The Importance Of Variable Speeds Under Extreme Pressure Loading In Molybdenum Disulfide Greases Using Four Ball Wear Tests.pdf

Tribology Transactions

ISSN: 1040-2004 (Print) 1547-397X (Online) Journal homepage: http://www.tandfonline.com/loi/utrb20

The Importance of Variable Speeds under Extreme Pressure Loading in Molybdenum Disulfide Greases Using Four-Ball Wear Tests Gabi Nehme To cite this article: Gabi Nehme (2013) The Importance of Variable Speeds under Extreme Pressure Loading in Molybdenum Disulfide Greases Using Four-Ball Wear Tests, Tribology Transactions, 56:6, 977-985, DOI: 10.1080/10402004.2013.816812 To link to this article: https://doi.org/10.1080/10402004.2013.816812

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Tribology Transactions, 56: 977-985, 2013 C Society of Tribologists and Lubrication Engineers Copyright  ISSN: 1040-2004 print / 1547-397X online DOI: 10.1080/10402004.2013.816812

The Importance of Variable Speeds under Extreme Pressure Loading in Molybdenum Disulfide Greases Using Four-Ball Wear Tests GABI NEHME Mechanical Engineering Balamand University, Lebanon North Deir El-Balamand, Lebanon

There is recent concern regarding grease behavior in extreme pressure applications. The research described here is aimed at providing good friction and wear performance while optimizing rotational speeds under extreme loading conditions. A design of experiment (DOE) was used to analyze molybdenum disulfide (MoS2 ) greases and their importance in reducing wear under extreme loading and various speeds conditions (schedule 1 and schedule 2 speeds). The lamellar structure of MoS2 provides very good weld protection by forming a layer that can be easily sheared under the applications of extreme pressures. An extreme load of 785 N was used in conjunction with different schedules of various rotational speeds to examine lithium-based grease with and without MoS2 for an equal number of revolutions. A four-ball wear tester was utilized to run a large number of experiments randomly selected by the DOE software. The grease was heated to 75◦ C and the wear scar diameters were collected at the end of each test. The results indicated that wear was largely dependent on the speed condition under extreme pressure loading, and thus a lower MoS2 concentration is needed to improve the wear resistance of lithium-based greases. The response surface diagram showed that the developed molybdenum disulfide greases exhibited both extreme pressure as well as good wear properties under various rotational speeds when compared to steady-state speed. It is believed that MoS2 greases under schedule 1 speeds perform better and provide an antiwear film that can resist extreme pressure loadings.

(petroleum oil, synthetic or vegetable oils), thickener (soaps, organic or inorganic nonsoap thickeners), and additives to enhance the performance and protect the grease and lubricant surfaces. The additives in greases have been in use over the years to achieve antiwear and load bearing capacity, to varying degrees of success. Chief among them are MoS2 and graphite (Gansheimer and Holinski (1); Misty and Bradbury (2); Tamashuasky (3)) for extreme pressure applications. In previous studies, different rotational speeds and extreme pressure loading were studied by Nehme and Dib (4) and Nehme (5), (6) using fully formulated oils and plain zinc dialkyldithiophosphate oils. It has been found that varying sliding speeds and contact loads will affect the tribofilm formation and additive interactions. A substantial effort was devoted to understanding these interactions by using a design of experiment (DOE) software model. The model was used to optimize several factors and responses for specific ranges of loads and rotational speeds. Previous studies showed that bearing damage by electrical wear has negative effects on deterioration of lubricating greases (Komatsuzaki, et al. (7)). More recent studies reported that the amount of MoS2 in a grease at low test loads and different variable loading conditions resulted in increased wear scars, indicating that MoS2 may actually behave abrasively and at extreme pressure conditions it helps increase the load wear index and the weld load (Aswath, et al. (8); Patel, et al. (9); Fusaro (10); Nehme (11)). The load wear index determines the load-bearing properties of lubricating greases according to the ASTM designation D2596-97; one steel ball was rotated against three stationary steel balls. The weld test, on the other hand, ascertains the lowest applied load at which the sliding surfaces seize and weld together. Analysis of the performance of molybdenum compounds in motor vehicles and their reactions with water and sulfur were extensively studied and were found to be of great importance in the enhancement of antiwear resistance films (Norton and Cannon (12); Scott, et al. (13); Braithwaite and Greene (14)). According to Gow (15), some 90% of all lubricant additives destroy the thickener structure of greases because they are often based on surface-active materials and this leads to what is commonly called the mayonnaise effect (softening and discoloring). He also mentions that of the remaining 10%, some 90% do not work. He ascribes this to the fact that the thickener

KEY WORDS Lithium-Based Grease; Wear; Extreme Load; Cyclic Frequencies; Aircraft-Grade Bearing

INTRODUCTION Lithium-based greases make up over 50% of the greases used in the industry today. Grease are mixtures of fluid lubricant Manuscript received February 5, 2013 Manuscript accepted June 14, 2013 Review led by Robert Errichello

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TABLE 1—DESIGN OF EXPERIMENTAL DATA THAT REPRESENT SEVERAL VARIABLES

Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Factor 1: Load (N)

Factor 2: MoS2 % Concentration

Factor 3: Schedule 1 or 2 Rotational Speeds

Response: Wear Scar Average Diameter (mm)

Response: Friction Coefficient: Steady vs. Unsteady

785 785 785 785 785 785 785 785 392.5 392.5 392.5 392.5 392.5 392.5 392.5 392.5

0 0 3 3 0 0 3 3 0 0 3 3 0 0 3 3

Schedule 1 Schedule 1 Schedule 1 Schedule 1 Schedule 2 Schedule 2 Schedule 2 Schedule 2 Schedule 1 Schedule 1 Schedule 1 Schedule 1 Schedule 2 Schedule 2 Schedule 2 Schedule 2

1.32 1.35 0.75 0.73 1.31 1.34 1.27 1.21 1.21 1.18 0.83 0.87 1.23 1.16 0.88 0.85

Fluctuations in friction Fluctuations in friction No fluctuations in friction No fluctuations in friction Fluctuations in friction Fluctuations in friction Fluctuations in friction Fluctuations in friction Fluctuations in friction Fluctuations in friction Minor fluctuations in friction Minor fluctuations in friction Fluctuations in friction Fluctuations in friction Minor fluctuations in friction Minor fluctuations in friction

material is almost always very polar (metallic soaps) and that the (also polar) extreme pressure (EP) additives will adhere to the soap structure rather than to the metal surface (Gow (16)). This is in contradiction to the results found by McClintock (17), who tested lubricant life of a number of greases and found an increase in life. Aswath, et al. (18) and Antony, et al. (19) show in an evaluation of high-performance grease that fluorinated additives and graphite have positive effects on EP pressure and antiwear resistance in complex grease formulations. Although leakage of grease when additives decrease viscosity can cause problems in products or manufacturing equipment under extreme pressure operations, few cases investigating the mechanisms and damage involved have been reported, with the exception of some reports on the behavior, distribution, or deterioration of grease in rolling bearings (Cann, et al. (20); Lugt (21)). Therefore EP additives could contribute greatly to the antiwear performance in roller bearing applications. As a rule of thumb, approximately 30% of the free volume of the bearing should be initially filled with grease (Lugt (21)). It is clear that this is much more than required to provide the bearing with a (fully flooded) lubricant film. In the beginning, excessive grease churning, or grease flow, takes place, which is responsible for the high-temperature peak caused by the churning component of the friction torque (Lugt (21)). Kaneta, et al. (22), using a scoop to ensure fully flooded conditions, have shown that the film thickness is indeed higher than the fully flooded film thickness. The goal of this study was to examine the influences of two different schedule speed conditions on the wear properties of lithium-based grease under extreme load and compare them to a steady speed of 1,200 rpm. Essentially, two different schedules were selected after extensive studies with number of rotations for each. In Schedule 1, the number of rotations at each rotational speed is halved so that going through the schedule twice gives a tribological exposure that is seemingly equivalent to going through schedule 2 once.

r

r

Schedule 1: 2,400 rpm, 1.875 min; 1,800 rpm, 2.5 min; 1,200 rpm, 3.75 min; 600 rpm, 7.5 min; 2,400 rpm, 1.875 min; 1,800 rpm, 2.5 min; 1,200 rpm, 3.75 min; 600 rpm, 7.5 min. Each rotational speed was conducted at equal number of revolutions for a total run of 36,000 revolutions. Schedule 2: 2,400 rpm, 3.75 min; 1,800 rpm, 5 min; 1,200 rpm, 7.5 min; 600 rpm, 15 min. Each rotational speed was conducted at equal number of revolutions for a total run of 36,000 revolutions.

This research examined different loads testing using chromeplated steel balls (bearing quality) that simulate the conditions of high-pressure contacts in real applications. The balls are aircraft grade E52100. During landings, the aircraft bearings are subjected to shock loads with very rapid acceleration. Most bearings should be protected from excessive wear and heat by a grease or lubricant. This research shows that at accelerating frequency and speed, the use of MoS2 greases will enhance the superior resistance to scuffing and scoring under an extreme load of 785 N as indicated by the wear scar data presented in Table 1. Aerospace ball bearings are not always easily accessible for routine maintenance but must perform flawlessly at a variety of speeds. Therefore, repeated frequencies and different speeds were used in these experiments to check the importance of MoS2 greases in preventing wear under EP conditions. Nehme (11) worked extensively on previous tests using different DOE combinations to check the performance of the grease lubricants with respect to variable and steady-state speeds. Different rotational speeds were studied previously on several oil combinations and proved effective in preventing wear depending on the process and applications (Nehme (11); Elsenbaumer, et al. (23)).

EXPERIMENTAL PROCEDURE Greases were prepared by weight percentage in batches of 200 g using a Kitchen Aid blender (4.5 quart capacity with power rating of 250 W). Lithium-based greases and technical fine-grade

Pressure Loading in Molybdenum Disulfide Greases

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TABLE 2—PROGRAM SETUP ADJUSTMENTS USING PLINT MACHINE SOFTWARE (TE92) FOR VARIOUS TESTING PROCEDURES

Comment Apply load Wait for 75◦ C Reset PID Run test Decrease rpm Reset PID Run test Decrease rpm Reset PID Run test Decrease rpm Reset PID Run test Stop motor

Step No.

Next Step

Loop Count

Step Time (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

2 3 4 5 6 7 8 9 10 11 12 13 14 15

0 0 0 0 0 0 0 0 0 0 0 0 0 0

CStp+ CStp+ 2 900 CStp+ 2 450 CStp+ 2 300 CStp+ 2 225 10

Data Mode (s)

10

10

10

10

Motor Enable

Load (N)

Specimen Temperature (◦ C)

Clutch Enable

Counter Reset

Speed (rpm)

Drive Enable

Disabled Disabled Disabled Enable Enable Enable Enable Enable Enable Enable Enable Enable Enable Disabled

785 OR OR OR OR OR OR OR OR OR OR OR OR OR

20 75 OR OR OR OR OR OR OR OR OR OR OR 0

Disengaged Disengaged Engaged Engaged Engaged Engaged Engaged Engaged Engaged Engaged Engaged Engaged Engaged Disengaged

Reset/on

0 0 0 2,400 1,800 1,800 1,800 1,200 1,200 1,200 600 600 600 0

Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled Enabled

Test Control Unit Current time Elapsed time Per test Current step Total time Per step Elapsed time Per step Residual time Per step Test status Test file Data file Data points In step Total data Start Stop

CStp+ = continuous step, PID = proportional integrator derivative, OR = operational relative

MoS2 were mixed together. Lithium-based greases have a higher melting point than calcium-based greases and are very common. The sequence of adding MoS2 to the grease and mixing is important for the final preparation. The mixture was mechanically mixed in the blender for 1 h. Two compositions of this grease with 3% MoS2 and without MoS2 were developed. Tests were carried out at the University of Texas at Arlington using a Plint four-ball wear tester (model TE92) according to the testing procedure D2266 that is implemented by ASTM standard. Four chromium steel balls of bearing quality and aircraft grade E52100 were used (Fig. 1). Three steel balls with a 1/2-in. diameter were clamped together and covered with grease, and the fourth was clamped in a ball chuck and the load was applied. The temperature of the grease was maintained at 75◦ C. A program was written for each specific condition to adjust for the speed and load according to the specific schedule. The setup of the program is provided by the Plint machine software where the operator can adjust the components and variables base on the test conditions shown in Table 2. The wear scar diameters were measured for the four balls and the average diameters were used in the DOE analysis using the response surface diagram software. Tests for the different factors considered in the analysis and the measured wear scar averages are presented in Table 1. The coefficient of friction and surface temperature of the cylinder as a function of the number of revolutions or time were measured directly using the Plint machine Compend software setup where the data are converted into an Excel file. Some of the data were graphed and compared at a later stage and some were used to compare friction fluctuations for different grease combinations (see Table 1). Posttest analysis such as wear scar diameters of the rotating and fixed balls were measured and examined using a JEOL JSM 845 scanning electron microscope (SEM) at the end of each test providing that the test balls are cleaned with hexane–acetone mixture to remove the debris and grease from the surface. The average wear diameter was calculated and input into the DOE matrix for analysis. The DOE methodology spans a wide range of analytical approaches developed for different situations or final targets (Barrentine (24)). A DOE is used when multi-objective optimization is sought; all responses and factors

used in the analyses should be converted into corresponding desirability functions. The desirability is high when the target values are approached simultaneously. Each target value is linked to its own desirability function. Therefore, the evaluation of the factors and responses base on the percentage probability vs. effects and analysis of variance were investigated. The optimized conditions were calculated using the desirability value.

RESULTS AND DISCUSSION Frictional Events and SEM Analysis of Greases with MoS2 Additives Figure 2 shows the frictional events that occur during typical MoS2 grease tests at extreme loads and the scanning electron images for the repeated various speed schedule tests. It presents the consistency of two repeated tests at different schedules by presenting the friction coefficients as a function of the number of revolutions. The stable antiwear film formed in schedule 1 at an extreme load of 785 N is responsible for the low friction coefficient. The dominance of the beneficial effects of the tribofilm for protection of the surface is very significant in schedule 1 and is reflected in the SEM images in Figs. 3c and 3d. The increase in friction and some inconsistencies in the repeated tests can be easily identified when using schedule 2, possibly due to the repeated breakdown of the protective antiwear film, which corresponds to the abrasive action of the debris present in the wear track (Figs. 3a and 3b). The breakdown region of the tribofilm shows a rapid increase in the friction coefficient and abrasive wear (Fig. 3). The fluctuations of friction coefficients under different speeds and loading regimes are summarized in Table 1 and clearly indicate the importance of schedule 1 speeds under extreme conditions because minor fluctuations were observed. The abrasive wear and protective antiwear film depend greatly on the various speed conditions under an extreme load of 785 N, whereas under a load of 392.5 N, the different variable speeds did not vary significantly the wearing conditions, as indicated in the average wear scar data in Table 1. The wear scar diameter did not vary and was approximately the same, especially when using 3% MoS2 additives. On the other hand, the variation was very clear under

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Fig. 1—Schematic of the four-ball wear test apparatus showing the continuously rotating ball and the three fixed balls covered with grease in the Plint model TE92 four-ball wear tester (color figure available online).

an extreme load of 785 N with and without MoS2 additives (Table 1). Therefore, the analysis was focused on various schedule speeds under an extreme load of 785 N. The overall variation in the repeated tests was less than 12%, which is insignificant considering the number of deterministic and nondeterministic variables in a tribological test. The wear scar diameters on the balls were measured and the average was used to optimize the data using DOE analysis. Figure 4 presents the variation in the friction coefficient for two repeated tests at steady-state speed of 1,200 rpm and load of 785 N. The repeated frictional events were not as consistent as the schedule 1 tests; the friction coefficients and SEM images (Fig. 5) indicated the importance of schedule speeds when used with MoS2 greases in specialty applications such as aircraft bear-

Fig. 2—Frictional events of lithium-based grease with 3% MoS2 at 785 N load and schedule 1 and 2 speeds for 36,000 revolutions (color figure available online).

ings. It shows that schedules 1 and 2 performed better when compared to steady-state speed with more improvement indicated in schedule 1 because the scanning electron images show wider wear track diameters and more abrasive particles in the steady-state speed tests. A careful examination of the wear surfaces shown in Fig. 5 indicates that an unstable tribofilm is formed with more asperity protrusions on the surfaces. These large deformations in the contact region of the steady-state and schedule 2 speed tests cause small cracks on the surface, where wear particles are formed and an adhesive transfer layer is formed on all balls. In abrasive wear the track is rougher and small metal scraps or debris can be seen. Abrasive wear will occur when the protective tribofilm breaks down and the two surfaces are under direct contact. As a result, a certain volume of surface material is removed and abrasive scratches are formed on the weaker surface; in this case, the tribofilms on the steel surfaces are wearing out. This corresponds to extreme conditions. Friction events in variablespeed schedule 1 tests were observed to be smoother and the plateau region lasted longer when compared to the same events at 1,200 rpm and schedule 2. It was also clear that schedule 1 rotational speeds played an important role in reducing friction and wear when compared to all other testing conditions. The stable antiwear film formed during schedule 1 frequencies is responsible for the steady-state friction behavior where it is reflected in the SEM images of the MoS2 grease. The wear scar diameter is very small compared to schedule 2 frequencies and steady-state frequency under the same loading conditions. The secondary electron image of the tribofilm is very smooth. In regions where a stable tribofilm is present the surface roughness is very low; the images indicate a featureless wear track with minimum abrasive wear and the presence of a wear-resistant film protecting the surfaces. It is more likely that the activation energy resulted from mechanical rubbing and the increase in temperature observed under extreme contact were responsible for the self-healing of the

Pressure Loading in Molybdenum Disulfide Greases

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Fig. 3—Scanning electron microscopy (SEM) images of lithium-based grease with 3% MoS2 at 785 N load and schedule 1 and 2 speeds for 36,000 revolutions (color figure available online).

tribofilm, which can be concluded from Fig. 3 and the secondary electron image in Fig. 5. Extreme contacts and thermal activation energy where tribofilms could results from surface interactions were studied extensively in oils under boundary lubrication (Gansheimer and Holinski (1); Nehme and Dib (4); Nehme (5); Aswath, et al. (8)) and similarities could be deduced under these conditions, because variable speeds and extreme load also suggest the formation of wear-protective films in particular cases (Nehme and Dib (4); Nehme (5)). The data shown for each condition in-

Fig. 4—Frictional events of lithium-based grease with 3% MoS2 at 785 N load and 1,200 rpm for 36,000 revolutions (color figure available online).

dicated a very high degree of repeatability of the tests. Therefore, the DOE analysis would be very significant when running analysis on the MoS2 grease samples.

SEM Analysis of Greases without MoS2 Additives The effects of 3% MoS2 greases on reducing wear under different speeds and different loads are clearly indicated in Table 1. There were large increases in wear scar diameter when greases without MoS2 additives were used. The breakdown of the protective film in the majority of these tests was due to abrasive wear, resulting in a rapid increase in friction coefficient and eventual increase in the wear scars. In most cases the wear scar indicated that abrasive wear is the mechanism of material removal as evidenced by the deep scratch marks on the surface shown in the SEM images in Fig. 6. There were also some indications of adhesive wear caused by plastic deformation introduced in the contact region. At this stage, large deformations caused small cracks on the surface, where wear particles are shown and adhesive transfer layers are formed on the balls as reflected in the lower and higher magnifications images for schedule 1 and 2 speeds. MoS2 is an extreme pressure additive that is used extensively in grease. Strong covalent bonding holds the Mo and S together (Fusaro (10)). Van der Waals bonding and multiple layers can be sheared easily along the S-S bond, providing physical lubrication. The particles of MoS2 are sheared and thus prevent the contacting layers from seizing together (Fusaro (10); Nehme (11)). The severity of the wear and material transfer at extreme load without MoS2 suggest that no lubrication film protected the metal surfaces and more metal-to-metal contact resulted where

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Fig. 5—Scanning electron microscopy (SEM) images of lithium-based grease with 3% MoS2 at 785 N load and 1,200 rpm for 36,000 revolutions: (a) represents test 1 of lithium-based grease mix; (b) represents test 2 of lithium-based grease mix under the same conditions (color figure available online).

extensive adhesive and abrasive phenomenon were dominant during the test, leading to both higher friction and wear. It appears that tribofilms were unstable under extreme loads and were unable to provide protection and were replaced by wear debris and excessive abrasive wear. This indicates that the wear debris formed because of pulling of asperities diminished the presence of the tribofilm in the absence of MoS2 in that specific lubrication regime.

Design of Experiment and Optimization Analysis and Discussion Design of experiment is the simultaneous study of several variables. Table 1 shows the wear scar diameters data for the grease blends used in the experiments with and without MoS2 under two different loads (392.5 N, 785 N) and different sched-

uled speed regimes. The data indicated an improvement in wear in schedule 1 compared to the schedule 2 test regime. The desirability, which is based on what is targeted in the model when the responses are evaluated before optimization, supports these findings. The desirability established by the model presents a clear improvement when schedule 1 is used under an extreme load of 785 N as reflected in Fig. 7. A concentration of MoS2 less than 3% or between 2.35 and 3% under schedule 1 variable speeds was sufficient to establish a good desirability of over 88% and the wear scar diameter was less than 0.9 mm (Figs. 7a and 7b). The 3D desirability study (Fig. 7b) suggests that 2.35% MoS2 is suitable and performs the same functions as the 3% MoS2 ; a saddle point in the 3D desirability study (Fig. 7b) of schedule 1 variable speed can be seen with a value of 0.881. The presence of MoS2 was crucial for all grease tests and its optimum concentration can be

Fig. 6—Scanning electron microscopy (SEM) images of lithium-based grease without MoS2 at different scheduled speeds and 785 N load: (a), (c) schedule 1 speeds at lower and higher magnifications (50 and 2000 ×); (b), (d) schedule 2 speeds at lower and higher magnifications (color figure available online).

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Fig. 7—(a) Optimization for schedule 1 and 2 variable speeds regarding desirability with respect to MoS2 percentage concentration and variations in load; (b) 3D desirability study for schedule 1 and 2 variable speeds with respect to MoS2 percentage concentration and variations in load (color figure available online).

reduced by more than 20% in machinery running at high variable speeds and extreme pressure without compromising the formation of a wear-resistant film. Reducing the MoS2 concentration is not sufficient when using schedule 2 because the desirability will be reduced significantly. Thus, setting longer time at specific frequency as in schedule 2 will involve a higher concentration of Molybdenum Disulfide grease, and 3% MoS2 will be necessary to

establish a minimum desirability of 42% under an extreme load condition, as indicated in Figs. 7a and 7b. The reduction of MoS2 to 2.35% in lithium-based grease did not significantly affect the wear protection in schedule 1. Perhaps most notable is the great similarity between the 2.35% and the 3% concentrations when using schedule 1, as evidenced by the disappearance of the large frictional events after a short time

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Fig. 8—Frictional events of lithium-based grease with 2.35% MoS2 at 785 N load and schedule 1 and 2 speeds for 36,000 revolutions (color figure available online).

through different tests, which is reflected in Figs. 2 and 8. This self-healing process is not evident from schedule 2 tests (Figs. 2 and 8). Even more dramatic effects are seen clearly in the highmagnification SEM images of Fig. 9 where abrasive wear is more dominant in schedule 2 speeds using 2.35% MoS2 . The secondary

electron images of the tribofilm surface from a region within the wear track show that the tribofilms are not uniform, and more scratches are observed at schedule 2 speeds where the film has been removed by interaction with wear debris. The scratches are caused by the sliding of wear debris generated during the test, resulting in some trenching. The response surface 3D plots in Fig. 9 explore two factors interactions (load and MoS2 concentration in grease) for various average wear diameter calculations. The central theme of these plots indicates that MoS2 concentrations are important when increasing the contact load under schedule 1 and 2 speeds, with a significant decrease in wear. The response surface diagram 3D plots greatly coincide with the targeted value plots shown in Fig. 7 where the friction coefficient under reduced MoS2 concentration was tested and the results are reflected in Fig. 8. The improvements in schedule 1 tests were clearly identified when compared to schedule 2 under the same loading conditions (Figs. 8 and 9). The results indicated that MoS2 is a good extreme pressure additive and is responsible for reduced wear under variable speed conditions. The higher magnification scanning electron images of the wear scar variations for 2.3% MoS2 greases under schedule 1 speeds indicated significant improvement. Wear debris was minimal and a very smooth surface resulted when compared to the schedule 2 SEM image.

Fig. 9—Response surface diagram and scanning electron micrographs of the wear scar variations for 2.3% MoS2 greases under schedules 1 and 2 at an extreme load of 785 N (color figure available online).

Pressure Loading in Molybdenum Disulfide Greases

CONCLUSION The friction and wear performance of MoS2 greases improves greatly under variable scheduled speeds along with extreme loading conditions. The friction coefficient stabilizes during schedule 1 variable speeds under an extreme load of 785 N. Lithium-based grease occupies over 50% of the currently used greases and therefore it was chosen as the reference grease. In this study, a DOE method was used to compare the wear performance of greases with MoS2 as an EP additive under different scheduled speeds and extreme contact loads. It is quite evident that the MoS2 concentration determines the reduction in wear associated with variable speeds, where the wear scars with MoS2 greases are much smaller than without MoS2 , as reflected in Table 1. The extent of wear is related to the variation in the friction coefficient during the tests. Flat, eventless regions of the friction coefficient associated with schedule 1 resulted in minimal wear, whereas regions with large frictional events related to schedule 2 and to steady-state speeds of 1,200 rpm resulted in increased wear. In this study, schedule 1 favored lower friction and thus a stable tribofilm formed and small amounts of abrasive wear were clearly reflected in the scanning electron images. The results indicated that grease with a blend of 3% MoS2 consistently exhibited superior wear performance than greases without MoS2 . Scheduled speeds played an important role in the analysis, resulting in reduced wear in several tests and enhancing the formation of an antiwear film when MoS2 is used in combination with lithiumbased greases. Schedule 1 speeds also showed a significant reduction in MoS2 additive without affecting the antiwear performance when using schedule 1 under extreme load.

ACKNOWLEDGEMENTS This research was supported by the University of Texas at Arlington and the University of Balamand at Lebanon. Testing assistance provided by the lab staff at the University of Texas and Dr. Pranesh Aswath is gratefully acknowledged.

REFERENCES (1) Gansheimer, J. and Holinski, R. (1972), “Study of Solid Lubricants in Oils and Greases under Boundary Lubrication,” Wear, 19(4), pp 439–449. (2) Misty, A. and Bradbury, R. (2002), “Investigation into the Effect of Molybdenum Disulphide and Graphite on the Load Bearing Capacity of Greases,” NLGI Spokesman, 66(3), pp 25–29. (3) Tamashuasky, A. (2002), “The Effect of Graphite Type, Purity and Concentration on the Performance of a Clay Filled Polyalpha Olefin Grease Based on Four Ball Wear (ASTM 2266) with Coefficient of Friction and Load Wear Index (ASTM D2596),” NLGI Spokesman, 65(12), pp 10–25.

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