Tribological Effects of BN and MoS2 Nanoparticles Added to Polyalphaolefin Oil in Piston Skirt/Cylinder Liner Tests
We report in this article the friction and wear results of polyalphaolefin (PAO 10) base oil with the addition of 3 wt% boron nitride (BN) and molybdenum disulfide (MoS2) nanoparticles with nominal size of 70 and 50 nm, respectively. The formulations were tested using cast iron cylinder liner segments reciprocating against aluminum alloy piston skirt segments at 20, 40, and 100 °C. The results showed that, at a load of 250 N and a reciprocating frequency of 2 Hz, BN did not lower friction whereas MoS2 nanoparticles were very effective at reducing both friction and wear, compared with the base oil. The viscosities of both formulations were similar to the base oil, which allowed for a direct comparison between them. Raman spectroscopy showed the formation of an aligned MoS2 layer on the cast iron liner surface, which most likely functions as a tribofilm. In the case of the cast iron liner tested with BN nanolubricant, no traces of BN were found. The effect of surfactants was also studied, and it was found that some surfactants were not only beneficial in dispersing the nanoparticles in oil, but also in producing some reduction in friction and wear, even when used as stand-alone additives in PAO 10.
KeywordsNanolubricants Nanoparticles Surfactants Additives Cast iron Cylinder liner Piston skirt Boron nitride (BN) Molybdenum disulfide (MoS2) Polyalphaolefin (PAO)
In the internal combustion engine, the piston skirt’s contribution to the total mechanical friction losses of the piston/cylinder system is significant and comparable to that of piston rings, which accounts for more than 40 % of the total mechanical engine losses [1, 2, 3]. Researchers have realized that typical materials and lubricants used today as well as a variety of traditional surface treatments may not provide sufficient protection to meet the requirements of reliability, performance, oil consumption, and emissions in the internal combustion engine [4, 5]. Depending on the operating conditions, reduction in friction and wear can be achieved through optimization of the surface topography of the piston rings or cylinder liner, coating the piston and piston rings with low friction films, or through the introduction of additive packages to lubricant formulations [4, 5].
Nanolubricants (dispersion of nano-sized materials in oils) are emerging concepts in lubrication, some of which have demonstrated reduction in friction and wear of moving parts [6, 7, 8, 9, 10, 11, 12]. Nanomaterial additives may help in counteracting friction, saving energy, increasing the efficiency of dynamic transmission, dampening vibrations, and reducing noise . While well-known solid lubricants, such as graphite, h-BN, and transition metal dichalcogenides, owe their lubricity to a unique layered structure, solid nanoparticles can engage additional lubrication mechanisms. Tribochemical film formation is often encountered, and even nanoscale ball bearing mechanisms have been proposed [6, 7, 8]. Furthermore, nanoparticles, due to their small size, may enter the contact area, which can have a positive effect. The main challenge with nanoparticles being used as additives in lubricants is their stability. Compatibility with the lubricating oil can be improved with surface modification techniques using organic compounds. Chalcogenides have been successfully prepared using a surface modification agent . Even though a great effort has been put forth by researchers, there are still difficulties associated with the synthesis of many nanoparticles and their stability in suspensions.
There are reports in the literature demonstrating tribological improvements due to the addition of nanoparticles, but no clear mechanisms for the improvements have been proposed. Evidences showing tribological benefits have emerged from laboratory experiments. Actual engine tests tend to be expensive and time consuming, and are sparse in the open literature . In this study, segments from a commercial piston/cylinder system were tribologically tested using a test rig that utilizes reciprocating motion and offers a fair compromise between actual and simple laboratory tribological tests. An investigation of surfactant ability to stably suspend BN and MoS2 nanoparticles was conducted, and the surfactant’s contribution to viscosity and tribological properties was studied. The cast iron samples subjected to friction tests at three temperatures were studied using Raman spectroscopy to identify chemical changes on the surface. Formation of tribofilms observed using Raman spectroscopy was correlated to the reduced friction and wear for some nanolubricant formulations.
2 Experimental Procedure
2.1 Samples and Equipment
Aluminum alloy piston composition
76.2 ± 0.33
16.16 ± 0.12
0.11 ± 0.03
0.36 ± 0.02
2.25 ± 0.03
4.39 ± 0.04
Topographical parameters of plateau-honed liner used in this study
Sa (roughness average)
Sq (root mean square)
Ssk (surface skewness)
Sku (surface kurtosis)
Sds(density of summits)
Spk (reduced peak height)
Sk (core roughness depth)
Svk (reduced valley depth)
The wear tracks on the liners were examined optically using an Olympus STM6 microscope after the tests were run.
Scanning electron microscope (SEM) images of the nanopowders used in this study were taken using Hitachi S-4700.
The dynamic viscosities of nanolubricants oil formulations at shear stress between 0.5 and 3.5 N/m2 and temperatures between 15 and 100 °C were measured using a Brookfield DV-II + rotational type viscometer (Brookfield Engineering Laboratories, Inc.) with the SC4-18 spindle (instrument error ~2 %).
The Raman microscope (Renishaw, the UK) with laser excitation of 633 nm was used at room temperature to examine the nanolubricants and the liner surfaces.
3 Results and Discussion
3.1 Suspension Stability and Viscosity of Nanolubricants
Surfactants used in this study
A continuous decrease in coefficient of friction was observed over time during the tests with pure PAO 10 at 20 °C (Fig. 5a). Morphological changes (wearing off the topmost features) in the surface are responsible for this behavior. The same trend in friction behavior was observed in PAO 10 with surfactant (Fig. 5b); however, the values of coefficient of friction were lower than those in pure PAO 10 throughout the test duration. It is suggested that interactions between the surfactant and the sample cause formation of a tribochemical film that, in addition to the slight increase of viscosity of the formulation due to the presence of surfactant, contributes to the reduction in the coefficient of friction. The trend was observed both at 20 and 40 °C. Furthermore, as will be discussed in the next section, little wear is measured on the skirt specimen at these temperatures which would further support the formation of a tribochemical film.
Introduction of BN nanoparticles (Fig. 5c) did not change the values of coefficient of friction significantly, but eliminated the gradual decrease of coefficient of friction observed in pure PAO 10 (Fig. 5a). This result might be attributed to the physical mechanisms of nanoparticles filling out the liner voids and valleys, thus providing a hydrodynamic effect and preventing morphological changes responsible for a decrease in coefficient of friction similar to that observed during the tests with pure PAO 10. Addition of the surfactant to the PAO 10 + BN mixture resulted in better stability of the suspension, as well as change in the friction behavior. However, it is clearly seen from Fig. 5d that the effect of the surfactant on friction is stronger than that of BN nanoparticles.
The coefficient of friction increased with temperature in the majority of the tested lubricant formulations (Fig. 5a–d). This is an expected result, because of a decrease of viscosity with temperature, leading to a shift from the mixed lubrication regime toward the boundary regime, where the increased asperity interaction results in s higher coefficient of friction. An exception to this temperature trend was observed for the formulation with MoS2 (Fig. 5e–f). At 20 and 40 °C, the frictions in PAO 10 + MoS2 were very similar to those in PAO 10 + BN formulations. A significant decrease in the coefficient of friction was observed in PAO 10 + MoS2 formulation as the temperature was increased to 100 °C. At this temperature, the contact is in the boundary lubrication regime. In this regime, as was observed in the previous cases, the friction was high. However, the presence of MoS2 resulted in a significant reduction in coefficient of friction (Fig. 5e). MoS2 is a well-known solid lubricant with a layered structure which can significantly reduce boundary friction. At lower temperatures, this was not observed because viscosity is higher, and therefore the oil plays a dominant role. It is also possible that there is thermal activation leading to the lubricity of MoS2, which is most likely related to the chemical interaction with the rubbing surfaces and formation of a tribochemical film. Formulations with both MoS2 and surfactant (Fig. 5f) demonstrated interference of the surfactant effect and the MoS2 effect. While at the lower temperatures of 20 and 40 °C the behavior is similar to the PAO 10 + BN + S#2, at 100 °C, the decrease in the coefficient of friction due to MoS2 appears to be counteracted by the surfactant, resulting in roughly the same coefficient of friction at all the temperatures. The surfactant stabilizes nanoparticle suspensions through the adsorption onto the nanoparticles, thus preventing agglomeration and settling of inorganic additives. The surfactant layer may also prevent or minimize interactions of MoS2 nanoparticles with rubbing surfaces, thereby hindering the formation of a tribofilm and resulting in higher coefficient of friction than MoS2 alone.
3.4 Raman Spectroscopy
Tribochemical films and the near-surface material govern friction and wear behaviors. Tribochemical films are formed during lubricated sliding contact as a result of reaction between the surface material, lubricant additives, and the environment. The effect of nanomaterial and surfactant additives to PAO 10 was further investigated using Raman microscopy to obtain a better understanding of the reduced friction and wear mechanisms.
Hexagonal BN has only one Raman-active high energy band at 1,366 cm−1 , which is well defined in the spectrum of BN nanoparticles’ suspension in PAO 10 (Fig. 9a). However, this peak is absent on the spectrum of cast iron tested with BN nanolubricant (Fig. 9a). Minor peaks on this spectrum may be indicative of chemical interactions between BN and cast iron, but they cannot be attributed to any of the original compositions. Therefore, it can be concluded that BN is not present at the tested cast iron interface. This result correlates well with the frictional performance of the lubricant containing BN.
Summary of Raman and resonant Raman peaks of MoS2
Raman peak, cm−1
32 (first order)
Vibration of two rigid layers against each other
Longitudinal acoustic mode, appears only for nanomaterials not for single crystals from scattering of zone-boundary photons activated by facet disorder.
286 (first order)
Vibration of atoms within S–Mo–S layer (not distinguishable in this study, most likely due to the random orientation of crystallites in powder sample).
383 (first order)
Vibration of atoms within S–Mo–S layer
408 (first order)
Vibration of atoms within S–Mo–S layer
b sum of two phonon process non-Raman optical phonon
The Stokes scattering of the b band has been attributed to a two-phonon Raman process of successive emission of a dispersive quasi-acoustic (QA) phonon and a dispersionless transverse-optical (TO) phonon. Both propagate along the c axis.
A2u or 2 × LA(M)
The two peaks around 460 cm−1 were demonstrated to be a superposition of two peaks 453 and 464 cm−1, with enhancement of the 453 cm−1 feature in the spectra of nanoparticles .
E1g(M) + LA(M)
Observed only for large >500 nm and bulk crystals
2 × E1g(G)
E2g1(M) + LA(M)
A1g(M) + LA(M)
2 × E2g1
A1g(M) + E2g1(M)
2A1g(G) or 2A1g(M)
Four first-order modes (32, 286, 383, and 408 cm−1) appear at all excitation energies, while other peaks shown in Table 4 are present only on the resonant Raman spectra and correspond to the coupling of phonon modes to electronic transitions associated either with the band-gap or the exotic states (multi-peak states that violate the perturbative pulse splitting law). The relative intensity of peaks in different samples can be used for the interpretation of morphological differences. For example, it was demonstrated that the intensity of A1g peak (408 cm−1) compared to the intensity of E2g1 (383 cm−1) peak increases as the particle size decreases [22, 23]. Superimposing the two peaks at ~460 cm−1, the relative intensity of the feature at 453 cm−1 increases as the particle size decreases . Also, additional low-frequency features observed at ~230 cm−1 in the nanocrystalline samples have been demonstrated to be characteristic of crystalline disorder, whereas they are absent in the spectra of fully crystalline samples .
The MoS2 resonance Raman peaks appear less intense in PAO 10 suspension compared with the tested cast iron sample, probably because of random orientation of the suspended nanoparticles and the scattering of PAO 10 molecules. It should be noted here that the MoS2 features are even less intense in the formulation with added surfactant. This could be due to better dispersion of nanoparticles in PAO 10 or stronger scattering of laser radiation, although no specific surfactant-related peaks were observed.
The difference between the spectra of the friction-tested cast iron samples with MoS2 nanolubricant with and without surfactant (Fig. 8) can be interpreted as a surfactant effect. Based on A1g and E2g1 peak ratios (Fig. 8), the size of MoS2 particles is larger in the film formed from the formulation with the surfactant. This conclusion based on smaller MoS2 particle sizes in the film formed without surfactant is confirmed by the increased contribution of 453 cm−1 peak in the spectrum without surfactant vs. higher contribution of 464 cm−1 peak when using MoS2 lubricant with the surfactant. It is possible that the surfactant prevents nanoparticles from degradation/shearing under severe condition of friction tests. Structural modifications of Mo–S–I nanowires under severe boundary lubrication conditions, which led to the formation of MoS2 sheets in the contact area, were previously correlated to lower coefficient of friction and reduced wear . Therefore, the MoS2 tribofilm formed without surfactant compared to that formed with surfactant is in agreement with the lower coefficient of friction (Fig. 5e, f) and stronger interaction with the cast iron surface observed using optical microscopy (Fig. 7e, f). Additional information can be extracted from the appearance of 420 cm−1 line on both (MoS2 with and without the surfactant) cast iron spectra. According to previous studies , this line is indicative of the long range crystal orientation with the c axis parallel with the laser (lamellar sheets perpendicular to the laser), therefore maximizing the Raman cross section of this line. This line is only present on the surface of tested cast iron samples, indicative of the lamellar alignment of MoS2 particles, but not observed on the spectra of the nanolubricant because of the random orientation of MoS2 nanoparticles in suspensions (Fig. 8).
Windom et al.  argued that microcrystalline MoS2 powders tend to undergo oxidation to yield MoO3 (peaks at 279, 820, and 994 cm−1) when subjected to high laser power, friction tests, and temperatures. The bands around 820 cm−1 observed on our samples with the tribofilms are sharp and quite well resolved, similar to the spectra of natural MoS2 crystal and are not likely due to oxidation. The absence of the other two peaks characteristic of MoO3 (278 and 994 cm−1) also confirms this assumption. Therefore, no oxidation of MoS2 during the friction tests was observed.
Friction and wear studies carried out using formulations of PAO 10 oil with different additives at 20, 40, and 100 °C examined the effects of BN and MoS2 nanoparticles as well as the effect of surfactant on the tribological behavior of gray cast iron cylinder liner segments against an aluminum alloy piston skirt segments. The results showed that BN did not offer any improvement in friction under the tested conditions, while the addition of MoS2 nanoparticles was very effective in reducing both friction and wear compared to the base PAO 10 oil. The wear measurements of the skirt samples support the trends observed in the friction tests. Furthermore, the use of surfactant proved not only beneficial in suspending the nanoparticles in the solution, but also lowered the friction and wear by itself. The analysis of Raman spectra showed that MoS2 nanoparticles added to PAO 10 oil form an aligned MoS2 tribofilm, which assisted in significantly lowering the coefficient of friction and reducing the wear of the sample. Surfactant appears to partially suppress the interactions between MoS2 additive and the cast iron surface. BN nanoparticles, having lamellar structure similar to MoS2, added to PAO 10 oil did not lower the coefficient of friction, and were not detected on the surface of cast iron after the test.
This study is a part of Industrial Technology Program #M68008852 supported by the US Department of Energy. The scanning electron microscopy was performed at the Electron Microscopy Center for Materials Research, and Raman Microscopy was conducted at the Center for Nanoscale Materials, at Argonne National Laboratory—The US Department of Energy Office of Science Laboratory—operated under Contract No. DE-AC02-06CH11357 by the UChicago Argonne, LLC. The authors would like to thank Eduardo Tomanik at Mahle Metal Leve S.A. for providing the samples used in this study.
- 1.Nakayama, K., Yasutake, Y., Takiguchi, M., Furuhama, S.: Effect of piston motion on piston skirt friction of a gasoline engine. SAE paper 970839 (1997)Google Scholar
- 2.Demas, N.G., Erck, R.A., Fenske, G.R.: Tribological evaluation of piston skirt/cylinder liner contact interfaces under boundary lubrication conditions. Lubr. Sci. 22, 73–87 (2010)Google Scholar
- 7.Bhushan, B., Gupta, B.K.: Handbook of tribology. McGraw-Hill, New York (1991)Google Scholar
- 16.Larson, R.G.: The structure and rheology of complex fluids. Oxford University Press, New York (1999)Google Scholar
- 17.Miller, F.A., Mayo, D.W., Hannah, R.W.: Course notes on the interpretation of infrared and Raman spectra. Wiley-Interscience, New York (2004)Google Scholar