Low-friction tribofilm formed by the reaction of ZDDP on iron oxide

ARTICLE IN PRESS Tribology International 39 (2006) 1538–1544 www.elsevier.com/locate/triboint Low-friction tribofilm formed by the reaction of ZDDP on iron oxide Kosuke Itoa,, Jean-Michel Martinb, Clotilde Minfrayb, Koji Katoa a Laboratory of Tribology, Graduate School of Engineering, Tohoku University, 6-6-01 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b Ecole Centrale de Lyon, LTDS, UMR 5513, 36 Avenue Guy de Collongue, 69134 Ecully Cedex, France Received 6 September 2005; received in revised form 10 January 2006; accepted 11 January 2006 Available online 10 March 2006 Abstract An iron oxide layer (mixture of Fe3O4 and FeO) was formed by water-vapor treatment on tool steel plates. Bearing steel cylinders were slid against the plates in PAO with and without 1% zinc dialkyldithiophosphate (ZDDP) at 80 1C. The friction coefficient was below 0.06–0.08 with ZDDP, which is more than 20% lower than that without ZDDP and about a half of a steel/steel contact under the same condition. The formation of multi-layered tribofilm of 30–130 nm on the iron oxide was identified by TEM. The bottom part of the tribofilm is a layer of 10–30 nm that contains Zn, Fe, S, P, and O with a gradient composition, where distribution peaks of Zn and S were found by EDX. r 2006 Elsevier Ltd. All rights reserved. Keywords: ZDDP; Iron oxide; Tribofilm 1. Introduction Low frictional loss in sliding engine components leads to the reduction of CO2 emissions through the improvement in fuel consumption, which is a strong social requirement for internal combustion engines. On the other hand, the reduction in NOx emissions is another requirement. Some of the NOx reduction technologies, however, are known to increase the wear of engine components [1]. To reduce the wear, the application of an iron oxide layer on the sliding surface by water-vapor treatment has been proven to be effective [1,2]. Past tribological studies on iron oxides are mostly on the dry sliding. Almost no study has ever focused on the frictional characteristics of the iron oxide, which is formed by the water-vapor treatment, in lubricants with an anti-wear additive, zinc dialkyldithiophosphate (ZDDP). Especially, how such iron oxide reacts with ZDDP and how the reaction products affect the tribological properties are not known. ZDDP has been the most effective anti-wear additive, thus indispensable for more than 60 years. No alternative is available to date. ZDDP forms a protective tribofilm on Corresponding author. E-mail address: kito@tribo.mech.tohoku.ac.jp (K. Ito). 0301-679X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2006.01.023 the ferrous surface. The tribofilm includes metal-polyphosphates, but its detailed structure has not yet been fully understood. In this study, the effect of the iron oxide layer formed by the water-vapor treatment on the friction coefficient is studied in lubricants with and without ZDDP, and the tribofilm formed on the iron oxide by the reaction with ZDDP is analyzed. 2. Experimental details A cylinder-on-flat, oscillating-type test apparatus, Cameron Plint tribotester, is used for the friction experiments. The cylinder specimen slides on the plate specimen in an oscillating motion. The oscillating frequency of the cylinder specimen is 7 Hz with a stroke of 7 mm. The total test duration is 1 h. The load was 50 N during the initial 5 min, and then increased to 350 N for the rest of the period. The cylinder-type specimen is 6 mm in diameter and 6 mm in length. It is made of NF 100C6 bearing steel that is equivalent of JIS SUJ2 or ASTM 52100. The Vickers hardness of the cylinder is 8.1 GPa. The sliding surfaces of the cylinders are polished using emery papers of #800, #1200, #2000, and #4000, and then finished with a ARTICLE IN PRESS K. Ito et al. / Tribology International 39 (2006) 1538–1544 diamond slurry with 1-mm grains. The resultant surface roughness is 0.05 mm Ra. The plate specimen has dimension of 10  8  2 mm. It is made of high-speed tool steel, JIS SKH51, that is equivalent of ASTM M2. The initial sliding surfaces of the plates are miller finished. The initial Rockwell hardness (in C scale) of the plate is 64. Then, a layer of iron oxide is formed on the surfaces by the water-vapor treatment at 600 1C for 2 h. Details on the process can be found elsewhere [1,2]. After the treatment, the surface is polished with diamond slurry with 1-mm grains. The resultant surface roughness is 0.1 mm Ra. The thickness of the oxide layer after the polishing is 2–5 mm, which contains about 50% of Fe3O4 and 50% of FeO. An optical microscope photograph of the plate is shown in Fig. 1. The white spots on the surface are hard phase particles such as carbides that are dispersed in the steel to sustain the hightemperature hardness. The plate specimen is soaked in an oil bath that is filled with a lubricant. The temperature of the lubricant is kept at 80 1C. Two types of lubricants are used: pure polyalphaolefine (PAO 6) and PAO 6 mixed with 1.0% of ZDDP (4-methyl-2-pentanol). PAO is one of the most typical synthetic base oils for engine oil. The maximum Hertzian contact pressure is 850 MPa and the minimum oil-film thickness calculated in the middle of the stroke by the simplified Dowson-Higginson equation [3–5] is 0.02 mm. Because the calculated oil-film thickness is smaller than the surface roughness, the condition is in the mixed lubrication regime. After the experiments, the specimens are cleaned in heptane for 10 min by using an ultrasonic cleaner. Then the wear tracks of the plates and cylinders are observed under optical microscopes using various magnifications. Electron probe microanalysis (EPMA) is conducted to identify the distributions of chemical elements on the plate surface. Fig. 1. An optical microscope photograph of the water-vapor treated steel plate surface after polishing. 1539 The cross sections of the near-surface regions of the plates are analyzed by a transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDX). Focused ion beam (FIB) method is used to make the thin films of the cross section for the analysis. The thin film samples are placed on a copper grid that has a carbon film on it to support the specimen to be observed. For this reason, carbon and copper are detected by the following EDX analysis. 3. Results The friction coefficient data of the tests in PAO with and without 1% ZDDP are plotted in Fig. 2. The friction coefficient, m, reaches a steady value of 0.10 after 1300 s in pure PAO. With ZDDP, the friction coefficient is more than 20% lower than that in pure PAO, which goes down to the lowest value of below 0.06 at around 500 s and reaches 0.08 at the end of the 1-h test. The optical microscope photographs of plates and cylinders of the tests without and with ZDDP are shown in Figs. 3 and 4, respectively. In pure PAO, the wear track of the cylinder is also discolored, while it does not with ZDDP. The wear tracks of the plates are severely discolored under the both conditions. Inside the wear track of the plate after the test in pure PAO, the formation of a crystalline tribofilm has been observed by a preliminary TEM observation, which will be analyzed further and reported in a future paper. Inside the wear track of the plate of the test in PAO with ZDDP, bluish and dark area can be seen (Fig. 4(a)). A TEM photograph of the cross section of a bluish area of the plate of the test with ZDDP is shown in Fig. 5(a). Fig. 5(b) is a typical example of the distribution of chemical Fig. 2. Friction coefficient of the iron oxide plate/steel cylinder couples of tests in pure PAO with and without 1% ZDDP. ARTICLE IN PRESS 1540 K. Ito et al. / Tribology International 39 (2006) 1538–1544 Fig. 3. Optical microscope photographs of (a) the cylinder and (b) the plate after the test in PAO. Fig. 4. Optical microscope photographs of (a) the cylinder and (b) the plate after the test in PAO with 1% ZDDP. Fig. 5. (a) A TEM photograph of the cross section on the bluish area in the wear track of the plate after the test in PAO with 1% ZDDP; and (b) the EDX analysis result showing the distributions of chemical elements along with the white line in (a). ARTICLE IN PRESS K. Ito et al. / Tribology International 39 (2006) 1538–1544 elements measured by EDX along the arrow shown in Fig. 5(a). A tribofilm of about 130 nm thickness exists on the iron oxide substrate. The tribofilm can be distinguished into three layers. The EDX spectra of the areas indicated in Fig. 5(a) on the middle and bottom layers of the tribofilm as well as the iron oxide layer are shown in Fig. 6. Semiquantitative analysis results on the spectra are shown in Fig. 7. The upper part is a thin (a few nm) layer, which is rather difficult to identify by the TEM photograph, but can be identified by the elemental distribution shown in Fig. 5(b). Zn, S, and P are included in the top layer, which have been found in all the locations that were analyzed. The middle part is a heterogeneous layer of about 100 nm. An electric diffraction pattern indicated that the middle layer was amorphous. The middle part of the layer includes P and O with a significant amount of C (Figs. 6 1541 and 7). The amount of Zn is negligibly small. Although a significant portion of C comes from the supporting carbon film, the relative amount in the middle layer is about 2 times that of other areas. It should be noted that almost no signal has been detected (see Fig. 5(b)) from the middle layer of the tribofilm. After the measurements for Fig. 5(b), a line having the brightness similar to the supporting carbon film appeared in the TEM image of the middle layer. It suggests that the rupture of the film occurred due to radiation damage. Probably, this is the cause that no signal was detected from the middle layer. In the EDX analysis, the electron beam was more focused, thus had higher energy density in Fig. 5(b) than the case for Fig. 6(1). Probably, local temperature rise occurred due to the high-energy electron beam. Similar change, in contrast, was also found after the measurement for Fig. 6(1), but it was not as significant as the case of Fig. 5(b). These results (1) Middle Layer of Tribofilm P, O, (C) Intensity (counts) 60 P C O 40 Cu Fe Zn 20 S Cr Fe Zn 0 0 5 10 15 20 Energy (keV) 200 Fe, Zn, S, O, P Intensity (counts) (2) Bottom Layer of Tribofilm Fe P O Cr 150 Zn S 100 Fe Cu C 50 Zn Cr Cr Mo 0 0 5 10 Energy (keV) 15 20 400 (3) Iron Oxide Layer Fe, O Intensity (counts) Fe 300 O Cr 100 Cr V Fe 200 C Si Cu Mo Mo 0 0 5 10 Energy (keV) 15 20 Fig. 6. EDX spectra on the areas specified as (1), (2), and (3) in Fig. 5(a), respectively, showing the chemical elements included in the areas. (Note: copper originated from the mesh to support the specimen and carbon partly originated from the carbon film on the copper mesh.) ARTICLE IN PRESS K. Ito et al. / Tribology International 39 (2006) 1538–1544 1542 100% 90% 80% C C C 70% at.% 60% 50% O 40% O 30% 20% 10% 0% S P O (1) P Fe (2) Fe Zn Cr (3) Fig. 7. Semi-quantitative analysis results on the spectrum shown in Fig. 6 showing the chemical composition of (1) the middle and (2) bottom layer of the tribofilm, and (3) the iron oxide. suggest that the middle layer contains carbon, quite possibly in the form of alkyl chains that are easily damaged by the local temperature rise. The bottom part is a layer of about 30 nm. The bottom layer includes Fe, Zn, P, S, and a large amount of O. The amount of oxygen in the bottom layer is about the same as that in the iron oxide layer. The distributions of Zn and S match very well, which peak in the bottom layer of the tribofilm. Although the concentration of Fe decays sharply, the area of high-Fe concentration overlaps with the peaks of Zn and S distributions. These facts suggest the existence of sulfides. By additional TEM observations on other areas inside the wear track, it has been found that the thickness of the tribofilm and that of the bottom layer of the tribofilm ranges from 30 to 100 nm and 10 to 30 nm, respectively. The main elements included in the bottom layer were the same regardless of the thickness. TEM cross sectional observation was also conducted outside the wear track. No layer was seen on the iron oxide substrate under the magnification same as Fig. 5(a). 4. Discussions 4.1. Effect of iron oxide on friction Minfray [6] reported that a friction coefficient of 0.11– 0.12 was obtained in a steel/steel friction couple lubricated with 1.0% ZDDP in PAO using the same cylinder-on-plate tribotester as in this study (Fig. 8). The test condition is also the same, except the plate material, and the PAO with ZDDP is from the same bottle. Under the same test condition, the steel/iron oxide contact resulted in the low friction (m0.06–0.08) as shown in Fig. 2. This means that the mechanism of the low friction is related to the characteristics of iron oxide. Under the same or similar conditions, such level of low friction coefficient has not been reported in the steel/steel contact unless using other type of ZDDP or a friction modifier such as MoDTC [7]. Fig. 8. An example of a typical friction coefficient of a steel/steel pair in PAO with 1% ZDDP [7]. The iron oxide could realize a low-friction tribosystem without friction modifiers. 4.2. Structure and composition of tribofilm The ZDDP tribofilm has a multi-layer structure (Fig. 5). The common characteristic of the tribofilm in any area is that a layer of 10–30 nm thickness that contains Zn, Fe, S, P, and O is available above the iron oxide substrate (i.e. the bottom layer of the tribofilm) although the ratios of these elements are not necessarily the same. The bottom layer of the tribofilm has a gradient composition (Fig. 5(b)) and had no or less radiation damage. The middle layer of the tribofilm is rather organic because it was damaged by the electron beam and includes a significant amount of carbon. The upper layer of the tribofilm includes similar elements as the bottom layer (i.e. Zn, S, and P). It is likely that the upper layer is originated from the bottom layer. During sliding, probably a part of the bottom layer is lost as the wear debris and then transferred to the cylinder, which is transferred back to the surface of the plate. The origin and function of the upper layer is a subject of future study. Martin et al. [8] proposed a schematic representation of the structure of ZDDP tribofilm on steel. Minfray [6,9] has later confirmed in the steel/steel contact that the overall tribofilm structure is almost the same as that by Martin et al. The lateral distributions of chemical elements have been reported by several studies [9–12]. Minfray conducted the depth profile analysis by XPS as shown in Fig. 9 [9]. Her tribofilm has been identified as zinc–iron polyphosphates with metallic sulfides, which are the tribochemical reaction products between the surface (i.e. Fe) and ZDDP itself. The film has a glassy structure (i.e. amorphous), but ARTICLE IN PRESS K. Ito et al. / Tribology International 39 (2006) 1538–1544 Fig. 9. XPS depth profile of chemical elements in the tribofilm on steel (adopted from [9]). no carbon was detected. Yin et al. [12], on the other hand, have also conducted the depth profile measurement on the ZDDP tribofilm formed on steel under a different test condition. Their result shows the same trend as Minfray’s except for the existence of carbon. Therefore, the middle part of the tribofilm is different from that formed in the steel/steel contact under the same test condition (i.e. Fig. 9), but may not be so unique to the iron oxide. In contrast, the formation of the tribofilm that is similar to the bottom layer found in this study has not been reported in steel/steel contacts. Although the existence of sulfides between the polyphosphate-based tribofilm and ferrous substrate is often mentioned in general (for example in Ref. [13]), they are typically ‘‘patchy islands’’ such as those reported in the Ref. [14] and not a continuous, thick layer seen on the iron oxide. Moreover, less Zn exists near the substrate in steel/steel contacts. In the case of Minfray’s study (Fig. 9), Zn content in the tribofilm is about 20 at% near the surface and about 5 at% near the steel substrate, which is the opposite trend to the result of this study. This trend is consistent with Yin’s result under a different test configuration. Therefore, the structure and composition of the bottom layer is unique to the iron oxide. It is known that zinc in ZDDP is replaced with metal cations (Mx+) easily by cation exchange reactions forming ‘‘MDDP’’ although the role of zinc is poorly understood [15]. MDDP is thermally less stable than ZDDP [13], which leads to the formation of the thermal film at an elevated temperature, typically above 50 1C. The thermal film on the native oxide layer on the steel surface then changes to a protective tribofilm by friction [6–8,16]. Based on these pieces of information, it seems that the function of zinc in a general steel/steel contact is simply to keep ZDDP molecules stable at room temperature and then make it unstable by replacing itself with other metal cations such as iron at an elevated temperature where ZDDP is expected to decompose. Recently, Mosey et al. [17] have shown by a computational simulation that zinc plays an important role in the polymerization of phosphate under high pressure. In any references, however, zinc is not expected to segregate 1543 on the surface of the substrate, which is the major difference from the case of the steel/iron oxide contact in this study. Preliminary experiments have shown that the adsorption characteristics of ZDDP were different between steel and the iron oxide, which will be analyzed in detail and reported in a future paper. The bottom layer of the tribofilm has a gradient composition (Fig. 5), which is probably a mixture of several compounds including sulfides, oxides, and phosphates. Although analysis on the chemical bonding information has not been made yet, there are several pieces of supporting evidence to identify the composition of the bottom layer of the tribofilm. Watanabe et al. [18] have conducted sulfidation experiments and reported that a porous FeS2 layer was formed on Fe3O4, while a dense FeS layer was formed on iron. They also reported that the reaction rate was much faster on Fe3O4. Therefore, FeS2 is one of the most probable forms of sulfides in the tribofilm. Fe3S4, which can be the same crystal system as Fe3O4, is another candidate although it is a less common compound. Recently, Wada et al. conducted detailed analyses on the composition of sulfides in the tribofilm of ZDDP on steel by using XANES [19]. They defined ‘‘sulfide ratio’’ as the ratio of the total sulfide without FeS2 to that with FeS2 and found a good correlation between the sulfide ratio and the friction coefficient (Fig. 10). They concluded that the friction is lower when more FeS2 is formed, although no scientific background has been given yet. The low friction observed on the iron oxide is possibly related to FeS2. The amount of oxygen in the bottom layer is almost the same as that in the iron oxide layer (Fig. 7). Oxygen near the middle part of the layer is probably a part of phosphates, while that near the substrate is in the form of Fe3O4 and FeO. To date, no information is available regarding the effect of oxides mixed with sulfides on the friction coefficient, which will be a subject of future study. Fig. 10. Effect of sulfides in tribofilm on friction coefficient (adopted from [19]). ARTICLE IN PRESS 1544 K. Ito et al. / Tribology International 39 (2006) 1538–1544 Generally, sulfur content in ZDDP tribofilm is much smaller than that in ZDDP [16]. Sulfur compounds that originated from the decomposition or reaction products of ZDDP are mostly released in oil. In the case of internal combustion engines, a part of such sulfur compounds is burnt in cylinder and lost (lube oil consumption), which flows to the downstream of the exhaust system and cause the deterioration of NOx reduction catalysts. In other words, sulfur compounds that can prevent wear or seizure are wasted and cause another problem. The improvement in the engine design for smaller lube oil consumption is being made, but it cannot be prevented completely. More effective use of sulfur in ZDDP is necessary if ZDDP cannot be replaced with others. The formation of the relatively thick and continuous layer of sulfur compounds on the iron oxide may lead to it. With the iron oxide by the water-vapor treatment on the surface, a low friction can be achieved and the necessary amount of ZDDP in engine oil for low wear may be reduced. 5. Conclusions An iron oxide layer that contains about 50% of Fe3O4 was formed by the water-vapor treatment on steel plates, which were slid against steel cylinders by using a cylinderon-plate-type tribotester in an oscillating motion under a mixed lubrication condition in PAO that contains 1% ZDDP. The following conclusions are made: 1. With iron oxide and a lubricant that contains ZDDP, the friction coefficient is below 0.06. 2. The iron oxide reacts with ZDDP as a result of sliding and forms a multi-layered tribofilm, which contains Zn and S in a layer of a gradient composition of Zn, Fe, S, P, and O on the surface. Detailed characteristics are summarized as follows: (1) The thickness of the tribofilm ranges from 30 to 130 nm. (2) The bottom layer of the tribofilm is 10–30 nm in thickness with a gradient composition of Zn, Fe, S, P, and O. (3) The distribution peaks of Zn and S exist in the bottom layer, which is unique to the iron oxide compared with the case of steel. Acknowledgments A part of this study was supported by the 21st century COE program, ‘‘International COE of Flow Dynamics.’’ The water-vapor treatment for plates was made with the cooperation of Nippon Piston Ring, Co. Ltd. and Hino Motors, Ltd. ZDDP was supplied by Asahi Denka Co., Ltd. The FIB and TEM observation were performed by Mr. Toru Itakura, Fujitsu Analysis Lab. 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