Tribological Behavior of NiAl-Layered Double Hydroxide

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Tribological Behaviors of NiAl-LDH Nanoplatelets as Oil-based Lubricant Additives Hongdong Wang, Yuhong Liu, Wenrui Liu, Rong Wang, Jianguo Wen, Huaping Sheng, Jinfang Peng, Ali Erdemir, and Jianbin Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10515 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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ACS Applied Materials & Interfaces

Tribological Behaviors of NiAl-LDH Nanoplatelets as Oil-based Lubricant Additives Hongdong Wang,†,

‡

Yuhong Liu,†,* Wenrui Liu,† Rong Wang,† Jianguo Wen,§ Huaping Sheng,§

Jinfang Peng,§ Ali Erdemir,‡,* Jianbin Luo†,* †

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

‡

Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

§

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United

States

KEYWORDS: Nano-additive, LDHs, Tribological properties, Oil-based

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ABSTRACT: Layered double hydroxides (LDHs) are a class of naturally-occurring inorganic minerals which are composed of divalent and trivalent metal cations. In this study, three different sized NiAl-LDH nanoplatelets were synthesized by varying crystallization time during the micro-emulsification process. The layered structure and three-dimensional size of nanoplatelets were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As lubricant additives, their tribological properties in base oil were evaluated using a ball-on-disk reciprocating tribometer under three different loads: 50, 100 and 150 N (which created peak Hertz pressures of 1.74, 2.16 and 2.47 GPa). Under contact pressures of up 2.16 GPa, not only did the friction coefficient (COF) decrease by about 10% after nano-LDHs were added, but also the wear performance improved substantially. These improvements resulted from a protective tribolayer formation on the contact interface, as revealed by detailed surface and structure analytical studies. In particular, cross-sectional TEM images revealed that the larger size nanoplatelets (NiAl-24h), rather than the smaller ones (NiAl-6h) showed the best and most stable tribological performance. This was mainly because of their higher degree of crystallinity, which in turn resulted in the formation of a tribofilm with much superior mechanical properties during sliding. Owing to the simple synthesis method and superior tribological properties as oil-based additives, nano-LDHs hold great potential for demanding industrial applications in the future.

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INTRODUCTION

Friction and wear phenomena are widespread in our daily life, and they have been frequently investigated under wide ranges of operating conditions in tribology field. However, unwanted friction and wear may lead to huge losses in terms of materials and energy. Especially in highly industrialized nations of the world, the energy and material losses due to friction and wear were estimated to account for as much as 5–7 % their gross national products (GNP).1 To address these issues, we usually utilized lubricants in the field of transportation, metal cutting, power generation, power transmission, and so on. Among all the lubricants, the mineral and synthetic-base oils were the most commonly used ones by industry due to their relatively high viscosity

and

pressure-viscosity

coefficients.

They

helped

insure

a

hydro-

to

elasto-hydrodynamic lubrication (HL to EHL, respectively) regime through the formation of a thick enough fluid film.

In the past few years, due to their unique chemical and structural attributes, a variety of nanomaterials have been used in oils to enhance their anti-friction and -wear properties.2-6 Among them, owing to their high specific surface area and relatively weak interlayer bonding (i.e., van der Waals), a number of two-dimensional layered materials such as MoS2,7, 8 WS2,9 and multilayer graphene10 were utilized as lubricant additives. The self-lubricating action of these materials was primarily due to an interlayer shear mechanism. In their layered structures, the atoms on each layer were closely packed and strongly bonded to each other, while the layers themselves were relatively far apart and the forces that held them together e.g., van der Waals, 3



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were weak. Strong interatomic bonding and packing in each layer provided very high strength and resilience, which reduced wear damage during sliding, while wide interlayer spacing and weak bonding ensured easy shear or slippage. Accordingly, in such layered materials, it was possible to observe nearly disappearing friction due to their special structural anisotropy.11 Recent studies have shown that some fine powders of the naturally-occurring minerals such as serpentine12, 13 and attapulgite14 may also function as good lubricant additives. Overall, these solids have exhibited excellent tribological properties during laboratory testing and industrial uses, mainly because of the formation of a highly protective and slick tribofilm on sliding surfaces.

Here, we present the friction and wear performance of a new class of layered double hydroxides (LDHs). Their general formula can be expressed as [M2+1−xM3+x(OH)2]x+ (An−)x/n· mH2O, where M2+ and M3+ are divalent and trivalent metal cations in the laminates, respectively, and An− is the interlaminar charge-balance anion. Due to their unique structures,15 shape-memory effect,16 as well as the diversity of chemical compositions,17 LDHs have been widely investigated in catalysis,18 functionalized films,19 flame retardant additives,20 water treatment,21 and other fields. LDHs possess not only a layered crystalline structure but also a unique chemical composition similar to highly surface-active mineral powders. Thus, it is of great interest to explore the potential of these attractive layered materials as lubricant additives. Meanwhile, different sized nanoplatelets were dispersed in oil-based lubricants and analyzed, so as to provide guidance for the selection of the most appropriate nano-additive in future applications. 4


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In this paper, we chose a micro-emulsification method to synthesize the LDH nanoplatelets whose size was controlled by changing the crystallization time. After LDHs were dispersed in base oil, the relationship between surface functionalization and dispersion efficiency has been investigated. Subsequently, the friction and wear performances of different oil samples were evaluated by a ball–on-disk tribometer in reciprocating mode. Their sliding contact surfaces were analyzed after the tests to ascertain specific composition of the tribofilms which resulted from the LDHs nano-additives. Furthermore, these tribofilms have been characterized by a high-resolution transmission electron microscope (HRTEM) to distinguish the impact of different sized nano-additives on tribological performance. The elastic modulus of the tribofilm (only a few tens of nanometer thick) was measured by AFM so as to determine the mechanical properties of the sliding contact surfaces. Finally, based on these results, the tribological behavior of nano-LDH additives and underlying lubrication mechanisms were proposed.

MATERIALS AND METHODS

Synthesis. A micro-emulsification method was adopted to synthesize the nano-LDHs ([Ni2Al(OH)6]Cl·mH2O).22 Deionized water was used throughout the experiment. The specific steps pursued in the preparation process were as follows. Firstly, a mixed solution of oleylamine (15 mL; Sigma-Aldrich) and 1-butanol (15 ml) was stirred under the protection of N2 atmosphere. Then, 15 ml mixed saline aqueous solution of NiCl2·6H2O and AlCl3·6H2O ([Ni2+]

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= 1.5 M, [Al3+] = 0.75 M) was prepared and added dropwise to the organic solution. At last, the obtained mixture was ultrasonically treated for 15 min, transferred into an autoclave, and heated at 120 °C. The incubation time was set to 6 h, 12 h and 24 h, respectively. The precipitation of nano-LDHs was achieved by centrifugation, washed three times using a mixture of ethanol and distilled water (1:1), and finally dried at 80 °C overnight.

Characterization of nano-LDHs. A Bruker D8 Advance diffractometer was utilized to obtain the dry powder X-ray diffraction (XRD) pattern in reflection mode (Cu Kα radiation, λ=1.54 Å) over a 2θ range of 3–70° to confirm the crystal structure. TEM and AFM were utilized to ascertain the structure and size of the platelets. The TEM images of the LDH nanoplatelets were obtained using a JEM 2010 microscope at an accelerating voltage of 120 kV. LDH nanoplatelets were ultrasonically dispersed in water; then the aqueous suspension was dropped on carbon films, and dried at 80 °C for 10 hours in air. The AFM images of LDH platelets were obtained using an atomic force microscopy (nano V, Veeco) in tapping mode at room temperature and 20 % relative humidity. The samples were prepared by dropping the aqueous solution of LDH (0.2 wt %) on a fresh silicon slice and dried at 80 °C overnight. N2 isotherms of LDH nanoplatelets were obtained with Quadrasorb SI (Quantachrome) at -196 °C. The Brunauer–Emmett–Teller (BET) method was adopted to calculate the specific surface area (SSA).

Tribological Experiments and Evaluation. The base oil (GTL8), which mainly consisted of hydrocarbon, was provided by Shell Co. Ltd. Its viscosity was 24.5 cP at 50 °C. The viscosity of base oil at 50 °C was obtained using a standard cone-and-plate rheometer (MCR301, Anton 6


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Paar Physica). The ball-on-disk tribological tests were performed using a SRV4 tribometer (Optimal, Germany) in reciprocating mode at 50 °C, at a stroke length of 2 mm, and the reciprocating frequency of 50 Hz for 30 min (a total sliding distance of 360 m). The applied load range was from 50 N to 150 N, and the room humidity was 30% RH. In the beginning of each experiment, a running-in process at 50 N was employed for 30 seconds. Each experiment was conducted at least three times to ensure repeatability. Both balls and disks were made of AISI 52100 bearing steel, whose elastic modulus, hardness, and Poisson’s ratio were 206 GPa, 64 Rockwell C, and 0.3, respectively. The diameter of ball was 10 mm with surface roughness (Sa) of about 8.4 nm. The disks were all polished so as to get the surface roughness (Sa) of about 7.6 nm. They were all ultrasonically washed with petroleum ether, acetone and ethanol, respectively. After tribological experiment, both sliding surfaces were ultrasonically cleaned and their wear scars were examined by an optical microscope (Olympus BX60), a 3D optical surface profiler (Zygo Nexview) and a scanning electron microscope (SEM, TESCAN LYRA3). The specimen for observing the cross section of tribofilm was prepared by focused ion beam electron microscope (FIB-SEM, TESCAN LYRA3). Then, the HRTEM image was obtained to observe its crystal structure using a JEM 2100F with an accelerating voltage of 200 kV. The elemental mapping was conducted by a cold field emission SEM (HITACHI, SU8220) and analyzed by energy dispersive X-ray spectrometer (Bruker, QUANTAX FlatQUAD). The modulus of tribofilm was measured using an atomic force microscopy (nano V, Veeco) in peak force tapping mode with the scan size of 10 × 10 µm.

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RESULTS AND DISCUSSION

Figure 1. (a) XRD patterns of nano-LDH samples and TEM images of (b) NiAl-6h, (c) NiAl-12h, and (d) NiAl-24h LDH width particle size distributions. The dashed lines are the fitting results of normal distribution.

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The nano-LDHs were all synthesized by a micro-emulsification method. However, for different sized LDH platelets, NiAl-nh (n=6, 12 and 24) were prepared by adjusting the hydrothermal reaction time to 6, 12 and 24 hours, respectively. First of all, in order to confirm the crystal structure of NiAl-LDHs, the XRD patterns of three different sized nanoplatelets were obtained and presented in Figure 1a. All of them exhibited a series of characteristic peaks of LDH, containing typical reflection peaks of rhombohedral phase. However, as compared to other samples, NiAl-24h LDH showed the best crystallinity, especially in the position of (110). According to the peak position of (003) Bragg reflection, the calculated value of basal interlayer distance d

(003) was

0.762 nm, which indicated that Cl− anions were intercalated in the interlayer

galleries. A hexagonal unit cell can be expressed by a = b= 2d (110) = 0.302 nm according to the position of (110) Bragg reflection, and c= 3d

(003)

=2.286 nm according to the position of the

(003) Bragg reflection.

The TEM images (see Figure 1b-d) showed the typical platelet-like morphology of nano-LDHs. These nanoplatelets were well dispersed and uniformly spread over on the substrate. The complete morphology of most nanoplatelets in TEM images were observed clearly, and the lateral size of each flake can be directly measured. After measuring the width of 30 different platelets and fitting the results by normal distribution, the average widths of NiAl-nh (n=6, 12 and 24) samples by statistical treatment were confirmed to be 12.53 (σ=1.83 nm), 15.86 (σ=2.07 nm) and 19.73 nm (σ=2.84 nm), respectively. It can be easily understood that the width of platelets will get larger on account of longer hydrothermal reaction time. 9



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Figure 2. (a) NiAl-24h LDH high-resolution TEM image and thickness distribution of analyzed particles. The dashed line is the fitting result of normal distribution. (b) NiAl-6h, (c) NiAl-12h, and (d) NiAl-24h LDH AFM images and corresponding cross-sectional profiles. (e) N2 isotherms of nano-LDHs. (f) NiAl-24h LDHs dispersed in base oil.

Through high-resolution TEM (HRTEM) imaging in Figure 2a, the layered structure of NiAl-24h can be observed and its average thickness can be obtained. The basal distance between interlayers was measured to be 0.76 nm, which corresponded well with the previous result from the XRD pattern. Based on the fitting result of normal distribution by measuring the thickness of 30 different platelets, the average thickness of NiAl-24h sample by statistical analysis was confirmed to be 8.59 nm. However, a large measurement error would occur when the thickness of NiAl-nh (n=6&12) samples was measured directly from HRTEM images. Therefore, with the aim to determine the thicknesses of nano-LDHs, the AFM images of three samples placed on a 10


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fresh silicon wafer were analyzed in Figure 2b-d. The thickness can be measured through cross-sectional profiles from corresponding lines. Here, the statistical method of measuring the thicknesses of nano-LDHs by AFM was similar to that of evaluating the widths by TEM. After calculation and analysis by selecting 30 nanoplatelets from AFM images, the thickness values of NiAl-nh (n=6, 12 and 24) samples by statistical analysis were confirmed to be 1.63 nm, 2.97 and 9.12 nm, respectively. In order to further confirm the dimensional sizes, the N2 isotherms of three different samples were obtained in Figure 2e to evaluate the specific surface area (SSA). The SSA values of NiAl-nh (n=6, 12 and 24) samples were calculated to be 138, 86 and 40 m2/g, respectively. This result corresponded well with the statistical data obtained from the previously performed direct measurements. Thus, we can draw a reasonable conclusion that, with the increase of hydrothermal reaction time, both lateral and vertical sizes of LDH platelets enlarged accordingly.

As for the lubricant nano-additives, it is essential to disperse well in the base oil and form a stable suspension for practical applications. Here, after LDH nanoplatelets were mixed at 1 wt % in proportion with the base oil, a homogeneous dispersion was achieved as shown in Figure 2f. In order to investigate the role of oleylamine as a surface modifier or surfactant in dispersion process, we washed the as-synthesized LDH nanoplatelets with ultrasonic pure ethanol for three times to remove the oleylamine on the surface of nano-LDHs, and then dispersed them in base oil for comparison. The FT-IR spectroscopy and their dispersion results were displayed in Figure S1. The main difference between two FT-IR spectra was the bending vibration of methyl group 11



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(δ

C-H)

at 1465 cm-1 and the hydrocarbon stretching vibration (ν

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C–H)

at 2854 and 2922 cm-1.

These values corresponded exactly to the characteristic peaks of oleylamine.23 Thus, it can be deduced that the oleylamine molecules were effectively removed from the surface of nano-LDHs. After they were dispersed in base oil and set aside for 12 h, it was obvious to find that nano-LDHs without surface-modification agglomerated quickly and precipitated out. However, the original sample was well dispersed and had no obvious agglomeration or precipitation. Thus, it was deduced that the good dispersion effect of nano-LDHs in base oil can be attributed to the successful surface modification by oleylamine. It was believed that the protonated head groups -NH3+ of oleylamine were linked with the polarized OH− groups of NiAl-LDH via electrostatic interaction to form nano-sized reactors during the synthesis process. The long-chain alkyl tails of oleylamine would orient themselves towards the base oil. Thus, the hydrophobic surface of nanoplatelets allowed the additive to be better dispersed within the base oil.

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Figure 3. Friction coefficient of four lubricants, including base oil, 1 wt % NiAl-6h, 1 wt % NiAl-12 h, and 1 wt % NiAl-24 h under the load of (a) 50 N; (b) 100 N; (c) 150 N.

The tribological properties of four lubricant samples (base oil, NiAl-6h, NiAl-12h and NiAl-24h) were evaluated and different loads (50, 100 and 150 N) were applied so as to determine the maximum contact pressure, under which these additives can still function or lower friction and wear. The friction coefficients (COFs) of AISI 52100 steel balls against AISI 52100 steel disks at 50 °C were provided in Figure 3. It was obvious that the final COFs of the samples decreased after the nano-LDHs were added to the base oil. The results of tribological experiments under the load of 50 N were shown in Figure 3a. The COF of test pairs showed a tendency to increase in the case of base oil with increasing time. When 1 wt % NiAl-24h 13



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nano-LDHs were added, the COF between two sliding surfaces remained relatively low and steady throughout the test, and it was about 11.6 % less than that of the base oil. Moreover, oils containing NiAl-6h and NiAl-12h additives showed positive effects on lowering and stabilizing COF. As for the COFs under the load of 100 N (corresponds to the maximum Hertz contact pressure of 2.16 GPa) in Figure 3b, the sample containing 1 wt % NiAl-24h nano-LDHs still possessed a minimum and stable COF value, about 9.8 % less than that of base oil.

However, as for the experimental results under 150 N (which corresponds to the maximum Hertz contact pressure of 2.47 GPa), the COFs of all samples with nano-LDH additives rose to a relatively higher value initially, but then decreased to lower values gradually. It was reasonable to speculate that the periods of relatively high COF were perhaps caused by much severe contact stresses of point contact geometry combined with the third-body effect of LDH particles during the initial running-in wear conditions. With the aim of further proving this assertion, we washed the ball and disk samples by ultrasonic ethanol after friction tests. Here, the wear scar was observed by an optical microscope and they were displayed in the Supporting Information Figure S2. As compared with the sliding steel surfaces lubricated by base oil (which displayed a nearly spherical wear scar with few shallow scratched along the sliding direction), the amount of wear was higher, and some deep scratches were found on rubbing ball contact spots after tests in nano-LDHs containing oils. For comparison, a formulated Mobil Pegasus 1005 oil (which uses advanced additive technology) was also tested under the same experimental conditions. As obvious from Figure S2e, a fairly large wear scar had also formed on ball side, further 14


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confirming that significant amount of wear would still occur under such sever contact pressures even with the use of a fully-formulated oil. Further, as in LDH-containing oil cases, a plenty of deep scratches were easily observed on the wear scar of ball after the sliding test as well as shown in Figure S2e. Based on these results, the observation of somewhat inferior wear performance in Figure S2 with additives under 150 N may be attributed to the relatively thinner tribofilm formation and its removal under such a high load (hence severe boundary condition). Thus, the nano-additives were not able to attain and maintain a continuous and protective tribofilm against wear.24 In fact, they may have aggravated the wear situations by triggering a third body wear process (deep and uneven wear marks along the sliding direction in Figure S2 were supportive of this hypothesis). Once formed, we believed that larger wear scars and tracks on ball and disk surfaces would significantly lower contact pressure (from 2.47 GPa to about 800 MPa), hence reduced contact severity and improved hydrodynamic efficiency or fluid film thickness, so as to lower friction toward the end of experiments (see Figure 3c).

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Figure 4. The optical images of the wear scars tested by (a) base oil; (b) 1 wt % NiAl-6h; (c) 1 wt % NiAl-12h; (d) 1 wt % NiAl-24h under the load 50 N. Three-dimensional topography of the wear scar on the ball, which was lubricated with (e) base oil, and (f) NiAl-24h under the load of 50 N. (g) Line scans of wear scars. The protective tribofilm looks much darker and denser in Figure b-d.

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On the contrary, it is interesting to find that, under the load of 50 N, the diameter of wear scar was much less when those solid surfaces were lubricated with additives, and no abrasive wear marks or scratches appeared (see Figure 4). The three-dimensional profiles (see Figure 4e&f) and line scans of wear scars (see Figure 4g) clearly revealed negligible wear damage when NiAl-24h sample was utilized as oil-based lubricant additive. The wear scar under the load of 100 N was also observed, and those with nano-LDH additives were slightly smaller (see Supporting Information Figure S3). In addition, some scratches were detected after test in 1 wt % NiAl-12h. Actually, the formation of scratches had a great relationship with the aspect ratio (width / thickness) of the nano-additives.25, 26 As an additive, the NiAl-12h sample with a high aspect ratio (~5.34) was very likely to scratch the solid surface due to its tumbling or self-flipping during the friction process. However, the NiAl-6h sample with its extremely thin features (only about ca. 1.63 nm) was easy to adsorb, and hence incorporated in the solid surface and thus played a more protective role; the NiAl-24h sample with a low aspect ratio (~2.16) was more rounded, and hence not as sharp to damage the solid surface during the lubricated rubbing process. These experimental results indicated that nano-LDH additives can effectively improve the tribological properties of the oil-based lubricant under normal conditions. Among them, the NiAl-24h sample exhibited the best and most stable tribological performance. Moreover, under harsh loading conditions, the nano-LDH additives were able to decrease contact pressure by creating larger conformal contacts, and thus lowered COF to avoid seizure or scuffing (which might be a desirable outcome in actual mechanical applications).

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Figure 5. The optical images of the wear track tested by (a) base oil; (b) 1 wt % NiAl-24h under 100 N. The SEM images of the wear track tested in (c) base oil; (d) 1 wt % NiAl-24h under 100 N. The protective tribofilm in Figure b&d shows a relatively denser (Figure d) surface morphology.

The physical topography and structure of sliding solid surfaces can have a direct effect on their tribological behavior. The wear tracks formed under the load of 100 N were observed by both optical microscopy and SEM, and presented in Figure 5. When compared with the track formed during tests in base oil (see Figure 5a&c), a relatively denser protective tribofilm can be found on the sliding surface (see Figure 5b&d) lubricated with NiAl-24h nano-LDH additives. In addition, their three-dimensional topography images can be seen from the Supporting Information Figure S4a&b. It can be found that most contour of the wear track was much higher than the disk surface when lubricated by the base oil. The extent of wear was far more obvious under the load of 150 N when comparing the Figure S4c&d in Supporting Information. The cross-sectional line profile of the wear track in Figure 6a also gave a clear comparison. This 18


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condition was typically called adhesion effect, which commonly occurred between two sliding solid surfaces of the same metal or alloys rubbing against one another.27 Due to the adhesion of the solid asperities, the sliding resistance of material will to some extent increase the friction in the sliding process.

Figure 6. (a) The cross-sectional profile of wear track after friction test under the load of 100N. (b) The depth and (c) surface roughness of wear track under different loads.

Through the analysis of each surface after test, the depth of the wear track with different additives and the roughness of each surface were collected and shown in Figure 6b&c. With the

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rise in load, all values of wear depth and surface roughness with additives increased accordingly, but there was almost no change in the surface roughness of wear track tested by base oil. Among these three samples, when NiAl-12h LDHs were added to the base oil for friction test, the largest values of wear depth and surface roughness were obtained. However, it was the best surface condition of wear track when NiAl-24h LDHs were dispersed in base oil as lubricant additives. These results were quite consistent with the friction experiments.

Figure 7. Analysis of the tribofilm formed on the wear track lubricated by 1 wt % NiAl-24h under the load of 100N. (a) Low magnification of cross-sectional TEM image; (b) EDS linear

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analysis along the wear track depth direction; (c) diffraction pattern of tribofilm; (d) HRTEM image of tribofilm.

In order to investigate the lubricating behavior of nanoplatelets in the base oil, we adopted FIB-SEM and HRTEM to directly observe the morphology of the tribofilm and determine its chemical composition. The cross-sectional TEM image of tribofilm on the wear track, lubricated by 1 wt % NiAl-24h sample, was displayed in Figure 7a. Obviously, about 20 nm thick tribofilm was evenly formed on the steel substrate after rubbing with LDH additives. The EDS linear analysis result (along the wear track depth direction) in Figure 7b showed that the elements Ni, Al and O enriched in the region of tribofilm. According to the previous studies,28-30 the phase of layered double hydroxide (LDH) will evolve into layered double oxide (LDO) at high temperatures. During the continuous reciprocation of rubbing process, it was quite possible that the LDHs embedded on the sliding surface were gradually oxidized to LDO due to the high temperature generated by the collision of asperities. The electron diffraction pattern in Figure 7c and the HRTEM image in Figure 7d revealed that the oxidized tribofilm formed on the sliding surface was mainly composed of iron oxides (ICSD collection code 15840), NiAl2O4 (ICSD collection code 9554) and cubic NiO (ICSD collection code 9866).30 The EDS mapping analysis for the wear track in Supporting Information Figure S5 showed a uniform element distribution of tribofilm on the wear track, suggesting that nano-LDHs indeed contributed to a friction-reducing layer on the solid surfaces.

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Figure 8. Analysis of the tribofilm formed on the wear track lubricated by 1 wt % NiAl-6h under the load of 100N. (a) Low magnification of cross-sectional TEM image; (b) EDS linear analysis along the wear track depth direction; (c) diffraction pattern of tribofilm; (d) HRTEM image of tribofilm.

To further understand the tribological behaviors of nano-LDHs in lubricant, we conducted a similar characterization on the tribofilm of the wear track, lubricated by 1 wt % NiAl-6h sample. As a result, about 50 nm thick tribofilm was formed on the substrate, and Figure 8 showed its cross-sectional TEM image. However, the electron diffraction pattern and the crystal structure of the tribofilm were not clearly observed by contrast. It was easy to understand that the thicker 22


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oxidized layer and inferior or incomplete crystallinity (due to the extremely small size of the NiAl-6h nanoplatelets) were most likely reasons for apparent ambiguity in electron diffraction and crystal structure analysis of the tribofilm. Considering the best tribological property of NiAl-24h sample and solid surface conditions after test under 50 and 100 N loads, it provided obvious evidence that the best crystal structure, rather than the minimum size, of the nano-LDH additives took a key effect.

Figure 9. Surface analysis using AFM in peakforce tapping mode. Surface topography of the tribofilm, lubricated by (a) base oil; (b) 1 wt % NiAl-6h; (c) 1 wt % NiAl-24h under the load of 100 N. (d) DMT modulus of the surface inside the wear track.

The mechanical property of the sliding solid surface was quite significant for the lubrication performance. Both the improvement of mechanical property of solid surface and the formation of

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intermetallic compounds can help to prevent the occurrence of adhesion between two sliding surfaces.27 Thus, it was necessary to further analyze the solid surfaces lubricated by different samples to determine their evolution during the rubbing process. Since the thickness of tribofilm was only a few tens of nanometers, the surface of wear track was characterized by AFM in order to obtain its accurate condition without the interference information of the steel substrate. The morphology of the wear track with different lubricant and the statistical result of DMT modulus inside the wear track were shown in Figure 9. When lubricated by the base oil, the adhesive wear on wear track can be easily observed due to stick-slip motion. The plastic deformation can be attributed to the transitory heat due to dissipated energy caused by rough surface or asperity collisions, resulting in relatively higher friction force or resistance to sliding.31 As for the lubricants with nano-LDH additives, the microstructure of the contact area looked more compact and no obvious stick-slip trace can be found. Then, peak force tapping mode was adopted to evaluate the modulus of the tribofilm formed with different nano-LDHs. As shown in Figure 9d, after rubbing with 1 wt % NiAl-24h sample, the mechanical property of solid surface obviously enhanced a lot. While the solid surface with 1 wt % NiAl-6h sample had no significant change in mechanical property compared to the surface with base oil. This result was quite related to the reaction between nano-additives and steel substrate.

According to the Hamrock−Dowson theory, the minimum film thickness can be evaluated by the formula,32, 33

24


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hmin = 3.63

U 0.68G 0.49 R (1 − e −0.68 k ) 0.073 W

(1)

where U = ηV / E ' R , G = α E ' , W = F / E ' R 2 . η is the shear viscosity of the fluid (~24.5 cP at 50 °C). The viscosity of base oil (GTL8) was measured under the shear rate from 10 to 1000 s-1 (see Figure S6 in Supporting Information). It was found that there was no significant difference in the value of viscosity with the change in shear rate. Thus, it can be considered that the base oil was a Newtonian fluid and did not exhibit the shear thinning rheology. V is the average relative velocity between two sliding surfaces (~200 mm/s). E′ is the effective modulus of elasticity. F is the applied load (100 N). R is the radius of the ball (5 mm). α is the viscosity-pressure coefficient (~2×10−8 Pa−1) and k is the ellipticity of the ball (~1). After calculation, the minimum lubricant film thickness was ~41.7 nm, which was much higher than the size of these nano-additives. Thus, it can be concluded that the addition of a small amount of the nano-additive had little effect on rheological behavior of the lubricant.

Figure 10. Schematic illustration of the proposed lubrication model with two kinds of nano-LDH additives.

As shown in Figure 10, the lubrication model of nano-LDH additives in the sliding process 25



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was proposed. In summary, smaller nano-additives were easily embedded in the defects and pits of sliding surfaces, forming a thick tribofilm. But it can hardly improve the mechanical property of contact surface due to its incomplete crystal structure and small size. However, the nano-additive with larger size and good crystal structure can be firmly combined with the substrate in the sliding process to ensure the surface smoothing and meanwhile improve its mechanical property, thereby reducing friction and preventing severe wear.

CONCLUSIONS

In this paper, three NiAl-LDH nanoplatelets with different sizes were synthesized by controlling the reaction time (6, 12 &24 h). Their three-dimensional sizes were confirmed by TEM and AFM images using a statistical method. After they were dispersed in base oil and tested by a commercial tribotester, the final COFs of these samples were reduced by about 10% depending on the contact loads. Furthermore, up to a contact pressure of 2.16 GPa, the wear resistance was also improved. Among them, the large nanoplatelets (NiAl-24h) showed the best and the most stable tribological performance, because of their more favorable crystal structure and the formation of a more protective tribofilm during sliding process with good mechanical properties. For the first time, this work confirmed the superior tribological behaviours of nano-LDHs as potential anti-friction and -wear additives in oil-based lubricant (especially under severe contact conditions), and thus may help in practical applications by industry.

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AUTHOR INFORMATION

Corresponding Author

*Jianbin Luo. Tel.: 86-10-62781385. Email address: [email protected].

*Ali Erdemir. Tel.: 630-252-6571. Email address: [email protected].

*Yuhong Liu. Tel.: 86-10-62792449. Email address: [email protected].

Author Contributions

Hongdong Wang conducted the experiments and finished the main manuscript. Rong Wang prepared the TEM lamella by FIB. All authors contributed to the analysis and discussion of the data, reviewed the manuscript, and have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was financially supported by the National Science Fund for Excellent Young Scholars (51522504), the National Natural Science Foundation of China (51335005,51321092) 27



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and Shell Global Solutions (US) Inc. Hongdong Wang wishes to acknowledge Chinese Scholarship Council (CSC) for its financial support.

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Table of Contents Graphic

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(a) XRD patterns of as-synthesized nano-LDHs and TEM images of (b) NiAl-6h, (c) NiAl-12h, and (d) NiAl24h LDH width particle size distributions. The dashed lines are the fitting results of normal distribution. 90x164mm (300 x 300 DPI)


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(a) NiAl-24h LDH high-resolution TEM image and thickness distribution of analyzed particles. The dashed line is the fitting result of normal distribution. (b) NiAl-6h, (c) NiAl-12h, and (d) NiAl-24h LDH AFM images and corresponding cross-sectional profiles. (e) N2 isotherms of as-synthesized nano-LDHs. (f) As-synthesized NiAl-24h LDHs dispersed in base oil. 180x92mm (300 x 300 DPI)



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Friction coefficient of four lubricants, including base oil, 1 wt % NiAl-6h, 1 wt % NiAl-12 h, and 1 wt % NiAl24 h under the load of (a) 50 N; (b) 100 N; (c) 150 N. 90x120mm (300 x 300 DPI)


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The optical images of the wear scars tested by (a) base oil; (b) 1 wt % NiAl-6h; (c) 1 wt % NiAl-12h; (d) 1 wt % NiAl-24h under the load 50 N. Three-dimensional topography of the wear scar on the ball, which was lubricated with (e) base oil, and (f) NiAl-24h under the load of 50 N. (g) Line scans of wear scars. The protective tribofilm looks much darker and denser in Figure b-d. 96x190mm (300 x 300 DPI)



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The optical images of the wear track tested by (a) base oil; (b) 1 wt % NiAl-24h under 100 N. The SEM images of the wear track tested in (c) base oil; (d) 1 wt % NiAl-24h under 100 N. The protective tribofilm in Figure b&d shows a relatively denser (Figure d) surface morphology. 90x65mm (300 x 300 DPI)


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(a) The cross-sectional profile of wear track after friction test under the load of 100N. (b) The depth and (c) surface roughness of wear track under different loads. 180x140mm (300 x 300 DPI)



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Analysis of the tribofilm formed on the wear track lubricated by 1 wt % NiAl-24h under the load of 100N. (a) Low magnification of cross-sectional TEM image; (b) EDS linear analysis along the wear track depth direction; (c) diffraction pattern of tribofilm; (d) HRTEM image of tribofilm. 180x127mm (300 x 300 DPI)


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Analysis of the tribofilm formed on the wear track lubricated by 1 wt % NiAl-6h under the load of 100N. (a) Low magnification of cross-sectional TEM image; (b) EDS linear analysis along the wear track depth direction; (c) diffraction pattern of tribofilm; (d) HRTEM image of tribofilm. 180x123mm (300 x 300 DPI)



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Surface analysis using AFM in peakforce tapping mode. Surface topography of the tribofilm, lubricated by (a) base oil; (b) 1 wt % NiAl-6h; (c) 1 wt % NiAl-24h under the load of 100 N. (d) DMT modulus of the surface inside the wear track. 90x89mm (300 x 300 DPI)


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Schematic illustration of the proposed lubrication model with two kinds of nano-LDH additives. 180x77mm (300 x 300 DPI)


Tribological Behavior of NiAl-Layered Double Hydroxide

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