The 7th International Tribology Conference, Tokyo 2015, was held on 16–20 September 2015 at Katsushika Campus of Tokyo University of Science, located at the east end of Tokyo Metropolitan, Japan.
At the ITC Tokyo 2015, over 600 registered participants from 30 countries and about 50 industrial exhibitors attended the conference. During the conference, about 400 oral talks and 200 posters were presented. Excellent papers submitted to the Tribology Online have subsequently undergone the normal peer-review process by multiple reviewers. Many of accepted papers were published in this Special issue, Part 1, and other papers will appear in the next Special issue, Part 2.
On behalf of the ITC Tokyo 2015 Conference Committee, and the Editorial Committee of Tribology Online, I, the Chair of the ITC Tokyo 2015, and the Guest Editor of Tribology Online, acknowledge the authors’ efforts to submit their papers to these Special issues. I also express my sincere thanks to the Editors, the Reviewers, and the Publication Coordinators for their efforts to complete the peer-review processes and the publication work.

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The sealing system radial lip seal represent a complex tribological system with high requirements to be fulfilled over a long time. The surface texture of shaft counterface has a great influence on the performance of radial lip sealing system. A usability evaluation of shaft counterface with standardized 2D roughness parameters is not possible, because they represent only limited functional properties of the shaft counterface. With optical measurement instruments, shaft counterfaces can be measured in 3D. By using 3D metrology shaft counterface data the circumferential direction can also be evaluated. Together with the 3D surface parameters according to ISO 25178 this provides a fully new approach to evaluate shaft counterface. After the measurement, the data must be preprocessed to obtain meaningful parameters and to get a uniform database regardless of the manufacturing process. Measuring the pumping rate of shaft counterfaces is a common but time-consuming method to describe the quality of radial lip seals. For the evaluation of the 3D surface parameter according to ISO 25178 correlation analyses were performed by using a linear regression analysis. The correlation analyzes have shown, that the 3D parameters have the potential to describe the quality of shaft counterfaces for sealing purpose.

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This study experimentally investigated the possibility of controlling the static lubrication characteristics of hydrodynamic journal bearings by cooling and heating locally. A test rig of a plain journal bearing was built to observe the pressure and temperature distributions, bearing eccentricity, attitude angle, frictional torque, and flow rate. The bush was assembled with three block parts in which polyoxymethylene plates were inserted to insulate heat. The rotating journal was supported by rolling element bearings, and a load was applied to the bearing bush by leverage and using dead-weights. A Peltier device and a rubber heater were mounted on the bearing bush. The temperature and pressure sensors were installed circumferentially in the bush. The eccentricity was monitored using contacting displacement sensors. The temperature of the bearing bush was changed such that the viscosity of the lubricating oil was controlled. The load, rotational speed, and oil viscosity were considered the main experimental parameters. When the bearing bush close to the minimum clearance was cooled, the fluid pressure changed and the bearing eccentricity decreased, suggesting an increase in minimum clearance.

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To extend tool service life, several types of tapping tool coated with hard films have recently been developed. However, the detachment of films from tool surfaces and chip snarling on tools remain barriers to further increases in tap service life and tapping speed. In this study, a tapping tool coated with a composite film composed of nickel-phosphorus (Ni-P)/silicon carbide (SiC) particles was developed, and its machining performance was experimentally investigated. The composite film was coated on a spiral tap using electroless plating. Tapping tests demonstrated that the new tapping tool prevented chip snarling at five times the conventional cutting velocity. At such high velocities, chip snarling occurred on other taps, including one with a conventional steam-treated surface, one coated with a titanium carbo-nitride film and one coated with Ni-P plating taps. Furthermore, due to its high resistance to wear, the tool service life of the tapping tool coated with the composite film was approximately two times longer than that of the conventional steam-treated tapping tool. These results indicate that the tapping tool coated with the Ni-P/SiC composite film has the ability to increase processing efficiency and extend tool service life.

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The performance of wire drawing is closely dependent on lubrication between die and metal wire. The behavior of lubricants between the die and the wire plays an important role in the lubricating performance for wire drawing. Therefore, the lubricant behavior should be made clearer by a direct observation of the drawing interface in die. The objective of this work is to understand the behavior of lubricants based on the chemical reactions for wire drawing. The drawing properties of these lubricants are discussed on the basis of the generation of metal soap. In situ observation of lubricant film during wire drawing was carried out using a micro-Fourier transform infrared spectroscopy (FTIR). The infrared (IR) spectra of oil film between die and metal wire can be obtained and oil film shape and concentration of additive can be estimated. It was found that oleic acid as an additive reacts with metal wire chemically and metal soap was formed. The concentration of metal soap was distributed to be high at the end of reduction zone under the dynamic conditions.

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The present paper describes a two-phase flow model used to study backward pumping of the low pressure fluid in the sealing interface of a mechanical seal. The Reynolds equation for a homogeneous mixture is solved as well as a transport equation used to determine the ratio of both fluids at each point. Several examples are presented to show that the second function of a seal (prevent the contamination of the process fluid by the operating environment) is not always insured because of possible reverse flows. The behavior of the seal can be greatly affected when it is partially lubricated by the low pressure fluid.

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The tribological properties of diamond-like carbon (DLC) films that were lubricated with an environmentally friendly ester-based oil were investigated. Sliding tests were performed on two types of DLC films, hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (ta-C) films, using a steel cylinder that was in contact with a steel disk coated with either a-C:H or ta-C under boundary lubrication. To compare the tribological properties of ester-based oils and other based oils, Trimethylolpropane (TMP) ester, polypropylene glycol (PPG), and poly-alpha olefin (PAO) were used as the test lubricants. The ta-C film that was lubricated with TMP ester exhibited the ultralow friction coefficient of 0.015; however, the a-C:H film that was lubricated with TMP ester did not exhibit the ultralow friction. On the other hand, results obtained by a quartz-crystal microbalance with dissipation indicated that the adsorption properties of based oils depended on the type of sensor material (carbon or iron) and the type of based oil used in the measurements. Moreover, the frequency change of based oils increased with decreases in the average frictional coefficients of the steel-steel tribopair and the DLC-steel tribopair. Results of surface analysis indicated that TMP ester formed tribofilms on the ta-C surfaces, which exhibited high viscosity and low friction. All these results suggest that the combination of the ta-C film and TMP ester is the new ultra-low-friction and environmentally friendly sliding system.

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In the elementary process of adhesive wear, wear elements, which are the elemental debris of wear particles, are generated at junctions of asperities, and subsequently grow into transfer particles between sliding surfaces through a mutual transfer and growth process. To elucidate the mechanism of adhesive wear and to establish a wear equation, it is necessary to investigate the characteristics of wear elements and transfer particles in more detail. In this study, changes in the numbers and shapes of wear elements for various metals were examined by means of scanning probe microscopy. In addition, the relationship between the numbers of wear elements and the adhesion forces, determined by friction force microscopy and force-curve measurements, was examined to evaluate the probability of generation of wear elements. The findings on the elementary processes of adhesive wear obtained from this study should be useful in establishing a future theory of wear.

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In the automotive industry one of the crucial issues is reduction of CO₂ emission to protect environment. A lot of researches have been done to reduce the CO₂ emission of cars by improving efficiency of engine and engine parts or lowering the weight. However, a significant reduction of CO₂ emission of cars can also be obtained from improvement of efficiency of transmission by reducing drag torque or spin loss in disengaged wet clutch. The reduction of drag torque can be achieved by introducing groove on facing of clutch disk and further reduction can be obtained by optimizing the shape of groove. Generally, the optimum shape of groove is decided by trial and error method which requires time, making sample, conducting a lot of tests and wasting a lot of raw materials. In this research, we proposed a new analytical model to predict the drag torque characteristics of a disengaged wet clutch of transmission without making any samples and performing any tests. The feature of the model: It bases on continuity and Navier-Stokes equation, a real time visualization investigation and laminar flow. The model is validated with tests results and the comparison result shows that they are very close to each other. The significance of the model is: it captures the multiphase drag torque characteristic which changes with changing gaps, speeds and oil flow rates. Moreover, the model is a green environment friendly tool to estimate drag torque characteristics without making any samples or conducting any drag tests.

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In tribological testing with simplified test geometries, the contact configurations usually consist of two bodies representing one material pairing. This contribution introduces an adapted ball-on-pyramid test configuration which allows the experimental investigation of combined contact configurations consisting of more than one material pairing. In the proposed test setup, a ball is loaded against four flat test samples positioned in a pyramid-like sample holder with an opening angle of 90°. The test samples are arranged on the circumference in a regular manner. The ball rotates uniformly around its vertical axis and slides over the samples. The coefficient of friction, the running-in wear, the linear wear rate, the wear scar diameter, and the weight loss of four different sample materials and their combinations are compared and discussed. The obtained results for combined contact configurations with more than one material pairing show that a manipulation of the friction and wear characteristics is possible.

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The influence of roughness orientation on the friction-speed characteristics of adsorbed films was investigated by changing the angle of the anisotropically striated roughness to the sliding direction of the counter surface. As the angle of the striated roughness becomes perpendicular to the sliding direction, the friction coefficient is decreased. The friction-speed characteristics of the adsorbed film in the transverse direction of the striated roughness having different height or shape of asperities were also investigated. The friction coefficient obtained in the low speed region decreased with increasing arithmetic mean peak curvature (S_pc) when stearic acid-formulated oil was used as a lubricant oil. The results demonstrated that the roughness orientation and the asperity shape of the transverse roughness were key factors for generating additional load-bearing pressure so that the adsorbed films could exert their intrinsic friction-reducing effects.

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Different concentrations of graphite powder (particle size < 20 μm) were added for the enhancement in mechanical and tribological properties of SU-8 polymer. The materials were studied as thick coatings with thickness in the range of ~ 35-40 μm on glass substrate. SU-8 and SU-8/graphite composite properties were examined using atomic force microscope (for nanoindentation test) and pin-on-disc tribometer (for friction and wear). The surface characteristics were studied using Optical Microscope, Goniometer and 3D Profilometer. At 10 and 20 wt% graphite concentration, it was found that several properties were enhanced such as elastic modulus ~ 2.2 times, marginal increment in hardness and approximately same water contact angle and same surface free energy (SFE) as compared with those of pure SU-8. More importantly, 10 wt% graphite concentration has given two-times lower steady-state coefficient of friction and ~ 10 times more wear life compared to those of pure SU-8. The 20 wt% composite gave higher coefficient of friction but lower wear rate than the 10 wt% composite.

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Long Fiber Thermoplastic-Direct (LFT-D), a new stamping method, has been developed to achieve the short cycle-time manufacturing of Carbon-Fiber-Reinforced thermoplastics (thermoplastic CFRP). In LFT-D method, to shorten cooling time is one of the effective way for achieving more productive manufacturing. During ejection sequence in the LFT-D, thermoplastic resin parts adhere stronger than previous method. High adhesion affects smoothness of the component surface, cycle-time and requested specifications of equipment. So, it is required to reduce the adhesion between the metal mold and thermoplastic resin. This paper reports the effect of nano-texturing on adhesion of thermoplastic resin against the textured steel around glassy-transition temperature. Experimental results show that the adhesion strength increases proportionally with the ratio of contact area. The lowest adhesion strength on nano-textured surface is 0.18 MPa, which is 66% lower than that of flat surface. It is clarified that surface adhesion energy increases with the ratio of contact area A_contact to circumference of contact area L_{circumference}.

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This paper describes a study on the effect of base oil type and thickener type on bearing torques with Li type greases. Using a ball bearing tribometer equipped with a torque sensor, friction torque behaviors were determined for model Li greases with different hydrocarbon type base oils. The results showed that the bearing torque changed with the formulation of the greases. For the clarification of the mechanisms, rheological parameter and film thickness of greases were evaluated, and the fluorescence technique was introduced to observe flow behaviors of the greases in and around EHL contact area by using a ball-on-disk tribometer, furthermore, thickener fiber structure was observed by an electron microscope. The abundance of grease around EHL contact area was different depending on thickener types. It is suggested that film thickness, flow behaviors, and thickener structure influence simultaneously on the bearing torque behaviors.

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Cast iron, aluminum metal and sintered aluminum-silicon (Al-Si) were slid against SUS440C stainless steel in n-hexane and in ethanol. In the case of the cast iron/stainless steel and aluminum metal/stainless steel pairs, friction and wear were lower in n-hexane than in ethanol. In contrast, the friction and wear for the sintered Al-Si/stainless steel pair were much higher in n-hexane than in ethanol. The results from morphological observations and chemical analyses of the worn surfaces suggested that the wear mechanism was influenced by the physical properties of the sliding materials and the tribochemical reactions between the sliding materials and lubricants.

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In order to clarify the plastic deformation mechanism of silicon carbide in cubic phase (3C-SiC), molecular dynamics (MD) simulations are performed on the nanoindentation using a spherical indenter. Transition from elastic deformation to plastic deformation has been confirmed by the phenomenon called pop-in in the load-displacement curves during nanoindentation. Dislocations on {1 1 1} slip planes are found during indentation. In order to analyze internal defects, common neighbor analysis (CNA) is slightly modified so that it is suitable for the analysis of slips of zinc-blend structure. In our method, the CNA is applied separately to sub-lattice of Si or C in the same SiC. By this method, structural changes are confirmed in a region with the shape of square pyramid when the pop-in behavior occurs. By measuring the atomic distances along the region of misalignment, it was confirmed that there is certainly atomic sliding by crystalline slip. Furthermore, it is found that, with increase of loading, dislocation loops spread along {1 1 1} slip planes.

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This paper presents a new method to evaluate the oil-film thickness in gears. By using high speed capacitance measurements in combination with an exact timing it is possible to detect the changes in capacitance along the path of contact. With knowledge of the contact area, the oil-film thickness can be calculated from the capacitance measurements. The design of the test rig relies on a good electrical insulation and accurate speed measurements. First test results show that the changing conditions in the contact can be detected by the measurement of the capacitance.

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The present study describes the tribological properties of a bronze alloy used as a sliding interface for operation under dry conditions. Many industrial slide bearings are made from bronze alloy. To achieve easy running-in and enhanced anti-seizure, solid lubricants are added to the alloys. However, it is difficult to simultaneously achieve easy running-in and anti-seizure with one solid lubricant. Thus, the combined effects of a solid lubricant and a dispersed sulfide layer on reducing and stabilizing the friction coefficient were also examined. The tribological properties of the resulting alloy were evaluated with a 3-ball on disc type testing apparatus under dry conditions and in air atmosphere. The surface state of the solid lubricant and dispersed sulfide layer was evaluated by observation of the morphology and phase states via atomic force microscopy. Distinctive features of a phase comprising mixed stiff and soft regular regions on the micrometer scale were observed. These two regular regions are considered to be formed from graphite and sulfide by burnishing. The effect of the dual phase state leads to achievement of both easy running-in and anti-seizure.

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Hydrogenated amorphous carbon (a-C:H) films exhibit excellent friction properties such as high mechanical hardness, high wear resistance, and low friction. a-C:H films have amorphous structures generally composed of sp²- and sp³-hybridized carbon, which bring about extraordinary friction properties. Some reports have focused on the low-friction mechanism of a-C:H films, concluding that it is induced by the existence of graphitized wear particles at the sliding interface. It is possible to consider the existence of graphitized wear particles as the most important factor in achieving the lower friction of a-C:H films. We focus on the effects of surface texture in trapping the graphitized particles at the sliding interface and discuss the role of surface texture with regard to the friction properties of a-C:H films. Micro slurry-jet erosion (MSE) surface machining was employed to manipulate the surface texture on a high-carbon chromium-bearing steel substrate, upon which a-C:H films were deposited. The friction properties of a-C:H films deposited on a mirror-like polished substrate (a-C:H/mirror-like) and on an MSE-produced substrate (a-C:H/MSE-produced) were compared using a reciprocating-type ball-on-disk sliding tester. From the results of friction testing, it is confirmed that a-C:H/MSE-produced films indicated lower friction coefficients compared with the a-C:H/mirror-like case. Scanning electron microscopy (SEM) and Raman spectroscopy were performed to study the friction improvement mechanism of the a-C:H/MSE-produced films. SEM revealed the existence of wear particles in the wear track of a-C:H/MSE-produced films. It is confirmed by Raman spectroscopic analysis that these wear particles’ structure was changed, adopting a graphite-like structure. From these results, it is possible to consider that the existence of graphitized wear particles induced lower shearing resistance at the sliding interface, enabling friction improvement.

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The planetary roller screw mechanism is used in the aeronautics industry for electro-mechanical actuators application. It transforms a rotational movement into a translation movement, and it is designed for heavy loads. The main components are made of martensitic stainless steel, and lubricated with grease. Like most usual rolling mechanisms, smearing and jamming can occur before the theoretical fatigue lifetime, especially in poor lubrication conditions. The actuated load is carried by small contacts between the threads of the screw, the rollers and the nut. The static single contact can be described as an ellipsoid on flat contact with high contact pressure (3-4 GPa). The motion consists of rolling with spin associated with side slip up to 10%. The aim of our study is to investigate the wear behavior of the WC/C coated contact for different operating and design parameters such as load, speed and slip ratio. The contact is simulated by a free rolling roller loaded on a rotating disk. A specific apparatus is used to create a contact with a side slip component, i.e. perpendicular to the rolling direction. The wheel rolling speed and the tangential force generated by the slip ratio are measured. The wear behavior of a WC/C carbon-based composite coating is investigated. It reveals progressive wear and cracking in the rolling direction, i. e. perpendicular to the sliding direction. A wear map has been drawn to establish the damage mode depending on the contact conditions.

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In Inertia Drive Motors, generated motion is based on stick-slip principle. Current analytical models are predictive enough to calculate qualitatively their optimal performances, such as maximal step size and speed, with relatively few input parameters. But, they do not take into account the contact life and temporal evolution of parameters as friction factor all along lifetime of IDM. So, analytical models reach their limits when precise predictions are necessary. This investigation aims at understanding wear mechanisms to model temporal evolution of friction. Such an understanding requires the reconstitution of the contact life through the evaluation of 1^st and 3^rd body flows. To do so, a new IDM-representative tribometer is designed. 1^st bodies - coated TA6V and polymer - are not see-through. They are replaced alternatively by an intermediate transparent 1^st body to observe the contact dynamically and in-situ. Friction factor, step size and mean speed are also measured. Preliminary results shows that wear profiles from real IDM and tribometer are similar. Direct observations bring out particles of TA6V coating are firstly snatched, then moves in contact and finally trigs others particle detachments.

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A premium threaded connection for oil country tubular goods (OCTG) has a metal-to-metal seal portion to ensure its gas tightness under high pressure and high temperature. The metal surfaces are coated with grease and then integrated by rotary sliding. Despite several studies, the gas tightness mechanism of the metal-to-metal seal has not yet been clarified. In this study, as the first step to clarify the mechanism, we evaluated how grease affects the gas tightness by fundamental tests considering rotary sliding and grease degradation due to high temperature. Our results showed that grease is mandatory for the gas tightness of the metal-to-metal seals and the gas tightness is affected by both types of base oils and solid additives of grease. The seals coated with the grease composed of mineral oil had better gas tightness than those coated with that composed of plant oil, even after degradation due to high temperature. In addition, grease containing soft metal particles had better gas tightness than grease containing hard inorganic particles because the extended soft metal particles adhering to the rough parts of the seal surfaces reduced the size of the leakage paths.

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Graphene oxide (GO) contains graphitic single-layered sheets, while oxidized wood-based nanocarbon (oWNC) is composed of graphitic nanoshells. Since both materials are thoroughly oxidized with many oxygen-containing functional groups, both GOs and oWNCs form stable dispersions in water. We found that lubrication by dispersions of GOs and oWNCs in water resulted in very low friction coefficients below 0.04 with only slight plate surface wear using a steel plate and tungsten carbide ball. After lubrication, thick tribofilms are formed on the sliding surfaces, which would decrease the friction. In this study, in order to clarify the tribological mechanisms that result in such low friction coefficients, we have performed tribological tests and analyzed the tribofilms formed by dispersions of GO in water using confocal laser scanning microscopy (CLSM) with an optical data storage system, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The GO tribofilm is composed of many carbons and some oxygen, derived from the disordered and aggregated GO sheets. The GO tribofilm have higher hardness and hydrophilic properties than the steel plates, which would provide low friction and wear. It is found that both the GO tribofilm and GOs are necessary to achieve the lowest possible coefficient of friction. It is likely that the oWNC tribofilm has similar friction and wear decreasing effects to that of the GO dispersion.

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Transporting heavy objects has a long history from the early civilizations thousands of years ago. However, it is difficult to investigate the ancient cases due to limited information. The experiments with reproductions of ancient sledges performed by Japanese tribologists provide quantitative information for the investigations of the ancient cases of heavy-load trasportation. The experimental results are used to verify two estimate methods with average pulling force and with average power. Then, two typical Chinese cases in the 15^th-16^th century of lubricated wood-on-ice sliding and wheeled vehicles were investigated with the two estimate methods, and compared with some typical modern cases of heavy-load transportation. The feasibility of the 16-wheel vehicle and the sledge sliding on lubricated ice in ancient China is verified.

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Interaction between thickener such as polyethylene glycol (PEG) and glycol antifreeze such as ethylene glycol (EG) was investigated by measuring the friction coefficient and the contact angle of the mixed solution of polyethylene glycol and ethylene glycol. The friction coefficient of undiluted EG was larger than that of undiluted polyethylene glycol with molecular weight 400 (PEG400), and the friction coefficient in the mixed solution of PEG400 and EG was expected to increase with an increase in the content of EG added to the mixed solution. However, the friction coefficient of the mixed solution at a sliding speed of 0.05 m/s showed the maximum value at EG content 70 mass%. The contact angle of the mixed solution of PEG400 and EG at 40°C increased with an increase in the EG content in the lubricant on the unheat-treated metal plate, but showed the maximum value at EG content 60 mass% on the metal plate which was heat-treated for 10 min at 190°C and then was cooled at room temperature. The agreement of both the EG content dependences of the friction coefficient and the contact angle might be explained by the intermolecular interaction between PEG400 and EG molecules which was induced on the metal oxide surface.

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An experimental approach was adopted to understand service damages such as crack initiation and surface degradation of bolted joints used in the junction technology of aircraft air bleed valve systems. A vibratory testing stage combined to Digital Image Correlation (DIC) focus on an experimental model device of valve body/actuator body junction. DIC coupled with in situ tribological observations and a Finite Element Model (FEM) has been used to identify more clearly local contact conditions. This approach has enlightened peeling-off and micro-slips instabilities under vibratory loadings, leading to third body formation in bolted joints contacts. The morphology of such third body is placed at the focus of damages observed in involved air bleed valve systems.

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Banana leaves are traditionally used to clean iron plates. This study evaluated the cleansing action of banana leaves by subjecting iron surfaces to a sliding regime on prepared leaves. Iron plates weighing 9, 16, and 23 N were soiled with polyethylene film at 135°C, 183°C, and 230°C. The surface morphology of banana leaves consisted of parallel ridges running along the leaf axis. Stomata and air pockets, the main biomimetic microstructures of banana leaves, were located between the spaces of the ridges. The cleansing action of the banana leaf surfaces (defined as dislodging of soil from the iron plate) was explained in terms of adhesive forces among the iron plate, polyethylene soil, and surface microstructure. The adhesion forces between the polyethylene soil and microstructures were the most significant. The operating forces were bio-adhesive in nature. The cleansing action increased as the loads and temperatures increased; the greatest cleansing action was obtained at 230°C (16 N). The morphological changes in the stomata and the air pockets during cleansing at different temperatures were tabulated.

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This paper discusses the stability of herringbone-grooved aerodynamic journal bearings supported by rubber O-rings. In this research, the dynamic stiffness and the damping properties of the O-rings are identified in a variety of operating and design conditions. The identification results are fitted with the empirical formulae previously proposed, and the distributions of the dynamic properties in a variety of support conditions are obtained. It is theoretically investigated how the stability of the bearing system changes with the dynamic properties. The bearing tests are also performed to verify the analytical results with the empirical formulae. The empirical formulae used here was reasonable for simulating the dynamic properties of the O-rings in a variety of bearing support conditions. The comparisons between the analysis and experiments verified the effectively of analyzing the stability of the bearing system by using the empirical formulae.

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A microfluidic device was fabricated to study the collision of bubbles with controlled approaching speed of 0.01-10 mm/s. The collision of bubbles resulted in instant coalescence, delayed coalescence and elastic collision as their approaching speeds gradually decreased. This velocity-dependent coalescence is attributed to the variation of the rotation speed of the bubbles under different approaching speeds, which is observed by adhering fluorescent particles on the bubble surfaces. The theoretical predictions illustrate that the rotation of the bubble helps to reduce the hydrodynamic pressures between two approaching bubbles, which accelerates the drainage of the thin liquid film in between and the coalescence.

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We have studied the effects of a textured structures on soft-materials, mainly to reduce friction between body tissues and plastic components used in medical devices. For this purpose, we investigated changes in friction between the structure of grooves on plastic and rubber, which was considered as body tissue. Silicone oil was used for the purpose of lubrication, and the changes in friction were observed from the point of view of aspect ratio. The results indicated that a grooved surface exhibited lower friction compared to a non-textured surface. Furthermore, directional sliding perpendicular to the grooves was more effective in reducing friction than that parallel to the grooves. In addition, the results indicated that the kinetic friction was changed by pitch and depth even if the aspect ratio was similar in a direction perpendicular to the straight grooves. Furthermore, the results indicated that a higher aspect ratio resulted in a lower kinetic friction when the pitch was fixed and the depth was changed in a direction perpendicular to the straight grooves. In this study, the frictional materials used were soft-materials that simulated the softness of the human body and medical devices. Thus, from the results mentioned above, we can conclude that textured structures can reduce friction for soft materials.

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The effect of different chemistries on cleaning of hard surfaces has widely been investigated. However, a fundamental understanding of the physical aspects involved in grime removal is yet to be established. A series of surface experiments are presented in this paper to determine the mechanical properties of cleaning processes. Experimental results showed that tribology is a powerful tool to investing the mechanical forces associated with cleaning of soiled surfaces. Analysis of protein-based soil removal from stainless steel suggests that friction coefficient decreases by nearly 28 and 75% for dry and wet samples, respectively, with an increase of 8 times in the normal load. The study also indicates a reduction of the coefficient of friction with an increase in frequency due to the reduced adhesion of counterface pin material on the sample and the possible localized softening of the soil surface. Tests performed using wet samples revealed that friction coefficient significantly decreases as a consequence of the water content. The effect of load on wet samples was mainly observed for short soaking periods (5 minutes). Specimens soaked for 45 minutes showed about 84% lower friction coefficient compared to dry samples but no significant dependency on the applied load. The volume of soil removed was higher for experiments presenting lower friction coefficients. Finally, wet cleaning promoted a higher cleaning efficiency requiring less energy to remove more soil. Overall it was established that purely mechanical forces are not sufficient to complete the cleaning process. Future work investigating the relationship of these tribological aspects with the chemistry of cleaning is to be expected.

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In this study, an experimental apparatus was developed where the relative humidity (RH) and oxygen concentration could be controlled between 0.01-10%RH and 1-100000 ppm, respectively. This apparatus was used to study the roles of water and oxygen molecules on the friction of carbon nitride (CN_X) coatings. The friction properties of Si₃N₄ balls sliding against CN_X (Si₃N₄/CN_X) under a nitrogen atmosphere were classified into the following three modes according to their average friction coefficients (μ_a) and the stability of the friction: Mode I, μ_a < 0.05 (stable); Mode II, 0.05 < μ_a < 0.3 (stable); and Mode III, 0.3 < μ_a (unstable). To achieve a low friction coefficient (e. g. < 0.05), the optimum RH and oxygen concentration were 0.1-1.0%RH and 100-10000 ppm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) showed that hydrogen and hydroxyl radicals derived from water molecules chemisorbed onto the worn surfaces of the CN_X coatings. Raman spectroscopy suggested that the structure of carbon on the worn surfaces was different from that of the deposited CN_X coating. We concluded that low friction (Mode I) arose from the water and oxygen molecules terminating carbon dangling bonds and structural changes in CN_X.

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Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight-bearing surfaces in total joint arthroplasty. However, the wear phenomenon of UHMWPE in knee and hip prostheses after total joint arthroplasty is one of the major restriction factors on the longevity of these implants. The authors focused on the microscopic machining surface asperities of the UHMWPE tibial insert of the virgin knee prosthesis as a factor influencing the wear mechanism of the UHMWPE tibial insert. In a previous study performed by the authors, on static contact analysis between the microscopic machining surfaces and the metallic femoral component, the influence of the microscopic machining surface asperities was noted on the stress and plastic strain of the UHMWPE tibial insert. In this study, repeated elasto-plastic contact analyses using the finite element method (FEM) between the machining surface asperities and the femoral component were performed in order to investigate the influence of elastic recovery of the machining surface asperities on the contact stress and plastic strain of the UHMWPE tibial insert. The analytical findings of this study suggest that the elastic recovery of the machining surface asperities of the UHMWPE tibial insert of the virgin knee prosthesis is another possible factor influencing the microscopic failure and/or microscopic wear behavior of the UHMWPE tibial insert in the knee prosthesis after total knee replacement.

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Tapered rolling bearings are applied to support differential gears in vehicles. Though it is categorized as radial bearing, axial load is required as preload to support the radial load while preventing the inner ring and the outer ring from separating. However, when axial load is applied to the tapered rolling bearing, starting friction becomes larger. This is due to the increase in sliding friction between roller end face and large flange face of the inner ring. In case of position-preload, the bearing width reduces due to the wear and the amount of preload dwindles. This phenomenon is called ‘preload loss’ but nowadays there is still no report regarding this phenomenon. This study explained the relationship among taper, bearing width and preload loss by observing the change of preload while rotating the tapered rolling bearing. The taper rolling bearing was installed using position-preload. Furthermore, we also propose a bearing mounting method which can reduce the preliminary preload loss based on the previous researches and viewpoints.

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In the present work an artificial neural network (ANN) model was developed to predict the wear rate and coefficient of friction of WC-12Co nanocomposite microwave clads. Various combinations of the transfer function and number of neurons in the hidden layer was used to optimise the neural network. The influence of nature of reinforcement, normal load and sliding distance on the wear rate of the conventional and nanostructured microwave clads was evaluated using the ANN model. The mean square error of 500 epochs was considered to evaluate the performance of the ANN model. The wear rate and coefficient of friction predicted through the ANN model was then compared with experimental results. The predictions of the ANN model were consistent with experimental results. It can be therefore concluded that ANN is an effective modeling technique to predict the wear rates of the WC-12Co microwave clads.

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Hydrogenated amorphous carbon (a-C:H) coating which is one of carbonaceous coatings shows ultra low friction coefficient (μ < 0.01) in dry Nitrogen gas. At present, it is expected to clarify low friction mechanism of a-C:H coating to apply a-C:H coating to industrial components. It was reported that hydrogen concentration, density, thickness and volume fraction of chemical bonds of surface layer (transformed layer) on bulk a-C:H coating during friction test were important to clarify low friction mechanism. But it is difficult to measure hydrogen concentration and density of transformed layer during friction test. In this study, we estimate the thickness and volume fractions of C-Csp², C-Csp³ and C-H bond of transformed layer during friction test by reflectance spectroscopy. In addition we calculate hydrogen concentration from volume fraction of C-H bond and filling factor related to density of transformed layer during friction test. After friction test, surface energy of transformed layer is measured by sessile drop method. As a result, it is suggested that a-C:H coating shows low friction coefficient by reducing real contact area due to densification of transformed layer and repulsion force in hydrogen-terminated surface.

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The authors tested the effects on the tribological characteristics of refrigeration oils under refrigerant atmospheres using a block-on-ring tribometer. Based on the results of a sliding test using the tribometer, it was found that tricresyl phosphate (TCP) in both a polyol ester (POE) and alkylbenzene (AB) oil formed a lubricating film on the sliding surface under R410A atmosphere. Surface analyses revealed that a phosphate film had been formed on the sliding surface. With the POE oil in particular, the anti-wear effect of TCP greatly reduced the width of the wear track. It was also found that R32 interferes with the formation of phosphate films to a greater extent than does R410A. It is thought that the high polarity and the high reactivity of R32 with nascent metal surfaces prevent TCP from adsorbing to the Fe of the sliding surface. These results show that not only refrigeration oils but also refrigerants can have a major effect on the formation of the phosphate film in refrigeration systems. This knowledge could be helpful when designing new refrigeration systems.

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Surfaces with different topographies are formed by the various types of manufacturing processes. To express the characteristics of these surfaces, the International Organization for Standardization (ISO) defines two-dimensional (2-D) surface roughness parameters, which are used for geometrical product specification (GPS). 2-D surface roughness parameters are widely used in engineering fields. In contrast, 3-D surface roughness parameters have been standardized with the development of measurement technologies. In the field of tribology, the relationship between the surface roughness parameters and tribological characteristics has been extensively studied. However, surface roughness parameters for the expression of tribological characteristics have not yet been reported. In this research, the fast Fourier transform (FFT) method was applied to evaluate the periodicity of sliding surfaces. Using the 2-D FFT method, the 2-D power spectral density (PSD) was obtained for actual surfaces. The relationships between the 2-D PSDs and the friction characteristics of anisotropic surfaces were examined. From these results, it was determined that all the data from the 2-D PSD are required to determine the optimal direction of sliding. A new 3-D surface roughness parameter to express these relationships was proposed. It was shown that this parameter is able to determine the sliding angle where the lowest or highest friction coefficients are attained for common industrial surfaces. This parameter could be used in the design of sliding surfaces as a GPS parameter to express friction characteristics.

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Poly (vinyl alcohol) (PVA) hydrogel is the candidate material for artificial cartilage. PVA hydrogels prepared by hybrid method of freeze-thawing (FT) and cast-drying (CD) method (PVA-hybrid gel) were developed, and lubrication property of PVA-hybrid gel was evaluated. Sliding pairs of an ellipsoidal PVA hydrogel and a flat glass plate were tested in reciprocating friction test. PVA-hybrid gels were prepared by temperature (T) and humidity (H) controlled drying process. Friction of PVA-hybrid gel prepared by drying process at T of 8°C and H of 80%RH gradually increased. PVA-hybrid gel prepared by drying process at T of 8°C and H of 50%RH showed low friction coefficient of 0.013 but it gradually increased with increase of sliding distance and surface damage was progressed. When the additional drying process at T of 20°C and H of 40%RH after the drying process at T of 8°C and H of 50%RH was provided, PVA-hybrid gel showed extremely low friction coefficient such as 0.006 and wear of this hybrid gel was slight. The additional drying process contributes to obtaining the surface layer of PVA-hybrid gel with low friction and low wear.

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Low, stable friction, a higher load-carrying capacity, and lower leakage of sealed blood are required for mechanical seals in implantable ventricular assist devices. One solution is to apply surface texture to the mechanical seal surface, consisting of self-mated silicon carbide. However, the effect of surface texture on the frictional properties of mechanical seals under blood sealing conditions has not yet been studied. Therefore, this study aimed to clarify the effect of surface texture on fundamental frictional properties of mechanical seals and to propose design concepts for mechanical seals in ventricular assist devices. The results show that surface texture increases the critical load of mechanical seals, although it also causes periodic peaks in friction. Further, it was found that surface texture induces the formation of denatured protein aggregates on the sealing surface, also inducing periodic surface friction peaks. The frictional properties of the mechanical seals were stabilized by creating small, dispersed concave features with wet blast fabrication, followed by coating with diamond-like carbon. Lower and more stable frictional properties were thus achieved, while simultaneously ensuring a higher critical load.

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Semi-solid lubricants, such as grease and gels, are mixtures of oil, thickeners, and gel agents. In this study, the concentration of the urea thickener and gel agent at the point of elastohydrodynamic lubrication (EHL) contact was investigated by in situ observation with a micro-Fourier transform infrared spectrometer (micro-FTIR). Micro-FTIR was used in conjunction with a ball-on-disk lubrication tester. The lubricant film was analyzed by micro-FTIR under dynamic conditions. Characteristic peaks correlating to the C-H stretching mode at 2850-3000 cm^-1 and the N-H stretching mode at 3000-3500 cm^-1 were found to originate from the oil and the urea thickener and gel agent, respectively. The concentration of the thickener and gel agent were estimated from these characteristic IR peaks. The results showed that the concentration of urea thickener and gel agent was increased drastically at the Hertzian contact region. Meanwhile, infrared spectra under hydrostatic pressure were obtained using a diamond-anvil cell. It was confirmed that the hydrogen bonding of the urea thickener and gel agent was stabilized under high-pressure conditions.

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We developed in situ observation and Raman analysis techniques for frictional interfaces and investigated the formation and removal behavior of the tribofilms that formed from a sulfur-containing extreme pressure (EP) additive on carbon steel. We detected an iron disulfide (FeS₂) tribofilm peak during the friction test and simultaneously detected D and G bands attributed to graphite-like products. Once scuffing occurred, both the FeS₂ peak and D and G bands disappeared. Therefore, the formation and removal of the tribofilm and carbonization of the additive were detected during the process that led to scuffing. We concluded that in situ Raman tribometry is an effective technique for investigating the formation process of tribofilms and the scuffing mechanism.

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This work studies the ultrafine subsurface layer produced in Al-Sn and Cu alloys in contact with AISI9840 steel. The tribological tests were conducted at room temperature without lubrication in a coaxial tribometer. A first microstructural characterization was performed using conventional scanning electron microscopy (SEM) and atomic force microscopy (AFM). Grain refinement mapping was performed by means of Electron Backscattering Diffraction (EBSD). The wear process, during which tribolayers are formed, leads to significant differences between the structure of the sub-micrometrical layer below the tribolayer and that of the bulk. SEM images show severe mechanical mixing in the surface of the worn zones, whereas EBSD observations allow identifying the mechanically homogenized layers as a sub-micro-crystalline material with a cellular structure aligned along the sliding direction.

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Friction characteristics of a finger pad sliding on a Si surface coated with organic molecular films, such as octadecyltrichrolosilane (OTS)-SAM and perfluoropolyethers (PFPE), having different film thicknesses were investigated to reveal the effect of film formation state on finger friction. The contact area of the finger pad was also measured using a black ink stamp to elucidate the interfacial phenomena between the finger pad and the surface. The results show that the friction coefficient of the finger decreased with increasing film thickness. This suggests that the friction coefficient was probably affected by surface conditions such as film formation state, even though the difference in film thickness was only a few nanometers. Assuming that the contact area remained unchanged while the finger slid across the surface, a linear relationship between the friction force and the contact area of the finger was observed. The slope of this approximately linear relationship was taken as the shear strength of the interface. The results demonstrated that the shear strength decreased as the film thickness of the coating on the Si surface increased; thus, the interfacial state of the solid was the dominant factor in determining the friction characteristics of the finger pad.

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A shaft with a low oil-wettable surface was found to show lower friction than one with a highly oil-wettable surface when used in oil-impregnated sintered bearings. Both the highly wettable shaft and the low wettable shaft were made of hardened stainless steel, with the latter also being coated with PTFE transfer film. A projector was used to observe bearing clearances where a large amount of oil droplets was observed on the low wettable shaft as compared to the highly wettable shaft. This suggested an oil-rich environment in the sliding area, resulting in low friction of the low wettable shaft. The large stability of droplets on the low wettable surface—stemming from a large contact-angle hysteresis between the oil and the low wettable surface—is probably responsible for the abundance of oil droplets in the bearing clearance for the low wettable shaft.

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The role of edge wear was studied in impact-abrasion testing conditions with an impeller-tumbler type test device. Three steels with different mechanical properties were tested at 30° and 90° sample angles using natural granite stone as abrasive particles. The edge and planar (inner) areas were carefully exposed to the same conditions by using tightly fitted two-part samples to obtain relevant information about the differences in their wear behavior. The role of edge-concentrated wear was dominant in all materials, and the edge wear rate was several times higher than the wear rate of the inner parts of the specimens. The difference in wear rate was particularly large in short tests and with the 90° sample angle. However, the dominance of the edge-concentrated wear decreased as the test duration became longer. The wear mechanism was largely microfatigue in all materials, but the samples tested at the 30° sample angle showed more microcutting in comparison to the 90° samples. Moreover, the role of microcutting was higher in the wear of the edge parts than in the inner parts.

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The final target of this study is to discover the concert effect between ZnDTP and ashless FMs (polar compounds) that improve friction reducing performance together with keeping the good antiwear performance of ZnDTP. Evaluation of tribological performance and ZnDTP tribofilm formation were performed with a ball-on-disk type rolling-sliding tribometer. The tribofilm generated on the disk was analyzed by FT-IR analysis, Auger Electron Spectroscopy (AES) and Electron Probe Micro Analysis (EPMA). It was found that amino compounds showed a blocking effect, in which the ZnDTP tribofilm formation was prevented and the friction coefficient increased, whereas carboxyl compounds showed a concert effect, in which the friction coefficient was reduced with maintaining ZnDTP tribofilm formation.

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Carbon-carbon (C/C) composites are widely used as sliding materials owing to their excellent mechanical properties. However, it is well known that the C/C composites exhibit an unstable frictional behavior called the friction transition, the main mechanism of which remains unclear. In this study we focused on the mechanism of the friction transition by examining the morphology of wear debris obtained using a thrust-cylinder-type tribotester. Moreover, to improve the tribological properties of C/C composites, we discussed the effect of surface texturing. The sliding results suggested that agglomeration of wear debris caused the friction transition. Furthermore, the friction transition was prevented by the effects of surface texturing.

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Tools used in hot forging operations are subjected to high wear during processing due to complex load conditions with high temperatures and mechanical loads. The resulting abridged service lifetime of the dies highly restricts the cost-effectiveness of the production process. Several investigations indicate that the condition of the tool topography during several forging cycles affects the formation of wear distinctly. Thus, an enhancement of the topography in order to reduce tribologically caused wear is expected to increase wear resistance. Within this investigation an appropriate surface structure of forging dies is to be defined. For this purpose, efficient production methods like turning, milling and blasting are applied to generate distinguished die surfaces concerning topographical structures. By investigating the worn-out surfaces, the optimal surface structure can be determined, taking into account topography parameters deduced from the Abbott-Firestone graph. A mechanical stabilization of the created topography is necessary due to the high mechanical and thermal loads during hot forging leading to distinctive topography alterations. Thus, plasma duplex treatments were carried out, consisting of plasma nitriding and hard coatings produced by PECVD coating technique. It can be shown, that the created topography of the forging dies is sustainable over high amounts of forging cycles and thus leads to a reduction of wear.

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In order to provide detail data on polyalphaorefin(PAO)’s viscosity for the tribological analysis under EHL lubrication conditions, viscosity-pressure-temperature correlation relation of wide range viscosity grade PAOs (from PAO2 to PAO100) was investigated employing a special compact pressure-generating apparatus called diamond-anvil cell. Pressure-viscosity coefficients were resulted to gradually decrease with increasing pressure and temperature up to 1 GPa at up to 150°C, and the values at 150°C were about a half of those at 40°C for all PAOs. The two representative temperature-pressure-viscosity correlation formulations were well regressed with the high-pressure viscosity data. By substituting the regression formulations for Blok’s pressure-viscosity coefficient, gradually increasing coefficient with viscosity grade were found out, and that of high viscosity PAO100 was about 1.5 times larger than that of low viscosity PAO2.

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