In Situ Studies of Cartilage Microtribology: Roles of Speed and Contact Area

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 and 3.2 mm, were used in the study. In situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: (1) at high sliding speed (V = 1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8–3.2 mm) despite the fourfold difference in the resulting contact areas; (2) the contact area was a strong function of the probe radius and sliding speed; (3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 to 5 mm/s; (4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V = 0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid–solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.     

A study on the friction properties of poly(vinyl alcohol) hydrogel as articular cartilage against titanium alloy

Many biomaterials are being developed to repair or replace articular cartilage. One of these materials, poly(vinyl alcohol) (PVA) hydrogel prepared from aqueous solution of the polymer by freezing and thawing method may exhibit the mechanical properties required to withstand the harsh environment of diarthrodial joints. To better understand how PVA hydrogel friction is affected by different variable factors, a three-factor, three-level designed orthogonal experiment was developed. Factors include lubricant, sliding speed, and normal load. Friction coefficient of the PVA hydrogel was found to depend significantly on load and sliding speed. Lubricant had little effects on the friction coefficient. Friction coefficient of the PVA hydrogel decreased with the increase of sliding speed and the friction coefficient approximately increased linearly with the increasing load. Average friction coefficient decreased from 0.0447 to 0.0379 while the sliding speed increased from 0.06 to 0.22 m/s. Average friction coefficient increased from 0.0276 to 0.0546, almost increasing one time, while the load increased from 5 to 15 N.     

The effect of glycosaminoglycan depletion on the friction and deformation of articular cartilage

Glycosaminoglycans (GAGs) have been shown to be responsible for the interstitial fluid pressurization of articular cartilage and hence its compressive stiffness and load-bearing properties. Contradictory evidence has been presented in the literature on the effect of depleting GAGs on the friction properties of articular cartilage. The aim of this study was to investigate the effect of depleting GAGs on the friction and deformation characteristics of articular cartilage under different tribological conditions. A pin-on-plate machine was utilized to measure the coefficient of friction of native and chondroitinase ABC (CaseABC)-treated articular cartilage under two different models: static (4 mm/s start-up velocity) and dynamic (4 mm/s sliding velocity; 4 mm stroke length) under a load of 25 N (0.4 MPa contact stress) and with phosphate-buffered saline as the lubricant. Indentation tests were carried out at 1 N and 2 N loads (0.14 MPa and 0.28 MPa contact stress levels) to study the deformation characteristics of both native and GAG-depleted cartilage samples. CaseABC treatment rendered the cartilage tissue soft owing to the loss of compressive stiffness and a sulphated-sugar assay confirmed the loss of GAGs from the cartilage samples. CaseABC treatment significantly increased (by more than 50 per cent) the friction levels in the dynamic model (p < 0.05) at higher loading times owing to the loss of biphasic lubrication. CaseABC treatment had no effect on friction in the static model in which the cartilage surfaces did not have an opportunity to recover fluid because of static loading unlike the cartilage tissue in the dynamic model, in which translation of the cartilage surfaces was involved, ensuring effective biphasic lubrication. Therefore the depletion of GAGs had a smaller effect on the coefficient of friction for the static model. Indentation tests showed that GAG-depleted cartilage samples had a lower elastic modulus and higher permeability than native tissue. These results corroborate the role of GAGs in the compressive and friction properties of articular cartilage and emphasize the need for developing strategies to control GAG loss from diseased articular cartilage tissue.     

The influence of continuous sliding and subsequent surface wear on the friction of articular cartilage.

Reciprocating motion friction tests were conducted upon cartilage-on-metal contacts while subjected to a constant load. Initial friction coefficients were compared with repeat friction coefficients following a sufficient load removal period. The repeat friction coefficients were marginally higher than the initial values and both were primarily dependent on the loading time. It was concluded that while a wear component had been identified, which modestly increased friction coefficients, the overriding parameter influencing friction was loading time. The authors postulate that fluid phase load carriage (being dependent on loading time) within the articular cartilage is largely responsible for low friction coefficients in the mixed and boundary lubrication regimes. This mechanism has been referred to as biphasic lubrication. Both synovial fluid and Ringer's solution were used as lubricants. Over the assessed 120 min loading time friction coefficients rose from 0.005 (for both lubricants) after 5 s to 0.50 and 0.57 for synovial fluid and Ringer's solution respectively. Synovial fluid was found to significantly reduce friction coefficients compared to Ringer's solution over broad ranges of the assessed loading times (p < 0.05). Stylus and non-contacting laser profilometry were successfully employed to provide reliable, quantitative and accurate measures of surface roughness. Laser profilometry before and after a continuous sliding friction test revealed a significant increase in surface roughness from Ra = 0.8 (+/- 0.2) micron to Ra = 2.1 (+/- 0.2) microns, (p < 0.0005); confirming that surface wear was occurring. Scanning electron microscopy (SEM) revealed the typical highly orientated collagen fibres of the superficial tangential zone. Environmental SEM (ESEM) of fully hydrated cartilage specimens provided largely featureless images of the surface which suggested that sample preparation for conventional SEM was detrimental to the authenticity of the cartilage surface appearance using SEM. Two distinct acellular, non-collagenous surface layers were identified using ESEM and transmission electron microscopy (TEM); respectively referred to as the boundary layer and surface lamina. The phospholipid/glycoprotein based boundary layer will provide boundary lubrication during intimate contact of opposing cartilage surfaces. The surface lamina, being a continuum of the proteoglycan interfibrillar matrix, is present to prevent fibrillation of the underlying collagen fibres. Both layers may contribute to the time dependent frictional response of articular cartilage. Although laser profilometry did reveal surface wear which was consistent with a small increase in friction, the primary variable controlling the friction coefficient was the period of loading.     

Analysis of fluid film formation between contacting compliant solids

Industrial compliant surface bearings and dynamic seals sometimes suffer severe damage during start up after long rest, and a similar problem is predicted for joint prostheses with compliant artificial articular cartilage. In this study, fluid film developing between compliant solids by sliding is analyzed numerically using a modified elasto-hydrodynamic lubrication theory which permits direct contact and cavitation. The result shows that the forefront of the fluid film moves in the same direction with sliding while direct contact remains until the fluid film takes the place of the entire contacting region. With an increase of compliance and the Stribeck number, the velocity of fluid film formation increases and approaches half of the sliding speed but never exceeds it. In other words, the minimum sliding distance for non-contacting condition is twice the initial contact width. Therefore, when a heavy load is applied to compliant surface bearings, the contact width will be large and the unlubricated region will remain long. Since a compliant material is not as strong as hard materials, it may be damaged during start up after a long rest. As the study has thus clarified the mechanism of damage which compliant surfaces experience during start up, effective methods to protect surfaces from damage will be found according to the theoretical backgrounds.     

Sliding velocity dependency of the friction coefficient of Si-containing diamond-like carbon film under oil lubricated condition

In this study we investigated the sliding velocity dependency of the coefficient of friction for a Si-containing diamond-like carbon (DLC-Si) film in an automatic transmission fluid (ATF) under a wide range of contact pressures. The DLC-Si film and a nitrided steel with a surface roughness, Rz_JIS, of around 3.0 μm were used as disk specimens. A high-carbon chromium steel (JIS-SUJ2) bearing ball was used as a ball specimen. Friction tests were conducted using a ball-on-disk friction apparatus under a wide range of sliding velocites (0.1-2.0 m/s) and contact pressures (P_max: 0.42-3.61 GPa) in ATF. The friction coefficients for the nitrided steel had a tendency to decrease with an increase in sliding veloicity under all the contact pressure conditions; however, the friction coefficients for the DLC-Si film were stable with respect to sliding velocities under all the contatct pressures. These results indicate that the DLC-Si film suppresses the stick-slip motion during sliding againt steel in ATF, which is a desired frictional characteristic for the electromagnetic clutch disks used under lubrication. Furthermore, the DLC-Si film showed a higher wear resistance and lower aggression on the steel ball specimen than the nitrided steel. There were less hydrodynamic effects on the friction coefficient for the DLC-Si film possibly due to maintenance of the initial surface roughness and its poorer wettability with the fluid. X-ray photoelectron spectroscopy (XPS) analysis of the sliding surfaces revealed that the adsorption film derived from the succinimide on the sliding surfaces of the DLC-Si film and the mating steel ball also contributed to the sufficient and less sliding-velocity-dependant friction coefficients.     

The Effect of Surface-Active Phospholipids on the Lubrication of Osteoarthritic Sheep Knee Joints: Friction

The synovial fluid aspirate from human joints that have experienced serious traumatic injury has been shown to have lower concentrations of phospholipids when compared with healthy joints. Previous studies provide evidence that synovial fluid constituents, specifically dipalmitoyl phosphatidylcholine (L-DPPC), are highly surface active, capable of rapidly depositing a layer of phospholipids onto glass. Such research has demonstrated that the adsorbed surface layers of synovial surfactant are excellent lubricants in vitro, significantly reducing the coefficient of friction under physiological loading in human knee joints. This study aimed to investigate the effect of concentration of L-DPPC lubricant solutions on the coefficient of friction of worn articular cartilage on steel. A pin-on-disc apparatus was used to measure the coefficient of friction of sheep-knee articular cartilage on steel under unidirectional sliding at physiological conditions of load and speed. Concentrations of L-DPPC solution between 100 times less and 100 times more than is normally present in synovial fluid were tested. All specimens were tested following a period of unlubricated induced wear. Trials were carried out at ambient temperature and between 33–37°C (representative of in vivo joint temperature). Friction measurement results demonstrated a reduction in the coefficient of friction of worn articular cartilage against steel with increasing concentrations of L-DPPC in lubricant solution.     

Tribocorrosion behaviour of Ni–SiC nano-structured composite coatings obtained by electrodeposition

The combined corrosion-wear degradation of nano-structured Ni–SiC coatings in sliding contacts immersed in electrically conductive solutions is investigated in situ by electrochemical techniques (open-circuit potential measurements, E_OC, the potentiodynamic polarization measurements, PD, and the electrochemical impedance spectroscopy). The coating thickness was 50 μm, with an average volume of dispersed phases inside nickel of 20%. The samples were tested in a cell, containing the electrolyte and electrodes, and mounted on a pin-on-disk tribometer, with the working surface of the specimen facing upwards. Both continuous and intermittent friction tests were carried out. In the intermittent tests, friction was applied periodically: during each cycle, friction was first applied for 2 s at constant sliding speed under constant normal load and then stopped during a latency time of 20 s or 0.5 s. Without friction, the free potential reaches a passive value after immersion in the test solution. When friction force is applied the free potential is shutting down to active values. Under friction the measured current, I can be considered as the sum of two partial currents: one generated by the wear track areas, where the passive film is destroyed and the surface is active; the other one linked to the surface not subjected to friction and that remains in the passive state. A localised corrosion process when subjected to friction in 0.5 M K₂SO₄ was not observed on nano-structured Ni–SiC composite coatings. The mechanical destruction of the passive film occurs in the wear track by friction and subsequent restoration of the film (repassivation) when friction stops. The wear volume loss increases with sliding forces.     

Research on swing friction lubrication mechanisms and the fluid load support characteristics of PVA–HA composite hydrogel

Hydrogel has been extensively studied for use as articular cartilage. This study aims to investigate fluid load support mechanism of polyvinyl alcohol–hydroxyapatite composite hydrogel. Finite element method is used to study swing friction lubrication mechanism and fluid load support. The friction coefficient increases with contact load and swing angle. The fluid flow has an important effect on the fluid load support, which decreases with an increase in contact load and swing angle. The fluid load support is very high (85%), and the hydrogel has low friction coefficient. It exhibits biphasic and self-generating lubrication mechanism.     

Influence of proteoglycan contents and of tissue hydration on the frictional characteristics of articular cartilage

In this work, the hypothesis that water content and substances present on the articular surface play an important role in lubrication through the formation of a layer with a high content of water on the articular surface is analysed. The hydrophilic properties of proteoglycans exposed at the articular surface and hydration of tissue are the main responsible factors for the formation of this layer. The role of the articular surface in the frictional characteristics of articular cartilage was examined using specimens (femoral condyles of pigs) with intact and wiped surfaces tested in intermittent friction tests. Results indicated that the intact condition presented low friction in comparison with the wiped condition. The measured water loss of the articular cartilage after sliding and loading indicated a gradual decrease in the water content as the time evolved, and rehydration was observed after the submersion of unloaded specimens in the saline bath solution. Micrographic analyses indicated the presence of a layer covering the articular surface, and histological analyses indicated the presence of proteoglycans in this superficial layer. The hydration of the cartilage surface layer and proteoglycan in this layer influence lubrication.     

Investigation of the friction and surface degradation of innovative chondroplasty materials against articular cartilage

Understanding the wear of the biomaterial-cartilage interface is vital for the development of innovative chondroplasty. The aim of this study was to investigate a number of biphasic materials as potential chondroplasty biomaterials. Simple geometry friction and wear studies were conducted using bovine articular cartilage pins loaded against a range of single-phase and biphasic materials. The frictions of each biomaterial was compared within simple and protein-containing lubricants. Longer-term continuous sliding tests within a protein containing lubricant were also conducted at various loading conditions to evaluate the friction and degradation for each surface. All single-phase materials showed a steady rise in friction, which was dependent on the loss of interstitial fluid load support from the opposing cartilage pin. All biphasic materials demonstrated a marked reduction in friction when compared with the single-phase materials. It is postulated that the biphasic nature of each material allowed an element of fluid load support to be maintained by fluid rehydration and expulsion. In the longer-term study, significant differences in the articular cartilage pin (surface damage) between the positive control (stainless steel) and the negative control (articular cartilage) was found. The potential biphasic chondroplasty materials produced a reduction in articular cartilage pin damage when compared with the single-phase materials. The changes in surface topography of the cartilage pin were associated with increased levels of friction achieved during the continuous wear test. The study illustrated the importance of the biphasic properties of potential chondroplasty materials, and future work will focus on the optimization of biphasic properties as well as long-term durability, such that materials will more closely mimic the biotribology of natural articular cartilage.     

Tribological Behavior of 1-Methyl-3-Hexadecylimidazolium Tetrafluoroborate Ionic Liquid Crystal as a Neat Lubricant and as an Additive of Liquid Paraffin

Ionic liquid crystal (ILC), 1-methyl-3-hexadecylimidazolium tetrafluroborate, was synthesized. The tribological behavior of ILC was evaluated using a four-ball machine at 80 °C. X-ray photoelectron spectroscopic analysis shows that ILC takes part in tribochemical reactions to generate tribochemical products such as B₂O₃, FeF₂, and/or FeF₃, and amine which form a protective film on sliding steel surface, resulting in reduced friction and wear. Besides, ILC 1-methyl-3-hexadecylimidazolium tetrafluoroborate is completely transformed from solid state to liquid crystalline phase at 80 °C, which facilitates the ordered arrangement of its long alkyl chain on sliding steel surface and helps to improve the tribological properties. When the ILC is used as an additive of liquid paraffin (LP), it contributes to reduce friction and wear and increase the load-carrying capacity of the base stock both at room temperature and 80 °C. The reason might lie in that a small amount of F from ILC takes part in tribochemical reactions to generate tribochemical products that form a protective film on sliding steel surface, and friction-induced heat accelerates the transition of as-synthesized ILC to a mesophase and the ordered arrangement of its long alkyl chain on sliding steel surface, both resulting in improved load-carrying capacity and anti-wear ability of the ILC.     

Influence of hyaluronic acid on the time-dependent friction response of articular cartilage under different conditions

Therapeutic lubricant injections of hyaluronic acid are a relatively recent treatment for osteoarthritis. Their efficacy, however, in vivo has been subject to much debate. Frictional properties of cartilage-cartilage contacts under both static and dynamic loading conditions have been investigated, using healthy cartilage and cartilage with a physically disrupted surface, with and without the addition of a therapeutic lubricant, hyaluronic acid. Most of the cartilage friction models produced typical time-dependent loading curves, with a rise in static friction with loading time. For the dynamic loading conditions the rise in friction with loading time was dependent on the spatial (and time) variation in the load on the cartilage plate. For sliding distances of 4 mm or greater, when the cartilage plate was unloaded during sliding, the dynamic friction remained low whereas, with shorter sliding distances, the dynamic friction increased with increasing loading time. Static friction was higher than dynamic friction (under the same tribological conditions). The 'damaged' cartilage models produced higher friction than healthy cartilage under equivalent tribological conditions. It was shown that hyaluronic acid was an effective boundary lubricant for articular cartilage under static conditions with both healthy and damaged cartilage surfaces. Hyaluronic acid was less effective under dynamic conditions. However, these dynamic conditions had low friction values with the control lubricant because of the effectiveness of the intrinsic biphasic lubrication of the cartilage. It was only under the tribological conditions in which the cartilage friction was higher and rising with increasing loading time because of depletion of the intrinsic biphasic lubrication, that the role of hyaluronic acid as an effective therapeutic lubricant was demonstrated.     

Analysis of biphasic lubrication of articular cartilage loaded by cylindrical indenter

Combination of theoretical biphasic analyses and corresponding experimental measurements for articular cartilage has successfully revealed the fundamental material properties and time-depending mechanical behaviors of articular cartilage containing plenty of water. The insight of load partitioning between solid and fluid phases advanced the prediction of the frictional behavior of articular cartilage. One of the recent concerns about biphasic finite element (FE) analysis seems to be a dynamic and physiological condition in terms of mechanical functionality as a load-bearing for articular joint system beyond material testing, which has mainly focused on time-dependent reaction force and deformation in relatively small and low speed compression. Recently, the biphasic FE model for reciprocating sliding motion was applied to confirm the frictional effect on the migrating contact area. The results indicated that the model of a cylindrical indenter sliding over the cartilage surface remarkably sustained the higher proportion of fluid load support than a condition without migrating contact area, but the effectiveness of constitutive material properties has not been sufficiently evaluated for sliding motion. In our present study, at the first stage, the compressive response of the articular cartilage was examined by high precision testing machine. Material properties for the biphasic FE model, which included inhomogeneous apparent Young's modulus of solid phase along depth, strain-dependent permeability and collagen reinforcement in tensile strain, were estimated in cylindrical indentation tests by the curve fitting between the experimental time-dependent behavior and FE model simulation. Then, the biphasic lubrication mechanism of the articular cartilage including migrating contact area was simulated to elucidate functionality as a load-bearing material. The results showed that the compaction effect on permeability of solid phase was functional particularly in the condition without the migrating contact area, whereas in sliding condition the compaction effect did not clearly show its role in terms of the proportion of fluid load support. The reinforcement of solid phase, which represented the collagen network in the tissue, improved the proportion of fluid load support especially in the sliding condition. Thus, a functional integration of constitutive mechanical properties as a load-bearing was evaluated by FE model simulation in this study.     

A study on tribological behaviour of transfer films of PTFE/bronze composites

Transfer films of PTFE/bronze composites with 5–30% volume content of bronze were prepared using a RFT friction and wear tester on surface of AISI-1045 steel bar by different sliding time (5–60 min). Tribological properties of these transfer films were studied using a DFPM reciprocating tribometer in a point contacting configuration under normal loads of 0.5, 1.0, 2.0 and 3.0 N. Thickness and surface morphology of the transfer films were investigated. It was found thickness of the transfer films slightly increased along with the increase of bronze content of corresponding composites. Increased sliding time of transfer film preparation is helpful to form transfer film with better ductibility and continuity, but sliding time almost has no effect on tribological properties of the transfer film. Higher bronze content in the composite improved tribological properties of the corresponding transfer film, i.e., reduced friction coefficient and prolonged wear life. All these transfer films are sensitive to load change. Their wear life becomes shorter along with the increase of load. SEM image of the worn surface show fatigue wear and adhesion wear have happened on the transfer film during the friction process. The author believe bronze in the transfer film effectively partaked in shear force applied on the transfer film and its good ductibility helped to improve tribological properties of the transfer films.     

固体润滑涂层在干摩擦及有油条件下的摩擦磨损性能

采用MRH-3环块磨损试验机对FM-510二硫化钼润滑涂层在于摩擦及有油条件下进行了摩擦磨损性能的考察和评价,评价结果表明：该涂层在干摩擦条件下具有低的摩擦系数、高的承载能力和长的耐磨寿命,摩擦系数随负荷增高而降低,随速度提高也降低。摩擦偶对双面涂膜比单面涂膜有更长的耐磨寿命,速度低时涂层的磨耗小,寿命长,可满足特定条件下的干摩擦工作要求,在有油润滑条件下二硫化钼基的FM-510润滑涂层可显减轻对偶磨损程度,摩擦系数比单独使用油润滑时大大降低。在难以形成连续的流体润滑薄膜,亦即不能形成流体动力润滑的情况下。摩擦偶对涂敷固体润滑涂层是解决其润滑问题的有效方案。     

Recent developments in the tribology of artificial joints

A history of the tribological development of artificial joints compares how these are lubricated with the mechanisms involved in human joints. It is concluded that while healthy human joints are lubricated by fluid film action, all current artificial joints at best are lubricated by mixed lubrication and hence wear is taking place throughout the life of the prosthesis. A new concept in artificial joints is described. Soft elastic layers simulate articular cartilage and if selected carefully can develop full fluid film lubrication with consequential low friction and minimal wear.     

Friction, wear and material transfer of sintered polyimides sliding against various steel and diamond-like carbon coated surfaces

Polyimide cylinders are slid under 50 N normal load and 0.3 m/s sliding velocity against carbon steel (R_a=0.2 and 0.05 μm), high-alloy steel (R_a=0.05 μm), diamond-like carbon (DLC, R_a=0.05 μm) and diamond-like nanocomposite (DLN, R_a=0.05 μm). Only for a limited range of test parameters, the friction of polyimide/DLN is lower than for polyimide/steel, while polyimide shows higher wear rates after sliding against DLN compared to steel counterfaces. The DLN coating shows slight wear scratches, although less severe than on DLC-coatings that are worn through thermal degradation. Therefore, also friction against DLC-coatings is high and unstable. Calculated bulk temperatures for steel and DLN under mild sliding conditions remain below the polyimide transition temperature of 180 °C so that other surface characteristics explain low friction on DLN counterfaces, as surface energy, structural compatibility and transfer behaviour. Friction is initially determined through adhesion and it is demonstrated that higher surface energy provides higher friction. After certain sliding time, different polyimide transfer on each counterface governs the tribological performance. Polyimide and amorphous DLC structures are characterised by C–C bonds, showing high structural compatibility and easy adherence of wear debris on the coating. However, it consists of plate-like transfer particles that act as abrasives and deteriorate the polyimide wear resistance. In sliding experiments with high-alloy steel, wear debris is washed out of the contact zone without formation of a transfer film. Transfer consists of island-like particles for smooth carbon steel and it forms a more homogeneous transfer film on rough carbon steel. The latter thick and protective film is favourable for low wear rates; however, it causes higher friction than smooth counterfaces.     

Dimples shape and distribution effect on characteristics of Stribeck curve

A purpose of this research is to study the influence of geometrical characteristics of the surface texture on the Stribeck curve in lubricating sliding.The tribosystem consists of the stationary block pressed at the required constant load 1800 N against the ring rotating at the defined speeds. Tests were conducted at increasing sliding speed of range 0.08–0.69 m/s. Every speed was maintained for 2 min. The test was carried out under conformal contact conditions. The sliding was unidirectional. The block was a part of a bearing sleeve hardened EN-GJS 400-15 cast iron with a hardness value of 50 HRC. The ring samples, 35 mm in diameter, were made from hardened 42CrMo4 steel of hardness 32 HRC. Some variants of specimen surfaces were created by burnishing technique. The area density of oil pockets S was in the range 7.5–20%. The dimples depth to length ratios were between 0.03 and 0.08. Ring surfaces with oil pockets of short drop, long drop and spherical shapes were tested.It was shown that with proper shape and dimensions as well as suitable area density of oil pockets the friction characteristic of the sliding pairs could be improved in comparison to non-textured surfaces.     

Scaling effects between micro- and macro-tribology for a Ti–MoS2 coating

The tribological properties of a Ti–MoS₂ coating (9 at% Ti) were studied at macroscopic length scales with an in situ tribometer and at microscopic length scales with a nanoindentation instrument equipped for microsliding experiments. Measurements were conducted in controlled environments at both low and high humidity (i.e. ～4%RH and ～35%RH). Reciprocating micro- and macro-sliding tests were performed with spherical diamond tip with a 50 μm radius and a sapphire tip with a radius of 3.175 mm, respectively. For both scales, the range of Hertzian contact pressures was between 0.41 GPa and 1.2 GPa. In situ video microscopy observations identified that the dominant velocity accommodation mode at macro-scale was interfacial sliding. However, an additional velocity accommodation mode, transfer film shearing, was also observed with higher humidity. Overall higher friction was observed with microtribology compared to macrotribology. The higher coefficient of friction was attributed to three different stages during the sliding process, which were identified with respect to different contact pressures, contact areas, tip shapes, and environmental conditions. The first two stages exhibited a solid lubrication behavior with some combination of interfacial sliding, transfer film shearing and microplowing. The transfer film thicknesses for these stages, normalized to the initial Hertzian contact radius, fell in a range of 0.001–0.1. For the third stage, the dominant VAM was plowing and the normalized transfer film thickness fell below this range. Comparisons between the two scales demonstrated that for dry sliding, microscopic contacts on Ti–MoS₂ deviate slightly from macroscopic behavior, showing higher limiting friction and microplowing. For humid sliding, microscopic contacts deviate significantly from macroscopic behavior, showing plowing behavior and absence of transfer films.