Subsurface damage during dry sliding wear of Al-Al3Ni eutectic alloy

Cylindrical Al-Al₃Ni eutectic alloy wear pins (10 mm in diameter) were slid against a polished steel surface in a pin-on-disc rotating machine under unlubricated conditions with bearing pressures of 6–60 kPa and a constant sliding speed of 70 m min^?1. Metallographic changes in the subsurface region of contact were examined by optical microscopy and microhardness measurements. In the bearing pressure range investigated the alloy exhibited “mild” wear in two linear regions identified as pure “oxidative” wear at low bearing pressures and oxidative with superimposed “metallic” wear at higher bearing pressures. Plastic deformation and fragmentation of the Al₃Ni phase occurred under all bearing pressures. However, in composites prepared by unidirectional solidification containing large Al₃Ni particles fragmentation was insignificant. In all other specimens the size of the fragmented particles in the subsurface region of contact was about 5 μm irrespective of the bearing pressure.     

Effects of normal load on single-pass scratching of polymer surfaces

The effects of normal load and the resulting scratch depth on scratch force profile, scratch hardness and the mechanisms of deformation and material removal for a number of industrially important polymers are studied. Upon scratching by a 30° angled conical tip, the mean tangential or scratch force is found to be linearly related to the normal load at lower speed (0.2 mm s⁻¹); however, at higher scratching speed (2.0 mm s⁻¹), there is a decrease in the slope of the scratch force versus normal load curve for all polymers. The phenomenon of stick-slip is severe at higher normal loads and scratch depths for the polymers that show ductile nature. The scratch hardness for softer polymers tends to decrease with normal load, whereas for harder polymers, scratch hardness increases for intermediate loads and tends to decrease at very high loads. The deformation mechanism, to a large extent, is insensitive to the imposed normal load or the depth of scratching; however, material removal and debris formation process depends upon the scratch depth.     

A new method to investigate the sliding wear behaviour of materials based on energy dissipation: W–25 wt%Cu composite

A new method is proposed to explain and monitor wear behaviour based on energy dissipation. The wear of a W–25 wt%Cu composite against 52100 steel was used to demonstrate this approach with pin-on-disc tests conducted under three normal loads. An energy-dependent criterion, namely, specific wear volume (wear volume/dissipated energy (mm³/J)), was defined to evaluate the wear of the composite. The specific wear volume can be used as a substitute for the traditional wear rate due to the simultaneous expression of several wear parameters and because of its strong dependence on the wear mode. The specific wear volume appears to be constant in any particular “wear mode” regardless of the active “wear processes”. In the wear of this composite, processes such as particle pull-out, mechanically mixed layer (MML) formation, crack propagation and delamination were observed. But, combination of these processes in each test had identical specific wear volumes. Thus, all of these wear processes were considered to be consecutive stages of the same wear mode: fatigue wear. The amount of dissipated energy and the volumetric loss increased with increasing normal load. Also, changing the normal load changed the rate of energy dissipation per unit sliding distance.     

An impact and erosion study of polyetheretherketone

Single-particle impacts by 4 and 5 mm diameter steel spheres on polyetheretherketone (PEEK) at various angles and speeds were studied using high speed photography and scanning electron microscopy. The following two material removal mechanisms were identified:

• 1.(1) the drawing out of filaments in oblique impact;
• 2.(2) material jetting in normal impact for speeds of approximately 480 m s⁻¹. A “deformation map” was constructed giving the types of crater formed by these spheres at given angles and speeds. Low speed (about 30 m s⁻¹) damage by quartz sand grains, sieved to give the size range 300–600 μm, was also studied. Various types of damage site were identified and the proportion of each for angles of impact ranging from 20° to 90° determined. The compressive stress-strain curve at a strain rate of 10⁴ s⁻¹ has been measured and a high speed photographic sequence of the deformation of a PEEK disc at a strain rate of approximately 2 × 10³ s⁻¹ is presented.

     

The study of nanoscratch and nanomachining on hard multilayer thin films using atomic force microscope

In this study, nanoscratching and nanomachining were conducted using an atomic force microscope (AFM) equipped with a doped diamond‐coated probe (DDESP‐10; VEECO) to evaluate the fabrication of nanopatterns on hard, Cr₂N/Cu multilayer thin films. The influence of normal force, scratch speed, and repeated scratches on the properties of hard multilayer thin films was also investigated. The nanoscratch experiments led researchers to establish a probe preparation and selection criteria (PPS criteria) to enhance the stability and accuracy of machining hard materials. Experimental results indicate that the depth of grooves produced by nanoscratching increased with an increase in normal force, while an increase in the number of scratches in a single location increased the groove depth but decreased friction. Therelationships among normal force and groove depth more closely resembled a logarithmic form than other mathematical models, as did the relationship between repeated scratching and its effect on groove depth and friction. The influence of scratch speed on friction was divided into two ranges. Between 0.1 and 2 µm/s, friction decreased logarithmically with an increase in scratch speed; however, when the speed exceeded 2 µm/s, the friction appeared stable. In this study, multilayered coatings were successfully machined, demonstrating considerable promise for the fabrication of nanopatterns in multilayered coatings at the nanoscale. SCANNING 34: 51–59, 2012. © 2011 Wiley Periodicals, Inc.     

Rail–wheel interaction monitoring using Acoustic Emission: A laboratory study of normal rolling signals with natural rail defects

This paper presents a laboratory study on the Acoustic Emission (AE) generated during railway wheel–rail track interaction, with a view to developing methods of in situ rail–wheel interaction monitoring using rail-mounted sensors. It is known that the physical processes of impact and wear generate AE and it was therefore expected that axle loads, speed and traction would influence the AE generated by an interaction and that the characteristics of “normal” interaction would be affected by wheel and/or track defects and/or any misalignment between rail and track.A set of laboratory experiments were carried out on a scaled test rig to characterise the continuous AE generated by a wheel rolling on a rail and, secondarily, to assess the effect on the AE characteristic of the natural defects present on the contact profile of the rail. The natural defects were of a relatively minor nature and their assessment serves as part of the calibration of background AE for experiments with more significant simulated defects.A simplified analytical model, devised for AE waves propagating from a moving source, based on “vehicle” speed and wave damping coefficients, has been developed for the test track and fitted to the measured results. As a wheel rolls towards a sensor and then away from the sensor the measured AE generally rises and falls in a predictable way. The effects of wheel and rail surface features were found to introduce deviations from this “background”, and a method to identify the location of surface defects, based on identifying peaks above the background is also demonstrated.     

Failure mechanisms and cutting characteristics of brazed single diamond grains

The knowledge about the cutting characteristics and the critical loading of brazed diamonds is essential for a safe and economic application of engineered grinding tools. Scratch tests were performed with single grains. The experiments were conducted with standard polyhedral diamond grains of different sizes, ranging from 300 to 850 μm, brazed with an Ag-Cu-based and a Cu-Sn-based active filler alloy onto a steel pin. Two failure mechanisms were revealed, namely “grain pullout” and “grain fracture”. Large grits mainly fail by grain fracture, whereas the smaller ones were mostly pulled out. This trend is supported by a simple mechanical model. The critical values, i.e. cutting force/scratch area, for grain fracture and grain pullout show a decrease with bigger grit size. Scratches are also analysed in terms of cutting characteristics. The dependency of the cutting and the normal force on the scratch area can be described by a power law with powers ranging between about 0.2 and 0.7, respectively. The measured cutting forces strongly depend on the rake angle, which was tested for ?19.5° and ?35.3°.     

Filling the gap of a plasma opening switch with plasma from an electroexploding wire in an external magnetic field

The dynamics of filling the interelectrode gap of a plasma opening switch (POS) using an electroexploding wire with a diameter of 4 μm (tungsten) and 6 μm (carbon) is considered. The wire was connected to coaxial electrodes of the POS perpendicular to the force lines of a longitudinal magnetic field, which was created by an external source. When a current was transmitted through the wire, the longitudinal speed of produced plasma was ≈10⁶ cm/s, and the azimuthal speed was ≈10⁷ cm/s (for tungsten) and ≥1.3 × 10⁷ cm/s (for carbon). As a result, a plasma “washer” was formed and the total quantity of particles in it was determined by the parameters of the wire and the POS gap. This result is the first step in a solution of “the first shot” problem.     

An experimental and theoretical investigation of pressures in four-lobe bearings

A four-lobe bearing 25.4 mm in diameter was tested with the load vector “on pad” and “off pad”. Static pressures were measured on the center line of the bearing and these experimental data are compared with two sets of theoretical results. The half-Sommerfeld and Reynolds boundary conditions were each used in the theoretical predictions. Of particular significance is the fact that the experimental data correlate best with the half-Sommerfeld solution. Differences between the data and the theoretical solution of the order of 10–35% and 15–100% are realized for the half-Sommerfeld and Reynolds boundary conditions respectively. Moreover, the trends of pressure versus rotational speed for the experimental data and the theoretical solution are the same for the half-Sommerfeld condition but sometimes differ for the Reynolds condition.     

Detection of Damage in Assembled Rolling Element Bearings

A nondestructive method, capable of detecting localized surface damage, such as small spalls, nicks, dents and scratches, on the balls (rollers) and races of assembled rolling-element bearings, is discussed. The method utilizes the “almost periodical” character of vibration peaks produced by rotating a damaged bearing at constant speed. These “almost periodical” peaks are compared to the random vibration peaks, generated by the undamaged portions of the rolling surfaces of the bearing. Instrumentation to accomplish this comparison is described. It includes a vibration test spindle with pickup and amplifier, electronic gating circuitry serving as frequency and phase discriminator, and electronic counters to count “almost periodic” and random vibration peaks. Different methods of interpreting the counts and evaluating the degree of bearing damage are discussed.     

Characterisation of DLC coating adherence by scratch testing

In order to characterize the adherence of DLC coatings (Diamond Like Carbon), scratch testing was performed on a unit equipped with sensors for normal and tangential forces, and an acoustic detector to detect the nucleation and the propagation of cracks. The system is also equipped with a microscope permitting observation of each event on the scratch according to the friction tangential force signal or the acoustic signal. The local microscopic observation allows identification of the damage with respect to the normal load. The test was performed with a Rockwell C indenter at the relative displacement speed, v=10 mm/min under a progressive normal load from 5 to 55 N.Coating failure appears in various modes, particularly the following: propagation of the cracks along the longitudinal edges of the scratch; propagation in front of the indenter; rupture along the maximum principal stress lines; and, detachment in the subsurface by shearing of the coating. The microscopic analysis of the evolution of the scratch under a progressive normal load permits identification of the various traces and the damage mechanisms of the coating.In this study, experimental results are shown for the scratch tests on bulk glass and DLC coating. Various modes of crack initiation, damage and rupture of these materials according to the critical normal load are presented. The analysis of the contact stress field distribution in bulk glass enables identification of the crack initiation and its propagation in the coating.     

The Effect of “Hot-Spot” Temperatures on the Unlubricated Wear of Steel

Experiments are described in which pins of low-alloy, medium-carbon steel are worn against disks of the same material under unlubricated sliding conditions. The friction and wear characteristics of this system are measured as functions of load and speed. The choice of loads and speeds was made in such a way as to obtain the entire range of “hot-spot” temperatures possible for the system. The results are then compared with those to be expected from a model of the wear process in which the wear at the contacting regions between the pin and the disk is closely associated with the oxidation of the metal in these regions. The temperature of oxidation is assumed to be the calculated “hot-spot” temperature. In order to make the results compatible with the proposed model, it is necessary to introduce a new parameter (having the dimensions of length) which is shown to increase steeply with increasing “hot-spot” temperature up to about 700 C. It then levels off, at about 10^?6 cm, for all hot-spot temperatures in excess of about 700 C. In this way, the hot-spot temperature is shown to be a very important variable in the wear of steel.     

Nanoscale Fabrication of the Ferroelectric Polymer Poly(vinylidene Fluoride with Trifluoroethylene) P(VDF‐TrFE) 75:25 Thin Films by Atomic Force Microscope Nanolithography

Thin films of an organic ferroelectric system, poly(vinylidene fluoride with trifluoroethylene) P(VDF‐TrFE, Kureha Corporation, Tokyo, Japan) 75:25 layers, have been deposited on highly ordered pyrolytic graphite and silicon dioxide by the horizontal Schaefer method of Langmuir–Blodgett techniques. It is possible to “shave” or mechanically displace small regions of the polymer film by using atomic force microscope nanolithography techniques such as nanoshaving, leaving swaths of the surface cut to a depth of 4 nm and 12 nm exposing the substrate. The results of fabricating stripes by nanoshaving two holes close to each other show a limit to the material “stripe” widths of an average of 153.29 nm and 177.67 nm that can be produced. Due to the lack of adhesion between the substrates and the polymer P(VDF‐TrFE) film, smaller “stripes” of P(VDF‐TrFE) cannot be produced, and it can be shown by the sequencing of nanoshaved regions that “stripes” of thin films can be removed. SCANNING 34: 404‐409, 2012. © 2012 Wiley Periodicals, Inc.     

The Assessment of Interface Adhesion of Cu/Ta/Black Diamond™/Si Films Stack Structure by Nanoindentation and Nanoscratch Tests

The interface adhesion of the Cu/Ta/Black Diamond? (SiOC:H, BD, low-k)/Si substrate films stack structure was investigated. During the nanoindentation tests, a series of indentations under varied maximum normal loads of 1–120 mN were carried out. Regular triangular marks were formed on the surface, and the material pileup around the marks was clearly observed. The delamination occurred first at the Cu/Ta interface with the critical normal load of about 3.14 mN. As the normal load increased to about 63.71 mN, the BD layer began to delaminate from the Si substrate, resulted from the propagation of the cracking within BD layer along the BD/Si interface. The failure behaviors of the stack structure during the nanoscratch tests were similar to that during the nanoindentation tests. At the scratch velocity of 500 μm/min, the critical normal loads for Cu/Ta and BD/Si interfaces delamination were around 15.55 and 27.44 mN, respectively. Furthermore, the critical normal loads were decreased with the increase of the scratch velocity. Due to the similarity between the nanoscratch test and the chemical mechanical polishing (CMP) process, these results implied that lower polishing speed was preferred to avoid the interface delamination during the CMP of Cu/low-k interconnect structure.     

Bubble sizes and shapes in a counter-current bubble column with pure and binary liquid phases

It is generally admitted that the “global-scale” behavior of bubble columns is imposed by the “local-scale” phenomena. For this reason, understanding the fluid dynamics in bubble columns relies on the precise knowledge of the so-called “birth and life” of bubbles. A-priori knowledge of the bubble sizes and shapes is required to characterize the “local-scale”, to understand the “global-scale”, to set-up and validate numerical models, as well as to support scaling-up methods towards the “industrial-scale”. This paper contributes to the present-day discussion by proposing an experimental research devoted to clarify the relationships between the bubble sizes and shapes, the integral flow parameters, and the liquid phase properties. The experimental study, based on a bubble-identification methods, was performed in a “large-scale” bubble column (inner diameter equal to 0.24 m, height equal to 5.3 m) operated in the batch and in the counter-current modes with pure (deionized water) and binary (mixture of ethanol and deionized water) liquid phases. The system was operated in the pseudo-homogeneous flow regime with superficial gas velocities in the range of 0.0037–0.0188 m/s and superficial liquid velocity, in the counter-current mode, equal to −0.066 m/s. In the different experimental runs, bubble size distributions and shapes were obtained at different radial and axial locations. The experimental observations have been presented, compared with literature correlations, used to develop novel correlations (to be applied in practical applications), compared with previously obtained experimental data and interpreted in a multi-scale point of view. The comprehensive dataset obtained within this research may be used to improve the validation of numerical approaches and, in particular, to tackle the unsolved issue of developing break-up and coalescence kernels.     

Effect of Air and Helium on the Head–Disk Interface During Load–Unload

The tribological characteristics of the head–disk interface are investigated during load–unload for air and helium-filled drives as a function of the pitch static angle and the roll static angle between slider and disk. A custom-made experimental tester inside a sealed environmental chamber was used to determine the regions of “safe” pitch static angle and “safe” roll static angle in air and helium environment during the load–unload process. The presence of head–disk contacts during load–unload were evaluated by measuring the acoustic emission signal and the decrease in rotational speed of the spindle. Scanning electron microscopy and optical surface analysis were used to investigate wear of the slider and the redistribution of lubricant on the disk surface after 10,000 load–unload cycles. The results indicate that the tribological performance of the head–disk interface is improved in helium environment compared to air environment.     

Fortpflanzung von oberflächenstörungen bei aufeinander abrollenden radial belasteten Rädern

Rolling test-rig trials were conducted in which railway wheelsets ran under normal load without slip and at constant speed upon rail profiled wheelsets. The wheelsets experienced occasional interference owing to the occurrence of periodic pitting or grooving in the running surfaces. Both mating wheels indicated an aliquant number of wear periods.Although of equal periodic length, the wear phenomena were of varying character such that they could not have been due to a reciprocal pressure process. It was demonstrated that the length and pitch of the wear periods could be explained as being due to surface wave pulses of propagation velocity c = 2950 m s^?1 circulating both wheels. Relations between wheel diameters, running speed and velocity c were formulated, under which (at adequate load application) periodic pitting or grooving could be expected to occur in the contact area. The surprising recurrence of extremely minute wear pits of less than 1 mm in size and the occurrence of pits and grooves at equidistant lateral intervals led to the assumption that, generally, interaction between acoustic emission and pitting can occur, in the course of which “coherent” ultrasonic waves are induced. On numerous occasions during test-rig trials, corrugations instead of pitting formed on one of the wheelsets and an even number of corrugations appeared on the rim. Circulating Rayleigh wave pulses seem to be involved in this phenomenon and the presence of even numbers seems to be due to a stationary surface wave on the wheel.     

Multifocus watermarking approach based on discrete cosine transform

Image fusion process consolidates data and information from various images of same sight into a solitary image. Each of the source images might speak to a fractional perspective of the scene, and contains both “pertinent” and “immaterial” information. In this study, a new image fusion method is proposed utilizing the Discrete Cosine Transform (DCT) to join the source image into a solitary minimized image containing more exact depiction of the sight than any of the individual source images. In addition, the fused image comes out with most ideal quality image without bending appearance or loss of data. DCT algorithm is considered efficient in image fusion. The proposed scheme is performed in five steps: (1) RGB colour image (input image) is split into three channels R, G, and B for source images. (2) DCT algorithm is applied to each channel (R, G, and B). (3) The variance values are computed for the corresponding 8 × 8 blocks of each channel. (4) Each block of R of source images is compared with each other based on the variance value and then the block with maximum variance value is selected to be the block in the new image. This process is repeated for all channels of source images. (5) Inverse discrete cosine transform is applied on each fused channel to convert coefficient values to pixel values, and then combined all the channels to generate the fused image. The proposed technique can potentially solve the problem of unwanted side effects such as blurring or blocking artifacts by reducing the quality of the subsequent image in image fusion process. The proposed approach is evaluated using three measurement units: the average of Q^abf, standard deviation, and peak Signal Noise Rate. The experimental results of this proposed technique have shown good results as compared with older techniques. Microsc. Res. Tech. 79:431–437, 2016. © 2016 Wiley Periodicals, Inc.     

DIUS-1.15M Mobile Hardware–Software Structuroscopy System

It is shown that by measuring the magnetic field in a specially shaped hole made in the magnetic circuit of a measuring transducer and the tangential component of the magnetic field in the transducer’s interpolar space near the surface of an object being magnetized, it is possible to locally determine the relative values of the magnetic properties of the test-object substance. The physical rationale is provided for a technique that makes it possible to determine the entirety of hysteresis characteristics and significantly expand the ranges of measurement of magnetic properties. The difference is established between the structural sensitivities of magnetic properties locally determined in terms of “flux–internal field” (the properties of matter) and “flux in the product–current in the windings” (the properties of “body”) coordinates. It is shown that it is possible and expedient to simultaneously measure and jointly analyze the magnetic characteristics of matter and “body”. Information is provided on the DIUS-1.15M mobile hardware–software system, intended for implementing single- and multiparameter techniques of monitoring the structure–phase and stress–strain states of ferromagnetic materials and objects based on the proposed measurement procedures.