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Abstract This paper reports the non-destructive evaluation of railway wheels via the Barkhausen noise technique across the wheel width. This study correlates non-destructive Barkhausen noise parameters with conventional destructive analyses expressed in terms of metallographic and SEM observations, microhardness profiles, residual stress, and average grain size measurements. The condition of the wheel surface was altered remarkably due to the real and long-term wheel operation. The results of investigations indicate non-homogenous distribution of Barkhausen noise emission as well as the corresponding surface state with respect to the wheel width. Severe plastic deformation and superimposed elevated temperatures alter the grain size considerably in the near-surface as well as the subsurface region. It was found that Barkhausen noise is strongly correlated with the average grain size within the Barkhausen noise sensitive depth, whereas the correlation with the residual stress distribution is quite poor. Highlights • Barkhausen noise emission is sensitive to the rail wheel damage. • Barkhausen noise emission is driven by grain refinement. • Wheel surface damage is asymmetrical with respect to the wheel width. [ABSTRACT FROM AUTHOR]

The paper presents a new concept for evaluation of surface damage of rails subjected to long-term cyclic loading. A rail cyclically loaded during 20 years of traffic was examined by combination of several non-destructive techniques with high sensitivity to surface damage, namely positron annihilation spectroscopy, X-ray line profile analysis and magnetic Barkhausen noise. The parameters of magnetic Barkhausen noise were correlated with the parameters describing the rail surface integrity, i.e. thickness, hardness, stress state and microstructure of the damaged layer. Good correlation between these parameters was observed and it has been demonstrated that thickness of the damaged layer can be measured non-destructively by the magnetic Barkhausen noise technique. Cyclic loading of the rail introduced a high density of dislocations into the sub-surface region and also vacancies which agglomerated into small clusters. Phase analysis of the rail surface revealed that repeated severe plastic deformation induced multiple phase transitions. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of friction and wear performance of quaternary CrSiCN coatings deposited on a hardened H-13 steel substrate by a plasma enhanced magnetron sputtering (PEMS) technique. Friction and wear tests were conducted with a reciprocating line contact between a hardened 4370 steel roller and coated and uncoated flat specimens under dry and lubricated conditions. The effects of coating thickness (1, 3.5 and 7.5 µm) on the mechanical properties, friction and wear performance were also assessed. In dry sliding, the friction of coated surfaces was about the same as for uncoated surfaces, except for the 1-µm coating, which had higher friction. Friction for coated surfaces under lubricated contact was in general higher than for uncoated surfaces. There was no measurable wear on any of the coated surfaces, under either dry or lubricated conditions. However, wear was higher on the steel roller counterface sliding against the coated surfaces, with the amount of wear proportional to the mating coating thickness. The effectiveness of formulated lubricant additives was also modified by the coating, resulting in major effects on friction and wear behavior. This reduction in lubricant additive efficacy is due to the fact that the additives were designed and optimized for ferrous surfaces. [ABSTRACT FROM AUTHOR]

Pin on disc wear tests on steel and alumina discs have been carried out using the NPL designed system which enables in situ monitoring of the wear and surface condition through the use of a line-scan camera microscope. In addition, the combination of a Linear Voltage Displacement Transformer (LVDT) and a non-contact optical aberration probe make it possible to monitor the wear of the disc and of the pin separately as the tests progresses. Accurately triggered optical micrographs acquired during the test are subsequently registered to produce videos showing the wear of the disc and the transport of debris along the scar. This enables the dynamics of the processes that take place at the contact interface to be visualised in real time during a test. When these techniques were applied to pin-on-disc wear tests with steel and alumina balls tested against alumina discs, insights were gained on the development of interfacial films as the tests proceeded. For the steel ball, these were formed from iron oxide that formed and reformed and moved across the wear surface, and for the alumina ball an interfacial layer was formed which filled depressions in the lapped surface of the alumina. These interfacial films had reduced aluminium content compared to the original alumina in the surface. [ABSTRACT FROM AUTHOR]

The damages of the surfaces of the hydraulic concrete structures are usually caused by abrasive action of the debris carried by the water. The action of the debris leads also to damages of the structural reinforcement. Therefore, selection of the repair materials is focused not only on the strength requirements, but also the abrasion resistance. On the other hand, there is very important that the majority of the repair works is conducted under water, at great depths. Therefore, the underwater concretes are increasingly used for the repairs. These concretes make possible repairing in the contact with water, with substantially limited wash-out losses of the cement paste. In this context, the important problem is utilization of the pozzolanic materials from recycling for the production of the underwater repair concretes, which allows for the limitation of the cement use. In this paper, the results of the testing of the abrasion resistance of the underwater repair concretes were presented. The concretes under investigation have contained 20%, 30%, 40% and 50% (of the cement mass) of the fly ashes from the fluidal beds. The fluidal fly ashes are the by-products of burning the coal in the energetic plants with the fluidal beds. Abrasion of the underwater concretes was tested using two methods. First method consists in the use of Boehme abrasion tester, which is used in EU countries for determination of the abrasion resistance of concrete. The second method is so-called underwater method according to the US Standard ASTM C1138. The method according to ASTM C1138 appeared to be more reliable in the evaluation of the abrasion resistance of the underwater concretes. The highest abrasion resistance showed the concrete with 30% (in relation to the cement mass) of the fluidal fly ashes. Due to the strong pozzolanic properties of the fluidal fly ashes, the substitution of the 30% of cement with the fluidal fly ash enables obtaining of the concrete with improved compressive strength and abrasion resistance as well as the workability which makes possible the proper conducting of the repair works under water. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the braking tribological characteristics of two kinds of metro brake shoe materials using a sub-scale frictional braking machine. The results indicate that the friction coefficient of braking interface has a close relationship with the velocity and pressure. The increase of velocity and pressure raises the temperature-rise of braking interface. The increasing velocity and pressure aggravate the wear of brake shoe rings. However, the wear loss of wheel rings decreases with increasing pressure. The braking surface of brake shoe rings exhibits adhesive wear and spalling, while obvious plowing and abrasive wear are dominant on the braking surface of wheel rings. According to the experimental results, it is concluded that brake shoe B material can be used to prevent and alleviate the occurrence and evolution of groove wear on wheel tread. This might be attributed to the higher percentage of FeC 3 versus FeS 2 in its composition compared with brake material A. Field testing, completed to 100,000 km thus far, has to be completed on these materials and further work should be performed for evaluating the effect of different brake shoe materials on the groove wear of wheel tread in the field. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the microstructure and wear behavior of laser cladding Co-based alloy coating on single wheel or rail material (the laser cladding coating is applied only to the wheel or to the rail) using a rolling–sliding wear test apparatus. The coating of laser cladding Co-based alloy consists of dendrite and eutectic. Single-treated wheel or rail specimen undergone the laser cladding treatment effectively decreases rolling friction coefficient and improve wear resistance of both wheel and rail materials. Single-treated wheel or rail roller presents mild wear damage. Furthermore, it is helpful to alleviate the surface damage and plastic flow of wheel and rail rollers. It is concluded that the laser cladding Co-based alloy coating is suitable for single treatment of wheel or rail material. However, further work should be carried out to clarify the rolling contact fatigue mechanism and to characterize the laser cladding Co-based alloy coating. [ABSTRACT FROM AUTHOR]

Abstract: Material removal rate from an automotive engine bore surface should not exceed a few nanometers per running hour. This corresponds to the ultra-mild wear (UMW) conditions. Understanding the role of the microstructure on the wear mechanisms in the UMW regime is essential for the development of lightweight automotive engines. In this work, sliding wear tests were performed on a hypereutectic Al–Si alloy containing 18.5wt.% Si under a light load of 0.5N and boundary lubricated conditions corresponding to the UMW regime using a ball (AISI 52100 steel)-on-disc tribometer. Sample surfaces were chemically etched to expose silicon particles. After sliding to 6×105 cycles, no measurable mass loss was detected using an analytical balance with an accuracy of 10−4 g. Wear damage was limited to the contact surfaces of the silicon particles. The calculated maximum contact pressure applied on the protruded silicon particles was less than the matrix hardness of the alloy, which was consistent with the experimental observations that large silicon particles in this alloy carried the applied load and prevented plastic deformation of the matrix. A eutectic Al–12% Si alloy with a softer aluminum matrix was tested under the same conditions and showed more extensive damage. In this alloy, silicon particles became embedded into the aluminum matrix, which in turn formed pile-ups near the particles and consequently were exposed to damage by the counterface. In 18.5% Si, neither silicon particle sinking-in nor damage to aluminum matrix in the form of plastic deformation and wear was observed. Accordingly, Al–18.5% Si was effective in maintaining a minimal surface damage under UMW conditions. [Copyright &y& Elsevier]

Abstract: Al–Si alloys that are intended for extended durability applications must operate under ultra-mild wear conditions. Understanding the mechanisms responsible for ultra-mild wear in cast Al–Si alloys is essential to any effort to accelerate the process of using these alloys in components operating under these conditions. Two eutectic grade Al–Si alloys with similar silicon percentages (11wt% Si and 12wt% Si) were tested. The alloys differed in matrix hardness, but had comparable silicon particle sizes and morphologies. Tests were conducted using a ball (AISI 52100 steel)-on-disc tribometer under light load (0.5N) in lubricated conditions corresponding to ultra-mild wear. The sample surfaces were chemically etched to expose hard particles. No material removal was detected by mass loss measurements in either of the alloys after sliding to 2.2×103 m (105 cycles). Sliding occurred on the top of silicon particles at first. Evidence of both the sinking-in of silicon particles, and the pile-up of plastically deformed material around the sunken-in particles was observed. A contact stress analysis based on the Greenwood and Tripp model suggests that the applied normal pressure on the hard phases was approximately equal with the matrix hardness of the harder alloy, but exceeded the matrix strength of the softer alloy. This is consistent with the observation that the Al–11% Si with the higher matrix hardness suffered less surface damage – less significant amounts of particle sinking-in and aluminum piling up – when compared with the Al–12% Si with softer matrix. [Copyright &y& Elsevier]

A new approach to conduct scratch tests on polymers is proposed. A brief review of the different definitions of scratch resistance and surface damages, currently available scratch test devices and the need for such a new approach is given. The new test methodology is developed using the concepts from materials science and solid mechanics, which include the consideration of material parameters, use of microscopy for image analysis and the finite element method (FEM). The consistency and reproducibility of test results are shown using a new scratch test device on two sets of neat and talc-filled polypropylene (PP) systems. Three different test conditions, i.e. linear load increase under constant speed, constant load under constant speed, and linear speed increase under constant load, have been conducted to determine the most effective, informative test conditions for evaluation of scratch resistance of polymers. The effect of loading and scratch speeds on the scratch-induced deformation of PP is also investigated using FEM simulations. Experimental observations and FEM results show a good qualitative correlation. The unique advantages of the new scratch test method for evaluating scratch resistance of polymers are discussed. [Copyright &y& Elsevier]

This study aims at investigating tribological characteristics of zirconium-based bulk metallic glass (BMG) using nanoindentation and nanoscratch techniques. The nanomechanical properties of Zr 65 Cu 15 Al 10 Ni 10 (at. %, Zr-based BMG) are first assessed via nanoindentation tests and simulations. The tribological performance of Zr-based BMG under various scratch conditions are investigated via nanoscratch tests with multiple ramp and constant normal loads. It is found that coefficient of friction (COF) irregularly increases with normal load due to the formation of fracture and chipping, which play crucial roles on the frictional behaviors of Zr-based BMG. The critical load for fracture and chipping formation is identified for the considered test conditions. Wear mechanism of Zr-based BMG under nanoscratching are investigated by observing the morphologies of scratch tracks through scanning electron microscope (SEM). The findings of this study can be beneficial for tailoring the surface of Zr-based BMG by using micro-machining and patterning techniques. • Nanomechanical deformation of Zr-based BMG was investigated via nanoscratch test. • Nanoscratch tests under multiple constant and ramp normal loads were performed. • Fracture and chipping play an important role in defining frictional behaviors of Zr-based BMG. • Coefficient of friction (COF) increases with applied load due to change in deformation behavior. [ABSTRACT FROM AUTHOR]

Mar is a type of subtle surface damage caused by a sliding object barely visible to human eyes. This minor damage phenomenon has rarely been systematically studied. Significant research efforts for the fundamental understanding of mar behavior in polymers are still needed. In this study, the mar behavior of a series of model amorphous polymers, i.e., polymethylmethacrylate (PMMA), polycarbonate (PC), and polystyrene (PS), were investigated based on a modified ASTM/ISO scratch testing methodology and a corresponding finite element method (FEM) modeling. Furthermore, the mar-induced visibility and material parameter relationships were established through a systematic FEM parametric study. Experimental results show that PMMA has the highest mar visibility resistance, indicated by lower surface roughness variation and low contrast between marred region and the background. The numerical analysis showed that the maximum principal plastic strain (ε 1 p) and total dissipated plastic energy (E p) can be considered for evaluating mar visibility resistance. Higher mar visibility resistance corresponds to lower ε 1 p and E p values. Based on these two criteria, the parametric analysis shows that mar visibility resistance increases with lower modulus, higher yield stress, higher hardening slope, and lower softening slope. The usefulness of the present study for the preparation of mar resistant polymers is discussed. • Experimental observation and FEM modeling have been carried out to study the mar behavior of amorphous polymers. • The ε 1 p and total E p on mar path are shown to be criteria for ranking mar visibility resistance of amorphous polymers. • Mar resistance can be improved with lower modulus, higher yield stress, higher hardening slope, and lower softening slope. [ABSTRACT FROM AUTHOR]

Machine components operating in a fluid under conditions of cavitation and hard particle erosion can be severely affected by wear, which may reduce the lifespans of the components. To understand this synergic behaviour, in this work, experimental and numerical approximations of the damage caused by a particle interacting with a spark-generated bubble were developed. The effects of particle size, particle material, bubble position, surface material and bubble size on the damage of a surface impacted by a particle propelled by the spark-generated bubble were evaluated. The experimental results show that under the tested conditions, the heaviest particles and larger bubbles caused more considerable damage, while the initial position of the bubble did not exert a significant influence. It was found that the relationship between the increase in the bubble size and the increase in damage was quadratic. Numerical simulations involving computational fluid dynamics (CFD) and explicit finite element analysis (FEA) of a particle interacting with bubble of several sizes were conducted. The findings exhibited good correlation with the experimental data which validated the proposed numerical models. Additionally, the simulation indicated that the damage on the surface was linearly related to the kinetic energy of a particle. Furthermore, it was identified that particles closer to the bubble nucleation point had higher velocities and could thus lead to more considerable damage to the surface; however, when the pressure inside the initial bubble was high (which produced larger bubbles), the bubble interface moved faster than the particle, and the particle was trapped by the bubble, which decelerated the particle and reduced the velocity of impact on the surface. The obtained results could help to explain the mechanism of interaction between the particle and the bubble and its correlation with solid surface damage. • The damage on a surface caused by a particle interacting with a bubble was studied. • Computational simulation of the phenomenon agreed with the experimental data. • The data showed a linear correlation between the damage and the kinetic energy. • Results can explain the synergic wear due to cavitation and hard particle erosion. [ABSTRACT FROM AUTHOR]