Many devices involve sliding contact where a rotating component slides over a stationary component. Examples include disk brakes, cam/valve lifters, and laboratory pin-on-disk machines. An essential feature of these devices is that a wear track forms on the rotating component that is cyclically exposed to frictional heating, temperature fluctuations, and enhanced chemical film formation. The objective of this paper is to develop a theoretical model and solution methodology to examine the thermal effects produced by friction between sliding contacts in systems with rotary motion. A new methodology based on a combination of control volume finite difference and the cellular automata concepts is developed. The method involves a cascading sequence of simple, explicit rules of evaluation, rather than complicated partial differential equations. Results using the general model developed in this study are presented in dimensionless form to show the importance of critical operating parameters. Implications for applications such as disk brake and cam/valve lifters are suggested.

An effective simulation technique for describing the spreading properties of molecularly thin lubricant films on magnetic disks has been developed. We propose a molecular precipitation method that can simulate initial molecule arrangement of the films dip-coated onto the disks. Reptation and Rouse models as the model of the molecular motion, and molecular insertion and molecular precipitation methods as the method for putting molecules in initial positions were compared. From the results of the spreading profiles and diffusion coefficients, it has been revealed that the molecular precipitation method combined with the Rouse model is effective in simulating the spreading of the lubricant films.

The paper provides some new insight of the molecular conformation of the PFPE with functional end groups on hard disk surfaces by considering the dip coating process. It is proposed that the change in the molecular conformation during the dip coating process can be divided into three stages: adsorbing stage, developing stage and drying stage. A formula of predicting the monolayer thickness of the PFPE lubricant on hard disk surfaces is also proposed.

This paper deals with the measurement of friction force exerted on molecularly thin lubricant film surfaces using a specially arranged pin-on-disk type friction tester. The measurements were carried out by sliding a 1.5-mm-diameter glass ball slider on a rotating disk surface with small loading force. Polar and non-polar PFPE lubricants were dip-coated on magnetic disks covered with diamond-like-carbon (DLC) film. Lubricant film thickness was varied to constitute multiple layered film structures on the DLC surface. To clarify the stratified effect of thin lubricant film on friction, a lightly loading force and a slow rotational speed were selected. The tested results showed that the friction force on non-polar lubricant surfaces increase slightly for mono-layer and multi-layer cases, while the friction force on polar lubricants show steady and gradual increase with increasing loading force. We conclude that friction force at small loading force is dependent intimately on the thickness, molecular weight and end-group functionality.

Brake squeal is an everyday problem, so it has been widely studied, mostly in vibration parlance studying modes, noise energy and frequencies. We propose to study the phenomenon from its origin — i.e. the contact — in a tribological approach that combines contact geometry, third body aspect and squealing initiation: thus, a temporal analysis was necessary. Experimental and numerical approaches have been used to identify influential parameters and they give similar results: a really strong correlation exists between the disk geometry and the squeal initiation.

The pumping effect which is proposed by the authors to explain the accumulation of liquid-like chemicals on the air bearing surface has been analyzed by taking into the account the incline of the air bearing surface. Numerical results show that the accumulation rate of the contaminant decreases as the incline of the air bearing surface increases and at the same time the maximum adsorbed film thickness shifts from the outlet to the inner surface.

In order to clarify the degradation mechanisms of the PFPE fluid under a head/disk sliding contact, decomposition reactions of z-dol fluids were investigated under triboplasma generation and in the atmospheres of oxygen, nitrogen and air in gas-discharge-plasma. The results showed that the PFPF degradation proceeds through intermolecular reactions with ambient air molecules by the tribomicroplasma.

An Al-Si alloy impregnated graphite composite (ALGR-MMC) containing a large amount of graphite (56 vol%) has been newly developed to improve the friction and wear characteristics of machine components such as clutches under dry sliding and journal bearings under lubricated sliding. The ALGR-MMC has the same self-lubricating performance as graphite in moist environments. It has good properties of thermal expansion, heat resistance and wear resistance. The objective of this study is to investigate the friction and wear of the ALGR-MMC under fretting conditions. Ball-against-disk type fretting wear tests for the ALGR-MMC (flat) in contact with bearing steel (ball) were conducted at various contact loads in moist air under unlubricated conditions. The friction coefficient was monitored during the tests. The area and depth of wear scars were measured after the tests or in the interrupted tests. From the friction and wear data, the effects of contact load on the fretting wear characteristics of the ALGR-MMC were evaluated with SEM observations of wear scars and EPMA analyses of wear surfaces.

In cryogenic machining, liquid nitrogen (LN2) is well recognized as an effective coolant due to its low temperature, however, its lubrication effect remains unknown. Our previous studies of the change in cutting forces, tool wear, chip microstructure, and friction coefficient indicate a possible lubrication effect by LN2. To verify proposed LN2 mechanisms and distinguish them, idealized disk-flat contact tests were performed. From the test results, the LN2 lubrication effect by altering material properties at low temperature was dependent on the material pairs. An uncoated carbide insert with a low carbon steel or titanium alloy disk test showed reduction of friction under LN2 cooling, but a coated insert increased the friction force. LN2 injection to form a physical barrier or hydrodynamic effect between two bodies is always effective to reduce friction force.

The turbulent flow of air generated by spinning disks in hard disk drives (HDD) is known to be major contributor to undesirable vibrations of the head stack assembly (HSA). The continuing demand for higher capacities in HDD has resulted in a continual increase in areal densities. Densities in excess of 100 Gb/ in 2 have been demonstrated and it is widely projected that conventional technology will ultimately achieve 1 Tb/in 2 . It is foreseeable that at such areal densities, a track density of 0.5 Million tracks/inch will be required, with each recorded bit being roughly 13 × 50 nm. Under such conditions, the tracking accuracy required is approximately 1.5 nm RMS (root mean square). These goals place stringent requirements on the positioning accuracy of the actuator while accentuating the need to mitigate the various sources of track misregistration (TMR).

Most hard disk drives manufactured in the last few years have Load/Unload (L/UL) technology. As opposed to the Contact Start/Stop (CSS) technology, L/UL technology has the advantage of improved areal density because of more disk space availability and better shock performance. The latter characteristic has significant benefits during the non-operational state of the hard disk drive since head/disk interactions are eliminated and the head is parked on a ramp adjacent to the disk. However, even if head/disk interactions are absent, other failure modes may occur such as lift-tab damage and dimple separation leading to flexure damage. A number of investigations have been made to study the response of the head disk interface with respect to shock when the head is parked on the disk ([1], [2]). In this paper, we address the effect of non-operational shock for L/UL disk drives.

The material requirements on aircraft engine mainshaft bearings increase due to an elevated speed index (bearing bore diameter multiplied by rotational shaft speed) and slip ratios [1]. The formation of reaction layers on surfaces in mechanical contact is strongly affected by the tribological loading conditions, the materials used, the lubricant, and the service temperature. An appropriate reactivity between material and lubricant in tribological systems decreases wear and friction and increases the durability [2,3]. Goal of the paper is to compare wear and friction properties of standard aerospace bearing steel AMS 6491 (M50) with that of the high strength stainless steel grade AMS 5898. The nominal chemical compositions are 0.82C-4.1Cr-1V-4.2Mo (wt%) and 0.3C-0.4N-15.2Cr-1Mo (wt%), respectively. In order to characterize the material behavior under pure sliding conditions, ball on disc (BOD) experiments were performed with a contact pressure of 1GPa and a sliding speed of 10 cm/s at room temperature and at 150°C. As lubricant the jet engine oil Mobil Jet II was used. It is assumed that the reaction layer formation depends on the material composition and is also effected by the counterpart and the lubricant additives. Thus, the experiments were performed with two different ball materials. The first ball material was the same standard aerospace bearing steel (M50) as mentioned above and the second was a ceramic (Si 3 N 4 ). The homogeneity and the distribution of the reaction layers as well as the wear rate were determined in the contact zone by means of optical profilometry, scanning electron microscopy (SEM) and Fourier transformation infrared spectroscopy (FTIR). The study showed that wear is significantly higher on the stainless steel grade compared to M50 (Fig. 1a and Fig. 1b). The dark areas in Fig. 2 are phosphorus rich regions on the wear track of M50. This reaction layer is mainly built up of phosphates, which result from the TCP lubricant additive. In the FT-IR spectra (Fig. 3) absorption bands at 1130 cm −1 (room temperature-BOD test) and at 1160 cm −1 (150°C-BOD test) are visible, which result from the P=O stretching [4]. The shift of the absorption band to lower wave numbers with decreasing test temperature is probably due to hydrogen bonding [5]. Contrary to AMS 6491 no measurable reaction layer was found on AMS 5898 after testing at room temperature (Fig. 1b). The friction coefficients of the two steels against Si 3 N 4 balls determined in the BOD tests are compared in Fig. 4. AMS 5898 shows an abrupt increase of the friction coefficient after a sliding distance of 3.5 m. The reason for that is a material transfer of disc material to the ceramic ball as can be seen in Fig. 5. This transfer material causes a plowing of the disc and thus, an increased wear can be observed. The different Cr-contents and consequently oxide layers of AMS 5898 and AMS 6491, which react differently with TCP, might explain this behavior. AMS 5898 does not sufficiently react with TCP at room temperature to form a protective layer. Consequently, material transfer and increased wear occurs. In case of AMS 6491 an increase of the operating temperature cause a change of the reaction layer (see Fig. 3) and to an increase in the wear rate.

Hard disk drives continue to increase the data storage density. The parameters of the head slider flying attitude must satisfy very strict performance goals. This paper focuses on the topic of the simulated annealing algorithm when it is applied to the problem of slider air bearing design in hard disk drives. The objective is to minimize the static flying height, to keep the pitch angle within a reasonable range, and to minimize the roll angle. The design variables include recess depth and geometry configuration of the slider air bearing surface. A typical tri-pad slider was taken as the physical model to demonstrate the validity of the optimal algorithm. Results show that simulated annealing is efficient for the optimization of the slider air bearing design. The static air bearing characteristics were enhanced significantly.

Tribological characteristics of A356 Al alloy-10 vol. % SiC P composite discs/brake pad has been studied under dry sliding conditions at sliding speeds in the range 2 to 5 m/s and at loads in the range 1–3 MPa. In these tests, disc of Al MMCs and pin of friction pad made of polymer based composite were used. Wear rates of Al MMC disc as calculated by weight loss method, found to be negative at high sliding speed and high load. Worn surface of disc has been analyzed using EDAX. SEM analyses of worn surfaces of composite disc infer transfer of material from pin to the disc resulting in the formation of tribolayers. Two types of tribolayers were observed on the worn surface, one having shiny appearance of copper rich layer and other is dark in colour consisting of Mg, S, Fe, Ba, Ca, Si, Cu, In and Al. In the later layers were rich in copper and appear as bright patchy layers under SEM. Coverage of copper rich layers increase all along and across the worn track at a sliding speed of 4 and 5 m/s in the load range 2 to 3 MPa. Atomic percent of copper increase with load and consequently affect the wear rate of disc. EDAX analysis of dark tribo layers on wear track of composite disc show continuous increase in the amount of Cu and Ba with increase in speed and load. Hence, wear rate of composite discs were relatively low under all test conditions. These results clearly indicate composition of friction material having profound influence on the wear rate of Al MMC discs.

This paper shows the effect of ultrathin lubricant films between sliding bodies on Acoustic Emission (AE) signals induced by the sliding friction. Experiments were conducted with a ball-on-disk friction tester to measure the friction coefficient, the raw AE signals and the root-mean-squarevalues of the AE signals (the AErms signals). The ball was a glass ball of 5mm diameter. The disk was a magnetic disk used for 2.5 inch HDD with a DLC protective layer on its surface, and was coated with PFPE Z-dol 4000 about 1.5nm thick. The AErms signals kept a low level for some time after the start of the test, and then increased. Its time variation was similar to that of friction coefficient. After the friction test, the circumferential distribution of the lubricant film thickness was measured with an ellipsometer. The distribution demonstrated the reduction of the lubricant film thickness at the circumferential position where the magnitude of AE signals became large. These facts showed that the AE signals correlated well with the lubricant film thickness.

This paper presents wear and friction properties of carbon-carbon composites (CCC) manufactured by resin transfer molding (RTM) process. During composite fabrication thickness stitching was employed to improve inter laminar tension and shear properties. Wear and Friction characteristics of carbon-carbon composites were evaluated using pin-on-disc set up. Results of test indicate that surface of composites having stitches in a perpendicular direction show increase in wear rates with increase in load.

In this paper, the start up condition in elastohydrodynamic lubrication was studied on a steel ball on glass disc contact lubricated with a mineral oil, using a previously developed mixed phase lubrication contact model. The numerical simulation demonstrated the contact geometry change in the transition from initial solid contact to final fully lubricated contact, as well as the load sharing variation between lubricated and solid contacts during the start up. The numerical results of contact geometry was then compared with an experimental study of the same start up situation published by Glovnea and Spikes [1]. The film thickness variations and the propagation of the solid-lubricated contact interface during the start up were compared. Furthermore, an analytical result of the propagation of the solid-lubricated contact interface was also generated by assuming the dominance of the wedge and squeeze terms in the Reynolds Equation, and was also compared with the numerical and experimental results. Good agreement exists among the analytical, numerical and experimental results. The good agreement proves the numerical model as a valid tool in studying the start up condition in elastohydrodynamic lubrication as the transition from solid contact to lubricated contact is of great importance when investigating the start up process and its effects on the overall lubrication performance.

Spring constants and damping coefficients of a molecularly thin liquid bridge of PFPE lubricant intervening between a diamond probe tip and diamond like carbon (DLC) surface of a magnetic disk are identified through regression analysis of tip damping vibration. Perfluoropolyether (PFPE) lubricants having functional end groups were used to form the liquid bridge between the DLC surface and a probe tip with the notably small curvature radius of 0.1 μm. The tip was both retracted from and extended toward the disk surface at four different, progressive distances to attain varied elongation of the bridge, and was made to vibrate at each step to provide damping waveforms. By applying regression analysis to observed waveforms, the spring constant and the damping coefficient of the liquid bridge were identified. It is interesting to note that both the absolute value of the spring constant and the frequency-multiplied damping decrease with bridge elongation, and after reaching the minimum, those values begin to increase.

Vapor phase lubrication (VPL) integrates media lubrication with the vacuum processing steps used throughout most of the hard disk media fabrication process. This avoids exposure of the unlubricated a-CH x overcoat surface to the ambient air and airborne contamination. In vapor lubrication the a-CH x surface can be oxidized under controlled conditions immediately prior to lubricant adsorption. The kinetics of a-CH x oxidation have been studied using x-ray photoemission spectroscopy in an apparatus that allows oxidation of freshly deposited a-CH x films. The dissociative sticking coefficient of oxygen is ∼10 −6 and the initial oxidation kinetics can be described by a Langmuir-Hinshelwood mechanism.

Measurement of complex hydrocarbon film thickness on engineering surfaces such as magnetic hard disks is difficult if the film thickness is below the peak to valley distance of the surface roughness. Instrument sensitivity and detection accuracy for most of the common tools for this purpose is limited. We have developed a novel calibration technique that allows accurate measurement of such films within 5%–10% accuracy.