-BN film sliding against the diamond indenter tended to decrease as the load increased, and was very low, exhibiting values of 0.03 ~ 0.065 at the maximum load of 4.9 N. Furthermore, the study confirmed that the friction coefficient of annealed c-BN film was lower than that of as-deposited c-BN film throughout the whole load range. Judging from the results of comparable investigations in which c-BN film came into contact with other materials such as c-BN compact, SiC and stainless steel, the wear performance and peeling resistance of the c-BN film proved to be significantly better in the case of contact with diamond.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):634-638. doi:10.1115/1.2831528.

To select tribological materials for use in clean environments, dust-generation properties from rolling-sliding elements made of silicon nitride ceramics and stainless steel were investigated. The rolling-sliding testing apparatus was used to investigate dust-generation properties of basic rolling-sliding elements. The number of dust particles generated increases rapidly with the percentage of sliding, or with what is called sliding rate in the rolling-sliding tests. Compared with pure martensitic stainless steel (440C) and silicon nitride (Si3 N4 ) , the combination of stainless steel and silicon nitride generates the least amount of dust PFPE grease lubrication drastically reduces the number of dust particles larger than 0.1 μm diameter. Dust generation increases with friction and wear volume. The generated dust volume is about 1/200 to 1/300 of the wear volume of a specimen without lubricant. This indicates that the dust particles were formed mainly by sliding.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):639-645. doi:10.1115/1.2831529.

Results are presented from an experimental study which demonstrates the feasibility of using an existing galling tester and test method as a screening device for evaluating thread compound additives for galling protection. The galling tester employs tubular pin and box specimen pairs fabricated from steel tubing used in the oil field. For the test thread compounds, four different additive packages were used, all with the same grease base of lithium stearate. For reference purposes, the lithium stearate grease base without any additives was tested as one of the thread compounds. The data obtained on contact stress at galling failure show that of the five thread compounds tested, the best galling protection is afforded by the API Modified thread compound, the industry standard for casing and tubing used by the oil industry. The other four compounds show lower galling failure contact stresses of varying degrees. Friction coefficient data obtained during the tests are also presented and discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):646-654. doi:10.1115/1.2831530.

An EHL approach to the rib-roller end contact in tapered roller bearings has been achieved by taking into account the non-Newtonian behavior of lubricants and thermal effects and with full consideration of the peculiar geometrical and kinematic conditions. Two kinds of geometrical configurations of rib and roller end were investigated: tapered rib/spherical roller end and spherical rib/spherical roller end. Optimal ratios of curvature radius of roller end to rib face were deduced. The film thickness, friction torque, lubricant temperature, and surface temperature at various speeds and loads were calculated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):655-659. doi:10.1115/1.2831531.

A “two-body,” elasto-plastic finite element model is employed to simulate repeated rolling contact in the presence of a surface irregularity. It is shown that the maximum Mises stress and equivalent plastic strain values in the substrate are related to the height of the pressure spikes. The results of the finite element calculations are used to derive generalizations about the influence of the indent geometry on the pressure spikes, peak cyclic plastic strains and their location below the surface. These relations can serve as guidelines for designing the depth and properties of surface coatings and modified layers.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):660-666. doi:10.1115/1.2831532.

A “two-body” elasto-plastic finite element model of two-dimensional rolling and rolling-plus-sliding has been developed to treat the effect of surface irregularities. The model consists of a smooth cylinder in contact with a semi-infinite half-space that is either smooth or fitted with one of two irregularities: a 0.4 μm deep groove, or a 7 μm deep groove. The model incorporates elastic-linear-kinematic-hardening-plastic (ELKP) and nonlinear-kinematic-hardening-plastic (NLKP) material constitutive relations appropriate for hardened bearing steel and the 440C grade. The calculated contact pressure distribution is Hertzian for smooth body contact, and it displays intense, stationary, pressure spikes superposed on the Hertzian pressure for contact with the grooved and ridged surface. The results obtained for the 0.4 μm deep groove are consistent with those reported by Elsharkawy and Hamrock (1991) for an EHD lubricated contact. The effect of translating the counterface on the half space, as opposed to indenting the counterface on the half-space with no translation, is studied. The stress and strain values near the surface are found to be similar for the two cases, whereas they are significantly different in the subsurface. Efforts have been made to identify the material constitutive relations which best describe the deformation characteristics of the bearing steels in the initial few cycles. ELKP material constitutive relations produce less net plastic deformation in the initial stages, for a given stress, than seen in experiments. NLKP model produces more plasticity than the ELKP model and shows promise for treating the net distortions in the early stages. Artificial indents were inserted on the running track of the cylindrical rolling elements and profilometer measurements of these indents were made, before and after rolling. These preliminary measurements show that substantial plastic deformation takes place in the process of rolling. The deformations of the groove calculated with the finite element model are compared to those measured experimentally.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):667-673. doi:10.1115/1.2831533.

An analysis is developed for the time-variable friction during the start-up of a rotor system. The analysis is based on a dynamic friction model that has been developed from the theory of unsteady lubrication and can describe the observed friction characteristics. The model reduces to the Stribeck curve of friction versus steady velocity, and shows hysteresis curves in oscillating velocity. The “Dahl effect” of a presliding displacement before the breakaway is also included. The results indicate that the friction characteristics and energy friction losses, during the start-up, depend on a set of dimensionless parameters that represent the bearing as well as the dynamic system. The study shows that appropriate design and operation can prevent stick-slip friction and minimize wear during start-up.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):674-678. doi:10.1115/1.2831534.

Measurements of thrust bearing friction torque are difficult because of the small values of the friction force in comparison with the axial load. Another problem of a thrust bearing test rig design is the necessity of accommodation of the axial load so that it does not interact with the frictional forces. Separation of fluid drag in the housing from torque in the bearing itself is also important. An analysis of a design in which the above-mentioned problems have been solved is presented. Theoretical analysis and calibration of the torque meter proved that the effect of axial load was successfully compensated for. Example of the results obtained due to the torque meter illustrate the application of the new design.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):679-684. doi:10.1115/1.2831535.

An experimental study shows that the traction coefficient of friction (μT ) is a function of the predicted air film height, the roller and film surface roughness, and the static coefficient of friction between the web and roller. The traction coefficient shows a noticeable decrease as the air film height becomes greater than the equivalent root mean square roughness of the web and roller and also becomes less dependent on the static coefficient of friction.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):685-690. doi:10.1115/1.2831536.

A perturbation analysis was made to study the effect of inertia on the film rupture in hydrodynamic lubrication using a modified Reynolds equation. A simplified boundary condition at film rupture proposed by Ota (1987) was employed. The theory was extended to investigate the wave number of the film rupture surface and to investigate the effect of gas bubbles included in the lubricant. Numerical calculations were carried out for a cylinder-plane bearing and are compared with previous experimental and numerical data. The effects of inertia on film rupture characteristics are clarified and the present numerical results are found to be in good agreement with earlier experimental data.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):691-695. doi:10.1115/1.2831538.

The stability of rigid rotors supported on finite oil rough journal bearings has been investigated using nonlinear transient method. The effect of various surface roughness parameters, viz., composite surface roughness, roughness orientation pattern, variance ratio on the stability is presented in the form of journal center trajectories. Although the stability is affected marginally with the increase in composite roughness, the variation is quite significant when the shear flow factor comes into the picture.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):696-701. doi:10.1115/1.2831539.

This paper describes a novel experimental procedure for measuring the four velocity or damping coefficients of an Oil Film Journal Bearing from imposed dynamic orbits. All four damping coefficients are derived from one imposed journal center dynamic orbit and, therefore, may be regarded as being obtained at the same time. The method requires the production of a “figure of eight” shaped orbit and utilizes the “cross-over” point therein. Coefficients are initially evaluated in a coordinate system which is chosen to align with the designated parts of the measured orbit. Each coefficient is then separately evaluated from single values of instantaneous imposed force and resulting journal center velocity. Coefficients are then converted into any other desired axes system. All the benefits of the original selected orbit technique are retained. The presented method places much less severe demands on experimental procedure. An available example illustrates the technique. A realistic uncertainty analysis for this example is included. Indications are provided which show how the orbit features may be adjusted to substantially reduce uncertainty.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):702-708. doi:10.1115/1.2831540.

A thermo-elasto-hydrodynamic analysis of an undercut journal bearing is presented whereby elastic deflection is introduced in a certain area of the bearing surface. The hydrodynamic pressure is computed from the generalized Reynolds equation, which takes into account thermal effects on viscosity. This is accomplished by solving the full energy equation for temperature. The elastic deflection is obtained from the elasticity equation. This study is then complemented with an elasto-hydrodynamic analysis of the full bearing. The controlled elastic deflection increases the bearing load carrying capacity and reduces friction.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):709-716. doi:10.1115/1.2831541.

With reference to a rigid symmetrical unbalanced rotor on lubricated journal bearings and adopting the short bearing theory, synchronous orbits and orbits with a 1/2 component described by the journal are determined as approximated solutions of the system of non-linear motion equations. The method also makes it possible to evaluate the stability of the above solutions and thus of the journal orbital motion. For different values of dimensionless unbalance of the rotor, examples of orbits thus obtained are given and, in the modified Sommerfeld number-stability parameter plane, the stability areas of the solutions are identified, in particular, the area of stability of the small synchronous orbits corresponding to the stable operating condition of the rotor-bearings system.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):717-723. doi:10.1115/1.2831542.

This paper solves the Reynolds equation by the finite difference method in a fixed coordinate system with the static load acted in the vertical direction. All static and dynamic characteristics (including load capacity, attitude angle, side flow, friction force, misaligned moments, and eight linear force coefficients) of a horizontally grooved bearing under different eccentricity and misalignment conditions are presented and compared with available experimental data. The effects of misalignment on all these bearing characteristics and on the stability of the rotor-bearing system are analyzed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):724-728. doi:10.1115/1.2831543.

A viscous and electrically conductive fluid separates two parallel plates. A magnetic field of intensity B0 is applied perpendicular to the plates. The top plate squeezes downward whereas the bottom plate moves laterally in its own plane. The nonsimilar problem is solved for the conditions of the top plate moving with constant velocity or constant force, while the bottom plate moves with a constant velocity. The effect of magnetic field on the braking action is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):729-736. doi:10.1115/1.2831544.

A Mroz image point, two surface, nonlinear-kinematic-hardening-plastic (MNKP) representation of bearing steel is inserted into a finite element model of 2-dimensional, line contact for pure rolling. The calculations are compared with previous results for the same contact pressure derived for elastic-linear-kinematic-hardening-plastic (ELKP) behavior. The residual stress, deformation, and the connection between continuing cyclic deformation, etching bands, and cracks are analyzed. Unlike the ELKP constitutive properties, the MNKP behavior displays a distinct transient region which results in higher residual stresses.

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):737-741. doi:10.1115/1.2831545.

Variation in friction and wear properties with relative humidity was obtained with an Sn pin sample on a Cu disk at a constant speed (0.4 m/s), load (6.4 N), and sliding distance (5 km), using a pin-on-disk apparatus. The influence of atmosphere on the tribological properties was investigated, including moisture ranging from 4% to 95 percent relative humidity (RH). It was found that the wear loss of the pin sample is very large at low humidity of around 5 percent RH, but it decreases and reaches saturation at about 50 percent RH. Factors characterizing the friction and wear at 50 percent RH were examined along with surface analysis of the disk. The results showed that the extensive transfer of Sn from pin to disk occurs during sliding and that the friction and wear behavior is determined by the friction and wear of an Sn sliding on Sn. An examination was carried out with an Sn pin sample on a stainless steel disk in comparison with an Sn-Cu couple. It was concluded that the friction and wear behavior is determined by the properties of the film transferred to the disk surface.

Commentary by Dr. Valentin Fuster

ERRATA

TECHNICAL BRIEFS

Commentary by Dr. Valentin Fuster
J. Tribol. 1995;117(4):744-747. doi:10.1115/1.2831547.
Abstract
Topics: Modeling
Commentary by Dr. Valentin Fuster

DISCUSSIONS

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