Search

A comprehensive investigation was conducted on the tribological properties of micro ball bearings comprised of borosilicate glass balls and polymer guide. Experiments were performed to assess the friction and wear behavior of the micro ball bearings with respect to operating conditions. Also, the effects of guide shape on the rolling resistance of the balls were investigated using FEM simulation. It was found that even under high load conditions the glass/polymer-based micro ball bearings showed a sufficiently low friction coefficient of ∼0.028. In addition, 10 million cycle durability test confirmed the excellent wear resistance of the micro ball bearings. Through FEM simulation, it was confirmed that the tribological properties of the micro ball bearings may be further enhanced by using ball guides with a U-groove rather than a V-groove shape. It is expected that the numerical and experimental results presented in this work will aid in development of micro ball bearings that would be useful for various practical applications.

Micro-scale wear characteristics of ultra-fine gold wires with a diameter of ~28 μm was investigated under a low contact force over a long sliding distance up to ~200 km. The tip of the wire was slid against a silicon wafer under a low normal force of 5 μN. Wear of the gold wire was quantified from the images of the tip obtained by a 3D laser microscope and a scanning electron microscope before and after the wear test. The dimensionless Archard relationship wear coefficient was in the order of 10 −7 which was extremely low considering that sliding was carried out in dry condition. Such a low wear characteristics was attributed to low contact pressure that could be achieved by formation of a conformal contact due to flattening of the wire tip as sliding wear progressed. Wear behavior of the gold wire could be characterized by formation of thin flakes of wear debris that were generated as a result of adhesive shear stress imparted at the sliding interface. As an effort to reduce the wear coefficient even further, octadecyltrichlorosilane self-assembled monolayer was deposited on the silicon to serve as a nano-lubricant. It was effective in improving the wear resistance by about two-fold.

Wear map of bearing steel lubricated by silver film has been constructed to delineate the wear transition behavior with the change in operating conditions. Experiments were performed in dry sliding conditions using two types of ball-on-disk type test rigs under the contact pressure of 100–1000 MPa and the sliding speed of 20–1000 mm/s in ambient air. For the silver coating, an ultra thin IBAD silver bond layer was firstly deposited on AISI 52100 steel surfaces and then a relatively soft silver film was deposited by a thermal evaporation method onto the first layer. This functionally gradient film showed a great improvement in the life, mainly owing to the better bond strength. In order to build up a general framework on the tribological behavior of the functionally gradient silver films, all test data were plotted on a map whose axes are contact pressure and sliding speed. As a result, three main regimes were clearly identified: (i) elastic/plastic deformation of silver coating without failure, (ii) mild wear regime after initial failure of silver coating and (iii) severe wear regime. In the mild wear regime, the contact surfaces were covered with transfer layers of agglomerated wear particles. The transfer layer acted as a protective layer and resulted in low friction after initial failure of the coating. The formation of transfer layer was suppressed by several destructive actions, when the sliding speed was high. And, above a critical sliding speed, no transfer layer was able to form. In the discussion, an empirical model that could explain the existence of critical sliding speed was proposed.

The fundamental tribological characteristics of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), octadecyltrichlorosilane (OTS), and single chain alkanethiol self-assembled monolayers (SAMs) with various chain lengths were investigated in order to identify the mechanical scribing condition for micro-machining applications. The concept of the novel surface micro-machining explored in this work is to mechanical scribe away the SAM resist coated on the workpiece surface where, pattern formation by subsequent chemical etching is desired. From the experimental results, it was found that the FDTS surface was damaged about 20% more rapidly than the OTS surface due to higher friction, even though the surface energy of FDTS was lower than that of OTS. Also, it was found that thiol on a copper surface could be removed even under a few nN normal load. The nano-tribological characteristics of alkanethiol SAM on various metals were largely dependent on the native oxide layer of metals. Based on these findings, FDTS and 1-hexadecanethiol (HDT) were chosen as the resists for silicon and metal surfaces, respectively. By using the mechano-chemical process with a diamond-coated tip, nano-patterns with sub-micrometer width and depth on surfaces of Au, Ag, Cu and Si could be fabricated.

To develop a flying-type optical head mounted on a slider with flying height below 100 nm for Near Field Recording (NFR) application where removability of the media is required, plastic media is preferred because of their manufacturing cost effectiveness. In this work the effects of substrate material, carbon protective coating, and lubricant on the tribological characteristics of the slider/media interface were investigated. For the substrate, glass, engineering plastic (PEEK), and polycarbonate (PC) were used. A thin carbon protective layer and a lubricant layer were deposited on the substrates. The tribological characteristics of carbon and lubricant layers were evaluated using an Atomic Force Microscopy (AFM). Also, the flyability test, drag test, Contact-Start-Stop (CSS) test, and load/unload test were performed for each substrate while monitoring the friction as well as acoustic emission signals. It was found that the flyability was the key factor that affected the durability of the disks. As for the carbon coating, better performance was observed in the order of a-C, a-C:H, and a-C:N. Finally, 5 nm lubricant thickness was better than 2 nm lubricant thickness in terms of durability. The results of this work are expected to serve as the basis for developing an optical media with high durability in NFR application using a flying slider.

In this paper, the effect of surface topography on the frictional behavior is investigated at micro/nano-scale in order to better understand the influence of asperity contact angle on friction. Experiments were performed to observe the variation in the frictional force as a spherical ball slides across a grooved surface. Specimens with single and multiple grooves of tens of micrometers in width were fabricated on silicon wafers. The frictional behavior between these specimens and steel balls of different diameters were observed with a tribometer built inside a scanning electron microscope (SEM). Normal load in the range of 20 mN and sliding speed of about 1–6 μm/s were applied in the experiments. It was shown that for relative ball/groove dimension that resulted in low contact angle, the overall frictional force was less than that observed for surface without the groove. Also, in situations where there was a great change in the contact angle stick–slip behavior could be observed. This stick–slip behavior is attributed to mechanical interlocking action. In addition to the above experiments, tests were conducted using lateral force microscopy (LFM). Unlike the previous finding that LFM output is dependent on the slope alone, it was found that the signal was more sensitive to the change in slope, especially when the slope was relatively large. Overall, both micro and nano-scale topographic effect on friction was similar. These results will ultimately aid in design of surface topography for micro-systems for best tribological performance.

An experimental study was performed to discover the effect of environment on the tribological behavior of Si-incorporated diamond-like carbon (Si-DLC) film slid against a steel ball. The films were deposited on Si (1 0 0) wafers by a radio frequency glow discharge of mixtures of benzene and dilute silane gases. Experiments using a ball-on-disk test-rig were performed in vacuum, dry air and ambient air. It was observed that coefficient of friction decreased as the environment changed from vacuum to dry air. Results also showed that low and stable friction related closely to the smoothening of track surfaces and the formation of silicon-rich oxide debris.

Basic understanding of asperity interaction with the counter surface during an abrasive wear process may be utilized to develop a precision machining process. This concept is implemented in this work using a diamond tip as the cutting tool and coated silicon wafer and metal specimens as the workpiece. The tip acts as a single asperity abrading the counter surface during the contact sliding motion. A precision machine based on computer numerical control was designed and built to perform such an abrasion process. The machine was instrumented to monitor the normal and frictional forces during the abrasion process. By optimizing the contact and sliding conditions the workpiece material could be removed at the micrometer scale. This technique was successfully developed to machine SiO 2 resist material coated on silicon wafer. Subsequent processing of the silicon by chemical etching produced micro-grooves that could otherwise be fabricated only by more complicated and capital intensive photolithographic methods. Also, in a different process, by feeding the diamond tip in the direction perpendicular to the sliding direction after each path, 100×100 μm square pockets with recess steps of 2 μm could be successfully fabricated on a brass surface. This intricate pattern can be used as a tool for molding micro-parts. This work demonstrates the capability of single asperity abrasion process in surface micro-machining.

In this work, the tribological behavior of octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3), which is a self-assembled monolayer (SAM) known for its low surface energy and hydrophobic property, deposited on silicon was investigated. Since the state of OTS depends heavily on the coating time, the effect of coating time on various tribological characteristics of OTS deposited on silicon (1 0 0) was of primary interest. The macroscopic frictional force of OTS deposits was also measured using a pin-on-disk type tribotester. The OTS deposited silicon specimens were slid against a 1.6 mm diameter STB2 steel ball under a normal load of 50 mN. It was found that the macroscopic friction coefficients for coating times >1 h ranged between 0.1 and 0.2 whereas the value for shorter coating time was as high as 0.7. We also found that the frictional behaviors of OTS coated silicon surface were less sensitive to humidity changes compared with bare silicon surface. Furthermore, the stiction of OTS coated silicon was much less sensitive to apparent area of contact, applied load, and soaking time. This work demonstrates the effectiveness of OTS film in reducing sliding friction of silicon micro-systems when deposited properly.

Rolling behavior of plasma-modified silver films was experimentally evaluated under various environments using a thrust ball bearing-typed test rig. The silver films were coated on AISI 52100 bearing races by a thermal evaporation method where the thickness was about 1.4 μm. Plasma surface modification was performed on the silver-coated specimens in order to change the water-wetting characteristics of the surfaces. Tests were performed in ambient air, dry air and vacuum under the normal load of 147 N and at the rotating speed of 31 mm/s, and the rolling resistances were measured with the contact cycles. The wetting angles were measured by Contact Anglemeter and the surfaces were analyzed by Fourier transformation infrared spectroscope and scanning electron microscope. Results showed that rolling resistance of plasma-modified silver-coated films was greatly affected by humidity. Rolling resistance of plasma-modified hydrophobic silver film showed its minimum value at a relative humidity of 80% while that of the modified hydrophilic silver film showed its minimum value at a relative humidity of 10%. It was found that the changes in wetting angle influenced remarkably the silver debris agglomeration and the tendency of contact patch formation. Analyses suggested that the dependence of the rolling resistance on humidity could be explained by the fact that adsorbed water vapors affected the wear debris agglomeration and resulted in the different behavior of rolling resistance with humidity.

The role of wear particles at the sliding interface on the frictional behavior of metals was investigated with respect to size and agglomeration characteristics using both pin-on-disk and pin-on-reciprocator tribotesters. The wear particle interaction at the sliding interface was monitored using a CCD camera. It was found that the friction coefficient significantly rises once a wear particle is produced and trapped at the interface. Wear particles of soft, ductile metal pairs go through a cycle of agglomeration and breakup, while wear particles of hard material pairs are less likely to agglomerate. The wear particles were smaller for the bidirectional sliding motion compared with those in the unidirectional sliding motion, which resulted in lower friction and wear characteristics. This difference is attributed to the higher probability of particle breakup in bidirectional sliding motion. The frictional behavior was also investigated when metal particles are introduced into the interface of Cu–Cu sliding pair. The friction coefficient increased with the introduction of Ni powder whereas it decreased with the introduction of Al powder. It is concluded from these results that frictional behavior could be altered at will by inserting appropriate particles into the sliding interface.

An experimental study was performed to discover the effect of a soft metallic coating on the behavior of rolling resistance. Pure silver was coated on AISI 52100 bearing steel by a thermal evaporation method, where the coating thickness ranged from 100 nm to 4400 nm. A thrust ball bearing-type test rig was used and the rolling resistance was measured in various environmental conditions such as ambient air, vacuum, dry air and controlled humidity. The surfaces were analyzed by SEM and EPMA. Results generally showed that minimum rolling resistance was obtained after the initial coating film was apparently worn. The decrease of rolling resistance resulted mainly from shakedown phenomena which occurred at the contact patches of agglomerated wear particles. It was also found that rolling resistance was affected strongly by environmental conditions. Results in various environmental conditions suggested that humidity related closely to the agglomeration of particles that affected significantly the rolling resistance. The influence of humidity on particle agglomeration was further investigated by using a plasma surface modification that changed the wetting angle of coated films.

The tribological behavior of ultra-thin soft metallic deposits on silicon substrates was investigated for low load sliding applications. Thermal evaporation and ion-beam sputtering deposition techniques were used to produce coatings of various apparent thicknesses (1–1000 nm). Sliding tests were conducted with gold and silver coatings on silicon substrates using a reciprocating pin-on-flat type apparatus to study the tribological behavior of these coated specimens. Various tribological parameters were examined for controlling friction and wear of soft metallic coatings under relatively low loads. Experimental results indicate that the film thickness, load and coating deposition technique affect the frictional behavior of soft metallic coatings. Microscopic observations showed that plowing is the main cause of friction for soft metallic coatings under relatively low loads. Based on the experimental observations, the film thickness and load dependency of soft coatings were analyzed using the plowing friction model with a modified hardness term. This analysis shows that the film thickness and load dependency of soft coatings can be reasonably explained using the plowing model.