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Tribological properties of polyalphaolefin oil modified with nanocarbon additives
DIAMAT-06306; No of Pages 6 Diamond & Related Materials xxx (2014) xxx–xxx
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Tribological properties of polyalphaolefin oil modified with nanocarbon additives N. Nunn a,b, Z. Mahbooba a,c, M.G. Ivanov b,d, D.M. Ivanov d, D.W. Brenner c, O. Shenderova a,b,⁎ a
International Technology Center, Raleigh, NC, United States Adámas Nanotechnologies, Raleigh, NC, United States c North Carolina State University, Raleigh, NC, United States d Ural Federal University, Yekaterinburg, Russian Federation b
a r t i c l e
i n f o
Available online xxxx Keywords: Friction modifier Antiwear additive Nanotribology Solid lubricant additives Nanolubricants Nanodiamonds Carbon nanotubes Nanographite
a b s t r a c t Enhancing the tribological properties and performance of lubricants with nanoparticle additives is an active area of research. Results of block-on-ring experiments examining tribological properties of polyalphaolefin (PAO) oil containing small amount of nanocarbon additives are reported in this study. Comparative analysis of coefficient of friction (COF) and wear was performed for PAO oil containing nanodiamond particles, onion-like carbon (OLC), single/multiwall carbon nanotubes (SWNT/MWNT) or nanographene platelets (NGPs). The performance of PAO oil samples containing nanodiamond additives was considerably different from those containing sp2 nanocarbon additives. The presence of sp2 nanocarbons reduced the COF of PAO by ~8–12 times. This effect on the COF was accompanied by a reduction in wear with the addition of carbon nanotubes and NGP; however, the wear was slightly increased in the presence of OLC. The presence of nanodiamond reduced the COF by ~70×, and the wear was significantly increased. The performance of nanocarbon particles in PAO in combination with molybdenum dialkyldithiophosphate (MoDDP) was also investigated. The combination of polishing capability of nanodiamonds with the protective action of MoDDP resulted in the reduction of both wear and COF as compared to pure PAO. In contrast, the combination of sp2 nanocarbons with MoDDP caused degradation of the PAO lubrication performance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Current oil-based technologies, which were developed with a focus on wear elimination, rely heavily on sulfur, phosphorous and/or chlorine based additives that have a propensity for bioaccumulation and environmental toxicity [1]. With the push towards more environmentally friendly components, modern oils, in addition to minimizing wear, must enhance fuel economy and the amounts of hazardous components used [2]. Therefore, new approaches for advanced lubricants are being sought, and new antifriction (AF) and antiwear (AW) additives are being developed. There have been a number of recent studies that have looked at the effect of adding various classes of nanoparticles to oil-based lubricants (i.e. “nanolubricants”) [3]. These classes have included metals and their oxides, molybdenum disulfide, tungsten disulfide, metal borates, fluorinated compounds, fullerenes [3–9], graphitic nanoparticles, and nanodiamond [3,10–21]. In general, these studies report significant reductions in friction (typically 10–20%) and wear, although the proposed mechanisms by which these effects are achieved differ. Mechanisms include changes in lubricant viscosity and thermal transport properties, the formation of protective surface films, and creation of smoother surfaces by physical removal of surface ⁎ Corresponding author at: Adámas Nanotechnologies, Inc., 8100 Brownleihg Dr. Suire 120, Raleigh, NC 27617, United States.
asperities or by filling in spaces between asperities. Aside from the “boundary lubrication” regime, benefits of nanolubricants were recently demonstrated in elastohydrodynamic lubrication, where reduced surface roughness in rolling contact was achieved through polishing by nanoparticles [9]. Ultrafine carbon-based additives play a special role among other nanomaterials due to their high biocompatibility and resulting reduced environmental impact; hence, they can be classified as “green” additives. Our group recently developed formulations of detonation nanodiamond (DND) particles in oils with particle sizes of nanodiamonds as small as 5–10 nm with exceptional colloidal stability [16–21]. The DND is introduced into the motor oil along with other typical antifriction and antiwear additives (but at much lower treatment rates than recommended), forming a synergistic composition. The coefficient of friction of a variety of widely used motor oils, as determined by block-on-ring tests, is reduced by a factor of up to 10 times following the addition of a DND-based additive [20]. The dramatic drop in the coefficient of friction is surprising in light of the tests being conducted with motor oils of class API SN or CJ-4, which already provide a high level of performance. Reduction in friction is accompanied by a 20–30% reduction in wear in laboratory tests [20]. OEM approved bench fuel efficiency tests on engines of passenger cars demonstrated up to a 4.5% fuel efficiency improvement by a top treatment of Mobil 5W30 SN with the nanodiamond-based additive developed for gasoline engine oils [19].
http://dx.doi.org/10.1016/j.diamond.2014.09.003 0925-9635/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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Based on our previous studies of the friction surfaces in tests performed with Mobil 5W30 oil containing DND, it was concluded that DND provides a significant polishing effect [20]. While results of using DND-based additives in fully formulated oils are very encouraging, mechanisms of DND action are, as of yet, unknown. The compositions of fully formulated motor oils are rather complicated, containing up to 20% of a wide variety of additives including detergents, dispersants, antioxidants, anticorrosion agents, AW/AF additives, and other components [22]. To reveal basic mechanisms of DND performance, simplified systems with well-known composition should be studied. In the present study, we used block-on-ring, high-load, long duration experiments to examine tribological properties of polyalphaolefin (PAO) oil containing small amounts of nanodiamond additives. PAO is a synthetic high performance oil often used as a base in modern lubricants. Based on the observation of synergism of DND with MoDDP in fully formulated oils [18–21], a set of experiments was also performed on testing MoDDP in combination with DND in PAO. Another set of experiments was performed with small addition of 0-, 1- and 2-dimensional nanocarbon particles in PAO. Because lubrication mechanisms are presumably different for sp3 and sp2 carbon particles, a difference in the performance of the nanocarbon additives was expected. In addition, sp2 nanocarbon additives are prospective candidates for nanolubricants on their own [23–25].
2. Experimental Detonation nanodiamond was synthesized by detonation of highenergy explosives, using an oxygen-deficient mixture of trinitrotoluene (TNT) with hexogen (RDX), in a closed steel chamber in the presence of a cooling medium [9]. The primary particle size of the DND is approximately 4–5 nm, and the sizes of the as-produced aggregated particles range from 200 to 300 nm. In this work, the initial polydisperse DND was de-agglomerated and fractionated. The resulting particles had average volumetric sizes of 5 nm when dispersed in de-ionized water and measured via dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instruments Ltd.). Stable, transparent, colloidal dispersions of DND in PAO oil (Spectrasyn-6, Exxon) were formulated using ultrasonic treatment in the presence of a proprietary dispersant. DND-based additives were used alone or in combination with molybdenum dialkyldithiophosphate (molybdenum di(2-ethylhexyl) phosphorodithioate). For tribotests, the DND additive initially prepared in PAO at 1 wt.% was mixed with pure PAO oil in ratios resulting in DND concentration of 0.01–0.015 wt.% in the final formulation. For production of OLC, polydisperse DND material was fractionated by centrifugation into 40 nm and 200 nm aggregates. These DND fractions were heated in vacuum (10−4 Torr) at 1650 °C for 3 h, producing OLC with aggregate sizes approximately corresponding to the sizes of the starting DND fractions (i.e. 40 nm and 200 nm). Multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT), purchased from Nanostructured & Amorphous Materials Inc., had a diameter of 8–15 nm and a length of 10–50 μm, and a diameter of 1–2 nm and a length of 5–30 μm, respectively. Nanographene platelets were purchased from Angstron Materials, Inc. (product #: N006-010-P). Platelet particles were 10–20 nm thick and had lateral dimensions of approximately 14 μm. The carbon and oxygen content was around 97.0% and 1.5%, correspondingly. Carbon-based nano-additives including MWNT, SWNT, 200 nm aggregates of onion-like carbon (OLC-L), 40 nm aggregates of onionlike carbon (OLC-S) and NGP were prepared at 0.5 wt.% concentration in PAO oil using a proprietary dispersant and then mixed at a ratio of 1:60 with pure PAO oil or with a PAO/MoDDP mixture for tribological tests. Part of the sample was left for a qualitative observation of colloidal stability. Colloidal stability of the samples, along with the stability of the same additives in industrial Mobil 5W30 oil is summarized in Table 1. After 24 h, only the SWNT and NGP formulations were unstable in PAO, and only NGPs were unstable in Mobil oil. The unstable samples
Table 1 Colloidal stability of carbon-based nano-additives in PAO, PAO with MoDDP and Mobil 5W30 oil. All combinations labeled as “stable” had suspensions with almost no sediment after 24 h, and all labeled “not stable” had a visual amount of sediment. Sample
In PAO
1:60 in PAO with M
1:60 in Mobil with M
MWNT SWNT OLC-L OLC-S NGP
Stable Not stable Stable Stable Not stable
Stable Not stable Stable Stable Not stable
Stable Stable Stable Stable Not stable
were still used in testing, but only the upper part of the sample (not containing the sediment) was used. Testing of the performance of the nanocarbon-based additives in PAO oil was performed using a block on ring test apparatus UMT-3 produced by CETR, USA (Fig. 1). A block (henceforth referred to as “sample/ s”) made from SAE 01 tool steel with hardness of Rc = 30 and flat friction surfaces with a mean surface roughness of 0.2 μm was used. The stainless steel rings used were 35 mm in a diameter and had a hardness of Rc = 60 and an average roughness of 0.3 μm. A rotational velocity of 200 rpm and a load of 300 N (corresponding to a contact pressure 220 MPa) were used for all tests. Each test lasted 5 h. During tests, the bottom part of the ring was immersed in oil (Fig. 1). Duplicate tests were performed for the majority of samples. Samples were cleaned with isopropanol (IPA) for further analysis after tribotests were completed. Scar profiles were characterized using an Alpha-Step IQ stylusbased surface profiler. Roughness maps and values were measured using a Zygo NewView 5000 3D optical profiler for wear scars in blocks and for ring surfaces. For each sample, roughness maps were collected at 3 areas, left, right and center, along a scar. On each map, roughness
Fig. 1. Schematics of the experimental setup for the block on ring tribological test.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
N. Nunn et al. / Diamond & Related Materials xxx (2014) xxx–xxx
was measured in 5 different spots over a distance of 50 μm. These roughness measurements were taken across areas that showed mostly uniform roughness so as to avoid excessive statistical error. The worn block surfaces were also analyzed with a Nikon optical microscope (Nikon Optiphot).
Table 2 Tribological characteristics of PAO-6 containing additives of DND and MoDDP. Average COF corresponds to the COF averaged over 5 h test. End COF corresponds to the friction coefficient at the end of the 5 h test. Error bar for the end COF measurements is ~0.005 and for wear scar measurements ~0.002 mm, correspondingly. Composition
Scar area, μm2
Average COF (end COF)
RMS scar
RMS ring
PAO 0.015% ND 0.15% M 0.015 ND/0.15% M 0.01% ND/0.1% M
9611 24,308 3823 3185 7012
0.12 (0.1) 0.037 (0.002) 0.058 (0.024) 0.099 (0.09) 0.045 (0.028)
0.084 0.106 0.053 0.085 0.055
0.223 0.283 0.202 0.298 0.195
3. Results Tribological characteristics (COF and scar area) of PAO oil containing nanodiamonds or different types of sp2 nanocarbon particles are presented below. Tests included: (i) PAO containing only nanocarbons, (ii) PAO containing nanocarbons in combination with MoDDP, and (iii) PAO containing only MoDDP. For nanodiamonds, in addition to friction and wear, roughness of the wear scar in a block and ring surface was also analyzed. 3.1. Tests of PAO containing ND and ND/MoDDP additives Plotted in Fig. 2 is the COF as a function of time for the pure PAO, PAO with DND, and PAO with the DND/MoDDP mixture. The results of the measurements of the COF and wear scar area for the samples are summarized in Table 2 and Fig. 3. For pure PAO oil, the COF did not appreciably change over time, remaining close to ~ 0.14. As can be seen from Fig. 2, a significant decrease in the COF over time was observed for the PAO containing 0.015% NDs. A sharp COF reduction took place at around 2 h during the test and stayed around an extremely low value ~ 0.002 (98.5% lower than the COF for pure PAO) for the remainder of the test. However, the resulting scar area at the addition of NDs was significantly increased by ~2.5 times (Table 2). The addition of only the MoDDP additive (0.15 wt.%) into PAO resulted in a decrease of the COF over time to a value ~0.024. A gradual decrease in COF was observed, as opposed to the sharp COF decrease observed in the case with ND. The addition of only MoDDP to PAO resulted in a more than 2× decrease in the wear scar in comparison with pure PAO (Table 2). The results of the tests for a mixture of DND and MoDDP are shown in Figs. 2 and 3, and Table 2. The DND/MoDDP mixtures were prepared in a ratio of 1 to 10 by weight (DND to MoDDP). Two concentrations of the additive combinations in PAO were tested, 0.015% DND with 0.15% MoDDP and 0.01% DND with 0.1% MoDDP. From Figs. 2 and 3, and Table 2, it can be seen that the results of the tests are sensitive to the amount of the additives, but both compositions show significantly improved characteristics as compared to pure PAO oil. The scar area is further reduced at combinations of 0.015% ND and 0.15% MoDDP as compared to 0.15% MoDDP alone; however, the COF reduction is insignificant for this combination. At lower amounts of nanoadditives, 0.01% ND and 0.1% MoDDP, the COF is low, ~0.028, but the scar size reduction is only ~27% as compared to pure PAO. Shown in Fig. 4 are optical micrographs of the scars taken at the same magnification. A well-pronounced, grooved texture was observed on the blocks initially because of vendor grinding/polishing. These grooves were directed parallel to the rotational direction of the rings. These grooves can be clearly seen on the undamaged parts (along the scar periphery) of the surface of the blocks in Fig. 4. These well-pronounced grooves were also observed on the
3
± ± ± ± ±
0.031 0.035 0.024 0.015 0.022
± ± ± ± ±
0.025 0.023 0.031 0.031 0.034
rings. Roughness was measured perpendicular to the grooves on the worn block and ring surfaces. Measurements of the roughness were averaged over three different spots (left, center, and right) and five data points for each spot along the scars of each block. The same measurement scheme was used for the rings, and the results for both blocks and rings are summarized in Table 2. Data from Table 2 demonstrate the lowest RMS roughness for the samples lubricated with 0.01% DND/0.1% MoDDP and 0.15% MoDDP in PAO. RMS roughness of the scar surfaces on the blocks and corresponding rings tested with 0.015% DND additives is about 26% higher than the roughness of the scars and rings observed in experiments performed with pure PAO. These trends are clearly seen from the roughness maps presented in Fig. 4. Roughness maps of the scar surfaces show high uniformity and smoothness for the scars formed during lubrication with 0.01% DND/0.1% MoDDP additive and 0.15% MoDDP in PAO. Roughness within scars formed in the presence of high concentrations of NDs (0.015%) is highly nonuniform. Optical micrographs (not shown) of the wear scars revealed that the steel blocks in the tests using only PAO oil experienced noticeable wear with deep abrasive grooves caused by the removal of the material from the contact. On the steel blocks lubricated using only NDs, abrasive grooves were even more pronounced; however, regions between the grooves were highly polished. These highly polished regions were elongated along the sliding direction. There were very few signs of wear on the surface of a wear scar in a block lubricated using 0.01% DND with 0.1% MoDDP additive. The grooves from the initial surface topography were much less pronounced, and the surface was more smooth and uniform in all directions. 3.2. Tests of PAO containing sp2 nanocarbons and sp2/MoDDP additives Plotted in Fig. 5 is the COF as a function of time for pure PAO and PAO containing other nanocarbon additives. The results of the measurements of the COF and wear scar area for the samples are summarized in Table 3 and Fig. 6. As can be seen from Fig. 5, a significant decrease in the COF over time was observed for the PAO containing onion-like carbon (OLC), carbon nanotubes, and nanographene platelets. PAO containing MWNT showed the biggest reduction in the end COF (~0.013). Scars formed in the samples containing MWNT, SWNT, and graphite nanoflakes in PAO all showed a 15% or greater reduction in area as compared to pure PAO, while both OLC samples exhibited an increase.
Fig. 2. Friction coefficient as a function of time for pure PAO-6 oil and for the oil with addition of DND (0.015%), MoDDP (0.15%) or a mixture of DND and MoDDP (0.01% DND with 0.1% MoDDP). Test time is 5 h, load 30 kg, rotation rate 200 rpm.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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4. Discussion
Fig. 3. Tribological characteristics of PAO-6 containing additives of DND and MoDDP (Mo on the diagram): COF corresponding to the friction coefficient at the end of the 5 hour test, area of the wear scar and RMS for the scar surface. Dilutions of the mixture of ND and MoDDP with PAO in 1:66 and 1:100 ratios, as denoted on the figure, correspond to the concentrations of the additives in the samples 0.015% DND with 0.15% MoDDP and 0.01% DND with 0.1% MoDDP.
This demonstrates that using only sp2 carbon additives, the lubricating performance of PAO oil can be improved. Each sp2 carbon-based additive was tested in combination with 0.16% of MoDDP in PAO, similar to tests performed with NDs. Table 3 summarizes COF and scar data for each test. The scar area for the reference sample tested with pure PAO was 9611 μm2, and each test for sp2 carbon-based additive in combination with 0.16% of MoDDP showed a scar area higher than the reference. In the presence of MoDDP, the COF was higher for each of the nanocarbon additives as compared to the test run with the nanocarbons without MoDDP. This demonstrates that the sp2 nanocarbon-based additives are not compatible with MoDDP when used in PAO oil. Optical micrographs (not shown) of the wear scars in the tests using only sp2 nanocarbon particles revealed a very similar appearance of the worn surfaces for all three types of the nanoparticles: carbon onions, nanotubes and graphene platelets. The grinding scratches from the initial surface topography were hardly seen in the contact area. The surface was smooth and uniform, providing an impression that nanocarbon material was smeared over the surface. For the steel blocks lubricated using nanocarbons with MoDDP, the wear was high and a surface topography similar to that of the pure PAO sample was observed.
Previously, we observed that a significant improvement of both friction and wear took place for the DND in combination with MoDDP additive when applied to fully formulated Mobil 5W30 motor oil [20,21]. A 90% improvement in the COF of the Mobil oil was observed in 7 hour tests at 300 N load and 200 rpm rotational speed [20]. The reduction in COF was accompanied by a reduction in the wear scar area by more than a factor of two. SEM observations revealed that scar surfaces in the sample treated with DND are much smoother than those lubricated by pure 5W30 Mobil oil. Roughness of the scar surfaces for the blocks tested with DND additive was about 35% lower than the roughness of the scars observed for pure oil experiments. It was speculated that DND can enhance delivery of other AF/AW additives (like MoDDP) to the freshly polished surfaces and thus promote the formation of a robust protective tribofilm. Molybdenum di(2-ethylhexyl) phosphorodithioate is an oil-soluble organic molybdenum additive containing sulfur and phosphorus. It improves the tribological performance of lubricants by forming sulfides (especially MoS2), oxides and phosphates (FePO4) on the metal surfaces [26]. In the current study, the same type of the block-on-ring test at the same operational conditions and similar content of the additives was performed in pure PAO oil instead of a fully formulated oil. The abrasive nature of nanodiamonds was very pronounced, resulting in a combination of large wear and extremely low friction coefficient (0.002) that was achieved at ~ 2 h after beginning of the test. This suggests that the majority of the wear took place within the first hours of the test before super low friction conditions were established. The relatively large roughness measured across the grooves on the steel surfaces needs to be explained in the light of the very low COF. It should be noted that this roughness is measured perpendicular to the grooves, while the rotational direction is along the grooves. Measurement of the roughness along the micronwide grooves with an optical profilometer is problematic due to alignment problem. Based on the optical analysis of the worn blocks, it can be currently assumed that deep micro-scars were formed at the initial stage of polishing (“work in” regime) while at the later stage, superpolished regions with enhanced hardness were formed on the steel surfaces along the sliding direction. While the micro-scars are responsible for the high roughness across the sliding direction, the COF is very low due to the existence of highly polished ridges occurring at the contact between sliding surfaces. A more detailed study, combining electron microscopy and atomic force microscopy will be required to explain this extremely low COF accompanied by large wear and roughness of the ND/PAO sample. Another interesting feature specific to ND used on its own is the abrupt decrease of the COF (Fig. 2) that was observed in multiple repeated experiments. This may correspond to the transition
Fig. 4. Optical micrographs of the scars (up row) and roughness maps of the scar surfaces (bottom row) formed in H30 block samples during tests with and without DND and MoDDP additives in PAO oil.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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Fig. 5. Friction coefficient as a function of time for pure PAO-6 oil and for the oil with OLC-L and OLC-S additives (a) and SWCNT, MWCNT and NGP additives (b). Test time is 5 h, load 30 kg, rotation rate 200 rpm.
from the “work in” regime to the highly polished regime; however, why this transition is so abrupt is not currently clear. The use of MoDDP along with ND resulted in the improvement of both friction and wear; however, the COF was not reduced as dramatically as was the case with the ND alone. The results are highly sensitive to the concentration of the ND and MoDDP in PAO, perhaps due to the different mechanisms of action of these two additives — polishing from the ND and the formation of a protective tribofilm from the MoDDP. A possible synergistic mechanism can be as follows. The MoDDP forms protective phosphate film initially in the regions between asperities, while NDs contribute to the efficient polishing of the asperities. After some time, a well-polished surface protected by a phosphate film is formed. NDs may be contained within this film, enhancing its strength. In the absence of MoDDP, NDs wear both asperities and unprotected regions between asperities, resulting in high wear — as observed in the present study. A concentration of 0.01% ND and 0.1% MoDDP seems close to optimal, providing high performance in combination with low MoDDP content, which is valuable from an environmental standpoint. Obviously, the composition of ND and MoDDP in PAO can be further optimized. In the fully formulated oil, the synergism of NDs and MoDDP (ZDDP) is more pronounced than in pure PAO when compared to the effect observed with the addition of only ND or only MoDDP [20]. This can be due to the presence of additional AF/AW additives in the motor oil for which NDs have high compatibility. Perhaps a protective tribofilm formed on the surfaces of sliding steel contacts has enhanced strength due to the presence of NDs than compared to those formed in the absence of NDs.
Another interesting aspect is related to well-known reactions between diamond and ferrous materials (for example, diamond tools are not used for cutting steel). This raises a question as to why ND oil additive works so well for enhanced lubricating performance. Perhaps the temperature at the sliding contacts is not high enough to cause a reaction between ND and Fe (N700 °C is required for bulk diamond [27]), and a formed tribofilm isolates NDs from the steel. However, it is also possible that a conversion of NDs to sp2 carbon does occur during lubrication of steel surfaces due to the catalytic effect of Fe, but this requires further investigation. Lubrication with sp2 carbon particles with a layered structure is also a popular topic of research [23–25]. Their strong intra-planar and weak inter-planar bonds have been recognized as offering a very good lubrication potential. In this study, using onions, carbon nanotubes and graphene platelets as additives to PAO, we observed significantly decreased the friction and wear compared to pure PAO. Friction surfaces were very smooth and uniform, regardless of the different morphologies of nanocarbons used (i.e. nanotubes, onions, or platelets). One possible explanation for the positive effect of sp2 carbon is due to the formation of a lubricious film based on the third-body material transfer through delamination and exfoliation of the layered carbon structure to the contact sliding surfaces, especially under high contact loads. Nevertheless, the combination of sp2 nanocarbon with MoDDP was detrimental for the lubrication performance. This can be attributed to the competing mechanisms of the tribofilm formation between sp2 layers and MoDDP. This raises concern about possible antagonism between
Table 3 Tribological characteristics of PAO-6 containing additives of sp2 nanocarbons and MoDDP. Average COF corresponds to the COF averaged over the 5 hour test, end COF corresponds to the friction coefficient at the end of the 5 hour test. Error bar for the end COF measurements is ~0.005 and for wear scar measurements ~0.002 mm, correspondingly. Composition
Scar area in PAO, μm2
Average COF (end COF) in PAO
Scar area in PAO with MoDDP 1:60, μm2
Average COF (end COF) in PAO with MoDDP 1:60
Pure PAO PAO with MoDDP MWNT SWNT OLC-L OLC-S Nanographene platelets
9611 – 7151 7790 10,829 12,100 6344
0.12 ± 0.025 (0.12) – 0.056 ± 0.035 (0.013) 0.063 ± 0.025 (0.034) 0.078 ± 0.007 (0.015) 0.081 ± 0.045 (0.016) 0.062 ± 0.035 (0.02)
– 3823 11,874 16,885 16,686 10,389 10,761
– 0.057 0.081 0.083 0.063 0.093 0.115
± ± ± ± ± ±
0.029 (0.024) 0.009 (0.072) 0.013 (0.07) 0.0278 (0.026) 0.01 (0.06) 0.007 (0.107)
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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performances were observed emphasizing different mechanisms of lubrication for sp3 and sp2 nanocarbons. In addition, the performance of nanocarbon particles in PAO in combination with molybdenum dialkyldithiophosphate was also investigated for the first time. Acknowledgments The present work was supported by the U.S. National Science Foundation under grant no. CMMI-1229889 and by the RFBR (grant 12-03-93937-G8_а). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. References Fig. 6. Tribological characteristics of PAO-6 containing additives of nanocarbons and MoDDP (Mo): COF corresponding to the friction coefficient at the end of the 5 hour test and area of the wear scar.
sp2 carbon additives and existing AW/AF components in modern oils and deserves further investigation. 5. Conclusion With the push towards more environmentally friendly, yet just as effective, lubricants being a major priority, this study sought to follow up our previous [10,16–21] work and the work of others [11–15, 23–25] with nanocarbon based nanolubricants. Whereas previous work sought to investigate the effectiveness of nanocarbon additives (primarily nanodiamond) in commercially available, synthetic motor oils [16–21], this study was primarily interested in investigating the synergy and tribological mechanisms between nanocarbon additives (sp2 and sp3), MoDDP, and an oil base (PAO). Thus, this study was performed in a more simplified system in the absence of a number of complex additives that are present in commercially available motor oils. Several important conclusions can be drawn. On their own, NDs caused a dramatic drop in the COF as compared to pure PAO and all other additives measured; however, the abrasive polishing nature of the NDs also caused a dramatic increase in wear. When NDs were paired with MoDDP, there was clearly a synergy between the two; however, it is concentration sensitive, with an apparently optimal concentration being around 0.01% ND/0.1% MoDDP. Nevertheless, the abrasive nature of the NDs still caused higher wear (albeit a lower average COF), and this is possibly due to the synergistic behavior of the polishing from the NDs and the formation of tribofilms from the MoDDP. Despite the higher wear, overall, the performance of the 0.01% ND/0.1% MoDDP mixture was comparable to the 0.15% MoDDP mixture, suggesting that the presence of NDs can reduce the amount of environmentally unfriendly MoDDP needed. Aside from the MWNTs, it appeared that most of the sp2 nanocarbon additives were less effective than NDs. Although the COFs of the sp2/PAO mixtures were comparable to those of MoDDP/PAO, the wear was higher for all cases (though less than ND/ PAO). It would seem that there is an antagonistic mechanism between the sp2 additives and MoDDP, thus limiting the effectiveness of the lubricant. At this stage, all proposed mechanisms are theoretical; therefore, more investigation is required (electron microscopy and atomic force microscopy) to further understand the true mechanisms behind the behavior of these nanolubricants. Prime novelty statement For the first time, comparative analysis of coefficient of friction and wear was performed for PAO oil containing sp3 or sp2 carbon nanoparticles: nanodiamond particles, onion-like carbon, single/multiwall carbon nanotubes or nanographene platelets. Considerably different
[1] M. Martin, N. Ohmae, Nanolubricants, John Wiley & Sons, Ltd., England, 2008, ISBN 978-0-470-06552-5. [2] A. Neville, A. Morina, T. Haque, M. Voong, Compatibility between tribological surfaces and lubricant additives — how friction and wear reduction can be controlled by surface/lube synergies, Tribol. Int. 40 (2007) 1680–1695. [3] L. Gara, Q. Zou, Friction and wear characteristics of oil-based ZnO nanofluids, Tribol. Trans. 56 (2) (2013) 236–244. [4] L. Sun, X.J. Tao, Y.B. Zhao, Z.J. Zhang, Synthesis and tribology properties of stearatecoated Ag nanoparticles, Tribol. Trans. 53 (2) (2010) 174–178. [5] M. Mosleh, N. Atnafu, J.H. Belk, O.M. Nobles, Modification of sheet metal forming fluids with dispersed nanoparticles for improved lubrication, Wear 267 (2009) 1220–1225. [6] L. Rapoport, V. Leshchinsky, I. Lapsker, Yu. Volovik, O. Nepomnyashchy, M. Lvovsky, R. Popovitz-Biro, Y. Feldman, R. Tenne, Tribological properties of WS2 nanoparticles under mixed lubrication, Wear 255 (2003) 785–793. [7] M. Mosleh, M. Ghaderi, Deagglomeration of transfer film in metal contacts using nanolubricants, Tribol. Trans. 55 (1) (2012) 52–58. [8] M. Mosleh, K.A. Shirvani, In-situ nanopolishing by nanolubricants for enhanced elastohydrodynamic lubrication, Wear 301 (2013) 137–141. [9] V.N. Mochalin, O. Shenderova, D. Ho, Y. Gogotsi, The properties and applications of nanodiamonds, Nat. Nanotechnol. 7 (1) (2012) 11–23. [10] M. Ivanov, D. Ivanov, Nanodiamond nanoparticles as additives to lubricants, in: O. Shenderova, D. Gruen (Eds.), Ch.8 in Ultrananocrystalline Diamond, 2nd ed. Elsevier, UK, ISBN: 9781437734652, 2012. [11] V.Yu. Dolmatov, Detonation nanodiamonds in oils and lubricants, J. Superhard Mater. 32 (2010) 14–20. [12] V.I. Zhornik, V.A. Kukareko, M.A. Belotserkovsky, Tribomechanical Modification of Friction Surface by Running-in in Lubricants with Nano-sized Diamonds, Nova Science Publishers, 2011. [13] V.E. Red'kin, Lubricants with ultradisperse diamond-graphite powder, Chem. Technol. Fuels Oils 40 (3) (2004) 164–179. [14] A. Puzyr, A. Burov, G. Selyutin, V. Voroshilov, V. Bondar, Modified nanodiamonds as antiwear additives to commercial oils, Tribol. Trans. 55 (1) (2012) 149–154. [15] M. Shen, J. Luo, S. Wen, The tribological properties of oils added with diamond nanoparticles, Tribol. Trans. 44 (3) (2001) 494–498. [16] M.G. Ivanov, V.V. Kharlamov, V.M. Buznik, D.M. Ivanov, S.V. Pavlyshko, A.K. Tsvetnikov, Tribological properties of the grease containing polytetrafluorethylene and ultrafine diamond, Friction Wear 25 (1) (2004) 99–103. [17] M.G. Ivanov, S.V. Pavlyshko, D.M. Ivanov, I. Petrov, O. Shenderova, Synergistic compositions of colloidal nanodiamond as lubricant-additive, JVST B 28 (4) (2010) 869–877. [18] M.G. Ivanov, S.V. Pavlyshko, D.M. Ivanov, I. Petrov, G. McGuire, O. Shenderova, Nanodiamond particles as additives in lubricants, Mater. Res. Soc. Symp. Proc. 1203 (1203-J17-16) (2010) 89–94. [19] M.G. Ivanov, D.M. Ivanov, S.V. Pavlyshko, I. Petrov, A. Vargas, G. McGuire, O. Shenderova, Nanodiamond-based nanolubricants, Fuller. Nanotubes Carbon Nanostruct. 20 (2012) 606–610. [20] O. Shenderova, A. Vargas, S. Turner, D.M. Ivanov, M.G. Ivanov, Nanodiamond-based nanolubricants: investigation of friction surfaces, Tribol. Trans. 57 (6) (2014) 1051–1057, http://dx.doi.org/10.1080/10402004.2014.933933. [21] M. Ivanov, Z. Mahbooba, D. Ivanov, S. Smirnov, S. Pavlyshko, E. Osawa, D. Brenner, O. Shenderova, Nanodiamond-based oil lubricants on steel–steel and stainless steel — hard alloy high load contact: investigation of friction surfaces, Nanosyst. Phys. Chem. Math. 5 (1) (2014) 1–6. [22] S.C. Tung, M.L. McMillan, E.P. Becker, S.E. Schwartz, Handbook of Lubrication and Tribology, Taylor & Francis Group, 2006. [23] L. Joly-Pottuz, B. Vacher, N. Ohmae, J.M. Martin, T. Epicier, Anti-wear and friction reducing mechanisms of carbon nano-onions as lubricant additives, Tribol. Lett. 30 (2008) 69–80. [24] D. Berman, A. Erdemir, A. Sumant, Graphene: a new emerging lubricant, Mater. Today 17 (2014) 1. [25] J. Kogovsek, M. Kalin, Various MoS2-, WS2- and C-based micro- and nanoparticles in boundary lubrication, Tribol. Lett. 53 (2014) 585–597. [26] Y. Yamamoto, S. Gondo, T. Kamakura, N. Tanaka, Frictional characteristics of molybdenum dithiophosphates, Wear 112 (1986) 79–87. [27] J. Field (Ed.), Properties of Natural and Synthetic Diamond, Academic Press, 1992.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Tribological properties of polyalphaolefin oil modified with nanocarbon additives N. Nunn a,b, Z. Mahbooba a,c, M.G. Ivanov b,d, D.M. Ivanov d, D.W. Brenner c, O. Shenderova a,b,⁎ a
International Technology Center, Raleigh, NC, United States Adámas Nanotechnologies, Raleigh, NC, United States c North Carolina State University, Raleigh, NC, United States d Ural Federal University, Yekaterinburg, Russian Federation b
a r t i c l e
i n f o
Available online xxxx Keywords: Friction modifier Antiwear additive Nanotribology Solid lubricant additives Nanolubricants Nanodiamonds Carbon nanotubes Nanographite
a b s t r a c t Enhancing the tribological properties and performance of lubricants with nanoparticle additives is an active area of research. Results of block-on-ring experiments examining tribological properties of polyalphaolefin (PAO) oil containing small amount of nanocarbon additives are reported in this study. Comparative analysis of coefficient of friction (COF) and wear was performed for PAO oil containing nanodiamond particles, onion-like carbon (OLC), single/multiwall carbon nanotubes (SWNT/MWNT) or nanographene platelets (NGPs). The performance of PAO oil samples containing nanodiamond additives was considerably different from those containing sp2 nanocarbon additives. The presence of sp2 nanocarbons reduced the COF of PAO by ~8–12 times. This effect on the COF was accompanied by a reduction in wear with the addition of carbon nanotubes and NGP; however, the wear was slightly increased in the presence of OLC. The presence of nanodiamond reduced the COF by ~70×, and the wear was significantly increased. The performance of nanocarbon particles in PAO in combination with molybdenum dialkyldithiophosphate (MoDDP) was also investigated. The combination of polishing capability of nanodiamonds with the protective action of MoDDP resulted in the reduction of both wear and COF as compared to pure PAO. In contrast, the combination of sp2 nanocarbons with MoDDP caused degradation of the PAO lubrication performance. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Current oil-based technologies, which were developed with a focus on wear elimination, rely heavily on sulfur, phosphorous and/or chlorine based additives that have a propensity for bioaccumulation and environmental toxicity [1]. With the push towards more environmentally friendly components, modern oils, in addition to minimizing wear, must enhance fuel economy and the amounts of hazardous components used [2]. Therefore, new approaches for advanced lubricants are being sought, and new antifriction (AF) and antiwear (AW) additives are being developed. There have been a number of recent studies that have looked at the effect of adding various classes of nanoparticles to oil-based lubricants (i.e. “nanolubricants”) [3]. These classes have included metals and their oxides, molybdenum disulfide, tungsten disulfide, metal borates, fluorinated compounds, fullerenes [3–9], graphitic nanoparticles, and nanodiamond [3,10–21]. In general, these studies report significant reductions in friction (typically 10–20%) and wear, although the proposed mechanisms by which these effects are achieved differ. Mechanisms include changes in lubricant viscosity and thermal transport properties, the formation of protective surface films, and creation of smoother surfaces by physical removal of surface ⁎ Corresponding author at: Adámas Nanotechnologies, Inc., 8100 Brownleihg Dr. Suire 120, Raleigh, NC 27617, United States.
asperities or by filling in spaces between asperities. Aside from the “boundary lubrication” regime, benefits of nanolubricants were recently demonstrated in elastohydrodynamic lubrication, where reduced surface roughness in rolling contact was achieved through polishing by nanoparticles [9]. Ultrafine carbon-based additives play a special role among other nanomaterials due to their high biocompatibility and resulting reduced environmental impact; hence, they can be classified as “green” additives. Our group recently developed formulations of detonation nanodiamond (DND) particles in oils with particle sizes of nanodiamonds as small as 5–10 nm with exceptional colloidal stability [16–21]. The DND is introduced into the motor oil along with other typical antifriction and antiwear additives (but at much lower treatment rates than recommended), forming a synergistic composition. The coefficient of friction of a variety of widely used motor oils, as determined by block-on-ring tests, is reduced by a factor of up to 10 times following the addition of a DND-based additive [20]. The dramatic drop in the coefficient of friction is surprising in light of the tests being conducted with motor oils of class API SN or CJ-4, which already provide a high level of performance. Reduction in friction is accompanied by a 20–30% reduction in wear in laboratory tests [20]. OEM approved bench fuel efficiency tests on engines of passenger cars demonstrated up to a 4.5% fuel efficiency improvement by a top treatment of Mobil 5W30 SN with the nanodiamond-based additive developed for gasoline engine oils [19].
http://dx.doi.org/10.1016/j.diamond.2014.09.003 0925-9635/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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Based on our previous studies of the friction surfaces in tests performed with Mobil 5W30 oil containing DND, it was concluded that DND provides a significant polishing effect [20]. While results of using DND-based additives in fully formulated oils are very encouraging, mechanisms of DND action are, as of yet, unknown. The compositions of fully formulated motor oils are rather complicated, containing up to 20% of a wide variety of additives including detergents, dispersants, antioxidants, anticorrosion agents, AW/AF additives, and other components [22]. To reveal basic mechanisms of DND performance, simplified systems with well-known composition should be studied. In the present study, we used block-on-ring, high-load, long duration experiments to examine tribological properties of polyalphaolefin (PAO) oil containing small amounts of nanodiamond additives. PAO is a synthetic high performance oil often used as a base in modern lubricants. Based on the observation of synergism of DND with MoDDP in fully formulated oils [18–21], a set of experiments was also performed on testing MoDDP in combination with DND in PAO. Another set of experiments was performed with small addition of 0-, 1- and 2-dimensional nanocarbon particles in PAO. Because lubrication mechanisms are presumably different for sp3 and sp2 carbon particles, a difference in the performance of the nanocarbon additives was expected. In addition, sp2 nanocarbon additives are prospective candidates for nanolubricants on their own [23–25].
2. Experimental Detonation nanodiamond was synthesized by detonation of highenergy explosives, using an oxygen-deficient mixture of trinitrotoluene (TNT) with hexogen (RDX), in a closed steel chamber in the presence of a cooling medium [9]. The primary particle size of the DND is approximately 4–5 nm, and the sizes of the as-produced aggregated particles range from 200 to 300 nm. In this work, the initial polydisperse DND was de-agglomerated and fractionated. The resulting particles had average volumetric sizes of 5 nm when dispersed in de-ionized water and measured via dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instruments Ltd.). Stable, transparent, colloidal dispersions of DND in PAO oil (Spectrasyn-6, Exxon) were formulated using ultrasonic treatment in the presence of a proprietary dispersant. DND-based additives were used alone or in combination with molybdenum dialkyldithiophosphate (molybdenum di(2-ethylhexyl) phosphorodithioate). For tribotests, the DND additive initially prepared in PAO at 1 wt.% was mixed with pure PAO oil in ratios resulting in DND concentration of 0.01–0.015 wt.% in the final formulation. For production of OLC, polydisperse DND material was fractionated by centrifugation into 40 nm and 200 nm aggregates. These DND fractions were heated in vacuum (10−4 Torr) at 1650 °C for 3 h, producing OLC with aggregate sizes approximately corresponding to the sizes of the starting DND fractions (i.e. 40 nm and 200 nm). Multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT), purchased from Nanostructured & Amorphous Materials Inc., had a diameter of 8–15 nm and a length of 10–50 μm, and a diameter of 1–2 nm and a length of 5–30 μm, respectively. Nanographene platelets were purchased from Angstron Materials, Inc. (product #: N006-010-P). Platelet particles were 10–20 nm thick and had lateral dimensions of approximately 14 μm. The carbon and oxygen content was around 97.0% and 1.5%, correspondingly. Carbon-based nano-additives including MWNT, SWNT, 200 nm aggregates of onion-like carbon (OLC-L), 40 nm aggregates of onionlike carbon (OLC-S) and NGP were prepared at 0.5 wt.% concentration in PAO oil using a proprietary dispersant and then mixed at a ratio of 1:60 with pure PAO oil or with a PAO/MoDDP mixture for tribological tests. Part of the sample was left for a qualitative observation of colloidal stability. Colloidal stability of the samples, along with the stability of the same additives in industrial Mobil 5W30 oil is summarized in Table 1. After 24 h, only the SWNT and NGP formulations were unstable in PAO, and only NGPs were unstable in Mobil oil. The unstable samples
Table 1 Colloidal stability of carbon-based nano-additives in PAO, PAO with MoDDP and Mobil 5W30 oil. All combinations labeled as “stable” had suspensions with almost no sediment after 24 h, and all labeled “not stable” had a visual amount of sediment. Sample
In PAO
1:60 in PAO with M
1:60 in Mobil with M
MWNT SWNT OLC-L OLC-S NGP
Stable Not stable Stable Stable Not stable
Stable Not stable Stable Stable Not stable
Stable Stable Stable Stable Not stable
were still used in testing, but only the upper part of the sample (not containing the sediment) was used. Testing of the performance of the nanocarbon-based additives in PAO oil was performed using a block on ring test apparatus UMT-3 produced by CETR, USA (Fig. 1). A block (henceforth referred to as “sample/ s”) made from SAE 01 tool steel with hardness of Rc = 30 and flat friction surfaces with a mean surface roughness of 0.2 μm was used. The stainless steel rings used were 35 mm in a diameter and had a hardness of Rc = 60 and an average roughness of 0.3 μm. A rotational velocity of 200 rpm and a load of 300 N (corresponding to a contact pressure 220 MPa) were used for all tests. Each test lasted 5 h. During tests, the bottom part of the ring was immersed in oil (Fig. 1). Duplicate tests were performed for the majority of samples. Samples were cleaned with isopropanol (IPA) for further analysis after tribotests were completed. Scar profiles were characterized using an Alpha-Step IQ stylusbased surface profiler. Roughness maps and values were measured using a Zygo NewView 5000 3D optical profiler for wear scars in blocks and for ring surfaces. For each sample, roughness maps were collected at 3 areas, left, right and center, along a scar. On each map, roughness
Fig. 1. Schematics of the experimental setup for the block on ring tribological test.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
N. Nunn et al. / Diamond & Related Materials xxx (2014) xxx–xxx
was measured in 5 different spots over a distance of 50 μm. These roughness measurements were taken across areas that showed mostly uniform roughness so as to avoid excessive statistical error. The worn block surfaces were also analyzed with a Nikon optical microscope (Nikon Optiphot).
Table 2 Tribological characteristics of PAO-6 containing additives of DND and MoDDP. Average COF corresponds to the COF averaged over 5 h test. End COF corresponds to the friction coefficient at the end of the 5 h test. Error bar for the end COF measurements is ~0.005 and for wear scar measurements ~0.002 mm, correspondingly. Composition
Scar area, μm2
Average COF (end COF)
RMS scar
RMS ring
PAO 0.015% ND 0.15% M 0.015 ND/0.15% M 0.01% ND/0.1% M
9611 24,308 3823 3185 7012
0.12 (0.1) 0.037 (0.002) 0.058 (0.024) 0.099 (0.09) 0.045 (0.028)
0.084 0.106 0.053 0.085 0.055
0.223 0.283 0.202 0.298 0.195
3. Results Tribological characteristics (COF and scar area) of PAO oil containing nanodiamonds or different types of sp2 nanocarbon particles are presented below. Tests included: (i) PAO containing only nanocarbons, (ii) PAO containing nanocarbons in combination with MoDDP, and (iii) PAO containing only MoDDP. For nanodiamonds, in addition to friction and wear, roughness of the wear scar in a block and ring surface was also analyzed. 3.1. Tests of PAO containing ND and ND/MoDDP additives Plotted in Fig. 2 is the COF as a function of time for the pure PAO, PAO with DND, and PAO with the DND/MoDDP mixture. The results of the measurements of the COF and wear scar area for the samples are summarized in Table 2 and Fig. 3. For pure PAO oil, the COF did not appreciably change over time, remaining close to ~ 0.14. As can be seen from Fig. 2, a significant decrease in the COF over time was observed for the PAO containing 0.015% NDs. A sharp COF reduction took place at around 2 h during the test and stayed around an extremely low value ~ 0.002 (98.5% lower than the COF for pure PAO) for the remainder of the test. However, the resulting scar area at the addition of NDs was significantly increased by ~2.5 times (Table 2). The addition of only the MoDDP additive (0.15 wt.%) into PAO resulted in a decrease of the COF over time to a value ~0.024. A gradual decrease in COF was observed, as opposed to the sharp COF decrease observed in the case with ND. The addition of only MoDDP to PAO resulted in a more than 2× decrease in the wear scar in comparison with pure PAO (Table 2). The results of the tests for a mixture of DND and MoDDP are shown in Figs. 2 and 3, and Table 2. The DND/MoDDP mixtures were prepared in a ratio of 1 to 10 by weight (DND to MoDDP). Two concentrations of the additive combinations in PAO were tested, 0.015% DND with 0.15% MoDDP and 0.01% DND with 0.1% MoDDP. From Figs. 2 and 3, and Table 2, it can be seen that the results of the tests are sensitive to the amount of the additives, but both compositions show significantly improved characteristics as compared to pure PAO oil. The scar area is further reduced at combinations of 0.015% ND and 0.15% MoDDP as compared to 0.15% MoDDP alone; however, the COF reduction is insignificant for this combination. At lower amounts of nanoadditives, 0.01% ND and 0.1% MoDDP, the COF is low, ~0.028, but the scar size reduction is only ~27% as compared to pure PAO. Shown in Fig. 4 are optical micrographs of the scars taken at the same magnification. A well-pronounced, grooved texture was observed on the blocks initially because of vendor grinding/polishing. These grooves were directed parallel to the rotational direction of the rings. These grooves can be clearly seen on the undamaged parts (along the scar periphery) of the surface of the blocks in Fig. 4. These well-pronounced grooves were also observed on the
3
± ± ± ± ±
0.031 0.035 0.024 0.015 0.022
± ± ± ± ±
0.025 0.023 0.031 0.031 0.034
rings. Roughness was measured perpendicular to the grooves on the worn block and ring surfaces. Measurements of the roughness were averaged over three different spots (left, center, and right) and five data points for each spot along the scars of each block. The same measurement scheme was used for the rings, and the results for both blocks and rings are summarized in Table 2. Data from Table 2 demonstrate the lowest RMS roughness for the samples lubricated with 0.01% DND/0.1% MoDDP and 0.15% MoDDP in PAO. RMS roughness of the scar surfaces on the blocks and corresponding rings tested with 0.015% DND additives is about 26% higher than the roughness of the scars and rings observed in experiments performed with pure PAO. These trends are clearly seen from the roughness maps presented in Fig. 4. Roughness maps of the scar surfaces show high uniformity and smoothness for the scars formed during lubrication with 0.01% DND/0.1% MoDDP additive and 0.15% MoDDP in PAO. Roughness within scars formed in the presence of high concentrations of NDs (0.015%) is highly nonuniform. Optical micrographs (not shown) of the wear scars revealed that the steel blocks in the tests using only PAO oil experienced noticeable wear with deep abrasive grooves caused by the removal of the material from the contact. On the steel blocks lubricated using only NDs, abrasive grooves were even more pronounced; however, regions between the grooves were highly polished. These highly polished regions were elongated along the sliding direction. There were very few signs of wear on the surface of a wear scar in a block lubricated using 0.01% DND with 0.1% MoDDP additive. The grooves from the initial surface topography were much less pronounced, and the surface was more smooth and uniform in all directions. 3.2. Tests of PAO containing sp2 nanocarbons and sp2/MoDDP additives Plotted in Fig. 5 is the COF as a function of time for pure PAO and PAO containing other nanocarbon additives. The results of the measurements of the COF and wear scar area for the samples are summarized in Table 3 and Fig. 6. As can be seen from Fig. 5, a significant decrease in the COF over time was observed for the PAO containing onion-like carbon (OLC), carbon nanotubes, and nanographene platelets. PAO containing MWNT showed the biggest reduction in the end COF (~0.013). Scars formed in the samples containing MWNT, SWNT, and graphite nanoflakes in PAO all showed a 15% or greater reduction in area as compared to pure PAO, while both OLC samples exhibited an increase.
Fig. 2. Friction coefficient as a function of time for pure PAO-6 oil and for the oil with addition of DND (0.015%), MoDDP (0.15%) or a mixture of DND and MoDDP (0.01% DND with 0.1% MoDDP). Test time is 5 h, load 30 kg, rotation rate 200 rpm.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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4. Discussion
Fig. 3. Tribological characteristics of PAO-6 containing additives of DND and MoDDP (Mo on the diagram): COF corresponding to the friction coefficient at the end of the 5 hour test, area of the wear scar and RMS for the scar surface. Dilutions of the mixture of ND and MoDDP with PAO in 1:66 and 1:100 ratios, as denoted on the figure, correspond to the concentrations of the additives in the samples 0.015% DND with 0.15% MoDDP and 0.01% DND with 0.1% MoDDP.
This demonstrates that using only sp2 carbon additives, the lubricating performance of PAO oil can be improved. Each sp2 carbon-based additive was tested in combination with 0.16% of MoDDP in PAO, similar to tests performed with NDs. Table 3 summarizes COF and scar data for each test. The scar area for the reference sample tested with pure PAO was 9611 μm2, and each test for sp2 carbon-based additive in combination with 0.16% of MoDDP showed a scar area higher than the reference. In the presence of MoDDP, the COF was higher for each of the nanocarbon additives as compared to the test run with the nanocarbons without MoDDP. This demonstrates that the sp2 nanocarbon-based additives are not compatible with MoDDP when used in PAO oil. Optical micrographs (not shown) of the wear scars in the tests using only sp2 nanocarbon particles revealed a very similar appearance of the worn surfaces for all three types of the nanoparticles: carbon onions, nanotubes and graphene platelets. The grinding scratches from the initial surface topography were hardly seen in the contact area. The surface was smooth and uniform, providing an impression that nanocarbon material was smeared over the surface. For the steel blocks lubricated using nanocarbons with MoDDP, the wear was high and a surface topography similar to that of the pure PAO sample was observed.
Previously, we observed that a significant improvement of both friction and wear took place for the DND in combination with MoDDP additive when applied to fully formulated Mobil 5W30 motor oil [20,21]. A 90% improvement in the COF of the Mobil oil was observed in 7 hour tests at 300 N load and 200 rpm rotational speed [20]. The reduction in COF was accompanied by a reduction in the wear scar area by more than a factor of two. SEM observations revealed that scar surfaces in the sample treated with DND are much smoother than those lubricated by pure 5W30 Mobil oil. Roughness of the scar surfaces for the blocks tested with DND additive was about 35% lower than the roughness of the scars observed for pure oil experiments. It was speculated that DND can enhance delivery of other AF/AW additives (like MoDDP) to the freshly polished surfaces and thus promote the formation of a robust protective tribofilm. Molybdenum di(2-ethylhexyl) phosphorodithioate is an oil-soluble organic molybdenum additive containing sulfur and phosphorus. It improves the tribological performance of lubricants by forming sulfides (especially MoS2), oxides and phosphates (FePO4) on the metal surfaces [26]. In the current study, the same type of the block-on-ring test at the same operational conditions and similar content of the additives was performed in pure PAO oil instead of a fully formulated oil. The abrasive nature of nanodiamonds was very pronounced, resulting in a combination of large wear and extremely low friction coefficient (0.002) that was achieved at ~ 2 h after beginning of the test. This suggests that the majority of the wear took place within the first hours of the test before super low friction conditions were established. The relatively large roughness measured across the grooves on the steel surfaces needs to be explained in the light of the very low COF. It should be noted that this roughness is measured perpendicular to the grooves, while the rotational direction is along the grooves. Measurement of the roughness along the micronwide grooves with an optical profilometer is problematic due to alignment problem. Based on the optical analysis of the worn blocks, it can be currently assumed that deep micro-scars were formed at the initial stage of polishing (“work in” regime) while at the later stage, superpolished regions with enhanced hardness were formed on the steel surfaces along the sliding direction. While the micro-scars are responsible for the high roughness across the sliding direction, the COF is very low due to the existence of highly polished ridges occurring at the contact between sliding surfaces. A more detailed study, combining electron microscopy and atomic force microscopy will be required to explain this extremely low COF accompanied by large wear and roughness of the ND/PAO sample. Another interesting feature specific to ND used on its own is the abrupt decrease of the COF (Fig. 2) that was observed in multiple repeated experiments. This may correspond to the transition
Fig. 4. Optical micrographs of the scars (up row) and roughness maps of the scar surfaces (bottom row) formed in H30 block samples during tests with and without DND and MoDDP additives in PAO oil.
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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Fig. 5. Friction coefficient as a function of time for pure PAO-6 oil and for the oil with OLC-L and OLC-S additives (a) and SWCNT, MWCNT and NGP additives (b). Test time is 5 h, load 30 kg, rotation rate 200 rpm.
from the “work in” regime to the highly polished regime; however, why this transition is so abrupt is not currently clear. The use of MoDDP along with ND resulted in the improvement of both friction and wear; however, the COF was not reduced as dramatically as was the case with the ND alone. The results are highly sensitive to the concentration of the ND and MoDDP in PAO, perhaps due to the different mechanisms of action of these two additives — polishing from the ND and the formation of a protective tribofilm from the MoDDP. A possible synergistic mechanism can be as follows. The MoDDP forms protective phosphate film initially in the regions between asperities, while NDs contribute to the efficient polishing of the asperities. After some time, a well-polished surface protected by a phosphate film is formed. NDs may be contained within this film, enhancing its strength. In the absence of MoDDP, NDs wear both asperities and unprotected regions between asperities, resulting in high wear — as observed in the present study. A concentration of 0.01% ND and 0.1% MoDDP seems close to optimal, providing high performance in combination with low MoDDP content, which is valuable from an environmental standpoint. Obviously, the composition of ND and MoDDP in PAO can be further optimized. In the fully formulated oil, the synergism of NDs and MoDDP (ZDDP) is more pronounced than in pure PAO when compared to the effect observed with the addition of only ND or only MoDDP [20]. This can be due to the presence of additional AF/AW additives in the motor oil for which NDs have high compatibility. Perhaps a protective tribofilm formed on the surfaces of sliding steel contacts has enhanced strength due to the presence of NDs than compared to those formed in the absence of NDs.
Another interesting aspect is related to well-known reactions between diamond and ferrous materials (for example, diamond tools are not used for cutting steel). This raises a question as to why ND oil additive works so well for enhanced lubricating performance. Perhaps the temperature at the sliding contacts is not high enough to cause a reaction between ND and Fe (N700 °C is required for bulk diamond [27]), and a formed tribofilm isolates NDs from the steel. However, it is also possible that a conversion of NDs to sp2 carbon does occur during lubrication of steel surfaces due to the catalytic effect of Fe, but this requires further investigation. Lubrication with sp2 carbon particles with a layered structure is also a popular topic of research [23–25]. Their strong intra-planar and weak inter-planar bonds have been recognized as offering a very good lubrication potential. In this study, using onions, carbon nanotubes and graphene platelets as additives to PAO, we observed significantly decreased the friction and wear compared to pure PAO. Friction surfaces were very smooth and uniform, regardless of the different morphologies of nanocarbons used (i.e. nanotubes, onions, or platelets). One possible explanation for the positive effect of sp2 carbon is due to the formation of a lubricious film based on the third-body material transfer through delamination and exfoliation of the layered carbon structure to the contact sliding surfaces, especially under high contact loads. Nevertheless, the combination of sp2 nanocarbon with MoDDP was detrimental for the lubrication performance. This can be attributed to the competing mechanisms of the tribofilm formation between sp2 layers and MoDDP. This raises concern about possible antagonism between
Table 3 Tribological characteristics of PAO-6 containing additives of sp2 nanocarbons and MoDDP. Average COF corresponds to the COF averaged over the 5 hour test, end COF corresponds to the friction coefficient at the end of the 5 hour test. Error bar for the end COF measurements is ~0.005 and for wear scar measurements ~0.002 mm, correspondingly. Composition
Scar area in PAO, μm2
Average COF (end COF) in PAO
Scar area in PAO with MoDDP 1:60, μm2
Average COF (end COF) in PAO with MoDDP 1:60
Pure PAO PAO with MoDDP MWNT SWNT OLC-L OLC-S Nanographene platelets
9611 – 7151 7790 10,829 12,100 6344
0.12 ± 0.025 (0.12) – 0.056 ± 0.035 (0.013) 0.063 ± 0.025 (0.034) 0.078 ± 0.007 (0.015) 0.081 ± 0.045 (0.016) 0.062 ± 0.035 (0.02)
– 3823 11,874 16,885 16,686 10,389 10,761
– 0.057 0.081 0.083 0.063 0.093 0.115
± ± ± ± ± ±
0.029 (0.024) 0.009 (0.072) 0.013 (0.07) 0.0278 (0.026) 0.01 (0.06) 0.007 (0.107)
Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003
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performances were observed emphasizing different mechanisms of lubrication for sp3 and sp2 nanocarbons. In addition, the performance of nanocarbon particles in PAO in combination with molybdenum dialkyldithiophosphate was also investigated for the first time. Acknowledgments The present work was supported by the U.S. National Science Foundation under grant no. CMMI-1229889 and by the RFBR (grant 12-03-93937-G8_а). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. References Fig. 6. Tribological characteristics of PAO-6 containing additives of nanocarbons and MoDDP (Mo): COF corresponding to the friction coefficient at the end of the 5 hour test and area of the wear scar.
sp2 carbon additives and existing AW/AF components in modern oils and deserves further investigation. 5. Conclusion With the push towards more environmentally friendly, yet just as effective, lubricants being a major priority, this study sought to follow up our previous [10,16–21] work and the work of others [11–15, 23–25] with nanocarbon based nanolubricants. Whereas previous work sought to investigate the effectiveness of nanocarbon additives (primarily nanodiamond) in commercially available, synthetic motor oils [16–21], this study was primarily interested in investigating the synergy and tribological mechanisms between nanocarbon additives (sp2 and sp3), MoDDP, and an oil base (PAO). Thus, this study was performed in a more simplified system in the absence of a number of complex additives that are present in commercially available motor oils. Several important conclusions can be drawn. On their own, NDs caused a dramatic drop in the COF as compared to pure PAO and all other additives measured; however, the abrasive polishing nature of the NDs also caused a dramatic increase in wear. When NDs were paired with MoDDP, there was clearly a synergy between the two; however, it is concentration sensitive, with an apparently optimal concentration being around 0.01% ND/0.1% MoDDP. Nevertheless, the abrasive nature of the NDs still caused higher wear (albeit a lower average COF), and this is possibly due to the synergistic behavior of the polishing from the NDs and the formation of tribofilms from the MoDDP. Despite the higher wear, overall, the performance of the 0.01% ND/0.1% MoDDP mixture was comparable to the 0.15% MoDDP mixture, suggesting that the presence of NDs can reduce the amount of environmentally unfriendly MoDDP needed. Aside from the MWNTs, it appeared that most of the sp2 nanocarbon additives were less effective than NDs. Although the COFs of the sp2/PAO mixtures were comparable to those of MoDDP/PAO, the wear was higher for all cases (though less than ND/ PAO). It would seem that there is an antagonistic mechanism between the sp2 additives and MoDDP, thus limiting the effectiveness of the lubricant. At this stage, all proposed mechanisms are theoretical; therefore, more investigation is required (electron microscopy and atomic force microscopy) to further understand the true mechanisms behind the behavior of these nanolubricants. Prime novelty statement For the first time, comparative analysis of coefficient of friction and wear was performed for PAO oil containing sp3 or sp2 carbon nanoparticles: nanodiamond particles, onion-like carbon, single/multiwall carbon nanotubes or nanographene platelets. Considerably different
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Please cite this article as: N. Nunn, et al., Tribological properties of polyalphaolefin oil modified with nanocarbon additives, Diamond Relat. Mater. (2014), http://dx.doi.org/10.1016/j.diamond.2014.09.003