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A new thermally cured polymer-graphene-zinc oxide-based solid lubricant is developed that reduces friction and wear significantly during the sliding wear of bearing steel under extreme contact pressure and long duration. The dry solid coating composite was made from a mixture of graphene, zinc oxide, and a specific industrial binder and then laminated on the surface of 52100 steel disks using the spin-coating technique. A ball-on-disk apparatus set to 1 GPa Hertzian pressure and a sliding distance of 500 m was used to examine the tribological properties of the coating. After ∼3000 cycles, the 15 μm thick coating created a significant reduction in the steel's coefficient of friction (approximately 82%) and wear loss compared to the uncoated surfaces. Following the triboligical examination, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy were conducted to determine the topography and morphology of the composite coating and resultant wear scars. It is revealed that the persistent protective coating on the disk surfaces was attributed to the adhesion influence of interfacial zinc oxide and graphene on the contact surfaces. Scratch testing of the new composite demonstrated a significant improvement in its adhesive properties on bulk interfacial surfaces compared to previously studied coatings.

In this study, the effects of refurbishing on rolling contact fatigue (RCF) in case carburized AISI 8620 steel were experimentally and analytically investigated. A thrust bearing test apparatus (TBTA) was designed and developed to simulate RCF. Initial RCF tests were conducted on AISI 8620 steel specimens to determine the baseline for pristine. Then new specimens were exposed to fatigue cycles equal to 90% of the L10 life of the pristine material. These specimens were then refurbished to the depths of 0.13b and 1.27b (b is the half width of Hertzian contact). The refurbished specimens were then subjected to RCF cycles in the TBTA until a spall appeared on the surface. The experimental results of refurbished specimens indicated a significant amount of fatigue life after refurbishing for both grinding depths. Moreover, it was observed that the remaining useful life of the refurbished test specimens was extended by increasing the depth of the regrinding. For the analytical investigation, a two-dimensional elastic-plastic finite element model was developed to estimate RCF life for pristine and refurbished specimens of case carburized steel. The characteristics of case carburized materials (e.g., variations in hardness and residual stresses) were incorporated in the 2D finite element model. In the present study, a continuum damage mechanics approach was employed to determine fatigue damage accumulation in original and refurbished domains. The results obtained from the experimental and FEA models for pristine and refurbished case carburized steel are in good agreement for both grinding depths.

This paper presents a continuum damage mechanics-based elastic-plastic FE model developed to quantify the rolling contact fatigue (RCF) life of refurbished bearings made from case carburized steel. Using the model developed for this investigation, case carburized steel fatigue performance was compared to that of through hardened steels. To simulate the characteristics of case carburized steel, a series of micro-indentation tests was performed to determine the hardness gradient and the case depth for the case carburized 8620 steel. The hardness gradient in the material was modeled by changing the yield strength as a function of depth. Therefore, the finite element modeling approach employed the von Mises plasticity-based model with kinematic hardening to incorporate the effect of material plasticity. Furthermore, the residual stress distribution resulting from the carburization process was modeled by modifying the damage evolution law. In order to simulate the refurbishing process, damage accumulation was calculated for a set number of fatigue cycles with the original bearing geometry. A layer of the original surface was then removed, but the fatigue damage accumulated prior to refurbishing was preserved. The refurbished geometry was then subjected to additional fatigue cycles until the damage was detected. The model as developed also accounts for the effects of topological randomness in the material microstructure through the use of Voronoi tessellations. The model was used to compare the RCF lives of refurbished bearings made from through hardened and case carburized bearing steel at contact pressures ranging from 2 to 3.5 GPa. The number of fatigue cycles prior to the refurbishing and the depth of material removal were varied to analyze their influence on refurbished life. It was found that greater regrinding depth (more than 0.5 times the half-width) and more fatigue cycles prior to refurbishing enhanced the total fatigue life of refurbished bearings. The model predicted that the ratio of the total RCF life of refurbished bearing to that of unrefurbished bearing is more for through hardened bearings than case carburized bearings with case depth of 500 μm. This is as expected, because in through hardened steel there is no case, so material properties are not affected by refurbishing. In the case hardened bearing steel, however, the refurbishing process removes part of the hardened case region and exposes the softer core to stress which reduces the fatigue performance of the refurbished bearing. Since this effect diminishes as the case depth increases; the increase in the fatigue life after refurbishing was larger for case carburized bearings with case depth of 1000 μm than through hardened bearings.

Although diamondlike carbon coatings like WC/a-C:H have a proven history of tribological performance in many mechanical applications, studies have reported that these coatings can sometimes experience high wear rates when operating in lubricants with large concentrations of phosphorous- and sulfur-based additives. In this study, WC/a-C:H and TiC/a-C:H coatings have been tribologically tested against AISI 52100 steel and Si3N4 counter faces under boundary lubrication in ISO 320 mineral oil, mineral oil with 1% aryl zinc dithiophosphate and 1% molybdenum dithiocarbamate, and a fully formulated wind turbine gear box oil. Results revealed that the wear coefficients of TiC/a-C:H were much smaller than those of WC/a-C:H in testing performed in lubricants with additives. A thionization wear mechanism was proposed to explain these results.

Wheelsets are removed from service for many reasons by North American Interchange Service. For example, journal bearings can be the cause of wheelset removal when operating temperature or acoustic signatures recorded by wayside detectors exceed certain limits. The Association of American Railroads (AAR) has established 13 failure progression modes to categorize the reason a bearing sets off one of these two detection systems. This study focuses on two failure progression modes: water etch caused by corrosion and issues associated with lubrication (grease). Greases for rail journal bearings are expected to satisfy a wide variety of requirements such as moisture tolerance, mechanical stability, vibration tolerance, range of operating temperatures and oxidation resistance, to name a few. This paper provides the reader an overview of several experiments and tests that were conducted with the goal of extending service life and reducing corrosion, including field tests used in the development of advanced journal bearing greases for the rail industry. One such new grease formulation was tested in the UK with good results.

Bearing refurbishing has become a popular method of extending the life of rolling element bearings. In the refurbishing process the raceways of the bearing may be ground to remove any surface damage prior to repolishing and reassembly with larger sized rolling elements. In the current study a continuum damage mechanics finite element model was developed to quantify the damage in original and refurbished bearings. After calculating the damage accumulation for a set number of contact cycles with the original bearing geometry, refurbishing is simulated by removing a layer of the original surface. The refurbished microstructural model is then subjected to additional computational contact cycles until a fatigue crack reaches the surface, signifying failure. This model preserves the fatigue damage accumulated prior to refurbishing and evaluates its influence on the refurbished bearing fatigue life. All refurbished bearing surfaces showed a significant amount of life after refurbishing with L10 lives from the point of refurbishment, varying from 20% to 94% of the original L10 life. The results indicate that the remaining life of the refurbished bearing population is inversely related to the time before refurbishing and is proportional to the depth of the regrinding. Results obtained from this investigation are in good agreement when compared to the Lundberg-Palmgren bearing life equation modified for analyzing the life of a refurbished bearing.

Poor or inefficient lubrication often gives rise to high friction and wear losses in machine components, which adversely affect their performance, efficiency, and durability. Many approaches are being explored to enhance the antifriction and antiwear properties of sliding machine components. In this study, the antifriction and antiwear properties of carbon spheres, synthesized from plastic waste by an autogenic process, were investigated as an additive to a poly-alpha-olefin (PAO-4 grade) oil. When dispersed at 1 wt% concentration, the carbon spheres reduced both friction and wear under boundary-lubricated sliding conditions. In particular, the reduction in wear was quite dramatic and appeared to be enabled by the formation of a fairly thick (≈200 nm) carbon-rich boundary film, the formation of which is attributed to tribochemical interactions between the carbon particles and sliding contact surfaces.

Lubricants containing additives that protect mechanical components against extreme pressure by reducing friction and wear are known as extreme pressure (EP) lubricants. In the current study, phosphorus-based EP lubricants with different additives (amine phosphate and phosphate ester) were tested in a steel ball-on-disc assembly under different EP conditions. The phosphate ester–steel interaction resulted in significantly higher wear and marginally lower friction than the amine phosphate–steel interaction. The tribological performance (especially wear) depended on the contact conditions. The tribofilm that formed on the steel surface with both EP lubricants consisted of organic compounds, oxides, and phosphates. The greater formation of the wear-resistant iron phosphate for the amine phosphate–steel interaction resulted in lower wear. The friction and wear performance for both EP lubricants depended upon surface roughness parameters along with the compounds that formed in the tribofilm.

This study demonstrated that surfactants can significantly improve the dispersion stability of milled boric acid particles over a broad range of operating temperatures. The tribological performance of boric acid and surfactant [sorbitan trioleate (STOMO)] mixed in the base oil was investigated under cylinder-on-plate line contact set-up using a reciprocating tribological test rig. Several characterisation techniques were used to understand particle distribution and size and composition of tribofilms that resulted from sliding contact. The results showed that surfactant prevented agglomeration of boric acid particles after the milling process and thus, provided good dispersion stability at elevated temperatures. The addition of surfactant in the base oil decreased the friction coefficients by 58 and 42% respectively, and reduced the wear scar diameter by 48 and 34% respectively, as compared to the base oil and fully formulated oil. In addition, the presence of a boron rich boundary film resulting f...

In this study, carbon based nanomaterials such as onion-like carbon (OLC), carbon nanotube and buckminsterfullerene (or buckyball) (C60) have been investigated as nanocolloidal additives for liquid lubricants. When tested under a range of contact pressures from 0·51 to 1·10 GPa, OLC provided the best overall friction reducing property in synthetic lubricant, although the friction reducing properties of all nanocarbon materials were dependent on the contact pressure. Detailed transmission electron microscopy revealed that the OLC kept its original structure intact and induced the formation during sliding of a thin ‘tribofilm’ composed of OLC, nanosized wear debris, amorphous carbon and graphitic layers on the wear debris. This tribofilm seems to be responsible for reduced friction under boundary conditions.

The objective of this study is to investigate the tribological and thermal properties of the recently developed Ag–MoS 2 nanoparticle-based multi-component lubricating system. To obtain greater tribofilm durability and to enhance the tribological and thermal properties of MoS 2 , a chemo-mechanical processing method has been developed for the synthesis of composite silver nanoparticles (Ag NPs) incorporated into MoS 2 nanoparticles (nMoS 2 ). In order to characterize and investigate thermal and tribological behavior of the Ag–MoS 2 hybrid system, four different Ag compositions were studied, ranging from 2 to 25 wt. % in MoS 2 . Different characterization techniques have been used to examine structural properties, silver content, particle size, and particle size distribution. The characterization results showed that the Ag NPs were successfully embedded into nMoS 2 with particle sizes lower than 300 nm for 90% of agglomerated particles and 100 nm for most of de-agglomerated particles with no phase change of the system. The increase of internal strain of nMoS 2 from the order of 0.002 to 0.02 was confirmed by XRD after the successful embedding of Ag NPs into nMoS 2 . In addition, the chemical and thermal analysis showed that silver molybdate was formed at temperatures above 450 °C, which demonstrates that this multicomponent system should be very effective for high temperature applications. The tribological tests revealed that nMoS 2 – 2 wt. % Ag results in 15–20% reduction in friction under boundary lubricated sliding conditions and 30–37% reduction in wear. Therefore, the addition of Ag nanoparticles at an optimum concentration can significantly enhance the thermal and tribological performance of the developed Ag–MoS 2 hybrid system.

In engine valve train systems, due to the sliding–rolling motion at the cam/follower interface, mixed and boundary lubrications dominate. It is in this regime that the effects of the lubricant chemistry at the molecular scale prevail over lubricant rheology effects in determining the friction and wear performance. The precise evaluation of friction and wear on the contact surfaces is essential to understand the tribochemistry of modern lubricant formulations and modern coating technologies. In the literature, a range of experimental tribometers have been used to simulate the tribology and tribochemistry of the cam/follower interface, leading to valuable information on the lubrication of this system. However, conventional tribometers such as pin on plate (PoP), pin on disc and disc on disc are not precise simulators due to their steady load operating conditions. This study looks into the opportunity of using the single cam tribometer (SCT) for better simulation of tribology and tribochemistry of engine val...

Gears and other mechanical assemblies are some of the key components for conversion of wind energy to electrical energy in wind turbines, but their durability and efficiency are severely impaired by some tribological issues like micro-pitting, wear, scuffing, and spalling. To address these issues, in this study, a combinational approach is proposed to incorporate surface treatment (electrochemical boriding) in coordination with the use of nano-colloidal lubricant additives. Boron nitride based solid lubricants were manufactured and flat gear steel samples were borided using a novel electrochemical boriding process. Combinations of nano-colloidal lubricant additives and borided surfaces are tested for their tribological performance, mainly friction and wear, over a wide range of contact conditions using a sliding contact linear reciprocating rig. Post-test surface analyses were carried out to investigate tribochemical interactions of colloidal lubricants with the steel surface and the chemical characterization of the tribofilms was investigated using XPS (X-ray Photoelectron Spectroscopy). Wear was also characterized and measured using optical profilometry. The borided surfaces enhanced the mechanical properties of the surface layer, leading to improved wear resistance. Moreover, it was observed that boron nitride stayed well dispersed within the oil and formed a stable tribofilm which was important to achieve improved tribological performance. The results of this current investigation are expected to aid in ongoing research efforts aimed at prolonging durability and efficiency of drivetrain components in advanced wind turbines.

Improvement of fuel economy and component durability as well as reduction of dependence on some harmful components of lubricants are the key concerns for automotive industries. In recent years, efforts have been made to use surface engineered surfaces in automotive components to produce environmentally friendly vehicles as well as to increase fuel economy. Since commercially available lubricants are mainly optimised for effective lubrication of ferrous materials, effective lubrication of novel materials, especially in the boundary lubrication/EP regime, is not well understood. Published research has also indicated that lubricant chemistry could have a detrimental impact on the intrinsic coating properties, changing their mechanical properties. Tribological systems working in boundary lubrication/EP conditions rely on lubricant–additive interactions to initiate surface reactions. The performance (friction-related efficiency and wear-dependent reliability) of tribological systems depends on the time taken to initiate surface reactions, the nature of the surface reactions and the stability of the films formed as a result of the surface reactions. This paper is a fundamental study of the lubrication and wear mechanisms of a WC-doped diamond-like-carbon (DLC) coating when lubricated in the extreme pressure (EP) regime. In the present work results from a tribochemistry study to understand the tribofilm evolution mechanisms for a DLC interface lubricated with a model oil containing a typical antiwear and corrosion inhibitor additive are presented. The performance was correlated with the wear performance. The wear measurements were carried out at small time-intervals along with the examination of the chemistry of the tribofilm using surface sensitive techniques; XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy. It was observed that surface reactions between the additive-coating surface varied considerably with time and these reactions significantly influence the tribological performance.

Many mechanical systems operate under mixed-boundary lubricated conditions. Moreover, these mechanical systems have highly loaded non-conformal contact surfaces, therefore significantly influencing friction/wear performance and also inducing various modes of failures. Hence, it is important to optimize selection of the lubricant since it plays a crucial role in controlling friction and affecting durability by forming tribochemical film on top of contact surfaces. The aim of the present work is to investigate the influence of extreme pressure additives, anti-wear additives, and corrosion inhibitors interacting at a steel/steel lubricated contact under extreme conditions and characterize the additive/surface interactions by advanced surface analysis techniques. The tribological testing was carried out employing a ball-on-disc test rig (ASTM G99-05) to measure friction. The wear on the balls was observed on the optical microscope after completion of the tribotest. Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy were used to access the chemistry on the surface. In this paper, synergistic effects and antagonistic effects in terms of tribological parameters (friction and wear) are discussed, and mechanisms of the interactions are explained by surface analysis. Implications for optimizing the system performance are discussed.