W. O. Winer

Molybdenum disulfide as a lubricant: A review of the fundamental knowledge

Abstract This report consists of a review and evaluation of the work presented in the literature over the past 40 years on the behavior of MoS2 as a lubricant. The discussion is concerned with the behavior of MoS2 as a lubricant, its frictional characteristics, the apparent reasons for these characteristics, and the physical and chemical properties which are important to its use as a lubricant. The low friction and easy cleavage of molybdenum disulfide is intrinsic to the material and a result of its crystal structure. Although it is clear that the presence of condensable vapors is not required for molybdenum disulfide to exhibit low friction, as it is in the case of graphite, it is true that condensable vapors play an important role in determining the friction and wear characteristics of a lubricant film of MoS2. The presence of oxidizing agents, particularly water vapor and oxygen, tends to reduce the wear life of the MoS2 film. This is particularly true in the case of burnished films. Although a great deal of knowledge exists concerning the behavior of molybdenum disulfide as a lubricant, there are still voids in our understanding of this subject.

A Rheological Model for Elastohydrodynamic Contacts Based on Primary Laboratory Data

A shear rheological model based on primary laboratory data is proposed for concentrated contact lubrication. The model is a Maxwell model modified with a limiting shear stress. Three material properties are required: Low shear stress viscosity, limiting elastic shear modulus, and the limiting shear stress the material can withstand. All three are functions of temperature and pressure. In applying the model to EHD contacts the predicted response possesses the characteristics expected from several experiments reported in the literature and, in one specific case where direct comparison could be made, good numerical agreement is shown.

Shear Strength Measurements of Lubricants at High Pressure

Measurements of lubricant shear rheological behavior in the amorphous solid region and near the liquid-solid transition are reported on three lubricants under pressure. Elastic, plastic and viscous behavior was observed. The maximum yield shear stress (limiting shear stress) is a function of temperature and pressure and is believed to be the property which determines the maximum traction in elastohydrodynamic contacts such as traction drives.

An Application of a Free Volume Model to Lubricant Rheology I—Dependence of Viscosity on Temperature and Pressure

Analyses of the dependence of lubricant viscosity on temperature and pressure, μ(T,P), have been carried out by using a modified WLF equation in which pressure effects on viscosity are given in terms of the pressure dependence of the glass transition temperature, Tg , and of thermal expansivity of free volume, αf . log μ(T,P)= log μg−C1•(T−Tg(P))•F(P)C2+(T−Tg(P))•F(P) where C1 and C2 are well known WLF constants, and μg is a viscosity at Tg . Tg (P) and F(P) are functions for describing the pressure dependence of Tg and αf , respectively. On the basis of the iso-viscous concept for Tg (P), μg has been assumed to have a constant value, 1 TPa•s, at any pressure (SCHEME I). SCHEME I yields a reasonable variation in Tg and αf with pressure for synthetic lubricants, while this analysis suggests a lower μg for mineral oils. In order to improve the applicability of the free volume model, a reference temperature Ts (P), at which the viscosity is 10 MPa•s, has been introduced instead of Tg (P) (SCHEME II). Analyses of dielectric transition for some lubricants and of μ(T,P) in the ASME Pressure-Viscosity Report have confirmed the excellent applicability of the present free volume model over wide ranges of temperature and pressure.

The Effect of Anion Vacancies on the Tribological Properties of Rutile (TiO2-x), Part II: Experimental Evidence

Friction and wear tests were completed with the (001) and (110) planes of single crystal rutile (TiO 2-x ) specimens sliding against selected ceramic counterfaces, in well-defined crystallographic directions. The purpose of the experiments was to investigate the environmental influences on anion vacancy formation, as related to a recent hypothesis connecting oxygen substoichiometry with predictable variations in the tribological properties of rutile. The data were obtained with two, entirely different test machines operating at various loads, speeds, temperatures, sliding directions and durations, as well as test specimen atmospheres. The results independently confirmed the predicted, anion vacancy-controlled formation of certain low and high lattice (strain) energy crystallographic shear systems (i.e., Magneli phases) and that their generation is overwhelmingly environment-dependent. The stoichiometry-controlled lattice energy of these rutile phases influences the surface and bulk shear strength (τS) of ...

Some Observations in High Pressure Rheology of Lubricants

Experimental data are presented on viscosity, elastic shear modulus, and limiting shear stress of 12 liquid lubricants. It is shown that transition histories do affect the limiting shear stress of the materials in the form of isothermal compression resulting in a lower density and lower limiting stress than isobaric cooling. The measured limiting shear stress agrees with EHD traction data at slide-to-roll ratios of 0.1 or more. In pressure viscosity measurements of the polymer solutions, it is found that for some temperatures, the pressure viscosity coefficient of the blend is slightly less than that of the base, which results in the crossing of the viscosity-pressure isotherms at high pressures.

The thermal aspects of oxidational wear

Abstract In this paper the early research on the thermal aspects of wear, where most of the attention had been on deducing the “hot spot” temperatures which occur between sliding surfaces, is briefly reviewed. More recent work in which the effect of oxide films on hot spot temperatures is deduced is then described. The division of frictional heat is also discussed. A surface model is used in which N individual contact areas (of radius a ) occur at any given time, each with an oxide film of thickness ξ formed at the contact temperature T c which occurs during the time of contact. By using an iteration process involving N , a , ξ and T c and experimental measurements of heat flow down the pin of a pin-on-disc wear test rig, it is shown that the resulting ξ values are consistent with the measurements of oxide film thickness. Recent experiments with photography and IR video recording are then described, which give some measure of validity to the calculated values of N , a and T c .

High-pressure rheology of lubricants and limitations of the Reynolds equation

Abstract A review of high pressure rheology leads to the conclusion that the results from rheometers may be used to generate empirical rate equations which are useful in modelling elastohydrodynamic traction. However, an analytical treatment of piezoviscous liquids reveals that the Reynolds equation adequately captures the mechanics of the piezoviscous liquid only when the shear stress is much less than the reciprocal of the pressure viscosity coefficient. Otherwise the cross film pressure gradient may be significant and secondary flows result.

Infrared Temperature Mapping in Elastohydrodynamic Lubrication

An improved technique for the measurement of temperature distributions in an EHD conjunction is presented. The technique reported here employs the infrared radiation emitted by the EHD conjunction and appears more rigorous, more reliable, and less cumbersome than that reported previously by the authors. Detailed mapping of fluid temperature (averaged through the thickness) and the ball surface temperature can be obtained. These temperature distributions have been reported for a naphthenic mineral oil for peak Hertz pressures of 1.05 and 1.51 GN/sq m (148 and 219 kpsi), at sliding speeds ranging from 0.35 to 12.7 m/s (13.4 to 500 ips) and a bath temperature of 40 C.

Lubricant limiting shear stress effect on EHD film thickness

A Grubin-like EHD inlet analysis utilizing a non-linear viscous fluid model with a limiting shear stress is reported. The shear rheological equation requirs only a low shear stress viscosity and the limiting shear stress both functions of pressure. Values employed for these properties are taken from measurements on typical lubricants. Reductions of EHD film thickness are found to be up to 40 percent compared with the standard Grubin prediction for typical operating conditions. Slide-roll ratio, limiting shear stress dependence on pressure, and atmospheric pressure value of limiting shear stress are new variables required to determine film thickness with the first two being more important than the last. The EHD film thickness is reduced by increasing slide-roll ratio and/or decreasing the pressure dependence of the limiting shear stress.