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Multifunctional nanofluids with 2D nanosheets for thermal and tribological management
Wear 302 (2013) 1241–1248
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Multifunctional nanofluids with 2D nanosheets for thermal and tribological management ˜ a-Paras b,1, T.N. Narayanan a,1, L. Garza b, C. Lapray b, J. Taha-Tijerina a,n,1, L. Pen b b J. Gonzalez , E. Palacios , D. Molina b, A. Garcı´a b, D. Maldonado b, P.M. Ajayan a a b
Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, USA Universidad de Monterrey, Av. Morones Prieto 4500 Pte. San Pedro Garza Garcı´a, N.L. 66238, Me´xico
a r t i c l e i n f o
abstract
Article history: Received 16 October 2012 Received in revised form 29 November 2012 Accepted 10 December 2012 Available online 8 January 2013
Development of multifunctional nanofluids for applications such as thermal management and lubrication is very essential for the advancement of many energy efficient modern industries. Here, we demonstrate the tribological improvement of two dimensional (2D) atomic sheets, such as hexagonal boron nitride (h-BN) and graphene (G), reinforced mineral oils over bare mineral oil, which was proven earlier by its better thermal performance. Latter, thermal and tribological properties of dielectric and metal-cutting lubricants reinforced with h-BN and G were investigated with a detailed study on their viscosity variation with temperature and nanofiller concentration. Two different tribological tests were performed for coefficient of friction and wear studies, namely ITEePib Polish method, and ASTM D5183. Results from these methods are compared and they show in agreement with each other. Tribological tests showed that the addition of nanofillers, even in small filler fraction, resulted in to a significant decrease in the wear scar and friction coefficient along with a huge enhancement in thermal performance. & 2012 Elsevier B.V. All rights reserved.
Keywords: Hexagonal boron nitride (h-BN) Graphene 2-D nanosheets Tribology Friction
1. Introduction Energy management is becoming more crucial for meeting the rising needs of mankind [1]. Nowadays, with increasing pressure of globalized markets and companies’ profit race, a dramatic search for materials having high energy efficient performance is being intensified. The advent of nanomaterials motivated scientist to address this, and to design various energy efficient systems, where these ultra-fine materials can play a seminal role in the energy management with lower amount of materials. A revolution in the field of nanofluids (NFs) was happened with the advent of thermal management fluids, a concept introduced by Choi in 1990s [2]. NFs contain smaller nanoparticles (ultra-fine particles) stabilized in a carrier fluid, and are attracted the attention of researchers to use them for thermal and electrical management and also for tribology. Friction plays a key role in diverse processes such as drilling, cutting, working pair components and mechanisms, among others; becoming more relevant in today’s life. Wear is the major cause of material and energy loss in mechanical processes, as components are in constant friction. Lubricants can be used to
n
Corresponding author. Tel.: þ1 7133485904. E-mail addresses: [email protected], [email protected] (J. Taha-Tijerina). 1 Contributed equally to this work.
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.010
minimize contact friction between components, resulting into considerable energy and tooling savings [3]. However, frictional heat generated, when two or more moving surfaces are in contact can degrade lubricants or oxidize them; thus, this heat generated should be dissipated rapidly. Recently, diverse nanoparticles were tested as reinforcements in common lubricants and metal-cutting fluids for their enhanced tribological properties [4–9]. Some of these studies show that the addition of nanoparticles to conventional lubricants can enhance the supporting force during loading and sliding movement, thus decreasing tribological issues such as friction and wear. Nanoparticles could be deposited on the rubbing surface and improve the tribological properties of conventional lubricants and metalcutting fluids, showing the contribution of friction and wear reduction by dispersed nanoparticles The addition of nanoparticles or nanoadditives to NFs has shown excellent results in tribological tests reducing wear and the frictional coefficient, also improving load carrying capacity [8–13]. Herna´ndezBattez et al. [9] studied the friction behavior of NiCrBSi coatings lubricated by CuO nanoparticles suspended in poly-alphaolephin (PAO6). Block-on-ring tests showed a decrease of up to 100% in the friction coefficient with 2 wt% CuO. Similarly, Yu et al. [11] reported improved lubricating properties by adding 0.2 wt% Cu nanoparticles to lubricant oil; in their study Cu formed a soft film by frictionshearing and high pressure reducing the coefficient of friction up to 20%. Wu et al. [13] examined the tribological properties of nanofluids
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Fig. 1. TEM and HRTEM images, respectively, of: G (a) and (c) (number of layers 10); h-BN (b) and (d) (number of layers 5).
Fig. 2. (top) Micro-Raman spectrum (633 nm excitation) of h-BN nanosheets depicting the E2g mode of B–N vibrations in h-BN nanosheets, and (bottom) in the case of exfoliated graphite (G) the reflection peaks are broadened, but still G consists of few layers of ordered graphene sheets.
nanoparticles on the worn surface, decreasing the shearing stress and friction coefficient. On the other hand, generated heat during the frictional loss needs to be draws out from the system in an efficient manner; thus a good lubricant must possess adequate thermal conductivity (TC). Various NFs [9,14] were tested in the past couple of years for their thermal performance and have been reviewed in our previous report [15]. Recently two dimensional nanomaterials (2D) got remarkable scientific attention due to their unique physical properties and high surface area [16]. The large scale wet chemical synthesis of these 2D nanomaterials attracted their applications in various composites. The bulk counter parts of some of these 2D materials such as graphene and hexagonal boron nitride (h-BN) are already known for their lubrication properties for a long time. Recently some of the researchers could able to synthesize stable nanofluids from these 2D materials in various carrier fluids [15,16]. In this report, we are proposing the multifunctional aspects of 2D-nanosheets based nanofluids, where they can do thermal management at the same time reducing the contact friction, even with very small filler fraction of nanomaterials. Tribological improvement of NFs with 2D-nanosheets is first substantiated with a mineral oil (MO) based nanofluidic system and then generalized with other common lubricating, metal-cutting/drilling NFs.
2. Materials and methods of API-SF oil (SAE30 LB51153) and base oil, with sphere-like CuO, TiO2, and nano-diamond nanoparticles. Their anti-wear behavior is attributed to the sphere-like morphology of the nanoparticles resulting in a rolling effect between the surfaces; and the deposition of the
In our work we performed a liquid exfoliation of micrometersized layered h-BN crystals (Sigma Aldrich), where we obtained 2D h-BN nanosheets ( 500 nm by 500 nm). This process allows us to obtain thin layer nanostructures containing a few atomic
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Table 1 Material properties. Materials
Properties
Base fluids Mineral oil (Nytro 10XN) EcoDraw HVE (1:6) Montgomery DB 4265 C-EX Metkut H1-EC Metkool 10131TA-S
Density (15 1C) 0.88 g/cm3 1.03 g/cm3 0.994 g/cm3 0.91 g/cm3 1.01 g/cm3
Nanoparticles Boron nitride (h-BN) Graphene (G) Test balls AISI 52100
Purpose/application Electrical insulating fluid Metal stamping lubricant Metal stamping lubricant Metal cutting fluid Metal cutting fluid
Morphology: 2D structures Size: 500 nm by 500 nm, 5 atomic layers Morphology: 2D structures Size: 500 nm by 500 nm, 10 atomic layers Chemical composition 0.98–1.1% C, 0.15–0.30% Si, 0.25–0.45% Mn, 1.30–1.60% Cr, o 0.025% P, o 0.025% S Diameter: 12.7 mm, 60 HRC
Note: base fluids viscosities will be further discussed.
Fig. 3. (a) and (b) Operation system of tribotester T-02: (1) ball chuck, (2) rotating ball, (3) stationary balls, and (4) ball pot [17,19].
layers ( 5–10 layers) that can be stabilized in mineral oil (MO) via molecular interactions as well as Brownian motion mechanism [15]. Similarly, graphite powder (Bay Carbon, SP-I) was processed, and we obtained 2D graphene nanosheets, typically containing 8–10 atomic layers, as shown in Fig. 1. Raman spectrum (Fig. 2) of h-BN at 1369 cm 1 originated from E2g mode of B–N bond vibration as seen in Fig. 2. The Raman spectrum of graphene shows the disorder-induced D peak at 1350 cm 1, G peak at 1595 cm 1 and 2D peak at 2695 cm 1. MO was reinforced with varying concentration by weight of hBN and graphene 2D-nanosheets, similarly lubricant and metalcutting fluids were also reinforced with 2D nanofillers. Basic characteristics of these fluids are shown in Table 1. The tribological properties of nanofluids were tested by a ITEePib [17] Polish method using a four-ball tribotester for testing lubricants under scuffing conditions and the ASTM D5183 method [18]. These two tests were compared in order to determine if the Polish method showed similar results to the ASTM method.
Fig. 4. Typical chart for scuffing initiation [19].
Table 2 Parameters of tests done using the T-02 tribotester. Parameters
ASTM D5183
ITEePib Polish method
Time Velocity (RPM) Temperature (1C) Applied force (N)
60 min 600 75 392
18 s 500 23 0–7200 (linear increment)
Each TC value is the average of at least 5 consecutive measurements, after reaching the thermal equilibrium in each case. 3.2. Viscosity
3. Experimental details
Shear viscosity studies were conducted with a TA Instruments ARES rheometer. Temperature dependent shear viscosity measurements for different concentrations of fluids reinforced with hBN (viscosity measurements of 0.01 wt% G also shown for comparison) were performed.
3.1. Thermal experimentation
3.3. Tribology experimentation
TC measurements on h-BN and G nanofluids of different weight percentages were carried out using a KD2 probe (Decagon Device Inc., model KD2 Pro). The measured TC values are compared with the base lubricants and fluids (TC of base fluid/lubricant is k0). The effective TC of the NF is keff. Experimental conditions were optimized to minimize error due to other mechanisms [15], such as free or forced convection.
A tribotester (T-02) with a four-ball fixture was used to determine the anti-wear properties of lubricants under pressure and controlled temperature. Machine operation involves four steel balls; one of them (top one) rotates applying a pressure with a load P, while rotating with a speed n, and under this ball, three stationary balls are secured by a holder as shown in Fig. 3. The ball test
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material was an AISI 52100 steel with a 12.7 mm diameter, and a hardness of 60 HRC. Using this tribotester, the ITEePib [16] Polish method for testing lubricants under scuffing conditions can be used to determine the friction torque, the maximum applied load, and the temperature of the lubricants; obtaining a similar chart as shown in Fig. 4. This chart helps to identify the extreme pressure properties of NFs, namely the time and load when the wear and the loss of film lubricant occur. The ITEePib Polish method shows a poz indicator, the pressure loss limit of lubricant film (see Fig. 4). The seizure appears when the film lubricant disappears because of the increment of load, at this moment the lowers and upper ball have a metal–metal contact. The wear scar diameter (WSD) of the 3 stationary balls are measured by an optical microscope and used to calculate poz. This parameter is calculated as follows: poz ¼ 0:52ðP oz =WSD2 Þ
ð1Þ 2
where, poz is the limiting pressure of seizure (N/mm ) (higher values are expected). Poz the seizure load (N), should be measured to the nearest 100 N.
The 0.52 coefficient results from the force distribution in the four-ball tribosystem. The greater poz value, the better action of the tested lubricant under scuffing conditions is [19]. ASTM D5183 [18] methodology can also be applied on this tribotester (T-02) to determine the coefficient of friction (COF) and WSD of diverse lubricants under constant load conditions. The parameters for each test are showed in Table 2.
4. Results and discussion Temperature-dependent TC for h-BN and G reinforced lubricants and metal-cutting fluids at various weight percentages are shown in Fig. 5. The keff of NFs increases with temperature (measurements performed from room temperature to 501C (323 1K), indicating the role of Brownian motion in TC enhancement and, at the same time they exhibit an enhancement with increase in filler fraction, revealing the role of percolation mechanisms as it is observed for other oil based samples[14]. These nanofluids show a temperature dependent variation in the thermal conductivity, indicating the role of nanoparticles in
Fig. 5. (a)–(d) Temperature-dependent effective thermal conductivity (TC) enhancement of various nanofluids (percentage of filler amount is mentioned). Pure base fluids show no significant variation in TC with temperature. All nanofluids show an enhancement in TC with temperature, indicating the contribution of Brownian motion and percolation mechanism in TC enhancement.
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thermal conductivity [20]. Moreover, for oil based samples, liquid layering at the particle/liquid can also contribute to the enhancement in TC [21,22]. In general, the viscosity plays an important role in these fluids. In our work it is observed that viscosity decreases significantly with increase in temperature, as it is shown in Fig. 6, while the enhancement in viscosity with the addition of the nanofillers is very small (in low filler fractions), which is an advantage of using low filler fractions since the increase in viscosity can decrease the keff of NFs, as well as flow characteristics of the fluids. It is also to be noted that nature of enhancement in TC with filler fraction and temperature is differ from fluid to fluid. Many factors such as fluids composition, viscosity, nature of fluids (morphology as well as interaction between fluid and nanofillers), among others can be the reasons for this difference. In this work, we found that, factors such as temperature and filler amount is more sensitive in determining the keff in low viscosity fluids. Fig. 7a and b shows the COF and steel balls WSD tribo-testing results on MO/h-BN and MO/G according to ASTM D5183 compared to bare MO, where those fluids were previously proven for their outstanding thermal performance [15]. For MO/h-BN and MO/G NFs
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at very low filler fraction (0.01 wt%) a decrease by 10% and 20%, respectively, was shown. The COF results from graphene fluid are similar to that reported by Eswaraiah et al. [10] showing a 20% reduction with 0.01 wt% of G, which is attributed to the formation of graphene nanobearings between the tester steel balls. Also, the 2Dnanosheets morphology could promote an ease sliding mechanism that contribute this effect. Moreover, these nanofillers were also tried to stabilize in MO with surfactant coating. Common surfactant such as oleic acid (OA) (2 vol%) is added to the fluid with extensive sonication (4 h). OA-stabilized MO based fluids were also tested for their tribological properties. Bare MO with OA addition did not show significant enhancement in keff (o2% at 50 1C). TC of OAcoated fluids neither show any change at the filler fraction of 0.01 wt%. However, COF and WSD shows a decrease compared to the surfactant-less material ( 6% more), indicating that OA may help to separate the nanoparticles thereby decreasing the agglomeration and/or additional layering due to OA can increase the lubrication properties. Later it is found that, agglomeration of nanofillers can adversely affect the lubrication properties of nanofillers.
Fig. 6. (a)–(d) Temperature-dependent viscosity variation of nanofluids. Small increments as adding filler fraction to nanofluids.
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Wear scars of steel balls for h-BN and G in MO are shown in Fig. 8. Common steel ball before the testing is shown if Fig. 8a as reference. Fig. 8b shows the nanolubricated with MO/h-BN upper tested steel ball wear scar (and the actual steel ball on the inset). An h-BN nanosheet is shown on the surface of tested steel ball in Fig. 8c, as also shown in Fig. 9 for EDAX analysis. Fig. 8d shows the MO/G wear scar as well. For metal-cutting fluids, results obtained by the ITEePib Polish and ASTM D5183 methods were compared and analyzed in order to determine which lubricant performed better under different
friction conditions. The main difference between these two methods is that the ITEePib Polish method tests lubricants under extreme pressure conditions for short time, while the ASTM D5183 works under constant loads for longer time. Table 3 shows the decrease on COF and steel balls WSD during tribo-testings according to ASTM D5183, compared to bare fluids. The WSD was calculated from the average diameter of the three lower-balls (see Fig. 3), at least 3 tests for each fluid, according to Dixon probabilistic methodology to generate statistical reliable results, were run. As lower the COF and WSD value, a better fluid
Fig. 7. COF and WSD enhancement of MO reinforced at 0.01 wt% of h-BN and G (ASTM D5183), measured values above the symbols.
Fig. 8. SEM images of steel balls, (a) before tribo-testing; tested using (b) MO/h-BN, showing the wear scar, (c) h-BN flakes on wear scar and (d) MO/graphene.
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Fig. 9. EDAX of test steel balls (top) for bare oil and (bottom) MO/h-BN.
Table 3 Decrease in WSD and COF of h-BN and G reinforced NFs obtained by the ASTM D5183 method. Tribology tests—ASTM D5183 method
WSD—f units: mm (%) EcoDraw Montgomery Metkut Metkool COF—l EcoDraw Montgomery Metkut Metkool
Pure
@0.01 h-BN
@0.05 h-BN
@0.10 h-BN
@0.01 G
@0.10 G
1.0692 (0%) 1.0606 (0%) 0.7520 (0%) 1.2018 (0%)
1.0499 ( 1.81%) 1.0481 ( 1.17%) 0.6207 ( 17.45%) 1.1019 ( 8.32%)
1.0430 ( 2.45%) 0.9930 ( 6.37%) 0.5539 ( 26.34%) 1.0933 ( 9.03%)
0.9826 ( 8.10%) 0.9732 ( 8.24%) 0.6352 ( 15.53%) 1.0527 ( 12.40%)
1.0665 ( 0.26%) 1.0500 ( 1.00%) 0.6267 ( 16.67%) 1.0946 ( 8.92%)
1.0044 ( 6.06%) 0.9769 ( 7.89%) 0.6178 ( 17.84%) 1.0658 ( 11.31%)
0.1337 0.1314 0.1207 0.1472
0.1275 0.1197 0.1352 0.1434
0.1247 0.1180 0.1105 0.1422
0.1399 0.1188 0.1046 0.1396
0.1327 0.1287 0.1197 0.1451
0.1276 0.1201 0.1049 0.1422
for tribological applications performance. In general, the addition of nanoparticles resulted in a significant decrease on these parameters. For instance, MO showed a decrease on COF and WSD up to 19.25% (m ¼0.1494) and 10.79% (WSD¼0.8141 mm) respectively, with the addition of 0.01 wt% G (Fig. 7). For metal-cutting NFs the addition of h-BN and G showed similar improvement values on both COF and WSD, with Metkut NFs showing the major reduction of COF and WSD, possibly due to the oleophilic behavior of 2D nanosheets. The tribological mechanism for this test, as explained by Mosleh et al. [23], may consist on nanoparticles filling valleys, and the shearing of trapped nanoparticles at the interface of contacting surfaces, thus making them smooth and lowering the frictional forces. Table 4 presents the results obtained for lubricants and metalcutting fluids, using the Polish method for calculating the extreme pressure (EP) properties of NFs, also known as the pressure loss limit of lubricant film Here the higher the poz the better the lubricant,
Table 4 Pressure loss limit results (poz) of NF films reinforced with h-BN and G obtained by the ITEePib Polish method. Tribology tests—ITEePib Polish method Pure
@0.01 h-BN
Polish—poz EcoDraw 755.14 231.60 Montgomery 3528.53 3255.30 Metkut 1079.16 2092.93 Metkool 3300.08 3338.99
@0.05 h-BN
@0.10 h-BN
@0.01 G
@0.10 G
2743.03 2956.86 2485.17 2931.56
2172.48 3257.70 2616.09 290.21
188.88 414.67 3664.80 3237.92 2395.40 2751.18 3546.40 3460.89
which works best under friction conditions. The COF is not included since it is unusable due to the sudden increment of COF, particularly when the lubricant film disappears and high pressure occur during
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the test (see Fig. 4). In contrast with the COF and WSD results obtained by the ASTM D5183 method there was no apparent correlation between the poz and the type and concentration of nanofillers. Metkut, with the highest viscosity, NFs showed the highest improvement on poz with the addition on nanoparticles. The load carrying capacity increased up to 142.42% and 154.94% with 0.10% h-BN and 0.10% G, respectively. This shows that a small addition of nanoparticles may result in a considerable enhancement on the tribological behavior. Here the tribological mechanism, as proposed by Hu et al. [24], may consist on the tribosintering of nanoparticles into the surface due to the extreme pressure effect. For lower viscosity lubricants, however, the poz values are less consistent and in general showed either much smaller increments or an increase of COF and WSD. This can be explained as the lower viscosities result in a much rapid sedimentation and re-agglomeration of nanoparticles.
5. Conclusions 2D nano-sheets of h-BN and G based thermally conducting fluids for lubrication and metal-cutting applications were developed. It is concluded that reinforcement of low filler fraction of 2D nanofillers can effectively reduce the contact friction. Two different tests, ASTM standard D5183, and a Polish method (from ITEePib) were performed for testing lubricants under conditions of scuffing and the results are in agreement each other, in general. The layering mechanism explained for the lubrication reduction with graphene engine oil fluid is more generalized to other layered materials based fluids such as h-BN. These multifunctional nanofluids can be better candidates for various energy management fields in future. Moreover, the advantage over the measurement time and material required with Polish method indicates that further lubrication testing on these fluids for other filler fractions can be undertaken with Polish method.
Acknowledgments J.T.-T. acknowledges the support from PGE and CONACYT (213780). L.P.-P. and D.M. acknowledge the support from UDEM VIAC grant 10T-PR098-12. M.A., T.N.N., and J.T.-T. acknowledge funding from the U.S. Army Research Office MURI grant W911NF11-1-0362, the U.S. Office of Naval Research MURI grant N000014-09-1-1066 on novel free-Standing 2D crystalline materials focusing on atomic layers of nitrides,oxides,and sulfides. Authors acknowledge the support from PGE, Emerson and Metalsa for supplying the lubricants and metal-cutting fluids for this research. References [1] R.E. Smalley, Future global energy prosperity: the terawatt challenge, MRS Bulletin 30 (2005) 412–417.
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Multifunctional nanofluids with 2D nanosheets for thermal and tribological management ˜ a-Paras b,1, T.N. Narayanan a,1, L. Garza b, C. Lapray b, J. Taha-Tijerina a,n,1, L. Pen b b J. Gonzalez , E. Palacios , D. Molina b, A. Garcı´a b, D. Maldonado b, P.M. Ajayan a a b
Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, USA Universidad de Monterrey, Av. Morones Prieto 4500 Pte. San Pedro Garza Garcı´a, N.L. 66238, Me´xico
a r t i c l e i n f o
abstract
Article history: Received 16 October 2012 Received in revised form 29 November 2012 Accepted 10 December 2012 Available online 8 January 2013
Development of multifunctional nanofluids for applications such as thermal management and lubrication is very essential for the advancement of many energy efficient modern industries. Here, we demonstrate the tribological improvement of two dimensional (2D) atomic sheets, such as hexagonal boron nitride (h-BN) and graphene (G), reinforced mineral oils over bare mineral oil, which was proven earlier by its better thermal performance. Latter, thermal and tribological properties of dielectric and metal-cutting lubricants reinforced with h-BN and G were investigated with a detailed study on their viscosity variation with temperature and nanofiller concentration. Two different tribological tests were performed for coefficient of friction and wear studies, namely ITEePib Polish method, and ASTM D5183. Results from these methods are compared and they show in agreement with each other. Tribological tests showed that the addition of nanofillers, even in small filler fraction, resulted in to a significant decrease in the wear scar and friction coefficient along with a huge enhancement in thermal performance. & 2012 Elsevier B.V. All rights reserved.
Keywords: Hexagonal boron nitride (h-BN) Graphene 2-D nanosheets Tribology Friction
1. Introduction Energy management is becoming more crucial for meeting the rising needs of mankind [1]. Nowadays, with increasing pressure of globalized markets and companies’ profit race, a dramatic search for materials having high energy efficient performance is being intensified. The advent of nanomaterials motivated scientist to address this, and to design various energy efficient systems, where these ultra-fine materials can play a seminal role in the energy management with lower amount of materials. A revolution in the field of nanofluids (NFs) was happened with the advent of thermal management fluids, a concept introduced by Choi in 1990s [2]. NFs contain smaller nanoparticles (ultra-fine particles) stabilized in a carrier fluid, and are attracted the attention of researchers to use them for thermal and electrical management and also for tribology. Friction plays a key role in diverse processes such as drilling, cutting, working pair components and mechanisms, among others; becoming more relevant in today’s life. Wear is the major cause of material and energy loss in mechanical processes, as components are in constant friction. Lubricants can be used to
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Corresponding author. Tel.: þ1 7133485904. E-mail addresses: [email protected], [email protected] (J. Taha-Tijerina). 1 Contributed equally to this work.
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.010
minimize contact friction between components, resulting into considerable energy and tooling savings [3]. However, frictional heat generated, when two or more moving surfaces are in contact can degrade lubricants or oxidize them; thus, this heat generated should be dissipated rapidly. Recently, diverse nanoparticles were tested as reinforcements in common lubricants and metal-cutting fluids for their enhanced tribological properties [4–9]. Some of these studies show that the addition of nanoparticles to conventional lubricants can enhance the supporting force during loading and sliding movement, thus decreasing tribological issues such as friction and wear. Nanoparticles could be deposited on the rubbing surface and improve the tribological properties of conventional lubricants and metalcutting fluids, showing the contribution of friction and wear reduction by dispersed nanoparticles The addition of nanoparticles or nanoadditives to NFs has shown excellent results in tribological tests reducing wear and the frictional coefficient, also improving load carrying capacity [8–13]. Herna´ndezBattez et al. [9] studied the friction behavior of NiCrBSi coatings lubricated by CuO nanoparticles suspended in poly-alphaolephin (PAO6). Block-on-ring tests showed a decrease of up to 100% in the friction coefficient with 2 wt% CuO. Similarly, Yu et al. [11] reported improved lubricating properties by adding 0.2 wt% Cu nanoparticles to lubricant oil; in their study Cu formed a soft film by frictionshearing and high pressure reducing the coefficient of friction up to 20%. Wu et al. [13] examined the tribological properties of nanofluids
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Fig. 1. TEM and HRTEM images, respectively, of: G (a) and (c) (number of layers 10); h-BN (b) and (d) (number of layers 5).
Fig. 2. (top) Micro-Raman spectrum (633 nm excitation) of h-BN nanosheets depicting the E2g mode of B–N vibrations in h-BN nanosheets, and (bottom) in the case of exfoliated graphite (G) the reflection peaks are broadened, but still G consists of few layers of ordered graphene sheets.
nanoparticles on the worn surface, decreasing the shearing stress and friction coefficient. On the other hand, generated heat during the frictional loss needs to be draws out from the system in an efficient manner; thus a good lubricant must possess adequate thermal conductivity (TC). Various NFs [9,14] were tested in the past couple of years for their thermal performance and have been reviewed in our previous report [15]. Recently two dimensional nanomaterials (2D) got remarkable scientific attention due to their unique physical properties and high surface area [16]. The large scale wet chemical synthesis of these 2D nanomaterials attracted their applications in various composites. The bulk counter parts of some of these 2D materials such as graphene and hexagonal boron nitride (h-BN) are already known for their lubrication properties for a long time. Recently some of the researchers could able to synthesize stable nanofluids from these 2D materials in various carrier fluids [15,16]. In this report, we are proposing the multifunctional aspects of 2D-nanosheets based nanofluids, where they can do thermal management at the same time reducing the contact friction, even with very small filler fraction of nanomaterials. Tribological improvement of NFs with 2D-nanosheets is first substantiated with a mineral oil (MO) based nanofluidic system and then generalized with other common lubricating, metal-cutting/drilling NFs.
2. Materials and methods of API-SF oil (SAE30 LB51153) and base oil, with sphere-like CuO, TiO2, and nano-diamond nanoparticles. Their anti-wear behavior is attributed to the sphere-like morphology of the nanoparticles resulting in a rolling effect between the surfaces; and the deposition of the
In our work we performed a liquid exfoliation of micrometersized layered h-BN crystals (Sigma Aldrich), where we obtained 2D h-BN nanosheets ( 500 nm by 500 nm). This process allows us to obtain thin layer nanostructures containing a few atomic
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Table 1 Material properties. Materials
Properties
Base fluids Mineral oil (Nytro 10XN) EcoDraw HVE (1:6) Montgomery DB 4265 C-EX Metkut H1-EC Metkool 10131TA-S
Density (15 1C) 0.88 g/cm3 1.03 g/cm3 0.994 g/cm3 0.91 g/cm3 1.01 g/cm3
Nanoparticles Boron nitride (h-BN) Graphene (G) Test balls AISI 52100
Purpose/application Electrical insulating fluid Metal stamping lubricant Metal stamping lubricant Metal cutting fluid Metal cutting fluid
Morphology: 2D structures Size: 500 nm by 500 nm, 5 atomic layers Morphology: 2D structures Size: 500 nm by 500 nm, 10 atomic layers Chemical composition 0.98–1.1% C, 0.15–0.30% Si, 0.25–0.45% Mn, 1.30–1.60% Cr, o 0.025% P, o 0.025% S Diameter: 12.7 mm, 60 HRC
Note: base fluids viscosities will be further discussed.
Fig. 3. (a) and (b) Operation system of tribotester T-02: (1) ball chuck, (2) rotating ball, (3) stationary balls, and (4) ball pot [17,19].
layers ( 5–10 layers) that can be stabilized in mineral oil (MO) via molecular interactions as well as Brownian motion mechanism [15]. Similarly, graphite powder (Bay Carbon, SP-I) was processed, and we obtained 2D graphene nanosheets, typically containing 8–10 atomic layers, as shown in Fig. 1. Raman spectrum (Fig. 2) of h-BN at 1369 cm 1 originated from E2g mode of B–N bond vibration as seen in Fig. 2. The Raman spectrum of graphene shows the disorder-induced D peak at 1350 cm 1, G peak at 1595 cm 1 and 2D peak at 2695 cm 1. MO was reinforced with varying concentration by weight of hBN and graphene 2D-nanosheets, similarly lubricant and metalcutting fluids were also reinforced with 2D nanofillers. Basic characteristics of these fluids are shown in Table 1. The tribological properties of nanofluids were tested by a ITEePib [17] Polish method using a four-ball tribotester for testing lubricants under scuffing conditions and the ASTM D5183 method [18]. These two tests were compared in order to determine if the Polish method showed similar results to the ASTM method.
Fig. 4. Typical chart for scuffing initiation [19].
Table 2 Parameters of tests done using the T-02 tribotester. Parameters
ASTM D5183
ITEePib Polish method
Time Velocity (RPM) Temperature (1C) Applied force (N)
60 min 600 75 392
18 s 500 23 0–7200 (linear increment)
Each TC value is the average of at least 5 consecutive measurements, after reaching the thermal equilibrium in each case. 3.2. Viscosity
3. Experimental details
Shear viscosity studies were conducted with a TA Instruments ARES rheometer. Temperature dependent shear viscosity measurements for different concentrations of fluids reinforced with hBN (viscosity measurements of 0.01 wt% G also shown for comparison) were performed.
3.1. Thermal experimentation
3.3. Tribology experimentation
TC measurements on h-BN and G nanofluids of different weight percentages were carried out using a KD2 probe (Decagon Device Inc., model KD2 Pro). The measured TC values are compared with the base lubricants and fluids (TC of base fluid/lubricant is k0). The effective TC of the NF is keff. Experimental conditions were optimized to minimize error due to other mechanisms [15], such as free or forced convection.
A tribotester (T-02) with a four-ball fixture was used to determine the anti-wear properties of lubricants under pressure and controlled temperature. Machine operation involves four steel balls; one of them (top one) rotates applying a pressure with a load P, while rotating with a speed n, and under this ball, three stationary balls are secured by a holder as shown in Fig. 3. The ball test
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material was an AISI 52100 steel with a 12.7 mm diameter, and a hardness of 60 HRC. Using this tribotester, the ITEePib [16] Polish method for testing lubricants under scuffing conditions can be used to determine the friction torque, the maximum applied load, and the temperature of the lubricants; obtaining a similar chart as shown in Fig. 4. This chart helps to identify the extreme pressure properties of NFs, namely the time and load when the wear and the loss of film lubricant occur. The ITEePib Polish method shows a poz indicator, the pressure loss limit of lubricant film (see Fig. 4). The seizure appears when the film lubricant disappears because of the increment of load, at this moment the lowers and upper ball have a metal–metal contact. The wear scar diameter (WSD) of the 3 stationary balls are measured by an optical microscope and used to calculate poz. This parameter is calculated as follows: poz ¼ 0:52ðP oz =WSD2 Þ
ð1Þ 2
where, poz is the limiting pressure of seizure (N/mm ) (higher values are expected). Poz the seizure load (N), should be measured to the nearest 100 N.
The 0.52 coefficient results from the force distribution in the four-ball tribosystem. The greater poz value, the better action of the tested lubricant under scuffing conditions is [19]. ASTM D5183 [18] methodology can also be applied on this tribotester (T-02) to determine the coefficient of friction (COF) and WSD of diverse lubricants under constant load conditions. The parameters for each test are showed in Table 2.
4. Results and discussion Temperature-dependent TC for h-BN and G reinforced lubricants and metal-cutting fluids at various weight percentages are shown in Fig. 5. The keff of NFs increases with temperature (measurements performed from room temperature to 501C (323 1K), indicating the role of Brownian motion in TC enhancement and, at the same time they exhibit an enhancement with increase in filler fraction, revealing the role of percolation mechanisms as it is observed for other oil based samples[14]. These nanofluids show a temperature dependent variation in the thermal conductivity, indicating the role of nanoparticles in
Fig. 5. (a)–(d) Temperature-dependent effective thermal conductivity (TC) enhancement of various nanofluids (percentage of filler amount is mentioned). Pure base fluids show no significant variation in TC with temperature. All nanofluids show an enhancement in TC with temperature, indicating the contribution of Brownian motion and percolation mechanism in TC enhancement.
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thermal conductivity [20]. Moreover, for oil based samples, liquid layering at the particle/liquid can also contribute to the enhancement in TC [21,22]. In general, the viscosity plays an important role in these fluids. In our work it is observed that viscosity decreases significantly with increase in temperature, as it is shown in Fig. 6, while the enhancement in viscosity with the addition of the nanofillers is very small (in low filler fractions), which is an advantage of using low filler fractions since the increase in viscosity can decrease the keff of NFs, as well as flow characteristics of the fluids. It is also to be noted that nature of enhancement in TC with filler fraction and temperature is differ from fluid to fluid. Many factors such as fluids composition, viscosity, nature of fluids (morphology as well as interaction between fluid and nanofillers), among others can be the reasons for this difference. In this work, we found that, factors such as temperature and filler amount is more sensitive in determining the keff in low viscosity fluids. Fig. 7a and b shows the COF and steel balls WSD tribo-testing results on MO/h-BN and MO/G according to ASTM D5183 compared to bare MO, where those fluids were previously proven for their outstanding thermal performance [15]. For MO/h-BN and MO/G NFs
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at very low filler fraction (0.01 wt%) a decrease by 10% and 20%, respectively, was shown. The COF results from graphene fluid are similar to that reported by Eswaraiah et al. [10] showing a 20% reduction with 0.01 wt% of G, which is attributed to the formation of graphene nanobearings between the tester steel balls. Also, the 2Dnanosheets morphology could promote an ease sliding mechanism that contribute this effect. Moreover, these nanofillers were also tried to stabilize in MO with surfactant coating. Common surfactant such as oleic acid (OA) (2 vol%) is added to the fluid with extensive sonication (4 h). OA-stabilized MO based fluids were also tested for their tribological properties. Bare MO with OA addition did not show significant enhancement in keff (o2% at 50 1C). TC of OAcoated fluids neither show any change at the filler fraction of 0.01 wt%. However, COF and WSD shows a decrease compared to the surfactant-less material ( 6% more), indicating that OA may help to separate the nanoparticles thereby decreasing the agglomeration and/or additional layering due to OA can increase the lubrication properties. Later it is found that, agglomeration of nanofillers can adversely affect the lubrication properties of nanofillers.
Fig. 6. (a)–(d) Temperature-dependent viscosity variation of nanofluids. Small increments as adding filler fraction to nanofluids.
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Wear scars of steel balls for h-BN and G in MO are shown in Fig. 8. Common steel ball before the testing is shown if Fig. 8a as reference. Fig. 8b shows the nanolubricated with MO/h-BN upper tested steel ball wear scar (and the actual steel ball on the inset). An h-BN nanosheet is shown on the surface of tested steel ball in Fig. 8c, as also shown in Fig. 9 for EDAX analysis. Fig. 8d shows the MO/G wear scar as well. For metal-cutting fluids, results obtained by the ITEePib Polish and ASTM D5183 methods were compared and analyzed in order to determine which lubricant performed better under different
friction conditions. The main difference between these two methods is that the ITEePib Polish method tests lubricants under extreme pressure conditions for short time, while the ASTM D5183 works under constant loads for longer time. Table 3 shows the decrease on COF and steel balls WSD during tribo-testings according to ASTM D5183, compared to bare fluids. The WSD was calculated from the average diameter of the three lower-balls (see Fig. 3), at least 3 tests for each fluid, according to Dixon probabilistic methodology to generate statistical reliable results, were run. As lower the COF and WSD value, a better fluid
Fig. 7. COF and WSD enhancement of MO reinforced at 0.01 wt% of h-BN and G (ASTM D5183), measured values above the symbols.
Fig. 8. SEM images of steel balls, (a) before tribo-testing; tested using (b) MO/h-BN, showing the wear scar, (c) h-BN flakes on wear scar and (d) MO/graphene.
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Fig. 9. EDAX of test steel balls (top) for bare oil and (bottom) MO/h-BN.
Table 3 Decrease in WSD and COF of h-BN and G reinforced NFs obtained by the ASTM D5183 method. Tribology tests—ASTM D5183 method
WSD—f units: mm (%) EcoDraw Montgomery Metkut Metkool COF—l EcoDraw Montgomery Metkut Metkool
Pure
@0.01 h-BN
@0.05 h-BN
@0.10 h-BN
@0.01 G
@0.10 G
1.0692 (0%) 1.0606 (0%) 0.7520 (0%) 1.2018 (0%)
1.0499 ( 1.81%) 1.0481 ( 1.17%) 0.6207 ( 17.45%) 1.1019 ( 8.32%)
1.0430 ( 2.45%) 0.9930 ( 6.37%) 0.5539 ( 26.34%) 1.0933 ( 9.03%)
0.9826 ( 8.10%) 0.9732 ( 8.24%) 0.6352 ( 15.53%) 1.0527 ( 12.40%)
1.0665 ( 0.26%) 1.0500 ( 1.00%) 0.6267 ( 16.67%) 1.0946 ( 8.92%)
1.0044 ( 6.06%) 0.9769 ( 7.89%) 0.6178 ( 17.84%) 1.0658 ( 11.31%)
0.1337 0.1314 0.1207 0.1472
0.1275 0.1197 0.1352 0.1434
0.1247 0.1180 0.1105 0.1422
0.1399 0.1188 0.1046 0.1396
0.1327 0.1287 0.1197 0.1451
0.1276 0.1201 0.1049 0.1422
for tribological applications performance. In general, the addition of nanoparticles resulted in a significant decrease on these parameters. For instance, MO showed a decrease on COF and WSD up to 19.25% (m ¼0.1494) and 10.79% (WSD¼0.8141 mm) respectively, with the addition of 0.01 wt% G (Fig. 7). For metal-cutting NFs the addition of h-BN and G showed similar improvement values on both COF and WSD, with Metkut NFs showing the major reduction of COF and WSD, possibly due to the oleophilic behavior of 2D nanosheets. The tribological mechanism for this test, as explained by Mosleh et al. [23], may consist on nanoparticles filling valleys, and the shearing of trapped nanoparticles at the interface of contacting surfaces, thus making them smooth and lowering the frictional forces. Table 4 presents the results obtained for lubricants and metalcutting fluids, using the Polish method for calculating the extreme pressure (EP) properties of NFs, also known as the pressure loss limit of lubricant film Here the higher the poz the better the lubricant,
Table 4 Pressure loss limit results (poz) of NF films reinforced with h-BN and G obtained by the ITEePib Polish method. Tribology tests—ITEePib Polish method Pure
@0.01 h-BN
Polish—poz EcoDraw 755.14 231.60 Montgomery 3528.53 3255.30 Metkut 1079.16 2092.93 Metkool 3300.08 3338.99
@0.05 h-BN
@0.10 h-BN
@0.01 G
@0.10 G
2743.03 2956.86 2485.17 2931.56
2172.48 3257.70 2616.09 290.21
188.88 414.67 3664.80 3237.92 2395.40 2751.18 3546.40 3460.89
which works best under friction conditions. The COF is not included since it is unusable due to the sudden increment of COF, particularly when the lubricant film disappears and high pressure occur during
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the test (see Fig. 4). In contrast with the COF and WSD results obtained by the ASTM D5183 method there was no apparent correlation between the poz and the type and concentration of nanofillers. Metkut, with the highest viscosity, NFs showed the highest improvement on poz with the addition on nanoparticles. The load carrying capacity increased up to 142.42% and 154.94% with 0.10% h-BN and 0.10% G, respectively. This shows that a small addition of nanoparticles may result in a considerable enhancement on the tribological behavior. Here the tribological mechanism, as proposed by Hu et al. [24], may consist on the tribosintering of nanoparticles into the surface due to the extreme pressure effect. For lower viscosity lubricants, however, the poz values are less consistent and in general showed either much smaller increments or an increase of COF and WSD. This can be explained as the lower viscosities result in a much rapid sedimentation and re-agglomeration of nanoparticles.
5. Conclusions 2D nano-sheets of h-BN and G based thermally conducting fluids for lubrication and metal-cutting applications were developed. It is concluded that reinforcement of low filler fraction of 2D nanofillers can effectively reduce the contact friction. Two different tests, ASTM standard D5183, and a Polish method (from ITEePib) were performed for testing lubricants under conditions of scuffing and the results are in agreement each other, in general. The layering mechanism explained for the lubrication reduction with graphene engine oil fluid is more generalized to other layered materials based fluids such as h-BN. These multifunctional nanofluids can be better candidates for various energy management fields in future. Moreover, the advantage over the measurement time and material required with Polish method indicates that further lubrication testing on these fluids for other filler fractions can be undertaken with Polish method.
Acknowledgments J.T.-T. acknowledges the support from PGE and CONACYT (213780). L.P.-P. and D.M. acknowledge the support from UDEM VIAC grant 10T-PR098-12. M.A., T.N.N., and J.T.-T. acknowledge funding from the U.S. Army Research Office MURI grant W911NF11-1-0362, the U.S. Office of Naval Research MURI grant N000014-09-1-1066 on novel free-Standing 2D crystalline materials focusing on atomic layers of nitrides,oxides,and sulfides. Authors acknowledge the support from PGE, Emerson and Metalsa for supplying the lubricants and metal-cutting fluids for this research. References [1] R.E. Smalley, Future global energy prosperity: the terawatt challenge, MRS Bulletin 30 (2005) 412–417.
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