Sliding friction induced atom diffusion in the deformation layer of 0.45% C steel rubbed against Tin alloy

Evolution of microstructure and compositions in worn surface and subsurface of 45 (0.45 mass% carbon) steel disc slid against tin-alloy-pin was analyzed by SEM, TEM and SIMS. The mechanical alloying layer and plastic deformation layer were formed in the sliding friction-induced deformation layer (SFIDL) of 45 steel. Ultra-refine and nano grains were detected in the worn surface layer. Elements of Sn, Cu and Sb, originated from the mating tin-alloy-pin, with diffusion depth of 35 μm, 11 μm and 4 μm, respectively, were detected in its SFIDL. Mechanisms accelerating atom diffusion in SFIDL were subsequently propounded.     

Wear behaviour of dental enamel at the nanoscale with a sharp and blunt indenter tip

Two different diamond nanoindenter tips, a rounded conical (～1200 nm radius) and a sharp cube corner (20–50 nm radius) were used to abrade bovine enamel. Square abraded areas (2 μm × 2 μm, 5 μm × 5 μm, 10 μm × 10 μm) were generated with loads that varied from 50 μN to 500 μN depending on the indenter tip. In addition normal and lateral forces were simultaneously measured along 10 μm single scratched lines with the sharp cube corner tip. SEM (scanning electron microscopy) and TEM (transmission electron microscopy) were also used to characterise the worn areas and debris. Two different wear mechanisms were observed depending on the geometry of the tip. The rounded tip generates a predominantly elastic contact that mainly compresses and plastically deforms the superficial material and generates severe shear deformation within the sub-surface material which, under certain conditions, fractures and removes material from the sample. The sharp tip cuts into and ploughs the enamel creating a wedge or ridge of material ahead of itself which eventually detaches. This sequence is repeated continuously for every passage of the sharp indenter tip. The different mechanisms are discussed in terms of abrading tip contact angle and enamel microstructure.     

Study on the dynamic and static softening phenomena in Al–6Mg alloy during two-stage deformation through interrupted hot compression test

The dynamic and static softening phenomena in Al–6Mg alloy were studied through interrupted two-stage hot compression test performed isothermally at 480 °C and strain rate range of 0.001–0.1 s⁻¹. The interruptions of 29 and 90 s were considered when the true strain reached 0.5. It was concluded that the effect of static softening on the flow stress was not highlighted by extending the interruption at a constant strain rate. Also, it was exhibited that softening rate highly enhanced with the strain rate decrement at a constant time. Moreover, the static and dynamic recrystallization was revealed as the dominant softening mechanisms at low and high strain rates, respectively.     

Fabrication of a resin-bonded ultra-fine diamond abrasive polishing tool by electrophoretic co-deposition for SiC processing

A resin-bonded ultra-fine diamond abrasive polishing tool is fabricated by electrophoretic co-deposition (EPcD), and the processing performance of the tool is evaluated in this study. The dispersion stability of suspensions is characterized by a laser particle size analyzer and settlement ratio. The cathodic EPcD of composite powder is realized by adding Al³⁺ into the suspension. The sintering temperature of composite coatings is determined by differential thermal analysis/thermogravimetry. The surface morphology of the composite coating is observed under a confocal microscope. Results show that uniform, dense, and smooth coatings with diamond and resin particles distributed homogeneously are obtained from the steel substrate. A large (Φ150 mm) polishing tool with a 20 μm-thick coating is successfully prepared using the above process. A smooth mirror surface of SiC wafer with a nanoscale roughness (4.3 nm) is achieved after processing with the ultra-fine diamond abrasive polishing tool.     

Effects of carbon fiber length on the tribological properties of paper-based friction materials

Four kinds of paper-based friction materials reinforced with carbon fibers of 100, 400, 600 and 800 μm were prepared by paper-making processes. Experimental results showed that the friction materials became porous with fiber length increasing. The friction torque curves were flat except the sample with 100 μm fibers. The wear rate of the sample with 100 μm fibers was only 1.40×10⁻⁵ mm³/J. Tiny debris and fine scratches formed in the worn surface were the reason for excellent wear resistance of friction pairs with 100 μm fibers. The friction pairs with 400, 600 and 800 μm fibers showed typically abrasive wear and fatigue wear.     

Measurement and statistical analysis toward reproducibility validation of AZ4562 cylindrical microlenses obtained by reflow

This paper presents the statistical analysis applied into the shape of microlenses (MLs) for validating the high-reproducibility feature of their fabrication process. The MLs were fabricated with the AZ4562 photoresist, using photolithography and thermal reflow processes. Two types of MLs arrays were produced for statistical analysis purposes: the first with a cross-sectional diameter of 24 μm and the second with a cross-sectional diameter of 30 μm, and both with 5 μm spacing between MLs. In the case of 24 μm diameter arrays, the measurements showed a mean difference in diameter of 2.78 μm with a standard deviation (SD) of 0.22 μm (e.g., 2.78 ± 0.22 μm of SD) before the reflow, and 2.34 ± 0.35 μm of SD after the reflow. For the same arrays, the mean difference in height obtained was, comparatively to the 5.06 μm expected, 0.76 ± 0.10 μm of SD before the reflow and 1.91 ± 0.15 μm of SD after the reflow, respectively. A mean difference in diameter of 2.64 ± 0.41 μm of SD before the reflow, and 1.87 ± 0.34 μm of SD after the reflow was obtained for 30 μm diameter MLs arrays. For these MLs, a mean difference in height of 0.71 ± 0.12 μm of SD before the reflow and 2.24 ± 0.24 μm of SD after the thermal reflow was obtained, in comparison to the 5.06 μm of height expected to obtain. These results validate the requirement for reproducibility and opens good perspectives for applying this fabrication process on high-volume production of MLs arrays.     

Tribology characteristics of magnesium alloy AZ31B and its composites

In this paper, wear characteristics of magnesium alloy, AZ31B, and its nano-composites, AZ31B/nano-Al₂O_3, processed by the disintegrated melt deposition technique are investigated. The experiments were carried out using a pin-on-disk configuration against a steel disk counterface under different sliding speeds of 1, 3, 5, 7 and 10 m/s for 10 N normal load, and 1, 3 and 5 m/s for 30 N normal load. The worn samples and wear debris were then examined under a field emission scanning electron microscopy equipped with an energy dispersive spectrometer to reveal its wear features. The wear test results show that the wear rates of the composites are gradually reduced over the sliding speed range for both normal loads. The composite wear rates are higher than that of the alloy at low speeds and lower when sliding speed further increased. The coefficient of friction results of both the alloy and composites are in the range of 0.25–0.45 and reaches minimums at 5 m/s under 10 N and 3 m/s under 30 N load. Microstructural characterization results established different dominant mechanisms at different sliding speeds, namely, abrasion, delamination, oxidation, adhesion and thermal softening and melting. An experimental wear map was then constructed.     

Uncertainty analysis of slot die coater gap width measurement by using a shear mode micro-probing system

A shear mode micro-probing system was constructed for gap measurement of a precision slot die coater with a nominal gap width of 90 μm and a length of 200 mm. A glass micro-stylus with a nominal tip ball diameter of 52.6 μm was oscillated by a tuning fork quartz crystal resonator with its oscillation direction parallel to the measurement surfaces. An on-line qualification setup was established to compensate for the influences of the uncertainty sources, including the water layers on the measurement surfaces. The measurement uncertainty of the measured gap width was estimated to be less than 100 nm.     

Effect of tool pin design on the microstructural evolutions and tribological characteristics of friction stir processed structural steel

In this research, friction stir processing (FSP) technique is applied for the surface modification of ST14 structural steel. Tungsten carbide tools with cylindrical, conical, square and triangular pin designs are used for surface modification at rotational speed of 400 rpm, normal force of 5 KN and traverse speed of 100 mm min⁻¹. Mechanical and tribological properties of the processed surfaces including microhardness and wear characteristics are studied in detail. Furthermore, microstructural evolutions and worn surfaces are investigated by optical and scanning electron microscopes. Based on the achievements, all designed pins were successfully applicable for low carbon steel to produce defect-free processed material. By the microstructural changes within the stirred zone, the processed specimen is obtained higher mechanical properties. This is due to the formation of fine grains as the consequence of imposing intensive plastic deformation during FSP; however, this issue is highlighted by using square pin design. In this case, minimum grain size of 5 μm and maximum hardness of 320 VHN, as well as, maximum wear resistance are all examined for the specimen modified by square pin.     

Plastic deformation of 25CrMo4 steel during wear: Effect of the temperature,the normal force,the sliding velocity and the structural state

This article follows a previous study on friction and wear of 25CrMo4 steel [N. Khanafi-Benghalem, K. Loucif, E. Felder, F. Delamare, Influence de la température sur les mécanismes de frottement et d’usure des aciers X12NiCrMoSi25-20 et 25CrMo4 glissant sur du carbure de tungstène, Matériaux et techniques 93 (2005) 347–362]. The aim of our work is to study in more details the process of plastic deformation and the wear rate of this steel in lubricated sliding against cemented tungsten carbide, process observed in the previous work. The considered parameters are the temperature T (from 20 to 200 °C), the normal force P (from 500 to 1500 N), the steel structure (normalised HV 220 and quenched/tempered HV 480 states) and the sliding velocity v (from 0.05 to 0.3 m/s). We measured the friction coefficient and the sample total volume loss. A displacement sensor follows the volume loss evolution during the test; this follow-up is approximate because of the sample plastic flow which leads to the formation of peripheral burrs. All the tests conditions generate a significant plastic deformation of the sample steel, even in the quenched/tempered state: it produces a marked increase of the surface hardness, the work hardened layer being much finer for the quenched/tempered state (15 μm) than for the normalised state (40 μm at 20 °C). For temperatures T ≤ 100 °C in normalised state, the wear follows the Archard's law with an increasing rate with temperature. For T ≥ 120 °C, the wear rate decreases during the test, the global volume of wear being a decreasing function of T. For the quenched/tempered state, the wear rate decreases with the increase of the normal force, this decrease is less than 30% of the normalised state value. The material heating during the wear tests is well correlated with the friction dissipated power, but remains small, except in extreme cases (v maximum, great friction at high temperatures). These results suggest the existence of two wear mechanisms: abrasion by sample debris and burrs emission by plastic flow. The abrasion is probably the dominating mechanism for the tests carried out at the lowest temperatures. The plastic flow becomes a significant component at the highest temperatures. Using a contact model, we discuss to what extent the influence of the temperature and the strain rate on the steel hardness and ductility could explain the temperature and the sliding velocity effect on wear. Other phenomena are probably present: the influence of the steel microstructure and the lubricant on the size and/or the number of particles responsible for abrasion.     

Miniaturized liquid film sensor (MLFS) for two phase flow measurements in square microchannels with high spatial resolution

Two miniaturized liquid film sensors (MLFS) based on electrical conductance measurement have been developed and tested. The sensors are non-intrusive and produced with materials and technologies fully compatible and integrable with standard microfluidics. They consist of a line of 20 electrodes with a purpose-designed shape, flush against the wall, covering a total length of 5.00 and 6.68 mm. The governing electronics achieve 10 kHz of time resolution. The electrode spacing of the two sensors is 230 μm and 330 μm, which allows measurements of liquid films up to 150 μm and 400 μm for sensors MLFS_A and MLFS_B, respectively. The sensor characteristics were obtained by imposing static liquid films of known thickness on top of the actual sensor. Further dynamic measurements of concurrent air-water flow in a horizontal microchannel were performed. The line of electrodes is placed across the flow direction with an angle of 3.53° from the direction of flow, allowing for a spatial resolution perpendicular to the flow of 14.2 μm for sensor MLFS_A and 20.5 μm for sensor MLFS_B. The high time and spatial resolution allows for fast and accurate detection of the presence of bubbles, and even measurement of film thickness and bubble velocity. Further information, such as the bubble shape, can be gathered based on the shape of the liquid layer underneath the bubble, which is particularly important for heat transfer studies in microchannels.     

Microstructure,mechanical properties and tribological behavior of tribofilm generated from natural serpentine mineral powders as lubricant additive

Surface-modified serpentine powders with an average size of 1.0 μm were dispersed into mineral base oil to improve the lubricating properties of oil, as well as to generate a thin tribofilm on the worn surface. SEM, TEM, nano-indentation and Stribeck testing were performed to study the morphology, microstructure, micromechanical properties and tribological behavior of the tribofilm, respectively. Results show that a nanocrystalline tribofilm, with a thickness of 500–600 nm, is formed on the worn surface under the lubrication of oil with 1.5 wt% serpentine. The film is mainly composed of Fe₃O₄, FeSi, SiO₂, AlFe and Fe-C compound (Fe₃C). A phenomenological model of the tribofilm generated by serpentine was developed based on the experimental results. The excellent mechanical properties, reinforced phase of embedded particles and porous structure of the tribofilm contribute to the reduction of friction and wear, especially in the case of boundary and mixed lubrication.     

Erosion-oxidation of uncoated,aluminized and chromized-aluminized 9Cr–1Mo steels under fluidized-bed conditions at 550–700 °C

Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of many components in the energy production sector, such as heat exchangers. In this paper, some results are presented for uncoated, aluminized and chromized-aluminized 9Cr–1Mo steel, subjected to air and to impacts by 200 μm silica particles at angles of 30° and 90° and speeds of 7.0–9.2 m s^?1 at 550 –700 °C, in a laboratory fluidized-bed rig, to determine whether or not aluminized and chromized-aluminized diffusion coatings could protect the steel under such conditions. Erosion-oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy.Under most conditions, the coatings provided some protection to the substrate: under 30° impacts, up to 650 °C, and under 90° impacts, at 700 °C, both coatings were effective, whereas under 90° impacts, up to 650 °C, only the chromized-aluminized coating gave significant protection. However, for 30° at 700 °C, the oxide scale on the substrate was protective and the coatings were not needed. Explanations for these observations are presented in this paper, in terms of interactions between the erosion and oxidation processes for the materials.     

A method of reducing windage power loss of a high-Speed motor using a viscous vacuum pump

A new structure for reducing the windage power loss of a high-speed rotor using a spiral grooved viscous vacuum pump combined with an aerodynamic step thrust bearing is proposed. The proposed structure can pump out the air from within the sealed space of the motor housing by using the pumping effect of the spiral grooves and thereby reduce the windage power loss of the rotor. In addition, a small gap was automatically formed between the rotor and the viscous vacuum pump by using the force balance between the aerodynamic step thrust bearing and the elastic material supporting the flat plate of the vacuum pump. It was numerically shown that the proposed structure could reduce the pressure in the sealed space of the motor housing to 0.02 MPa at 30,000 rpm at a gap of 10 μm. In addition, the calculated results at 10,000 rpm were compared with the experimental results and showed good agreement and the proposed structure was very useful in reducing the windage power loss of a high-speed motor.     

Analysis of impact energy factors in ductile materials using single particle impact tests on gas gun

Wear is surface damage that involves progressive material loss due to relative motion between the contacting surfaces. Removal of material by action of impacting particles is known as erosion. Single particle impact tests were conducted using small particles (95–100 μm) and impact velocity 90 ms⁻¹. A new technique has been developed to measure the impact crater using Laser Scanning Confocal Microscope (LSCM). Depth of craters was calculated based on the impact parameters and the material properties and compared with measured values. The variations are discussed with the high strain-rate deformation and energy loss in the material through strain energy and heating.     

Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in situ composite

Resistance to wear is an important factor in design and selection of structural components in relative motion against a mating surface. The present work deals with studies on fretting wear behavior of in situ nano-Al₃Ti reinforced Al–Ti–Si amorphous/nanocrystalline matrix composite, processed by high pressure (8 GPa) sintering at room temperature, 350, 400 or 450 °C. The wear experiments were carried out in gross slip fretting regime to investigate the performance of this composite against Al₂O₃ at ambient temperature (22–25 °C) and humidity (50–55%). The highest resistance to fretting wear has been observed in the composites sintered at 400 °C. The fretting wear involves oxidation of Al₃Ti particles in the composite. A continuous, smooth and protective tribolayer is formed on the worn surface of the composite sintered at 400 °C, while fragmentation and spallation leads to a rougher surface and greater wear in the composite sintered at 450 °C.     

Friction-induced ultra-fine and nanocrystalline structures on metal surfaces in dry sliding

Sliding friction tests of pin-on-disc type were carried out for carbon steel, pure iron and pure copper, and the microstructure and hardness near the sliding surfaces were investigated in detail. It was found that patchy transfer layers with ultra-fine (<200 nm) structures were produced on the disc surfaces. Nanocrystalline grains of 30–50 nm were identified for carbon steel, and submicron sized grains of 100–150 nm were observed in pure copper. The thicknesses of the ultra-fine structures were in the range of 10–50 μm, depending on the specimen material, sliding speed and applied load. The hardness near the sliding surface of pure iron was increased compared with the matrix. It was suggested that the hardening was due to the very fine structure formed by severe plastic deformation, but not due to phase transformation caused by thermal effects.     

A new concept for producing ultrafine-grained metallic structures via an intermediate strain rate: Experiments and modeling

A novel experimental methodology to produce ultrafine-grained metallic microstructures, which is applied on aluminum is proposed in this work. In fact, the ultrafine-grained aluminum polycrystal is made from commercial purity powder by a combination of hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2 μm. The material is then subsequently impacted, using a falling weight at a maximum impact velocity of 9.2 m/sec. The resulting material shows a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture perpendicular to the impact direction. The mechanical properties of the impacted material are then characterized under compression tests at room temperature under a strain rate of 10^?4 s^?1. The effect of the change of the deformation path on the mechanical response parallel and perpendicular to the impact direction is also investigated. These results are discussed in relation with microstructure. Further, a new extension of a micromechanical approach developed by Abdul-Latif et al., [2] is proposed to predict the grain size effect on the enhancement of the mechanical strength of polycrystals. Within the framework of small strain hypothesis, the elastic anisotropy of the grain and grain rotation are neglected for the sake of simplicity. The local inelastic deformation heterogeneity is determined through the slip theory. It is assumed that the yield strength increases linearly with decreasing grain size as in Hall–Petch relationship. It is obviously recognized that the model with its new extension describes fairly well the effect of the grain size on the strain–stress behavior of the sub-micrometer aluminum.     

An ultra high-pressure test rig for measurements of small flow rates with different viscosities

Within the framework of a research project regarding investigations on a high-pressure Coriolis mass flow meter (CMF) a portable flow test rig for traceable calibration measurements of the flow rate (mass - and volume flow) in a range of 5 g min⁻¹ to 500 g min⁻¹ and in a pressure range of 0.1 MPa to 85 MPa was developed. The measurement principle of the flow test rig is based on the gravimetrical measuring procedure with flying-start-and-stop operating mode. Particular attention has been paid to the challenges of temperature stability during the measurements since the temperature has a direct influence on the viscosity and flow rate of the test medium. For that reason the pipes on the high-pressure side are double-walled and insulated and the device under test (DUT) has an enclosure with a separate temperature control. From the analysis of the first measurement with tap water at a temperature of 20 °C and a pressure of 82.7 MPa an extensive uncertainty analysis has been carried out. It was found that the diverter (mainly due to its asymmetric behaviour) is the largest influence factor on the total uncertainty budget. After a number of improvements, especially concerning the diverter, the flow test rig has currently an expanded measurement uncertainty of around 1.0% in the lower flow rate range (25 g min⁻¹) and 0.25% in the higher flow rate range (400 g min⁻¹) for the measurement of mass flow. Additional calibration measurements with the new, redesigned flow test rig and highly viscous base oils also indicated a good agreement with the theoretical behaviour of the flow meter according to the manufacturers׳ specifications with water as test medium. Further improvements are envisaged in the future in order to focus also on other areas of interest.     

Direct measurement of bottom shear stress under high-velocity flow conditions

The objectives of the present research are to accurately measure bottom shear stress under high-velocity flow conditions. To achieve high-velocity flow conditions, a laboratory-scale flume has been specially built in which flow velocity can reach over 3 m s⁻¹. Also an instrument that can directly measure bottom shear stress has been developed and validated. Then, the flow resistance has been estimated by simultaneously measuring flow velocity and bottom shear stress. It appears that the shear stress is indeed proportional to velocity squared and also to Reynolds number. On the other hand, Manning's n value and the skin friction factor are more or less uniform across all experimental cases.