Study of Surface Films of ZDDP and MoDTC With Crystalline and Amorphous Overbased Calcium Sulfonates By XPS
By Costello, Michael T Urrego, Roberto A; Hunter, Maureen
Keywords Overbased Sulfonate. Sulfurized Olefin. Molybdenum Dithiocarbamate, XPS, Four-Ball ER Surface Chemistry, Metalworking Lubricants
The role of amorphous or crystalline overbased detergent and sulfur containing AW/EP additives on the surface films created in a tribometer was investigated. In the crystalline overbased sulfonate containing samples the weld point was higher than for the equivalent blend using an amorphous overbased sulfonate. The calcium carbonate core of the overbased sulfonates was deduced to be present in the surface film when combined with either ZDDP or MoDTC. For samples containing ZDDP the crystalline overbased sulfonate, reduced the phosphate film thickness, while for the MoDTC it reduced the MoS2 film thickness. In both cases significantly more carbon, likely as calcium carbonate (which acts to improve the EP/AW properties of the lubricant), was present when using the crystalline overbased calcium sulfonate compared to the amorphous overbased calcium sulfonate.
Understanding the relationship between lubricant composition and the effects on the film structure created in a tribological system is essential in the development of new lubricant additives. The additive ZDDP (zinc dialkyldithiophosphate) has been widely used in industrial and engine oils for over 40 years for both the antiwear and antioxidant properties.’ The ZDDP’s commonly used are composed of either primary or secondary aliphatic dithiophosphates with varying lengths (C3 to C 12). In recent years, a variety of surface techniques have been used to study the surface films created by ZDDP in tribometers. It appears the rate of formation of the tribofilm produced by ZDDP on a metal surface during rubbing is low enough to avoid corrosive wear, but sufficiently high enough to maintain it during rubbing.
In general it has been observed that ZDDP reacts to form metal phosphates and metal sulfides on rubbed metal surfaces.2-7 The phosphate films can form glassy solids,” as well as a network of polyphosphate chains of varying length, depending on the conditions of the testing.9 These films serve to protect the surface of the metal from scuffing and wear. The S from the dithiophosphate either incorporates into the phosphate films or decomposes to form FeS films of varying thickness depending on the conditions of the experiments.3 For example, in FT-IR and XPS imaging in samples of ZDDP it was confirmed that the phosphate film thickness increased with increasing load in ball-on-disk tribometers.5,6 10
The study of the formation of the films deposited on the metal surfaces of a tribometer by MoDTC (molybdenum dithiocarbamate) and its analogs is also quite extensive. In general it has been observed that the MoDTC decomposes to forms sheets of MoS? on the surface, as well as, a mixture of molybdenum oxysulfides, which serve to prevent wear.4, 11
Over the past few decades the antiwear properties of overbased sulfonates were found to be a result of the formation of protective films caused by the deposition of inorganic carbonate on the surface of the metal in the contact.1216 The effect of this deposited film is analogous to the effects of films deposited by traditional phosphorus and sulfur containing antiwear additives like ZDDP.
Surprisingly, the formation of films on metal surfaces by overbased detergents and sulfur containing antiwear additives is less studied and understood. In four-ball wear tests it was demonstrated that overbased detergent decreased the EP/AW performance of ZDDP, due to competitive adsorption between the detergents and the ZDDP1 while in Falex seizure load tests there was a positive synergy.17 In the XPS and the XANES studies of the metal surfaces worn by a pin-on-disk tribometer using blends of ZDDP with overbased calcium phenate or salicylate, it has been shown that there is competition for the surface and that the overbased detergent breaks down to form carbonate, which is deposited on the surface of the metal. It was found that the Ca^sup 2+^ ions replaced the Zn^sup 2+^ in the polyphosphate structure of the film and, as a result, in the case of low amounts of overbased detergent long- chain polyphosphate films were formed, while in the case of high amounts of overbased detergent short-chain polyphosphate films were formed. This chain shortening effect resulted in a diminution of the antiwear properties of the ZDDP.9
We know that the combination of overbased sulfonate and a sulfurized olefin which contains active sulfur (such as the pentasulfide R-S-S-S-S-S-R linkage) which displays a large synergy in Four-Ball EP testing due to an interaction between the overbasing and sulfurized olefin.1″ This can be observed in the surface films formed in the Reichert and Four-Ball tribotesters by XPS.19 In the present study we investigated the effects of overbased sulfonate on other sulfur containing antiwear additives. We found that the surface films created by the overbased sulfonates and MoDTC or ZDDP display a small AW/EP improvement in the Four-Ball EP testing compared to the base oil. After the four-ball testing was completed the scars on the test pieces were analyzed using XPS scanning and depth profiling to determine the composition of the wear film produced. We attempted to determine whether the films formed were similar to films produced in the absence of overbased sulfonate or were affected by an interaction between the two sulfur containing additives.
The test samples were prepared using a 100 SUS naphthenic oil. sulfurized olefin (SO), molybdenum dithiocarbamate (MoDTC)1 amorphous (C400A) and crystalline (C300C) overbased calcium sulfonate, and zinc dialkyldithiophosphate (ZDDP). Descriptions of the components used are included in Table I . A description of the 100 SUS diluent oil used is included in Table 2 (see page 22). The ASTM D2783 test. “Measurement of ExtremePressure Properties of Lubricating Fluids (Four-Ball Method)” was performed on all blends and the test pieces from the highest non-seizure load were then analyzed using both XPS depth profiling and survey scans.
On each ball, the scar area was examined initially using XPS by low-resolution survey scans to determine which elements were present and the concentration of those elements. Depth profiles of the elements were performed to a depth of -300 ? at a sputter rate of 57 A/min relative to SiO?. The quantification of the elements was performed using the atomic sensitivity factors for a Physical Electronics Model 5700LSci ESCA spectrometer. The approximate escape depth (3XsinB) of the carbon electrons was 70A.
The analytical test conditions for the XPS depth profiling are as follows:
Instrument ………….. Physical Electronics 5700LSci
X-ray source …….. Monochromatic Al Ka at 1486.6 eV
Source power ……………………….. 350 watts
Analysis region ………………… 0.8 mm diameter
Exit angle ………………………………. 45[degrees]
Charge correction ……………………….. none
Charge neutralization ………….. electron flood gun
Ion sputtering ……… 3 kV Ar’. 3.5 mm x 3.5 mm raster
Sputter rate …………………… 57 A/min for SiO2
Illuminated region ………………… 2 mm X 6 mm
Prior to sputtering, the outermost surfaces of the scar areas on the six ball samples all contained various amounts (
Results and Discussion Four-Ball EP Results
The four-ball test data demonstrates that the addition of AW/EP additives increases the weld load relative to the base oil (refer to Table 3). Unlike the sulfurized olefin containing samples,” the results of the overbased calcium sulfonate blends reveal that there is far less synergy when using ZDDP or MoDTC (refer to Table 3). The samples containing the ZDDP or the MoDTC displayed weld points that were one load higher than the base oil. The addition of overbased sulfonate slightly improved the weld load relative to the base oil and the crystalline C300C samples in combination with both the ZDDP and the MoDTC possessed a weld point that was one load higher than the amorphous C400A samples.
The relatively small improvement in the weld point for the ZDDP and the MoDTC, compared to the large synergy between overbased sulfonate and sulfurized olefin (SO), is probably attributable to the activity of the sulfur used in the blends.18 Evidently the sulfur in the sulfurized olefin (SO) is more active than the sulfur in the ZDDP and the MoDTC, which is tied up in the dithiophosphate and dithiocarbamate ligands attached to the metal centers. The results for the load wear index match the trends observed in the weld point data and based on these results there is clearly, relatively very little interaction of the overbased sulfonates and the ZDDP or the MoDTC.
Samples A through F that contained blends of ZDDP and MoDTC were tested using a Four-Ball EP Rig. The test balls from the runs with the highest non-seizure load were then analyzed using an XPS survey scan and depth profiling.
Surface Prior to Sputtering
The surface concentration was measured by XPS and at the outermost surfaces (see lable 4) prior to sputtering, the chemical states detected on the scar areas of the six samples were mainly C- (C1H) with small amounts of C – O, C=O, and O – C=O, oxygen, and traces of iron as FeCv In addition, traces of sulfur as SO^sup -3^ (x=3 or 4), sulfur as S^sup -2^, calcium as Ca^sup +2^, phosphorus as PO^sup 1^, molybdenum as Mo^sup +4^ and Mo^sup +6^ were also detected on some, but not on all the surfaces. On five of the six samples (except sample D), traces of CC^sub 1^^sup 2-^ might be present at the near surface regions. Due to the overlap of the S2s peak with the Mo3d peak in some spectra, the concentration of Mo was calculated using the Mo3p3 peak. On the outermost surface the sample containing the ZDDP (A) we were unable to detect Zn and P1 but when samples with both the ZDDP and overbased sulfonate (C and E) were analyzed we found evidence of Zn (refer to Table 4), as well as P on the amorphous overbased calcium sulfonate sample. In addition, the presence of Ca was also now detected. In the survey scan of the sample containing MoDTC (B) the Mo was observed on the surface, but when samples with both the MoDTC and the overbased sulfonate (D and F) were analyzed there was less Mo present (refer to Table 4). These concentration differences may indicate possible interactions between the components
Surface Depth Profile
After the surfaces were surveyed they were then sputtered with Af to reveal a surface concentration depth profile. The average concentration profiles of the worn surfaces after the friction tests with each additive package can be seen in Table 5. It was observed that C, Fe, and O are present on all the balls. It is well known that nitrogen, chlorine, sulfur, phosphorus and molybdenum compounds are mostly responsible for the AW/EP performance and that the presence of antiwear compounds such as sulfides, phosphates, and chlorides enhance the load-carrying capacity of the blends.
Calcium was detected at all depths for samples C, D, E and F. But on sample A and B, a trace of calcium (detection limits -Ol atom %) was detected only at some, but not at all depths. Moreover, various amounts of sulfur most likely as S^sup -2^ were detected at all depths for all the six samples.
Phosphorus was detected at all depths for samples A and E. On sample A, the two P2p peaks were observed after sputtering. One at ~129 eV was consistent with a putative phosphide, and the other peak at ~133 eV was consistent with a phosphate (PO.). On sample E, phosphorus appeared to be mainly as PO. after sputtering. On Sample C there is no clear evidence of P on the surface.
In the depth profiling for the sample containing ZDDP (A), there was evidence for the presence of P, while for the sample containing ZDDP and C300C (C), there was very little evidence of P, but Ca was present. In addition, the amount of carbon in the depth profile is higher than in the ZDDP (A) samples and this may be evidence for the presence of additional CaCd depositing on the surface. Contrary to the crystalline overbased sulfonate results, for the samples containing ZDDP and the amorphous C300A (E) there was both Ca and P present. This may indicate a competition between the carbonate (from the overbased sulfonate) and phosphate (from ZDDP) on the surface. We conjecture that the larger particle sized crystalline carbonates preferentially adsorb to the surface compared to the smaller particle sized amorphous carbonates. While the crystalline material promotes more carbonate on the surface, the amorphous material promotes more phosphate.
In the depth profiling average concentration estimates, the sample containing the MoDTC possessed significant concentrations of Mo and S, while for the sample containing MoDTC and overbased sulfonate, there was a much smaller concentration of these elements (refer to Table 5). In the sample containing MoDTC and the crystalline C300C (F). there was significantly less Mo in the profile and significantly more Ca than the sample containing the amorphous C400A (D). It appears that the CaCO^sub 3^ from the crystalline C300C more effectively displaces the Mo on the surface than the amorphous C400A. This effect has also been observed previously in the XPS results of sulfurized olefins and overbased sulfonates.”
Binding Energy Depth Profile
As is observed from Figure I for the sample containing ZDDP (sample A) there is an Ols peak at 531 eV, which is consistent with the formation of metal oxides, as well as evidence of metal sulfides from the S2p peak at 162 eV. The spectrum also displays a P2p peak at 1 33 eV, which is actually the combination of the closely spaced P2pl and P2p3 peaks, indicating the presence of metal phosphates. There is also a small difficult-to-resolve peak at 129 eV, which is tentatively assigned to a phosphide. The observation of phosphates and sulfides on the surface indicate that the ZDDP adsorbed onto the metal surface and decomposed under the tribological conditions of this test. The C1s peak at the 284 eV binding energy can be attributed to the organic carbon in the system. In fact, the depth profile displays a large concentration of carbon on the top few layers, which is probably a hydrocarbon layer, followed by a large drop in concentration attributable to carbon from the anti- wear additives as the sputtering depth increases
In Figure 2, in the sample containing ZDDP + C300C (C) there is an Ol s peak at 531 eV, which is consistent with the formation of metal oxides, as well as the evidence of metal sulfides from the S2p peak at 162 eV. Surprisingly, the spectrum does not display a significant P2p peak at 133 eV indicating the presence of metal phosphates. The shortening of the polyphosphate chains formed from the tribochemical reaction of ZDDP with the metal surface has been previously observed in XANES spectra of ZDDP and overbased sulfonate.” In these spectra it also appears that the crystalline C300C is more strongly deposited on the metal surface than the smaller particle sized C400A amorphous overbased sulfonate and has inhibited the formation of the polyphosphate chains.
The putative mechanism proposed for this inhibition has been the scission of the polyphosphate chains by the formation of calcium phosphate. The determination of the carbon moiety on the surface is problematic, since we have no direct evidence of CaCO3 from the CIs spectra. The CIs peak at the 284 eV binding energy can be attributed to the organic carbon in the system, but there is no evidence of carbon as a carbonate (CO3^sup 2-^ at the 289 eV binding energy from the overbased sulfonate. Since the carbonate from the antiwear films are notoriously weak in XPS they cannot be easily detected by this method (XANES is a more sensitive method for the detection of carbonate).9 As a result, the Ca2p3 peak at 347 eV binding energy which is indicative of Ca^sup 2+^ could be evidence not only for CaO or Ca3(PO4)2, but also for CaCO3 on the surface. In addition, unlike the ZDDP spectra, the CIs spectra for ZDDP + C300C displays a more gradual decay in concentration over the depth profile and contains significantly more carbon, which may be a result of the decomposition of the overbased sulfonate.
In Figure 3 for the sample containing ZDDP + C400A (E) the spectra are similar to Figure 2. There are peaks for metal oxides at OIs (531 eV), metal sulfides at S2p ( 162 eV) and Ca^sup 2+^ at Ca2p3 (347 eV). Interestingly, there is a significant P2p peak at 133 eV indicating the presence of metal phosphates, which is not observed in Figure 2 for the ZDDP + C300C (C) spectra. The implication is that the C400A has not deposited CaCO3 as effectively into the contact as the C300C, which may promote the disruption of the polyphosphate chains by the formation of the calcium phosphate. The same CIs peak at 284 eV binding energy could be attributed to the organic carbon in the system since there is no evidence of carbon as a carbonate (CO3^sup 2-^) at the 289 eV binding energy from the overbased sulfonate. Alternatively, this could also be the result of a weak carbonate signal in the XPS masking the CaCO^sub 3^ which cannot be easily detected by this method.9 The Cls spectrum displays a high concentration of carbon in the top layers, with a rapid decrease through the depth profile similar to the ZDDP (A) profile. This may be an indication that the C400A has less interaction on the surface than the C300C.
In Figure 4, for the sample containing the MoDTC (B), there are peaks for the metal oxides at Ols (531 eV), metal sulfides at S2p (162 eV) and Ca^sup 2+^ at Ca2p3 (347 eV). In addition, there is the well defined spin-coupled doublet of Mo3d5 (227 eV) and Mo3d3 (231 eV), which is indicative of the oxidized Mo^sup 4+^ species, most probably from MoS^sub 2^ formed on the surface as well as a small Mo^sup 6+^ peak (232 eV) at the surface attributable to MoO^sub 3^. The Cls peak at 284 eV binding energy can be attributed to the organic carbon in the system. In fact, the depth profile displays a large concentration of carbon on the top few layers, which is probably a hydrocarbon layer, followed by a large drop in concentration consistent with the carbon from the antiwear additives. These spectra indicate the surface is mostly composed of a MoS^sub 2^ layer covered by a thin MoO^sub 3^ layer.
In Figure 5, for the sample containing MoDTC and C400A (D), there are peaks for metal oxides at Ols (531 eV), metal sulfides at S2p (162 eV) and Ca^sup 2+^ at Ca2p3 (347 eV). There is also the well defined spin-coupled doublet of Mo3d5 (227 eV) and Mo3d3 (231 eV), which is indicative of the fact that the MoS^sub 2^ formed on the surface which is overlapped by a small S2s peak (226 eV). Additional evidence for the formation of a MoS^sub 2^ species is confirmed in the Mo3p1 (415 eV) and Mo3p3 (398 eV) peaks. The C1s peak at the 284 eV binding energy can be attributed to either the organic carbon in the system or carbon in the CaCO^sub 3^. While the signal of the carbon in the CaCO^sub 3^ is not strong in XPS, the concentrations of Ca in the Ca2p3 spectra are consistent with the presence of CaCO^sub 3^. These spectra indicate that the surface is mostly composed of a film of the MoS^sub 2^ and the CaCO^sub 3^ with very little evidence of the MoO^sub 3^ formation. In Figure 6 for the sample containing the MoDTC and the C300C (F) there are peaks for metal oxides at O1s (531 eV), metal sulfides at S2p (162 eV) and Ca^sup 2+^ at Ca2p3 (347 eV). There is also the well defined spin- coupled doublet of Mo3d5 (227 eV) and Mo3d3 (231 eV), which is indicative of MoS^sub 2^ formed on the surfaces, but these peaks are smaller than the peaks present in the amorphous C400A overbased sulfonate spectra (D). Additional evidence for the formation of a MoS^sub 2^ species is confirmed in the Mo3p1 (415 eV) and Mo3p3 (398 eV) peaks. The CIs peak at the 284 eV binding energy can be attributed to either the organic carbon in the system or carbon in the CaCO^sub 3^. While the signal of the carbon in the CaCO^sub 3^ is not strong in XPS, the concentrations of Ca in the Ca2p3 spectra are consistent with the presence of CaCO^sub 3^. These spectra indicate the surface is mostly composed of a thin layer of MoS^sub 2^ and CaCO^sub 3^with very little evidence of MoO^sub 3^ formation. The absence of significant Mo peaks in the spectra is indicative of the competition between the overbased sulfonate and the MoDTC for the metal surface. As with the ZDDP spectra, it appears that the C300C prevents the deposition of molybdenum on the surface of the metal, which in turn reduces the amount of the MoS^sub 2^ that can be formed.
In the XPS data it appears that the interaction of the overbased calcium sulfonates with ZDDP or MoDTC is antagonistic toward film formation of the sulfur containing species. The calcium carbonate core of the overbased sulfonates was deduced to be present in the surface film when combined with either ZDDP or MoDTC based on the XANES literature and XPS data. For samples containing ZDDP. C300C reduced the phosphate film thickness more than the amorphous C400A, In the XPS depth profile experiments for the MoDTC, the C300C reduced the MoS^sub 2^ film thickness and, as observed in the ZDDP system, the larger particle sized crystalline C300C was more effective in terms of EP performance than the amorphous C400A. As a result, since the calcium carbonate is known to act as an effective EP/AW additive, the Four-Ball EP performance of the blends was not degraded as much for the C300C system regardless of the antagonism toward film formation of the sulfur containing species.
Editor’s Note: Albert Einstein once said, One of the most beautiful experiences you can have is to gaze at a mystery. . .to ask how does it work.” Understanding the role additives play in a finished product is fundamental to formulating different products for different applications. But figuring out the mechanisms of additive synergies and antagonisms can sometimes feel like a holiday morning. Some days you just can’t wait to get to the lab to see if the pending test results fit your theory. In this month’s Editor’s Choice paper, authors Michael Costello and Roberto Urrego use X-ray Photoelectron Spectroscopy (XPS) to determine the composition of the wear films produced in the ASTM D 2783 Four-Ball EP tester using various combinations of two overbased calcium sulfonates with either ZDDP or MoDTC. Their results give greater insight into the chemistries and mechanisms occurring during the formation of the various tribofilms. I hope you enjoy reading this paper. I did.
– Dr. Maureen Hunter,
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Michael T. Costello (STLE member) and Roberto A. Urrego (STLE member), Chemtura Corp., Middlebury, Conn.
Copyright Society of Tribologists and Lubrication Engineers Aug 2007
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