Lubricity of High Water Content Aqueous Gels

Aqueous gels such as biopolymer gels, mucus, and high water content hydrogels are often qualitatively described as lubricious. In hydrogels, mesh size, ξ, has been found to be a controlling parameter in friction coefficient. In the tribology of aqueous gels, we suggest that the Weissenberg number (Wi) is a useful parameter to define different regimes, and following the original formulations in rheology, Wi is given by the polymer relaxation time (ηξ³/k_BT) multiplied by the shear rate due to fluid shear through a single mesh (V/ξ): Wi?=?ηVξ²/k_BT. At sliding speeds below a Weissenberg number of approximately 0.1, Wi?<?0.1, the friction coefficient is velocity-independent and scales with mesh size to the ??1 power, µ ∝ ξ^?1. De Gennes’ scaling concepts for elastic modulus, E, give a dependence on polymer mesh size to the ??3 power, E ∝ ξ^?3, and following Hertzian contact analysis, the contact area is found to depend on the mesh size squared, A ∝ ξ². Combining these concepts, the shear stress, τ, and therefore the lubricity of aqueous gels, is predicted to be highly dependent on the mesh size, τ ∝ ξ^?3. Studies aimed at elucidating the fundamental mechanism of lubricity in biopolymer gels, mucus, and hydrogels have wrestled with comparisons across mesh size, which can be extremely difficult to accurately quantify. Using scaling concepts relating polymer mesh size to water content reveals that shear stress decreases rapidly with increasing water content, and plots of shear stress as a function of swollen water content are suggested as a useful method to compare aqueous gels of unknown mesh size. As a lower bound, these data are compared against estimates of fluid shear stress for free and bound water flowing through a mesh size estimated by the water content of the gels. The results indicate that the strong dependence on lubricity is likely due to a synergistic combination of a low viscosity solvent (water) coupled to a system that has a decreasing friction coefficient, modulus, and the resulting contact pressure with increasing water content. Although the permeability, K, of aqueous gels increases dramatically with water content (and mesh size), K ? ξ²/η, the stronger decrease of the elastic modulus and subsequent decrease in contact pressure due to an increase in the contact length, predicts that the draining time under contact, t, actually increases strongly with increasing water content and mesh size, t ∝ ξ². Consistent with the finding of extremely high water content aqueous gels on the surfaces of biological tissues, these high water content gels are predicted to be optimal for lubrication as they are both highly lubricious and robust at resisting draining and sustaining hydration.     

Grooved surface texturing by electrical discharge machining (EDM) under different lubrication regimes

The aim of the present research work was to investigate the effectiveness of grooved surface texturing with a rhombic geometry under different lubrication regimes. Tribological investigation under unidirectional sliding was focused on the effect of texturing parameters including pattern area density on the coefficient of friction under different lubrication regimes, achieved by varying sliding speed and lubricant viscosity. Grooved patterns with different textured area densities were produced on steel samples by electrical discharge machining. Results of this investigation showed that under boundary lubrication, textures resist sliding thus resulting in increased friction. The largest improvement of friction reduction was observed under hydrodynamic lubrication, for low‐viscosity oil when using the textured disc with 21% pattern area density. The reduction of the coefficient of friction if compared with the untextured surface was of approximately 24%. Examination of the sliding surfaces has not shown any quantifiable wear for the contact conditions studied.     

Effect of surface texturing on the elastohydrodynamic lubrication analysis of metal-on-metal hip implants

With the development of surface processing techniques, applications of bearing components have been introduced with specific surface textures to take advantage of lubrication.An advanced numerical model was established to simulate the mixed EHL problem of a metal-on-metal hip prosthesis with dimpled surface texturing. The surface texture with simple cylindrical dimples was numerically simulated, under both steady state and walking conditions.The present results showed that surface texturing may have a potentially beneficial effect on the reduction of asperity contact ratio and the improvement of lubrication performance of metal-on-metal hip replacements, particularly under predominant boundary lubrication conditions.     

磨料粒度对表面微织构纯钛干摩擦性能的影响

为了提高钛及钛合金钻具在超深钻探、深海钻探和外太空钻探工程中的减摩抗磨性能。利用激光表面加工技术在工业纯钛（TA2）表面制备了不同参数的点阵微织构。采用MS-T3000摩擦磨损试验机测试了微织构钛合金在不同粒度模拟月壤作用下的摩擦学性能。利用扫描电子显微镜和能谱分析仪分析磨痕形貌及元素含量。研究结果表明：当磨料粒度小于微织构点阵的直径时,磨料压入微织构点阵里,磨料具有滚动和滑动两种运动方式。当粒度大于微织构点阵的直径时,磨料不能完全压入微织构点阵里,磨料对微织构TA2表面产生了滑动犁削作用。由于两种磨料磨损的作用机理不同,同等条件下,小粒度的磨料作用下的微织构TA2的摩擦因数和磨损率较大粒度磨粒作用下的最大减少量分别为50%和53%。考虑磨料粒度与微结构的匹配性,可以大大降低摩擦减少磨损。     

Determination of a coupling equation for milling parameters based on optimal cutting temperature

Nanofluid minimum quantity lubrication (NMQL) is one of the main modes of sustainable manufacturing. It is an environment-friendly, energy-saving, and highly efficient lubrication method. With the use of nanoparticles, the tribological properties of debris–tool and workpiece–tool interfaces will change. However, spectrum analyses of force and power spectral density (PSD) of surface microstructures are limited. In the present work, the milling force, friction coefficient, specific energy, surface roughness, and surface microstructure of debris were evaluated in milling of 45 steel for different lubrication conditions, namely, dry, flood, minimum quantity lubrication, and Al₂O₃ NMQL. Results demonstrated that compared with other lubrication conditions, NMQL achieves minimum milling force peak (F_x?=?270 N, F_y?=?160 N, F_z?=?50 N), friction coefficient (μ?= 1.039), specific energy (U?= 65.5 J/mm³), and surface roughness value (R_a?=?2.254 μm, RS_m?=?0.0562 mm). Furthermore, a spectrum analysis of the milling force and PSD of the surface microstructure was conducted for validation. The spectral analysis of milling force revealed that NMQL obtained the lowest milling force and amplitude in the middle-frequency region, thereby indicating the minimum abrasion loss of the tool. Meanwhile, the PSD analysis indicated that NMQL had the lowest proportional coefficient in the low-frequency region (0.4766) and the highest proportional coefficient in the high-frequency region (0.0569). These results revealed that the workpiece surface gained by Al₂O₃ NMQL obtained higher wave fineness than other working conditions. By combining with the lowest R_a, NMQL contributes the best workpiece surface quality. Therefore, machining experiments using NMQL showed the best lubrication performance.     

The Effect of Laser Texturing of Steel Surfaces and Speed-Load Parameters on the Transition of Lubrication Regime from Boundary to Hydrodynamic

Laser surface texturing (LST) is an emerging, effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were carried out on pin-on-disk test apparatus at sliding speeds that ranged from 0.15 to 0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two types of oil with different viscosities (54.8 cSt and 124.7 cSt at 40°C) were evaluated as lubricants. Electrical resistance between flat-pin and laser-textured disks was used to determine the operating lubrication regime. The test results showed that laser texturing expanded the range of speed-load parameters for hydrodynamic lubrication. LST also reduced the measured friction coefficients of contacts that operated under the hydrodynamic regime. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.     

The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact

Laser surface texturing (LST) is an emerging effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were conducted with a pin-on-disk apparatus at sliding speeds in the range of 0.015–0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two oils with different viscosities (54.8 and 124.7 cSt at 40 °C) were used as lubricants. The test results showed that laser texturing expanded the contact parameters in terms of load and speed for hydrodynamic lubrication, as indicated by friction transitions on the Stribeck curve. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.     

Geometric Shape Effects of Surface Texture on the Generation of Hydrodynamic Pressure Between Conformal Contacting Surfaces

Micro-surface texturing design is becoming an important part of surface engineering since engineering practices and analyses have indicated that surface textures may significantly affect the tribological performance of contact interfaces. Advances in the manufacturing technologies of surface finishing and micro-machining, such as laser surface texturing, photolithography, and etching, LIGA process, have made it possible to fabricate different fine structures on various engineering surfaces. Though the influence of micro-surface texturing on hydrodynamic lubrication has been widely investigated over the last decade, such an influence may be complicated and difficult to characterize with only a few statistic surface parameters. Thus, very little attention has been paid to the effects of different textural shapes and orientations on hydrodynamic lubrication, which is the main topic of this article. A theoretical model based on a single dimple was established to investigate the geometric shape and orientation effects on the hydrodynamic pressure generated between conformal contacting surfaces. Using the Successive Over Relaxation method, the average hydrodynamic pressure generated by the texture pattern with the dimples in shapes of circle, ellipse, and triangle at different orientations to the direction of sliding are obtained. The results indicate that geometric shape and orientation have obvious influences on load-carrying capacity of contacting surfaces. With the same dimple area, area ratio and dimple depth given in this research, ellipse dimples perpendicular to the sliding direction showed the best load-carrying capacity. This result agrees with previous experimental results very well.     

A technique for forecasting the durability of rolling bearings and the optimum choice of lubricants under flood-lubrication and oil-starvation conditions

Effective conditions of friction in units that ensure performance and maximum longevity have been created by flood lubrication under the conditions of elastohydrodynamic (EHD) lubrication. In connection with this, prolonging the service life of friction units by ensuring local requirements to the amount of oil in the contact area and establishing the optimum choice of lubricating material in terms of its viscosity ratings is a topical problem for rolling bearings. Thus, it is suggested to determine the parameter of the lubricating layer (λ) values for the conditions of flood lubrication (λ_o) and progressing (λ_o.s ) and catastrophic (λ _c.s ) oil starvation for a wide spectrum of lubricating materials of different viscosity, since, in oil starvation mode, the efficiency of EHD conditions is violated, the lubrication mode is disturbed, and the bearing capacity of the EHD lubricating layer is lost.     

Triboengineering properties of ceramic oxide coatings under boundary friction against steel

The triboengineering properties of ceramic-oxide coatings are studied during boundary friction against steel 45. Ceramic-oxide coatings formed by microplasma oxidation can be used within a wide range of pressures (8–36 MPa) and sliding velocities (0.05–2 m/s) to ensure lower friction coefficient and wear rate of the materials in contact as compared to a steel-steel pair. The improved triboengineering characteristics of tribojoints with ceramic oxide are attributed to the formation of nanostructures (with 40–100 nm elements) and surface smoothing up to R _a = 5.1 nm. The nanostructure and frictional surface morphology of steel are found to be a result of oil I-40A modification by high-dispersed wear products of ceramics in the friction zone playing the role of a nanosize additive to the lubricating material.     

Effect of Oxygen on the Self-formation of Carbonaceous Tribo-layer with Carbon Nitride Coatings under a Nitrogen Atmosphere

The ball-on-disk friction and wear tests of CN _X coatings (CN _X /CN _X ) were conducted under a nitrogen atmosphere with controlled relative humidity (RH) (3.4–40.0%RH) and oxygen concentration (100–21 × 10⁴ ppm) in this study. We found that the specific wear rate of CN _X coating on ball (W _b), which could give stable and low friction coefficient (<0.05), was below 3.0 × 10^?8 mm³/Nm. Average friction coefficients (µ _a) and W _b of CN _X /CN _X increased (µ _a: 0.02–0.33, W _b: 1.6 × 10^?8–2.4 × 10^?7 mm³/Nm) with increasing oxygen concentration (230–211,000 ppm) as well as RH (4.7–21.1%RH) under a nitrogen atmosphere. However, the W _b remained low value below 2.3 × 10^?8 mm³/Nm regardless of oxygen concentration (100–207,000 ppm) of a nitrogen atmosphere (3.4–3.9%RH) when CN _X -coated balls were slid against a hydrogenated CN _X (CN _X :H) coatings (CN _X /CN _X :H). Besides, the CN _X /CN _X :H achieved low and stable friction coefficient below 0.05 under a nitrogen atmosphere (10,000 ppmO₂) regardless of increasing RH up to 20%RH. Raman analysis indicated that the structure of carbon on the top surface of CN _X coating was changed from as-deposited CN _X coating in the case of low friction coefficient (<0.05). Furthermore, TOF-SIMS analysis provided the evidence that the carbon derived from CN _X -coated disk was considered to diffuse into the ball surface, and it mixed with the carbon derived from CN _X -coated ball on the wear scar, which formed the chemically bonded carbon tribo-layer. Low friction coefficient (<0.05) with CN _X coatings under a nitrogen atmosphere was achieved due to self-formation of the carbon tribo-layer.     

Cutting-force components in turning by tools with no cutting tip

Turning by tools that are characterized by a linear or curved cutting blade but have no cutting tip is studied experimentally. The influence of the depth and cutting speed, the supply, and the cutter inclination on the components P _z and P _y of the cutting force is investigated in inverse and direct cutting.     

A numerical study on the transient aspects of porous flows and the applicability of Darcy’s friction flow relation

In recent developments of shale reservoirs, it is important to estimate the permeabilities of hydraulic fractures accompanying the Non- Darcy effects and geometric changes. Accordingly, a new permeability estimation method that considers the varying geometric features under different flow regions is demanded. To this end, the present study introduces the generalized Darcy’s friction flow relation, especially for examining the friction factor-Reynolds number (f · Re) relationship of porous flow, which is originally used in general internal friction flow analyses. Moreover, simple hydraulic fractures comprising structured microbeads are simulated via computational fluid dynamics during fracture aperture variations under different flow conditions from laminar to turbulent. Frictional flow features, e.g., the preservation characteristics of f · Re values, are examined under different geometry and flow conditions, and the transient flow characteristics are investigated using streamline analyses. Consequently, it is verified that the f · Re values vary slightly in proportion to the geometric changes caused by aperture reduction in each medium. Even though the variations in the f · Re values are much smaller than the permeability variations, it seems to be contrary to our expectation. Otherwise, the almost linear-variation aspects of f · Re values were observed in both directional flow cases. The linear-variation aspect of f · Re values is expected to be useful in the permeability-variation estimations in porous media with changing basic geometric factors, such as hydraulic fracture closing. Moreover, it is demonstrated that regardless of aperture reduction in the same type of medium, each porous flow has a very similar power-law relation between f and Re values when the flow velocity changes from the laminar to the turbulent condition. This aspect can be effectively used for obtaining permeability estimations of the varied media, particularly under different flow conditions.     

Effect of laser textured dimples on the lubrication performance of cylinder liner in diesel engine

Laser surface texturing (LST) technique was utilised on a cylinder liner in a diesel engine. In order to analyse the effect of LST micro‐dimples on the lubrication and friction properties of cylinder liner–piston ring (CL–PR), we developed a new mixed lubrication model on the basis of the average Reynolds equation and asperity contacts equation. The model can consider the coupling effects between the surface roughness of non‐texturing regions and micro‐dimples and the synergistic effects of multi‐micro‐dimples. The results show that cylinder liner surface by LST can form effective hydrodynamic lubrication effect in most regions of the strokes, only near the dead points, the friction pair is in mixed lubrication state, asperity contact plays a major role in balancing the external load and the asperity friction force is obvious. The micro‐dimple parameters were optimised to obtain a better lubrication effect with the following optimised results: r_p = 30–60 µm, S_p = 0.2–0.4 and e = 0.03–0.1. Copyright © 2012 John Wiley & Sons, Ltd.     

Modeling and analysis of mechanical Quality factor of the resonator for cylinder vibratory gyroscope

Mechanical Quality factor(Q factor) of the resonator is an important parameter for the cylinder vibratory gyroscope(CVG). Traditional analytical methods mainly focus on a partial energy loss during the vibration process of the CVG resonator, thus are not accurate for the mechanical Q factor prediction. Therefore an integrated model including air damping loss, surface defect loss, support loss, thermoelastic damping loss and internal friction loss is proposed to obtain the mechanical Q factor of the CVG resonator. Based on structural dynamics and energy dissipation analysis, the contribution of each energy loss to the total mechanical Q factor is quantificationally analyzed. For the resonator with radius ranging from 10 mm to 20 mm, its mechanical Q factor is mainly related to the support loss, thermoelastic damping loss and internal friction loss, which are fundamentally determined by the geometric sizes and material properties of the resonator. In addition, resonators made of alloy 3J53 (Ni42CrTiAl), with different sizes, were experimentally fabricated to test the mechanical Q factor. The theoretical model is well verified by the experimental data, thus provides an effective theoretical method to design and predict the mechanical Q factor of the CVG resonator.     

Assessment of critical level of shear stresses in tribocouples of Аl<Subscript>2</Subscript>O<Subscript>3</Subscript> and metal details of ITER blanket modules

The shear strength of plasma-sprayed electroinsulating Аl₂O₃ coating has been determined for blanket modules of a thermonuclear reactor in a friction couple with 316L(N)-IG steel and BrAZhNMts9-4-4-1 bronze. An analysis of the experimental and calculated data on the influence of shear stresses on the electric insulation of blanket modules has revealed the need to decrease the coefficient of friction to permissible level [f] = 0.28 in the range of contact pressures Р = 3–500 MPa.     

Generalized model of vibro-isolation systems with seated human body for single-axis transmission of vibration

A generalized model of vibration isolation systems with a human body in a seated position is developed. The system dynamics is modeled for a single-axis transmission of vibration, that is, longitudinal x, lateral y and vertical z, to limit whole-body vibration exposure. The developed model can be successfully employed to describe the propagation of vibration waves from the source to a vibrating object. The proposed mathematical model can be widely used to protect a human body against vibration.     

Calculating the wear resistance of metals in frictional pairs 3. Calculating the wear rate of steel in roller pairs with rolling friction and slip

On the hypothesis that slipping friction has the primary influence on the wear of steel in a rolling frictional pair with slip, the dimensionless wear rate I of wheel steel in the driven roller and the dimensional wear rate W (expressed in terms of I) are calculated. Satisfactory agreement of the theoretical and experimental wear rates is observed in the regions of weak, pronounced, and catastrophic wear.     

Spectroscopy of Single AlInAs Quantum Dots

A system of quantum dots based on Al _x In_1?xAs/Al _y Ga_1?yAs solid solutions is investigated. The use of Al _x In_1?xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al _x In_1?xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al _x In_1?xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al _x In_1?xAs quantum dots.     

Textured Surface Hydrodynamic Lubrication: Discussion

We discuss on a recently presented theory of textured surface hydrodynamic lubrication (Scaraggi, Phys Rev E, 2012). The model, based on the Bruggeman effective medium approach, allows to analytically evaluate the effects of a generic texture shape, distribution, and area density on the macroscopic hydrodynamic characteristics of the contact, such as friction and supported load. In this study, we apply the cited theory to practical cases, and in particular we derive the flow and shear stress tensors for two limiting conditions, i.e., for isotropic (circular inclusion in isotropic medium) and perfectly anisotropic (infinite slit inclusion) flow conductivities. These results are then used to perform near-optimum design calculations for the simplest case of one- and two-dimensional thrust-bearing geometries. Finally, a comparison with published results is presented and discussed. The developed theory may be a very useful tool in the process of evaluating the lubrication performances of sliding microtextured surfaces and for the near-optimum design of a textured pair, where texturing could be achieved by both physical (e.g., microstructuring) and chemical surface manipulation.