In this study, porous materials were applied to the surface materials of electrical sliding contacts to produce long and stable operations. The proposed sliding contacts consist of a rotating circular grooved porous disk and a stationary rider having a spherical surface. We conducted the experimental analysis from two perspectives. The first perspective was to inspect the effect of the design parameters on the electrical conductivity and the frictional characteristics; this was done by performing tests under various loads applied on the sliding contacts. The second perspective was to investigate the durability and stability of the sliding contacts by using the tests for a long operating time under a constant applied load. Our experimental results proved that the porous disk generated a lower and a more stable contact voltage and frictional force than the conventional nonporous solid disk. This effect is significant when a large curvature radius of grooves is provided on the disk surface. These results are attributable to the reduction of the hydrodynamic fluid force by the porous material and the grooves; the porous material yields lower hydrodynamic pressure due to the seepage of the lubricant oil across the interface between the oil film and the porous matrix and the grooves enhance the reduction of the hydrodynamic effect because of oil leakage to the downstream region. These factors reduce the oil film thickness between the disk and rider and facilitate the metal contact, and thereby a porous grooved disk generates higher electrical conductivity than a conventional solid disk.

Misalignment between the shaft and the bearing of a journal bearing set may be inevitable and can negatively impact journal bearing performance metrics in many industrial applications. This work proposes a convex profile design of the journal surface to help counteract the negative effects caused by such a misalignment. A transient mass-conserving hydrodynamic Reynolds equation model with the Patir–Cheng flow factors and the Greenwood–Tripp pressure–gap relationship is developed to conduct the design and analysis. The results reveal that under transient impulse loading, a properly designed journal profile can help enhance the minimum film thickness, reduce mean and peak bearing frictions, and increase bearing durability by reducing the asperity-related wear load. The mechanism for the minimum film thickness improvement due to the profile design is traced to the more even distribution of the hydrodynamic pressure toward the axial center of the bearing. The reason for the reductions of the friction and wear load is identified to be the decreased asperity contact by changing the lubrication regime from mixed lubrication to nearly hydrodynamic lubrication. Parametric studies and a case study are reported to highlight the key points of the profile design and recommendations for profile height selection are made according to misalignment parameters.

Diamondlike-carbon (DLC) coatings have received a lot of research attention by physicists and engineers, especially in the past 25 years. Attempts to use such materials in tribological applications have achieved variable success. The rationale for this work was to investigate the wear durability of three types of DLC coatings applied to hardened and tempered bearing steel and subject them to realistic high pressure cyclic loading under oil lubricated conditions for long duration. A thrust bearing design was deployed for this purpose. The wear and friction behavior of the DLC coated materials relative to uncoated materials was compared when using base (additive free) oils and typical autoengine formulated oils. The type of oil used made no difference to the dynamic friction and oil temperature for all the material and oil combinations used. Durability of the coated and uncoated roller bearings was determined by the type of material. For the uncoated bearings, life was limited after very many test cycles (approaching a billion) via classical rolling contact fatigue pitting. For all the DLC coated rollers life was governed by wear of their coatings. In the case of the tungsten doped DLCs (a-C:H:W), these were worn progressively and uniformly via microabrasion, whereas the nondoped ta-C and a-C DLC coatings were principally worn via delamination and tearing. The latter effect was relatively rapid and was considered to be initiated by blistering of the coating, a process that was probably driven by the high elastic energy/internal stress within the nondoped coating materials. The durability to delamination and tearing of the ta-C coatings was slightly lowered in formulated oil compared to tests made in base oil. Overall, for the test conditions used here, there was no apparent benefit in using DLC coatings.

Hydrogenated diamondlike carbon (H-DLC) coatings provide excellent wear resistance and low friction for bearing applications. However, the use of such coatings with aqueous lubricants could pose some difficulties due to the hydrophobic nature of the surface. A thrust bearing tribometer was used to compare performance of hydrophilic and hydrophobic surfaces in hydrodynamic lubrication with a mixture of water and glycerol as the lubricant. Hydrophobic surfaces on both runner and bearing were achieved with the deposition of H-DLC films on titanium alloy surfaces. Hydrophilic surfaces were created through modification of H-DLC surface with covalently bonded heparin. Several possible combinations of hydrophobic and hydrophilic surface conditions were used on the bearing and runner surfaces to provide full-wetting, partial-wetting, and half-wetting conditions. The experimental results confirmed that load support is still possible, when the bearing is half-wetted or partially wetted. However, the full-wetted bearing combination (i.e., Reynolds no-slip boundary condition) provided the highest load support. Introduction of slip at the surface resulted in a lower measured torque. Heparin treatment resulted in a lower than expected static friction and friction in full lubrication regime. The durability of coated surfaces was evaluated in a series of start–stop tests and in impact tests. The results confirmed that the coatings are stable and survive the test regiment that exceeded 50 test cycles; whereas the uncoated titanium alloy bearing surfaces were damaged after ten test cycles.

The frictional characteristics of some basic additive systems used in automatic transmission fluids (ATFs) were assessed using a variable speed friction test machine. The test evaluates the coefficient of friction in unidirectional sliding contact over a range of speeds, loads, and temperatures. Tests were performed on a steel-versus-clutch friction material tribocouple, mainly to establish the link between the lubricant/surface interactions and additive system performance. A new parameter that measures the positive μ -v curve slope speed range was applied, together with friction coefficient ratio index, to show the antishudder property. It is suggested that this is a good complimentary method to the friction coefficient ratio index evaluation. Lubricant formulation is important to the overall friction performance. Both friction modifiers and detergents are shown to be important to the ATF antishudder performance. Oil absorption ability of the post-test friction material surface is proposed to be an important parameter to indicate the antishudder durability in the test. The detergent and antiwear agents are also suggested to be responsible for the decreased oil absorption and pore plugging of the friction material surface.

There has been a lot of attention on the effect of lubricant additives on the friction at carbon coated surfaces. But only few papers have addressed the effect of additives on the durability of some diamondlike carbon DLC coatings. This paper presents a systematic study assessing the additive/additive and additive/surface interactions, and their influences on the durability of a low hydrogen-containing ( 15 at . % hydrogen) metal-free DLC coating (a-C:15H). In this study, lubricating oils containing a zinc dithiophosphate (ZDDP) antiwear additive and/or organomolybdenum friction modifiers (moly dimer and moly trimer) were used. Tribological tests were carried out in a pin-on-plate tribometer under boundary lubrication conditions. To understand the effect of additives, tribofilms formed on the wear tracks were analyzed using surface sensitive analytical techniques such as atomic force microscope, scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. Results showed that the ZDDP formed a zinc phosphate containing an ultrathin antiwear tribofilm, which offered excellent durability/wear protection to the a-C:15H coating. However, the antiwear performance of this additive was compromised when it was used with moly dimer or moly trimer. Surface analysis revealed that unlike steel surfaces, MoS 2 formed on the DLC surfaces had negligible influence on friction, while the low friction DLC wear debris had strong influence on friction. Abrasive wear was found to be the dominating wear mechanism in the cases when additives showed poor wear protection on the a-C:15H coating.

In order to improve the durability of metal-film magnetic tapes, novel lubricants, A20H, X-1P, and X, a modified phosphazene, were deposited on the tapes. The adhesion, friction, and wear of the unlubricated/lubricated tapes were investigated using an atomic force microscope (AFM). The degradation of the lubricants was studied using a mass spectrometer in high vacuum. The durability of various unlubricated/lubricated tapes was compared in ambient and in humid air. The AFM test results show that the A20H lubricated tape exhibited lower adhesion and friction than X-1P and X lubricated tapes. The lubricants were believed to be mainly degraded by tribochemical reaction and mechanical shear in high vacuum. In high humidity air, the various lubricated tapes exhibit higher friction than in ambient air. By comparing the tribological performances of the various lubricated tapes to metal particle (MP) tape, it was found that the lubricated metal-film tapes exhibit lower adhesion, friction, and wear than the MP tape.

We numerically investigated contact characteristics of a contact pad with a rough disk surface and the possibility of contact/near-contact sliders, using a single-degree-of-freedom (1-DOF) slider and a random wavy surface model with random roughness. Contact characteristics of a contact pad are numerically calculated based on a modified Greenwood-Williamson model, considering the bulk deformation of the surface due to all other asperity contact forces. It was found that contact stiffness and other characteristics are mainly determined by asperity contact, to the extent that the contact pad penetrates into the upper standard deviation of asperity peak height. However, the contact stiffness tends to approach a constant value as the pad penetrates into the average asperity height because the bulk deformation becomes predominant. From the numerical simulations of a 1-DOF air bearing slider model in contact and near-contact regimes over a random wavy surface with random roughness, a typical example of design condition of disk surface waviness in terms of the tracking ability and wear durability are shown, and the possibility and difficulty of a contact/near-contact slider is discussed. Finally, we analyzed meniscus effects on the contact characteristics and found a hysteresis process of the touch down and take off of a slider due to the meniscus force.

The authors developed a computer simulator for in-contact head slider motion where a 3-DOF model of head-suspension assembly was introduced and effects of meniscus force between the slider and the disk were considered. The contact force between a contact pad and a disk surface was taken into account and the dynamic characteristics of an in-contact head slider were discussed from the viewpoints of both bouncing vibration and wear durability by not only 3-DOF head-suspension assembly model over a sinusoidal disk surface undulation but also 1-DOF head-suspension assembly model over a sinusoidal disk surface undulation. In addition to the critical frequency of bouncing, the critical frequency of pressure was introduced. As a result of 3-DOF simulation and 1-DOF analysis, it was found that there exists an optimum surface energy at which the stability of bouncing vibration and wear durability becomes highest under perfect contact condition. Furthermore, the behavior of the optimum point for several design parameters were made clear.

The problem of a ball bearing in normal contact between two transversely isotropic coated substrates is investigated using the finite element method (FEM). A three-dimensional finite element model is developed that accurately determines the steady friction force in low-speed bearing systems containing soft layered solid lubricant films. Extensive numerical results, which are verified using Hertzian contact theory and laboratory experiments, are obtained at 540 operating conditions by varying coating material, coating thickness, normal load, ball material, and ball radius. Friction force results generated from the FEM are normalized by introducing the dimensionless transversely isotropic coating parameter, ξ. A numerical expression for the normalized friction force in coated ball bearings is determined by curvefitting the results of the 540 simulations performed. The relevance of such an expression, as related to the durability of low-speed bearings, is subsequently ascertained and discussed.

In this study, the authors numerically analyzed the bouncing vibrations of a two-degree-of-freedom (2-DOF) model of a tripad contact slider with air bearing pads over a harmonic wavy disk surface. The general features of bouncing vibrations were elucidated in regard to the modal characteristics of a 2-DOF vibration system and design parameters such as contact stiffness, contact damping, air hearing stiffness, the rear to front air bearing stiffness ratio, static contact force and the coefficient of friction. The design of a contact slider was discussed in terms of tracking ability and wear durability. In addition, two sample designs of a perfect contact slider with sufficient wear durability were also presented.

In this paper, the design condition of a single-degree-of-freedom (1DOF) contact slider over a random wavy surface of a disk is analyzed statistically. It is numerically confirmed that when the slider is in contact with a disk, spacing variation histograms are close to Gaussian even if surface waviness histogram is not close to Gaussian. The design condition of the slider and the surface waviness necessary for perfect contact sliding is derived from 3 σ s < δ , where σ s and δ are the standard deviation of spacing variation and the static penetrating depth, respectively, and it is verified both numerically and experimentally. The necessary condition for wear durability under uniform contact pad pressure is also derived. An example of the slider-to-disk interface design which satisfies both perfect contact sliding and wear durability is shown. In order to evaluate σ s adequately, frequency response of slider must be integrated over the range from f r / 2 to 2 f r where f r is the contact resonance frequency. It is found that a disk surface should be extremely smooth to satisfy both the perfect contact sliding and wear durability conditions. [S0742-4787(00)03001-0]

The general characteristics of the bouncing vibrations of a IDOF contact slider model over the surface of a harmonic wavy disk were studied both by computer simulation and theoretical analysis. The necessary design conditions for a contact slider and the surface of a disk were discussed in terms of perfect contact sliding and wear durability. It was found that the bouncing vibrations change with the amount of waviness amplitude A(f r ) at the contact resonant frequency f r ( = ( 1 / 2 π ) k c / m ) relative to static penetration depth δ, or f r relative to limiting critical frequency f cl , above which the downward acceleration of the surface of a disk is larger than that of a slider due to slider load. When the contact stiffness is large enough so that δ < A(f r ) (f cl < f r ), the slider bounces with a large amplitude similar to an elastic impact in a wide frequency range. When the contact stiffness is small enough so that δ > A(f r ) (f cl > f r ), bouncing vibrations occur near the contact resonance, similar to the resonance of a nonlinear soft spring system. Here, the bouncing vibration can be completely eliminatedby increasing the contact damping ratio and decreasing the slider mass and the waviness amplitude.

This study is a computational analysis of the bouncing vibration of a point contact slider model over computer generated random disk surfaces and the design conditions of slider to disk interface parameters necessary for contact recording. The Gaussian random surface of a disk with various standard deviations and frequency characteristics is generated by using a modified midpoint displacement algorithm. From the calculated results of bouncing vibration of a slider for various parameter values, it was found that the decrease in contact stiffness and increase in slider load can significantly reduce the bouncing vibration as well as the increase in contact damping and the smoothness of the surface. It was also found that the bouncing vibration spectrum of a contact slider over a simulated disk surface agreed closely with the experimental results presented in a previous study by the authors. The maximum and rms values of the spacing and the contact force were examined for various design parameters. The design conditions of the contact pad to the disk interface were discussed in terms of tracking ability and wear durability for slider loads of 0.5 mN and 5 mN.

This paper concerns an experimental investigation of fretting behavior using grease lubrication for ball (ball bearing steel) -flat (low alloyed steel) contact. The influences of the imposed displacement amplitude, contact geometry, pressure, and frequency are emphasized. In order to better understand the effect of grease lubrication at the interface, a glass counterface was used to visualize the contact; sliding tests were used to identify the effect of a large displacement amplitude. Dynamic loading analysis, in combination with subsequent metallographic examinations, have shown that fretting damage depends strongly upon fretting conditions and upon grease durability. Lubrication conditions and palliative effect in fretting are discussed. Results have shown an important effect of normal force, displacement amplitude, and frequency of motion on the lubricating effect of grease. The separation of grease into two phases in the contact during friction, creating bubbles, appears to be an important phenomenon.

Two of the most difficult issues to resolve in current design of head/disk interface in magnetic recording devices are stiction and durability problems. One method of overcoming these problems is by implementing a technology known as load/unload, where the system is designed so that the slider never touches the disk surface. One potential problem with this type of system is slider/disk contact induced disk defects. The objective of this paper is to show that the likelihood of disk scratches caused by head/disk contacts during the load/unload process can be significantly decreased by rounding the edges of the air-bearing surface. Using the resistance method, we observe that head/disk contacts burnish the corners of the slider and thereby decrease exponentially with load/unload cycles. A well burnished slider rarely causes any disk damage thus resulting in an interface with significantly higher reliability. A simple Hertzian contact stress analysis indicates that the contact stress at the head/disk interface can be greatly decreased by increasing the radius of curvature of the air-bearing surface edges.

The objective of this paper is to demonstrate, from experiments and modeling, how and why molecular orientation of functional end groups of perfluoro-polyether (PFPE) lubricants play an important role in the tribological performance of thin film magnetic disks. These disks typically have an amorphous carbon overcoat upon which a thin lubricant layer is deposited using dip-coating technique. Glancing-angle FTIR (Fourier Transform Infra-Red Spectrometry) is used for measuring molecular orientation of planer functional end groups. A molecular orientation index (MOI) was defined as 1 for randomly oriented functional end groups. The MOI is mathematically derived as 3 (maximum) for lubricant molecules oriented with their functional end groups perpendicular to the surface, and as 0 (minimum) if lubricant molecules oriented with their functional end groups parallel to the surface. The MOI is shown to depend on processing conditions and lubricant film thickness. The tribological performance of the lubricant films was evaluated using drag-mode contact start-stop testing. It was found that wear durability of the lubricant films (~2 nm) with MOI ~ 1.5 is a few times better than those with MOI ~ 0.5 to 1.0. No significant difference in the amount of bonded lubricant film was detected over the range of MOI studied. Nor was there a detectable relationship with hydrophobicity. It was inferred from decreased MOI values due to thermal effects and storage time that a smaller MOI value corresponds to a lower free energy state of the lubricant film. Interestingly, MOI values for bonded lubricant films for Process A are found to be close to 3.0, suggesting that almost all functional end groups in the bonded films are oriented perpendicular to the carbon surface, close to 2.0 for process B, and close to 0 for process C, meaning that almost all functional end groups in the bonded films from process C are oriented parallel to the carbon surface. Relationship between physical/chemical bonding configurations and MOI values are graphically presented in detail. Based on this relation, a simple model on lubricant film structures for the three processes studied is presented. The model MOI values agree very well with measured MOI values as a function of lubricant thickness for all three processes, and the model also appears to account for the observed tribology performance for the MOI values studied (0.5 ~ 1.5).

Recently, laser texturing has captured the attention of head/media interface engineers in the hard disk drive industry because it provides precision in the landing zone placement while eliminating the transition zone of a mechanically textured landing zone. It also offers excellent tribological performance in terms of low CSS stiction and good durability. These advantages make it the solution of choice for high-end magnetic hard disk drives. This paper models the effects of laser bumps and laser textured disk surfaces on the Headway AAB slider’s flying characteristics. Two commonly used laser bump profiles (“Sombrero” and “Volcano” types) and various texture patterns are numerically generated in the simulator. The slider’s dynamic responses to these moving laser bumps and textures under two outer rail flying conditions are simulated, and the effects of various bump/texture parameters on the slider’s fly height, pitch, roll and their modulations are discussed. The laser texture mechanism is explained by examining the air bearing pressure profiles induced by the moving laser textures.

Hydrogenated carbon films (CH x ) with different hydrogen content percentages have been examined. Drag tests on CH x coated disks, using 50 percent Al 2 O 3 /TiC sliders, with and without carbon coating on the slider air bearing surfaces (ABS), were conducted in an ultra high vacuum chamber equipped with a mass spectrometer. Mass fragments of lubricant released from the head disk interfaces were recorded in real time along with friction measurements. The results show that a higher hydrogen content in the carbon overcoat can improve wear durability by reducing the friction coefficient and affecting the chemical reactions between the sliders and the lubricant. A carbon overcoat on the slider ABS can protect Z-dol lubricant from catalytic reaction with the Al 2 O 3 in the slider material. The wear durability at the head disk interface is controlled by combined mechanical and chemical factors, which are defined by the atomic structures of the contacting surfaces.

Amorphous carbon nitride coatings of thickness of 5 and 30 nm were deposited onto 65 and 95 mm magnetic thin-film rigid disks surfaces using single-cathode and dual-cathode magnetron sputtering systems containing nitrogen-argon plasmas. Under optimum deposition conditions, amorphous carbon nitride coatings can be synthesized on ultrasmooth thin-film disks with no significant pinholes at thickness down to 5 nm, with hardness 22–28 GPa (compared to 7–12 GPa for amorphous carbon), and r.m.s. roughness as low as 0.25 nm. These amorphous carbon nitride coatings were shown to have better contact-start-stop performance and three-to-four times better pin-on-disk contact durability compared with amorphous carbon overcoats under identical testing conditions. Amorphous carbon nitride appears to be a promising candidate overcoat material for replacing amorphous carbon in the next-generation magnetic thin-film rigid disk systems.