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1.
An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge. 相似文献
2.
The pure squeeze thin film elastohydrodynamic lubrication motion of circular contacts with effects of surface forces and surface roughness taken into account is explored under constant load conditions. The coupled transient stochastic Reynolds, elasticity deformation, the load balance, surface forces (hydrodynamic, solvation and van der Waals pressure) and lubricant rheology equations were solved simultaneously by using the finite difference method and the Gauss–Seidel iteration method. The simulation results reveal that the differences between radial type roughness and circular type roughness problems are apparent as the film thickness is thinner than 5 nm. The oscillation phenomena in pressure and film thickness come mainly from the action of solvation forces. The effects of surface forces become significant as the film thickness becomes thinner. The film thickness with circular type roughness is thicker than that with radial type roughness. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
3.
Molecular dynamics simulations are used to study the tribological performance of a lubricant mixture containing hexadecane base oil and 5% zinc dithiophosphate (ZDDP) under molecular confinement conditions. The influence of ZDDP additive and the surface-lubricant interaction on the mechanical and thermal interfacial response are studied in detail. Results show that mechanical and thermal slips are reduced by increasing the surface energy. Simulations also demonstrate the migration of ZDDP molecules and their adsorption onto the solid surface resulting in a remarkable suppression of mechanical slip compared to pure hexadecane. Consequently, the effective shear rate is higher and so is the friction. 相似文献
4.
The thermo-hydrodynamic behaviour of lubricant films strongly depends on the operating conditions (pressure, speed), on the physical properties of the lubricant, on the temperature level and, furthermore, it is often nonlinear. The thermo-hydrodynamic study of a thin fluid film confined between two parallel planes on relative motion is presented in this paper. The case considered is that of not fully developed thermal conditions and physical properties of the lubricant independent of temperature. A 2D analytical solution is proposed to determine the temperature distribution. The physical parameters affecting the characteristic length, beyond which the flow becomes thermally developed, are determined. 相似文献
5.
In the paper an experimental study of compression of rough surface in the presence of lubricant and in dry state is presented. The evolution of roughness parameters was analyzed. In the lubrication conditions an existence of asymptotic state was observed where roughness parameters and real contact area stabilize due to hydrostatic response of lubricant. The simplified model was proposed allowing to predict the roughness parameters and real contact area of the deformed surface in the asymptotic state. The model is based on analysis of topography of the undeformed surface and compression of a singular lubricant pit. The analysis was performed both numerically and experimentally. 相似文献
6.
Muhannad Mustafa Nusrat J. Chhanda M. Mahbubur Razzaque 《Tribology International》2010,43(1-2):202-209
A numerical model for the hydrodynamic behavior of an oscillating squeeze film between a rubber surface and a rigid surface is presented. The effects of roughness of the rubber surface on the hydrodynamic force and the leakage flow rate in the squeeze film are analyzed. A modified Reynolds equation, Laplace equation and a three-parameter viscoelastic constitutive equation are solved simultaneously to obtain the pressure distribution in the squeeze film and the deformation of the rubber surface. Equations are discretized into finite difference equations and solved by Gauss-Siedel iteration method. It is found that increasing roughness of the surface profile significantly increases the hydrodynamic force accompanied by a small decrease in the leakage flow rate. Spatial distribution of the roughness of the rubber surface has no significant effect on the leakage flow rate or hydrodynamic force. The results obtained from the presented simple model are compared with the experimental results available in the literature and a very good agreement is found. 相似文献
7.
Wear life of bonded molybdenum disulfide film lubricant applied on discrete sections of chromium electroplated to an aluminum alloy substrate under extreme contact pressure was investigated. Block-on-ring tests with ridge-shaped, hardened stainless steel blocks and lubricant coated rings were carried out in a nitrogen atmosphere for several kinds of ring materials including this under-coating. All tests except those for the discontinuous chromium under-coating indicated large or unstable friction soon after the tests started. Discontinuous chromium coating demonstrated a stable friction coefficient of around 0.05 for a long time and the lubricant film remained intact after the test. This is because discontinuous chromium coating on aluminum substrate deformed along the shape of the ridged-block and contact pressure was decreased to several hundred MPa. 相似文献
8.
In many applications (e.g. roller, barrel or needle bearings) surface features exhibit longitudinal alignment to the direction of motion. These features are produced by surface finishing techniques in the circumferential direction and are associated with line or very wide elliptical contact geometries. In such a case, the contact length in the direction of motion is considerably shorter comparing its width and the effect of a longitudinal roughness could significantly influence the lubrication film formation. Recent experimental studies have indicated less severe effect of a longitudinal roughness on lubrication film formation in the comparisons with that observed with transversely orientated roughness caused by the inlet perturbation. Nevertheless, these experimental studies have been focused on the behaviour of artificially produced asperities within a circular contact. The quantitative experimental study of longitudinal real surface roughness within a line contact has not been realized yet. That is why, in this study, the line contact formed between a steel tapered roller and glass disc is observed within an optical test rig and the effects of real surface roughness on lubrication film formation are studied. Experiments carried out under pure rolling conditions have shown that the depth is the key parameter that influences the effect on the film thickness. If the roughness features are shallow, the lubrication film shape within the contact follows the shape of the surface closely. However, the groove having only about 800 nm in depth divided the line contact into two parts that behave as two separate line contacts. Such an effect can increase the risk of the wear of rubbing surfaces as the lubrication film thickness between the real machine components can be significantly lower than expected. 相似文献
9.
Various phenomena are revealed under EHL and micro-EHL conditions, such as the properties of the lubricant under high pressure, traction, and the load-bearing capacity of the lubricant film, and are discussed in the present paper. A new lubrication regime, thin film lubrication, has been discussed. The theoretical and practical significance of research on thin film lubrication is elaborated. Finally, the characteristics describing thin film lubrication and its main research directions are suggested. 相似文献
10.
运用光干涉法相对光强原理,通过纳米级润滑膜厚度测量仪测量了不同温度、不同载荷下纳米间隙中环境友好润滑剂季戊四醇油酸酯的油膜厚度。探究了膜厚与速度、载荷、温度之间的关系,观察了薄膜润滑现象。结果表明:在对数坐标系下,膜厚与速度具有一定的线性关系,在速度较高时,线性关系更强,润滑剂具有流体效应。载荷对膜厚的影响远小于速度对膜厚的影响,温度对膜厚的影响主要表现在温度越高时,润滑剂的黏度越低,润滑剂的膜厚越薄。 相似文献
11.
The surface chemistry of chloroform as an extreme-pressure lubricant additive at high concentrations
Carbon tetrachloride is an extremely good extreme-pressure (EP) lubricant additive at low concentrations (<3 wt% chlorine) since it can react to form a high-melting-point Fe3C antiseizure layer. In contrast, small hydrogen-containing additive molecules (CH2Cl2, CHCl3) decompose to form FeCl2 which melts at ~940 K and limits the maximum seizure load to ~3500 N as measured hi a pin and v-block apparatus. However, both thermodynamic calculations and results of a Mössbauer analysis of an iron foil heated in CHCl3 at 830 K indicate that iron carbide can be formed from chloroform. In addition, it is also found in that case that a plot of seizure load versus concentration, after initially forming a plateau, once again increases with higher additive concentrations (>4 wt% chlorine) in accord with the idea that a higher melting point carbide film can be formed. It has been shown previously that asymptotes in the plot of removal rate versus applied load correspond to melting of the interfacial anti-seizure film. When using 9.0 wt% chlorine from chloroform as the additive, a drastic increase in removal rate is found at an interfacial temperature of ~940 K corresponding to the melting of FeCl2 and an additional asymptote is evident at ~1500 K due to the melting of Fe3C in accord with the thermodynamic and Mössbauer results. 相似文献
12.
非晶硅薄膜太阳能电池主要采用掺氟氧化锡(FTO)导电玻璃作为基板,但FTO薄膜雾度较低、表面形貌无法优化,导致无法得到较优的陷光结构,从而限制了太阳能电池的转换效率。为了进一步提升太阳能电池的转换效率,探讨了替代型的掺铝氧化锌(AZO)薄膜,通过优化前段磁控溅射镀膜工艺和后段湿化学蚀刻工艺,用以平衡AZO薄膜的光电性能和雾度,从而获得具有理想表面形貌的AZO导电玻璃,使其成为理想的非晶硅薄膜太阳能电池的基板材料。实验表明,经工艺优化后制作的AZO导电玻璃可提升光电转换效率。 相似文献
13.
Friction and adhesion measurements between surfaces of cross-linked, stiff polymers of varying roughness against smooth, bare
mica surfaces were carried out in dry air as well as in the presence of lubricating oil. The nominal (macroscopic) contact
area varies with the applied load according to the Johnson, Kendall and Roberts (JKR) theory, yet shows significant hysteresis
due to the irreversibility arising from the loading/unloading curves of multiple asperities. Upon introducing the oil between
the surfaces, the critical shear stress is reduced to zero due to the elimination of the adhesion force. However, the effect
is less noticeable on the friction coefficient. Lastly, the effect of increasing the (RMS) roughness was greatest over the
first few nanometers due to the diminution of the adhesion-controlled contribution to the friction, after which a further
increase in roughness had less dramatic effects. A model is presented to account for the observed adhesion hysteresis during
repeated loading/unloading cycles of purely elastically deforming rough surfaces.
Bruno Zappone and Kenneth J. Rosenberg made equal contributions. 相似文献
14.
《Lubrication Science》2017,29(8):557-566
High‐speed optical imaging is introduced as a visualization method to investigate the film‐flow properties of liquids with different surface tensions and viscosities (water, poly‐alpha‐olefin oil, and glycerol) over free rotating surfaces with different surface energies and polarities (steel, and two different diamond‐like‐carbon ‐ DLC coatings, i.e. DLC, and F‐DLC). It was found that the polar surface energy strongly influences the structural dynamics of the liquid film's flow and the film's slip. Namely, a decrease in the polar surface energy results in a less stable film with de‐wetting areas and breakups into streams, as well as in a larger amount of film slip, which was most clearly expressed by the F‐DLC. It was also found that the combination of a high surface tension and a low viscosity provides the largest amount of liquid slip, with the most obvious breakup of the liquid film being observed with water, which clearly exhibits these properties. 相似文献
15.
16.
Molecular dynamics (MD) simulations were conducted in order to study the dynamic behavior and traction of bilayer lubricating
films of n-hexane, cyclohexane, and n-hexadecane. Lubricants were confined between bcc iron surfaces with and without transverse grooves of mono-atomic depth.
Once the system equilibrated statically, one of the solid surfaces was moved to shear the film. The results demonstrated that
the traction coefficient was governed by structures of the films, which depended on the molecular structures of the lubricants
and on the atomic scale geometry of the solid surfaces. Traction was high when interfacial slip between lubricant layers and
solid walls occurred. Evolution of the layered structure by gradual rearrangement of the molecules and resulting slip between
the lubricant layers, caused significant reduction in the traction coefficient. The atomic steps enhanced the molecular rearrangement
of n-hexadecane, while they retarded or inhibited those of n-hexane and cyclohexane resulting in a relatively higher traction coefficient for stepped surfaces. Molecular orientation
of the normal alkanes under shear is described by the orientational order parameter, which has a strong correlation with the
traction coefficient. The steady state traction coefficient of all the three simple hydrocarbons was highest when both of
the surfaces had steps, and lowest when both of the surfaces were flat. 相似文献
17.
In this paper, a theoretical study of the effect of surface roughness on the hydrodynamic lubrication of couple-stress squeeze film between a sphere and a flat plate is presented on the basis of Christensen's stochastic theory for hydrodynamic lubrication of rough surfaces. The modified Reynolds equation accounting for the couple stresses and the surface roughness is mathematically derived. The modified Reynolds equation is solved for the fluid film pressure and the bearing characteristics, such as the load carrying capacity and the time–height relationship, are obtained. It is found that the surface roughness considerably influences the squeeze film characteristics. The load carrying capacity and squeeze film time are found to increase for an azimuthal roughness pattern as compared to the corresponding smooth case, whereas the reverse trend is observed for a radial roughness pattern. 相似文献
18.
An experimental and numerical study of cold rolling lubricated by O/W emulsion has been carried out. The strip rolling experiment
was carried out on a Hille experimental rolling mill with a view to study the performance of emulsion lubrication in terms
of practical rolling parameters. Accordingly, rolling parameters such as rolling force and torque were measured. The experimental
measurements compare favourably with the computed results from a numerical scheme developed by the authors. The scheme, based
on a two-phase lubricant model, is capable of calculating the oil concentration at any point within the inlet zone and work
zone, rolling pressure, film thickness, and fractional contact area ratio associated with strip rolling under mixed film lubrication
at different rolling speeds. Using this scheme, the intertwined effects of an emulsion’s parameters such as: oil concentration,
mean oil droplet size, and rolling speed on strip rolling were investigated. The numerical study encompassed the mixed film
regime for speed, S ranges from 10−4 to 10−2, supply oil concentration level λds from 1 to 10%, and oil droplet size D
S from 5 to 10. Experimentally, the differences between water, oil and emulsion-lubricated rolling are not discernible except
for film thickness. At a low speed of 10 RPM, force and torque of water-lubricated rolling are marginally higher than oil-
or emulsion-lubricated ones. However, the difference between emulsion and neat oil is not apparent. The numerical results
show the occurrence of a moderate oil concentration increase in the inlet zone followed by a sharp one at the beginning of
the work zone. The effect of the concentration process is predominantly seen in the film thickness and the lubricant pressure
whilst its effect on the total pressure is less pronounced. The analysis of the results suggests that it is possible to lower
the emulsion oil concentration without any adverse effect on the rolling process. This principle can be used to control the
outlet lubricant film thickness and hence the surface quality of the rolled strip. 相似文献
19.
Wang Xin-jie Bai Shao-xian Huang Ping 《Frontiers of Mechanical Engineering in China》2006,1(3):370-373
A new mathematical model for thin film lubrication is established by taking into account the effect of an electric double
layer. In the present paper, experiments are carried out on a self-made tester. With a composite block and a rotating disk,
influence of electric double layer on thin film lubrication is studied. Two different methods are used to reconstruct the
field of electric double layer so as to change its effect. One is to change the ionic concentration of lubricants by adding
additives, and the other is to apply an external electric field on friction pairs. According theoretical analysis, both the
methods will apparently change the electro-viscosity of the lubricant film so as to change the lubrication performances. After
theoretical calculation of electro-viscosity is amended according to the experimental results, the equations of electro-viscosity
are presented. The results show that the equivalent viscosity of fluid induced by the effect of electric double layer apparently
increases with the decrease of thickness of the film while the lubrication film is thin enough. The effect of electro-viscosity
is weakened as the thickness of the film increases. Moreover, the effect of electro-viscosity increases with the increase
of external electric field at first. When the voltage reaches a certain value, the electro-viscosity begins to decrease.
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Translated from Tribology, 2005, 25(6) (in Chinese) 相似文献