The shear thickening fluid exhibits a non-Newtonian characteristic, i.e. the viscosity follows a power law with the increasing shear rate. The shear thickening polishing technology has been proposed and developed to realize the precision finishing, utilizing the slurry with free abrasives prepared by the shear thickening fluid. In this paper, the formation mechanism and evolution of surface roughness in the cylindrical shear thickening polishing process is investigated in the elastohydrodynamics theoretically and experimentally. The elastohydrodynamic properties and the surface roughness are modeled by a novel solution method, based on the integration of polishing pad’s elastic deformation and slurry’s hydrodynamics. The Taguchi orthogonal tests of polishing the cylindrical AISI 52100 steel with the foam polyurethane pad were conducted, and the experimental surface roughness Ra of 9.689 nm was achieved under conditions of loading force 10 N, α-Al2O3 powder with grain size 1 μm and concentration 20 wt%, starch powder with concentration 25 wt%. According to the comprehensive analysis of variance and level average response, it is revealed that the slurry’s viscosity has a great influence on the surface roughness. The effecting mechanisms of processing factors on the surface roughness are interpreted from the perspective of effects transition in aspects of viscosity, pressure distribution and single grain’s cutting depth, interestingly exhibiting significant differences from that of traditional polishing methods.

A single strain path (SSP) and reciprocating strain path (RSP) burnishing processes are implemented on copper surfaces using a multi-ball surface burnishing tool. Dry wear tests are performed on both machined samples to investigate the effect of different strain paths on the wear behavior. A backscattered scanning electron microscope and nanoindentation are operated to characterize the microstructure and mechanical properties between samples machined via different strain paths. Molecular dynamics models are developed to explore the mechanism that enhances wear resistance through the gradient grain structure in the surface layers machined by SSP and RSP. The wear test results show that the wear scar volumes of the SSP treated samples are about 70% of the original samples, while the RSP treated samples are about 60%, indicating that the RSP further enhances the wear resistance of copper. Compared to SSP machining, RSP machining activates the Bauschinger effect in the copper to produce the gradient structure with a larger thickness (500 μm–700 μm) and greater hardness (1.72 GPa–1.82 GPa). Consequently, RSP machining further improves the resistance to crack initiation and expansion, resistance to plastic deformation, total strain capacity, and elastic recovery in the wear process, which is why samples machined with RSP have better wear resistance.

During micro-milling, the regular top-burrs usually bend in a direction laterally opposite to the milled feature. This top-burr morphology changes as the cutting edge condition progressively deteriorates with inherent tool wear. An edge-chipped tool tends to produce unusual top-burr that resembles an incompletely separated chip. Such chip-like burr also has a tendency to flow inside the micro-milled feature. This article, for the first time, explores the integral roles of worn-out tool geometry, micro-milling mechanics and process parameters on chip-like burr formation on Ti–6Al–4V during machining using 0.5 mm diameter TiAlN-coated WC-6Co tools under minimum quantity lubrication. Round-edges of the coated micro-mill progressively pass through break-in wear, steady abrasive wear, adhesive wear, coating delamination, and edge-chipping. A comprehensive analysis is presented here to explore the mechanism of chip-like burr formation considering the changes in tool geometry and corresponding variations in chip-tool-work tribological contact, cutting forces, stress distribution, burr root thickness, burr nano-hardness and surface roughness. Further, an algorithm for digital image processing is developed for efficient and reliable measurement of the burr size and corresponding inwardly-bent fraction. Subsequently, the roles of process parameters are assessed to explore the scopes of controlling the inward bending of the chip-like burrs.

Despite the great attention that WECs have received from researchers over the last few years, the earliest stages of this failure are still under debate. The wide variety of drivers studied, and the different methodologies employed to reproduce this failure mode, raise the question; do different methodologies lead to different initiation mechanisms? The present work aims to understand the most premature stages of WEC formation in 100Cr6 steel discs tested under RCF conditions and pre-charged with hydrogen. Firstly, three key aspects in the WEC formation process were identified, which have been extensively studied by other authors, but at intermediate or late stages of formation. These aspects are, (1) microstructural alteration formation first steps, (2) newly formed grain growth and (3) carbide dissolution. Secondly, samples representative of early stages were analysed by SEM/EDX/EBSD. The results of this research suggest that the early stages of these aspects are in agreement with the intermediate and late stages presented by other authors in the case of both WEC critical oil lubricated bearings and failed field bearings. Therefore, tribometer tests preceded by hydrogen uptake seem to recreate the tribological conditions found in wind turbine bearings. In addition, a very detailed failure initiation mechanism is proposed in this work.

A rail corrugation development calculation model that can consider the non-uniformity of material properties near rail surface was established to study the effects of rail decarburized layer on the corrugation growth. Firstly, the elastic modulus and hardness at different vertical positions in the rail decarburized layer were measured using nano-indentation technique. Then a two-dimensional wheel-rail rolling contact theory model that can consider the non-uniformity of material properties was established based on elasticity theory and hierarchical model. A numerical table for fast calculation of wheel-rail contact was established. Finally, the rail corrugation development with the presence of rail decarburized layer was simulated by considering wheel-rail contact with non-uniform material properties in the vehicle-track dynamic simulation. The results show that the decrease of elastic modulus and hardness of rail decarburized layer material would accelerate the development of corrugation. The corrugation growth rate with the presence of rail decarburized layer increases 43%. The maximum peak-to-valley depth of rail corrugation is about 9 times larger on the rail with decarburized layer than that on the rail without decarburized layer. Although the hardness plays a much more significant role in the corrugation development than the elastic modulus, the elastic modulus still has some influence on the growth rate of corrugation depth. The maximum depth of corrugation considering the non-uniformity of both elastic modulus and hardness is 11.7% larger than that considering only the non-uniformity of hardness.

Wheel acoustic roughness of one metro line in China shows that the wheel polygonal wear (WPW) on both sides of the wheelset is asymmetrical. The dominant harmonics of the left and right wheels are 12–13, but with large differences in acoustic roughness levels. To analyse the causes of the asymmetrical WPW (AWPW), an investigation of the acoustic roughness of the wheels and rails and a modal analysis of the wheelset–track system are carried out. In addition, a long-term wear model considering the flexibility of the wheelset and track is established to provide insight into the initiation and evolution of the AWPW. The results of the experiments and simulations show that the natural asymmetric vibration mode in the wheelset–track system with a frequency of 83 Hz is the main cause of WPW, which can be excited on sharp curves. The difference between the lengths of the left and right curved tracks with small radius is the cause of AWPW. The residual WPW after reprofiling and rail corrugation with a wavelength of about 200 mm are the main sources of excitation for the asymmetric natural vibration.

Development of anti-corrosion and wear-resistant materials in FLiNaK molten salt is vital to the service safety and reliability of the various mechanical systems for the molten salt reactor. In this work, a WC-12Co coating was fabricated by detonation gun spraying technique. Its microstructure, mechanical properties and tribological behaviors in FLiNaK molten salt were studied. A ball-on-disk rotary configuration was selected for the tribological test, in which the WC-12Co coating disks slid against Al2O3, SiC, Si3N4 and steel counterface materials in FLiNaK molten salt and low-oxygen environment at 600 °C. The results showed that the WC-12Co coating displays a low porosity of 0.71%, a bonding strength of above 44 MPa and good anti-corrosion in molten salt. FLiNaK molten salt provides the good lubricity and inhibits the oxidation wear of the WC-12Co coating, contributing to the low friction coefficient of 0.097–0.17 and wear rate of (4.95–8.40) × 10−7 mm3N−1m−1.

Rolling contact fatigue (RCF) can damage and reduce the life of the wheel and rail materials due to the degradation of surfaces in contact. The complex combination of RCF with corrosion can also contribute to the rapid surface degradation of wheel and rails materials. It is a serious problem when a railway system's environmental conditions can lead to severe corrosion.Even in some cases, severe corrosion caused by rainwater was observed in some open warehouse facilities. This work aims to investigate the influence that corrosion by artificial rainwater has on the generation and propagation of cracks caused by RCF. Twin disk tests were conducted on disk specimens manufactured from wheel and rail sections with corrosion and without corrosion zones on their contact surfaces. Corrosion on the wheel and rail disk specimens was accelerated by the potentiodynamic anodic polarization (PAP) process. The results show that PAP tests can promote pits and micro-cracks formation with different morphologies and sizes in both materials. These surface defects in rail material can grow and accelerate the cracks from the surface and down deep into the subsurface. Finally, pitting and corrosion were removed by the wear action in the wheel material. However, longer and deeper cracks were observed. Results showed that corrosion significantly influences RCF development on rail material.

Rail samples were tested under various scenarios as part of a complete analysis of the rail grinding process. Two main group of tests were performed which assessed the following: 1) preventive and corrective maintenance on fresh rail samples and 2) post-grinding tribological performance of the ground samples. The results allowed further knowledge to be acquired with regards to the performance and effectiveness of the grinding process and its effect on the surface quality of the rail samples. Results indicated a correlation between White Etching Layer and the formation of cracks and defects. Additionally, the harder grades were found to retain larger quantities of White Etching Layer upon completion of the rolling/sliding testing due to the hardness gradient between the White Etching Layer and bulk material, promoting the formation of cracks.

Understanding the wear micro-mechanisms of metallic binders is a prerequisite to designing durable cemented carbides for drilling applications. Here we studied the oxidational wear behaviors of WC-based composites encompassing particularly the CoAl binder (α phase, comprised of 5.7 wt% Al and bal. Co) and the CoFeAl binder (B2 phase) with same Al content. Unlike the oxidation of Co in the CoAl, selective oxidation of Al occurred unexpectedly in the CoFeAl, which weakened the binder integrity and aggravated the wear losses. Such unique behavior was rationalized computationally, where the Al2O3 formation was attributed to the low energy barrier to Al diffusion in the B2 structure. These findings could lay a useful basis for future development of robust cemented carbides.

Although the nanocrystalline Ni–W coatings offer superior wear resistance, under sufficiently higher loads and strain rates they often suffer from the strain localization which leads to a severe wear damage. Further, the presence of tensile residual stresses exaggerates the situation. Herein, we report a facile approach of microstructural gradation to circumvent this problem. Present study investigates the sliding wear performance of gradient nanocrystalline Ni–W coatings prepared by pulse electrodeposition. By tailoring the W-content along the deposition direction, a concomitant modulation in the microstructure, hardness and elastic modulus was obtained as revealed by the depth-wise glancing incidence XRD and nanoindentation, respectively. The gradient Ni–W coatings have demonstrated a low coefficient of friction (CoF) and superior wear resistance over their homogeneous counterparts. The micro-Raman spectroscopy has indicated the formation of tribo-oxide layer composed majorly of CoWO4, WO3 and NiO on the surface of the wear tracks. Based on the crystal-chemical model, the lower CoF of gradient Ni–W is attributed to the presence of a stable and adherent tribo-oxide. Furthermore, the enhanced wear resistance of the gradient Ni–W coating is ascribed to the lower tensile residual stresses, strain delocalization and its higher thermal diffusivity.

Machining-induced surface integrity evaluation and functional performance are considered as the most representative problems for evaluating the machining quality of Ni-based superalloys (for example, Inconel 718). Although high-speed ultrasonic vibration cutting (HUVC) has been proved to be effective for improving the machinability of Inconel 718, the surface integrity and its effect on functional performance (for example, wear resistance) are still unclear. By applying scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and white light interferometry (WLI), the surface integrity induced by HUVC and the wear resistance of the machined surface were comprehensively evaluated; the grain refinement, hardness increase, and wear mechanism of the machined surface were thoroughly studied, which were compared with the conventional cutting (CC) process. Compared with the CC process, the HUVC process achieves superior grain refinement, thicker surface plastic deformation layer, lower surface roughness Ra and Rz, higher hardness and compressive residual stress, and thicker hardening layer. The wear rate of the machined surface processed by HUVC is reduced by at most 24.01%. This study shows that the HUVC process can be potentially used to improve the wear resistance, which can effectively achieve grain refinement, surface roughness reduction, and wear resistance improvement while reducing the machining cost and process procedure.

Particular matter (PM) and squeal noise emitted from disc brakes are essential research subjects in the automotive industry. This study investigates the effect of braking conditions on the PM emissions and brake squeal of low metallic (LM), semi-metallic (SM) and non-asbestos organic (NAO) brake friction materials (BFMs) with a pin-on-disc tester. Braking conditions significantly affect the total number concentration (TNC) for all three BFMs. Generally, the geometric mean diameter (GMD) for NAO BFM is smaller than that for SM and LM BFMs. Moreover, the effect of braking conditions on the GMD for SM and LM BFMs is higher than that for NAO BFM. The speed has a more significant effect on GMD than contact load. The braking conditions can significantly affect the PMC. SM and LM BFMs exhibit larger particle mass concentrations (PMCs) than NAO BFM in most cases. All three BFMs show comparable A-weighted sound pressure levels (SPLs). The influence of braking conditions on brake squeal for SM and LM BFMs is stronger than that for NAO BFM.

The useful service life of metallic structures is affected by the exposure to the meteorological phenomena. In this sense, the continuous impact of particles suspended in the atmosphere could produce microscratches on the surfaces of these components, generating several wear degradations. These damaged surfaces can be repaired in order to increase their service life, avoiding component replacement and reducing the environmental impact. Recently, coatings deposition by cold spray has been revealed as a promising strategy for repairing damaged components. Therefore, the main objective of this work is focused on the analysis of wear resistance against micro-scratching in metallic coatings. For this purpose, 316L stainless steel powders were deposited onto low carbon steel S355J2 by cold spray, varying the pressure and temperature of the propellant gas. Nanoindentation tests were carried out to determine hardness and elastic modulus. The wear rates and the friction coefficients were measured through microscratch tests using a Berkovich tip. The wear mechanisms were analysed evaluating the scratch grooves in the scanning electron microscope. Finally, the relationship between hardness and wear rate was determined.

1-Octanethiol (OT) capped ZnS nanoparticles (OT-ZnS) as lubricating oil additives were investigated by tribological tests including wear tests and rheological measurements. The wear tests were performed on lubrication states using a custom design wear test device consisting of a rotating pin on a cylinder geometry. A rotating 1.2379 cylinder (steel counterpart) and a stationary C40 pin were used as a pair of test material for the wear test. The stationary pin was pressed against a rotating cylinder under lubrication conditions. It was observed that the synthesized OT-ZnS NP effectively improved the lubrication performance of 10W engine oil and the surface properties of the test materials. The addition of OT-ZnS NPs to the 10W base oil increased its viscosity and decreased the contact angle (CA) values. The results showed that the wear loss and the wear scar dimensions were both the lowest, while 0.0125 (wt.%) of OT-ZnS NPs were treated with the base oil. The mass loss of the pins was reduced to more than 6.3%. The wear reduction was observed at about 27% and the length of wear scar in the minor axis was reduced to 11.4% when a load of 100 kg was applied. The images of wear scars showed that the OT-ZnS NPs additives could prevent severe contact with the interacting surfaces. OT-ZnS NPs partially filled on the grooves and adhered on the wear scar surfaces and promote tribofilm formation, thereby it improve the tribological performance of 10W base oil, especially under extreme pressure conditions. Sulfide film (sulfuration layer) adhered to the wear scar surfaces and decrease the wear.

The cavitation-erosion (CE) damage features and a microstructural peculiarity of the CoCrFeNi high entropy alloy (HEA) are presented in this paper. While this HEA is theoretically predicted and often experimentally demonstrated to be a single-phase solid solution, noticeable elemental segregation has been observed in both the interdendritic regions and dendrite interiors in this study. This segregation causes a subtle reduction of lattice parameter, and the interdendritic segregated regions are frequently CE damage initiation sites. In the dendrite interiors, oxygen-rich particles with elemental segregation as their wrappings and the small speckles of elemental segregated regions show different CE damage features. In the dendrite interiors, CE-induced martensite formation is negligibly, and slip lines and their intersections are CE damage initiation sites. While a few CoCrFeNi-based HEAs have been shown to be CE-resistant in the literature, the CE resistance of the CoCrFeNi base itself is not high.

A project is discussed for which the trackwork specification required that the phenomenon of rail corrugation on metro systems be considered, with mitigation measures proposed for each corrugation mechanism. The trackwork contractor was obliged to consider these mitigation measures in design of the trackwork. Detailed monitoring of several test sections of track was undertaken bi-annually to quantify the severity of corrugation and its development. The detailed monitoring, supplemented by in-cab noise recordings that were made over the complete system, demonstrated that corrugation was minimal after two years of service.

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 Sq 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 Sq 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.

The article presents a fully coupled 3D thermomechanical model of a railway vehicle disc brake for calculations of temperature, stress, contact pressure and wear distributions. Five specially designed composite organic friction materials associated with a cast-iron ventilated brake disc were analyzed. The performed computer simulations correspond to the operational parameters of braking carried out on a full scale dynamometer test stand at constant braking power and constant vehicle velocity. These conditions were obtained by correcting the clamping force in relation to changes in the coefficient of friction. In the finite element (FE) model of the disc brake, the pad wear depth distribution was determined on the basis of the Archard's law taking into account the specific wear rate, contact pressure and sliding velocity. To create the numerical model with the geometry deformation allowing for friction material loss due to wear, advanced techniques were adapted. The geometric model of the brake includes the complex shapes of the brake pad holder, brake pad and ventilated disc. The wear constants of the tested friction material were obtained by means of the FE simulation and the measurement of the pad weight loss before and after the test on a full scale dynamometer test stand. The distributions of temperature, stresses, contact pressure and accumulated wear depth, obtained from the finite element analysis, in combination with the measured changes in the clamping force and coefficient of friction during braking, enabled to establish relationships between the properties of materials, operational parameters of braking and geometrical features of the brake components.

The prediction of wheel-rail rolling contact fatigue (RCF) crack initiation is a critical task in rail operation, particularly when material surface defects are present at the wheel-rail interface. A FE (finite element) model for wheel-rail rollers with indents, representing defects, was developed, and a novel crack initiation prediction procedure was proposed. In addition, a full-scale wheel-rail simulation model for a rail with an indent was established for investigating the effects of indent size. The results indicated that the maximum contact pressure on the rail material first occurred at the leading edge of the indent and then moved to the trailing edge with an increased value. The crack initiation criterion was determined by comparing the experimental and calculated results. The RCF crack initiated first at the trailing edge of the indent and later at the leading edge. In addition, the crack initiation life decreased with an increase in the indentation depth. To obtain the same contact pressure as that calculated for the small indent in the disc-on-disc case, the indent size needed to be doubled for the full-scale wheel-rail case. The predicted crack initiation life in the full-scale wheel-rail model was similar to that in the wheel-rail roller model.