The 5^th World Tribology Congress, WTC2013, was held at Torino, Italy, on September 8-13, 2013. The papers presented at the WTC2013, and submitted to the Tribology Online have subsequently undergone the normal peer-review process by multiple reviewers and finally 17 papers were accepted and published in this Special issue.



On behalf of the Editorial Committee, I, the Editor-in-chief of Tribology Online, acknowledge the authors’ efforts to submit their papers to this Special Issue. I also express our sincere thanks to the Editors, the Reviewers, and the Publication Coordinators for their effort to complete the peer-review processes and the publication work.

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In the present study, two types of nine-recess hybrid journal bearings (HJBs) with different recess geometries, i.e., the Young Leaf Mark and the square, were operated at a rotational speed of up to 67,000 rpm using the cryogenic journal bearing test apparatus at JAXA to clarify influences of the journal rotating direction on the cryogenic HJB. Liquid nitrogen was supplied to the test bearings with a pressure of up to 4 MPa. The test bearings were 60 mm in diameter and 25 mm in width, with a 0.055 mm clearance and a recess area ratio of 0.186. Based on the experimental results the dependence of the rotor vibration on the rotational speed was confirmed to be affected by the journal rotating direction. The test HJBs with the positive sweepback angle of the recess leading edge showed the smallest rotor vibration in the high-rotational-speed operation where the hydrodynamic effect was predominant.

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Gecko has remarkable ability to control its adhesion and friction during running to realize a swift climbing or running on vertical walls or upside down ceilings. This ability has received considerable interest of researchers. During the recent decade, significant progress has been made in understanding the mechanism and the biomimetic fabrication of gecko-inspired dry adhesives. While people generally desire a strong and reversible adhesion property, this article described the progress in the investigation of the properties of gecko hierarchical structures, and the corresponding theoretical understandings, and then presented our recent progresses on the flexible controlled adhesion and friction of gecko surface and biomimetic surfaces and a demonstration of a hybrid clamp to ascertain the mechanical property of control principles is also involved. Gecko-inspired adhesives with flexibly controlled adhesion and friction performances have been developed. Macroscopic sliding of those adhesive surfaces with anisotropic hierarchical microscopic structures in a desired direction could be carried out and engage or disengage a flexible control of strong adhesion and friction, and acquire an “intelligent” adhesion.

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This paper describes experimental and theoretical verification in impact response of optimized grooved bearing designed by topological optimization. Firstly, in order to measure the impact response characteristics accurately temperature compensation method for air film thickness of bearing is proposed and confirmed by experiment of bearing reaction force with air film thickness. Then, in this study, impact experiments were conducted with the optimized bearing and spiral grooved bearing and compared with theories of linear and non-linear analyses. As a result, the optimized bearing showed the high impact resistance compared with spiral grooved bearing. In addition, it was confirmed that the results of non-linear analyses are good agreement with those of experiment. Finally, based on above results, the impact response calculations of both bearings by non-liner theory under more high impact force condition were conducted. As a result, it is found that the response of spiral grooved bearing generates the self excited vibration. On the other hand, the response of optimized bearing showed damping waveform with time even under the high impact force conditions.

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A new type high temperature self-lubrication TiC/FeCrWMoV metal ceramic was fabricated successfully by applying an innovating technology which molten solid lubricant (60Pb40Sn-15Ag-0.5RE) was infiltrated into metal ceramic preforms with an interpenetrating network using a vacuum high pressure infiltration furnace. The friction and wear behaviors of the composites were investigated using a pin-on-disk high temperature wear testing machine at different temperature (up to 800°C). The compositions, images and structures of worn surfaces were analyzed by means of scanning electron microscope (SEM), energy dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD). The self-lubrication mechanisms of the composites were discussed. The experimental results indicated that during elevated temperature sliding, the solid lubricants were squeezed out of the micropores to the frictional surfaces to form PbMoO₄, PbO, SnWO₄, Ag₂WO₄ and Ag₃Sn. The formation of lubrication film containing of these oxides and of intermetallic compounds was the main reason that the composites had good self-lubrication properties at high temperature. It was considered that the micro-pores on friction surface would be the crucial factors determining the self-lubricating properties of the self-lubrication composites.

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In our previous research, it was suggested that a sort of copper molybdate Cu₃Mo₂O₉ generated on the friction track of aluminum bronze coated with MoO₃ powder reduced friction under high temperature conditions. In order to study the lubricity of copper molybdate as high temperature lubricant, the lubrication properties of two kinds of copper molybdate powders, CuMoO₄ and Cu₃Mo₂O₉, were studied by comparing with the lubricity of CuO and MoO₃ powders. These powders were supplied to the sliding surface of the stainless steel specimens as solid lubricant and the friction test was conducted at various ambient temperatures ranging from room temperature to 700°C. Cu₃Mo₂O₉ and CuMoO₄ powders were synthesized by heating the mixture of CuO and MoO₃ powders in their own specific heating conditions and identified by XRD. Both of Cu₃Mo₂O₉ and CuMoO₄ powders showed lower friction coefficient and smaller wear amount of specimen with increasing ambient temperature. On the other hand, CuO and MoO₃ powders showed poor lubricating ability and high wear amount of specimens comparing with two kinds of copper molybdate powders. It was suggested that the enhanced maintaining abilities of copper molybdate on the stainless steel substrate at high temperature and generation of unoxidized copper under high temperature conditions could enhance lubricating abilities.

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The objective of this study is to evaluate the boundary lubrication performance of ionic liquids under high vacuum and low temperature by taking the cosmic space environment into consideration, as a screening stage prior to evaluating lubrication performance in actual space mechanisms. The boundary lubrication performance of ionic liquids was evaluated at room temperature with a reciprocating linear motion tribometer, and at low temperature (from –80°C to room temperature) with a unidirectional rotation tribometer. Low-temperature rheometry was also carried out. Ionic liquids showed a supercooling state and crystallization. This crystallization was prevented by mixing different ionic liquids together. The equimolar mixture of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (EMI-TFSA), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (BMI-TFSA), and 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (HMI-TFSA) showed no crystallization in our experiment. The antiwear performance of this sample oil mixture was similar to that of MAC and much better than that of PFPE at low temperatures. This mixture prevented metal contact at –80°C, most likely due to its high viscosity and high adsorption of molecules.

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This research is focused on improving the wear resistance property of stainless steel by using the technique of friction reforming as well as determining the influence of the mated materials. The powder intrusion and friction stir mechanisms are utilized in the surface friction reforming process. Then, wear tests are conducted to examine the wear properties in three different conditions: dry, oil-lubricated, and artificial seawater. Furthermore, investigations of the mated materials are performed. As a result, the modified material presents moderate effects under dry and oil-lubricated conditions. However, in artificial seawater, the modified material exhibits excellent wear resistance properties; the wear volume of the modified material is about one-fourth of that of the substrate material. As for the mated materials, they have a significant influence on the wear properties.

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Numerous researchers have focused on the relation between kinetic friction and the surface properties of materials, such as their surface roughness and chemical state, because kinetic friction is the loss of kinetic energy at a sliding interface. Contrary to conventional wisdom, recent theory has predicated that, given the fact of phonon energy dissipation, kinetic friction depends on the properties of bulk atoms in a solid, not only on the surface properties. However, this expectation has not been proven. Here we show evidence of this idea via atomic-scale experiments and simulations. We compared the kinetic frictions of isotopically distinct single-crystal diamonds, which differ only in atomic mass, using atomic force microscopy and observed that the friction of ¹³C diamond is lower than that of ¹²C diamond by approximately 3%. Simulations and theoretical analysis reproduce this result well, suggesting that the lower friction of ¹³C diamond originates from the inhibition of the energy-dissipative phonon by a heavier atom mass. This discovery provides a design concept of low-friction materials by tuning the energy-dissipation process with modification of the inner-solid properties; i.e. phonon properties.

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Under severe bearing conditions, deposits form on the loaded part of bearing pads and if this happens excessively, it could lead to unacceptable rises in temperature and vibration. Deposits are mainly organic compounds, formed as oxidation by-products, which are insoluble in lubricating oil and can be the precursor to deposits. In this study, the deposit precursor in lubricating oil was investigated in detail and the deposit production mechanism was demonstrated. Furthermore, the difference of oil type effects and the dependence of conditions on deposit formation are also considered.

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This paper investigates the friction and wear properties of the fresh and soot-contaminated API CF-4 and CI-4 diesel engine oil using the SRV4 tribometer. It was found that the comprehensive wear properties of CI-4 oil was better than those of the CF-4 oil. Tests of the fresh and contaminated CF-4 oil in combination with two types of rings of different surface treatment showed discriminations, and these discriminations also reflected on the CF-4 oil tests with test rings of different surface coatings.

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This paper presents some tribo-troubleshooting case studies of bearings and seals in actual machinery, proposing a new practical method to learn how to design and operate machinery free from tribo-troubles, based on unique analyses of these case studies. The analyses focus on the initiators that triggered the chain of events leading to the eventual problems, the various promoters that expedited the process and some human factors involved, rather than the direct causes of each case study. Understanding only the relation between direct cause and effect won’t suffice to prevent recurrence of the same tribo-troubles, but understanding the total process from the initiator to the final problem inclusive of promoters and human factors is much more important. Creating new databases consisting of similar case studies with the unique analyses incorporated will definitely contribute to giving birth to trouble-free tribo-design methodology.

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The protein film adsorbed on joint prosthesis material affects tribological property. It is expected that optimum protein film structure could improve friction and wear of joint prosthesis materials. Therefore, it is important to understand and to clarify the effect of protein film on tribological property. In the rubbing combination of ultra-high molecular weight polyethylene (UHMWPE) and CoCrMo alloy, the adsorption behavior of bovine serum albumin (BSA) and its effect on frictional property during friction were investigated by using electrochemical method. Friction force and electric potential were measured under open circuit potential (OCP) condition, but friction force and electric current were measured under –0.2 V applied potential condition. Under OCP condition, adsorbed BSA was peeled off at first rubbing, after that BSA re-adsorbed on surface under shear force condition. BSA adsorbed film reconstructed by shear force was sustained during rubbing. It is considered that the conformation of adsorbed BSA under shear stress was changed from that of static adsorption, because readsorbed BSA molecule did not show exfoliation from metal surface, which means that adsorption strength was increased from static adsorption. Under applied potential condition where wear performance of BSA film is expected to be evaluated, BSA showed the desorption at first rubbing, and gradual decrease in desorption amount. Therefore, it is confirmed that BSA film on metal surface under rubbing condition is reconstructed during rubbing. The adsorbed film formed during rubbing is stable and has an optimally-adapted structure for shear force and can sustain its property during rubbing.

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Characteristics of friction and adhesion/cohesion between snow and SS400 steel in a shearing velocity range of 2.5-25.4 m/s, as well as in snow itself in a shearing velocity range of 0.5-2.5 m/s, were analyzed by using a specially developed annular shearing type experimental analysis system in a normal stress range of 20-60 kPa. The friction coefficient between melt forms and SS400 was smaller than that between rounded grains and SS400 at the same shearing velocity. The adhesive force per unit area of the rounded grains and the melt forms with respect to SS400 and the cohesive forces per unit area in the rounded grains and the melt forms tended to be larger with smaller shearing velocity in the transient state, whereas those in the steady state showed the opposite tendency, except for the adhesive force of the melt forms in the transient state and that of the rounded grains in the steady state in the case where the shearing velocity was 25.4 m/s. In addition, the friction coefficients in the rounded grains and the melt forms were larger with decreasing shearing velocity.

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We report a short note of preliminary and new experimental results that the generation of ammonia (i.e. nitrogen fixation) can occur on tribologically activated surfaces formed as a result of the frictional sliding of pure titanium when this is performed in a mixed gaseous environment of dinitrogen and dihydrogen at a total pressure of 6 × 10^-4 Pa. Gaseous ammonia, synthesized at the tribologically activated surface on the actual area of contact and/or on fine debris of wear elements with a size of the order of ten or a few tens of nanometers, was detected by analyzing the partial pressures for each species between amu 1 and amu 50 present in the environmental gases during the sliding friction: this analysis was performed using a quadrupole mass spectrometer (QMS). This process, however, depends on the mode of wear, i.e. it is expected to occur only under mild conditions of wear.

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High performance polymer composites have been intensively investigated for tribological applications in air, but rarely in hydrogen environment. Author’s previous benchmark of composites in liquid hydrogen (LH₂) showed that graphite filled polymers have beneficial friction behaviour in this extreme condition. Therefore, further investigations have been undertaken in hydrogen environment. This paper presents first results obtained with polyimide composites filled with different types (natural, synthetic) and amounts of graphite in air, vacuum and hydrogen environments. A particular attention is taken to the influence of hydrogen on graphite as well as on the polymer matrix.

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Hard particulate fillers are used to improve the mechanical and tribological properties of thermoplastic resins. In this study, we investigated the fundamental tribological behavior of polyamide 66 (PA66) resin composites containing rice bran ceramics (RBC) particles at a wide range of pressure-velocity (Pv) values under dry conditions. Irrespective of Pv value, PA66/RBC composite exhibited relatively low friction coefficients (0.4), whereas the friction coefficients for pure PA66 increased. In addition, the PA66/RBC composite better prevented an increase in the surface temperature compared with pure PA66 at high Pv values. To distinguish the effect of the flowability of the resin, surface temperatures normalized by the glass transition temperature were defined. The specific wear rates of pure PA66 changed with the normalized surface temperature because the wear mode transited. Low specific wear rates (< 0.3 × 10^–8 mm²/N) were obtained for the PA66/RBC composite without the transition of the wear mode against the normalized surface temperature. These wear rates were reduced by 55-86% compared with those of pure PA66.

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The effects of energy-saving hydraulic fluids are confirmed by not only many actual hydraulic systems but also bench tests using hydraulic pump. We have studied some laboratory evaluation methods that are simpler than bench test using hydraulic pump. In this paper, we report some results of roller on disk test, which reflects the tribological condition of a piston pump. Specimen Roller was made from metals used for the piston of a piston pump, and specimen disks were made from seven different metals used for the cylinder block of a piston pump. The evaluation condition of hydraulic fluid by the roller on disk test was established. In this condition, the widely used anti-wear fluid has higher friction coefficient than energy saving type, and in energy saving type, the friction coefficients become lower with time in all metal combinations. It agrees very well with the energy-saving effect of hydraulic fluids in an actual hydraulic system.

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