Molecular tribology of lubricants and additives

Knowledge of the bulk viscosity provides little guidance to predict accurately the interfacial shear strength and effective viscosity of a fluid in a lubricated contact. To quantify these differences between bulk and thin-film viscosity, an instrument was developed to measure the shear of parallel single crystal solids separated by molecularly-thin lubricant films. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. Studies with lubricant additives cast doubt on the usefulness of the concept of a friction coefficient for lubricated sliding.     

Investigation of the strain distribution with lubrication during the deep drawing process

A lubrication/friction model can be implemented in FEM codes to predict the contact area ratio, friction coefficient and strain distribution in lubricated deep drawing process. In the lubrication analysis, the surface roughness effect on lubrication flow is included by using Wilson and Marsault's average Reynolds equation that is appropriated for mixed lubrication with severe asperity contact. With regard to the asperity contact theory, the well-known flattening effect is considered. Friction is expressed in terms of variables such as lubricant film thickness, sheet roughness, lubricant viscosity, interface pressure, sliding speed, and strain rate. The proposed lubrication/friction model combined with a finite element code of deep drawing process to predict the contact area ratio, friction coefficient and strain distribution. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

Friction characteristics of a ferritic-austenitic stainless steel during lubricated reciprocating motion

This report describes friction measurements of stainless steel against stainless steel during lubricated, small-amplitude reciprocating motion. The experimental investigation was divided into two parts. First, four different lubricants were evaluated using a response surface design, during which the average contact pressure and the sliding velocity were varied. Secondly, a 2⁴ factorial design with three replicate runs was performed. Here, the coefficient of friction in the initial stage and the duration of that stage were studied. The independent variables were the average contact pressure, sliding velocity, surface roughness and type of lubricant. In the early state (stage I), the value of the frictional force is controlled by plowing of the surfaces by asperities. In many lubricated contacts, this is the practically useful stage. The experimental results from the response surface design show that the duration of stage I depends on the type of lubricant. Adhesive wear can take place before 100 cycles. The factorial design indicates that the coefficient of friction in the initial stage is affected by the type of lubricant, surface roughness and the simultaneous change of the surface roughness and type of lubricant. The duration of the initial stage is affected by a change in the surface roughness, average contact pressure and a simultaneous change in average contact pressure and surface roughness. A two-parameter Weibull analysis was performed on the data from the factorial design. For the tests where lubricant no. 3 was used, a mixed distribution was indicated for the duration of stage I. This mixed distribution indicates that a weakest-link process as well as a healing process were involved.     

Instantaneous Coefficients of Gear Tooth Friction

In a gear contact as simulated on a roller test machine, the instantaneous coefficient of friction follows the concept of transition from boundary to hydrodynamic lubrication. The coefficient has been found to increase with increasing load and to decrease with increasing sum velocity, sliding velocity, and oil viscosity as each of these quantities is varied individually. The viscosity was determined by the temperature of the oil entering contact and the viscosity-temperature characteristics of the lubricant. The results have been combined in a formula which closely represents the data. When this formula is used in gear scoring calculations, the same type of U-shaped load-speed curve is obtained as has been found on several gear test rigs.     

On the equivalent friction factor in hydrofilm extrusion

Hydrofilm extrusion is a kind of hydrostatic extrusion which uses a minimum amount of oil as lubricant and pressure-transmitting medium. In hydrofilm extrusion energy dissipation in the fluid lubricant between the die and working material corresponds to sliding friction in ordinary lubricated extrusion using solid lubricants. Utilizing the upper-bound theorm, an “equivalent” friction factor is defined so that the overall frictional effect between the die and working material can be conveniently investigated in terms of geometrical parameters and press velocity. On the basis of this definition, the effects of various process parameters on the frictional characteristics in hydrofilm extrusion are discussed. It is consequently found that the dominant contribution to frictional energy dissipation is made by reduction of area and press velocity. Die length is found to have very little influence on the equivalent friction factor in so far as it is longer than billet diameter.     

Bridging the Gap Between the Atomic-Scale and Macroscopic Modeling of Friction

A short survey of a modern view on the problem of friction from the physical viewpoint is presented. An atomically thin lubricant film confined between two substrates in moving contact has been studied with the help of molecular dynamics (MD) based on Langevin equations with coordinate- and velocity-dependent damping coefficient. Depending on model parameters, the system may exhibit either the liquid sliding regime, when the lubricant film melts during sliding (the “melting-freezing” mechanism of stick-slip motion), the “layer-over-layer” sliding regime, when the film keeps a layered structure at sliding, or the solid sliding regime, which may provide an extremely low friction (“superlubricity”). Atomic-scale MD simulations of friction, however, lead to a “viscosity” of the thin film, as well as to the critical velocity of the transition from stick-slip to smooth sliding, which differ by many orders of magnitude from the values observed in macroscopic experiments. This contradiction can be resolved with the help of the earthquakelike (EQ) model with a continuous distribution of static thresholds. The evolution of the EQ model is reduced to a master equation which can be solved analytically. This approach describes stick-slip and smooth sliding regimes of tribological systems within a framework which separates the calculation of the friction force from the atomic-scale studies of contact properties.     

In situ studies of the lubricant chemistry and frictional properties of perfluoropolyalkyl ethers at a sliding contact

In situ analyses of lubricated sliding contacts were performed by interfacing an ultraviolet Raman spectrometer to a ball-on-flat tribotester. The sliding contact was simulated by rotating a sapphire window that is transparent to ultraviolet radiation against a stationary ball. Various loads were transmitted to the contact center through the ball. A branched perfluoropolyaklyl ether (Krytox 479) and two linear perfluoropolyalkyl ethers (Fomblin 491 and Fomblin 497) have been studied under various loads at a 10 cm/s sliding speed. Krytox and a Fomblin of lower viscosity, Fomblin 497, decomposed to amorphous carbon upon sliding on a chrome steel ball but no amorphous carbon was detected from Fomblin 491. The amount of amorphous carbon at the contact area during sliding was a balance of formation and removal rates. It is postulated that surface activity of the chrome steel ball was the main cause for the lubricant degradation. The lubricant degradation at the chrome steel/sapphire interface was found to slightly increase the kinetic coefficient of friction at the contact center. However, catastrophic scuffing was not observed.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

Some aspects of the friction and wear of electrical sliding contacts under conditions of boundary lubrication

The friction characteristics of lubricated electrical sliding contacts are considered. Data are presented concerning the effects of the current, the lubricant properties, the velocity and the load on the friction and wear behaviour of the contacts. In light-current electrical contacts the effectiveness of the lubricant does not depend on its conductivity but on its ability to prevent the formation of a non-conductive film by lubricating action. Under semifluid lubricating conditions the heavy current acts by discharge through the lubricant film, and the lubricant conductivity generally determines the friction and wear characteristics of the contact. Colloidal metal particles produce additional conductivity in the clearance between the contacting surfaces, prevent electrical erosion and, in some cases, form plastic films which decrease the coefficient of friction and the wear rate of the contact.     

Quantitative Viscosity Mapping Using Fluorescence Lifetime Measurements

Lubricant viscosity is a key driver in both the tribological performance and energy efficiency of a lubricated contact. Elastohydrodynamic (EHD) lubrication produces very high pressures and shear rates, conditions hard to replicate using conventional rheometry. In situ rheological measurements within a typical contact are therefore important to investigate how a fluid behaves under such conditions. Molecular rotors provide such an opportunity to extract the local viscosity of a fluid under EHD lubrication. The validity of such an application is shown by comparing local viscosity measurements obtained using molecular rotors and fluorescence lifetime measurements, in a model EHD lubricant, with reference measurements using conventional rheometry techniques. The appropriateness of standard methods used in tribology for high-pressure rheometry (combining friction and film thickness measurements) has been verified when the flow of EHD lubricant is homogeneous and linear. A simple procedure for calibrating the fluorescence lifetime of molecular rotors at elevated pressure for viscosity measurements is proposed.     

渐开线直齿轮的摩擦激励及振动阻尼分析

齿轮传动中的润滑油膜一般为非线性粘滞体（Ｒｅｅ－Ｅｙｉｎｇ体）。运用部分膜承载热弹流理论计算齿轮传动中的滑动摩擦力和滚动摩擦力。齿轮的振动阻尼力是齿面滑动摩擦力中的一部分,是齿轮振动角位移的非线性函数。为了便于工程应用,使用线性阻尼系数。它是齿轮几何尺寸和压力油膜的粘度及膜厚的函数。     

Mapping Shear Stress in Elastohydrodynamic Contacts

A new method has been, devised for investigating the theological properties of lubricant films in two-dimensional EHD contacts. A lubricated, sliding contact is produced between a sapphire flat and a steel ball. Thermal infrared emission microscopy is then employed to obtain 2-D maps of the variation of temperature rise due to friction across the contact. These maps are then used in conjunction with moving heal source theory to produce maps of energy dissipation and thus shear strength, of the lubricant film across the contact.

A series of mixtures of two lubricants, one giving high traction and one with low traction, have been studied using this technique to investigate the influence of lubricant, blending on shear stress and traction.     

Friction and Wear Properties of Micro Textured DLC Coated Surfaces in Boundary Lubricated Sliding

In the present study, the friction and wear properties of boundary lubricated textured surfaces were investigated. The capability to feed lubricant into the interface of a sliding contact and to isolate wear particles was related to the shape, size and orientation of the texture patterns. Well-defined surface textures of square depressions or parallel grooves of different widths and distributions were produced by lithography and anisotropic etching of silicon wafers. Subsequently the wafers were PVD coated with thin, wear resistant DLC coatings, retaining the substrate texture. The surfaces were evaluated in reciprocating sliding against a ball-bearing-steel ball under starved or amply lubricated boundary lubrication conditions.     

Tribological behavior of reciprocating friction drive system under lubricated contact

The dynamic friction and wear behaviors are investigated in reciprocating friction drive system using a 0.45% carbon steel pair. The effects of various operating parameters on the traction force, stick and slip time, and friction modes are examined under the lubricated contacts. Moreover, the critical operating conditions in classifying three friction modes are also established. Results show that the fluid friction induced by the shearing of lubricant dominates the variation of traction force and produces the positive slope γ at the first period of slip in the traction force–relative sliding velocity curve. The γ value decreases at higher driver speed during stick-slip motion due to the thicker fluid film and shear thinning effect. The γ value increases due to the asperity interactions as the friction region is transferred from stick-slip to sticking with normal load from 196 to 980 N. Furthermore, it is also found that the static friction force is independent of stick time for the tangential loading rate ranged from 1.12 to 16.8 s⁻¹. The transition region produces the severest wear under the different driver speeds, but the wear is insensitive to the friction regions and the severe wear only occurs at higher normal load due to the action of Hertzian contact.     

Roughness and Lubricant Effect on 3D Atomic Asperity Contact

Large-scale molecular dynamics simulations were performed to study the sliding process of rough surfaces with and without lubricant. In the dry contact, a linear relationship has been observed between the load and the contact area for surfaces with large root mean square (RMS) roughness. However, it becomes nonlinear when the RMS is small. In the presence of adhesion, small roughness results in a large friction force when the surfaces are flattened and the contact area reaches 60 %. In order to confirm this observation, nonadhesive models have been established with an observation that under the combined influence from roughness and adhesion, the contact area plays a crucial role to determine whether the dry sliding is under the domination of roughness or adhesion. In the lubricated sliding, an increase in friction force has been found for the partially lubricated condition because the asperity contact still accounts for a great deal of resisting force. Besides, the lubricant exerts a comparable resisting force to the sliding.     

Temperature—The Key to Lubricant Capacity

Failure of a nonreactive mineral oil can be predicted by Blok's formula for determining the maximum temperature between two bodies in rolling and sliding contact. Evaluation of many lubricants on a geared roller lest machine revealed that the lubricant failure for any particular lubricant-material combination occurs at a constant, critical contact temperature over wide ranges of load, sliding velocity, surface velocity, specimen temperature, film thickness, and viscosity grade. Coefficient of friction can be predicted by a parameter involving the unit load, inlet viscosity, sum velocity, and sliding velocity. The load capacity of a lubricant varies inversely with specimen temperature for a constant set of lest conditions. Electrical resistance measurements across the contact zone aided in identifying the lubricant failure point and in revealing the action of two deposit-forming additives.     

The elastohydrodynamic transition speed for spheres sliding on lubricated rubber

An elastohydrodynamic number derived elsewhere in the literature [1] characterizes the onset of hydrodynamic support for a rigid sphere sliding on a lubricated viscoelastic base. This number includes elastic properties of the base track, in contrast with previous studies where such have been neglected. A generalized coefficient of sliding friction has been defined as the actual coefficient of friction divided by the tangent modulus of the viscoelastic material. Experimental plots of the coefficient of friction versus sliding speed for spheres sliding on lubricated rubber are shown to produce a relatively sudden decay in coefficient at the transition speed from “dry” to elastohydrodynamic contact. These plots in turn fit closely on a master curve of generalized coefficient of friction versus the elastohydrodynamic number.The inclusion of surface roughness on the sphere produces both a higher value of the generalized coefficient prior to the transition speed and a higher sliding velocity at which the transition itself occurs. Furthermore, the rate of decay for the generalized coefficient of friction appears distinctly greater for rough spheres. The overall effect of roughness is to reduce the difference between the dry and wet coefficients of sliding friction. Random abrasion of the spheres with emery paper of known grit size appears to be an effective method of inducing surface roughness on the spheres. The nature of all the experimental curves may be satisfactorily explained by squeeze-film theory.An important application of the sliding of smooth and rough spheres on a lubricated flexible base is the sliding/slipping behaviour of automobile tyres on a wet road surface during normal rolling.     

Thermal activation in boundary lubricated friction

The friction coefficients for copper pairs lubricated with fatty acids and fluorinated fatty acids have been measured over a wide range of sliding speeds and temperatures. Sliding speeds in the range 10⁻⁷−10⁻² m s⁻¹ and temperatures in the range 4.2–300 K were used. The friction coefficients near 300 K are generally low and increase with sliding speed, while the friction coefficients at low temperatures are markedly higher and relatively independent of velocity. Each lubricant's friction vs. velocity behavior over the temperature range 150–300 K can be described by a friction-velocity master curve derived from a thermal activation model for the lubricant's shear strength. The activation energies deduced from this friction model are identical to those obtained in the same temperature range for a vibrational mode associated with low temperature mechanical relaxations in similarly structured polymers. These results suggest that thermally activated interfacial shear is responsible for the fatty acids' positive-sloped friction vs. velocity characteristics at low sliding speeds near room temperature.     

Nonlinear thermodynamic model of boundary friction

Melting of an ultrathin lubricant film confined between two atomically flat surfaces is studied. An excess volume parameter is introduced, the value of which is related to the presence of defects and inhomogeneities in the lubricant. Via minimization of the free energy, the Landau-Khalatnikov kinetic equation is obtained for this parameter. The kinetic equation is also used for relaxation of elastic strains, which in its explicit form contains the relative shear velocity of the rubbing surfaces. With the numerical solution of these equations, a phase diagram with domains corresponding to the sliding and dry stationary friction regimes is built at a fixed shear velocity. A simple tribological system is used to demonstrate that in the dynamic case, three friction regimes can occur, namely, dry, stick-slip, and sliding friction. It is shown that a lubricant can melt when the shear velocity exceeds a critical value and with elevation of its temperature. The dependence of the dynamic friction force on the pressure applied to the surfaces, the temperature of the lubricant, and the shear velocity is considered. It is shown that growth of pressure leads to the forced ordering and solidification of the lubricant.     

TRIBOS: A Performance Evaluation Tool for Traction-Transmitting Partial EHD Contacts

The details are given of a computer model for performing a state-of-the-art tribological assessment of the performance of a lubricated concentrated rolling/sliding/spinning/contact comprising general anisotropic rough surfaces. The name chosen for this program is TRIBOS.

It computes: 1. The contact ellipse dimensions and area

2. The elastohydrodynamic (EHD) film thickness both at the plateau and at the constriction that forms at the rear of a lubricated concentrated contact under fully flooded (un-starved) and isothermal lubricant inlet conditions

3. The apportionment of the applied load between the asperities and the lubricant film

4. The magnitude and direction of the tractive force transmitted between the contacting bodies by the combined effects of (a) shearing of the fluid film and (b) coulomb friction between contacting asperities

5. The mean number of asperity contacts and the real contact area, i.e. the total contact area of the elastically deformed asperities

6. A film thickness correction factor accounting for lubricant starvation in the contact inlet

7. A film thickness correction factor accounting for a viscosity decrease of the inlet oil due to fluid heating

8. An index of surface fatigue behavior

The program is a synthesis of computational tools from the current literature for the computation of fluid film thickness and traction, and a general asperity simulation model for the elastic contact of anisotropic rough surfaces. In the example given, it is used to perform a comparative evaluation of the performance of 18 combinations of 9 surface roughnesses and 2 lubricants in a traction drive contact.