Range Of Normal Loads Research Articles

Prediction of contact surface temperature between rough sliding bodies – numerical analysis and experiments

PurposeThe paper's aim is to predict numerically the contact temperatures between two rough sliding bodies and to compare with the experimental results.Design/methodology/approachAn elastic contact algorithm is used to analyze the normal contact between two nominally smooth surfaces. The algorithm evaluates real contact area using digitized roughness data and the corresponding contact pressure distribution. Using finite element method a steady state 3D temperature distribution at the interface between the sliding bodies is obtained. Using infrared (IR) imaging technique, experiments were carried out to measure the contact temperature distribution between rough rubbing bodies with a systematic variation of surface roughness and operating variables.FindingsContact temperature distributions over a wide range of normal load, sliding velocity and surface roughness have been obtained. It was seen that the maximum contact temperature expectedly increases with surface roughness (Sa values), normal load and sliding velocity. The results also indicate that the “hot spots” are located exactly at the positions where the contact pressures are extremely high. Temperatures can be seen to fall drastically at areas where no asperity contacts were established. The temperature contours at different depths were also plotted and it was observed that the temperatures fall away from the actual contact zone and relatively high temperatures persist at the “hot spot” zones much below the contact surface. Finally it is encouraging to find a good correlation between the numerical and experimental results and this indicates the strength of the present analysis.Research limitations/implicationsExperimental accuracy can be improved by using a thermal imaging camera that measures emissivity in situ and uses it to find the contact temperature. The spatial resolution and the response time of the camera also need to be improved. This can improve the correlation between numerical and experimental results.Practical implicationsOne of the major factors attributed to the failure of sliding components is the frictional heating and the resulting flash temperatures at the sliding interface. However, it is not easy to measure such temperatures owing to the inherent difficulties in accessing the contact zone. Besides, thermal imaging techniques can be applied only with such tribo‐pairs where at least one of the contacting materials is transparent to IR radiation. In practice, such cases are a rarity. However, the good correlation observed between the numerical and experimental results in this work would give the practicing engineer a confidence to apply the numerical model directly and calculate contact temperatures for any tribo‐material pairs that are generally seen around.Originality/valueA good correlation between the numerical and experimental results gives credence to the fact that the numerical model can be used to predict contact temperatures between any sliding tribo‐pairs.

The effect of loading on surface roughness at the atomistic level

One of the key points to better understand the origins of friction is to know how two surfaces in contact adhere to one another. In this paper we present molecular dynamics (MD) simulations of two aluminium bodies in contact, exposed to a range of normal loads. The contact surfaces of both aluminium bodies have a self-affine fractal roughness, but the exact roughness varies from simulation to simulation. Both bodies are allowed to have an adhesive interaction and are fully deformable. Tracking important contact parameters (such as contact area, number of contact clusters, and contact pressure) during a simulation is challenging. We propose an algorithm (embedded within a parallel MD code) which is capable of accessing these contact statistics. As expected, our results show that contact area is increasing in proportion with applied load, and that a higher roughness reduces contact area. Contact pressure distributions are compared to theoretical models, and we show that they are shifted into the tensile regime due to the inclusion of adhesion in our model.

Wear Behavior and Mechanism of a Cr-Mo-V Cast Hot-Working Die Steel

The wear behavior and mechanisms of a Cr-Mo-V cast hot-working die steel with three microstructures (tempered martensite, troostite, and sorbite) were studied systematically through the dry-sliding wear tests within a normal load range of 50 to 300 N and an ambient temperature range of 298 K to 673 K (25 °C to 400 °C) by a pin-on-disk high-temperature wear machine. Five different mechanisms were observed in the experiments, namely adhesive, abrasive, mild oxidative, oxidative, and extrusive wear; one or more of those mechanisms would be dominant within particular ranges of load and temperature. The transition of wear mechanisms depended on the formation of tribo-oxides, which was related closely to load and temperature, and their delamination, which was mainly influenced by the matrix. By increasing the load and ambient temperature, the protective effect of tribo-oxides first strengthened, then decreased, and in some cases disappeared. Under a load ranging 50 to 300 N at 298 K (25 °C) and a load of 50 N at 473 K (200 °C), adhesive wear was the dominant wear mechanism, and abrasive wear appeared simultaneously. The wear was of mild oxidative type under a load ranging 100 to 300 N at 473 K (200 °C) and a load ranging 50 to 150 N at 673 K (400 °C) for tempered martensite and tempered troostite as well as under a load of 100 N at 473 K (200 °C) and a load ranging 50 to 100 N at 673 K (400 °C) for tempered sorbite. At the load of 200 N or greater, or the temperatures above 673 K (400 °C), oxidative wear (beyond mild oxidative wear) prevailed. When the highest load of 300 N at 673 K (400 °C) was applied, extrusive wear started to dominate for the tempered sorbite.

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.

Microtribological Performance of Au–MoS2 and Ti–MoS2 Coatings with Varying Contact Pressure

Solid lubricant coatings with co-sputtered metal and MoS2 have shown favorable macrotribological properties at a wide range of contact stresses and humidity levels. These materials are also candidates for use in microcontacts and micro-electromechanical systems (MEMS), but their performance at this scale is poorly understood. For this study, microtribological properties of Au–MoS2 and Ti–MoS2 coatings, with varying metal additives of less than 15 at%, were examined using a nanoindentation instrument. Titanium and gold were chosen for this study as metal additives due to their different influence on the mechanical properties of the coating. The hardness and reduced modulus of the coatings increased with the addition of metal, when compared to pure MoS2. Reciprocating microscratch tests were performed with two spherical diamond tips (50 and 10 μm radii) in dry air. A range of normal loads were used between 0.2 and 5.0 mN. Friction and wear measurements were analyzed with respect to the variation in the contact pressure and compared to literature studies performed at the macroscale. Correlations were found between the coating mechanical properties, tip-coating adhesion, interfacial shear strength, and the formation of transfer films and tribofilms.

Micro-scale sliding contacts on Au and Au-MoS 2 coatings

The microtribological properties of Au and Au-MoS 2 coatings were examined using a nanoindentation instrument. MoS 2 was chosen for this study as an additive to Au due to its positive influence on the mechanical and tribological properties. Reciprocating microscratch tests were performed using a diamond indenter with a tip radius of 50 μm and a range of normal loads between 0.2 mN and 5.0 mN. The friction and wear results, with respect to the two coatings, were correlated to different velocity accommodation modes and levels of adhesion. It was found that the addition of 20 mol% MoS 2 to Au reduced the adhesion and limiting friction and also improved the wear resistance significantly. This coating shows potential for applications in microcomponents and microswitches due to its wear resistance, relatively low friction and good electrical conductivity.

The influence of Strontium addition on wear properties of Al–20 wt% Si alloys under dry reciprocating sliding condition

The modification response of Al–20 wt% Si alloys was studied with different Sr addition levels. The wear properties of Al–Si alloys modified by Sr were investigated by using a UMT-3 reciprocating wear machine over a range of normal loads, i.e., 2, 5, 8 N, at the frequency of 50 Hz. The worn morphologies of the wear surface were examined by scanning electron microscopy (SEM). The results show that the primary Si and the interdendritic eutectic matrix are obviously changed after the Sr modification. The frictional coefficient greatly changes at a low load and mains unnoticeably at a high load. In addition, the frictional coefficients of the unmodified alloys decrease with increasing of the load, but those of the modified alloys increase when Sr addition level exceeds 0.04 wt%. The wear volumes of Al–Si alloys are inversely proportional to the hardness. The wear behaviors are optimal in the range of 0.04–0.06 wt% Sr. Adhesive wear is the main mechanism for the unmodified alloys, whereas abrasion wear characterizes in the modified alloys.

Friction and Wear Properties of Sintered Cu Alloy Impregnated with Thermo-Reversible Gel-Lubricant

The friction and wear properties of a sintered porous Cu alloy impregnated with either Thermo-Reversible Gel-Lubricant (TR Gel-Lube) or base fluid (PAO) were investigated. Friction tests were conducted under a wide range of normal loads (0.98 N, 9.8 N, and 19.6 N) and sliding velocities (0.2 m/s, 0.5 m/s, 1.0 m/s, and 2.1 m/s), using a high-carbon chromium steel (JIS-SUJ2) bearing ball with 8 mm diameter as a ball specimen. The friction coefficients of the sintered Cu alloy impregnated with TR Gel-Lube were lower than those of the alloy impregnated with the base fluid under a wide range of normal loads and sliding velocities. Particularly, the friction coefficient of the sintered Cu alloy impregnated with TR Gel-Lube had extremely low value - less than 0.05 at high normal load and high sliding velocity. The specific wear rates of the sintered Cu alloy impregnated with base fluid and that impregnated with TR Gel-Lube had low values - less than 10-8 mm2/N under a wide range of normal loads and sliding velocities. There was no significant difference in wear rates between the two impregnants.

Performance of the resurfaced hip. Part 1: The influence of the prosthesis size and positioning on the remodelling and fracture of the femoral neck

Hip resurfacing is an established treatment for osteoarthritis in young active patients. Failure modes include femoral neck fracture and prosthesis loosening, which may be associated with medium-term bone adaptation, including femoral neck narrowing and densification around the prosthesis stem. Finite element modelling was used to indicate the effects of prosthesis sizing and positioning on the bone remodelling and fracture strength under a range of normal and traumatic loads, with the aim of understanding these failure modes better. The simulations predicted increased superior femoral neck stress shielding in young patients with small prostheses, which required shortening of the femoral neck to give an acceptable implant-bone interface. However, with a larger prosthesis, natural femoral head centre recreation in the implanted state was possible; therefore stress shielding was restricted to the prosthesis interior, and its extent was less sensitive to prosthesis orientation. With valgus orientation, the implanted neck strength was, at worst, within 3 per cent of its intact strength. The study suggests that femoral neck narrowing may be linked to a reduction in the horizontal femoral offset, occurring if the prosthesis is excessively undersized. As such, hip resurfacing should aim to reproduce the natural femoral head centre, and, for valgus prosthesis orientation, to avoid femoral neck fracture.

Adhesion, friction and wear characterization of skin and skin cream using atomic force microscope

Skin cream is commonly used to improve skin health and create a smooth, soft, and moist perception by altering the surface roughness, friction, and adhesion of skin surface. In this study, a systematic characterization of the friction and adhesion properties of skin and skin cream was carried out, which is essential to develop better skin care products and advance biological, dermatology, and cosmetic science. Since cream rheology is expected to be a function of its thickness as well as the velocity and normal load during its application, friction and adhesion experiments were performed at a range of cream film thickness, velocity, and normal load in order to study their effect on virgin skin and cream treated skin. Since environmental dependence of skin and skin cream is of importance, the effect of relative humidity and temperature on the coefficient of friction and adhesive force was also studied. Durability of the virgin skin and cream treated skin was studied by repeated cycling tests. These experiments were performed by using an atomic force microscope (AFM) and a macroscale friction test apparatus in order to study the scale effects. Friction and wear mechanisms under various operating conditions are discussed.

Feasibility Study for Real Time Measurement of Wheel-Rail Contact Using an Ultrasonic Array

Failure of a wheel-rail contact is usually by wear or fatigue of either component. Both mechanisms depend on the state of stress, which in turn depends on size and location of the contact patch. In this work, the feasibility of an ultrasonic approach for measuring the contact, real time on a rail, has been evaluated. The approach is based on the physical phenomenon of ultrasonic reflection at an interface. If the wheel and rail surfaces make contact, and are under high stress, they will transmit an ultrasonic pulse. However, if there is no contact, or the contact is under low stress, then the wave is completely or partially reflected. By measuring the proportion of the wave reflected, it is possible to deduce the extent of the contact area and also estimate the pressure distribution. In a previous work (Marshall, Lewis, Dwyer-Joyce, Olofsson, and Bjorklund, 2006, “Experimental Characterisation of Wheel-Rail Contact Patch Evolution,” ASME J. Tribol., 128(3), pp. 493–504), static wheel-rail contacts were scanned using a transducer to build up a two-dimensional (2D) map of the contact. The procedure was time consuming and could in no way be used for measurements online. In this work, a method is presented that could be used at line speeds, and so provide wheel-rail contact measurements in field trials. The scan is achieved by using an array transducer that performs a one dimensional electronic line scan. This, coupled with the speed of travel of the contact patch past the sensor location, enables a 2D map of the contact to be produced. Specimens were cut from wheel and rail sections and loaded together hydraulically in a biaxial frame. An array transducer was mounted beneath the rail specimen. The array transducer consisted of 64 ultrasonic elements that could be pulsed independently, simultaneously, or with controlled phase difference. The signals were reflected back from the contact to effectively produce a line scan. The transducer was physically moved to simulate the translation of the contact patch and so generate a series of 2D reflection profiles. Contacts under a range of normal and lateral loads have been measured and compared with some simple results using a pressure sensitive film. While the map produced by ultrasonic reflection is relatively coarse, the results agree well with measurements from the pressure sensitive film. The work concludes with a discussion of how this array measurement procedure might be implemented at full line speed, and what resolution could potentially be achieved.

Fingerprints are unlikely to increase the friction of primate fingerpads

It is generally assumed that fingerprints improve the grip of primates, but the efficiency of their ridging will depend on the type of frictional behaviour the skin exhibits. Ridges would be effective at increasing friction for hard materials, but in a rubbery material they would reduce friction because they would reduce contact area. In this study we investigated the frictional performance of human fingertips on dry acrylic glass using a modified universal mechanical testing machine, measuring friction at a range of normal loads while also measuring the contact area. Tests were carried out on different fingers, fingers at different angles and against different widths of acrylic sheet to separate the effects of normal force and contact area. The results showed that fingertips behaved more like rubbers than hard solids; their coefficients of friction fell at higher normal forces and friction was higher when fingers were held flatter against wider sheets and hence when contact area was greater. The shear stress was greater at higher pressures, suggesting the presence of a biofilm between the skin and the surface. Fingerprints reduced contact area by a factor of one-third compared with flat skin, however, which would have reduced the friction; this casts severe doubt on their supposed frictional function.

Junction Growth and Energy Dissipation at the Very Early Stage of Elastic-Plastic Spherical Contact Fretting

The contact area, friction force, and relative displacement evolution at the very early stage of fretting are investigated experimentally. Copper and steel spheres of various diameters are loaded against a hard sapphire flat by a range of normal loads deep into the elastic-plastic regime of deformation. A reciprocating tangential loading is then applied with a maximum loading below the static friction to avoid gross slip. Real-time and in situ direct measurements of the contact area, along with accurate measurements of the friction force and relative displacement, reveal substantial junction growth and energy dissipation mainly in the first loading cycle. The so-called “slip amplitude” is found to be attributed to residual tangential plastic deformation rather than to interfacial slip. Elastic shake-down is observed for the 2.5% hardening steel spheres while plastic shake-down is observed in the case of the elastic perfectly-plastic copper spheres.

Computer simulation of the two-body abrasion process modeling the particle as a paraboloid of revolution

Two-body abrasive wear is a process with strong stochastic characteristic. Abrasive particle geometry, distribution and worn surface morphology can only be statistically determined and analytical models therefore always cause large inaccuracy. In this research, the earlier model of a particle as pyramid with a hemispherical tip has been replaced by a paraboloid model of revolution. In the pyramid model the normal load cannot be large enough to penetrate beyond the height of the hemisphere. Generally, in practice the hemisphere tip is quite small, and it readily penetrates into the surface. A new particle model has, therefore, been devised to extend the normal load range. New contact equations are proposed for the particle geometry used in the present model. The Monte Carlo method and finite element methods (FEMs) have also been combined to calculate the wear rate of the material during simulation. It is found that the linear wear rate increases continuously during the running-in process and reaches a constant value after some travel distance. Computed roughness and worn surface morphologies are in agreement with the experimental data. Finally, a comparison between simulated and experimented wear rates has also made. Both data matched very well.

Characterization of the mechanical properties of a dermal equivalent compared with human skin in vivo by indentation and static friction tests

The study of changes in skin structure with age is becoming all the more important with the increase in life. The atrophy that occurs during aging is accompanied by more profound changes, with a loss of organization within the elastic collagen network and alterations in the basal elements. The aim of this study is to present a method to determine the mechanical properties of total human skin in vivo compared with dermal equivalents (DEs) using indentation and static friction tests. A new bio-tribometer working at a low contact pressure for the characterization the mechanical properties of the skin has been developed. This device, based on indentation and static friction tests, also allows to characterize the skin in vivo and reconstructed DEs in a wide range of light contact forces, stress and strain. This original bio-tribometer shows the ability to assess the skin elasticity and friction force in a wide range of light normal load (0.5-2 g) and low contact pressure (0.5-2 kPa). The results obtained by this approach show identical values of the Young's modulus E(*) and the shear modulus G(*) of six DEs obtained from a 62-year-old subject (E(*)=8.5+/-1.74 kPa and G(*)=3.3+/-0.46 kPa) and in vivo total skin of 20 subjects aged 55 to 70 years (E(*)=8.3+/-2.1 kPa, G(*)=2.8+/-0.8 kpa).

Effect of stir-casting on the microstructure and adhesive wear characteristics of cast Al—Si—Cu alloy

This paper describes the microstructure, hardness, and wear characteristics of cast aluminium—silicon alloy produced by a stir-casting technique. A hypoeutectic (Al—7% Si—0.8% Cu) alloy was cast under different stirring speeds. The stirring of semi-solid metal (SSM) slurries was performed under continuous cooling conditions from liquidus temperature. Wear characteristics of the alloys under investigation were studied using a pin-on-flat wear system over a range of normal loads (10—40 N) at constant average sliding speed (0.2 m/s) against cast iron and stainless steel counter surfaces. Stir-casting resulted in a material with higher microhardness and lower wear rates than conventional cast alloy. Both the stir-cast and conventional cast alloy showed higher weight loss against the stainless steel compared with that against the cast iron counter surface. Optical microscopy of the conventional cast and stir-cast alloys showed that the stir-casting results in spheroidization and refinement of primary alpha aluminium and modification of eutectic silicon. The scanning electron microscopy of wear surfaces was carried out to investigate the mode of wear. Attempts were made to establish the structure property relationship.

Adhesive wear of stir cast hypereutectic Al–Si–Mg alloy under reciprocating sliding conditions

This paper describes an attempt to enhance the wear properties of hypereutectic cast aluminium–silicon alloys produced by semi-solid metal (SSM) processing technique. The rheological experiments on SSM slurries were performed under continuous cooling condition from liquidus temperature. Wear characteristics of alloy under investigation were studied using pin on flat wear system over a range of normal load (10–40 N) at constant average sliding speed (0.2 m/s) against cast iron and stainless steel counter surface. Stir cast alloy showed lesser weight loss compared to conventional cast alloy. Stir cast and conventional cast alloys showed higher weight loss against the stainless steel as compared to that against cast iron counter surface. Optical microscopy of the conventional cast and stir cast hypereutectic alloy has shown that stir casting causes refinement of primary silicon particles and modification of eutectic silicon compared to conventional cast alloy. The scanning electron microscopy of wear surfaces was carried out to investigate the mode of wear.

Preparation and tribological tests of thin fluoroorganic films

Adhesion, friction and consequent wear of sliding surfaces are the basic problems that limit the performance and reliability of microelectromechanical devices. Lubrication of these nano- and microscale contacts is different from traditional lubricants. Self-assembled monolayers (SAMs) chemically bonded to the substrate are considered to be the best solution of lubrication. The majority of these monolayers are hydrophobic providing low friction, adhesion and wear. Chemical vapor deposition was used to grow a fluorosilane film on silicon Si(1 0 0) and a condensed monolayer of 3-mercaptopropyltrimethoxysilane (MPTMS) on Au(1 1 1). The films were characterized by means of a contact angle analyzer for hydrophobicity, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for identification of thin fluoroorganic monolayers deposited on silica surfaces and condensed monolayer MPTMS. Adhesion and friction measurements were performed using atomic force microscopy (AFM) and compared with measurements performed using a microtribometer operating in millinewton (mN) normal load range. Nanotribological measurements indicated that silica and MPTMS modified by fluorosilanes have the lowest friction coefficient and indicated a decrease friction coefficient with increasing fluoric alkyl chain length.

Synthesis and Tribological Properties of Tubular NbS2 and TaS2 Nanostructures

NbS2 and TaS2 nanostubes were prepared via solid state reaction method, and their tribological properties were studied over a wide range of normal loads and sliding velocities. The size distributions and shapes of sulfides were characterized using transmission electron microscopy and scanning electron microscopy. The results indicated that the NbS2 and TaS2 compounds had tubular structures with the mean diameter of 100 nm and length of 10 mm. The effect of NbS2 and TaS2 nanotubes added in commercial lubricating oil T40, was initially studied using the instrument MS-T3000. The results showed that the antiwear ability of NbS2 and TaS2 as additives was obviously better than that of the normal lubricating oil in ambient conditions. The general frictional properties of the solid lubricant containing TaS2 is better than those of the solid lubricant containing NbS2. The frictional mechanism was discussed.

Influence of CrC addition in Ni-Cr-Si-B flame sprayed coatings on microstructure, microhardness and wear behaviour

In the present paper the influence of the addition of chromium carbide (CrC) particles on the microstructure, microhardness and abrasive wear behaviour of flame sprayed Ni-Cr-Si-B coatings deposited on low carbon steel substrate has been reported. Wear behaviour of the coatings was evaluated with a pin-on-block wear system against SiC abrasive medium (120 & 600 grades) over a range of normal load (5–20 N). It was observed that the wear behaviour is governed by the material related parameters (microstructure, microhardness of coating) and test parameters (abrasive grit size and normal load). The addition of CrC reduces the wear rate three to eightfold. Wear resistance was greater against coarse abrasives at high loads than against fine abrasives. Heat treatment of both unmodified (1004) and modified powder (1004-10%CrC, 1004-20%CrC) coatings deteriorated the abrasive wear resistance. SEM study of wear surfaces showed that wear of the coatings largely takes place by groove formation, plowing and scoring. Electron probe micro analysis (EPMA) of the coating was carried out for composition and phase analysis.