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White etching areas (WEA) and white etching cracks (WEC) are frequently linked to premature bearing failure in conventional high carbon bearing steels like 100Cr6 (SAE 52100). In contrast, no WEA/WEC has yet been reported for the high nitrogen bearing steel X30CrMoN15-1 (SAE AMS 5898). Thus, the present study proves for the first time that X30CrMoN15-1 is also susceptible to develop WEA/WEC under rolling contact fatigue (RCF) when pre-charged with hydrogen. RCF tests conducted in parallel without hydrogen pre-charging resulted in RCF damage only, which identifies hydrogen as an active agent for WEA/WEC formation in X30CrMoN15-1. These findings correspond to the fact that hydrogen diffusion during RCF is often considered to cause or accelerate the formation of WEA/WEC. Additionally, it is observed that the M 2 (C, N) and M 23 C 6 precipitates of the martensitic microstructure of the X30CrMoN15-1 do not entirely decompose during the WEA formation process as observed for M 3 C precipitates in 100Cr6. In conclusion, the results for X30CrMoN15-1 strongly suggest that the formation of WEA is driven by a hydrogen-activated local severe plastic deformation process, which initiates continuous dynamic recrystallisation, leading to the characteristic nano-ferritic grains observed in WEA. Also, the highly stable and self-regenerating passive chromium-oxide layer of X30CrMoN15-1 mitigates the risk of WEA/WEC failure during typical RCF operation by hindering the formation and adsorption of ionic hydrogen. Hence, this study emphasises the importance of protecting the base material against hydrogen ingress to delay WEA/WEC formation. [Display omitted] • White etching areas and cracks (WEA/WEC) under rolling contact fatigue in X30CrMoN15-1 have been proven for the first time. • Testing reveals that hydrogen is an active agent in WEA/WEC formation in X30CrMoN15-1. • Precipitates from X30CrMoN15-1, such as M 2 (C, N) and M 23 C 6 , are also found within the WEA. [ABSTRACT FROM AUTHOR]

Polymer composites are increasingly used as frictional components due to their self-lubricating properties. However, they tend to exhibit severe wear during the running-in stage, thus significantly impairing their service life. In the present work, a dual-pin-on-disk (DPOD) method is developed in order to reduce the running-in wear of a polytetrafluoroethylene/alumina (PTFE/Al 2 O 3) composite when sliding against GCr15 bearing steel by introducing an assistant polymer component during the running-in stage. The results show that the running-in wear rate of the main polymer pin is reduced by up to 77.26 %. This is because the assistant pin causes excess wear debris to be released into the wear track and provides a higher shear force, thereby promoting the tribological chemical reactions and facilitating the generation of tribofilms on the worn surfaces. In addition, the processed polymer achieves a wear rate similar to that of the conventional single pin condition (∼3 × 10−7mm3/Nm) in the subsequent steady-state stage. The results of the present work are expected to increase significantly the wear life span and application prospects of the polymer composites. • The severe running-in wear of the PTFE/Al 2 O 3 composite was reduced by 77.26 % through a dual-pin-on-disk (DPOD) method. • The DPOD method provide excess wear debris and an auxiliary shearing stress to improve the quality of tribofilms. • The DPOD running-in method has little effect on the steady-state tribological performance of the polymer composite. [ABSTRACT FROM AUTHOR]

The wear resistance of metallic shims under heavy load is crucial for the friction behavior of friction damper in civil engineering. This study verifies the potential of the strengthening grinding process (SGP) to enhance the wear resistance of metallic materials under heavy load. A multidisciplinary effort is shown in this paper to investigate the wear mechanism of SGP-treated GCr15 bearing steel in terms of the application scenario of civil engineering. The SGP treatment uses a multiphase abrasive comprising steel balls, corundum powder, and strengthening liquid to impact the surface of the GCr15 bearing steel. Then, a dry friction sliding test was performed on the polished, sandblasted, and SGP-treated GCr15 bearing steel. The microtopography and microstructure of these three friction shims with different treatments before and after the wear test were compared and analysed. The test results indicated the feasibility of SGP in enhancing the wear resistance of GCr15 bearing steel friction shims under heavy loads in terms of the application scenario of civil engineering. The SGP-treated sample had a more stable friction coefficient (0.475) and slighter wear compared to the other two samples due to its enhanced layer with a gradient structure and a thickness of approximately 23 μm. • The strengthening grinding process of GCr15 bearing steel was investigated. • An enhanced layer with a gradient microstructure and surface micro-pits was fabricated. • The friction shim treated by the strengthening grinding process exhibited excellent wear resistance. [ABSTRACT FROM AUTHOR]

In practical applications, the lubricating performance of grease gradually deteriorates owing to the inclusion of foreign contaminants. This study investigates the impact of environmental contaminants, including silicon dioxide (SiO 2) particles and water content, on the degradation of lithium-base grease and its subsequent effect on the surface damage of bearing steel material. The experimental results reveal an interactive effect between water content and particles in grease. The presence of particles and water contamination in separated or mixed conditions leads to increased friction, wear, and pitting compared to the base grease. The wear mechanisms of the two contaminant factors are different as follows: the particles predominantly function as third bodies, which enhance the oil film thickness between contacting surfaces while inhibiting the formation and growth of a protective tribofilm from the base grease, resulting in elevated friction and wear, and severe surface damage. Additionally, the addition of suitable water content in grease can absorb friction heat produced by the particles, mitigating friction and wear. Vibration exhibits behavior similar to friction, where both friction and vibration increase proportionally in the absence of water contamination. This observation suggests that vibration can be used as a valuable indicator for monitoring friction in contaminated conditions. • Presence of environmental contaminants in grease results in high wear and surface damage. • 0.3 wt% water in grease mitigates wear and friction caused by particle contamination. • Particles act as third bodies, while optimal water content absorbs heat, reducing friction. • Vibration exhibits similar behavior to friction under the contamination conditions. [ABSTRACT FROM AUTHOR]

In a recent work, after conducting disc-on-disc tests under the influence of hydrogen, the authors identified microstructure alterations, the so-called dark grooves, as possible initial stages of WEC formation. In this work, the aim is to investigate whether dark grooves also constitute the initial stages of WEC in cylindrical roller thrust bearings (CRTBs) tested under the influence of WEC-critical oils. BEM simulations of the roller/washer contact were conducted to identify the coordinates of the maximum subsurface shear stresses, the region in which the first stages of failure are likely to start. Subsequently, CRTB tests were carried out, where prematurely stopped bearings were characterized considering the BEM results. The initial signs of failure i.e., the dark grooves consisting of nanopores and nanograins, were found in the maximum subsurface shear stress regions indicated by the BEM simulations. • Dark grooves a first step in the white etching crack formation • Dark grooves a microstructural alteration formed by pore-clusters • Dark grooves grow and coalesce giving rise to vertical cracks with very little WEA • Dark grooves found in the most mechanically stressed material regions [ABSTRACT FROM AUTHOR]

A dicationic liquid (DIL) was synthesized and used as a base lubricant. IL-grafted graphene oxides (MGO) were manufactured as an additive to the DIL lubricant. Then, the MGO/DIL was applied as a lubricant for the surface contact between a stainless steel ball and bearing steel. The tribological performance of MGO/DIL was measured and compared to those from a monocationic liquid (MIL) based lubricant. The MGO/DIL lubricant produced better friction and wear performance at room temperature but poor wear at 100 °C. Through novel instrumentation techniques, it could be found that when DIL is exposed to air at higher temperatures, more oxygen is adsorbed into DIL, which accordingly reacts with underlying bearing steel, resulting in critical surface oxidation and wear. • The DIL [(C 1 im) 2 C 6 ][NTf 2 ] 2 lubricants and IL-grafted GO additive were synthesized • Wear and COF of steel alloy were investigated at room temperature and 100 °C • Compared to MIL, the DIL showed better thermal stability but poor friction and wear • Due to its doubly charged cation, the DIL produced severe wear at high temperature [ABSTRACT FROM AUTHOR]

The radial piston hydraulic motor is widely used due to its ability to provide high torque output and stable speed control. However, it exhibits poor lubrication performance in low-speed, heavy-load conditions. Specifically, the friction and wear performance of the piston-roller friction pair directly affect the motor's starting torque, operational efficiency, and service life. To address the friction and wear issues of the piston-roller friction pair, a self-lubricating composite bearing bush is introduced between the friction pairs. The present study investigates the wear and friction properties of self-lubricating material sliding against bearing steel(GCr15SiMn). The materials are neat Poly Ether Ether Ketone (PEEK), PEEK with 10% Polytetrafluoroethylene (PTFE) and15% Potassium Titanate Whiskers (PTW). Additionally, the role of filler (i.e., nano-silica) in the PEEK-PTFE-PTW-bearing steel hybrid wear system is investigated. The composite materials have undergone surface modification using a silane coupling agent (KH550) to enhance the adhesion performance between the filler and the matrix. Imitating the working condition of the piston-roller friction pair, experiments are conducted under boundary lubrication conditions on a ring-to-disc tribometer. The worn surfaces were examined using confocal laser scanning microscopy and scanning electron microscopy. The results indicate that self-lubricating materials filled with 2% and 3% nanoscale silicon dioxide achieved the best tribological performance. • When the amount of wear is too small, the volume wear can be calculated using confocal laser scanning microscopy. • Nanoparticles on the surface of the material can play a role in alienating methylene compounds. • The feasibility of using self-lubricating material for piston-roller friction pair of hydraulic motor is studied. • The formation of a solid-fluid lubrication film with low shear strength in boundary lubrication is the key to reduce friction. [ABSTRACT FROM AUTHOR]

Friction and wear are the main reasons for energy dissipation and the premature failure of mechanical assemblies; thus, it is desirable to achieve extremely low friction and wear in a variety of practical applications. The results in this work demonstrate that 4-dodecylphenol functionalized magnesium oxide (MgO) nanolubricant is capable of achieving extremely low friction (about 0.05) and wear rate (100 μm3/N·m) for bearing steel tribopairs. Magnesium oxide nanoparticles are well dispersed in base oil as 4-dodecylphenol could interact with particles through the phenol group via charge transfer. The superior tribological performance originates because MgO nanoparticles could exist stably between the friction interfaces, preventing the direct contacts of asperity peaks and reducing the real contact area. Furthermore, the interlayered MgO nanoparticles are also beneficial to smoothing the friction surfaces via the micro-polishing process, effectively reducing the specific contact pressure and hindering the occurrence of serious adhesive wear. Careful observation of the wear surfaces on tribopairs demonstrates the role of MgO nanoparticles in tribological performance improvement and the lubrication mechanism. Our result is beneficial to paving the way to the large-scale application of MgO nanolubricants and the realization of extremely low friction and wear. • The 4-dodecylphenol modifying agent could effectively disperse MgO nanoparticles in PAO4 oil. • The tribological experiments demonstrate the superior friction and wear-reducing ability of MgO nanolubricant. • The well-dispersed MgO nanoparticles could not only separate but also micro-polish the friction surfaces. [ABSTRACT FROM AUTHOR]

The aim of this work is to determine the rolling contact fatigue (RCF) strength of steel AISI 52100 (DIN 100Cr6) as a common bearing steel in different heat treated conditions. In order to study and evaluate the steel cleanliness and its influence on the fatigue behavior the results are compared to AISI L3 (DIN 102Cr6) as a cold-working tool steel containing higher amounts of non-metallic inclusions as a consequence of a different metallurgical processing. In this work experimental results obtained from both materials with martensitic and bainitic microstructures are presented. In addition, the inclusion content in each material was analyzed using extreme value statistics, and the endurance limits are predicted based on the maximum expected inclusion in the maximum loaded volume as a critical crack initiating defect. The predicted RCF values are in good agreement with the experimental results. Irrespective of the microstructure, the hardness after heat treating dominates the endurable Hertzian pressure. The second important parameter, which determines the endurance limit, is the cleanliness of the steel. Furthermore, the crack pattern in the surface and microstructure of the RCF specimens were analyzed by optical microscopy. The results show no microstructural modifications such as dark etching regions or white etching bands. However, the formation of butterflies was detected around non-metallic inclusions in both steels. • Systematical study of the classical rolling contact fatigue behavior of steel AISI 52100 and AISI L3. • Determination of the endurance limit at ultimate load cycles of N G = 108. • Investigation of the influence of steel cleanliness and heat treatment conditions on the fatigue behavior. • Prediction of the endurance limit based on the extreme value statistics of non-metallic inclusions. [ABSTRACT FROM AUTHOR]

The dry rolling/sliding wear of nanostructured bainite has been investigated and compared with that of a conventional quenched and tempered bearing steel. In order to elucidate the role of retained austenite on the wear performance, high silicon hypereutectoid bearing steel with an identical alloy composition was heat treated to obtain different microstructures with similar hardness and different amounts of retained austenite. The results indicate that the nanostructured bainite can meet the minimum hardness requirements for bearing applications. Moreover, the nanostructured bainite outperformed the tempered martensitic steel in terms of wear resistance. The work hardening capacity and thus wear resistance increases due to the transformation of retained austenite into martensite. The results of XRD analyses show that the higher stability of retained austenite and strength of bainitic ferrite leads to better wear performance. It is demonstrated that the stability of retained austenite outweigh the influence of retained austenite content on wear resistance. Adhesion and oxidation were identified as the main wear mechanisms. In addition to microstructure, surface oxidation also plays a prominent role in determining the wear resistance. • The nanostructured bainitic steel shows a lower wear rate than that of quenched and tempered steel. • Wear is affected more by the retained austenite stability than the volume fraction. • Surface oxidation significantly affects the wear and friction of the steels. • During the wear process, some globular carbides fractured and voids nucleated. [ABSTRACT FROM AUTHOR]

Abstract Hybrid bearings have rings made of steel and rolling elements made of bearings grade silicon nitride ceramic. Due to the higher modulus of elasticity of ceramic compared to steel, the ellipse of the Hertzian contact is slightly smaller than that in a conventional bearing. This results in about a 12% increase in the contact pressure between the ceramic ball and the raceway. Following the standard Rolling Contact Fatigue theory, a higher contact pressure leads to a reduction of the dynamic load-carrying capacity and service life of the bearing. However, long experience in the use of hybrid bearings shows that, in most applications, hybrid bearings clearly outperform traditional bearings made entirely out of steel. This paper addresses this issue by applying a novel approach to Rolling Contact Fatigue. Central to the new method is the survival probability of the raceway surface which is explicitly formulated into the basic life equations of the rolling contact. This allows tailoring the contribution of the stress system close to the rolling surface to better represent the ceramic-steel interaction, which has been proven to be substantially more favourable in the case of a hybrid ceramic-steel contact. The comparison between experimentally obtained hybrid bearings fatigue lives and lives predicted using the new calculation model indicates good agreement. It is found that an increase of the fatigue resistance of the raceway surface of hybrid bearings can, in most cases, compensate for the additional stress present in the subsurface region of the rolling contact. The new methodology, by providing a direct account of the survival probability of the ceramic-steel interface gives a better representation of the expected performance of hybrid bearings, opening new opportunities for the reliable use of this type of bearing in modern machinery. [ABSTRACT FROM AUTHOR]

White Etch Cracking (WEC) is a severe and unpredictable failure mode that has been frequently observed in, among others, bearings for wind energy applications. In this work a novel, patented, WEC resistant bearing steel solution is developed through high temperature solution nitriding, followed by a custom heat treatment. Conventional medium carbon, 13 wt% Cr martensitic stainless steels were subjected to the novel process to obtain a nitrogen-rich case. A series of soft annealing, austenitization, cryogenic and tempering treatments converted the nitrogen-rich region into a fine-grained case containing nano-sized precipitates. Test bearings and tribometer specimens were treated with the same sequence of treatments. These specimens were tested under two different accelerated WEC failure promoting conditions. In contrast to standard through-hardened bearing steel the nitrided specimens showed no WEC formation, not even after prolonged testing. • A high temperature nitriding process was developed for bearing steel. • An ultrafine microstructure was obtained in stainless steel after nitriding. • Laboratory tests indicated the nitrided steel was immune to white etch cracking. • This replicates the performance of expensive high-nitrogen steels. [ABSTRACT FROM AUTHOR]

Wind turbine gearbox (WTGB) bearings suffer from premature failures through white etching cracks (WECs). To investigate causes of this failure mechanism, we tested two types of 100Cr6 bearing steel components. We carried out rolling contact fatigue experiments on standard washers and material extracted from a WTGB bearing. These differed strongly in morphology and orientation of non-metallic inclusions (NMIs) with respect to the raceway, especially of MnS-type. While the standard washer did not exhibit WECs formation, the WTGB bearing did - even under moderate conditions (1.9 GPa Hertzian pressure, 107 cycles). We therefore conclude that MnS-inclusions are a key factor in initiation of WECs in WTGB bearings. • Novel RCF test rig enables testing of wide range of materials. • WECs are generated in 100Cr6 steel from a WTGB bearing without additional drivers. • Standard 100Cr6 material did not show premature WEC formation. • We attribute this to differences of MnS orientation with respect to the race way. [ABSTRACT FROM AUTHOR]

Surface roughness is significant to friction and wear. However, the influence remains elusive, especially at the nanoscale. This study aimed to explore the effect of the nanoscale surface roughness on sliding friction and wear. For bearing steel tribo-pairs, the surface roughness can affect the lubrication state transition. The lower the surface roughness is, the easier it is to enter mixed lubrication and full film lubrication. Moreover, reducing the surface roughness can decrease friction and wear in mixed lubrication. At 94 mm/s sliding speed, as the surface roughness S q decreases from 769.7 nm to 21.1 nm, the average friction coefficient first remains unchanged and then decreases by 20%, while the wear rate decreases by 85%. However, further reducing the surface roughness may result in friction failure, raising friction and wear. At 10 mm sliding radius, about 20∼40 nm S q can lead to the minimal friction and wear. For the mechanism, as the surface roughness decreases, the asperity contact diminishes. Therefore, friction and wear decrease. However, when the surface roughness passes the critical point, asperities cannot fully accommodate the interfacial wear debris. Consequently, friction failure occurs. The findings may provide an enhanced understanding of the role of surface roughness in tribology. [Display omitted] • The surface roughness can affect the lubrication state transition. • Reducing the surface roughness can decrease friction and wear in mixed lubrication. • Too low surface roughness may result in friction failure, raising friction and wear. [ABSTRACT FROM AUTHOR]

The purpose of this work was to study effects of different grain size distributions on fretting wear behavior of a low carbon steel flat specimen against a bearing steel ball. The grain structure of the low carbon steel was varied to produce a gradient microstructure consisting of fine grains at the surface and coarse grains in the interior. The novel method of surface treatment involved a multi-step technique of compressing a series of hardened steel rods on the surface and then recrystallizing those surfaces. Four similar kinds of microstructures as observed at different depths were reproduced on the surface of the steel samples by changing the heat treatment cycles after deformation. Worn specimens were characterized using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), Electron back scattered diffraction (EBSD) and optical profilometry. Wear resistance at all the loads was found to be highest for the fine-grained steel specimen compared to that of the coarser-grained specimens. The wear resistance of a bimodal microstructure was considerably higher due to the presence of fine as well as strained grains. Higher wear resistance for the finer grain microstructures was attributed to higher degree of interaction with the grain boundary regions. The wear behavior at the surfaces with different microstructures could well be correlated with similar types of microstructures developed at different depths. [ABSTRACT FROM AUTHOR]

The purpose of this work is to demonstrate how a new acoustic emission (AE) technique can be used to monitor friction surface degradation in a four-ball tribosystem under different types of lubrication. The AE method is based on a novel signal spectral categorization technique, and it was used to identify concurrent degradation processes in bearing steel. The correlation of AE features with the development of specific microstructural features on the contact surfaces has been used to identify the AE "signature" of specific damage mechanisms, and thus to monitor the progression of wear. The proposed approach enables the construction of a chronology of lubricant and/or contacting material degradation during tribological testing with a high degree of confidence. Furthermore, it provides an efficient means for automated wear monitoring and for real-time, non-supervised interpretation of the state of wear in a given tribosystem. [ABSTRACT FROM AUTHOR]

As a key component, axle bearings play an important role in transmission systems. Motivated by the rapid growth of railway network and the significance of bearing safety, we report here how different operating duration may alter the macroscopic and microscopic mechanical property of high speed train axle bearing's outer ring made of GCr15 steel, a high-carbon chromium bearing steel known for its superb contact fatigue resistance, good dimensional stability and excellent corrosion resistance. We characterized the structural and mechanical properties of the load carrying zone at operating duration of new, 1.2 Mkm and 2.4 Mkm bearing. In contact to the as-received ones, the outer ring samples show clear grain refinement, exhibit inhomogeneous hardness distribution and show a decrease of about 42% for the tensile elongation after 2.4 Mkm operation. [ABSTRACT FROM AUTHOR]

The purpose of this work was to expand the applications of molybdenum disulfide (MoS 2 ) nanosheet additives by investigating their effectiveness and advantages for lubricating line contacts between bearing steel and medium carbon steel surfaces. The friction and wear effects of MoS 2 nanosheet additives were investigated using block-on-ring tests, and compared with those from zinc dialkyldithiphosphates (ZDDP). Both additives were combined with ISO VG 32 white oil as base fluid. Results suggest 0.25 wt% MoS 2 nanosheets exhibited similar friction coefficient, oil temperature and wear scar width with those of ZDDP at low loads; while the reductions of friction coefficient and wear scar width were 28.6% and 34.3%, respectively, compared to the behavior of ZDDP at high load (459 N). Furthermore, the wear scar widths of the ZDDP containing oil significantly increased in 2 h tests, while they remained constant with a smaller size for 0.25 wt% additives of MoS 2 nanosheets at a load of 326 N. Scanning electron microscope observations showed some pits on the wear scar lubricated with ZDDP, compared with spalls for 0.06 wt% MoS 2 nanosheets. The wear scar was smooth and had a patched film for tests using 0.25 wt% MoS 2 nanosheets. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analysis revealed that the patched surface film was composed of MoS 2 and phosphate, and that film reduced friction coefficient and wear. The results highlight how high concentration MoS 2 nanosheets exhibit significant advantages over ZDDP in reducing friction and wear in high load in line contact. [ABSTRACT FROM AUTHOR]

The dry sliding wear behavior of ZrC particles and carbon fiber reinforced 2024Al matrix composite rubbed against AISI52100 bearing steel was investigated at different loads and sliding velocities. The wear rate of the composite increased with increasing load, while an opposite trend was recorded as the velocity increased. With increasing load and speed, the wear mechanism changed from major abrasion wear, slight adhesive wear, and oxidative wear to dominant oxidative wear, secondary abrasion, and adhesive wear. The improved wear resistance of the composite over that of the 2024Al alloy is attributed to the steady mechanically mixed layer supported by the harder and stronger composites, as well as the self-lubricating carbon fibers. • The wear rate of C f (Ni)–ZrC/2024Al composite increases with the load and decreases as the sliding speed goes up. • The wear behavior of C f (Ni)–ZrC/2024Al composite is controlled by abrasion wear, adhesive wear and oxidative wear. • The improved wear resistance of C f (Ni)–ZrC/2024Al is due to steady mechanically mixed layer and self-lubricated carbon fiber. [ABSTRACT FROM AUTHOR]

Observing the importance of intermetallic materials in mechanical and metallurgical applications, the present work investigates the micro-abrasive wear behaviour in a model–carbide reinforced iron aluminide system. Fe–30Al–6Cr (at.%) iron aluminide alloy with carbon additions " as cast " specimen was tested. Micrographs revealed a continuous network of eutectic chromium carbides at the interdendritic regions of the D03 ordered aluminide matrix. AISI 52100 bearing steel sphere of diameter 25.4 mm (1″) was used for wear tests as counter body. Micro-abrasive slurries were prepared with micro-abrasive particles of black silicon carbide – SiC or alumina – Al 2 O 3 , with distilled water or glycerin, in four possible combinations of materials ⇒ "Al 2 O 3 + H 2 O distilled", "Al 2 O 3 + glycerin", "SiC + H 2 O distilled" and "SiC + glycerin". Further, keeping the normal force constant and together with different levels of micro-abrasive slurries compositions and sliding distances, a factorial experiment was designed. Result analysis showed that wear volume increased with an increase in micro-abrasive slurry concentration, independently of the type of micro-abrasive particle and liquid. However, the micro-abrasive slurries prepared with SiC and distilled water provided larger wear volumes than the volumes of wear reported under the micro-abrasive slurries formulated with Al 2 O 3 and glycerin. The reason is attributed to the high hardness of SiC particles resulting in high abrasion, whereas the Al 2 O 3 – glycerin slurry lubrication effect restricted high wear. Wear micrographs revealed a change in worn surface morphology from " grooving micro-abrasion " to " rolling micro-abrasion " due to an increase in sliding distance and micro-abrasive slurry concentration. • Synergism between concentration , abrasive material and liquid influenced the Fe-30Al-6Cr (at.%) tribological behaviour. • There were transitions between abrasive wear modes as a function of the abrasive slurry composition and sliding distance. • Actuation map of abrasive wear modes was organised as a function of the abrasive slurry composition and sliding distance. • The abrasive slurry composition influenced the severity with which the Fe–30Al–6Cr (at.%) intermetallic alloy was worn. In total, seventy-two (72) micro-abrasive wear tests by rotative ball were performed. Each experiment was carried out without interruption and the micro-abrasive slurry was continuously stirred and dripped between the test sphere and the specimen, during the experimental runs – additionally, the micro-abrasive slurry was not reused throughout the micro-abrasive wear tests, from one experiment to another. [ABSTRACT FROM AUTHOR]

The impacts of the surface ultrasonic rolling process (SURP) on the surface morphology, surface microstructure, residual stress distribution, and rolling contact fatigue (RCF) properties of G20Cr2Ni4A carburized bearing steel were studied in this research. The results show that when the RCF rolling direction is perpendicular to the grinding direction, the fatigue peeling of the raceway surface is more serious. SURP not only reduces surface roughness but also enhances the residual compressive stress and hardness of the surface layer, allowing it to resist the initiation and propagation of surface cracks and enhancing RCF life. Fatigue cracks are easily formed between the ground layer and the matrix, according to microscopic analyses of fatigue crack initiation and propagation on the original sample surface. These cracks have the potential to spread down the ground layer, causing the shallow layer of approximately 1 μm to spall off. They also promote crack propagation into the matrix and shorten fatigue life. SURP can cause the ground layer to peel off, making fatigue cracks on the surface more difficult to form and enhancing contact fatigue life. • SURP changes the initiation position of RCF cracks in bearing steel. • SURP improve the ability of internal microstructure to resist crack propagation. • SURP makes the tempered layer undergo plastic deformation and strengthened. • SURP can reduce surface ground defects. [ABSTRACT FROM AUTHOR]

A copper-based carbon quantum dots (Cu-CQDs) and its films on the surface of steel discs were prepared successfully by the hydrothermal method and simply immersion into Cu-CQDs solution method, respectively, in order to develop a functional additive and high performance of bearing materials. Thus obtained Cu-CQDs has an average particle size of approximately 10 nm. The physical and chemical properties of the prepared Cu-CQDs and its films were studied using various modern instruments like SEM, RAMAN, XRD, etc. An UMT-2 tribometer was conducted to investigate the tribological behavior of lubricated thin films. Here, the films played an important role in modifying of friction and wear properties of bearing steel discs. The lower friction coefficient and wear rate of film-coated steel discs were achieved lubricated with 0.5 wt% Cu-CQDs. The tribological mechanisms of Cu-CQDs and films were ascribed to the formation of lubrication films including the components of Cu-CQDs and the friction-induced formation of transfer film, respectively. • A novel self-assemble Cu-CQDs film was successfully prepared via simply immersion method. • The tribological behavior of Cu-CQDs film lubricated with the different media was evaluated. • The Cu-CQDs film has excellent wear resistant and friction reduction properties. • Friction and wear mechanism of Cu-CQDs film is ascribed to the formation of the transfer film. [ABSTRACT FROM AUTHOR]

Comparative experiments were performed to demonstrate how the lubricated sliding friction and wear behavior of a bearing steel can be affected by the type of relative motion. Data from rotary motion (ball-on-disk tests) were compared to that for reciprocating motion (high frequency ball-on-flat tests) using the same contact pressure, average velocity, temperature and humidity. Two lubricants were compared. The first was aqueous glycerol and the second was aqueous glycerol containing 0.1% w nanodiamonds. The aqueous glycerol solution showed superlubricity effects in rotary testing because its friction coefficient was lower than 0.01. This effect was attributed to a stable protective matrix comprised of free and bound water layers in aqueous glycerol and a thick film lubrication condition. Under reciprocating conditions, it is proposed that the protective matrix was damaged due to the repetitive application of compressive stress and boundary lubrication condition. As a result, there was no superlubricity behavior observed in reciprocating tests. In the rotary test method, the nanodiamond additives (dispersed in glycerol) and wear debris accumulate at the contact zone. That mixture grows into a large abrasive particle mix. However, during reciprocation there are fewer nanodiamonds and wear debris particles in the contact zone. They polish and smooth the surface, as indicated by the post wear surface images. The abrasive nature of nanodiamonds increases the micro polishing effect and thus decreases friction, significantly. It is shown that the lubricity behavior of a lubricant and additive can be influenced by the direction of motion used in the test method. [ABSTRACT FROM AUTHOR]

The dry sliding friction and wear behavior of Cu-MoS 2 composite pins against bearing steel (AISI 52100) discs were investigated in order to determine the optimal additive content. The MoS 2 content ranged from 0–40 vol%. The microstructure and composition of the worn surface, wear debris and transfer layer were observed and analyzed in detail to reveal the dominant friction and wear mechanisms. It is confirmed that MoS 2 is an effective lubricant for copper matrix composites against steel because the friction coefficient decreased from 0.67 to 0.18 when adding 20 vol% of the MoS 2 . The wear rates of the composites tended first to increase then decrease as the MoS 2 content increased. The high wear rates of Cu-5 vol% MoS 2 and Cu-10 vol% MoS 2 composites are mainly attributed to the cracks associated with the MoS 2 particles, and the decrease of wear rate for composites with MoS 2 above 20 vol% is derived from the formation of a nearly continuous lubricating film on the worn surface. The formation mechanism for MoS 2 lubricating film has been clarified by observing its microstructure and characterizing its distribution in the worn surface. [ABSTRACT FROM AUTHOR]

The purpose of this investigation was to study microstructural evolution leading to the formation of wear debris and white etching areas (WEA) in the subsurface of AISI 52100 steel during rolling contact fatigue (RCF). The specimens were tested using a roller-on-roller geometry testing machine. Surface damage was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Lamellar structures generated during the rubbing of crack faces give a clear picture of how wear debris is formed. In contrast to the previously reported WEA, which is composed of nano-ferrites generated under the mechanism of recrystallization, the WEA produced in the current RCF condition consists of an amorphous phase with nanocrystallites dispersed. Based on this, it is proposed that WEA can be classified into deformed WEA and transformed WEA depending on whether phase transformation occurs. In addition, the relationship between white etching crack (WEC) and WEA was discussed in detail. The result suggests that WEC does not initiate WEAs as it has been accepted. Instead, they form under different mechanisms. [ABSTRACT FROM AUTHOR]

Laser alloying with boron and solid lubricants (CaF 2 or BaF 2 ) was used in order to produce the self–lubricating layers on 100CrMnSi6–4 bearing steel. The surface of the base material was coated with a paste consisting of alloying material blended with a diluted polyvinyl alcohol solution. Then, this surface was re-melted by the laser beam using TRUMPF TLF 2600 Turbo CO 2 laser. The produced layer consisted of the two zones: the re–melted zone (MZ) and the heat affected zone (HAZ). The re–melted zone was composed of eutectic mixture (iron borides with martensite) and CaF 2 or BaF 2 particles, locating close to the surface. In the HAZ, the martensite, retained austenite, pearlite and sorbite were observed. The layers were free of cracks or gas pores. The microhardness of the re–melted zone was equal to about 800 HV and was lower than that-characteristic of the laser-alloyed layer only with boron. During the wear resistance test, the tribofilm was created on the worn surface. This tribofilm consisted of the calcium fluoride or barium fluoride, which was smeared on the surface. This way, the solid lubricant reduced the mass loss of mating parts and improved their tribological properties. [ABSTRACT FROM AUTHOR]

Bearings are widely used in different industries which demand different quality requirements to ensure a long life with low wear rates. In this sense, bearing components should present combination of low surface roughness and tight dimensional deviations, as well as good lubricant retention capacity. So, grinding is an alternative to achieve this combination, however, one of the challenges is the large amount of heat generated in the grinding wheel-workpiece interface when using conventional abrasives grinding wheels, which can lead to surface finish deterioration and thermal damages, thereby reducing the component's life. Thus, several cooling-lubrication techniques have been tested in the past decades to enhance the cooling-lubrication properties of cutting fluids, with a highlight in recent years for the benefits of adding solid particles with good tribological properties to improve grinding efficiency. In this context, this work sought to evaluate the roughness and surface morphology of the bearing steel SAE 52100 (60 ± 2 HRC) after grinding with multilayer graphene (MLG) platelets dispersed in two different cutting fluids (synthetic and semi-synthetic). Tests with traditional MQL (without solid particles) and conventional techniques (flood) were also carried out for comparison. The output variables investigated were surface roughness parameters Ra, Rz, Rku, Rsk, Rk, Rpk, and Rv, images of machined surfaces through a scanning electron microscope (SEM), and the specific grinding energy. The results indicated that the base fluid type significantly affected the grinding performance. Tests with semi-synthetic fluid having MLG decreased the Ra parameter of the ground surface by 9% compared to the traditional MQL technique, with a higher Rvk and negative skewness. For the synthetic fluid also with MLG, about 29% increase in the Ra parameter was observed compared to MQL without nanoparticles, although it resulted in higher values of Rpk with more positive skewness. Presence of MLG in semi-synthetic and synthetic fluids reduced the specific grinding energy by 7% and 14%, respectively. Also, less intense plastic deformation and material adhesion was observed when grinding with MLG. • The type of base fluid strongly influenced grinding performance of the multilayer graphene (MLG) nanofluid applied with MQL technique, indicating a change on nanoparticles action mechanism. • For the vegetable-based semi-synthetic base fluid, MLG addition favoured tribo-films formation at the contact zone, thereby improving surface finishing and lubricant retention capacity of ground surface. • Wider and deeper grooves were observed at the machined surface after grinding with MLG dispersed in the synthetic fluid even though reducing the specific grinding energy, thus indicating an abrasive-like behavior of MLG nanoparticles. [ABSTRACT FROM AUTHOR]

Atomic layer deposition (ALD) was employed to deposit highly conformal and uniform nanolaminate coatings of ZnO/Al 2 O 3 /ZrO 2 /Al 2 O 3 on M50 bearing steel substrates for sliding wear testing, as well as on CrN coated M50 bearing steel cylindrical rods and cups for three-ball on rod rolling contact fatigue (RCF) testing. The nanocrystalline ZnO top layer was structurally-engineered to achieve low surface energy, textured (0002)-oriented grains. To achieve this preferred orientation, amorphous Al 2 O 3 was deposited as a seed layer, while crystalline ZrO 2 acted as a high toughness/load bearing layer. The ALD nanolaminates reduced both the sliding wear rate (2 × 10-7 mm3/N·m) and enhanced the RCF resistance of M50 steel out to 6 million rolling contact fatigue cycles with no signs of spall. Based on cross-sectional transmission electron microscopy studies inside the wear surfaces, it was determined that sliding and rolling-induced plastic deformation was observed in ZnO layer where basal plane stacking faults were sheared parallel to the sliding and rolling directions. This intrafilm shear velocity accommodation mode mitigates the friction, reduces wear, and inhibits brittle fracture. Therefore, ALD ZnO/Al 2 O 3 /ZrO 2 /Al 2 O 3 nanolaminates are candidate coatings that provide both sliding and RCF resistance in moving mechanical assemblies that require thin (∼10-300 nm), uniform and conformal solid lubricants. • Atomic layer deposition of ZnO/Al 2 O 3 /ZrO 2 was successful on M50 bearing steel. • The top ZnO layer acted as a solid lubricant during sliding and rolling contacts. • Low sliding wear rates down to 2 × 10-7 mm3/N·m was achieved after annealing. • Improved rolling contact fatigue resistance was exhibited with the oxide coatings. • Diffraction and microscopy techniques revealed solid lubrication mechanisms. [ABSTRACT FROM AUTHOR]

The effects of laser texturing technology on the tribological performance of tapered roller bearings in terms of fatigue life, friction, and wear are studied for the first time. The influence of diameter, height and density of the circular-shaped dimples is thoroughly analyzed, first on flat samples, and the most encouraging results are upscaled to components. This research demonstrates the advantages that surface texturing can bring to the most demanding applications involving rolling surfaces under lubrication regime. The experiments show a torque reduction up to 18% and very promising results in fatigue life tests. • Textures have been successfully manufactured on laboratory specimens and finished bearings by ultra-short pulse laser. • Dimple design affects the final tribological performance, being height the most critical feature: the lower height the worse. • A tribological improvement up to ∼18% and ∼8% at variable low speed & load, respectively, was achieved. • A ∼18% improvement in high-speed friction test has been achieved compared to untextured finish product. • Promising behavior on fatigue life was observed, producing a life enhancement under high-demanded test conditions. [ABSTRACT FROM AUTHOR]

This study explores the feasibility of employing MAX phase composite as impregnated solid lubricants and/or as bulk material for advanced high temperature tribological applications. The development of microstructure of Ti-based MAX phase composites Ti 3 SiC 2 –TiC and Ti 3 AlC 2 –Ti 2 AlC fabricated by spark plasma sintering and their dry sliding tribological properties against bearing steel, Si 3 N 4 , and Al 2 O 3 counterfaces were investigated using a ball on disc tribometer at 400 °C. An orientation relationship between the Ti 3 SiC 2 matrix and TiC particle was established wherein Ti 3 SiC 2 /TiC interphase is a coherent boundary from which Ti 3 SiC 2 grows epitaxially from TiC. Against steel, the MAX phase composites – in stark contrast to Si 3 N 4 and Al 2 O 3 – exhibited a negative wear behaviour due to the excessive wear and subsequent sintering of the transferred Fe particles at the sliding interface. Intrinsic lubrication mechanisms involving tribo-oxidation, tribochemical reaction, and mechanochemical reaction played a vital role in the friction and wear properties. The friction and wear properties of the Ti 3 AlC 2 –Ti 2 AlC MAX phase composite was superior to the Ti 3 SiC 2 –TiC composite owing to better oxidation kinetics governing the oxide growth and their retention. Evidently, the wear of the MAX phase is expected to proceed upon extensive deformation incorporating a range of microscale micromechanisms. • Ti 3 SiC 2 /TiC interface is a coherent interface with low interfacial energy. • Tribo-oxidation enhances tribosintering • Oxidation kinetics plays a role in tribofilm spallation. • The role of counterface material was studied. • Wear of MAX phase proceeds by deformation. [ABSTRACT FROM AUTHOR]

In electric vehicles' powertrains, bearings can be susceptible to failure more rapidly than some of the other components mainly because of accelerated fatigue, corrosion and wear. Specifically, due to much severe operating conditions (including high speeds, torques, temperatures) of such vehicles combined with the presence of an electrical environment in which uncontrollable or stray electrical currents discharge through the contact interfaces can further exacerbate the situation and reduce their service lives by triggering much severe adhesive, abrasive and oxidative wear. In this work, we explore the effects of DC current discharges through the contact interfaces of AISI 52100 bearing steel using a novel two-electrode cell-based micro-abrasion tester under electrified dry and boundary lubricated contact conditions. A mineral base oil (as a reference), a gear oil and an automatic transmission fluid (ATF) with different dielectric strengths were tested under unelectrified and DC electrified (1.5 A) conditions. Overall, our test results suggest that passing DC current increases coefficient of friction (CoF) and wear of dry sliding contacts while it slightly reduces CoF but increases wear significantly for lubricated contacts due to accelerated oxidation by electrification. SEM, Raman spectroscopy and non-contact optical profilometry were used for analyzing the physical and chemical nature of worn surfaces and understanding of the specific wear modes and mechanisms. • DC current causes significant alterations in CoF and wear of bearing steel. • Wear increased by electrification due to a more rapid and potent oxidation of the surfaces. • The oxides produced by dry and lubricated sliding corresponded to hematite (α-Fe2O3). • A relationship of dielectric strength of oils to CoF and wear is presented. • The novel modified micro-abrasion tester was useful to simulate electrified lubricated sliding contacts. [ABSTRACT FROM AUTHOR]

The degradation of parts by wear is inevitable in service, but a reduced rate is encouraged by materials with high abrasion resistance. This paper investigates the scratch resistance of a supersolidus liquid-phase sintered, X190CrVMo20-4-1 martensitic stainless steel densified in a vacuum, and a nitrogen atmosphere after heat treatment. Micromechanical properties of the constituents of the steel samples obtained via nanoindentation supplemented the scratch experiments. Worn grooves on the steel samples were examined using a confocal laser scanning microscope and a scanning electron microscope. A good agreement was among the mechanical properties, scratch hardness, specific scratch wear rate, and instantaneous coefficient of friction of the various microstructural constituents in the steel samples. Nitrogen-sintered steel sample, deep cryogenically treated, had a better scratch resistance owing to the presence of a eutectic network of M 7 C 3 carbides and M(C, N) carbonitrides in a hardened, martensitic matrix. An effective load transfer between the hard phases and the matrix is essential for the excellent abrasive resistance of the steel. • Tribological characteristics of martensitic stainless steel is influenced by processing environment and heat treatment. • Superior wear resistance by slipping is achieved with higher matrix hardness to support hard phases. • Higher COF is encouraged by excessive sticking of less hardened matrix and carbide fracture. • Scratch depth profile is influenced by the load bearing capability of the steel based on their heat treatment. • Deep cryogenic treatment influenced an overall increase in the load bearing capability of tool steel. [ABSTRACT FROM AUTHOR]

During the grinding process, a large proportion of the cutting energy is converted into heat. Since not all the heat can dissipate into the cooling lubricant, the material structure of the workpiece can suffer from thermally-induced damage, which has a negative effect on component quality. The generation and distribution of heat is influenced by the change of the grinding wheel topography due to wear. A means of wear analysis for grinding wheel topographies has been developed to describe and explain the wear condition of the grinding wheel and is presented in this paper. For this purpose, creep feed grinding investigations were conducted on bearing steel with electroplated cBN grinding wheels with two different grain specifications. Static and kinematic topography parameters describing the topography changes of the grinding wheels due to wear and their influence on the thermo-mechanical load were investigated in the tests. The static parameters such as the Abbott-Firestone curve or the wear volume make it possible to quanitfy the wear mechanisms. By determining the kinematic topography parameters such as engagement areas or angles, the change in engagement conditions could be derived, which is important for determining the heat flows into the workpiece. • Identification and quantification of wear mechanisms of cBN grinding wheels during creep feed grinding. • Change in the thermo-mechanical load depending on the wear behavior. • Change of static and kinematic topography parameters during wear-induced topography change. • Explanation of the relationships between the grinding wheel wear and the thermo-mechanical load for cBN grinding wheels. [ABSTRACT FROM AUTHOR]

The focus of this work was to evaluate the use of a WC/a-C:H coating, specifically designed to withstand rolling contact fatigue, to mitigate micropitting damage on through-hardened AISI 52100 bearing steel. This was accomplished by conducting experiments with various combinations of coated discs and rollers using a three-disc-on-roller rolling contact fatigue machine. In addition to testing coated and uncoated contacts, discs and rollers with three hardness levels and different surface roughness values were also evaluated. Upon the completion of this work, it was observed that the onset of micropitting damage was affected by the hardness of the asperities on the mated roller and disc samples. It was also observed that the coated surface can cause abrasive wear damage on an uncoated surface if the coating is applied to a surface that has a roughness R q ≥0.5 μm. If the coating is applied to a smoother roller surface ( R q ≤0.26 μm), the coating is able to mitigate micropitting damage on the roller surface without causing high abrasive wear rates on the disc surfaces. [ABSTRACT FROM AUTHOR]

Oxidation is critical to metal chemical mechanical polishing (CMP). In this work, the role of oxidation in the material removal of GCr15 bearing steel during CMP was thoroughly investigated. Six representative oxidizers were selected to acquire the material removal rate (MRR) data. There are three distinct MRR trends. The oxidizers can be divided into three categories based on the MRR and surface composition results. For the first, with the concentration of H 2 O 2 or KIO 3 increasing, the MRR first sharply increases, then decreases and stabilizes; For the second, as the concentration of KClO 3 or NaClO increases, the MRR first increases and then levels off; For the third, with the concentration of K 2 S 2 O 8 or Fe(NO 3) 3 increasing, the MRR almost linearly increases. According to the reaction kinetics and steady state, a fundamental MRR model considering iron oxidation and removal processes was built, and then applied to the three categories of oxidizers combined with the specific oxidation paths. The fitting results agree with the MRR data. Moreover, the key to realizing efficient removal of GCr15 steel is Fe2+ for H 2 O 2 , KIO 3 , KClO 3 and NaClO, while Fe 3 O 4 for K 2 S 2 O 8 and Fe(NO 3) 3. The findings can deepen the understanding of oxidation in metal CMP. • The six representative oxidizers for GCr15 steel CMP can be divided into three categories. • A fundamental MRR model considering iron oxidation and removal was built. • The key to efficient removal of GCr15 steel is Fe2+ for H 2 O 2 , KIO 3 , KClO 3 and NaClO, while Fe 3 O 4 for K 2 S 2 O 8 and Fe(NO 3) 3. [ABSTRACT FROM AUTHOR]

In rolling contact bearings, contact occurs between the rolling surfaces over a small region. Hence, high contact stresses are induced in the region even under a small load. These stresses are cyclic in nature. Over a long period of time, the surface gets fatigued and surface fatigue wear ensues. The experimental investigation carried out in the laboratory earlier consumed a long time. Therefore, a numerical approach is resorted to in this paper. To study this aspect, the stress field in the contact surface region is analyzed under elastohydrodynamic lubrication with line contact considering the friction effect. The bearing steel microstructure is considered as a mixture of martensite and retained austenite having entirely different mechanical properties and yield behavior. Using finite element analysis, the stress field has been evaluated taking into account two volumetric fractions (8% and 20%) of retained austenite in heat treated AISI 52100 bearing steel. A simplified approach neglecting the effect of strain induced transformation of austenite is attempted in the first phase. The depth at which the failure is expected to occur is compared with the experimental values of earlier investigation. The predicted values are marginally higher than the experimental values. [ABSTRACT FROM AUTHOR]

Abstract: Current study has been carried out to compare the development of microstructure in deformed zone of M50 bearing steel during abrasion. Three-body abrasive wear was carried out using abrasive test apparatus as per ASTM G 65-85 with applied load of 13kg. Silica sand of dimension 200±50μm was used as abrasives. The effect of deformed features in abrasion resistance has also been delineated. Crystallographic lath pocket oriented sheared features were observed after abrasion of martensite. Very less fractions of crystallographic sheared features were observed after abrasion of ferrite. The abrasion did not produce dislocation cell structure in ferrite. Ferrite associated just more dynamic recovery while martensite was concomitant to greater dynamic recrystallization. Major toughness enhancement by abundant sheared features and dynamic recrystallization in martensite resulted in higher abrasion resistance of martensite than ferrite. [Copyright &y& Elsevier]

In this study, the combined treatment technology of shot peening (SP) and ultrasonic surface rolling (USR) were applied to the surface modification of AISI 52100 steel. The results revealed that SP, USR and combined treatment effectively improved the microhardness and residual stress of the alloy; however, SP and combined treatment were often accompanied by increased surface roughness. A nanocrystalline structure layer was prepared on the surface of the material through combined treatment, and the reason for the formation of the nanocrystalline structure layer was explained in detail. Compared with ceramic beads, the combined treatment of SP with steel beads was beneficial. The surface hardness and residual stress were improved while the minimum surface roughness was obtained. Under this process, the RCF life was significantly increased. Studies have shown that with the increased of RCF life, crack initiation transferred from the surface to the subsurface, and the crack depth increased significantly. • Improve the rolling contact fatigue life of bearing steel through combined treatment. • Dislocation entanglement induces grain refinement. • The depth of crack propagation increases with the increase of fatigue life. • The large-angle crack propagation is beneficial to improve the fatigue life. • Surface roughness is the most important factor affecting rolling contact fatigue life. [ABSTRACT FROM AUTHOR]

White Etch Cracking (WEC) is a severe and unpredictable failure mode affecting bearings in various industrial sectors. In this work, accelerated WEC laboratory tests have been performed using FE8 type test rigs with ceramic rollers to test the WEC resistance of different bearing materials, materials quality and roughness. It is demonstrated that the test method can reliably and consistently provoke WEC in commercially available washers. Tests using washers with different roughness values did not show significant changes in the time to failure. Tests of through hardened bearing steel with a low content of inclusions resulted in a significantly longer time to failure compared to the baseline. Through hardened washers with a black oxide coating did not improve the WEC life as the coating was worn away during testing. Tests with two types of carbo-nitrided washers gave significantly longer time to failure, of which one type in particular showed high resistance towards WEC formation. • Tests of White Etching Crack resistance for several bearing materials was conducted. • Bearing steel with low inclusion content showed improved performance. • Carbo-nitrided steels showed significant improved performance. • Black oxide coating was worn off during testing and did not improve performance. • Lower roughness did not show improved performance. [ABSTRACT FROM AUTHOR]

Abstract: Friction and wear behaviors of W–C:H coatings with different tungsten contents sliding against bearing steel balls at different air humidities were investigated. The worn out surfaces of steel balls and coatings were analyzed with the aid of scanning electron microscopy (SEM) and Raman spectroscopy. A tribolayer composed of a graphite-like material mixed with tungsten and iron oxides was observed on the friction surfaces of the steel balls. The chemical and phase compositions of the tribolayer, which depend both on the tungsten content in coatings and air humidity, determine the tribological properties of the W–C:H coating in a frictional contact with bearing steel. At average air humidity (50%), those coatings that contain less than 10at% of tungsten in a frictional contact with steel exhibit favorable tribological properties. The friction coefficient of frictional contacts under test reaches a low value (f∼0.01) at a low air humidity and increases with humidity of up to ca. 0.2. The best tribological properties in a wide range of air humidity (5–90%) have been found for W–C:H coatings with the tungsten content between 2 and 5at%. [Copyright &y& Elsevier]

Abstract: In the present investigation a series of orthogonal hard turning tests were conducted to study the effects of tool wear and cutting parameters (cutting speed and feed rate), on white and dark layer formation in hardened AISI 52100 bearing steel, using PCBN inserts. Experimental results were presented including quantification of tool wear and microstructure analysis of the machined surfaces. The experimental results were compared with a newly developed finite elements (FE) model that enables to capture the effect of cutting conditions and tool wear on the microstructural changes occurring at the machined surface. The results showed that cutting regime parameters and, especially, tool wear affect noticeably white and dark layers formation. [Copyright &y& Elsevier]

Abstract: The non-lubricated, ball-on-flat sliding friction and wear properties of in situ (TiB2 +TiC)/Ti3SiC2 composites against bearing steel ball were investigated. The load was varied from 10 to 30N. The material without TiB2 shows an increasing friction coefficient with the increasing load. However, for (TiB2 +TiC)/Ti3SiC2 composites, the friction coefficient shows an insensitivity to load. Both friction coefficient and wear rate initially increase and then dramatically decrease with TiB2 content increasing from 0 to 20vol%. (TiB2 +TiC)/Ti3SiC2 composites with 15vol% and 20vol% TiB2 show excellent wear resistance, whose friction coefficient and wear rate are much lower than those of TiC/Ti3SiC2 composite. The enhanced wear resistance is mainly attributed to the facts that the hard TiB2 and TiC particles nail the surrounding soft matrix and decentralize the shear stresses under the sliding ball, a self-lubricating oxide debris forms on the wear surface, and the wear mode converts from adhesive wear to abrasive wear. [Copyright &y& Elsevier]

Rolling-sliding contact fatigue experiments were performed on through hardened (TH) 100Cr6 and surface induction hardened (SIH) C56E2 bearing steels to study the effect of heat treatment procedure and hardness difference on their wear and/or surface-initiated damage. The heat-treated microstructures included tempered martensite without the presence of retained austenite. It was found that initial stress distribution below the surface of TH specimens remains close to zero with respect to the depth, while, employing SIH resulted in relatively high compressive residual stresses. The wear damage of the specimens was characterized for negative ΔH (i.e., specimen's hardness minus counterpart's hardness) subjected to rolling-sliding contact in a twin-disc configuration operated under a mixed lubrication condition. According to the wear mechanisms and damages assessed, three regions were distinguished: i) mild wear and onset of micropitting, ii) transition region from mild to severe micro-pitting and iii) severe micro-pitting wear. Decreasing the ΔH resulted in a gradual increase in the wear rate of TH specimens, while, the increase in the wear rate of SIH specimens was delayed; with a same absolute hardness, TH specimen with a ΔH of −84 HV already reached the third wear region, while a SIH specimen with a ΔH of −150 HV still operates in the second region. Inspection of the affected subsurface unveiled the response of worn microstructures to the etchant and how the micro-cracks could possibly form within the wear affected zone. The results obtained were mainly explained based on the state of the residual stresses, possible contributions of the microstructural features, and wear behavior (material removal) of the specimen. • Rolling-sliding contact fatigue of Through Hardened (TH) 100Cr6 and Surface Induction Hardened (SIH) C56E2 bearing steels. • Influence of the residual stresses on the wear performance of the bearing steels. • Specific wear rates were plotted as function of hardness difference between the specimen and counterpart. • Study of the surface and subsurface of the wear tracks. • Superior wear performance of SIH treated medium carbon bearing steel (DIN C56E2). [ABSTRACT FROM AUTHOR]

The tribological properties of ionic liquids (ILs) have been widely investigated, and their excellent performance has been acknowledged by many researchers. However, their lubricity is closely linked to operating conditions. For wider applications, the complexity of their behavior must be understood. In this study, we compared the tribological properties of ILs at temperatures of 30 °C and 80 °C. Two ammonium ILs with different cation side-chain lengths, methyl-trioctylammonium [N1888] and tributylmethylammonium [N4441] and the same anion bis(trifluoromethylsulfonyl)imide [NTf2] were selected. Their tribological properties were investigated in the lubrication of three different friction pairs: bearing steel–bearing steel E 52100, bearing steel–stainless steel 304, and bearing steel–aluminum alloy 6082 T6. Tribotests were performed using a ball-on-plate reciprocating tribometer for 30 min at 4 N, 15 Hz, and a 1 mm stroke length. The worn surfaces were analyzed using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated diminished lubricity at higher test temperatures. The performance difference between the investigated temperatures depended on the friction pair, being almost negligible in bearing steel–bearing steel friction pairs and significantly different for bearing steel–aluminum friction pairs. The wear reduction ability was more sensitive to the test temperature than friction did. The investigated ILs were superior to the 5W–40 motor oil at both temperatures. [Display omitted] • Tribological performance on ILs decreased with increased test temperature. • The wear reduction ability of ILs was more sensitive to the test temperature compared to friction. • Tribofilm formation was faster at the low test temperature. • The lubricity of the investigated ILs was superior to that of 5W–40 oil at both temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The C–W2B5 composite was fabricated by reaction hot-pressing of B4C and WC powders. The effect of applied load on the friction and wear behavior was investigated by the block-on-ring test in air. In particular, the composition and microstructural characterization of the mechanically mixed layers (MML) and wear debris formed during sliding wear were studied using scanning electron microscopy (SEM) with EDX and X-ray diffraction (XRD). The results revealed that the friction coefficients and wear rates of the composite increased with increasing testing load. The MML and wear debris generated from the sliding system had similar microstructural features and were composed of a mixture of ultrafine grained structures which mainly consisted of W2B5, Fe and Fe2O3. The formation mechanism of the MML and their influences on wear mechanism of the multiphase material were also studied on the basis of the microstructural observations. [Copyright &y& Elsevier]

If an isotropic rough surface is intersected with a plane near the highest points of the surface, the resulting intersection will comprise a set of approximately circular islands whose size distribution gives some information about the nature of the contact process for the rough surface. Statistical arguments are used to predict the distribution of line segment lengths obtained when a random straight line intersects a random distribution of circular islands. The resulting expression leads to an Abel equation, which can be inverted explicitly to predict the distribution of island radii from the distribution of length segments above the given height in a two-dimensional profilometer output.The resulting expression is tested by comparing its predictions with the bearing area island distribution for an isotropic fractal rough surface defined by the random mid-point displacement (RMD) algorithm. Also, by adding extra scales in the RMD algorithm, the effect of measurement resolution on the mean island radius and the predicted distribution is explored. [Copyright &y& Elsevier]

The selection of bearing steel surfaces for use with silicon nitride rolling elements within hybrid bearings is critical to the performance and life of such components, which have potential applications in advanced high speed aircraft. The wear and friction performance of these combinations is a major factor currently being considered for the next generation hybrid bearings. This paper reports on hybrid bearing contacts that have lubricated Si3N4 elements, which have been loaded against various bearing steels under pure sliding contact conditions on a fully instrumented pin-on-disc wear test rig. The wear and friction performance of Si3N4 has been compared to a baseline case of bearing steel M50 ball sliding against a M50 disc. Both hybrid and steel on steel contacts were lubricated by an aircraft engine oil Mobil Jet II. Wear mechanisms were determined by post-test analysis of the pin wear scars, disc wear surface and wear debris using optical microscopy, surface profilometry and FEG-SEM (scanning electron microscopy). The wear rates of Si3N4 sliding against different bearing steels are ranked by performance and related to their wear mechanisms, hardness and microstructure. Typical sliding contact wear mechanisms were found for the steel on steel combination while Si3N4 sliding against steel showed that transgranular and sub-micron-cracking mechanisms predominate. Evidence of material transfer (steel onto the silicon nitride) was found. Friction values for the various combinations are also reported and found to be substantially lower (μ=0.04) than bearing steel on bearing steel combinations (μ=0.17). The disc and pin wear was monitored on-line by an electrostatic wear sensor, LVDT and laser displacement probe, a friction strain gauge, and an infrared thermometer. Correlations between wear rate and charge generation/level, friction, contact temperature, and disc hardness are presented. [Copyright &y& Elsevier]

Experiments are conducted for investigating the effect of normal pressure, sliding amplitude and oscillating sliding frequency on friction, wear coefficient and threshold energy for wear activation of 52100 high carbon bearing quality steel (ASTM A295) during a false brinelling test. The cylinder-on-cylinder set-up is used. The investigation showed that the friction coefficient decreases with increasing normal pressure and increases with increasing sliding amplitude; however, there is no influence of an increase in oscillating sliding frequency. Similarly, the wear coefficient decreases with increasing normal pressure and increases with increasing sliding amplitude; however, unlike the friction coefficient, it increases with an increase in oscillating sliding frequency. Different conditions do not have a significant influence on the threshold energy for wear activation. By incorporating a varying friction and wear coefficient with changing sliding parameters, the analytical prediction for false brinelling are improved by 10–50% over the constant parameter model. • The effect of varying conditions on friction, wear coefficient was investigated. • The effect of different parameters on threshold energy for wear activation was investigated. • The results were then used to improve and validate the false brinelling models developed previously. [ABSTRACT FROM AUTHOR]

In this study, the tribological properties of an AlSi10Mg alloy that was fabricated using laser powder bed fusion, an additive manufacturing technique, was investigated in both the as-built (but stress-relieved) and T6-treated condition. The stress-relieved condition consists of elongated Al grains and a dendritically-arranged nano-crystalline Si-phase. T6 treatment, a conventional heat treatment for cast AlSi10Mg alloys, caused this Si phase to dissolve, re-precipitate and coarsen, which resulted in a loss of hardness, toughness and strength. Dry sliding wear tests were conducted against both a bearing stainless steel and a much harder SiC, and with the sliding direction either perpendicular or parallel to the building direction. In almost all conditions it was found that the T6 treatment either had no statistically significant effect on the wear rate or was detrimental. • In the stress-relieved condition, the wear performance is isotropic. • Wear anisotropy occurred in the T6 condition. • The wear rate against steel is more severe than against SiC. • Frictional response is not necessarily indicative of wear performance. • T6 treatment is generally not recommended for sliding wear performance. [ABSTRACT FROM AUTHOR]

The aim of this study is to study the wear performance of maraging 18Ni-300 steel, fabricated via selective laser melting, with respect to the building direction. The wear resistance of the alloy was investigated with various sliding velocities, including high sliding speeds up to 1 m/s. The samples were subjected to ball-on-disk type wear tests using AISI 52100 high-carbon-steel balls as the counterpart material both in the as-built and post heat-treated states. Abrasive wear was revealed to be the dominant wear mechanism at the relatively low sliding speed range of 25–100 mm/s. Under these wear conditions, the wear of the maraging steel induced by selective laser melting was nearly isotropic. Contrastingly, at the relatively high sliding speeds of 500 and 1000 mm/s, the plastic flow of the maraging steel manifested near the worn surface, and the delamination of the hardened area became the predominant wear mechanism. In these cases, the wear resistance of the maraging steel was significantly influenced by the heat treatment and wear loading direction parameters. The anisotropic wear resistance in the high-speed sliding wear scenario was explained by the anisotropic tensile properties of the material. Therefore, special consideration should be given to the relationship between building direction and contact surfaces when using SLM maraging steel for wear environments with high-speed sliding. • Dependence of wear on building direction and sliding velocity for selective laser melted maraging steel was investigated. • Near isotropic wear behavior at relatively low sliding speed becomes anisotropic with increasing sliding speed. • Aging heat treatment improves the wear resistance of the alloy only when sliding speed is high. [ABSTRACT FROM AUTHOR]