The 6th World Tribology Congress (WTC 2017) was held from September 17 to 22, 2017, in Beijing. It was hosted by Chinese Tribology Institute (CTI) and organized by the State Key Laboratory of Tribology of Tsingnua University. It was the largest WTC ever since the first one was held in London in 1997, with more than 1200 papers from all over the world presented (including oral and poster presentations). The great success of WTC 2017 was celebrated not only by the participants but also by the beautiful weather in Beijing during the week.

CTI kindly accepted Tribology Online as one of the official journals for publication, and the Japanese Society of Tribologists (JAST) called for papers for WTC Special Issues. We have received more than twenty papers from eleven countries. These papers have been peer reviewed as usual, and the papers accepted for publication is published in this Special Issue, Part 1, and in the Special Issue, Part 2, to be released later this year.

The papers in this issue are from materials tribology to lubricants and lubrication mechanisms, and from fundamental studies to practical studies. Although the number of the papers included in this issue is not many, they reflect a wide spectrum of the tribology researches and the internationally active atmosphere of this area.

On behalf of the editorial board of Tribology Online, the Editor-in-Chief would sincerely like to thank all the contributors to this Special Issue, and hope the readers find these papers interesting and useful.

Nickel Aluminum Bronze (NAB) alloy specimens subjected to Friction Stir Processing (FSP) with and without particulate addition were subjected to tribological testing in a reciprocating sliding tester which can perform sliding test on specimens immersed in sea-water. Stainless steel ball was slid against flat coupons of NAB as cast, and Friction-Stir-Processed NABs, under sea-water to study the wear performance considering their prospective use in marine bearing applications. The wear expressed in terms of the wear scar area measured using an image processing method indicates that the performance of the Friction-Stir-Processed NABs in sea-water environment is inferior to that of the un-processed NAB, even though the processed NABs have relatively superior hardness. While wear tests in dry condition indicate that the wear resistance for the friction stir processed samples is high, under saline environment the trend was found to be reversed, as their wear resistance was found to be low. The reduction in wear resistance in sea-water is explained in terms of the (electrochemical) corrosive wear mode being active in the saline medium. The surface damage as revealed from the SEM characterization of wear scars were found to correlate with the experimental deductions.

A non-Newtonian numerical solution system for the transient thermo-elastohydrodynamic lubrication (TEHL) model has been proposed for a helical gear pair with the finite line contact theory. This model is used to comprehensively show the transient effect, temperature effect, operating condition and the non-Newtonian behaviors on the lubricating performance. The results indicate that, the transient effect is insignificant except the area of engaging-in and engaging-out along the line of action (LOA). Besides, due to the pitch point position on the contact line, the temperature distributions in different meshing positions along LOA is significantly differentiated. In the single tooth contact region, the variation of film thickness is delayed for the load variation. By introducing the thermal effect, the film thickness has been reduced and the difference of friction coefficient is significant between the isothermal and thermal model. The effect of input torque and speed is significant for the lubrication performance. As the input torque increases, the squeeze effect has been enhanced during the engaging-in region. Finally, the influence of characteristic shear stress of the Eyring fluid on the pressure and film thickness is negligible. The results indicate that the TEHL model with non-Newtonian fluid predicts more realistic friction coefficient.

In this study, friction and wear properties of Al-Sn based bearing alloy with no overlay, bronze lining coated with Al-Sn based overlay and PAI based overlay, bronze lining coated with Sn based overlay and Pb-containing bearing material have been investigated using five different engine oil formulations including pure PAO 6 base oil and different viscosity grade oils containing anti-wear additive using an Optimol SRV® high-temperature reciprocating friction and wear test machine. It has been found that when Al-Sn based lining is lubricated with plain base oil, wear is lower compared to those with oils containing additives. For the other bearing materials, oils containing additives have shown improved friction and wear performance compared with that of the pure base oil.

Two different nitriding techniques, named plasma nitriding and fluidized bed nitriding, were applied for surface modification of ductile cast iron NAAMS-D6510. Two test methods (i.e., Pin-on-disc tribotest and inclined sliding wear test) were used to generate different tribological contact-loading conditions. While the pin-on-disc (POD) test had a vertical contact and relatively low stress applied on the mating surfaces, the inclined sliding wear test created a tilting contact with extremely high pressing pressure. The cross-sectional hardness and nitrogen concentration profiles along the case hardened layers were measured using a microhardness tester and energy dispersive spectroscope (EDS), respectively. Surface profilometry and scanning electron microscopy (SEM) were employed to measure and observe wear tracks after the tribological tests. The test results showed that the plasma nitrided sample was subjected to more severely abrasive/adhesive wear with numerous surface fatigue cracks, compared to the sample treated by fluidized bed nitriding, during the highly stressed inclined sliding wear test. This phenomenon was found to be attributed to their different hardness and nitrogen concentration profiles. A smaller hardness gradient between the compound layer and the case diffusion zone, and a thicker case hardened layer (provided from the fluidized bed nitriding in this study) could improve the wear resistance and the surface fatigue cracking resistance against the high contact loads. On contrast, less difference could be observed between the two different nitriding-treated samples during the POD tests. Moreover, the oxidation layer on the top of the compound layer of plasma nitrided sample may be beneficial for friction reduction under a low contact pressure condition like the POD sliding, but it would be soon removed and lost its advantage under the high contact pressure condition as for the inclined sliding case.

Marine water-lubricated rubber bearing is used to support the stern shaft of surface vessels and underwater vehicles, whose friction vibration and noise directly influence the stealth, security of the vessels and both physical and psychological health of occupants. In order to investigate the this problem, the kinds of water-lubricated bearings with different rubber material were tested on the bearing testing machine, and friction coefficient, noise, vibration frequency and amplitude were measured under different working conditions. The results show that friction coefficient of water-lubricated rubber bearings are affected with the rubber material, operating speed, applied load and cooling water temperature, which results in the change of bearing vibration state. The component content of carbon black and self-lubricated materials can affect the deformation size, frictional performances and friction coefficients of bearings. The higher the speed, the smaller the friction coefficient, and friction vibration state is changed. With the increase of the specific pressure in some range, the contact pressure between tested bearing and shaft decreases due to the augment of the contact area so that the friction coefficient and vibration amplitude are changed. As cooling water temperature is enhanced, sometimes friction vibration declines, but the critical speed producing noise increases due to softened rubber.

One of the solutions to reduce friction between two sliding surfaces in automobile engine is by applying DLC coating to engine components. However, it is critical that the effect of lubricant additives to DLC coating to be clarified before hands as to avoid components failure. In this study, tribological friction test between SUJ2 balls and as-deposited plus UV irradiated a-C:H coatings was conducted to clarify the effect of ultraviolet irradiation to DLC coating in four different additives added lubricant oils. AFM, nano-indentation hardness test, ellipsometery, Zygo, and EDS-SEM were used to investigate the effect of UV irradiation to a-C:H DLC before and after friction test. Prior the friction test, the results showed that UV irradiation presented no significant change in terms of hardness and roughness but the irradiation did penetrate into topmost surface of the a-C:H coating to several degree and created dangling bonds available to interact with lubricant additives elements. Friction test results showed that UV irradiated a-C:H coatings presented lower friction coefficient than as deposited a-C:H coatings. Worn surface analysis revealed that UV irradiated a-C:H coatings attracted more lubricant additives element to attach on its surface thus created thicker tribofilm on its own surface and its counter materials, resulted in lower friction coefficient than the as-deposited a-C:H coatings.

A squeeze film test apparatus was used to study the mechanism and characteristics of the irregular typewriter noise in engine main bearings oil films. To simulate the dynamic load of the bearing, the transparent moving plate in the test apparatus was moved upward and downward rapidly. The high-speed camera system was used for photographing oil film of the test area. The test apparatus could collect five kinds of signals, sound, displacement, vibration acceleration, force, and pictures of squeeze oil simultaneously during the test running. The irregular noise appeared in the test and the maximum value of irregular noise’s sound pressure reached 3 Pa. And the clear pictures of cavitation when irregular noise occurred were taken simultaneously and consecutively via high-speed camera system. The results show that noise occurs under the condition where the collapsed cavitation reached the atmosphere in the falling process of the moving plate. The parameters such as vibration exciting signals, lubricating oil quantity, and oscillation frequency were tested in experiments. There are negative correlations between lubricating oil quantity and the intensity and occurrence rate of the noise. The noise is most prone to occur under square wave excitation and at the oscillation frequency of 12 Hz.

In the present study, a flow visualization experiment in the clearance between a clear floating-ring and a rotating journal was carried out in order to clarify the effects of the recess geometry on the flow characteristics. Five types of the floating-rings with different recess geometries, i.e., the plane rhombus, the plane circle, the revise circular Young Leaf Mark, the dome and the revise dome geometries, were operated at a rotational speed of up to 40,000 rpm using liquid nitrogen as the working fluid to observe the interaction between source flow from the recess and rotating flow induced by the journal rotation in the clearance. The cavitation cloud inside the clearance was induced by viscous frictional heating and the pressure drop. The influence of the sweepback angle (θ_s) of the recess leading edge on the flow characteristics was investigated by comparing flow coefficient (C_f) and the cavitation cloud area ratio (Ac) obtained from the visual image. Based on the experimental results, it may be confirmed that the effects of the recess geometry on the flow characteristics of the cryogenic hybrid journal bearings are mainly caused by the influence of the θ_s value on the flow condition.

Ionic liquids have been used as novel high-performance lubricants. However, fluorine-based ionic liquids cause corrosion of materials by the formation of FeF₂ and FeF₃. This study focuses on cyano-based ionic liquids, as they have low corrosion tendencies owing to the absence of fluorine in their structures. However, cyano-based ionic liquids are hydrophilic, and hence, the effect of ambient conditions (e.g., atmospheric moisture) on the tribological properties needs to be investigated. This study reports the tribological properties of two types of cyano-based ionic liquids 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium tricyanomethane under air, vacuum, and dry air environments. Their friction coefficients were found to be dependent on the ambient condition. Both the ionic liquids exhibited low friction coefficients in vacuum. The time-of-flight secondary ion mass spectrometry results indicated that the anion was adsorbed on the worn surface under vacuum conditions, and the adsorbed anion exhibited a low friction. In dry air, the cation was adsorbed on the worn surface, and the adsorbed cation exhibited a higher friction coefficient than the adsorbed anion. In air, the cyano-based ionic liquids caused corrosion of the sliding material and exhibited a high friction coefficient, because of the absorption of moisture from air.

Tribofilms are critical for the management of wear and the control of friction. This study focused on the tribological properties of tribofilms to clarify their antiwear and low-friction mechanisms. By monitoring tribofilm growth and measuring friction-coefficient mapping, we found that a tribofilm was generated in a dynamic process of formation and wear. The friction coefficient decreased with an increase in film thickness. The friction coefficient distributions of the tribofilms depended on the chemical nature more than the surface morphology. ZDDP formed a tribofilm with good antiwear properties, whereas MoDTC accelerated the tribofilm formation and decreased the friction coefficient.

Nitriding is one of the most popular heat treatments widely used in hydraulic sliding parts of automotive or construction machinery components. Since the nitrided steels shows great wear resistance due to its high hardness, it is widely applied to various sliding parts in the hydraulic system of construction machinery. During operation, these parts are usually lubricated with fully formulated oils, which include various additives such as dialkyldithiophosphate (ZnDTP), dispersant, detergent, etc. In this study, the tribological features of two kinds of nitrided surfaces lubricated with fully formulated oil and sample oils were evaluated by changing the combination of additive element formulations, and the effect of additive formulation on tribofilm formation and frictional property were investigated in detail. As a result, it was revealed that there is a large interaction between the nitrided surfaces and additives, the coexistence of ZnDTP and detergent affects the phosphorous-derived tribofilm formation on nitrided surface as well as their frictional properties. In particular, the low frictional characteristics of nitrided compound layer was characterized by the smoothed surface and low tribofilm formation on the track.

The influence of synovial fluid pressure in wedge-film gap on the biphasic lubrication property of articular cartilage during sliding motion has not been discussed in previous studies. We hypothesized that synovial fluid pressure generated in a wedge-film shaped gap just ahead of cartilage contact region suppresses the effusion of interstitial fluid in articular cartilage and that this enhances biphasic lubrication ability of articular cartilage. In the present study, we determined fluid pressure distribution in the wedge-shaped gap. We, then, also performed a friction analysis to compare the coefficient of dynamic friction of standard poroelastic model with that of synovial fluid pressure (SFP) model. In SFP model, the effusion of interstitial fluid from articular cartilage was regulated in accordance with the pressure difference between measured synovial fluid pressure and analyzed interstitial fluid pressure at the surface. Experimental results revealed that the maximum fluid pressure increased with the increase of sliding speed. The coefficient of dynamic friction was lower in SFP model than in standard model with the largest difference of 25% observed at 30 mm/s. Effusion of interstitial fluid from articular cartilage near the periphery of contact region was more suppressed at higher sliding speeds, which resulted in a better lubrication property due to higher interstitial fluid pressurization. These results suggested that synovial fluid plays important roles not only in hydrodynamic lubrication but also in biphasic lubrication by suppressing the effusion of interstitial fluid in articular cartilage.

Modern engineering surfaces are cut, grind, and polished to create surfaces that are isotropic, uniform, and homogeneous to enable easy assembly and manufacture. Commonly used surface descriptors such as Ra are based on these characteristics. In the 1990s, the magnetic hard disk drive storage industry began to use multiscale directionally aligned roughness to achieve its lubrication requirements successfully, and this leads to the concept of designed surface topography and textures to enable additional functionalities. The design and fabrication of surface topography to impart directionality, uniform reactivity, and patterned geometric dimples or protrusions can enhance or expand the surface properties to meet specific desirable application needs. Adding thin films, coatings, multilayer nanocomposites on the surface can alter the surface physical and chemical characteristics and enhance durability. Properly designed surface textures (dimples, grooves, and discrete shapes) can reduce friction, create drag reduction, as well as control the real area of contact and heat transfer characteristics. In terms of designing friction reducing surfaces for an application, the dominant friction mechanisms need to be understood and proper surface designs applied to reduce the dominant friction generation. Generally, roughness perpendicular to the sliding direction will increase frictional resistance while increasing the interfacial temperatures. Multiscale roughness parallel to the sliding direction, depending on the speed and load of the contact, will reduce scuffing and reduce friction. In this paper, we use surface textures coupled with diamond-like-carbon thin films and bonded chemical layer to effect friction reduction of piston ring-cylinder liner friction.