Tribological properties of graphene nanoplatelets or boron nitride nanoparticles as additives of a polyalphaolefin base oil

In this work, antifriction and antiwear capabilities of hexagonal boron nitride nanoparticles (h-BN) or graphene nanoplatelets (GnP) as additives of a polyalphaolefin neat oil (PAO 40) were studied at pure sliding conditions. For this purpose, eight PAO 40 nanodispersions were prepared: four nanodispersions with h-BN and four others based on GnP. The mass concentrations of these dispersions are 0.25, 0.50, 0.75 and 1.00 wt% of h-BN and 0.05, 0.10. 0.25 and 0.50 wt% of GnP, having all of them a good stability against sedimentation (at least 96 h). Tribological assays were carried with prepared nanolubricants as well as with PAO 40 base oil at 20 N load. All nanolubricants based on h-BN or GnP showed lower friction coefficients in comparison to the non-additivated neat oil, with a maximum decrease in friction of 21% for the 0.50 wt% GnP nanodispersion. Regarding the produced wear, all disks lubricated with nanolubricants showed lower wear than those lubricated using PAO 40. The greatest wear reduction in wear track width (22%) was also achieved for the 0.50 wt% in GnP nanolubricant. Moreover, through the confocal Raman microscopy and roughness analyses of worn disks it can be concluded that the wear reductions are due to the surface repairing and tribofilm formation mechanismsS

Tribological behavior of electric vehicle transmission oils using Al2O3 nanoadditives

Antifriction and antiwear performances of Al2O3 nanoparticles (NPs) as additives of an automatic transmission fluid, ATF, are presented in this research. For this purpose, four nanodispersions were formulated: ATF + 0.05 wt% Al2O3 NPs, ATF + 0.10 wt% Al2O3 NPs, ATF + 0.15 wt% Al2O3 NPs and ATF + 0.20 wt% Al2O3 NPs to identify the optimal concentration of additive. Tribological experiments were taken at pure sliding conditions, with the formulated nanolubricants and the ATF, under a working load of 20 N. The four nanolubricants tested resulted in lower friction coefficients than those obtained using ATF, reaching a maximum reduction of 6 % with the ATF + 0.10 wt% Al2O3 nanolubricant. The tribological pairs tested with the Al2O3 nanolubricants show lower wear than those tested with the ATF, having the best wear decrease with the ATF + 0.10 wt% Al2O3 nanolubricant, with reductions of 45, 57 and 78 %, respectively, in diameter, depth and area of the wear scar. Furthermore, by means of confocal Raman microscopy, roughness evaluation and SEM-EDX of the worn tribological specimens, it can be determined that mending, tribo-sintering as well as rolling mechanisms occur.This research is supported by Xunta de Galicia (ED431C 2020/10) and by MCIN/AEI/10.13039/501100011033 through the PID2020-112846RB-C22 project. JMLdR is grateful for financial support through the Margarita Salas program, funded by MCIN/AEI/10.13039/501100011033 and “NextGenerationEU/PRTR”. Furthermore, authors are also grateful to Repsol Lubricants for providing the ATF and to RIAIDT-USC for its analytical facilities.S

Tribological synergies among chemical-modified graphene oxide nanomaterials and a phosphonium ionic liquid as additives of a biolubricant

In the present work, antifriction and antiwear synergies of two functionalized graphene oxides (reduced graphene oxide, rGO, and reduced graphene oxide modified with octadecylamine, rGO@ODA) with a phosphonium ionic liquid (IL) as additives of a biodegradable ester base oil, BIOE, were investigated. For this aim, four BIOE nanodispersions were formulated: two nanodispersions without IL and two others with IL (hybrid nanolubricants), being the nanodispersions: BIOE + 0.05 wt% rGO, BIOE + 0.05 wt% rGO@ODA, BIOE + 1 wt% IL + 0.05 wt% rGO and BIOE + 1 wt% IL + 0.05 wt% rGO@ODA, and all of them showing good temporal stability (at least 3 weeks), especially those containing the ionic liquid. Tribological tests, under pure sliding conditions, were made with the aforementioned nanolubricants and with neat BIOE under a 20 N working load. All hybrid and non-hybrid nanolubricants resulted in lower friction coefficients compared to BIOE oil, with the largest friction reduction of 34% achieved by the 1 wt% IL + 0.05 wt% rGO nanolubricant. Discs lubricated with the prepared nanolubricants showed less wear than discs lubricated with BIOE, achieving the highest reduction (34%) in wear track width also for the 1 wt% IL + 0.05 wt% rGO nanolubricant. In addition, through confocal Raman microscopy, as well as roughness analysis of tested discs, it can be concluded that surface repairing, synergistic effect and tribofilm formation mechanisms occur.Authors thank Verkol Lubricantes for providing us with BIOE base oil and acknowledge the assistance of the RIAIDT-USC analytical abilities. This research was supported by Xunta de Galicia (ED431C 2020/10) and by MINECO and the ERDF programme through ENE2017-86425-C2-2-R project. FG acknowledges the Juan de la Cierva Incorporación 2017 program, the Xunta de Galicia (2016-2019, ED431G/09, Centro singular de investigación de Galicia), and Fondo Europeo de Desarrollo Regional (FEDER) for financial supportS

Tribological properties of dispersions based on reduced graphene oxide sheets and trimethylolpropane trioleate or PAO 40 oils

The main goal of this work is to study the tribological properties of nanolubricants formed by trimethylolpropane trioleate (TMPTO) or a polyalphaolefin (PAO 40) base oils with reduced graphene oxide sheets (rGO). This reduction was carried out in order to have a good stability of the nanoadditives in the fluids. For this aim, rGO nanopowders were prepared by thermal reduction of graphene oxide (GO) powders using KOH/ethanol as reducing agent. Tribological behavior of nanolubricants based on TMPTO and on PAO 40 oils with 0.05, 0.10, 0.25 and 0.50 wt% of rGO was evaluated. The tribological tests were carried out with a tribometer operating in ball on disk configuration and rotational mode under a working load of 20 N at room temperature. In order to analyze the wear track through the width, depth and cross-section area of the scar, a 3D optical profilometer was used. The best antifriction performance was found for 0.25 wt% rGO nanodispersions, with a 24% and a 20% enhancement for the PAO 40 and the TMPTO base oils, respectively. Moreover, for this last nanodispersion, a reduction of 24% in the wear track width was obtained.This work was supported by both the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (ERDF) programme through ENE2014-55489-C2-1-R and ENE2017-86425-C2-2-R projects. Moreover, this work was funded by the Xunta de Galicia (GRC ED431C 2016/001). The three last funders also financed the acquisition of the 3D Optical Profile Sensofar S Neox (UNST15-DE-3156). JMLR acknowledges Xunta de Galicia for a Principia contract. FG is grateful for financial support from the Xunta de Galicia (Centro Singular de Investigación de Galicia accreditation 2016–2019, ED431G/09) and the European Union (ERDF). Authors would like to thank the use of RIAIDT-USC analytical facilities.S

Chemically modified nanomaterials as lubricant additive: time stability, friction, and wear

This work reviews the results on the temporal stability of nanodispersions containing chemically modified nanoadditives: carbon-based nanomaterials, metals, metal oxides, metal sulfides, nanocomposites, among others. Morphology, size, coating type and concentration of the NPs as well as the viscosity of the base oil are the main characteristics that affect stability. Coated spherical NPs with mean diameter lower than 20 nm are the most common among those which led to nanolubricants with stabilities longer than two months. The tribological results of the nanolubricants with stabilities higher than one month are also reviewed. Regardless the base oil, chemically modified nanoparticles reduced wear more than friction, reaching reductions of friction up to 75% and wear volume up to 99%. As for the tribological mechanisms involving chemically modified nanoparticles, the formation of adsorbed or tribochemical reaction films on worn surfaces were reported more oftenThis work is supported by MCIN/AEI/10.13039/501100011033 and by the European Regional Development Fund (ERDF, FEDER in Spanish) through the ENE2017-86425-C2-2-R and the PID2020-112846RB-C22 projects as well as by Xunta de Galicia (ED431C 2020/10). JMLDR acknowledges the grant of the Margarita Salas program, funded by MCIN/AEI/10.13039/501100011033 and “NextGenerationEU/PRTR”S

Synergistic effects of hexagonal boron nitride nanoparticles and phosphonium ionic liquids as hybrid lubricant additives

Hybrid nanostructure combinations of ionic liquids (ILs) and hexagonal boron nitride (h-BN) nanosheets are proposed as additives to lubricants to enhance their tribological performance and their temporal stability. Trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate (IL1), tributylethylphosphonium diethylphosphate (IL2) and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (IL3) are the ILs (at 1 wt%) that have been combined, each separately, with h-BN nanosheets (at 0.1 wt%), were added as hybrid additives in a polyalphaolefin base lubricant, PAO 32. Some of the nanodispersions remain stable up to 240 days after preparation. Increases in both viscosity and density owing to h-BN nanoparticles and/or ILs are lower than 7% and 0.3%, respectively. The tribological performance (friction and wear) of the several developed nanolubricants were investigated using a tribometer with a ball-on-three-pin configuration and a 3D optical profiler, respectively. The results showed that the hybrid additives improve, in general, the tribological performance of the lubricant rather than being used separately, thus synergistic effects have been found. Raman spectroscopy conducted on the worm surface showed protective IL and h-BN tribofilms. PAO 32 + IL1 + h-BN is the nanolubricant that exhibits the best tribological performance of those studied in this work.The authors acknowledge Repsol for providing us the PAO 32 sample. Authors would also thank RIAIDT-USC for the use of analytical facilities, especially to Mr. Ezequiel Vázquez for his useful advice. This work was supported by the Spanish Ministry of Science, Innovation and Universities and the ERDF programme (FEDER in Spanish) through ENE2017-86425-C2-2-R project, and by the Xunta de Galicia (ED431E 2018/08, ED431D 2017/06 and GRC ED431C 2016/001). These funders also financed the acquisition of the 3D Optical Profile (UNST15-DE-3156)S

Thermophysical and tribological properties of dispersions based on graphene and a trimethylolpropane trioleate oil

The objective of this work is to study the tribological and thermophysical properties of nanolubricants composed by a trimethylolpropane trioleate (TMPTO) oil, and graphene nanoplatelets (GnP). For this aim, nanolubricants based on TMPTO with 0.05, 0.10, 0.25 and 0.50 wt% of graphene nanoplatelets were prepared. The dependence on temperature and concentration of the viscosity, density and speed of sound was determined by means of a rotational viscometer and two mechanical oscillation densimeters. Likewise, the antifriction and antiwear properties of these nanolubricants were analyzed. For this purpose, tribological tests were carried out at room temperature in a tribometer operating in ball on plate configuration and in reciprocating mode under a working load of 2.5 N. A 3D optical profilometer was used to analyze the wear track through the width of the scar. As regards thermophysical behavior of density and viscosity, both increase as the concentration of nanoparticles increases, whereas the speed of sound slightly diminishes when the GnP concentration increases. The best antifriction-antiwear performance was obtained for the nanolubricant containing 0.25 wt% in GnP.This work was supported by both the Spanish Ministry of Economy and Competitiveness and the ERDF programme through ENE2014-55489-C2-1-R and ENE2017-86425-C2-2-R projects. Moreover, this work was funded by the Xunta de Galicia (AGRUP2015/11 and GRC ED431C 2016/001). The three last funders also financed the acquisition of the 3D Optical Profile Sensofar S Neox (UNST15-DE-3156). JMLR acknowledges Xunta de Galicia for a Principia contract.S

Improving the tribological performance of a biodegradable lubricant adding graphene nanoplatelets as additives

This research is based on the investigation of the tribological properties of a biodegradable polymeric ester lubricant (BIOE) additivated with two different pristine graphene nanoplatelets (GnPs), named as GnP7 and GnP40. These GnPs have lateral sizes of 7 and 40 μm, and thickness of 3 and 10 nm, respectively. Four different nanoadditive loadings: 0.015, 0.035, 0.055 and 0.075 wt% have been used. Stability of nanolubricants has been investigated using the visual control and the refractive index evolution during time, revealing a slightly better stability for the nanolubricants formulated with the largest lateral size graphene nanoplatelets (GnP40). The influence of the thickness and lateral size of these carbon-based nanoadditives on the antifriction and antiwear capabilities of BIOE is analysed. For this purpose, rotational friction tests were taken with the eight nanolubricants under a 20 N working load and 340 m sliding distance. All nanolubricants showed friction coefficients and worn area lower than those previously reported for the unadditivated BIOE. As regards friction, the ideal loading for both GnPs was 0.055 wt% GnP, being the best anti-friction behaviour obtained using GnP40 as additive (up to 26% reduction), whereas the 0.055 wt% GnP7/BIOE nanolubricant leads to the best anti-wear capability with wear reductions up to 56%. Finally, from Raman microscopy and roughness assessments on the worn surfaces, it can be determined that the good tribological performance of nanolubricants is owing to the protective film formation and surface repairing mechanismThis research was supported by the “Ministerio de Ciencia, Innovación y Universidades” (Spain) and the FEDER program through the ENE2017-86425-C2-1/2-R project and by Xunta de Galicia (ED431C 2020/10). Authors would like to thank Verkol Lubricantes and Avanzare Innovacion Tecnologica S.L. for providing the BIOE base oil and pristine graphene nanoplatelets, respectively. Authors would like also to thank the help of RIAIDT-USC analytical facilities. M.J.G.G. and J.I.P. are grateful for the support of the Xunta de Galicia for the postdoctoral (reference ED481B-2019-015) and predoctoral fellowship grantsS

Synergy between boron nitride or graphene nanoplatelets and tri(butyl)ethylphosphonium diethylphosphate ionic liquid as lubricant additives of triisotridecyltrimellitate oil

In this work, the synergy between an ionic liquid (IL) and nanoparticles as additives of lubricants was studied. For this purpose, four dispersions based on graphene nanoplatelets, GnPs, or nanoparticles of hexagonal boron nitride, h-BN, with or without the IL tri(butyl) ethylphosphonium diethylphosphate in an ester type base oil, triisotridecyltrimellitate (TTM), were prepared and tribologically analyzed as potential nanolubricants. The mass concentration of the nanoadditives is 0.1 wt%, whereas for the IL it is 2 wt%. The prepared blends were stable for three weeks. New density and viscosity values show that both properties slightly increase with the addition of IL and/or nanoparticles. Tribological tests were performed under a normal load of 20 N for TTM, the four dispersions and the TTM + 2 wt% IL mixture. With respect to base oil, a maximum friction reduction of 33% was achieved for TTM/IL/GnP nanodispersion. The best antiwear performance also corresponds to this same nanodispersion with a wear track width reduction of 44% and a strong decrease of the average cross sectional area of 65%, both respect to those obtained with the neat oil. In the case of wear scar depth, the maximum reduction is 32% for TTM/IL/h-BN nanodispersion. In addition, the values for roughness of worn surfaces tested with both TTM/IL/GnP and TTM/IL/h-BN nanodispersions are lower than those corresponding to the neat oil, to the TTM/IL mixture and to those of the corresponding binary dispersions. Hence, positive synergies between the IL and GnP or h-BN as additives of TTM were found. Confocal Raman microscopy demonstrates tribofilm formation and mending effect on worn surfaces.This work was supported by MINECO and the ERDF programme through 401 ENE2014-55489-C2-1-R and ENE2017-86425-C2-2-R projects, and by the Xunta de Galicia 402 (ED431E 2018/08, ED431D 2017/06 and GRC ED431C 2016/001). These funders also 403 financed the acquisition of the 3D Optical Profile (UNST15-DE-3156).S