Tribological Properties of Synthetic and Biosourced Lubricants Enhanced by Graphene and Its Derivatives: A Review

Abstract

This review explores the tribological properties of biosourced lubricants (biolubricants) enhanced by graphene (Gr) and its derivatives and hybrids. Friction and wear at mechanical interfaces are the primary causes of energy loss and machinery degradation, necessitating effective lubrication strategies. Traditional lubricants derived from mineral oils present environmental challenges, leading to an increased interest in biolubricants derived from plant oils and animal fats. Biolubricants offer high biodegradability, renewability, and low toxicity, positioning them as ecofriendly alternatives. This work extensively reviews the role of Gr-based nanoadditives in enhancing the lubrication properties of biolubricants. Gr with its exceptional physicomechanical properties has shown promise in reducing friction and wear. The review covers various Gr derivatives, including Gr oxide (GO) and reduced Gr oxide (r-GO), and their performance as lubrication additives. The discussion extends to Gr hybrids with metals, polymers, and other 2D materials, highlighting their synergistic effects on the tribological performance. The mechanisms through which these additives enhance lubrication, such as the formation of protective films and improved interactions between lubricants and tribopairs, are examined. Emphasis is placed on the environmental benefits and potential performance improvements of Gr-based biolubricants. Finally, by analyzing current research and technological trends, the paper outlines future prospects for optimizing lubricant formulations with Gr-based nanoadditives, aiming for more sustainable and efficient tribological applications.

Figure 1

Structure of the current review paper.

Figure 2

Overview of the BIOE-based lubricant test results. (a) Mean friction coefficients (μ) for the BIOE-based lubricants. (b) Width of the wear tracks on the disks lubricated with various BIOE-based lubricants. (c) 3D surface topography of the wear tracks on surfaces lubricated with BIOE/GnP7 lubricants with different GnP7 weight fractions (reprinted from ref (195). Copyright 2022 Elsevier).

Figure 3

Comparative analyses of selected lubricant samples. (a) Relationship between the brake power and specific fuel consumption for samples S1 (100 wt % 15W40 oil) and S3 (formulated blend of 98 wt % coconut oil, 1.9 wt % oleic acid, and 0.1 wt % Al₂O₃-G). (b) Exhaust gas analysis results for blends S1 and S3 (reprinted with permission from ref (120). Copyright 2023 Elsevier). (c) Average coefficient of friction outcomes for the Pongamia oil (PO) and mineral engine oil (MO) with varying GnP concentrations (0–0.2 wt %). (d) Wear scar diameter (WSD) results for the PO and MO oils across the same range of GnP concentrations (reprinted from ref (215). Copyright 2021 MDPI).

Figure 4

Surface imagery of wear scars on balls subjected to a 150 N load. (a) Rapeseed oil (RO) + 10% beeswax, (b) RO + 20% beeswax, (c) RO + 30% beeswax, (d) RO + 30% beeswax + antioxidants/antiwear nanoadditives, (e) RO + 30% beeswax + antioxidant/antiwear nanoadditives + GO, (f) RO + 30% beeswax + antioxidant/antiwear nanoadditives + graphite, (g) COM (reference commercial biological grease). Analysis of wear on the balls at 150 N load: (h) wear spot diameter, (i) wear volume (reprinted with permission from ref (108). Copyright 2023 Elsevier).

Figure 5

Friction and wear analysis of the AISI52100-AISI52100 steel pairs. (a) Friction coefficients for pairs lubricated by Codonopsis pilosula/multilayer Gr (P+MGr) and P greases under loads of 250 and 300 N. (b) Friction coefficients at varying temperatures for the same sliding pairs lubricated with P+MGr and P greases. (c) Wear rates for the AISI 52100 steel lubricated with P and P+MGr greases at a constant temperature of 25 °C under different loads. (d) Wear rates at a fixed load of 100 N across various temperatures. (e) Schematic representation of the lubrication mechanisms of the P grease at room temperature and (f) elevated temperature (reprinted with permission from ref (142). Copyright 2020 Elsevier).

Figure 6

Overview of the GO-based dispersions and tribological testing. (a) Schematic illustration detailing the synthesis processes for GO–D and GO-T. (b) Digital images showing the dispersion of GO, GO-D, and GO-T (0.05 wt %) immediately and 20 days postsonication. Diagrammatic representation of friction processes involving steel balls in GO-D and GO-T dispersions: (c) friction process, (d) rubbing surface of steel balls, (e) adsorption film formation by GO-D and GO-T sheets, and (f) tribochemical film formation (reprinted with permission from ref (176); Copyright 2018 Elsevier). SEM images depicting surface conditions of steel balls after tribology tests for (g) K1 (PTFE:10g/PPTA:0.35g/GO:0.5g), (h) K2 (PTFE:10g/PPTA:0.7 g/GO:0.5 g), (i) K3 (PTFE:10g/PPTA:1.4 g/GO:0.5 g), and (j) K4 (PTFE:10g/PPTA:2.8 g/GO:0.5g) at a scale of 50 μm (reprinted with permission from ref (221). Copyright 2023 Elsevier).

Figure 7

Analysis of the sedimentation, friction, and wear in lubricant testing. (a-b) Sedimentation test results for trimethylolpropane (TMP) ester/GO/oleic acid (OA) dispersions immediately following the mixing process. (c-d) Results after a period of 3 days, with the left side of each image showing samples without and the right side with OA. In each image, glass vials are arranged from left to right displaying TMP ester samples with increasing GO concentrations of 0.05, 0.1, and 0.5 wt %. (e) Average COF for all lubricants tested (reprinted with permission from ref (212). Copyright 2023 Elsevier). (f-i) SEM images of the wear on cast iron plates lubricated with TMP, TMP+graphene (G), TMP+ionic liquid (IL), and TMP+G+IL, respectively. Reprinted with permission from ref (224). (j) Comparative analysis of friction coefficients and wear track widths (WTW) for BIOE and BIOE lubricants. (k) 3D surface topography showing cross-sectional profiles of worn scars for all tested BIOE lubricants (reprinted from ref (225). Copyright 2021 Elsevier).

Figure 8

(a) Schematic representation of the synthesis process for the chitosan-g-PNIPAM copolymer. (b) Diagram illustrating the fabrication steps for the GO/chitosan-g-PNIPAM nanohybrids. (c) Conceptual schematic of the friction mechanism proposed for the GO/chitosan-g-PNIPAM nanohybrids when used as an additive in water-based lubrication systems (reprinted with permission from ref (71). Copyright 2021 Elsevier).