Effect of contact load and lubricant volume on the properties of tribofilms formed under boundary lubrication in a fully formulated oil under extreme load conditions

Wear 268 (2010) 1129–1147

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Effect of contact load and lubricant volume on the properties of tribofilms formed under boundary lubrication in a fully formulated oil under extreme load conditions Gabi Nehme b , Ramoun Mourhatch a,∗ , Pranesh B. Aswath a,∗ a b

Materials Science and Engineering Department, 500 West First Street, Rm. 325, University of Texas at Arlington, Arlington, TX 76019, United States University of Balamand, Deir El-Balamand, El-Koura, Lebanon, United States

a r t i c l e

i n f o

Article history: Received 13 March 2009 Received in revised form 29 December 2009 Accepted 7 January 2010 Available online 15 January 2010 Keywords: Boundary lubrication Tribofilms Contact loads Focused ion beam Transmission electron microscopy Nanomechanical properties XANES

a b s t r a c t The effect of contact load and volume of lubricant in the time to tribofilm breakdown and extent of wear was examined in fully formulated oil with zinc dialkyl dithiophosphate as the main anti-wear additive. The mechanism of tribofilm formation and breakdown was followed carefully by monitoring the friction coefficient over the duration of the test. Tribological tests conducted with limited lubricant exhibited a direct relationship between the amount of lubricant and time to tribofilm breakdown with tests with smaller amounts of lubricant exhibiting shorter lifetimes. In addition the number of cycles for breakdown of the tribofilm is inversely proportional to the applied load at a fixed amount of lubricant. It is also shown that higher contact loads (405 N with maximum Hertzian contact pressure of 2.77 GPa) resulted in premature breakdown of the tribofilm while lower contact loads (297 N with maximum Hertzian contact pressure of 2.5 GPa) resulted in no failure even after 100,000 cycles and the presence of a stable tribofilm. Focused ion beam cross sections of the tribofilm reveal that at higher contact loads the tribofilm is patchy with local regions as thick as 1 ␮m while at lower contact loads (297 N) the tribofilms are typically 100–200 nm thick. Transmission electron microscopy analysis of the wear debris indicates a larger fraction of the crystalline particles at higher contact loads of 405 N are Fe2 O3 . This is true even though the duration of the test is quite short indicating higher friction and higher contact temperature results in rapid oxidation of the wear debris to Fe2 O3 . Fe2 O3 is significantly more abrasive and results in rapid bread down of the tribofilm. Analyses of wear debris of tests conducted at 297 N indicate smaller number of oxide crystallites with a chemistry of Fe3 O4 within a non-crystalline matrix. XANES analysis indicates that at loads of 405 N sulfur is present as sulfates both at the surface and interior of tribofilms while at loads of 297 N it is present as sulfates at the surface and a mixture of sulfates and sulfides in the interior. Phosphorous K-edge XANES spectra indicate phosphorous is present as a mixture of short chain Zn and Fe phosphates at both loads. Fe L-edge spectra indicates that at 405 N the Fe is present largely as FeSO4 and FePO4 with some oxides of Fe while at 297 N it is present as a mixture of FeS and iron phosphates together with Fe oxides. The Zn L-edge spectra indicate that it is present primarily as phosphates and not as either oxides or sulfides in the tribofilms. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The lubricant in an internal combustion engine serves several purposes, important among them are maintaining fuel economy, reducing wear and providing corrosion protection [1]. The interaction of the lubricant with the solid surface results in the formation of a tribofilm with physical and chemical properties that are distinct [2]. The work of Godot [3] established the importance of a transfer film that was formed as a consequence of the interaction

∗ Corresponding author. E-mail address: [email protected] (P.B. Aswath). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.01.001

of the lubricating media with the metal surface. Willermet et al. [4], Martin et al. [5], Bancroft and co-workers [6], Spikes [7] among others established the importance of tribofilms on the surface in boundary lubrication. Most of the wear in an engine occurs during start up where a thin layer of lubricant coupled with a pre-existing transfer layer or tribofilm is the only protection against metal on metal contact. The effectiveness of boundary lubrication is dependent on many variables that include the anti-wear additives and hardness of the contact surfaces [8–10]. With the drive towards improved fuel economy, lower viscosity oils are being used, which increases the role played by additives in protecting tribological surfaces under boundary lubrication. The main anti-wear agent used in engine oils

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is zinc dialkyl dithiophosphate that decomposes thermally resulting in the formation of various compounds that include soluble organic sulphides, organo-thiophosphates and organo phosphate which under tribological conditions of high pressure and temperature form oil insoluble components such as zinc polyphosphates on surfaces as tribological films [4,6,7,11–15]. The thickness and coverage of these films on the surface is important in determining the wear resistance under boundary lubrication[16]. The tribological performance of a system under boundary lubrication is complicated by contributions from lubricant chemistry, thermal effects, dynamic loading, speed, temperature and material effects [17]. In addition the many test methods employed to evaluate boundary lubrication such as reciprocating pin on plate [18], ball on disc [19], spinning ball on disc [20], pin on disc [21] and four ball wear [22] making it difficult to compare the different results. Boundary lubrication occurs under conditions of extreme stress and/or insufficient lubrication. All the prior studies of boundary lubrication have been conducted under low speeds to ensure boundary conditions at area of contact. In these approaches the lubricant responsible for the formation of the protective film is circulated from a larger pot and there is in effect fresh lubricant being constantly supplied to the area of contact. In this study, the role of the amount of lubricant and contact load on extent of wear was examined. In addition, the role of contact load on the formation and breakdown of tribofilm was studied using focused ion beam microscopy, scanning electron microscopy and transmission electron microscopy. The mechanical properties of the tribofilm were examined using nanoindentation, nanoscratch and nanowear tests. The chemical makeup of the tribofilm as a function of contact load was evaluated using XANES spectroscopy. 2. Experimental procedure A Plint T53 SLIM modified ball on cylinder lubricity evaluator (Phoenix Tribology, England) was used for all tests. A SAE Timken steel cylinder (64–66 HRC, 60 mm outer diameter) was the moving body that rotates in a clockwise direction. The surface finish of the cylinders was examined using a profilometer (Mahr Perthometer M1). The cylinders were cleaned with acetone and hexane to remove any machining oil that was present. The as-received cylinders have inconsistent surfaces with Ra or surface roughness ranging from 0.2 to 0.4 ␮m, and Rz or asperities height ranging from 3 to 5 ␮m. These cylinders were subsequently polished using a 1500 grit polishing paper for 5000 cycles at 700 rpm at a load of 5 N using neutral base oil as a lubricant. The cylinder was then removed and ultrasonicated in acetone to remove the polishing paper abrasive, neutral base oil and any wear debris. After cleaning, the surface roughness was measured to ensure that the Ra ranged between 0.2 and 0.25 ␮m. The severity of the boundary test typically results in annealing of a stationary steel ball resulting in excessive wear and decrease in contact stress. To ensure that the wear occurs on the steel cylinder and not the ball, a 12.5 mm tungsten carbide ball (79 HRC) was used as the counter body, the ball did not exhibit much wear resulting in little changes in the contact stress during the test. A transfer layer was generated on the stationary tungsten carbide ball at the region of contact of the ball and cylinder resulting in tribological contact between the transfer layer and the moving cylinder. All tests were conducted using a single 10W30 lubricant with a phosphorous level of 0.1 wt.% and sulfur content of 0.25 wt.%, details of the lubricant chemical composition are provided in Table 1. The cylinder and tungsten carbide ball were mounted on the Plint machine and measured quantity of desired oil was applied. A medical grade syringe was used to dispense oil in micro-droplets of ≈5 ␮l at the area of contact between the ball and cylinder with a load of 385 N at the contact point (Hertzian contact pressure of

Table 1 Composition of oil. Element

Composition (ppm)

Sulfur Zinc Phosphorous Silicon Aluminum Chromium Copper Manganese Iron Nickel Lead Tin Sodium Boron Calcium Magnesium Molybdenum Barium Cadmium

2637 1002 1009 12 25 4 6 2 7 3 9 9 24 122 2450 18 81 1 2

2.72 GPa). The total amount of oil dispensed varies based on the test but ranged between 40 and 80 ␮l. The cylinder was hand rotated by 30◦ and another droplet applied and the process repeated until the entire oil was dispensed, the cylinder was hand rotated to ensure the creation of continuous layer of oil over the circumference of the cylinder. Dead weights were placed on the lever arm to achieve the desired contact loads for the test. Test conditions were input to the software and the test was conducted at 700 rpm. Friction coefficient as a function of the number of cycles was measured and plotted. In order to examine the role of contact load on friction and wear behavior a series of wear tests were conducted at different values of contact load ranging from 297 to 405 N (maximum Hertzian contact load of 2.5–2.77 GPa). The tests were run till failure which is defined as rapid excursion of friction coefficient to over 0.15 and an associated burn off of the lubricant due to the local increase in temperature at contact area. After the completion of the tribological test the cylinder was washed with hexane–acetone mixture to remove the debris and oil from the surface and saved for analysis. Post-test analysis such as the wear profile was measured using a stylus profilometer (Mahr M1 Perthometer); the tribological surface was examined using a JEOL JSM 845 Scanning Electron Microscopy (SEM) operating in secondary electron imaging mode coupled with energy dispersive spectroscopy (EDS). In order to measure the thickness of the tribofilms selected tribological samples were mounted in a focused ion beam (FIB) microscope (Zeiss Leo Focused Ion Beam 1540 XB Microscope). An area of uniform tribofilm on the wear track was isolated and an area of 8 ␮m × 10 ␮m was milled using a gallium ion beam in a direction perpendicular to the motion of the cylinder. The depth of the sputtered area in each case was approximately 4 ␮m. A secondary electron micrograph revealing the cross section of the tribofilm was generated using the secondary electron detector located at an angle of 54◦ from the direction vertical to the surface of the sample. At the end of a few tribological tests, the droplet of used oil was harvested and deposited on a 3 mm copper grid with a polymer film coating. Acetone was used to remove the organic matter and the residual wear debris was examined using a JEOL 1200 EX Scanning Transmission Electron Microscope in bright field imaging mode and electron diffraction pattern of the wear debris was acquired using selected area diffraction. Nano indentation, nano scratch and scanning nano wear tests were performed on tribofilms generated from wear tests conducted at 297 and 405 N using a Ubi 1 Hysitron Triboscope® . The normal

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force resolution of the Triboscope® is 3 nN at 1 ␮N load, the maximum normal force possible is 10 mN and the maximum load rate is 50 mN/s. The normal displacement resolution of the Triboscope® is 0.0004 nM with a maximum displacement of 5 ␮m. The lateral force resolution (in scratch mode) is 0.5 ␮N and maximum lateral force measureable is 2 mN. The lateral displacement (in scratch mode) is 3 nm with a maximum lateral displacement of 15 ␮m. The data from nano indentation was used to calculate the reduced elastic modulus and hardness of the tribofilm. During indentation, the applied load was increased incrementally with periodic partial unloading and the load and displacement of the indenter were recorded. The intent of this approach is to measure the hardness at different stages of indentation as well as the reduced modulus as a function of penetration depth of the indenter. The quasi-static nano indentation tests were performed with a NorthStar® cube corner probe with a tip radius of 40 nm. Scratch tests were performed using a Hysitron Ubi 1Triboscope® in scratch mode. The Triboscope® is load-controlled and displacement sensing in the normal direction to the sample surface, while simultaneously displacement-controlled in the lateral direction parallel to the sample surface. Scratch tests were performed in 30-s from 0 to 5000 ␮N linear ramping load with a 90◦ conical probe with 2 ␮m tip radius on the tribofilm sample. During a scratch test, a normal force is applied to the indenter tip as a function of time in accordance with the scratch load function, while the tip is also driven towards the predetermined lateral position within a specified amount of time. Normal force, normal displacement, lateral force, and lateral displacement are measured and recorded as a function of time. From these four parameters, material properties and film adhesion characteristics can be deduced. Scanning wear tests were performed using the instrument’s in situ Scanning Probe Microscopy (SPM) mode. In this mode, wear regions were created by raster scanning the indenter tip across the sample surface while maintaining a normal force of 75 ␮N. Scanning wear tests were performed using 90◦ cube corner tip with 2 ␮m tip radius. An area of 4 ␮m square was selected for scanning and four passes were carried out in each case subsequent to the scanning wear tests the tip was used in Scanning Probe Mode (SPM) at a load of 0.5 ␮N to acquire topographical information of the worn region of the tribofilm. Phosphorus K-edge and sulfur K-edge X-ray absorption near edge structure (XANES) spectra were obtained at the 800 MeV Aladdin storage ring at the University of Wisconsin, Madison using the double crystal monochromator (DCM) beam line covering the region of 1500–4000 eV. The photon resolution at the P K-edge and S K-edge was 0.3 and 0.5 eV respectively. Oxygen K-edge, Fe L-Edge and Zn L-edge XANES spectra were obtained at the 2.9 GeV storage ring at The Canadian Light Source, Saskatoon, using the Spherical Grating Monochromator (SGM) beam line covering the region of 250–2000 eV. The photon resolution was 0.2 eV. XANES spectra were recorded both in fluorescence yield (FY) and total electron yield (TEY) modes. 3. Results and discussion 3.1. Calculation of lubricating film thickness At low values of Stribeck Number (N/P where  is the viscosity of the oil, N is the speed and P is the applied pressure) corresponding to low viscosity or speed and/or high loads, boundary lubrication predominates. In order to account for influence of surface roughness and film thickness, one can define , the Lambda ratio. The  ratio (h/ Ra ) is the ratio of effective lubricant film thickness (h) at the point of contact  to the composite surface roughness of the contacting bodies ( Ra ).  values at or below unity indicate asper-

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ity contact and boundary lubrication regime. In order to calculate the thickness of the lubrication film four models of film thickness that included the Grubin equation [23], Dowson’s–Higginson Equation [24], Archard–Cowking equation [25], and Hamrock–Dowson equation [26]. Grubin’s equation [23], H = 1.95G0.73 U 0.73 W −0.091 Dowson–Higginson [24], H = 1.6G0.6 U 0.7 W −0.13 Archard–Cowking [25]

 H = 2.04 1 +

2Rx 3Ry

−0.74 G0.74 U 0.74 W −0.1

Hamrock–Dowson [26] H = 3.63(1 − e−0.68 k)G0.5 U 0.68 W −0.1 where H is the ratio of film thickness to the equivalent radius in rolling direction (h/Rx ). U, G and W are the speed, materials and load parameters respectively. Rx and Ry are equivalent radii in the sliding direction and transverse to it respectively. Using the appropriate values of the physical constants and material properties the calculated values of lubricant film thickness (h) at the point of contact ranges from 41 to 78 nm based on the choice of the four equations above irrespective of the contact load. In addition, with the surface roughness of the cylinders in the range of 0.2–0.25 ␮m, the value of  « 1 and boundary conditions are satisfied in all cases. This would indicate that the properties of the tribofilm formed on the surface would play a very important role in determining wear behavior. 3.2. Mechanism of boundary lubrication at extreme loads Fig. 1 shows the friction events that occur during a typical boundary lubrication test at extreme loads. The early stages of the test process marked as (A) are largely abrasive due to the wearing of the asperities on the surface of the cylinder as a tribofilm has not been formed yet. The time it takes for the anti-wear additive zinc dialkyl dithiophosphate (ZDDP) to break down and form a uniform protective layer is approximately 500–1000 revolutions (≈1 min into the test). The stable anti-wear film formed during the end of region A is responsible for the subsequent steady state friction behavior in region B. In region B there is a dominance of the beneficial effect of the tribofilm for protection of the surface counteracting the detrimental effects of the debris formed in region A. The stable film has been shown by several earlier studies to be composed of polyphosphates of Zn and Fe and sulfides and sulfates of Zn and Fe. The duration of region (B) with steady state friction is a strong function of several factors, which include the amount of lubricant and the applied load. When the protective film breaks down there is a steep rise in the friction coefficient as shown in region C. This rise in friction results in the further breakdown of the ZDDP and the reestablishment of the protective anti-wear film. The formation of this protective film results in the decrease in the coefficient of friction as shown in region (D) and it remains stable in region (E). Region (E) corresponds to a balance between the formation of the stable anti-wear film and the abrasive action of the debris present in the wear track. The eventual breakdown of the film corresponds to the exhaustion of the ZDDP in the oil coupled with increase in the extent of wear debris. This break down region is region (F) that shows a rapid increase in friction coefficient. The duration and failure of the test depend greatly on the nature of the protective anti-wear film, amount of lubricant used and the applied load.

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Fig. 1. Frictional events in boundary lubrication: (A) represents the beginning of the test and the break in process, (B) represents the steady state friction with a stable tribofilm, (C) represents the onset of scoring, (D) represents the reactivation of anti-wear film and healing process, (E) represents the steady state friction after the healing process with a stable tribofilm (F) represents the steep rise of friction and failure after the breakdown of the tribofilm.

3.3. Repeatability of the boundary lubrication tests Boundary lubrication occurs under conditions of extreme stress and or insufficient lubrication. In order to truly examine boundary lubrication, a closed system was chosen where a specified amount of oil was applied to the contact surface and the boundary lubrication has to be provided by this layer of oil. In order to establish this method as a viable method to study boundary lubrication, multiple tests were conducted under similar conditions to establish repeatability and validity of the test method. Fig. 2(a) shows friction coefficient as a function of number of cycles for four tests conducted under identical conditions with cylinders with surface finishes between 0.2 and 0.25Ra . In each test 50 ␮l of oil was applied to the contact area between the ball and the cylinder as detailed in the experimental procedure. The boundary lubrication tests were conducted under a load of 385 N (maximum Hertzian contact pressure of 2.72 GPa). The number of cycles to failure in these tests were found to range between 15,500 and 17,000 cycles, a variation of less than 10%, which is insignificant considering the number of deterministic and non-deterministic variables in a tribological test. The wear scar on the cylinder was measured using the profilometer at six locations 60◦ apart. Fig. 2(b) shows one such representative profile for each of the four tests. These four profiles are from four different tests indicating that the tests are extremely repeatable. The wear volume is calculated by multiplying the wear area with the circumference of the cylinder and was found to range between 1 ± 0.2 mm3 for all the 4 tests. The difference in wear volume also lies within 20% between the four tests, which indicates reproducibility of the boundary lubrication test. The three outcomes measured from these four tests, time to final breakdown, wear profile and wear volume clearly indicate that the test is viable and useful to examine the mechanism of boundary lubrication.

3.4. Effect of amount of oil in boundary lubrication The amount of oil used in the boundary lubrication tests was chosen to simulate conditions that exist when oil has drained off

from the surface. In order to estimate the amount of oil that is present under conditions where oil has drained from the surface, cylinders were cleaned with acetone and weighed carefully. They were then dipped in engine oil for a period of 10 min and then laid out on an absorbent paper for a period of 24 h and then weighed

Fig. 2. (a) Friction co-efficient as a function of number of cycles for four boundary condition tests conducted under identical conditions at a load of 385Newtons (corresponding to a Hertzian contact pressure of . . . GPa) with 50 ␮l of oil. A, B, C and D refer to the four different tests showing very similar performance and times to failure between 14,500 and 16,500 cycles. (b). Profilometric traces of the wear surfaces of the four boundary condition tests conducted under identical load of 385 N and 50 ␮l of oil. A, B, C and D refer to the four tests showing near identical profiles.

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Fig. 3. Ball on cylinder tests conducted under boundary lubrication with different amounts of oil. The plots show the variation of friction coefficient as a function of number of cycles. (A) Represents the variation in friction coefficient of two tests using 40 ␮l of oil, (B) Represents the variation in friction coefficient of two tests using 50 ␮l of oil, (C) Represents the variation in friction coefficient of two tests using 60 ␮l of oil, (D) Represents the variation in friction coefficient of two tests using 80 ␮l of oil.

again. The difference in weight yielded the amount of oil that was present in the form of a thin film boundary layer. The amount of oil measured as boundary lubricant was found to be between 50 and 80 ␮l. Boundary lubrication tests were conducted with 40–80 ␮l oil to examine the role of the amount of oil in the boundary layer on the durability of the tribological film formed. The oil was applied using the procedure detailed in the experimental section and. all tests were conducted at a fixed load of 385 N. Fig. 3 presents the variation of friction coefficients for the tests with 40, 50, 60 and 80 ␮l of oil as a function of number of cycles, in addition the number of cycles to final breakdown of the boundary layer is also shown by the rapid excursion in friction coefficient at the end of the test. It is evident that the onset of catastrophic wear represented by the increase in friction coefficient appeared first in the 40 ␮l test due to the inability to form a stable anti-wear tribofilm. This resulted in the final breakdown after 11,000 cycles and possibly exhibited lubrication starved conditions throughout the test. The test conducted with 50 ␮l oil in the boundary layer resulted in a delay of the failure to 15,000 cycles. When 60 and 80 ␮l of oil is used, a stable tribological film is formed resulting in an increase in time to breakdown to between 20,000 and 22,000 cycles for 60 ␮l of oil and 28,000 cycles for 80 ␮l of oil. When more than 80 ␮l of oil was used the excess oil was spun off during the first few cycles and results were similar to the tests with 80 ␮l of oil. The data shown in Fig. 3 are two repetitions of each condition indicating very high degree of repeatability of the tests. The wear profiles for the four conditions with 40, 50, 60 and 80 ␮l of oil are shown in Fig. 4. It is immedi-

Fig. 4. Profilometric traces of the wear surfaces of the four boundary condition tests conducted under identical load of 385 N and different amounts of oil in the boundary layer. (A) Represents the variation in wear profile of two tests using 40 ␮l of oil, (B) Represents the variation in wear profile of two tests using 50 ␮l of oil, (C) Represents the variation in wear profile of two tests using 60 ␮l of oil, (D) Represents the variation in wear profile of two tests using 80 ␮l of oil.

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ately evident that the most severe conditions occur for tests with the least amount of oil that failed in the shortest duration of time. As the amount of oil is increased the wear profile indicates a less severe wear condition and smaller wear areas. Fig. 5(a) shows the relationship between the thickness of the oil layer on the cylinder (not contact film thickness which range from 40 to 80 nm) and the wear volume at the end of the test. The thickness of the oil layer was calculated from the width of the wear film observed in the tests and the amount of oil applied for each of the tests. The width of the oil layer was essentially the same in all cases, increasing the amount of oil resulted in an increase in the thickness of the layer. There is an inverse relationship between the amount of oil in the layer and the wear volume at the end of the test indicating that the thickness of the oil layer (i.e. amount of oil present) and not the film thickness at the point of contact (i.e. calculated from the different models) is critical in determining the strength and durability of the protective tribofilm formed on the surface of the cylinder. 3.5. Archard’s wear equation Archard’s seminal work on adhesive wear postulated a linear wear rate where the wear rate was controlled by the number of points (asperities) in contact and the wear process proceeded by the forming and breakdown of these contacts [27,28]. The effective contact area in this case is the sum of all the points of contact. A wear equation is defined by V =k

L×d H

where V is the wear volume, L is the load, d is the sliding distance, H is the hardness of the wearing material and k is the wear coefficient. Archard’s original interpretation of k indicated that k might either be viewed as a measure of the ratio of number of contacts that plastically deform to the number of contacts that deform elastically. Alternatively, k can be a measure of the number of times an asperity has to be in contact before it becomes a wear particle. Others have examined the hypothesis that asperities are in both elastic and plastic contact and only those that are plastically deformed result in creation of wear particles. Finkin [29] also arrived at a linear rate equation using a Coffin-Manson model of fatigue failure criterion for the formation of wear particles. Kimura et al. using different surface finishes and oil film thickness showed that the extent of wear is a function of both surface roughness and oil film thickness [30]. In a study of impacting surfaces Rabinowicz and Hozaki [31] derived a relationship based on Archard’s equation and have shown that in the presence of a lubricant the extent of wear is significantly reduced. Archard’s wear coefficient was plotted as a function of amount of oil used in the boundary lubrication test and is shown in Fig. 5(b). It is evident from this figure that there is an exponential drop in the magnitude of k as the amount of lubricant available increased before it leveled off at 80 ␮l, when larger amounts of lubricant was used, excess amounts were spun off and the extent of wear is independent of lubricant volume beyond 80 ␮l. This diagram establishes that there is a critical amount of lubricant volume required before minimum wear is achieved. Fig. 5(a) indicates that if the lubricant volume is distributed evenly across the wear track, the thickness of the oil film (0.05 mm) is more than sufficient to meet the requirement for minimum film thickness based on the earlier calculations (≈50 nm from Table 1) even when 40 ␮l of oil is used. This indicates that the minimum volume of oil required to minimize wear is driven by the availability of anti-wear additive to form an effective tribofilm that protects the surface. Under conditions of boundary lubrication, presence of sufficient antiwear additive in the formulation is of critical importance to maintain stable and viable tribofilms that protect the surface.

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Fig. 5. (a) Wear volume at the end of the test as a function of the volume of oil in the boundary layer. Superimposed is the variation of boundary oil film thickness as a function of volume of oil in the boundary later. All of the tests were conducted at a fixed load of 385 N. (b) Wear coefficient as a function of volume of oil. All of the tests were conducted at a fixed load of 385 N.

3.6. Medium and extreme pressure lubrication Boundary lubrication tests were conducted between normal loads of 297 and 405 N (that correspond to maximum Hertzian contact pressure between 2.5 and 2.77 GPa) using 50 ␮l of fully formulated oil. Friction coefficient as a function of number of cycles were recorded and the wear profiles were measured. Fig. 6 is a plot of the friction coefficient as a function of number of cycles for the 375, 385, 395 and 405 N load tests. Fig. 7 is a plot of the friction coefficient as a function of number of cycles for the 297, 307, 317, 336, 356 and 365 N normal load tests. Table 2 provides details of test conditions and number of cycles to failure. These tests indicate that the time to breakdown of the tribofilm decrease with an increase in the contact load. Tests conducted at 297 N did not fail even after 100,000 cycles where as tests conducted at loads of 405 N failed

after as little as 15,000 cycles. In addition, the friction coefficient of tests conducted at the higher loads are larger. Tests conducted at loads of 375 N or larger can be considered to be extreme loading conditions as they never exhibited a propensity to form a stable tribofilm as reflected by their high coefficient of friction for the duration of the test. On the other hand tests conducted at 365 N or lower exhibited a stable region where the presence of a stable tribofilm provided a region of stable coefficient of friction. Fig. 8 shows the profilometric traces of representative wear profiles of all the 10 boundary lubrication tests. The profiles indicate that even though the large load tests were conducted only for short periods of time (e.g. the 405 N test was run for only 15,000 cycles) they had the widest and deepest wear scars in comparison to the low load tests (e.g. 297 N run for 100,000 without failure) that had the narrowest and shallowest wear scars. The applied load rather

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Fig. 6. Ball on cylinder tests conducted at Extreme Pressure with boundary lubrication of 50 ␮l of oil in all cases. The plots show the variation of friction co-efficient as a function of number of cycles for four different contact loads of 375, 385, 395 and 405 N.

Fig. 8. Profilometric traces of the wear surfaces of the boundary condition tests conducted under identical amount of oil (50 ␮l) in the boundary layer but with different contact loads carried out to failure. (A) 405 N (B) 395 Newton (C) 385 N (D) 375 N (E) 365 N (F) 356 N, (G) 336 N (H) 307 N (I) 317 N, (J) 297 N. The number of cycles to failure in each case is provided in Table 2.

Fig. 7. Ball on cylinder tests conducted at Intermediate Pressure with boundary lubrication of 50 ␮l of oil in all cases. The plots show the variation of friction coefficient as a function of number of cycles for six different contact loads of 297, 307, 317, 336, 356 and 365 N.

than the duration of the test controls the formation and stability of the tribofilm and the extent of wear. The number of cycles to final breakdown of the tribofilm has an inverse linear dependence on the applied load as shown in Fig. 9(a). Wear volume at the end of the test is calculated by integrating the area of the wear track over the circumference of the cylinder and is also plotted as a function of the applied load in Fig. 9(a). The wear volume at failure is linearly dependent on the applied load under boundary lubrication. Table 2 Test conditions to examine the role of contact load for a fixed amount of oil (50 ␮l). Contact load (N)

Maximum Hertzian contact pressure (GPa)

# of cycles to failure

297 307 317 336 356 365 375 385 395 405

2.5 2.53 2.55 2.6 2.65 2.68 2.7 2.72 2.75 2.77

100,000a 95,000 96,200 66,000 51,300 45,400 27,500 19,800 19,180 15,000

a

No failure was observed.

Fig. 9. (a) Wear volume at the end of the test for boundary lubrication tests conducted at different contact loads. Also plotted is the number of cycles for final failure of the boundary lubrication tests as a function of applied contact load. (b) Wear coefficient as a function of contact load. All of the tests were conducted with 50 ␮l of oil under boundary lubrication conditions.

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Fig. 10. Low magnification secondary electron image of the wear track for the boundary lubrication test conducted with 50 ␮l of oil and a contact load of (a) 297 N (b) 336 N (c) 385 N and (d) 405 N.

Using information from the test in Archard’s wear equation, the wear coefficient was calculated for all the tests conducted at the different loads and plotted in Fig. 9(b). There are two distinctive linear regions in the plot one from 297 to 365 N and another from 365 to 405 N. The first region exhibits a graduate increase in k with increase in load and the second region exhibits a very rapid increase

in k with increase in load. There are two distinctive mechanisms of wear exhibited in these two regions, in the low load region (<365 N) the extent of wear increases gradually with increase in load and the likely mechanism of wear is driven by abrasive wear. On the hand in the high load region (>365 N) there is extensive wear and the likely mechanism is adhesive wear. In all tests the wear debris is

Fig. 11. High magnification secondary electron image of the wear track for the boundary lubrication test conducted with 50 ␮l of oil and a contact load of (a) 297 N (b) 336 N (c) 385 N and (d) 405 N.

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Fig. 12. Phosphorous EDS maps of the wear track for the (a) 297 N and (b) 405 N loaded boundary lubrication test conducted with 50 ␮l of oil. Sulfur EDS maps of the wear track for the (c) 297 N and (d) 405 N loaded boundary lubrication test conducted with 50 ␮l of oil, the corresponsing SEM images are shown in Fig. 10(a and d) for the 297 and 405 N loads respectively.

trapped in the 50 ␮l of oil that was originally used in the test and repeatedly delivered to the point of contact. After the completion of the wear test the cylinders were cleaned with a mixture of acetone and hexane and examined in a scanning electron microscope. Figs. 10 and 11 show representative secondary electron images of the wear surfaces of the cylinders at the end of the tribological test, tested at loads of 297, 336, 385 and 405 N. The secondary electron images in Fig. 11(a) indicate a featureless wear track at a load of 297 N with a little abrasive wear. The P and S EDS maps of the wear track shown in Fig. 12(a and b) indicates the presence of more phosphorous than sulfur on the wear surface indicating the presence of a tribofilm. It is likely that the tribofilm on the surface is composed of organic polyphosphates that has been shown to occur at low to intermediate loads [14]. At a load of 405 N there is extensive amounts of adhesive wear as shown in Fig. 11(d) and pull out of wear debris from the surface. The P and S EDS maps shown in Fig. 12(c and d) indicate a larger sulfur content compared to phosphorous. Other studies [32] have also indicated that at extreme pressures organic polyphosphate tribofilms are unable to provide protection and are replaced by a FeS solid tribofilm which is more adherent and more stable. Fig. 8, that shows the corresponding profilometric trace also indicates a deep wear track with an irregular surface profile at 405 N and a shallow smooth wear surface at 297 N. 3.7. Tribofilms in boundary lubrication Wear under boundary loading conditions is very extensive if the anti-wear film is weak or the lubricant is depleted. Under these conditions even the smaller scars in the wear tracks showed distress. Under non-catastrophic conditions, anti-wear additives such as zinc dialkyl dithio phosphate decompose yielding products that react with the metal surface to form a film that is either highly wear

resistant. Fuller et al. [33] have carried out a spectroscopy study using X-ray absorption near edge spectroscopy (XANES) to identify sulfur and phosphorus containing species. They have shown that the tribofilms consist of long chain polyphosphates at lower loads. In the current study under boundary condition, analysis of the surfaces with secondary electron microscopy and energy dispersive spectroscopy mapping shown in Figs. 10–12 indicate an increase in wear when one or more of the following conditions are met: there are large fluctuation in friction coefficient, an early scoring event, a smaller quantity of oil on the surface boundary and extreme loading condition. In addition, when an efficient tribofilm is formed it results in a low and stable coefficient of friction which delays the onset of catastrophic wear at lower loads. Adhesive wear is caused by large plastic deformation by dislocations that are introduced in the contact region under compression and shearing. These large deformations in the contact region cause small cracks on the surface, where wear particles are formed and an adhesive transfer layer is formed on the stationary ball. This corresponds to severe loading conditions as shown in Fig. 12(c and d). In abrasive wear the track is relatively smooth and small regions where there is abrasive pull out can be seen. Abrasive wear will occur when the protective tribofilm breaks down due to third body (wear debris) interaction with the tribofilm. As a result, a certain volume of surface material is removed and abrasive scratches are formed on the weaker surface, in this case the tribofilm on the steel surface. This type of behavior is seen in Fig. 12(a and b) at loads below 365 N. In the current study the stationary tungsten carbide ball has a little transfer film on the surface and the bulk of the wear at 405 N load is of the adhesive kind while at 297 N it is abrasive wear. In addition from EDS mapping it is evident that at lower loads there is more phosphorous in the tribofilms while at higher loads there is more sulfur in the tribofilm.

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Fig. 13. (a) Secondary electron image of the wear track of the cylinder tested at 297 N for a period of 100,000 cycles. The entire surface is covered by a smooth tribofilm. There are very few asperities on the surfaces. (b) Trench cut into a region within the tribofilm in (a) using a FIB showing the cross section of the wear track. The wear track thickness is approximately 180 nm.

A careful examination of the wear surface at high magnification formed after 100,000 cycles at 297 N shown in Fig. 13(a) indicates a stable tribofilm is formed with a few asperity protrusions on the surface. The FIB cross-section of the tribofilm in a direction perpendicular to direction of motion is shown in Fig. 13(b). The secondary electron image of the trench indicates that the tribofilm is very smooth with a thickness of 180 nm. This thickness of tribofilm measured here is consistent with several other studies [7,34–36] where typical thickness of the tribofilms is in the range of 100 nm. Wear debris harvested from the wear track is deposited on a polymer film and cleaned with hexane and examined in the TEM. The bright field electron image shown in Fig. 14(a) and the corresponding selected area diffraction image in Fig. 14(b) indicates that the film is largely amorphous with the presence of nanocrystalline particles embedded within it (the ring pattern arise from the nanoparticles and the matrix is essentially amorphous with no pattern associated with it). The SAD pattern was indexed and indicates that the particles are Fe3 O4 . Other studies of wear debris have indicated that wear

Fig. 14. (a) Bright field transmission electron micrograph of the wear debris from the test conducted at a load of 297 N for a period of 100,000 cycles. There are several crystalline particles embedded within the amorphous matrix. (b) Selected area diffraction pattern from the region shown in (a). The pattern indicates the presence of only Fe3 O4 embedded with the wear debris.

debris is a mixture of Fe and Fe oxides [11,25,26,37] and under severe boundary lubrication conditions Fe2 O3 forms [38]. Examination of the wear surface of the cylinder tested at 405 N indicates a large amount of irregularities such as scratches and pullout with the presence of a larger number of asperities as shown in the secondary electron image in Fig. 15(a). In addition, it is evident that the tribofilm formed is patchy with good coverage at some locations and significant wear at other locations. Focused ion beam trench of a region that has a stable tribofilm near the center of the

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Fig. 15. (a) Secondary electron image of the wear track of the cylinder tested at 405 N for a period of 15,000 cycles. Note the large patches of stable tribofilm separated by regions where the tribofilm has been abraded off. There are also a large number of asperities on the surfaces. The region where a focused ion beam has been used to trench the wear track is also shown. (b) Trench cut into a region within the tribofilm in (a) using a FIB showing the cross section of the wear track. The wear track thickness is approximately 1 ␮m.

wear track shown in Fig. 15(b) indicates a tribofilm with a thickness of approximately 1 ␮m, which is significantly thicker than the film formed at 297 N. The nature of the test which is conducted with a limited amount of oil (50 ␮l) and under severe boundary conditions, where the extensive wear debris that if formed is constantly reintroduced to the contact point results in the debris being digested and compacted into the tribofilm near the center of the contact (in this test the wear debris is trapped within the drop of oil used in the test and at higher load extensive amount of debris is generated). In regions where the film is present it is well compacted and sintered. Free wear debris harvested still suspended in the oil at the end of the test is examined in the TEM. Bright field electron image of the debris shown in Fig. 16(a) and the corresponding selected area diffraction shown in Fig. 16(b) indicate the formation of a significantly larger number of crystalline particles embedded within an amorphous matrix. These crystalline particles were identified as a mixture of Fe2 O3 and Fe3 O4 . The presence of the Fe2 O3

Fig. 16. (a) Bright field transmission electron micrograph of the wear debris from the test conducted at a load of 405 N. Note the large number of crystalline oxide particle dispersed within an amorphous glass matrix. (b) Selected area diffraction pattern from the region shown in (a). The pattern indicates the presence of both Fe2 O3 and Fe3 O4 embedded with the wear debris.

has been shown to be abrasive and very detrimental to the wear process [39]. It is quite evident that it is not the duration of the test that determines the number of these crystalline oxide particles in the wear debris but rather the applied load. The test conducted at 405 N was very short (<15,000 cycles) in comparison to the test conducted at 297 N, which had not failed even after 100,000 cycles. The debris from the test conducted at 405 N has a much higher density of crystalline Fe2 O3 particles as well as Fe3 O4 particles while the test conducted at 297 N had very few Fe3 O4 particles in an amorphous matrix. In a study of boundary lubrication of steel surfaces with moisture and ZDDP it was shown that the predominant oxide present is Fe3 O4 and is incorporated into the tribofilm [40]. Based

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Fig. 17. Load vs. Time profile of the incremental load nano-indentation test.

on the wear debris analysis it can be postulated that at extreme loads (405 N) there is significant oxidation of the wear debris resulting in the formation of the more abrasive Fe2 O3 while at lower loads the nanoparticles of oxide remain in their lower oxidation state of Fe3 O4 . 3.8. Nanoscale properties of tribofilms Nanoscale properties of tribofilms provide important insights into the abrasion resistance of tribofilms. These properties include hardness and modulus of the tribofilms as a function of thickness as well as the scratch resistance of tribofilms and wear resistance of the tribofilms. 3.8.1. Hardness and reduced elastic modulus as a function of indenter penetration into the tribofilm Nanoindentation test with incremental loading with periodic unloads were conducted to measure hardness and reduced modulus. Fig. 17 is the load function used in the nanoindentation tests. The reduced elastic modulus was calculated using the Oliver-Pharr method [41] and hardness was calculated by getting the ratio of applied load to the projected area of indentation. The reduced elastic modulus (E* ) was calculated by using the following equation: (1 − i2 ) (1 − s2 ) 1 = + ∗ Es E Ei where Es is the measured modulus of the film and s is Poisson’s ratio assumed to be 0.3. Ei is the modulus of the indenter, 1170 GPa and i is the Poisson ratio of the indenter, which is 0.07. In the incremental load tests, the reduced modulus and hardness was calculated at each unloading point. Fig. 18(a–c) are the hardness as a function of penetration depth and reduced modulus as a function of penetration depth for the tribofilms formed at 297 and 405 N. The reduced modulus profile of the two tribofilms are quite similar with the surface being much more compliant with a reduced modulus of 100 GPa. Within a thickness of 100 nm the reduced modulus levels off at 160 GPa that is slightly below the modulus of steel (200 GPa). This indicates that the surface material has a lower modulus in comparison to the material deeper in the tribofilm.

Fig. 18. Nanoindentation of tribofilms formed at 297 and 405 N (a) Load versus displacement plot for incremental load test to estimate the hardness and reduced modulus as a function of penetration depth of indenter. (b) Reduced modulus as a function of penetration depth of indenter (normal displacement) of the tribofilm (c) Hardness as a function of penetration depth of indenter.

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Fig. 19. Scratch tests of the tribofilms formed at 297 and 405 N scuffing load (a) Normal force as a function of normal displacement (b) Lateral force as a function of lateral displacement and (c) Coefficient of friction as a function of lateral displacement.

This data is consistent with other studies have shown modulus of tribofilms that are in the range measured here [36,42,43]. The hardness of the tribofilm as a function of indenter penetration depth is shown in Fig. 19(c). In both cases the surface of the tribofilms is softer than the bulk of the tribofilm with hardness increasing with indenter depth. Earlier studies on nanoindentation of tribofilms have yielded contradictory results based on the test conditions and nature of nanoindentation test used. Bec et al. [42] in their study of ZDDP in a wear study using a reciprocating Amsler machine at a mean Hertzian pressure of 0.36 GPa showed the presence of a soft alkylated phosphate layer on the surface that was as thick as 900 nm and had a relatively low hardness of between 0 and 0.08 GPa. This soft layer was removed on imaging indicating it was a weakly bonded layer. When examining the tribofilm after removal of the surface alkylated phosphate layer Bec et al. [42] postulated that the increase in hardness with increasing indentation depth could

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be explained by considering the tribofilm as a “smart” material. This indicates that under pressure, the tribofilm hardens resulting in higher hardness at greater depth. Their data indicated that hardness of the film increased from 4 to 8 GPa in the harder regions that formed as pads and from 1 to 3 GPa in valley regions. Aktary et al. [43] in their study with a cylinder on flat system with a load of 225 N at 100 ◦ C and ZDDP concentration of 1.49 wt.% showed that the film morphology evolved with rubbing time. However, they showed that the modulus and hardness were relatively constant at the tested indentation load of 100 mN with a cube corner indenter in all cases at approximately 90 GPa for reduced modulus and a hardness that ranged between 2.5 and 5 GPa suggesting that the tribofilm is composed of polyphosphates. Nicholls et al. [36] in their study of cylinder on flat system with a load of 220 N (0.5 GPa Hertzian pressure for line contact) at 100 ◦ C showed that the modulus of the film using a Berkovich indenter was approximately 90 GPa and the structure was composed of long chain polyphosphates. On the other hand a recent study by Somayaji et al. [34] conducted with secondary ZDDP with a nominal concentration of 0.1 wt.% P in base oil in a ball on cylinder contact condition at contact loads approximating 3.4 GPa indicated that the surface of the tribofilm was significantly harder than the bulk. The hardness and modulus were measured using a cube corner indenter with a tip radius of 40 nm with different applied loads. Hardness near the surface in the range of 15 GPa and modulus near the surface of approximately 200 GPa were achieved when just ZDDP was used. However, in the presence of antioxidants the behavior was reversed with the hardness near the surface being lower than the bulk of the tribofilm similar to the observations of Bec et al. [42] and Nicholls et al. [43]. In another study by Ye et al. [44] on fully formulated oils it was shown that surfaces of tribofilms are softer than the bulk. This contradictory observation can be resolved by the observation that most of the earlier studies were conducted at lower Hertzian loads yielding softer tribofilms. When just ZDDP is used at high pressure it is possible to form a heavily crosslinked polyphosphate film as suggested by Muser et al. [45]. In their study Muser et al. [45] using molecular dynamics modeling showed that the hardness of the tribofilms are a function of contact loads, which increases the extent of cross linking resulting in very hard tribofilms. resulting in very hard tribofilms. In a recent study Muser et al. have shown that in the presence of moisture or other additives it is possible for the extent of cross linking to be reduced resulting in softer tribofilms [46]. In addition, there are some clear distinctions between the two tribofilms formed at the two different loads. It is clearly evident that in the tribofilm formed at 297 N there is a plateau region at around 100 nm corresponding to the interface between the tribofilm and the substrate where as in the case of the tribofilm formed at 405 N the hardness continuously increases with depth as the indenter remains in the tribofilm (thickness of tribofilm formed at 297 N is ≈100 nm compared to 1000 nm when loads of 405 N are used). The hardness of the underlying steel substrate is approximately 8 GPa. These differences may be attributed to the fact that at higher contact loads it is possible to cross-link the poly phosphate chains to a greater extent as the indenter penetrates as suggested by Muser et at. [45] and Bec et al. [42] who treated the films as a smart structure that stiffens with applied load. 3.8.2. Nanoscratch behavior of tribofilms The breakdown of the tribofilm occurs when a hard abrasive particle that usually is wear debris is trapped between the tribofilm and the contacting surface. The wear debris is oxidized fragments of the tribofilm shown in Figs. 14(a and b) and 16(a and b) composed of Fe3 O4 and Fe2 O3 . When the hardened wear debris scratches the tribofilm it in turn creates additional wear debris that is oxidized which compounds the abrasive wear. The nanoscratch test can be used where the tribofilms resistance to scratch can be evaluated by

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using a hard diamond indenter. In this test the axial load and lateral displacement is prescribed while the axial displacement and lateral load are measured. Fig. 19(a–c) represent the normal force as a function of normal displacement, lateral force as a function of lateral displacement and friction (ratio of lateral force to normal force) as a function of lateral displacement respectively. From Fig. 19(a–c) it is evident that there are two distinctive regions in the scratch test conducted on the tribofilm formed at 297 N. At a depth of approximately 130 nm, there is an abrupt change in slope in the load as well as a sharp drop in the coefficient in friction in the 297 N tribofilm, that we believe corresponds to decohesion of the tribofilm from the surface and the indenter is now encountering the substrate material. The FIB analysis clearly indicated that the typical thickness of the tribofilms formed at 297 N is approximately 180 nm and matches well with the measurements made here (≈100 nm), the differences between 100 and 200 nm in thickness can be attributed to selection of location for measurements. For loads below 400 ␮N, the normal displacement in the tribofilm formed at 297 N is much higher than the one formed at 405 N indicating that it is easier for the indenter to scratch the tribofilm formed at 297 N. The underlying substrate below the tribofilm is a medium carbon alloy steel that has been hardened to 64 HRC (≈6–7 GPa) and has carbide particle dispersed within the matrix resulting in an irregular scratch profile based on the phase in contact with the indenter. On the other hand, the scratch test within the tribofilm formed at 405 N shows a steady increase in penetration depth with increase in axial force as shown in Fig. 19(a). At the end of the scratch test the total depth of penetration in the tribofilm formed at 405 N is significantly smaller than within the tribofilm formed at 297 N indicating that the tri-

bofilm at 405 N is more abrasion resistant than the film formed at 297 N. 3.8.3. Nanowear behavior of tribofilms A measure of the wear resistance of a tribofilm can be evaluated by conducting a Nanowear test. In each case a cube corner tip was used, a fixed load of 75 ␮N and four passes over an area of 2 ␮m × 2 ␮m was conducted. At the end of the test the profile of the worn area on the tribofilm was imaged using the tip in a scanning probe mode with a load of 0.5 ␮N. Fig. 20(a and b) is the scanning probe image and cross section at the center of the nanowear scar on the tribofilm formed at 297 N. Fig. 21(a and b) is the scanning probe image and cross section at the center of the nanowear scar on the tribofilm formed at 405 N. Fig. 22(a and b) is the scanning probe image and cross section at the center of the nanowear scar of the substrate. Comparing the three figures it is evident that the tribofilm formed at 405 N has the greatest resistance to wear as the depth of nanowear scar is only 60 nm while the depth of the nanowear scar on the tribofilm formed at 297 N is 550 nm, deep into the substrate. When compared to the nanowear profile of the substrate material, the wear on the tribofilm formed at 297 N is comparable. The tribofilm formed at 405 N is patchy and does not cover all regions of the surface, however, in regions where coverage is present it is very thick and the wear resistance of the tribofilm is significantly better than what is observed at a load of 297 N. Molecular dynamics modeling of the tribofilm by Muser et al [45] has indicated that pressure induced cross linking of the polyphosphates is possible at higher loads and may be one explanation for the improved

Fig. 20. Scanning wear of the tribofilm formed at 297 N normal load. A 90◦ cube corner tip with a tip radius of 2 ␮m was used at a load of 75 ␮N. Four passes were scanned at this load. (a) Scanning probe image of the worn area after the can and (b) profilometric trace of the cross section.

Fig. 21. Scanning wear of the tribofilm formed at 405 N normal load. A 90◦ cube corner tip with a tip radius of 2 ␮m was used at a load of 75 ␮N. Four passes were scanned at this load. (a) Scanning probe image of the worn area after the can and (b) profilometric trace of the cross section.

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Fig. 22. Scanning wear of the steel used in the tribological tests. An area outside the wear scar was chosen for comparison. A 90◦ cube corner tip with a tip radius of 2 ␮m was used at a load of 75 ␮N. Four passes were scanned at this load. (a) Scanning probe image of the worn area after the can and (b) profilometric trace of the cross section.

nanowear resistance of the tribofilm formed at 405 N compared to 297 N.

Fig. 23. (a) Phosphorous K-edge XANES spectra in total electron yield mode of model compounds and tribofilms. (b) Phosphorous K-edge XANES spectra in fluorescent yield mode of model compounds and tribofilms. Tribofilms were formed at 297 and 405 N.

3.9. XANES analysis of tribofilms Tribofilms formed in tests conducted at 297 and 405 N were examined at the Canadian Light Source using the Spherical Grating Monochromator (SGM) beam line to examine the Fe-L edge, Zn-L edge and O–K edge and at the Synchrotron Radiation Center the Double Crystal Monochromator (DCM) beam line was used to examine the P K-edge and S K-edge. 3.9.1. Phosphorous K-edge XANES Fig. 23(a and b) are the total electron yield and fluorescent yield spectra of the model compounds Zn3 (PO4 )2 and FePO4 as well as the tribofilms formed at 297 and 405 N contact load. Earlier studies on tribofilms formed at high loads using ZDDP alone have shown that the tribofilms are made up of short chain polyphosphates of Fe and Zn [35]. The TEY spectra at the K-edge of P provides information from the top 30–50 nm of the tribofilm and from Fig. 23(a) it is evident that spectra matches FePO4 better than Zn3 (PO4 )2 , however, there is enough overlap of the peaks to indicate that both species are likely present near the surface of both tribofilms. On the other hand the FY spectra that provides a measure of the chemistry and coordination of P deeper in the tribofilm (up to 500 nm) indicates that the white line matches Zn3 (PO4 )2 compared to FePO4 , however, the pre-edge present is unique to the FePO4 and is present in both tribofilms. However, other studies using P K-edge have shown

that when overbased calcium sulfonates are used with thiophosphates they are incorporated as calcium phosphates in tribofilms [47]. The P K-edge spectroscopy can be used to distinguish between Ca-phosphates and Fe-phosphates with Ca-phosphates having a distinctive post edge to the right of the white line. In Fig. 23(b) that is the FY spectra at the P K-edge there is evidence to suggest that at 405 N the tribofilm has a post edge shoulder typical of Ca phosphates. It can then be concluded that some Ca is incorporated at higher loads into the tribofilms as Ca3 (PO4 )2 or Ca may replace the Zn or Fe cation in the polyphosphate chains [48]. 3.9.2. Sulfur K-edge XANES spectra Fig. 24(a and b) shows the TEY and FY sulfur K-edge spectra of the model compounds ZnS, ZnSO4 , FeS, FeS2 , FeSO4 and Fe2 (SO4 )3 and tribofilms formed at 297 and 405 N. From the spectra of the model compounds it is evident that there is good differentiation between the sulfates and the sulfides with the sulfates having a single distinctive peak at 2482 eV while the sulfides have a pre-edge peak that occurs between 2468 and 2473 depending on the nature of the sulfide. It is impossible to distinguish between the Zn and Fe sulfates as their K-edge sulfur spectra are identical. The sulfur Kedge TEY peak also provides information on the top 30–50 nm of the tribofilm and examining the spectra it is evident that the S near the surface of the tribofilm formed at 405 N is primarily in the form of a

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Fig. 24. (a) Sulfur K-edge XANES spectra in total electron yield mode of model compounds and tribofilms. (b) Sulfur K-edge XANES spectra in fluorescent yield mode of model compounds and tribofilms. Tribofilms were formed at 297 and 405 N.

sulfate while the one formed at 297 N has a mixture of sulfates and some ZnS. The FY spectra of sulfur indicates essentially the same outcome with the film formed at 405 N being made up of sulfates while the film formed at 297 N having a mixture of sulfates and Zn and Fe sulfides. The relative proportion of sulfides in the FY spectra is higher than the one in the TEY for tribofilms formed at 297 N indicating that deeper down there is a larger proportion of sulfides. The presence of sulfates only in the tribofilm formed at 405 N is not unexpected as the extensive amount of wears coupled with higher contact temperatures at higher loads results in the oxidation of the Fe and Zn sulfides to their respective sulfates. 3.9.3. Oxygen K-Edge XANES spectra Fig. 25(a and b) are the TEY and FY oxygen K-edge spectra of the model compounds Fe2 O3 , FeSO4 , Fe2 (SO4 )3 , FePO4 , ZnSO4 , Zn3 (PO4 )2 , ZnO and tribofilms from tests at 297 N and 405 N. The oxide, sulfate and phosphate peaks of oxygen are in general quite broad as shown in the TEY spectra. The location of the pre-edge and actual peak position for the different compounds show some distinctive differences. The Fe2 O3 , FePO4 and to smaller extent Fe2 (SO4 )2 and FeSO4 show distinctive pre-edge peaks. The pre-edge in Fe2 O3 at 530 eV arises from transition of 2p electron in oxygen to the available 3d levels in Fe while the white line peak at 542 eV arises from the transition of 2p electron of oxygen to the 4s and

Fig. 25. (a) Oxygen K-edge XANES spectra in total electron yield mode of model compounds and tribofilms. (b) Oxygen K-edge XANES spectra in fluorescent yield mode of model compounds and tribofilms. Tribofilms were formed at 297 and 405 N.

4p states in Fe [49,50]. It has also been shown by de Groot that the spectra for Fe3 O4 are very similar to Fe2 O3 and it is difficult to distinguish between the two [49]. The pre-edge peaks seen in FePO4 , Fe2 (SO4 )2 and FeSO4 have the same origin as in the case of the oxides but the white lines are at lower energies near 538 eV as compared to the 542 eV in the oxides. The tribofilms formed at 297 N show clear evidence of the presence of oxides of Fe within them and possibly some FePO4 and sulfates of Fe while the film formed at 405 N has a much smaller pre-edge peak. The FIB analysis and SEM analysis of the tribofilms indicates the presence of a stable tribofilm at 297 N while it is patchy and non-uniform at 405 N. The oxygen present in the tribofilm formed at 405 N is more likely composed of either sulfates or phosphates of Zn and Fe and it is not easy to distinguish between the two using O K-edge spectra. The FY spectra of O at the K-edge show the presence of sharp peaks even with the standard oxides, sulfates and phosphates that is highly unusual. The peaks for all these compounds are generally broad without sharp peaks. The vacuum in the chamber used for the XANES analysis was maintained at 5 × 10−7 torr, which is more than sufficient for most analysis. However, the small residual amount of oxygen present in the chamber is ionized by the beam and as the FY detector is placed far from the sample the spectra from the ionized oxygen gas is superimposed on the spectra from the tribofilm. The detector used in the FY acquisition was new with extremely sensitive channel plates resulting in acquisition of the ionized oxygen

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Fig. 26. (a) Iron L-edge XANES spectra in total electron yield mode of model compounds and tribofilms. (b) Iron L-edge XANES spectra in fluorescent yield mode of model compounds and tribofilms. Tribofilms were formed at 297 and 405 N.

spectra, the actual concentration of oxygen in the chamber varied from sample to sample within a narrow range resulting in slightly different intensities of the background ionic oxygen spectra and they are not subtracted as a blank spectra was not recorded with every sample that was tested. This issue is not important in the TEY spectra as the collection ring in TEY is placed very close to the specimen and does not acquire much signal from the ionized oxygen. The FY spectra also shows a pre-edge very similar to TEY spectra with the difference that this pre-edge is distinctive in both the 297 and 405 N tested tribofilms. This indicates that both tribofilms deeper down have significant levels of oxides (Fe2 O3 and/or Fe3 O4 ) as well as sulfates of Fe. 3.9.4. Fe L-edge XANES spectra Fig. 26(a and b) are the TEY and FY iron L-edge spectra of the model compounds Fe2 O3 , Fe2 (SO4 )2 , FeS, FePO4 , FeSO4 as well as the tribofilms formed at 297 and 405 N. The Fe K edge has the white line at approximately 710.75 eV and a pre-edge at approximately 709 eV. The relative intensity of the pre-edge is a strong function of the chemistry. FePO4 has the smallest pre-edge while FeSO4 has a pre-edge that has a higher intensity than the white line. The TEY spectra of the tribofilms formed at 297 and 405 N show some distinctive differences. The tribofilm formed at 405 N has a pre-edge that has a higher intensity than the white line indicating that the primary form of Fe is in the form of FeSO4 while the presence of oxides of Fe cannot be ruled out as evidenced by the O K-edge.

Fig. 27. (a) Zinc L-edge XANES spectra in total electron yield mode of model compounds and tribofilms. (b) Zinc L-edge XANES spectra in fluorescent yield mode of model compounds and tribofilms. Tribofilms were formed at 297 and 405 N.

Smaller amounts of FeS and FePO4 are also present confirmed by the S K-edge and P K-edge spectra. On the other hand, the tribofilm formed at 297 N is largely made up of Fe2 O3 , FePO4 and FeS and to a smaller extent FeSO4 . The FY spectra of the tribofilms are more diffuse with two broad peaks that cover the entire region that covers the phosphates, oxides, sulfates and sulfides with the main peak position favoring the sulfates and to a smaller extent the oxides and phosphates. However, from the K-edge XANES spectra (both TEY and FY) of P and S it is evident that there is strong presence of P deeper in the tribofilm in the form of phosphates and to a smaller extent the presence of S in the form of sulfates in the tribofilm formed at 405 N and a mixture of sulfides and sulfates at 297 N. Hence, we can conclude that near the surface of the tribofilm, Fe is present in the form of phosphates, sulfides and oxides when formed at 297 N but tends to be converted to sulfates at the surface at loads of 405 N. Deeper within the tribofilm the Fe is present as a mixture of oxides, sulfates, sulfides and phosphates at both contact loads. 3.9.5. Zn L-edge XANES spectra Fig. 27(a and b)shows the TEY and FY iron L-edge spectra of the model compounds Zn3 (PO4 )2 , ZnO, ZnS and ZnSO4 together with spectra from tribofilms formed at 297 and 405 N. The model compounds show some distinctive differences making it possible

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to distinguish between the different chemistries. Both Zn3 (PO4 )2 and ZnSO4 exhibit sharp edges at ≈1023 eV while the edges associated with ZnO and ZnS are made up of multiple peaks and the location of the white line is shifted to ≈1025 eV. The spectra from Zn in both TEY and FY for both tribofilms are weak as evidence by the larger signal to noise ration of the spectra. However, it is clearly evident that neither ZnO nor ZnS are present in the tribofilm. The white line for ZnSO4 is slightly to the right of the Zn3 (PO4 )2 and there is some fine structure at 1028 and 1034 eV in the ZnSO4 that is absent in the Zn3 (PO4 )2 . Comparing these two spectra with the spectra from the tribofilm, it is evident that the most likely form of Zn is Zn3 (PO4 )2 . An earlier study [35] using L-edge XANES of P and S had shown that when just ZDDP was used at extreme loads the phosphates are present in the form of short chain phosphates. 4. Conclusion The extent of wear is related to the variation of friction coefficient during the test. Flat eventless regions of the friction coefficient resulted in minimal wear whereas regions with large fluctuation in friction coefficient resulted in increased wear. The latter condition indicates the repeated breakdown and formation of the tribofilm results in rapid depletion of the antiwear additive ZDDP in the oil. Wear under boundary lubrication is controlled by the nature and effectiveness of the tribofilms formed on the surface. The extent of wear under boundary lubrication at a fixed load is directly proportional to the applied load with a linear increase in the extent of wear as the applied load is increased. The time for final break down of the protective tribofilm is inversely dependent on the applied load with larger loads resulting in shorter lifetimes. In addition, the time to failure and the extent of wear is dependent on the amount of lubricant used and contact load. In the current study, a study of the chemistry of the tribofilm using EDS mapping indicates that the tribofilms formed at lower loaded of 297 N were richer in phosphorous while tests conducted at 405 N had a greater amount of sulfur. Lower loads favored stable tribofilm formation and small amounts of abrasive wear while at higher loads adhesive wear is favored with some regions with stable tribofilm and others without. FIB analysis of the tribofilm indicates that at higher loads regions with tribofilm patches were thicker (up to 1000 nm) compared to films formed at 297 N which were typically 100–200 nm thick. Based on TEM analysis of the wear debris, it is evident that having thicker films does not result in improved wear as the higher loads have a larger concentration of abrasive Fe2 O3 particles together with Fe3 O4 compared to lower loads that have smaller number of oxide particles made up only of Fe3 O4 . The wear debris analysis indicates that the extent of oxidation of the debris and formation of Fe2 O3 nanoparticles embedded in them is largely dependent on the contact load and not the duration of the test. XANES analysis of the tribofilms indicate that at the surface of the tribofilms sulfates of Fe and Zn are present at both 297 and 405 N while deeper down in tribofilms formed at 297 N we have a sulfides of Fe present as well. Phosphorous is present as short chain phosphates of Fe and Zn at both loads. Analysis using Fe L-edge and O K-edge spectra confirm that the tribofilms formed at 405 N contain sulfur in the form of sulfates and to a smaller extent in the form of oxides of iron while the tribofilms formed at 297 N contain sulfides of Fe and some oxides of Fe as well. Zn L-edge indicates that Zn in the tribofilm is present as phosphates and to a lesser extent in the form of sulfates at both loads. Acknowledgements Support provided by the State of Texas as part of a Technology Development and Transfer Grant and Platinum Research Organi-

zation LLC is gratefully acknowledged. The authors would like to thank Prof. Ronald L. Elsenbaumer and Dr. Harold Shaub for useful discussions. XANES analysis was conducted at SGM Beamline at the Canadian Light Source and the DCM beamline at The Synchrotron Radiation Center at Madison Wisconsin. Assistance provided by Mr. Tom Reiger at Canadian Light source and Dr. Narayan Appathurai is gratefully acknowledged.

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