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Space Industry commonly uses molybdenum disulphide (MoS2) as a solid lubricant film on gears in a variety of mechanisms. These mechanisms are often exposed to humid air during fabrication, integration into higher-level assemblies, and storage. It is generally accepted that operating MoS2 films in humid environments has a detrimental effect on the film's lifetime compared to operation in vacuum. However, the effect of exposure of such films to humid air with subsequent operation in vacuum is not well understood. In this study, a sputtered MoS2 dry film lubricant coating was applied to steel gears, which were then tested in a vacuum environment. Half of the spur gears coated with MoS2 were exposed to approximately 58 percent relative humidity for up to 77 days before testing. The other half set of gears were not exposed to humid air before testing. All gears were tested in a vacuum gear test rig. Gear tooth forces and the radial displacement of the gears were recorded during testing. These data together with photos, videos, Scanning Electron Microscope (SEM) inspections of worn gears, and Energy-Dispersive X-ray (EDX) spectra were analysed. For purposes of this study, the coating durability was defined as the time until the start of the coating degradation as was evident from increase of friction. Assessment of the results obtained from all experiments indicate that gears exposed to humid air prior to operation in vacuum showed nearly 36 percent reduction in coating durability, on average. SEM and EDX analyses of wear debris indicate mainly MoS2 was present with minor traces of other elements. The worn surfaces of gears show significant content of MoS2 still remaining in the gear teeth's contact area.

We focus this study on the dynamics of a Rubber-Layer Roller Bearing (RLRB) for vibrational base isolation. By exploiting viscoelastic friction arising from rigid cylinders rolling on concave rigid plates padded with rubber layers, the device is able to provide a nonlinear damping behavior. Specifically, we consider the case of a two degrees of freedom system, in which an inertial mass is base-isolated from ground motion by mean of RLRB. We firstly investigate the system dynamic response under sinusoidal ground motion, then we focus on a real earthquake excitation. In both cases, the system equipped with nonlinear RLRB device presents smaller inertial load and displacement compared to the case of a system exploiting linear damping. Further, we show that an optimization can be performed on the physical parameters of the device (i.e. the viscoelastic material relaxation time and the cylinders spacing) in order to further enhance the system dynamic behavior.

A load-sharing-based mixed lubrication model, applicable to cam–roller contacts, is developed. Roller slippage is taken into account by means of a roller friction model. Roughness effects in the dry asperity contact component of the mixed lubrication model are taken into account by measuring the real surface topography. The proportion of normal and tangential load due to asperity interaction is obtained from a dry contact stick–slip solver. Lubrication conditions in a cam–roller follower unit, as part of the fuel injection equipment in a heavy-duty diesel engine, are analyzed. Main findings are that stick–slip transitions (or variable asperity contact friction coefficient) are of crucial importance in regions of the cam where the acting contact forces are very high. The contact forces are directly related to the sliding velocity/roller slippage at the cam–roller contact and thus also to the static friction mechanism of asperity interactions. Assuming a constant asperity contact friction coefficient (or assuming that gross sliding has already occurred) in highly loaded regions may lead to large overestimation in the minimal required cam–roller contact friction coefficient in order to keep the roller rolling. The importance of including stick–slip transitions into the mixed lubrication model for the cam–roller contact is amplified with decreasing cam rotational velocity.

Bearing steel under severe loading condition undergoes substantial subsurface microstructural alterations known as Dark etching regions and white etching bands. White etching bands (WEBs) develop after hundreds of millions of stress cycles in bearing components and have been reported for several decades but the formation mechanism of white bands is not fully elucidated. Current research presents a systematic rolling contact fatigue (RCF) testing in a rotary tribometer under accelerated conditions, where rolling cycles are simulated in a 4-ball test configuration. The post RCF investigations have been carried out to understand the formation mechanism of WEBs in a ball-on-ball point contact load. WEBs have been characterised with the help of nanoindentation and Energy-dispersive X-ray spectroscopy analysis. The quantitative analysis of WEBs growth with subsurface stress field has revealed that the unique orientations of white bands are governed by the plane of maximum relative normal stress along the contact track. Moreover, the accelerated growth and reversal of WEBs sequence at elevated temperature have revealed that the WEBs formation is dependent on temperature/load combination. The observed growth of lenticular carbides in current research is also compared with dislocation gliding model and the role of carbon diffusion within WEBs is highlighted.

A significant fraction of operational cost in oil sands mining is directly or indirectly related to undercarriage component wear. One of the strategies to minimize the cost is to repair/rebuild the worn components. Recent advancement of welding technology offers an economical way to achieve this goal. Depending on the involved wear mechanisms, the selection of appropriate overlay material can considerably extend the service life of worn undercarriage components. Due to their excellent wear resistance, tungsten carbide based overlays is a unique choice for this purpose. In this study, three WC-based overlays with different carbide types were selected for evaluation. Wear resistance was assessed via low-stress abrasion (ASTM G65 ) and rolling contact abrasion (RCA). RCA tests were conducted with two slip ratios, where 0.3% slip ratio simulates pure rolling/high-stress contact fatigue, and a 3.0% slip combines fatigue and abrasion as observed in undercarriage components. Based on the wear test results, it was found that, despite their good low-stress abrasion resistance, WC-based overlays display high rolling contact abrasion damage and is not suitable for the restoration of undercarriage components. SEM observation on wear scars correlates the wear performance with carbide type and corresponding mechanisms.

Although both wear and fatigue are inevitably associated with the wheel/rail contact, they are normally studied as different phenomena and both are commonly considered excluding. Wear of railway wheels and rails is usually studied by relating wear rate to the T γ or T γ / A parameters or by plotting the so called “wear maps”, where different rolling parameters are related to the wear rate. On the other hand, fatigue in wheel/rail contact is mainly studied from the surface and subsurface stress or deformation fields and from the crack growth rate. However, one of the main causes of wear in rolling contact is the loss of material due to delamination processes that have their origin in the presence of surface fatigue cracks. So, if wear and fatigue are related, it should be possible to study both of them using the same method. In this paper, the fatigue index, based on the shakedown theory and used to predict the surface initiated fatigue of railway wheels, is proposed as a means to study the wear rate in rolling contact. The fatigue index is directly related to the apparition and growth of fatigue cracks at the surface and, thus, directly related to the peeling and spalling processes that constitute the severe and catastrophic wear mechanisms. The data collected from a series of twin-disc tests show that the wear rate correlates with the fatigue index following an exponential evolution independently of the normal load. Furthermore, a fatigue index around 0 marks the onset of severe wear.

Commonly used four-ball testing configuration when used to investigate rolling contact behavior of polyetheretherketone (PEEK) produced some unexpected peculiarities, namely direct contact between ceramic balls. Direct contact between lower three balls is normally not expected to take place when elastic materials (ferrous or ceramics) are tested. However, due to the viscoelastic nature of PEEK surface damage of the ceramic balls were observed leading to the production of wear debris which in turn facilitated premature failure of PEEK components of the assembly. It is postulated that caution is required when using the four-ball testing configuration to assess rolling contact performance of viscoelastic materials, such as engineering polymers. The research reported in this paper was supported by funding from Gdansk University of Technology, Gdansk, Poland(Grant DS-2012).

In this study, the tractive rolling contact problem between a rigid cylinder and a graded coating is investigated. The main objective of this study is to investigate the effect of the stiffness ratio, the coefficient of friction and the coating thickness on the surface contact tractions, the surface in-plane stress, the stick zone length and the creep ratio parameter that may have a bearing on the fatigue life of the component. Assuming that the shear modulus varies exponentially through the thickness of the coating, the governing integral equations associated with the rolling contact problem are constructed. Furthermore, it is supposed that the contact patch is controlled by a central stick zone accompanied by two slip zones. The conventional Goodman approximation is employed in order to decouple the governing singular integral equations. Finally, the numerical solution of the integral equations is obtained by applying the Gauss-Chebyshev integration method. (c) 2011 Elsevier Ltd. All rights reserved.

In this paper, starting from results of Part I (Afferrante et al. [1] ), we develop a procedure to evaluate the frequency dependent receptances of the energy dissipation at the contact interface, which is the quantity of interest in corrugation studies as the most known wear laws assume the local wear proportional to the frictional dissipation. These results are applied to the calculation of the growth rate of corrugation in railway tracks with an initial sinusoidal undulation. In particular, the wheel inertia and the transient contact mechanics effects are considered using a continuum description of the rail (hence neglecting pinned–pinned resonance due to the beam bending between two successive supports) by the simplest model (the Euler beam) which we can use to describe the normal receptance of the rail. The presence of a finite partial slip zone in the contact area can significantly modify the results, partially explaining the scatter of the experimental data collected in the literature. In particular, we found that partial slip affects the predicted apparent wavelength of highest growth of corrugation, and with respect to the full stick conditions, higher tractive ratio increases significantly the growth factors and non-linearity, showing an unexpected absolute maximum of growth at intermediate velocities, and the “resonance-free” regime becomes increasingly not a constant “frequency”, spanning a range of frequency between 700 and 1500 Hz even for a given system and set of loads. Only a full investigation involving all other resonances in the system may clarify further the correspondence with experimental values, since the corrugation “enigma” may be due to a combination of effects.

Diamond-like carbon (DLC) is being advanced as an excellent surface modification technology for a growing number of applications because of its low friction coefficient and excellent wear resistance. Due to the excellent characteristics of certain mechanical elements, such as a bearing, high levels of functionality are expected. Unfortunately, it is difficult to take full advantage of such high functionality because of insufficient endurance under high-pressure operating conditions. In order to make use of DLC as a bearing surface coating, the mechanism of DLC delamination under high-pressure conditions needs to be clarified. This paper discusses the mechanism of delamination of the intermediate layer of a DLC coating on a bearing in a test rig and by numerical simulation using FEM analysis. Test results revealed that the smaller the Young's modulus value of the intermediate layer (a factor affecting durability) of the DLC coating, the greater the resistance to delamination.

Thin wall ductile iron castings are being used in the industry as a way to improve the strength to weight ratio of machine parts. The high cooling rate, suffered by thin wall parts during the solidification process, promotes several microstructural changes, such as, carbide precipitation and a noticeable nodule count increment. The present work, studies the effect that the increase in nodule count has on the rolling contact fatigue resistance of ductile iron with different matrix microstructures. Ductile cast iron test samples, with nodule counts ranging between 150 and 1400 nod/mm2 were obtained. The samples were then heat treated in order to obtain three sets of different ADI grades and also a quenched and tempered set. The rolling contact fatigue properties were evaluated by using a flat washer type test rig. Different relations of the contact area versus the nodule size were obtained by using three different counterparts. The results show that an increase in the nodule count promotes a noticeable increase in the rolling contact fatigue life, being more important for the quenched and tempered samples. The relation between contact area surface and nodule size was the main variable influencing the rolling contact fatigue life. Fil: Dommarco, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: Jaureguiberry, A. J.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: Sikora, Jorge Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina