Ni3Al-based composite containing Ag/Cu modified hBN nanosheets (Cu-hBN)/Ag-Cu-hBN have been fabricated by vacuum hot press sintering and their tribological performance has been evaluated at RT, 200, 400, 600 and 800 °C at a constant load of 10 N and a sliding speed of 0.2 m/s using a ball-on-disc rotary tribometer against a Si3N4 ball. The main aim of the study is to examine the occurrence of a synergetic action between Ag and Cu-hBN in attaining low friction and anti-wear properties from room temperature to 800 °C. The results indicated a significant reduction in the coefficient of friction by the addition of a combination of Ag and Cu-hBN in comparison to either Ag or Cu-hBN. Ni3Al-Ag-Cu-hBN attained the lowest coefficient of friction (∼0.19) and wear rate (1.85 x 10-5 mm3/Nm) at 800 °C, which has been ascribed to the supportive lubrication provided by Ag and Cu-hBN. The investigation also revealed that Ag provided lubrication at temperatures below 500 °C, whereas Cu-hBN and other lubricious oxides like Ag2MoO4, Ag2Mo2O7, MoO3, CuO and NiMoO4 provided low friction and anti-wear properties beyond 500 °C. Self-lubricating Ni3Al-Ag-Cu-hBN composites offer themselves as a potential candidate for high-temperature applications where the components work under relative sliding motion.

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.

The lubricating effect of h-BN nanosheets, dispersed in pure water without the use of any chemicals, was investigated under fretting contacts. The resistance against oxidation of sliding pairs was also considered using AISI 52100 and AISI 304 steel pairs separately, given that oxidation is inevitable on steel surfaces. In the partial slip regime, effects of the type of lubricant and oxidation on wear and friction were insignificant. In the mixed slip regime, the mending effect of h-BN counteracted the oxidation of sliding surfaces, resulting in mild wear. In the gross slip regime, the mending effect collapsed due to oxide debris, resulting in severe wear. Moreover, the results showed that the presence of h-BN nanosheets influenced the transition of fretting regimes. These findings suggest that aqueous-dispersed h-BN nanosheets can be an effective eco-friendly lubricant additive, particularly when fretting conditions enable a stable mending effect and sliding surfaces exhibit high corrosion resistance.

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.

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.

Copper matrix self-lubricating composites (CMSCs) are commonly used as sliding contacts in aerospace craft for their good electrical conductivity and wear resistance. In efforts to further improve performance and prolong the service life of CMSCs, the effect of WS2 nanosheet addition ranging from 10 to 25 wt% on the mechanical and tribological properties of Cu-WS2 self-lubricating composites was analyzed in this work. The dry sliding wear of composites against beryllium copper counter discs was measured at the load of 1 N. The increase in WS2 nanosheet leads to a decline in the strength of Cu-WS2 composite, whereas it improves the tribological properties presented as the reduction of friction coefficients and wear rates. Compared to Cu-WS2 with submicron WS2 sheet, Cu-WS2 with WS2 nanosheet shows both improved mechanical and tribological properties. The wear rate of Cu-WS2 composite with 25 wt% WS2 nanosheets is 61.2% lower than that with 25 wt% submicron WS2 sheets. The addition of WS2 nanosheet reduces the size of WS2 agglomerates and generates finely dispersed WS2 particles in Cu-WS2 composites. It not only promotes lubrication film continuous and smooth, but also inhibits the initiation and propagation of cracks at the worn subsurface by improving the strength of Cu-WS2 composite.

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 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.

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.

Running-in is essential for improving the performance and lifetime of a machine. This study proposed a two-stage running-in procedure based on the lubrication regime and nanoparticle effect to increase running-in efficiency; the method was verified using a disk-on-block experiment. The first stage involved wearing down the high peaks of the surface asperities in relatively severe mixed lubrication. The second stage was a mild wear process, in which the shape of the asperity peak was trimmed with soft CuO particles in three-body mixed lubrication. The CuO concentrations were divided into three concentration variations (0.1 wt%, 0.5 wt%, and 1.0 wt%) to investigate the effect of CuO concentration on the smoothing surface processes. The tribological parameters, including the surface roughness value, peak radius of asperity, plasticity index, adhesion index, real contact area, true friction power intensity (TFPI), and modified true friction power intensity (MTFPI) index, were investigated to evaluate the friction coefficient, wear, and anti-scuffing performance. The result after first stage showed that the surface roughness value decreased to similarly low values for the different initial roughness values. After the second stage of the trimming process, the asperity peak radius, contact angle, real contact area, plasticity index, and anti-scuffing improved considerably. The obvious reduction in the friction coefficient in the second stage was due to the decrease in ratchet friction, deformation, and plowing friction. These methods effectively enhanced the efficiency of the running-in process.

Polycrystalline diamond (PCD) is one of the hardest man-made materials with excellent wear resistance, also a preferred friction pair material for sliding bearings on geological drilling and deep-sea energy development equipment which has demanding requirements for extreme load carrying and abrasive wear resistance. However, the friction and wear characteristics of PCD self-mated conformal contact sliding bearings are still not fully understood. PCD self-mated pairs may exhibit some special tribological characteristics, such as abnormal friction vibrations, even if the roughness of the friction surfaces is at the nanometer level, resulting in PCD damage and reliability problem of bearings. In order to reveal the cause of friction vibration and the wear mechanism of PCD self-mated pairs, the friction and wear behaviors of PCD self-mated pairs in air and water lubrication conditions were hereby studied using a ring-on-ring test apparatus. The friction torque at different rotational speeds and loads was measured, the wear surface features of the PCDs were examined, and the composition of the solid film formed during the test at the friction interface was identified. The results show that PCD self-matching pair tends to generate severe friction vibration due to the adhesion of diamond grains rather than the increase of friction interface roughness under the dry friction condition. Water matters considerably in reducing the adhesion. The wear forms of PCDs are adhesive and abrasive wear in dry friction, and abrasive wear in water. PCD with large diamond grains is provided with a better wear resistance.

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.

In situ SEM scratch testing of DLC and Si-doped DLC deposited on Si wafers has been conducted using sharp 1 and 5 μm radii diamond asperities, enabling the stages of deformation and wear of DLC coatings to be evaluated in real time. With increasing load, initial plastic deformation and tensile cracking in the scratch track progresses to the propagation of radial and lateral cracks, and full coating spallation. In situ SEM imaging reveals nucleation of radial cracks in the DLC coating around the front and side of the moving asperity, followed by lateral crack propagation both ahead of, and behind, the asperity contact zone. Post-mortem FIB cross sectioning reveals microcracking and lateral cracks in the silicon substrate below DLC coatings prior to coating spallation. The DLC failure mechanisms are influenced by asperity geometry, with notable DLC coating lift up/delamination events occurring during the smaller 1 μm radius asperity scratch tests. The sharper 1 μm radius asperity required ∼20% of the applied load, and higher contact pressure, to initiate spallation during scratching compared the larger 5 μm asperity, indicating that smaller radii asperities are significantly more likely to cause DLC coating spallation, although the spallations they generate were observed to be, on average, smaller.

This study aimed to reduce cavitation erosion effects by improving the surface properties of high-alloyed steel cast parts using the tungsten inert gas (TIG) surface physical modification technique. Local surface melting was performed at different linear energies (El = 4080–8880 J/cm) by varying the current between 100 and 200 A at a constant voltage of 10.2–11.1 V. Hardness increased from 210 to 390 HV5 when a TIG current of 150 A with a linear energy of El = 6630 J/cm was implemented.Using this technique, a surface layer with increased resistance to cavitation erosion was formed. This new surface absorbed large amounts of impact energy owing to a favourable combination of microstructural changes, leading to improved elastic and plastic properties, work hardening, cracking, and failure response. The cavitation erosion performance of the re-melted surface layer was analysed using a piezoceramic vibrating device according to the ASTM G32–2016 standard. Following TIG surface re-melting, the average penetration depth of erosion and the cavitation erosion rate increased by approxmately 6.8 times. Based on optical and electronic metallographic analyses, hardness measurements, and X-ray diffraction, it was shown that the morphology of the surface layer following cavitation erosion tests was affected. Microcraters tended to develop at locations where carbide particles from the alloying elements were previously present. At a current of 150 A, the depth of the microcraters reached values of approximately 15 μm; however, microcrater depth reached 10 μm in the austenite matrix. Based on these investigations, an understanding of the mechanisms that result in the improvement of the resistance to erosion by cavitation of cast high-alloy steels whose surfaces were re-melted using the TIG technique is developed.

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.

Spacecraft landings on Earth's moon are highly susceptible to lunar regolith particle impingement leading to abrasion and premature failure of structural components. Herein, abrasive wear of Al 6061 and Ti6Al4V, critical aerospace alloys in two lunar simulants environments viz. JSC-1A and Greenland Anorthosite (GA) under extreme temperature conditions were examined. The highest wear loss was observed for softer Al 6061 without simulant conditions at all temperature ranges, while harder Ti6Al4V has the highest wear loss when sliding with JSC-1A. Wear loss increased under high temperature (HT) conditions due to thermal softening in alloys. The synergy of material removal and addition in three-body wear showed that wear volume quantification via the conventional method is improbable. A novel technique to quantify “true wear volume” incorporating embedding fraction (Ef) is developed. The results showed that JSC-1A regolith, owing to sharper morphology than GA, creates more embedding at RT and HT by 12% and 15% on Al 6061 and 23% and 29% on Ti6Al4V. A non-dimensional embedding index (Ei) is postulated, which can potentially serve as a tool to establish abrasion caused by lunar dust for future explorations.

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.

The present study has developed new insights into the wear transition mode of cermets and provides a better understanding of the role of abrasive characteristics in three-body abrasive wear. Two sintered and fully densified cermets, NbC–Ni (NbC–12Ni–10Mo2C) and WC-Co (WC-9.5Co) with similar micro-hardness (HV30 ≈ 13.5 GPa) were selected in this study. The abrasion response of cermets against different abrasive characteristics namely size (67–245 μm), hardness (silica, alumina and SiC), and shape (round and angular) was experimentally investigated according to the ASTM G65 standard. The test results showed that the specific wear rate (SWR) of the cermets increased significantly with increasing abrasive size and hardness. The size effect shows that the dominant wear micro-mechanisms shift from binder removal (rolling) to mixed binder-carbide extrusion (sliding) as the abrasives particle size changes from smaller (67 μm) to larger (245 μm). The hardness ratio between the abrasives and cermets (Ha/Hs) highlights that silica and alumina abrasives provide mild wear (Ha/Hs 1.5). Both cermets exhibited similar SWR during mild wear, but NbC–Ni showed 7 times higher SWR than WC-Co during severe wear. The abrasives shape effect does not show a significant difference on the wear rate of the cermets.

The effects of mould temperature (cooling temperature) on molten HDPE (Hostalen GC 7260) during manufacture, is evaluated in this paper. HDPE gears were produced at varying mould temperatures using Injection Moulding. Optimised injection values for melt temperature, injection volume, hold pressure, and hold time were obtained, and then held constant while the mould temperature was altered.Analysis on how the mould temperature affected peak melting points and crystallinity were then carried out using differential scanning calorimetry (DSC). These revealed that crystallinity improved as the mould temperature was increased from 22 °C to 65 °C. Gears produced at similar cooling temperatures were then meshed on a gear test rig and run at 1000 rpm, using different torque loadings. Their wear rates, and modes of failure were then analysed, and comparisons were made to ascertain how the differing mould temperatures employed during the injection moulding manufacturing process affected their wear characteristics. Topographical analysis of worn gear teeth was performed using scanning electron microscopy (SEM). It was noted that gear tooth wear and failure was dependent on the mould temperature employed during the manufacturing process. Gears produced at 65 °C showed improved tooth surface wear resistance at lower loads (0.5 Nm and 1 Nm) compared to those produced at 22 °C, but were more likely to fail through tooth fracture at the pitch line due to excessive material removal. Gears produced at lower mould temperatures, on the other hand, exhibited better wear resistance for higher loads (3 Nm and 4 Nm), compared to those produced at higher mould temperatures, and were more likely to fail due to material flow. The results show a correlation between mould temperature, crystallinity, and gear performance. Based on wear rate responses of gears produced at differing mould temperatures to the application of varying torque loadings, a Mould Temperature to Torque Reference Chart for HDPE is presented.

The formation mechanism of the white etching layer (WEL) on the rail steel material has been confirmed via two different routes; thermal-induced WEL by phase transformation and mechanical-induced WEL by severe plastic deformation. In this study, a combination of thermal and mechanical processes was performed to examine the synergistic interaction of two formation mechanisms of the WEL on rails. An experiment was conducted on the rail steel material using a Gleeble thermo-mechanical simulation system. The results confirmed that the martensitic WEL is formed by phase transformation from rapid quenching at a temperature below the critical point of material phase transformation, under 550 °C, when high hydrostatic pressure is applied. The application of combined thermal and mechanical action induced martensite with the presence of retained austenite. The microstructure of the transition zone between the WEL and the base material contains partial and full dissolution of cementite, broken pearlite lamella, and dislocation density, confirming that both thermal and mechanical processes are involved in the phase transformation of the material.