Adhesion Friction Coefficient Research Articles

A simple method to control nanotribology behaviors of monocrystalline silicon

A simple method was proposed to control the nanotribology behaviors of monocrystalline silicon against SiO2 microsphere by adjusting relative humidity (RH). Experimental results indicated that adhesion work, friction coefficient, and nanowear of silicon against SiO2 microsphere significantly varied between 60% and 90% RH. Under 60% RH, adhesion work was 119 mN/m, and friction coefficient was about 0.53. However, adhesion work and friction coefficient decreased to ∼70 mN/m and ∼0.3 under 90% RH, respectively. An apparent wear track ∼13 nm deep formed on the silicon surface under 60% RH, whereas no obvious wear scar was observed on the silicon surface under 90% RH. Analysis indicated that such tribological behaviors were due to different water condensations on the silicon surface under 60% and 90% RH. Under 60% RH, the water that condensed on the surfaces of the silicon sample and SiO2 tip mainly consisted of ice-like water. As a result, adhesion work was enlarged by the breaking force of the ice-like water bridge in the contact area. Given that a ≡Si–O–Si≡ bonding bridge easily formed between the silicon surface and the SiO2 tip with the help of water condensation under 60% RH instead of 90% RH, the friction coefficient was large and the nanowear of the silicon sample was severe under 60% RH. These results may help elucidate the nanotribology behaviors of silicon and facilitate the tribological design of dynamic microelectromechanical systems working under humid conditions.

Effect of Nitrogen Flow Rate on Structure and Adhesion Strength of Magnetron Sputtered Ti-Si-N Nanocomposite Films

Ti-Si-N nanocomposite films were prepared by co-sputtering Ti and Si targets in Ar/N2 gas atmosphere. The effect of N2 flow rate on the structure, adhesion strength and friction coefficient of the deposited films was studied by using X-ray diffraction, atom force microscope, field emission scanning electron microscopy and multi-functional tester for material surface properties. The Ti-Si-N films had a fine, smooth and compact structure with TiN nanograins embedded in an amorphous Si3N4 matrix. The nanocomposite films exhibited (200), (111), (220) and (222) reflections with a dominant orientation of the (200) reflection. When the N2 flow rate increased, the film structure was refined. It was found that both interfacial adhesion strength and friction coefficient depended on the N2 flow rate, and the best values were exhibited by the nanocomposite film produced at N2 flow rate of 15 sccm, perhaps contributed to a finer and smoother structure of this deposited film. Keywords: Ti-Si-N, Sputtering, Nanocomposite film, Microstructure, Adhesion strength, Friction coefficient

Influence of Cr content on the micro-structural and tribological properties of PLD grown nanocomposite DLC-Cr thin films

Improved adhesion and low friction coefficient of diamond-like carbon (DLC) films has been a major challenge which otherwise would have opened door to many engineering applications. Amorphous DLC films doped with different concentrations of Chromium (Cr) (5, 10, 15 & 20 at%) were deposited on to stainless steel (AISI SS304) substrate using Nd-YAG pulsed laser deposition technique (PLD). Raman spectroscopy was used to calculate the internal stress of the films. DLC with lower Cr concentration (5 at%) showed significant internal stress reduction (3 GPa) compared to DLC with higher Cr concentration. Formation of Cr3C2 nanoclusters in the DLC matrix was confirmed by XPS and TEM analysis which enhanced the adhesion strength of the film. DLC films with lower Cr concentration exhibited ultra-low friction coefficient (0.04–0.09) and the value increased significantly in higher Cr concentration. An optimum metal concentration in DLC film leads to improved adhesion and friction behaviour that houses many engineering applications.

Forces on micron-sized particles randomly distributed on the surface of larger particles and possibility of detachment

Numerical calculations based on the Lattice-Boltzmann method were performed for a particle cluster consisting of a large carrier particle covered with small spherical drug particles which were exposed to a constant velocity air flow. Prior to these studies, the simulations were validated based on a test case where a single particle is situated on a plane wall and exposed to a linear shear flow. The present simulations were compared with analytical results and other simulations. Moreover, the required small particle resolution and the domain size were properly selected based on an extensive numerical study. The diameter of the carrier particles was 100μm, while the fine particles had diameters of 3μm and 5μm, respectively. The range of Reynolds numbers considered was between 1 and 200. Moreover, the coverage degree of the carrier by the small particles was varied in the simulations between 10% and 50%, and this influence on the detachment was determined. From these simulations the fluid dynamic forces on the drug particles were extracted in dependence of the angular position in order to estimate the possibility of drug particle detachment. Detachment might occur through lift-off, sliding or rolling. Lift-off was found to be not possible due to the acting small normal forces even at Re=200. The probability of sliding and rolling detachment in dependence of the angular position was estimated based on measured adhesion properties, i.e. van der Waals force, adhesion surface energy and friction coefficient. With these studies it is aimed to understand drug particle detachment from carrier particles in an inhaler device as a basis of modelling and optimising dry powder inhalation.

Sliding of a smooth indentor over a viscoelastic half-space when there is friction

The spatial contact problem of the sliding, at a constant velocity, of a smooth indentor along the boundary of a viscoelastic half-space is considered. Shear stresses, related to the contact pressure by the Coulomb–Amonton friction law and due to surface adhesion interaction forces, act in the contact area. Shear deformation of the base is described by an integral operator with an exponential kernel, which is characterized by a single relaxation time. An integral equation is constructed for determining the unknown contact pressures, to solve which the boundary elements method is employed. Calculations are carried out for two forms of indentor: in the form of a paraboloid of revolution and in the form of an elliptic paraboloid. The dependence of the pressure distribution, the dimensions and displacement of the contact area with respect to the axis (or plane) of symmetry of the indentor, and also the dependence of the mechanical component of the friction force on the rate of sliding, the parameters of the viscoelasticity and the adhesion friction coefficient in the region of the contact interaction, are analysed.

Effect of PTFE Addition on the Properties of Electroless Ni-Cu-P-PTFE Deposits

Abstract The effect of PTFE addition on electroless Ni-Cu-P-PTFE deposit was investigated, such as surface morphology, phase composition and microhardness, as well as adhesion strength and friction properties. The electroless Ni-Cu-P-PTFE deposits with different PTFE contents were prepared on mild steel (1015) substrate surface by adjusting different process parameters. The surface morphologies and microhardnesses of the deposits were investigated by SEM and MH-6 Vickers, respectively. Adhesion strength and friction coefficient were measured by using MFT-4000 multifunctional surface material performance instrument. The results show that the deposition rate of the Ni-Cu-P-PTFE deposit increases with the increase of PTFE concentration. Further experiments indicate that the PTFE particles co-deposited in the composite deposit will reduce the microhardness due to their softness properties and loose structure. With the increasing of PTFE concentration, the adhesion strength decreases. Moreover, the addition of PTFE particles reduces the friction coefficient, and the friction coefficient is in inverse proportion to the content of PTFE particles in the deposits, but not to the concentration of PTFE in the bath.

Slab Upwarping of Twin-Block Slab Track on Subgrade–Bridge Transition Section

This study is a preliminary analysis of the causes of upwarping of reinforced concrete slabs of a twin-block slab track, specifically on the subgrade–bridge transition section. The analysis considers the structural features and force characteristics of slab track on the basis of site investigations and numerical simulation, and a corresponding repair method is proposed. It is determined that the temperature and temperature gradient of the slab, the connection status between the slab and the hydraulically bonded layer (HBL), and the position of the terminal spine are the main factors that lead to the upwarping of the slab. Model tests on the force transmission parameters between slab and HBL are carried out to measure the adhesive strength of the cohesive contact and the friction coefficient for the frictional contact. The adhesive strength is found to range from 0.803 to 1.57 MPa. The actual friction coefficient with large dispersion varies significantly from 1.5 to 3.0 because of the influence of the strength of the material and the friction roughness. The analysis output shows that the upwarping of the slab decreases with the reduction of temperature and temperature gradient of the slab, but the upwarping increases with the decrease in adhesive strength and friction coefficient. Increasing the number of terminal spines with 4-m spacing, which is optimized after the spacing effect, contributes to the reduction of the slab upwarping. A reasonable layout scheme for anchor pins can significantly improve the integrity and stability of track structure affected by slab upwarping. From the output of the analysis, a repair method that involves setting additional anchor pins between the slab and HBL is thus recommended.

Micro-scale investigation on belt finishing cutting mechanisms by scratch tests

Belt finishing is an abrasive process that involves various complicated tribological phenomena. To understand the micro-mechanisms in belt finishing, simulation by scratch tests on AISI 52100 steel alloy was proposed.Firstly, scratching using perfect indenter geometry was performed by simulating the real movements and the cutting conditions of belt finishing. Multi-pass scratch and parallel interacting scratch tests were carried out. The influence of the attack angle and the sliding cycle on the deformation behavior and the overall friction coefficient were systematically studied. It was found that with repeated scratches, material is cutting by the fatigue of the instable wedges. A three-dimensional analytical model was established to determine the relationship between the adhesion friction coefficient and the plastic deformation friction coefficient. It was found that the adhesion effect is more influential at small attack angles than at high attack angles.Secondly, the simulation of belt finishing was made by scratch tests at low and high speed with a real grain. With the same conditions, a grain cut less material than a perfect indenter which could be due to the adhesion phenomenon. At high speed, a single grain produces more scratches on the surface. Flow stresses and plastic deformation are more severe.

Effects of nano-additive TiO2 on performance of micro-arc oxidation coatings formed on 6063 aluminum alloy

Ceramic coatings were fabricated on aluminum alloy substrates by micro-arc oxidation (MAO) in silicate electrolytes doped with different concentrations of TiO2 nano-additive. Effects of nano-additive concentration on the structural and mechanical properties of the MAO coatings were analyzed. The results revealed that some nano-particle were incorporated into the resulting coating during the MAO process, while there was a reasonable concentration for the TiO2 nano-additive. With increasing the nano-additive concentration to 3.2 g/L, the adhesion value increased, while mean friction coefficient and mass loss decreased. A further increase of nano-additive deteriorated the adhesion and mean friction coefficient values, which was consistent with the micro-hardness tests.

Micromechanical scribing and indentation behavior of injection-molded polymeric carbon nanotube (CNT) nanocomposites

Polymeric carbon nanotube (CNT) nanocomposites have unique mechanical, electrical, and thermal properties. Anisotropy can be induced depending on the alignment of the CNT fillers within polymeric composites, which is known to affect material properties. In order to investigate the effects of CNT alignments in micromechanical scribing using a single crystal diamond tool, a microindenter–scriber system was developed. Multiwalled carbon nanotube–polystyrene (MWCNT–PS) samples with varying CNT concentrations were prepared through a microinjection molding process, where the injection enables the partial alignment of CNTs in the flow direction through high shear stress. A mechanistic scribing force model was proposed based on the material properties that could be obtained using the microindentation techniques. Scribing experiments were performed in the parallel and perpendicular directions to the CNT alignment. Forces in three axes were measured and analyzed to identify three unknown parameters—the shearing, plowing, and adhesion friction coefficients. The resulting coefficients for scribing perpendicular to the CNT alignment showed distinguishable trends from scribing parallel to the CNT alignment as the CNT loadings increased. Their linear trends in relation to the material properties identified from indentation techniques can be used to predict microscribing parameters and resulting cutting forces, in combination with the proposed mechanistic model.

Influence of exfoliating facial cleanser on the bio-tribological properties of human skin

In daily life, cosmetics can play important roles in facial skin moisturizing, whitening and sun protecting. Unfortunately, the aged skin stratum corneum (SC) often impedes the penetration for cosmetics. Regular removal of the extra aged horny cells can facilitate the penetration for cosmetics and improve the performance of the skin. However, no study has assessed the detailed effects of exfoliating facial cleanser on the friction properties of human skin and the degree of aged SC removal. In this paper, the effects of three kinds of scrub facial cleanser with different sizes, amounts and hardness scrub particles on the tribological properties of human skin have been investigated in vivo using a UMT-II tribometer under the simulated face washing conditions. The hardness of the scrub particles were measured by using a Nano-Hardness/Scratch Tester. The results showed that the aged SC has all been removed by the three kinds of scrub cleanser under the reciprocating sliding wear mode, which resulted in decrease of skin surface roughness and increase of skin conductance and hydration. Thus, the adhesion force and friction coefficient between the hydrated skin and a probe increased due to the increased contact area and skin hydration. Compared with the other two cleansers, the cleanser with two kinds of size and moderate hardness scrub particles made the skin to deform more elastically and removed more aged SC, which led to greater decrease in skin surface roughness and greater increase in skin conductance, hydration and adhesion. Hence the skin friction coefficient and penetration were higher and the moisturizing persistence lasted longer time after the same moisturizer was used. This indicated that the properties of scrub particles in cleanser played important roles in skin tribological behavior and penetration. The results may be instrumental in developing and testing skin cosmetics.

Application of a simple surface nanocrystallization process to a Cu–30Ni alloy for enhanced resistances to wear and corrosive wear

Abstract A simple surface nanocrystallization process combining repeated hammering and recovery treatment was applied to Cu–30Ni alloy for improved resistance to corrosive wear in a NaCl solution. This study was motivated by aquaculture applications of copper alloys, which require not only the resistance to pure corrosion but also that to corrosion involving wear such as scratching. The synergy of corrosion and wear may significantly accelerate surface failure of copper alloys in the marine environment. We nanocrystallized the surface of the copper alloy by repeatedly hammering the surface followed by a recovery treatment that turned dislocation cells into nano-sized grains, which considerably strengthened the surface. The high-density grain boundaries largely enhanced atomic diffusion and thus improved the passivation capability of the nanocrystallized surface. As a result, the nanocrystallized surface possessed higher resistances to corrosion, wear and corrosive wear. In this work, particular emphases were put on effects of the hammering duration on the thickness of nanocrystallized surface layer and variations in corrosion, wear and corrosive wear rates as a function of the distance from top surface to bulk. Variations in other relevant properties with the distance were also investigated, including the adhesive force, friction coefficient and hardness. The relationships between the properties and depth help to estimate the durability or service life of the nanocrystallized surface layer. Efforts were made to clarify the mechanisms responsible for observed phenomena and relationships.

Tribological behavior of low-carbon steel depending on treatment and structural state

The work presents the results of tribological study of low-carbon steel (up to 0.2 wt % carbon) in three states: initial (hot-rolled), heat-treated (refined), and refined and further subjected to severe plastic deformation by equal-channel angular pressing. After the various treatment types, the steel samples are found to be in different structural states with different hardness values and oxygen content in the surface layer. These factors exert a complex effect on the material’s tribological properties in contact with tool steel. The lowest adhesive strength, friction coefficient, and wear rate are shown by refined and plastically deformed steel.

Tribological properties of the low-carbon steels with different micro-structure processed by heat treatment and severe plastic deformation

Abstract This paper presents the results of tribological investigations conducted on steel 20 with the carbon content of up to 0.2%. The steel was studied in the three conditions: initial (hot-rolled), after heat-treatment (quenching + tempering) and after heat treatment with subsequent severe plastic deformation (SPD) performed by equal channel angular pressing technique (ECAP). It was stated that after various treatments the material acquires various structural conditions and possesses various strength properties and has a considerable difference in oxygen content in the surface layer. This influences the tribological properties during the contact with tool steel. The lowest values of adhesive bond shear strength, friction coefficient and wear rate are demonstrated in the material after martempering with subsequent SPD by ECAP technique. As a result of complex investigations, it has been revealed that severe plastic deformation by equal channel angular pressing technique enhances efficiently the strength of the low-carbon steel due to grain structure refinement observed by the sample of the low-carbon steel. This considerably influences the decrease of the friction coefficient and its adhesive component. The increased oxygen content on the material surface after SPD compared to both initial condition and after heat-treatment contributes to stronger passivation of the surface due to formation of oxide films, which together with strain hardening contributes to decrease of the friction coefficient in the specified temperature range, and increase of wear resistance as well. The surface of the investigated material after SPD treatment by the ECAP technique possesses a highest bearing capacity and requires more time for wearing-in in friction assemblies. Oxygen content increase in the form of metal oxides on the surface of low-carbon steels is accompanied by a decrease of the adhesive component of friction coefficient.

Molecular Dynamics Study on Friction Due to Ploughing and Adhesion in Nanometric Scratching Process

In this study, three-dimensional MD simulations are carried out to study the nanometric scratching process. The ploughing friction coefficient and the adhesion friction coefficient are distinguished for the first time using MD simulations. The contribution of chip to friction coefficient is also evaluated. The simulation results show that the macroscale theory can qualitatively evaluate the ploughing friction coefficient, but it slightly overestimates the ploughing friction coefficient on the nanoscale for the scratching depths studied. It is found that the adhesion friction coefficient is independent of the scratching depth as predicted by macroscale theory. It is also found that the contribution of chip to friction coefficient is independent of the scratching depth and cannot be neglected on the nanoscale.

Analysis of the manufacturing chain of CVD diamond coated shaft type cutting tools

Hypereutectic aluminium silicon alloys, e.g. casted AlSi17Cu4Mg, are commonly used in the automotive and aeronautical industries. These alloys consist of hard, abrasive silicon particles in a soft aluminium matrix and thus place high mechanical loads on the tool during machining processes. Polycrystalline Diamond or CVD (chemical vapour deposition) diamond based cutting tools can be used for the high speed machining of these alloys due to their high hardness and wear resistance. Diamond thin film coatings of different film morphologies are commonly applied on cemented carbide tools using Hot Filament CVD. The distinguishing characteristic to other coatings is utmost hardness resulting in high resistance to abrasion, low tendency to adhesion and low friction coefficient. The manufacturing of CVD diamond coated shaft type cutting tools is challenging due to the complex design of the cutting edges and the demanding stress behaviour during tool application. The influencing parameters of substrate type, chemical and mechanical substrate pre-treatment as well as diamond film modification on the tool cutting performance are discussed. The manufacturing route of CVD diamond coated thread milling drills is analysed with the use of material and tribological tests. The complex thread manufacturing tools are then applied in the machining of AlSi17Cu4Mg, whereby the tool performance is characterised with respect to their wear behaviour, the process forces and temperatures as well as the workpiece quality.

Influence of Ag Content and Nanograin Size on Microstructure, Mechanical and Sliding Tribological Behaviors of Ag-DLC Films

Diamonds like carbon (Ag-DLC) films of nine Ag contents were deposited on (100) Si substrate in a mid-frequency dual-magnetron system. The influence of Ag content and nanograin size on the microstructure, mechanical properties and sliding tribological behaviors of the films has been investigated. It is found that (i) the Ag nanocrystallites were dispersed in the amorphous DLC matrix, and increasing Ag content resulted in an upward tendency of the Ag nanocrystallite size; (ii) the films with Ag content ranging between 3.3-11.4 at% and with Ag nanograin sizes in range of 5.4-16.8 nm exhibited higher hardness, lower intrinsic stress, better adhesion, and lower friction coefficient and wear rate as compared with those of 14.7-23.6 at% Ag content and 19.7-34.7 nm nanograin sizes; and (iii) the best combined properties were achieved for the film deposited with 8.7 at% Ag and with grain size 12.9 nm.

Nanotribological properties of novel lubricants for magnetic tapes

Two classes of novel lubricants, perfluoropolyethers (PFPE) and ionic liquids (ILs), were deposited on metal film magnetic tapes. The adhesive force and coefficient of friction of lubricated and unlubricated tapes were investigated at the nanoscale with an atomic force microscope (AFM) as a function of various humidity and temperature conditions. Microscale tests with a ball-on-flat tribometer were also performed in order to study the length-scale effects on friction. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated by monitoring the friction force, surface potential and contact resistance with the AFM. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) experiments were conducted to determine the chemical species that affect intermolecular bonding and as an aid in interpreting how the lubricant film tribological properties vary with the environmental conditions. Z-TETRAOL, one of the PFPEs, was found to exhibit the lowest adhesion and friction among the lubricant films studied. The ionic liquid 1,1′-(pentane-1,5-diyl)bis(3-hydroxyethyl-1 H-imidazolium-1-yl) di[bis(trifluoromethanesulfonyl)imide)] exhibited comparable nanotribological properties with the PFPEs. This is attributed to the presence of hydroxyl groups at its chain ends, which can hydrogen bond with the surface similar to PFPEs.

空気圧制動系における非線形粘着状態観測器

This paper shows that the VSS observer has the possibility to estimate adhesion force using wheel slip simulation and wheel slip experiments. It has been shown that the disturbance observer was effectives in estimation of adhesion force. To apply a strain gauge to the brake caliper in order to measure the braking torque is difficult, because the delicate parts on bogies are to be avoided at the train depot. When all the axles of vehicles slip, the wheel slip prevention cannot work sufficiently. Moreover, the brake systems of railway vehicles possess strong nonlinear characteristics with variable parameters, such as adhesion force and friction coefficient of brake materials. It is important that the estimation method has robustness against such disturbance and parameter changes, and it is known that the Variable Structure System (VSS) observer has robustness against parameter change, disturbance input and so on. However, there have been no research reports about a Variable Structure System (VSS) observer for the brake system or wheel slip preventing methods in a railway vehicle. In order to compare the theoretical value of adhesion force with estimated value by a VSS observer, wheel slip simulation was preformed. Simulation results show that the VSS observer can estimate the adhesion force efficiently. Moreover, estimation experiments of adhesion force using VSS observer were performed, and it has been clarified that the VSS observer can efficiently estimate the tangential force coefficient from the experimental results.

CrossRef Listing of Deleted DOIs

A technique to produce cast Al-11.8 pct Si alloy composites containing up to 40 vol pct (15 pct by weight) dispersions of 125 µm size coconut shell char particles is described. The technique consists of stirring shell char particles into the vortex created by mechanical stirring of melts and subsequent casting of composite melts in suitable molds. The composite melts were also pressure die cast at a pressure of 100 MPa into cylindrical castings. The incorporation of large volume fraction of shell char particles is aided by (a) preheating of the particles to about 500 ‡C to 600 ‡C for two hours before introduction into the melts, and (b) alloying of Al-11.8 pct Si melts with 3 to 6 pct Mg. Electron Probe Micro Analysis (EPMA) analysis indicated an Mg enriched region around dispersed char particles in the composite indicating that prealloying with Mg probably improves wetting between char particles and the melt. Dispersions of 15 pct wt of char particles lead to decreases in hardness (from 85 BHN to 55 BHN), compression strength (from 542.30 MPa to 218.68 MPa), U. T. S., (from 164.16 MPa to 63.75 MPa), and electrical conductivity (from 27.8 pct I ACS to 11 pct I ACS). However, since these decreases are accompanied by a decrease in density, specific strength values of Al-11.8 pct-shell char composites are adequate for a variety of applications. Adhesive wear rates and friction coefficient values at low sliding speeds (0.56 m per second, and at loads of 10 N and 60 N) decrease with increase in wt pct of char particles under dry conditions.