Low Normal Loads Research Articles

Tribological performance of silicone composite coatings filled with wax-containing microcapsules

The effect of wax-containing microcapsules incorporated in silicone composite coatings deposited on aluminum (Al) alloy substrates on the tribological performance of the coatings was systematically investigated. The wax-containing microcapsules were prepared via in situ polymerization. The tribological behavior of the composite coatings was evaluated using ball-on-disk tribological test. It was found that the increase in microcapsule concentration in the composite coatings apparently reduced the friction coefficient of the coatings because the lubricant released from the broken microcapsules during the tribological test of the coatings lubricated the rubbing surfaces. The results showed that the silicone composite coatings rubbed by a smaller Cr6 steel ball (3mm diameter) under a lower normal load (100mN) produced higher friction coefficients via reduced complication of their underlying strong substrates compared to the same coatings tested against a larger Cr6 steel ball (6mm diameter) under a higher normal load (1N).

S116051 原子間力顕微鏡を用いた繰り返ししゅう動における単分子吸着膜の変形破壊挙動の観察

Island-shaped self-assembled monolayer (SAM) of octadecyl-trichlorosilane was repeatedly slid with the atomic force microscope (AFM) to clarify the deformation and fracture behavior, employed as model for localized adsorption films in practical boundary frictional conditions. There were two types of the island-shaped adsorption film; one is well-assembled monomolecular film with 2.2nm in thickness, another is less-assembled monomolecular film with about 0.8nm in thickness. The well-assembled monomolecular film can respond like as solid film with large rigidity, and then, has Coulomb frictional property. On the other hand, the less-assembled film can respond like as elastic material if relatively low normal load is applied. In this case, friction coefficient also linearly depends on the applied normal load. Moreover, if the normal load increases by certain level, kinetic response of the film indicates plastic behavior. That is, there is irreversible reduction of film thickness due to the applied load and its associated increase in friction coefficient.

S115023 微小領域におけるトライボロジー特性

This paper describes tribology in micro-scale order that is influenced by low friction force caused by low normal load. Recently, micromachines have been used in various fields. Tribolobical property in micro-scale such as micromachines move is known to be different as compared to macro-scale tribology. The aim of this research is to clarify microtribology and evaluate it. We selected diamond stylus tip and glass wafer as specimens in this research. In this experiment, friction force between two specimens is measured by using scratch tester. Besides, we observed friction groove and measured its depth with SEM. In conclusion, some knowledge about microtribology has clarified.

Transition of Wear Mechanisms of Plasma Source Nitrided AISI 316 Austenitic Stainless Steel Against Ceramic Counterface

A single high-nitrogen face-centered-cubic (f.c.c.) phase (γN) layer formed on the plasma source nitrided AISI 316 austenitic stainless steel at a nitriding temperature of 450 °C for a nitriding time of 6 h. An approximately 17 μm-thick γN layer has a peak nitrogen concentration of about 20 at. %. Tribological properties of the γN phase layer on a ball-on-disk tribometer against an Si3N4 ceramic counterface under a normal load of 2 and 6 N with a sliding speed of 0.15 to 0.29 m/s were investigated by friction coefficient and specific wear rate measurement. Worn surface morphology and wear debris were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The microhardness of the γN phase layer on the nitrided stainless steel was measured as about 15.1 GPa. The change in the friction coefficient of the γN phase layer on the stainless steel was dependent on the applied normal load, which was associated with that in the specific wear rate. Under a lower normal load of 2 N, the lower specific wear rate of the γN phase layer with a sliding speed of 0.15 m/s was obtained as 2.8 × 10−6 mm3/N m with a friction coefficient of 0.60. Under a higher normal load of 6 N, the lower specific wear rate with a sliding speed of 0.29 m/s was 7.9 × 10−6 mm3/N m with a friction coefficient of 0.80. When the applied load increased from 2 to 6 N, a transition of the wear mechanisms from oxidative to abrasive wear was found, which was derived from the oxidation reaction and the h.c.p. martensite phase transformation of the γN phase during the wear tests, respectively.

Micro-abrasion wear testing of triode plasma diffusion and duplex treated Ti–6Al–4V alloy

In this paper micro-abrasion wear testing is used to evaluate the wear resistance of triode plasma diffusion-treated, single-layered TiN-, CrAlN-, and WC/C-coated and duplex-diffusion and coated Ti–6Al–4V under uniform three-body rolling abrasion. Nanoindentation, Knoop microhardness, mechanical surface profilometry, optical microscopy, scanning electron microscopy and atomic force microscopy, were used to characterise the surfaces under investigation. Optimum testing conditions for rolling abrasion were established by varying the test parameters and resultant severity of contact. Very low normal loads and high volume fractions of particles in the abrasive slurry are necessary to obtain predictable and reproducible results. Relatively coarse SiC abrasive particles, having a mean diameter of around 3μm, appear more suitable for micro-abrasion testing of the samples investigated, compared to finer Al2O3 particles. Problems associated with the measurement of the scar volume and subsequent calculation of the wear rate for hard coatings deposited on relatively soft metals like titanium are identified, and suitable testing and measurement techniques are suggested. Three-dimensional wear scar maps generated by mechanical stylus profilometry were used to measure the wear volumes. Under the test conditions used, wear coefficients can be determined from perforating and non-perforating tests, although perforating tests provide more consistent results. Triode plasma diffusion treatments, plasma-assisted (PA) PVD TiN and PAPVD CrAlN can reduce the specific wear rate of Ti–6Al–4V, while PACVD-based WC/C coatings do not provide suitable protection against abrasive wear. The combination of triode plasma oxynitriding diffusion treatments and PVD coatings to create duplex treatments can also lead to further reductions in the coating wear coefficient when compared to non-duplex coatings deposited on non-pretreated substrates.

Tribological properties of polymer/silica composite coatings for microsystems applications

The tribological properties of composite coatings consisting of silica nanoparticles dispersed in either a poly-methylmethacrylate (PMMA) or polystyrene (PS) matrix were assessed. The experiments were conducted using a reciprocating type of micro-tribotester under relatively low normal loads ranging from 5 to 15 mN. Results indicated that the wear resistance of PMMA could be significantly improved by adding silica particles at the cost of increased friction coefficient from 0.27 to 0.4. The effect of silica content on the wear resistance of PS was less apparent. Such outcome was attributed to the difference in the compatibility between silica and the polymer matrix.

Improvement of the tribological properties of pure Ti by laser cladding intermetallic compound composite coating

Intermetallic compound composite coating was fabricated on pure Ti substrate by laser cladding. The microstructure and tribological properties of the prepared intermetallic compound coating sliding against AISI-52100 steel ball at different normal loads were evaluated. The morphologies of the worn surfaces were analysed by means of scanning electron microscopy and three-dimensional non-contact surface mapping. It is shown that the microhardness and wear resistance of the pure Ti substrate are greatly increased after laser cladding intermetallic compound coating, due to the formation of hard phases in the composite coating. The intermetallic compound composite coating, involving only mild abrasive wear and adhesive wear under lower normal load, and abrasive, adhesive and microfracture under higher normal load, when sliding against the AISI-52100 counterpart.

Characterizing moist food-powder flowability using a Jenike shear-tester

A range of commercially-available crop seeds and food powders were tested using a Jenike shear-tester. At low normal loads, all the materials are found to change their strength characteristics regularly, with increasing moisture content resulting in higher values for shear stresses. However, under high normal loads, an opposite trend was observed for some materials, for which there is the potential of a lubricating effect to develop, resulting in unpredictable bed behavior in a silo. Low moisture contents reduced shear-stress oscillations occurring in the bed of dry material considerably. At higher moisture contents, an unacceptably long horizontal shear displacement was needed to establish steady-state shearing conditions. This may render the Jenike shear-tester inappropriate in the examination of powders of higher moisture content, especially those composed of easily deformable particles. NMR analysis proved that chemical composition of water surrounding the moistened seeds was changing during the time and that may affect the granular bed flow characteristics.

In vitro study on the wear behaviour of human tooth enamel in citric acid solution

Abstract With a worldwide increase in the ingestion of acidic soft drinks, the wear of human teeth in acidic oral environment is becoming normal, especially in young people. Previous studies mostly focused on the friction and wear behaviours of human teeth in the artificial saliva, and wear behaviours in acidic environment are still far from being well understood. In this paper, the wear behaviour of human tooth enamel against titanium alloy in 0.001 M citric acid solution (pH 3.20) has been investigated using a reciprocating wear machine. Three normal loads, 10 N, 20 N, and 40 N, were used. Results showed that in the citric acid solution, a honeycomb-like structure appeared on the surface of enamel as a result of severe dissolution of the enamel rods, and the surface hardness decreased. Under a low normal loading level, the surface softening of enamel caused by erosion dissolution played a significant role in its wear behaviour, and the wear mechanism of enamel was dominant by adhesion delamination. Hence, enamel wear in the citric acid solution was significantly higher than in the artificial saliva. With the load increasing, brittle fracture by the loading force aggravated, and obvious flake delamination happened. Enamel wear tended to be characterized mainly by mechanical removal. As a result, both the wear morphology and wear loss of enamel in the citric acid solution were similar to those in the artificial saliva at a high normal load of 40 N. These results indicated that there existed a competitive mechanism between the mechanical action and the chemical action when enamel wear occurred in the citric acid solution. The effect of erosion on the wear behaviour of enamel was more obvious under a low load than under a high load. The results would help extend understanding of the tooth wear process in acidic environment and provide a more rational explanation for the mechanism of tooth wear in the mouth.

Plastic hardening in cubic semiconductors by nanoscratching

The effect of scratch proximity on the resistance to plastic deformation in InP (100) crystals under low normal loads has been studied using atomic force microscopy (AFM) and transmission electron microscopy. Plastic flow has been observed for scratches performed with an atomic force microscope along ⟨110⟩ and ⟨100⟩ crystallographic directions. Plastic hardening has been determined from AFM measurements of the scratch depth and width, as a function of the distance between parallel scratches. For relatively low loads, hardening is found to be independent of the crystallographic direction of the scratch. Significant hardening takes place for scratch separations of less than ∼80 nm. Analysis of the microstructure indicates that hardening occurs due to the interaction of dislocations generated at adjacent scratches and acting on different slip planes.

Dry sliding wear behaviour of medium carbon steel against an alumina disk

The dry sliding wear behaviour of a medium carbon steel against an alumina disk was studied in different heat treated conditions. The wear mechanism in the hardened and tempered steel at 30 N and 40 N loads involved an initial adhesive wear followed by an intermediate oxidative wear regime and reappearance of the adhesive wear regime on removal of oxide layer. At 45 N load, the mechanism changed to abrasive wear along with the existence of intermediate oxidative wear of shorter duration. On the other hand, at lower (30 N) and higher (45 N) normal loads the wear mechanism in forced air cooled steel consisted of alternate regimes of adhesive wear and oxidative wear. However at 40 N load, the wear mechanism involved an initial oxidative wear regime for a long duration followed by an adhesive wear regime. The hardened and tempered steel with higher initial hardness and strength, exhibited better wear resistance at lower normal loads (30 N and 40 N) than the forced air cooled steel. However, its wear resistance at higher load (45 N) became inferior to the forced air cooled steel due to lesser extent of work hardening, shorter duration of oxidative wear regime and the abrasive wear being operative.

Characteristics of Extrusive Wear and Transition of Wear Mechanisms in Elevated-Temperature Wear of a Carbon Steel

The dry sliding wear of a medium carbon steel with different microstructures was measured under the normal load range of 50–150 N at 400°C by a pin-on-disc high-temperature wear setup. The wear behavior and wear mechanism were systematically studied; in particular, the characteristics of extrusive wear and the transition of wear mechanisms were investigated. Under low normal loads, the wear is oxidative type wear. Once the normal load reached a critical value, a mild-to-severe wear transition occurred, and subsequently an extrusive wear prevailed. The mild-to-severe wear transition depended on the microstructure of matrix; the critical normal load of the transition was 112.5 N for tempered sorbite, 125 N for lamellar pearlite, and 137.5 N for tempered martensite and tempered troostite. As oxidative wear prevailed, a thick oxide layer about 20–30 μ m and a plate-like wear debris with regular outline were recognized. However, as the extrusive wear occurred, the wear rate abruptly increased but the friction coefficient was reduced. The extrusive wear predominated due to thermal softening of the matrix and presented a superthin oxide layer (less than 0.5 μ m) and low oxide content on worn surfaces, accompanied by the appearance of ribbon-like wear debris.

Prediction of Fretting Fatigue Life of Aluminum Alloy LY12CZ

Stress in the interface of contacts was calculated applying finite element software/ABAQUS, with which the Ruiz fretting damage parameter was obtained and the location for crack formation was found. A new model for predicting fretting fatigue life has been presented based on friction work. The rationality and effectiveness of the model were validated according to the contrast of experiment life and predicting life. At last influence factor on fretting fatigue life of aerial aluminum alloy LY12CZ was investigated with the model. The results revealed that fretting fatigue life decreased monotonously with the increasing of normal load and then became constant at higher pressures. At low normal load, fretting fatigue life was found to increase with increase in the pad radius. At high normal load, however, the fretting fatigue life remained almost unchanged with changes in the fretting pad radius. The bulk stress amplitude had the dominant effect on fretting fatigue life. The fretting fatigue life diminished as the bulk stress amplitude increased.

Investigation on wear mechanism of thermochemically fabricated W–Cu composites

W–25 wt%Cu nanocomposite powder was prepared by thermochemical co-precipitation method. Consolidation procedure was performed at 1200 °C under hydrogen atmosphere and led to the densification up to 98% theoretical density. Wear test was carried out with a pin-on-disc apparatus under 5, 10, and 15 N normal loads. Volume of the lost material and also the wear rate increased by increasing the normal load while, friction coefficient showed a reverse record and decreased by increasing the load. Formation of mechanically mixed layers (MMLs) and their successive deformation and work hardening was found to be the origin of the friction coefficient decrease at increased normal loads. At low normal load just shallow scuffs and particle pull out were observed in the surface and sub-surface studies of the wear sample. But at higher normal loads large flakes of the matter were cut off and lost accompanying deep grooves and branched cracks. Continuous cyclic deformation at sufficiently high normal loads, could give rise to MML formation, sub-surface deformation, crack nucleation and propagation, and finally delamination of the composite.

Effects of Polypropylene Fibers on the Shear Strength of Sandy Soil

This paper presents the effect of two types of polypropylene fibers on shear strength parameters of sandy soil. To achieve the goals of this research, a sandy soil was obtained from a depth of 40 cm from the natural ground surface around American University of Sharjah. Two types of polypropylene fibers; one highly flexible with flat profile and the other with relatively high stiffness and crimpled profile were used in this study with four different aspect ratios (L/D) for each type. The initial physical properties of the sand such as specific gravity, angle of internal friction and shear strength were obtained in accordance with American Standard for Testing and Materials (ASTM). The sandy soils were mixed with the two types of fibers at different percentages by dry weight of the sand and four different aspect ratios. The test results of the study showed that the shear strength of the sand increased with increasing the flexible flat profile fibers content. Also it was noticed that by increasing the aspect ratio of the flexible flat profile the angle of internal friction and the shear strength increased. The crimpled profile fiber increased the shear strength of the sand under high normal load and has small or no effect on shear strength of the sand at low aspect ratio under low normal load.

The frictional behavior of UHMWPE films with different surface energies at low normal loads

The relationship between initial coefficient of friction, μ i and surface energy (or attractive force) for ultra-high molecular weight polyethylene (UHMWPE) film is modeled and experimentally measured at low normal loads (15–75 mN). The results show that the initial coefficient of friction increases exponentially with increasing attractive force between a silicon nitride ball and visco-elastic soft UHMWPE film. There is a strong influence of the normal load on this relationship for the polymer surface in the above-mentioned load range. The magnitude of the initial coefficient of friction decreases for UHMWPE film in the higher normal load regime because of its visco-elastic and soft nature as earlier proposed by Bowden and Tabor.

Atomic-scale simulations on the sliding of incommensurate surfaces: The breakdown of superlubricity

Molecular dynamics simulations of frictional sliding in an atomic force microscope (AFM) show a clear dependence of superlubricity between incommensurate surfaces on tip compliance and applied normal force. While the kinetic friction vanishes for rigid tips and low normal force, superlubric behavior breaks down for softer tips and high normal force. The simulations provide evidence that the Frenkel-Kontorova-Tomlinson (FKT) scaling applies equally to a more realistic three-dimensional (3D) incommensurate AFM model except in the limit of very low stiffness and high normal load. Unlike the FKT model in which the breakdown of superlubricity coincides with the emergence of the metastable states, in the 3D model some metastable states appear to reduce frictional force leading to nonmonotonic dependence of force on normal load and tip compliance. Metastable states vary with the slider positions and the relative stabilities of these metastable states result in varying transition mechanisms depending on sliding velocity.

On the Tangential Displacement of a Surface Point Due to a Cuboid of Uniform Plastic Strain in a Half-Space

The elastic solution of a tangentially loaded contact is known as Cerruti’s solution. Since the contact surfaces could be easily discretized in small rectangles of uniform shear stress the elastic problem is usually numerically solved by summation of well known integral solution. For soft metallic materials, metals at high temperature, rough surfaces, or dry contacts with high friction coefficient, the yield stress within the material could be easily exceeded even at low normal load. This paper presents the effect of a cuboid of uniform plastic strain in a half-space on the tangential displacement of a surface point. The analytical solutions are first presented. All analytical expressions are then validated by comparison with the finite element method. It is found that the influence coefficients for tangential displacements are of the same order of magnitude as the ones describing the normal displacement (Jacq et al., 2002, “Development of a Three-Dimensional Semi-Analytical Elastic-Plastic Contact Code,” ASME J. Tribol., 124(4), pp. 653–667). This result is of great importance for frictional contact problem when coupling the normal and tangential behaviors in the elastic-plastic regime, such as stick-slip problems, and also for metals and alloys with low or moderate yield stress.

Microscopic Friction Studies on Metal Surfaces

Atomically flat and clean metal surfaces exhibit a regime of ultra-low friction at low normal loads. Atomic force microscopy, performed in ultra-high vacuum on Cu(100) and Au(111) surfaces, reveals a clear stick-slip modulation in the lateral force but almost zero dissipation. Significant friction is observed only for higher loads (∼4–6 nN above the pull-off force) together with the onset of wear. We discuss the minor role of thermal activation in the low friction regime and suggest that a compliant metallic neck between tip and surface is formed which brings upon the low, load-independent shear stress.

An exploration of the elastohydrodynamic— hydrodynamic transition through film thickness measurements in rolling point contacts

The numerical solution of the elastohydrodynamic lubrication (EHL) problem is often presented in charts where dimensionless parameters and asymptotic relationships are used. Among them, the so-called isoviscous rigid asymptote is especially interesting because it defines the transition between elastohydrodynamic and hydrodynamic regimes, or between piezoviscous—elastic and isoviscous—rigid problems, respectively. In other respects, the combination of moderate contact pressures (i.e. moderate piezoviscosity, low normal load, and/or large radii of curvature) and high viscosity and speeds (i.e. high hydrodynamic effect) is quite often found in working mechanisms, for instance in most flange contacts in roller bearings. Surprisingly, only a few papers have analysed and discussed these operating conditions. Based on experimental results obtained in the range of the Moes parameters 15< L<50 and 2< M<160 for pure rolling circular point contacts, the aim of this work is first to show how film thickness distributions are affected when the piezoviscous elastic to isoviscous rigid transition is approached. In this context, the relative values taken by the central and minimum film thicknesses will be particularly studied. The second objective is to compare our results with those predicted by elastohydrodynamic and hydrodynamic solutions for non-conformal contacts found in the literature. Finally, the exploration of mild EHL is dealt with and a former publication in which electrohydrodynamic very thin films and highly loaded thick film regimes were studied is completed.