COST EFFECTIVE DETERMINATION OF LUBRICANT PROPERTIES THAT INFLUENCE FILM FORMATION AND ENERGY EFFICIENCY. A 2015/2016 SAIF Project Report

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1 1 COST EFFECTIVE DETERMINATION OF LUBRICANT PROPERTIES THAT INFLUENCE FILM FORMATION AND ENERGY EFFICIENCY A 2015/2016 SAIF Project Report Thomas J. Zolper University of Wisconsin-Platteville Abstract Lubricant rheological properties such as the pressure-viscosity index α and the shear modulus G are necessary to accurately model the film forming ability, hydrodynamic friction, and wear in tribological applications. However, measurement of these properties is costly and requires specialized equipment to obtain reliable data. The SAIF grant recipient describes the use of extant molecular-rheological and rheological-tribological models to predict these rheological properties and their effect on bearing film thickness measurements. The work supplements ongoing research that is being undertaken by a group of universities, private industries, and government agencies to develop lubricant additives that minimize wear and film friction, and thereby enhance energy efficiency. 1. Introduction Lubricants function by forming fluid films to separate the components of tribological interfaces and thereby reduce wear. Modern lubricants are obtained from natural and petroleum-derived hydrocarbons (mineral oils) as well as synthetic hydrocarbon- and other polymers. Poly-α-olefins (PAO: Figure 1a) are synthetic hydrocarbon-based lubricants that are commonly used to reduce hydrodynamic friction and wear in automotive engines. PAOs have limited ability to reduce hydrodynamic friction, however, certain additives such as olefin copolymers (OCP: Figure 1b) can

2 2 induce temporary shear-thinning, which reduces hydrodynamic friction and thereby increases energy efficiency. a b Figure 1: Molecular structures of (a) poly-α-olefins (PAO) and (b) olefin copolymer (OCP). Shear-thinning is often viewed as undesirable, but fuel efficiency requirements have caused the reassessment of its potential benefits [1, 2]. Fluids with properly developed shear-thinning properties can improve energy efficiency and thereby reduce power consumption [1-4]. The SAIF grant recipient proposed an evaluation of the non-newtonian fluid properties of PAO-OCP mixtures based on his prior collaborations with Northwestern University, the Department of Energy, Argonne National Laboratory, University of Akron, General Motors, and Valvoline Corporation. Table 1 lists the research activities in the original SAIF proposal. Some of the tasks were already underway and other activities were modified based on the results and identified needs of the research group. Table 1: Timeline of steps for rheological property determination by computational methods. Task Start Time End Time Responsibility Mol. Mass, Density & Viscosity July 2014 Oct Northwestern Univ. Film Thickness and Friction August 2014 Nov Northwestern Univ. Shear-Viscosity Data Oct Dec Argonne Nat l Lab Shear- and Pressure-Viscosity Jan Mar Univ. of Akron Analyze Rheological Data/Models Mar May 2015 Applicant (UWP) Write Computer Search Algorithm Apr July 2015 Applicant (UWP) Research Report and Journal Article July 2015 Sept Applicant (UWP)

3 3 2. Results: Mass, Density, Viscosity, Film Thickness and Friction in Non-Newtonian Fluids Several molecular, rheological and tribological measurements were necessary to fully characterize the PAO-OCP mixtures studied here. Details of the locations and procedures for all relevant measurements are provided in the peer-review manuscript that resulted from this SAIF grant [5]. Please note, all figures in this document are subject to copyright by the ASME Journal of Tribology and should not be publicly distributed Molecular Mass, Density and Viscosity Molecular mass distributions of PAO-OCP mixtures were measured using gel permeation chromatography at Northwestern University. Table 2 lists the lubricant type, molecular masses, polydispersity indices (PD), and contents of all the lubricants tested. Table 2 Type, molecular mass, polydispersity, and weight fractions of the constituents of the PAO-OCP mixtures examined in this study Sample Mw (g/mol) PD Weight Fraction (%) Pure OCP PAO 2 PAO 4 OCP-A 316, PAO PAO OCP-B PAO-OCP PAO-OCP Density and viscosity measurements were made using a constant temperature bath. Supplemental shear- and pressure-viscosity measurements were made at the University of Akron at representative temperatures, pressures, and shear-rates. Table 3 lists the density, viscosity and pressure-viscosity indices of the present lubricants at 303 K, 348 K, and 398 K. The PAO density is typical of low molecular mass hydrocarbons at the respective temperatures. The densities of the PAO-OCP mixtures are also in the typical range because of the major contributions of the base fluids to the mixture density.

4 4 Table 3 Density, viscosity, and pressure-viscosity index of the PAO-OCP mixtures Sample Density (g/cm 3 ) Viscosity (mpa s) Pressure-Viscosity (GPa -1 ) 303 K 348 K 398 K 303 K 348 K 398 K 303 K 348 K 398 K PAO OCP-B ~ ~ ~ PAO-OCP PAO-OCP Figures 2 a-c illustrate the viscosity variation as a function of pressure at temperatures of 303 K, 348 K, and 398 K [5]. The plots depict the expected increase in viscosity with increasing pressure and OCP content; they also show the decrease in viscosity with increasing temperature. a b

5 5 c Fig. 2 Viscosity vs. pressure for (a) PAO 4, (b) PAO-OCP 10, and (c) PAO-OCP 20 at 303 K (triangles), 348 K (squares), and 398 K (diamonds) The reciprocal asymptotic pressure-viscosity index (Eq. 1) is recommended for use in the Hamrock-Dowson equation because it represents a greater portion of the pressure-viscosity curve than other piezoviscous descriptors. It is used to calculate the pressure-viscosity indices listed in Table Film Thickness and Friction 1 1 P * 0 p 0 iv, as P (1) A PCS tribometer at Northwestern University was used to measure lubricant film thickness and friction coefficients. Measurements were made for each fluid from 303 to 398 K as the disk velocity U was varied from m/s to 4.35 m/s. Figures 3 a-c present the measured (symbols) and calculated (lines) film thicknesses of the PAO 4 and PAO-OCP mixtures as a function of entrainment speed at temperatures of 303 K, 348 K, and 398 K [5]. T

6 6 a b c Fig. 3 Measured (symbols) and calculated (lines) film thickness vs. entrainment speed for (a) PAO 4, (b) PAO-OCP 10, and (c) PAO-OCP 20 at 303 K (triangles), 348 K (squares), and 398 K (diamonds) at Σ = 0.5

7 Critical Stresses Table 4 lists the approximate high-pressure densities of PAO-OCP 10 and PAO-OCP 20 at the mean contact pressure and test temperatures. Table 3 also lists the theoretical critical stresses of OCP A, PAO 2, and PAO 4 at each of the temperatures tested and the weight fractions listed in Table 1. The authors list these approximations to emphasize the orders of magnitude of difference in the critical stresses of the low mass PAOs and the high mass OCP additive. Table 4 Approximate density ( P 0. 36GPa ) of mixtures and calculated critical stresses of the constituent fluids Density (g/cm 3 ) Critical Stress (Pa) Sample 303 K 348 K 398 K Constituent 303 K 348 K 398 K OCP-A PAO-OCP PAO E E E+5 PAO E E E+6 Density (g/cm 3 ) Critical Stress (Pa) Sample 303 K 348 K 398 K Constituent 303 K 348 K 398 K OCP-A PAO-OCP PAO E E E+5 PAO E E E+6 Figures 4 a-c depict the mean shear stress of the PAO 4 and PAO-OCP mixtures as a function of strain rate at temperatures of 303 K, 348 K, and 398 K. Each point in Figure 4 corresponds to the respectively measured film thicknesses in Figure 3. a

8 8 b c Fig. 4 Mean shear stress vs. strain rate for (a) PAO 4, (b) PAO-OCP 10, and (c) PAO-OCP 20 at 303 K (triangles), 348 K (squares), and 398 K (diamonds) at Σ = Friction Coefficients Figures 5 a-c illustrate the variation of the friction coefficient of pure PAO 4 and PAO-OCP mixtures with film thickness, which effectively reflects the lubrication regimes. It is evident that friction decreases with increasing entrainment speed until the film thicknesses exceeds the composite roughness of the ball and disk, shown by the dashed vertical line located at about 30 nm.

9 9 a b c Fig. 5 Friction coefficient vs. film thickness for (a) PAO 4, (b) PAO-OCP 10, and (c) PAO- OCP 20 at 303 K (triangles), 348 K (squares), and 398 K (diamonds) at Σ = 0.5

10 10 3. Conclusions This research was undertaken to compare the lubrication properties of PAO-OCP mixtures, by investigating the effects of the OCP content on viscosity, film formation, and friction coefficient. The major findings are as follows: 1) Increasing the OCP content in the PAO base oil causes an increase in viscosity and pressureviscosity index as a function of temperature and pressure. 2) The critical stress of the high-mass OCP-A component of the mixtures is exceeded by the interface shear stress at low entrainment speeds. Thus, the low-shear viscosity of the mixtures shear-thins to that of the base fluid, resulting in a decrease in film thickness (Figs. 3 b-c) and shear stress (Figs. 4 b-c). 3) The EHD friction of PAO-OCP mixtures is similar to that of the PAO base oils due to non- Newtonian shear-thinning; however, if it could increase in proportion to the low shear viscosity of the mixtures, the EHD friction should be substantially higher than measured. Acknowledgements Research at Northwestern University and Argonne National Laboratory was supported by US Department of Energy (DOE; Grant DE-EE ). References [1] Bronshteyn, L. A., and Kreiner, J. H., 1999, "Energy efficiency of industrial oils," Tribology Transactions, 42(4), pp [2] Coy, R., 1998, "Practical applications of lubrication models in engines," Tribology International, 31(10), pp [3] Taylor, R., Dixon, R., Wayne, F., and Gunsel, S., 2005, "Lubricants & energy efficiency: Lifecycle analysis," Tribology and Interface Engineering Series, 48, pp [4] Akbarzadeh, S., and Khonsari, M., 2008, "Performance of spur gears considering surface roughness and shear-thinning lubricant," Journal of Tribology, 130, p [5] Zolper, T., He, Y., Delferry, M., Shiller, P., Doll, G., LotfizadehDehkordi, B., Lockwood, F., Marks, T., Chung, YW., Greco, A., Erdemier, A., and Wang, Q., 2017 Investigation of Shear- Thinning Behavior on Film Thickness and Friction Coefficient of Polyalphaolefin Based Fluids with Varying Olefin Copolymer Content, Journal of Tribology, 139, p