Friction reduction and viscosity modification of cellulose nanocrystals as biolubricant additives in polyalphaolefin oil

Carbohydrate Polymers 220 (2019) 228–235

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Friction reduction and viscosity modification of cellulose nanocrystals as biolubricant additives in polyalphaolefin oil

T

Ke Lia,b, Xiao Zhangb,c, Chen Dud, Jinwan Yangb,c, Bolang Wud, Zhiwei Guob,c, Conglin Dongb,c, ⁎ Ning Lind, , Chengqing Yuanb,c a

Intelligent Transport Systems Research Center, Wuhan University of Technology, Wuhan, 430063, China Reliability Engineering Institute, National Engineering Research Center for Water Transport Safety, MOST, Wuhan, 430063, China c School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, 430063, China d School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, 430070, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Cellulose nanocrystals Lubricant additive Rheological property Tribological performance

With the increasing requirement of environmental protection, the development of lubricating materials with non-toxicity and good biodegradability becomes more and more significant. As the novel green nanomaterial derived from natural cellulose, cellulose nanocrystals (CNCs) in the present work were prepared from native cotton and added into polyalphaolefin (PAO) base oil as the lubricant additive. To improve the compatibility of CNCs with PAO, the surface of CNCs were grafted by stearoyl chains, which entangled with polyolefin chains and led to a good dispersibility and stability of the colloidal solution. This hybrid oil with the elevated viscosity improved the formation of lubricant film in the boundary lubrication regime. Combining with the mending effect of CNC particles on the surface roughness and scars, both the friction and the wear were dramatically reduced. Specifically, the introduction of 2 wt% modified nanocrystals (mCNC) in PAO base oil reduced the coefficient of friction (COF) by 30%. The results of this study suggest that cellulose nanocrystal is a promising ecofriendly and effective lubricant additive.

1. Introduction To reduce the energy loss and the wear induced by the friction, lubricants are widely employed in mechanical systems. The most common used lubricants are lubricating oils, which consist of base oils and additives with various functions such as friction modifiers, antiwear agents, extreme pressure additives, viscosity index improvers, pour point depressants, detergents and so on (Minami, 2017). Besides the required technical characteristics, in recent years the pollution and environmental health aspect of lubricants drives more and more attentions. It has been estimated that around 1 million tons of loss lubricants (20% of the total market) are released into the environment every year (Delgado, Quinchia, Spikes, & Gallegos, 2017). Although the lubricant additives are minor portions, their influence to environment is also very considerable in environmentally-sensitive applications (Nagendramma & Kaul, 2012). Conventional additives (e.g. zinc dialkyl dithiophosphates, calcium sulphonates and tricresyl phosphates) contain heavy metals and harmful elements such as P, S and Cl (Hewstone, 1994). Therefore, the demand of ecofriendly lubricant additives is of

great significance. To reduce the toxicity and improve the biodegradability, many efforts have been made on molecular design of novel additives with less or no harmful components. As ashes formed by metallic composition would damage the biodegradability of the oil, Mo element in molybdenum dialkyl dithiocarbamate (MoDTC) was reported to be replaced by boron element for environmentally friendly use (Xu, Li, Sun, & Xue, 2013). Similarly, sulfur atoms in traditional anti-wear additives were exchanged with oxygen atoms and nitrogen atoms but kept similar tribological characteristics (Gao, Liu, Song, & Dai, 2018). Another example is the development of halogen-free ionic liquids as lubricant additives, with the purpose of reducing their toxicity and hazardous effect to the environment (Gusain, Gupta, Saran, & Khatri, 2014; Nagendramma, Khatri, Thakre, & Jain, 2017). Another approach is to introduce the nanoparticles as lubricant additives, which has different working principles from that of conventional oiliness agents. The conventional additives normally undergo a chemical process to bind atoms at surfaces, which needs a certain induction period and is mostly irreversible (Tang & Li, 2014). In contrast,

⁎ Corresponding author at: School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Luoshi Road #122, Wuhan, 430070, China. E-mail addresses: [email protected], [email protected] (N. Lin).

https://doi.org/10.1016/j.carbpol.2019.05.072 Received 26 February 2019; Received in revised form 23 May 2019; Accepted 24 May 2019 Available online 28 May 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.

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the film formation of nanoparticles is largely mechanical process, which will be more durable and less likely to react with other additives (Shahnazar, Bagheri, & Abd Hamid, 2016). Up to now, many types of nanoparticles such as metals (Cu, Fe, Pd), metal oxides (TiO2, SiO2, ZnO), boron-based particles (calcium borate, zinc borate, boric acid) and carbon materials (diamond, graphene, carbon nanotube) have been studied in the application of lubricant additives (Zhang, Wei et al., 2018; Chen, Liu, & Luo, 2016; Zhao, Jiao, Chen, & Ren, 2014; Zhao, Zhao, Li, Wang, & Liu, 2014; Lin, Wang, & Chen, 2011). A variety of mechanisms including rolling effect (Wu, Tsui, & Liu, 2007), protective film (Hu et al., 2002), mending effect (Liu et al., 2004) and polishing effect (Xu, Zhao, & Xu, 1996) have been proposed to explain the remarkable nano-lubricating effect as well. Although plenty of research have reported the advantages of nanoparticles as ecofriendly lubricant additives, there are still many challenges restricted to their applications. The first problem is the aggregation of nanoparticles in the lubricating systems due to their high surface energy. The agglomeration cannot enter the contact gap of the mating surfaces to improve the lubrication and, even worse, may produce abrasive wear in some cases (Kalin, Kogovsek, & Remskar, 2012). Another challenge is that the density of solid nanoparticles (e.g. Cu 8.9 g/cm3, TiO2 4.2 g/cm3, diamond 3.2 g/cm3) are usually much higher than base oils (0.85˜0.95 g/cm3), which leads to the serious sedimentation of particles in the oil (Gulzar et al., 2017). Therefore, various surfactants or modification techniques have been studied to obtain a homogeneous and stable dispersion system (Zhang, Wei et al., 2018; Lin et al., 2011). Moreover, the high production cost of nanoparticles should be also considered to make their applications more economically feasible (Shahnazar et al., 2016). Recently, the concept of ecofriendly additives from natural sustainable resources attracts much attention of researchers. As the most important biopolymers in the nature, cellulose has many attractive features such as renewability, non-toxicity, biocompatibility, and biodegradability (Grishkewich, Mohammed, Tang, & Tam, 2017), hence cellulose and its derivatives have been introduced into the lubrication in last several years. It has been reported that cellulose pulp can be used as thicken agents in biodegradable greases (Gallego, Arteaga, Vaencia, Diaz, & Franco, 2015; Nunez, Martin-Alfonso, Valencia, Sanchez, & Franco, 2012); ethyl cellulose is a potential viscosity modifier for vegetable oils (Quinchia, Delgado, Reddyhoff, Gallegos, & Spikes, 2014); and the addition of cellulose esters into the base oil can improve the friction reduction (Singh, Sharma, & Singh, 2014) and wear resistance (Zhang, Qiao et al., 2018). As the rigid nanomaterial from natural cellulose, cellulose nanocrystal (CNC) is the highly-crystalline product treated by the hydrolysis to remove the amorphous and semi-crystalline regions of cellulose fibrils (Thomas et al., 2018). According to their sources, CNCs commonly process the rod-like morphology with 5–50 nm in diameter and 100–1000 nm in length (Klemm et al., 2018). Compared with traditional inorganic and metallic nanoparticle additives for lubrication, CNCs have many unique advantages. One of the most important characters of CNCs is their desired mechanical strength (elastic modulus ranges from 100 to 200 GPa) but light weight (density =1.46 g/cm3), which will benefit their dispersion in the base oil. Furthermore, the plentiful hydroxyl groups on the surface of CNCs supply active groups for their functional modification to enhance their compatibility with the base oil (Habibi, 2014). Finally, the renewability, low cost and nontoxicity of CNC contribute its promising candidate in the industrial production and application than inorganic and metallic nanoparticles (Klemm et al., 2018). Recently, CNC was reported as the novel ecofriendly lubricant additives in engine oil. (Awang, Ramasamy, Kadirgama, Najafi, & Sidik, 2019), and the aqueous CNC suspension was used in the water-based lubricants (Shariatzadeh & Grecov, 2019). In the present study, CNCs were prepared from native cotton by hydrochloric acid hydrolysis, and added into polyalphaolefin (PAO) base oil with the purpose of anti-

friction and anti-wear. To improve the compatibility of CNCs with PAO oil, the surface modification was proposed to CNCs by the grafting of stearoyl chains. The dispersibility, rheological behavior and tribological performance of the PAO/CNC hybrid oil were successively studied. The improvement and mechanism in the friction and wear using CNC lubricant additives were proposed and discussed as well. 2. Materials and methods 2.1. Preparation and surface modification of CNCs The CNCs were isolated from native cotton by the typical hydrochloric acid hydrolysis, according to our previous study with little modification (Zhang, Liu et al., 2018). Briefly, the natural cotton was treated by the diluted NaOH solution (2 wt%) overnight to remove the impurity and lignin component. The purified cellulose fibers (20 g) were hydrolyzed in 700 mL of 4 M HCl solution (37%, Aladdin Corporation, China) for 5 h at 80 °C. The obtained suspension was purified by successive centrifugation (6000 rpm) and water washing to remove the free acid. After the homogenization for 5 min (ART DS-20, Germany), the purified suspension containing CNCs was further treated by the dialysis in water for three days, and released the powders by the freeze-drying treatment. The surface modification was performed on the CNCs based on chemical reaction between hydroxyl groups and stearyl chloride (Aladdin Corporation, China), taken the previous reports as references (de Menezes, Siqueira, Curvelo, & Dufresne, 2009; Sun, Fang, & Tomkinson, 2000). The CNCs aqueous suspension was solvent-exchanged from water to acetone and finally to N, N-dimethylformamide (DMF) for three times to obtain the dispersed CNCs in DMF (0.5 wt%). A small amount of LiCl was added in the 100 mL CNCs suspension to adsorb the trace moisture, and the nitrogen gas was introduced in the flask 30 min to remove the oxygen for subsequent reaction. The desired amount of stearyl chloride (6.5 mL) was dissolved in 20 mL DMF, and carefully transferred into the suspension using a syringe. The reaction was started after the addition of 0.1 g 4-dimethylaminopyridine (DMAP) and 2.5 mL trimethylamine (TEA) catalysts (pre-dissolved in DMF) at 80 °C for 24 h under the magnetic stirring. After the modification, the resultant suspension was washed by DMF for three times and purified by centrifugation to remove any unreacted reagent. 2.2. Preparation of hybrid oil based on PAO and modified cellulose nanocrystals (mCNC) The hybrid oils were prepared by the compounding of PAO (SpectraSyn 8, Exxon Mobil, USA) as the base oil and pristine CNC or surface-modified CNC (mCNC) as the lubricant additive. The dried CNC or mCNC powders were re-dispersed in tetrahydrofuran (THF) at the 0.5 wt% concentration after the ultrasonication for 15 min, and then carefully dropped into the PAO/THF solution with continuous magnetic stirring for 1 h. After the removal of THF solvent by the rotary evaporation, the hybrid oils of PAO/mCNC and PAO/CNC were obtained with the codes of PAO/mCNC-x and PAO/CNC-x according to the loading levels of nanocrystals (x) varying from 0.1 wt% to 2 wt%. 2.3. General characterizations The surface modification on cellulose nanocrystals were investigated by the characterizations of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The dried CNC and mCNC powders were performed on a FTIR iS5 spectrometer (Nicolet, Madison, U.S.A.) in the range 4000–400 cm−1 and a ESCALAB 250Xi equipment (Thermo Fisher Scientific, U.S.A.) for XPS analysis. The influences of surface modification on the morphology and crystalline property were studied using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The CNC or mCNC suspension was 229

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decrease of the peak at 3344 cm−1 (hydroxyl groups) can be observed by the comparison between mCNC and CNC spectra. Furthermore, an additional peak appeared at 1734 cm−1 on the spectrum of mCNC, ascribed to the stretching vibration of ester groups from surface grafting. More evidence for the surface modification can be observed on the XPS results (Fig. 2B). Attributed to the grafting of carbon-carbon chains on surface, the C/O ratios of nanocrystals increased from 1.20 of CNC to 1.31 of mCNC. The C 1s signals from XPS results can be treated by the peak decomposition to define the different carbon-based bonds. In comparison with the spectra of Fig. 2 C and D, the additional C4 peak located at 288.3 eV can be observed for OeC = O bonding in spite of the low intensity due to the surface reaction (Lin & Dufresne, 2013). All other peaks of C1 (283.7 eV), C2 (285.3 eV) and C3 (286.6 eV) ascribed to CeC, CeO and OeCeO bonding exhibited the intensities change because of the surface modification of the CNC and mCNC. Attributed to the grafting of hydrophobic carbon-carbon chains on the surface, the water contact angle of mCNC increased to 67° in comparison with that of 38° for the hydrophilic surface of CNC (as shown in Figure S1). It is essential to regulate the surface chemistry and polarity to enhance the possible interaction and compatibility between modified nanocrystals and PAO base oil. The dispersive stability of mCNC and CNC in hybrid oils was evaluated by the photographs at different time intervals. As shown in Figure S2, the PAO/mCNC-2% retained a homogeneous dispersion and stability during 5 h for the subsequent tribological tests. However, the visible sedimentation appeared at the bottom of hybrid oils containing 2 wt% pristine CNC at 20 min standing, resulting from the incompatibility and self-aggregation of hydrophilic cellulose in the PAO base oil. The results of hybrid oils at all CNC or mCNC concentrations are summarized in Table S1. The improved dispersion of mCNC in the hybrid oils may be explained by the possible entanglement from grafted C18 chains and polyolefin chains, which held and supported the dispersion of lightweight cellulose nanoparticles in the oil. The rheological behavior of lubricating oils is very important for their tribological applications. The shear-dependent viscosities of all tested oils are illustrated in Fig. 3. It can be found that PAO/CNC samples with the four tested CNC concentrations show very similar properties as the pure PAO, which is a typical Newtonian Fluid with a constant viscosity (≈ 85 mPa·s) at the entire range of measuring shear rates. However, the addition of mCNC increased the viscosity of the hybrid oil, which became more and more obvious with the increasing mCNC concentrations. Especially for the PAO/mCNC-2% sample (Fig. 3D), its viscosity increased to a much higher level than PAO/CNC2% and PAO (e.g. 3300, 380 and 84 mPa·s at γ˙ = 1 s−1 of three oils respectively), and a very strong shear-thinning behavior was observed. Besides the viscosity, the viscoelasticity of colloidal solutions is also a significant property (Zhang, Qiao et al., 2018). Thereby the storage modulus G′ and loss modulus G′′ of all samples were measured as functions of shear stress (Fig. 4). The results suggest that PAO itself behaved a pure viscous fluid with a constant G′′ and nearly zero G′. The addition of CNC into PAO had little influence on the oil moduli except a slight increase of G′ of PAO/CNC-2% sample. In the case of PAO/mCNC samples, it had higher G′ and G′′ values than PAO/CNC samples at the same doping concentrations. This tendency turned stronger with the increasing nanoparticle concentrations. When the mCNC concentration reached 2 wt%, G′ became higher than G′′ below a critical shear stress value (≈ 0.4 Pa), indicating a substantial elastic behavior of PAO/ mCNC-2%. The results of rheological measurements suggest that when the doping concentration of mCNC in PAO oil reaches a critical level a colloidal solution can be formed with better stability than unmodified CNC dispersions. As PAO/mCNC-2% sample shows the best dispersive stability, it was selected as the lubricant in the following tribological measurement. For references purposes PAO/CNC-2% and pure PAO were tested as well. Fig. 5 depicts the rotary friction test of three oils on steel surfaces. The coefficient of friction (COF) of PAO slightly decreased from 0.11 to 0.10

diluted as 0.01 wt% and negatively stained with 2% (w/v) uranyl acetate for the TEM observation on a Tecnai G2 F30 instrument (FEI, U.S.A.) at 300 kV. The XRD analysis was performed on a D8 Advance Xray diffractometer (Bruker, Germany) with diffraction angles (2θ) ranging from 5° to 45°. After the surface modification, the water contact angle of mCNC and CNC were measured, which was performed on an instrument (Powereach, China) with the 100 μL volume of water droplet by a microinjection needle. The nanocrystals’ powder was compacted as a plate-like sample with a smooth surface under a pressure of 20 MPa before the measurement. Regarding the dispersive stability of CNC and mCNC in the PAO, all the fabricated hybrid oils were left standing in room temperature at intervals for observation and photograph. The homogeneous dispersion and compatibility of nanocrystals in the hybrid oil can be evaluated by the absence or presence of visible sedimentation. 2.4. Rheological measurement The rheological tests of oils were measured using a cone-plate rheometer (MCR102, Anton Paar) with a cone of 25 mm in diameter and 2° in angle at 25 °C. Before the measurement of the oil samples, the rheometer was calibrated using a standard silicone oil supplied by Anton Paar. Then dynamic viscosities of oils were measured using the rotary mode at varied shear rates from 1 to 1000 s−1. Their viscoelastic behavior was analyzed according to the elastic modulus G′ and loss modulus G′′ of oils measured by the oscillating mode at a constant frequency of 10 rad/s and varied shear stresses from 0.01 to 10 Pa. All the tests were repeated three times. 2.5. Tribological measurement The rotary friction tests of oils were performed in a ball (diameter =12.7 mm) -on-disc mode using a Multi-Functional Tribometer (MFT5000, Rtec-Instruments, USA) at room temperature (25 °C). Both the ball and disc specimens were made of steel (GB GCr15, i.e. DIN 100Cr6) with a surface roughness (Ra) of 20 nm and a hardness of HRC 60. Before the friction test a volume of the tested oil (5 μL) was placed on the disc, then the disc was rotated at a track radius of 5 mm. As nanoparticle additives are normally considered to improve the friction in the boundary lubrication regime, the friction test in the present work was set under a relatively high load (60 N, corresponding to a contact pressure of 1.56 GPa) and a small speed (20 rpm, corresponding to a linear velocity of 10 mm/s). After the friction test, the surface morphology of steel discs was examined using scanning electron microscopy (SEM, Ultra Plus, Zeiss, Germany) and laser scanning confocal microscopy (LSCM, VK-X1000, Keyence, Japan). 3. Results With the aim of enhancing the compatibility between cellulose nanocrystals and PAO base oil, the carbon-carbon chains (C18) were grafted onto the surface of nanocrystals (Fig. 1A). The original morphology was preserved after the surface modification for nanocrystals, as observed by the rod-like nanoparticles with 100–300 nm length and 10–20 nm width from TEM images (with the measurement on the Nano Measurer software) in Fig. 1B and C. Meanwhile, both CNC and mCNC exhibited the typical crystalline features of cellulose I with 2θ of 14.9°, 16.5°, 22.8° and 34.5° ascribed to the planes of 11¯0 , 110 , 200 and 004 (Li, Tao et al., 2018; Li, Zhang et al., 2018). According to the Segal equation calculation (Segal, Creely, Martin, & Conrad, 1959), the crystalline index of CNC and mCNC were higher than 75%, indicating the maintenance of crystalline integrity before and after the surface modification. The surface modification on cellulose nanocrystals was confirmed by the results of FTIR and XPS. As shown in Fig. 2A, the intensity 230

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Fig. 1. Surface modification on cellulose nanocrystals (A); TEM images of pristine CNC (B) and mCNC (C); XRD patterns of nanocrystals before and after modification (D).

smoother, indicating a weaker stick-slip effect. After the friction test, the surface morphology of three steel discs was analyzed using both scanning electron microscopy and laser scanning confocal microscopy (Fig. 6). It can be seen that the wear scars

during the testing time (2 h). PAO/CNC-2% showed a smaller COF than pure PAO, which was about 0.09 in the end of the test. When PAO/ mCNC-2% was applied, not only the friction was much smaller than the other two samples (COF ≈ 0.07), but also the friction curve was much

Fig. 2. The FTIR spectra (A) and general XPS spectra (B) for the nanocrystals before and after modification; the XPS decomposition of C 1s signal into its constituent contributions for mCNC (C) and CNC (D). 231

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Fig. 3. The shear-dependent viscosities of hybrid oils containing CNC or mCNC at the concentrations of 0.1 wt% (A), 0.5 wt% (B), 1 wt% (C), 2 wt% (D).

the case of pure PAO. To figure it out, a micro area (94 μm × 70 μm) selected from the bottom of each wear scars were analyzed (Fig. 7B˜D). The surface roughness of the cases of pure PAO, PAO/CNC-2% and PAO/mCNC-2% were 0.169 μm, 0.097 μm and 0.051 μm in Sa and 1.587 μm, 0.981 μm and 0.830 μm in Sz respectively. This result shows that the addition of CNC, both modified and non-modified, can make the lubricated surface smoother than the case of the pure oil.

of three discs had similar width, but the wear track of PAO/mCNC-2% seemed much shallower than those of the other two oils. Then the geometric profiles of wear tracks are shown in Fig. 6A. The wear tracks of pure PAO, PAO/CNC-2% and PAO/mCNC-2% were 432 μm, 406 μm and 408 μm in width and 6.03 μm, 5.23 μm and 3.57 μm in depth respectively, which approved that the PAO/mCNC-2% led to smallest wear on the steel surfaces. Another information can be found from Fig. 7A is that the worn area generated by PAO/CNC-2% and PAO/mCNC-2% looked smoother than

Fig. 4. The shear-dependent modulus of hybrid oils containing CNC or mCNC at the concentrations of 0.1 wt% (A), 0.5 wt% (B), 1 wt% (C) and 2 wt% (D). 232

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the grooves and scars of the friction surface and compensate for the loss of mass, which is known as the mending effect. This mechanism is supported by the fact that the surfaces lubricated by PAO/CNC-2% and PAO/mCNC-2% were much smoother (Ra =97 nm and 51 nm) than the case of pure PAO (Ra = 169), and this surface roughness level was reasonable for the compensation using CNCs (diameter 10˜20 nm). Moreover, when PAO/mCNC-2% and PAO/CNC-2% are compared, their lubrication performance and mechanism are not identical. The grafted oleophilic chains on the surface of CNCs made their dispersion in PAO oil form a stable colloidal solution. The increased viscosity of the hybrid oil facilitated the formation of lubricant film and the separation of solid contacts in the boundary lubrication regime, which was revealed by the subdued stick-slip effect on its friction curve. Thus, besides the mending effect as nanoparticles, mCNC also works as a viscosity modifier and leads to a further lower friction and smaller wear than PAO/CNC-2%. The overall lubrication mechanism of PAO/mCNC hybrid oil is illustrated in Fig. 8. As the initial attempt on the use of cellulose nanocrystals in the application of lubricant additive, in future the grafting density and the molecular structure of the grafted chains, together with the surfacemodification technique and the doping technique, need to be investigated to further improve the colloidal stability of CNCs in base oils. On the other hand, a too rigid colloidal solution might result in an excessively high viscosity, which also does not benefit the lubrication. The balance of the viscosity should be considered and the shear-thinning is preferable, which can provide a high load-carrying ability at low velocities and a small shear resistance at high velocities (Li, Tao et al., 2018; Li, Zhang et al., 2018). Therefore, a series of friction tests with various operating conditions should be studied in the furfure work as well.

Fig. 5. The rotary friction tests (60 N, 20 rpm, 5 mm sliding radius, 25 °C) of pure PAO, PAO/CNC-2% and PAO/mCNC-2% on steel surfaces.

4. Discussion Without changing the geometry and crystal structure, the surfacegrafting of stearoyl chains enhanced the hydrophobicity of cellulose nanocrystals and strongly improved their compatibility with the base oil. When surface modified cellulose nanocrystals (mCNC) concentration was relatively high (2 wt% here), the entanglement between the stearoyl chains on mCNC surface and the PAO chains works like a physical crosslinking and forms a colloidal solution, which results in the different properties of PAO/mCNC-2% from PAO/CNC-2%, such as the improved dispersive stability, the obvious elastic behavior, the increased viscosity and the superior tribological performance. By incorporating CNCs into the base oil, both the friction and the wear were reduced. The rolling effect is often proposed to explain the friction reduction using spherical nano-additives such as nanodiamonds (Chen et al., 2016) and fullerene-like particles (Cizaire et al., 2002). However, this mechanism is not so likely in the case of CNCs, due to their rod geometry. A more convincing explanation is that the CNCs fill

5. Conclusions The present work investigated a novel ecofriendly lubricant additive using CNCs prepared from natural sources through a hydrolysis process. The obtained CNCs were surface modified by the grafting of stearoyl chains, and then added into PAO oil as the lubricating additive with various concentrations from 0.1 wt% to 2 wt%. The obtained PAO/ mCNC-2% hybrid oil exhibited a good dispersive stability and increase

Fig. 6. The SEM and LSCM images of wear scars on the steel discs after the friction tests using pure PAO (A, D), PAO/CNC-2% (B, E) and PAO/mCNC-2% (C, F). 233

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Fig. 7. The geometric profiles of wear scars on the steel discs after the friction tests (A); surface roughness of the bottom area in each wear scar lubricated by pure PAO (B), PAO/CNC-2% (C) and PAO/mCNC-2% (D).

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Fig. 8. Schematic of proposed lubricating mechanism of PAO/mCNC hybrid oil.

of viscosity due to the entanglement of grafted stearoyl and PAO chains. Compared with PAO/CNC-2% and pure PAO oil, the hybrid oil PAO/ mCNC-2% showed the lowest friction (COF of 0.07) and smallest wear loss, which is attributed to two mechanisms. (i), the compensation of CNC nanoparticles for the roughness and scars of sliding surfaces leads to a mending effect; (ii), the good colloidal stability of modified CNCs in the base oil improves the boundary lubrication as a viscosity modifier. From the results of this study, it can be concluded that cellulose nanocrystal is a promising lubricant additive especially for green applications.

Acknowledgments This study was supported by the National Natural Science Foundation of China (51605351 and 51603159). The authors also thank the support of the Fundamental Research Funds for the Central Universities (WUT: 191044005).

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2019.05.072.

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