*Corresponding author. Tel.: +1 5196612111 ext. 86341; Fax: 5196613022

E-mail address: mkasrai@uwo.ca; tsham@uwo.ca

1

Preparation of platy Co/Al hydrotalcites using aluminum hydroxide and investigation of

their tribological properties in base oil

Dong Zhao1, 2, Masoud Kasrai2*, Tsun-Kong Sham2*, Zhimin Bai1, Fuyan Zhao1, Shuo Li1

1School of Materials Science and Technology, China University of Geoscience (Beijing), 100083 Beijing, PR China

2Department of Chemistry, Western University, London, ON, Canada, N6A 5B7

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Abstract: In this study the synthesis of Co/Al hydrotalcite (layered double hydroxides: LDH) using

insoluble Al(OH)3 by hydrothermal method is described. The syntheses were conducted under various

conditions. The as-prepared Co/Al-CO32--LDH was characterized by X-ray diffraction (XRD) and scanning

electron microscope (SEM). The tribological properties of the products were evaluated in base oil using

four-ball and plint friction testers. The LDH thus synthesized displays perfect hexagonal plate-like

morphology having a disk diameter in a range of 0.2-1.5µm with a thickness of 40nm. Addition of the

product Co/Al-CO32--LDH, as additive to the base oil, significantly reduces the friction coefficient (22.3%)

and wear (26.1%), compared to base oil alone. The morphology and chemistry of the worn surface were

characterized by SEM, atomic force microscope (AFM) and X-ray absorption near edge structures

(XANES) spectroscopy. The results show that the structural distortion of LDH induced by

mechanochemical processes during friction consumes the friction force between the rubbed surfaces and as

a result the friction and the wear is reduced. Thus our results show that the LDH sheets absorbed on the

worn surface can prevent direct contact between the friction pairs (surfaces) and decreases the roughness of

the worn surface.

Keywords: Co/Al hydrotalcites; antifriction; tribochemistry; XANES.

1. Introduction

Layered double hydroxides (LDHs) are a class of layered anionic clays having the general formula of

[M(1-x)2+Mx

3+(OH)2](An-)x/n·zH2O, where M2+ (Mg, Mn, Fe, Co, Ni, Cu, Zn, etc. ) and M3+(Al, Fe, Co, Cr,

etc. ) are metal cations and An- is the charged compensating anion(CO32-, NO3

-, Cl-, organic anions, etc.).1-2

LDHs have attracted considerable attentions owing to their potential applications as adsorbents3-6,

catalysts7-8, flame retardant additive9 and pharmaceuticals10-11. The most common method for the

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preparation of LDHs is co-precipitation at various or constant pH, followed by aging at certain temperature,

which involves the steady mixing of a solution of cations and a base solution with vigorous stirring.12-13 To

avoid the rapid precipitation of the more insoluble metal hydroxide in solution, urea has been used as the

base source to obtain homogeneous precipitation of the components.14-16 Also, insoluble oxides and

hydroxides such as MgO, Al2O3, Mg(OH)2, Al(OH)317-21 have been utilized as the metal cation source. This

method reduces the number of nuclei and obtains large size and well crystallized LDHs. In the past, Co/Al

LDHs had been synthesized by co-precipitation22-24 and homogeneous precipitation25 methods with soluble

salt as metal sources. As compared with Mg/Al LDHs, Co/Al LDHs as transition metal-bearing LHDs are

known to have broader technological applications attributable to their special electronic, optical, catalytic

and magnetic properties.26-33 The synthesis, property and application of Co/Al LDHs have been studied

widely, but the synthesis of Co/Al LDHs with Al(OH)3 has not been reported in the literature.

In tribology studies, the oil-soluble antifriction and antiwear additives have been studied extensively.

However, the extreme pressure properties of some inorganic particles have been found to be superior to

some organic ones.34 In terms of minerals as additives for lubricating oil, layered structure minerals such as

serpentine possess good anti-wear properties and thermal stabilities.35-36 Our previous work shows that the

addition of Mg/Al and Co/Al hydrotalcite to lubricant oil can decrease the friction coefficient and improve

wear protection.37-38 Zhang et al.39-40 studied the tribological behaviors of serpentine and Cu powders on

steel surface under sliding friction. The outcomes show that a protective film containing metal oxides can

be formed when the mineral particles was used as lubricant additives. This oxide layer possesses high wear

resistance and self-lubricating ability. Wang et al.41 reported that the CoO surface oxide layer exhibits good

wear resistance and antifriction performances. Our study reported here demonstrates that the Co/Al LDH is

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a potential candidate for lubricant additive. However, there is no detail on the chemistry and morphology of

the worn surface and the frictional behaviour of Co/AL LDH particles.

In this study, the preparation of Co/Al-CO32--LDH by hydrothermally treating Co(NO3)2·6H2O and

Al(OH)3 in water is reported. It is shown that using Al(OH)3 as Al precursor results in the production of

micro-particle LDH that exhibits almost perfect hexagonal plate-like morphology. A reaction scheme for

the formation of the Co/Al LDH structure with Al(OH)3 is proposed. The effects of the synthesis conditions

on the phase composition, particle size and morphology of Co/Al LDHs are presented. Application of

LDHs as a base oil additive at room temperature was investigated. The XANES (X-ray Absorption Near

Edge Structure) spectra were recorded to characterize the chemical information of the worn surface to

understand the tribological behavior of LDH particles under friction.

2. Experimental methods

2.1 Materials

Raw materials used in the synthesis of Co/Al LDHs are Co(NO3)2·6H2O, Al(OH)3 powder, NaOH and

Na2CO3. All are analytical grade. Distilled water was used to prepare solutions.

2.2 Synthesis of Co/Al LDHs

A series of Co/Al LDHs samples were synthesized by a hydrothermal method. The following solutions

were prepared. Solution A was prepared by dissolving Co nitrate with mol ratios of Co:Al=2:1, 3:1, while

maintaining the concentration of Co2+ at 0.5mol/L. Solution B containing sodium carbonate

[m(Na2CO3)=10g/L] was dissolved in a NaOH solution of which the concentration of OH-1 was 1mol/L.

Both solution A and B were added simultaneously to a teflon autoclave with Al(OH)3 powder whose weight

was determined by the ratio of Co/Al and stirred with a glass rod for 1 minute.

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Two samples with Co/Al mol ratio of 2:1 and 3:1 were prepared under 130oC for 16h. Four samples

with Co/Al mol ratio of 2:1 were prepared under different temperature (70oC, 90oC, 110oC, 150oC) for 16h.

Four samples with Co/Al mol ratio of 2:1were prepared under 110oC for different times (4h, 8h, 12h, 16h).

2.3 Characterization of Co/Al LDH

All samples were characterized by power X-ray diffraction using a Rigaku diffractometer (Cu Kα

source, λ=0.15406nm, operated at 40kV and 100mA, step size 0.02° and scanning speed 8°/min). The

percentage crystallinity of hydrotalcite at different temperature and time were calculated by comparing the

summation of integral intensities of the (003) and (006) planes, in which the product has maximum

intensities as reported in literature16. SEM images were taken by HITACHI S-4800.

2.4 Tribological properties of Co-Al LDH

The lubricating oil (CD15W-40) was used as the base oil, which has a boiling point of 300oC,

viscosity of 110.6mm2/s at 40oC, viscosity of 15.2mm2/s at 100oC and a viscosity index of 228. Oil blends

were prepared by adding LDH powder (0.5 wt%) to the base oil and dispersed using an ultrasonic bath for

10 min. The extreme pressure (EP) friction and wear tests were carried out on a four-ball machine for 1

hour at room temperature and at the speed of 1200rpm/min and a load of 392N. The ball with a diameter of

12.7mm is made of GCr15 steel. Antiwear (AW) tests were carried out on a pin-on-flat Plint machine for 1

hour at room temperature at the sliding speed of 25Hz and load of 220 N. The steel plate (AISI 52100) with

dimensions of 7 mm radius and 3.5 mm thickness were polished using 1-µm diamond paste followed by 0.1

µm diamond paste. The cylinder (AISI 52100) has a radius of 3 mm and a length of 11 mm. The data

reported in this study are the average of three independent measurements under identical conditions. The

coupons were rinsed with hexane before analysis.

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2.5 Worn surface analysis

The morphologies and elemental distribution of the worn surfaces were characterized by scanning

electron microscopy (SEM, LEO 1540, Nanofabrication Laboratory at UWO) equipped with Energy

dispersive X-ray spectroscopy (EDX). AFM topographic images were collected for all the samples in

ambient conditions, using a Nanoscope Ⅲa equipped with a MultimodeTM head (Vecco, Inc. USA). These

AFM images were obtained in a tapping mode using a monolithic silicon cantilever possessing a spring

constant of around 42 N/m and a resonance frequency of around 320 kHz. Images were captured

continuously at a scan rate of 0.25 Hz. The surface roughness parameters of Ra were obtained by processing

100 × 100 µm2 AFM images of the tribofilms. X-ray absorption measurements were conducted at the

spherical grating monochromator (SGM) beamline at Canadian Light Source (CLS). X-ray absorption near

edge structure (XANES) spectra were obtained in detection modes of total electron yield (TEY) and X-ray

fluorescence yield (TFY) with spot size of 1000 µm × 100 µm (Horizontal × Vertical). All model

compounds were commercial products and used without further purification.

3. Results and discussion

3.1 Phase composition and structure

3.1.1 Samples synthesized with different Co/Al ratio

XRD patterns of the products synthesized with different Co/Al ratios are shown in Fig.1. The single

phase formation was confirmed by XRD pattern of Fig. 1a for the Co/Al ratio of 2:1. The diffraction pattern

shows the characteristic reflections of LDH materials with sharp (00l) and (11l) peaks indicating that the

sample is a well-crystallized LDH. The basic layer structure of LDHs is based on that of brucite [Mg(OH)2]

in which some of the divalent cations were replaced by trivalent cations, the layers stack on top of one

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another to form the three-dimensional structure. The brucite-like layers in LDHs may be stacked in

different ways. The basal spacing of three basal reflections at low angles can be used to determine the

stacking sequences in LDH. The basal spacing of d003(0.759 nm), d006(0.379 nm) and d012(0.259 nm) shows

a good relation as d003 ≈2d006≈3d012, respectively, indicating that the LDH is with rhombohedral 3R

polytype.42 Thus the lattice parameter c0 and a0can be determined by c0=3d003 and a0=2d110, respectively,

which are shown in Table 1. The pattern reveals that the Co/Al LDH has sharp symmetric peaks for (003),

(006), (110), (113) planes which are characteristic of clay minerals possessing layered structure23,43. When

the Co/Al ratio in the starting solution was changed to 3:1, the reflection peaks corresponding to LDHs are

similar to those in Fig. 1a. However, an impurity phase appeared in the pattern. These small particles on the

surface of plates are thought to be the β-Co(OH)2 phase. Excess Co present in the reaction solution may

undergo side reaction in the presence of excess Na(OH), resulting in the formation of compounds other

than LDH26. We can observe from Table 1 that d003 increases from 0.759nm to 0.762nm with increasing the

Co/Al ratio from 2:1 to 3:1, which can be attributed to the decrease in Al sites inducing a decrease in

electrostatic attraction between the positive hydroxide layers and negative interlayers44. The sample with

higher Co content induce the increase in the value of a0 for the larger ionic radius of Co2+ (0.074nm)

compared to Al3+(0.05nm).45 Comparing the value of a0 (0.307-0.309 nm) of LDHs with that of Co(OH)2

( 0.314-0.318 nm), the smaller a0 of LDHs confirms that the Al3+ cations with smaller ions radius has

substituted Co2+ into the LDHs layers.46

3.1.2 Samples synthesized under different crystallization temperature

The effect of reaction temperature on the phase composition, crystallinity and particle size is

manifested through the precipitation reaction and aging process of the LDH synthesis. According to the

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X-ray diffraction patterns as shown in Fig.2, all the samples are well crystallized and exhibit sharp and

symmetric peaks at low 2Ɵ angles, but a small amount of β-Co(OH)2 was present in products at 90oC,

Al(OH)3 was present at 70 oC, and both occurred in products at 150 oC. The average crystallite size (D003)

along [003] direction shown in Table 2 was calculated according to Scherree’s formula47. For the LDH with

3R stacking sequence along the c axis, the number (N) of layers stacked along the c axis of the unit cell can

be determined by D003 and unit cell thickness c (N= D003 / c). When the temperature increased from 110 oC

to 150oC, the layers stacked along the c axis maintained the same number, indicating that temperature

seems not to be a crucial parameter affecting the growth rate of hydrotalcite along the c axis when it is

higher than 110 oC. When the temperature was 70 oC, the impurity phase Al(OH)3 gave an XRD pattern

identical with that of synthetic gibbsite, which did not appeared for the preparation using a soluble

aluminium salt as an Al sources. The appearance of Al(OH)3 could be due to incomplete dissolution of the

insoluble Al source under 70 oC.48 When the temperature was 150 oC, the appearance of β-Co(OH)2 was

detected, which is also reported by Bai et al..38 However, Kayano et al.25 reported the presence of Co3O4 at

the same temperature. For the pure LDHs products, the higher the hydrothermal temperature, the higher the

crystallinity is. But the difference is not significant.

3.1.3 Samples synthesized under different crystallization times

XRD data (Fig. 3, Table 3) show that pure Co/Al LDHs can be prepared after aging for 12h and more,

and the crystallinity increases with prolongation of crystallization time. It also can be seen that when the

crystallization time is shorter than 8h, a small amount of impurity Co(OH)2 will be generated. Yang et al.49

reported a systematic investigation of the formation mechanism of LDH. These results showed that,

amorphous colloidal hydroxide aluminum is formed firstly; then, the divalent cations are incorporated into

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the sheet of oxide-hydroxide aluminum. Therefore, the present of Co(OH)2 could be due to the incomplete

incorporation of Co2+ into the oxide-hydroxide aluminum sheets because of the insufficient preparation

time. This phenomenon is different from our previous synthesis using Al nitrate salt.38 Our previous study

showed that the extension of crystallization time to 7 h could induce Co(OH)2 in the final product. From the

correlation of Co(OH)2 appearance with crystallization time, it could be speculated that Co and Al cations

could be quickly incorporated into LDHs phases when soluble metal salts are used as metal sources;

furthermore, the formation of LDHs could be slowed down and Co(OH)2 appear in shorter crystallization

times when insoluble Al(OH)3 powder are used as the Al precursor. For the samples (12h and 16h) without

other crystalline phases, the crystallinity of LDHs increases with crystallization increasing time. But the

difference is not significant. Therefore, which aging time would be the optimum depends on ones purposes

and we prefer 12h. To visualize the influence of crystallization time on the particle size, SEM images were

obtained for three samples. As shown in the SEM images of Fig. 3, the particle sizes of LDHs were around

40-150nm and 0.2-1.5 µm for 4h and 12h respectively.

3.2 Implication of growth mechanism

The differences in the crystallite phase composition are obviously related to the reaction and

crystallization pathways during the LDHs formation. As we observed, when the alkaline solution was

added into the salt solution with Al(OH)3 the suspension color was bluish-gray in the initial stage(10

seconds) and then changed to rose-red, implying a spontaneous transformation from initial metastable

α-Co(OH)2 to β-Co(OH)2.50 In strong bases (the final pH of reaction solution was around 11.5), cobalt (Ⅱ)

hydroxide accepts additional hydroxide ions41 to form [Co(OH)6]4- and the [Al(OH)4]

- was the predominant

Al species in the reaction solution because the pH value of suspension was larger than 11.17 Therefore, the

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growth morphology of Co/Al LDHs could be consistent with the model of ACP (anion coordination

polyhedron) growth units. The possible growth process contributing to the Co/Al LDH formation can be

envisaged as follows: Growth units [Co(OH)6]4- and [Al(OH)6]

3- first incorporate into metal layers in the

same plane, then metal layers absorb CO32- and H2O, and the process circulates51. Finally, the growth

becomes Co/Al hydrotalcite. The dissolution of Al(OH)3 to [Al(OH)4]-, hydration process of Co2+ to

α-Co(OH)2 and phase conversion of α-Co(OH)2 to β-Co(OH)2, all slow down the nucleation and growth

rate of Co/Al LDH, that is why the pure Co/Al LDH could only be obtained after more than 8 hours. The

slower nucleation rate attributed to the formation of large and perfect hexagonal plate like Co/Al-LDH is

shown in Fig. 3 (SEM image 12h). The crystal growth mechanism is proposed and illustrated in Fig.4.

3.3 Characterization of the product obtained under optimized conditions

Based upon the foregoing results on various synthetic parameters, a Co/Al ratio of 2:1, crystallization

temperature at 110oC and an crystallization time of 12h were chosen as optimum synthesis conditions to

produce Co/Al-CO32- LDH using Al(OH)3. The lattice parameters of sample prepared using these optimized

conditions are: a0=b0=0.306nm, c0=2.286nm, d-spacing=0.762nm. Since the thickness of the hydrotalcite

layer is evaluated to be 0.48 nm, the gallery height which is defined as the difference between the d-spacing

and the layer thickness42 is around 0.28nm for LDH.

3.4 Wear performance of Co/Al LDH

Figure 5A shows the friction coefficient for oil with and without added Co/Al LDH as a function of

testing time obtained on a four-ball machine under extreme pressure (EP) condition. The coefficient of

friction is a proxy for energy loss caused by friction. Decreasing energy loss is equally important as

reducing wear. It can be seen from Fig. 5A that base oil blended with Co/Al LDH shows a smaller friction

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coefficient than lubricating oil alone. The friction coefficient, using base oil alone, slowly increases from

0.1 to 0.11 with rubbing time. The wear scar width for the test was 0.46 mm. In contrast, the friction

coefficient test with oil blended with Co/Al LDH decreases with rubbing time and becomes stable around

0.08 which is substantially smaller (22.3%) than the test without Co/Al LDH. The same is true for the wear

scar diameter of 0.34 mm which is smaller by 26.1% than oil alone. Previous studies by our group have

shown that the particle size plays an important role on the performance of LDHs on friction and wear.38 The

smaller the particle size, the better antifriction and antiwear performance is.

In order to obtain antiwear (AW) conditions, a plint friction tester was used. Fig.5B shows the friction

coefficient obtained from lubricating oil with and without Co/Al LDH. It reaches around 0.06 compared to

0.07 without LDH and it has a much smoother profile.

In summary, the friction tests clearly show that the addition of Co/Al LDH to oil can significantly

reduce friction and improve wear properties of lubricating oil under EP and AW conditions. These results

are similar to those obtained using layered structure inorganic nano powders such as serpentine52 and WS253.

However, there is no report on the characterization of tribofilm generated by oil blended with LDH.

Post-analysis of the worn surface helps understand the mechanism of tribofilms formation derived from

additives under friction. Therefore, the surface analysis was performed on worn surfaces formed by

tribology experiments to study the tribological behaviour of LDH.

3.5 Worn surface characterization

3.5.1 Worn surface morphology and elements distribution

SEM equipped with EDX was utilized to analyze the morphologies and elemental distribution of the

worn surface. Typical SEM images and EDX spectra of worn surface lubricated with base oil are shown in

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Fig. 6 (1). It is obvious to see the numerous scratches on the worn surface along the sliding direction. It

shows severe abrasive and adhesive wear36. In the enlarged morphology (Fig. 6 (1)-b), inside (A) and

outside (B) the wear track regions were analyzed by EDX respectively. The results showed the presence of

Fe, O and C elements on the worn surface, in which Fe and C elements are from the steel bulk and oxygen

is from the original oxide film on the steel disc. It is noticed that the quantity of oxygen in the track is

smaller than that outside the track, which suggests that the original oxide film was removed during the

friction as a result of severe wear. This may account for the high friction coefficient of base oil alone used

as lubricant.

Figure 6 (2) displays SEM images and EDX spectra of worn surface lubricated by base oil blended

with LDH. It is easy to observe the sliding direction in micrograph Fig. 6 (2)-a. It seems that the surfaces

outside and inside the wear tracks are smoother than the corresponding ones formed by base oil alone in

Figure 6 (1). The enlarged micrograph of Fig. 6 (2)-a is shown in Fig. 6 (2)-b showing the LDH sheets

adsorbed on the worn surface. The EDX analysis shows the presence of Co and Al elements from LDH in

the worn surface. Thus, we can say that the LDH sheets could adsorb on the rubbing surfaces and prevent

the direct contact between metal sliding pairs. Moreover, the low-strength (Van der Waals force) between

LDH’ layers also could decrease the friction force along the sliding direction. These reasons may account

for the lower friction coefficient of base oil blended with LDHs.

3.5.2 AFM image of the worn surface

The morphologies of the worn surface were studied by atomic force microscopy. The 100 × 100 µm2

images were collected at five different spots close to the center of the worn surface. The surface roughness

Ra is analyzed from the whole area of AFM image by the NanoScope control software. The surface

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roughness of the sample was obtained by the average of the Ra of five different spots. The scale bar is

shown on the right side of Fig. 7. The dark regions have low topography and the bright regions have high

topography. The Ra of the worn surface lubricated by base oil and the combination of base oil and LDH are

41.9 (±4.8) nm and 29.5 (±3.9) nm respectively. The tribofilm formed by base oil alone is rougher than that

lubricated by base oil containing LDH. The scratches along the sliding direction could be the result of the

asperity between rubbing pairs during the friction process. The section analysis shows that the marked

scratch has a maximum depth of 204.6 nm and a width of 12.5 µm for base oil sample. The maximum

depth of film formed by base oil with LDH is around 94.8 nm, which is much smaller than base oil sample.

This result suggests that LDH plates can prevent asperity contact and as a result the formation of fewer

deep scratches in the film.

3.5.3 XANES characterization of the worn surface

Al K-edge

To confirm the impact of LDH on the tribological performance of base oil, the normalized Al K-edge

XANES spectra of the worn surface, LDH and model compound α-Al2O3 in which Al atoms are in

octahedral sites54 were investigated. The results are shown in Fig. 8. In the TEY mode, the Al K-edge

spectrum of LDH is identical to the Al K-edge spectrum of hydrotalcite reported in the literature ( Doyle et

al., hydrotalcite in Figure 3).55 Comparing with the featured peaks of α-Al2O3, the dominant peaks b and c

of LDH are assigned to octahedral AlO6. The Al K-edge spectrum of the worn surface exhibits a broad peak.

The signal-to-noise ratio of the worn surface spectrum is worse than that of LDH and the model compound.

It should be noted that the proportion of Al in the worn surface is relatively very small (only 2%) as was

shown by EDX result in Fig. 6(2). Therefore, the spectrum for the worn surface was noisy and weak. In the

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surface sensitive TEY spectra, the main peak position of the worn surface aligns well with broad peak b of

LDH, indicating that AlO6 is predominantly present at near surface of the worn film. For the bulk sensitive

FY spectra, the main peak has moved to lower energy by 2.5 eV, and does correspond to LDH and α-Al2O3.

According to the literature the main peak can be assigned to tetrahedral AlO4. This suggests that the local

geometry of Al atoms at the bulk of the worn film has changed. This gives an evidence that the structure of

fractional LDH changed markedly by shearing force along the sliding direction after friction. This

mechanochemical process could consume the shearing force between rubbed surfaces then decrease the

friction coefficient.

O K-edge

The oxygen K-edge XANES spectra of model compounds and worn surfaces are shown in Fig. 8. The

TEY spectra for the worn surface formed by base oil shows the same feature as Fe2O3 especially at peaks a,

b and f positions and the signal from Fe2O3 is stronger than that of steel surface. This suggests that the main

chemical species on the worn surface lubricated by base oil is Fe2O3. The steel surface during the friction

test is oxidized and the presence of Fe2O3 is expected. In the spectrum of worn surface lubricated by base

oil blended with LDH, the intensity of peaks a and b is much reduced. The broad peak aligns well with the

broad one in steel surface and the peak f from Fe2O3 is not evident. These results suggest that the film

generated by base oil blended with LDH was less oxidized than that lubricated by base oil alone and thus

the addition of LDH can protect the worn surface from oxidation. It also should be noted that the energy

position of peaks a' and b' in the spectrum of base oil with LDH moved to lower energy than peaks a and b

which were assigned to Fe2O3 and appeared in the spectrum of base oil sample. It can be due to the

appearance of Co and Al in the oxidized worn film. The FY spectra probe the bulk of the tribofilm (>

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200nm).56 The FY spectrum of the steel surface is noisier than the TEY spectrum indicating that the iron

oxide film on the steel surface is thin. The FY spectrum of steel surface lubricated by base oil is similar to

the TEY one suggesting that the oxidation process during the friction is severe and a thicker oxide film

could be formed. After addition of the LDH, the FY signals from iron oxide decreased, which gives

evidence that the LDH could prevent the oxidation of the rubbed surfaces during friction. That could be

ascribed to the notion that LDH sheets absorbed on the rubbing surfaces prevent the surface from oxidation

hence the smaller friction force derived from the sliding between LDH layers results.

Co L-edge

Compared to Al K-edge spectra, Co L-edge spectral features of the worn surface are much stronger

due to the higher proportion of Co in the film (see EDX result in Fig. 6 (2)). The results are displayed in

Fig. 8 along with model compound spectra: Co3O4 representing tetrahedral Co2+ and octahedral Co3+

cations and CoO representing octahedral cation (Co2+). The spectra contain two distinct regions separated

by 16.0 eV due to the core-level spin orbital splitting of the 2p3/2 and 2p1/2 orbitals.57 Peaks a, b, c, d and e

in LDH align very well with those in spectrum of CoO, confirming that the Co atoms in LDH are in

octahedral form. However, there is a new peak d' not found in CoO. The peak d' in LDH aligns well with

that in Co3O4 spectrum indicating the presence of octahedral Co+3 in LDH sample. Comparing with CoO,

the peak d' of Co3O4 could be due to the appearance of Co3+ octahedral.58 The Co K-edge XANES (not

shown here) of LDH and model compounds show that there is no Co3+ in LDH. Thus, the peak d' appeared

at higher energy position of LDH can be ascribed to the higher electronegativity of Al3+ cations than that of

Co2+ cations which have been partially replaced by Al3+ at octahedral sites. For the oil sample, the energy

positions of peaks a, b, c and d align very well with those in spectrum of LDH. This suggests that the cobalt

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on the worn surface is in the Co2+ oxidation state.59 The most notable changes is that the intensity of peak b

and c became higher than peak d, the peak d' disappeared and the peak e shifted to lower energy. These

changes (main features moved to lower energy position and no oxidation state change) are in line with Al

K-edge TEY spectrum of oil sample, which can be ascribed to the structure distortion induced by

mechanical force under friction. The octahedral symmetry of Co and Al were distorted by shear force and

vertical load under friction, and then the electrostatic interaction between Co-O and Al-O became weaker.

Therefore, the lower absorption energy results for the Co 2p-3d dipole transition.60 This observation

confirms that the energy consumption originating from crystal structure distortion induced by mechanical

process is an important reason for friction coefficient decrease.

4. Conclusions

In this paper, for the first time, Co/Al LDH hexagonal platelets has been synthesized using insoluble

Al(OH)3 as Al precursor via a hydrothermal method. The tribological performances of base oil blended

with LDH have been investigated using a four-ball and pin-on-disk friction and wear testers. SEM, EDX

and XANES have been used to characterize the morphology, element information and chemical nature of

the worn surface respectively.

The following conclusions can be drawn from the results:

1) The dissolution of Al(OH)3 to [Al(OH)4]-, the hydration process of Co2+ to α-Co(OH)2 and the

phase conversion of α-Co(OH)2 to β-Co(OH)2 could all slow down the nucleation rate and increase the

crystal growth rate, therefore, it is easy to prepare large platy and perfect hexagonal plate-like Co/Al LDH

particles. Using Al(OH)3 the precipitation reaction is homogeneous and slow even when NaOH is used as

precipitation reagent, which will prevent the formation of cobalt ammonia complex in the products when

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using ammonia as a hydrolysis agent.

2) The systematic monitoring of the experimental parameters such as Co/Al mol ratio, crystallization

temperature and crystallization times, shows that the optimal conditions are as follows: Co/Al mol ratio of

2:1, crystallization temperature at 110oC and reaction time 12h.

3) Using Co/Al LDH as lubricant additive can significantly improve the tribological properties of base

oil by reducing friction coefficient, wear and surface roughness. The LDH sheets can adsorb on the rubbing

surfaces to prevent direct contact between metal sliding pairs and thus prevent asperities contacts. The

low-strength shearing for the weak interatomic interactions between LDH’s interlayers and the crystal

structure plays an important role to consume the friction force then decrease the friction coefficient and

reduce surface wear.

4) Morphologies and chemistry of the worn surface were characterized by SEM, atomic force

microscope (AFM) and X-ray absorption near edge structures (XANES). The results show that the structure

distortion of LDH induced by mechanochemical process during friction consumes the friction force

between rubbing surfaces in contact and as a result friction and wear is reduced. Thus the LDH can prevent

the steel surface from oxidation during friction.

Acknowledgment

Research at the University of Western Ontario is supported by NSERC, CFI, OIT and CRC. Thanks to

Western Nanofabrication Laboratory for SEM imaging and Canadian Light Source (CLS), where the

synchrotron measurements were conducted. The support from the China Scholarship Council is gratefully

acknowledged. Research at China University of Geosciences (Beijing) is supported by the Fundamental

Research Funds for the Central Universities (2652013039).

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Page 21 of 34C

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22

Table caption

Table 1 Lattice parameters of products synthesized with different Co/Al ratios.

Table 2 XRD parameters of products synthesized under different crystallization temperature.

Table 3 XRD parameters of products synthesized under different crystallization time.

Figure caption

Fig. 1. XRD patterns and SEM images of products synthesized with different Co/Al ratio under 130℃ for

16h. a-Co/Al=2:1, b-Co/Al=3:1.

Fig. 2. XRD patterns of products synthesized with Co/Al ratio of 2:1 under different temperature for 16h.

Fig. 3. XRD patters and SEM images of products synthesized with Co/Al ratio of 2:1 under 110℃ for

different time.

Fig. 4. Suggested scheme for Co-Al LDHs formation from Co(NO3)2﹒6H2O and Al(OH)3.

Fig. 5. Friction coefficient as a function of time (A: four-ball, 392N, 1200r min-1, 60min; B: plint friction

tester, 220N, 25Hz, 60min).

Fig. 6. SEM images of worn surface. (1) SEM morphology (a, b) and the corresponding EDX patterns (A,

B) of the worn surface lubricated with base oil: Micrograph (b) is for high-magnification of area (a);

Spectra A, B are for regions A and B in micrograph (b). (2) SEM morphology (a, b) and the corresponding

EDX pattern (A) of the worn surface lubricated with base oil blended with LDHs: Micrograph (b) is for

High-magnification of area (a); Spectra A are for regions A in micrograph (b).

Fig. 7. Representative AFM topography images and section analyses of the worn surface, (a) base oil, (b)

base oil with LDHs.

Fig. 8. XANES spectra of model compounds and tribofilms measured in TEY and FY modes (Oxygen

Page 22 of 34C

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23

K-edge, Aluminum K-edge and Cobalt L-edge).

Page 23 of 34C

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Table 1 Lattice parameters of products synthesized with different Co/Al ratios.

Sample FWHM(003)/º d003/nm d110/nm a0/nm c0/nm

a 0.220 0.759 0.154 0.309 2.277

b 0.231 0.762 0.153 0.307 2.286

FWHM(003) is the peak width at half-height of diffraction peak (003).

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Table 2 XRD parameters of products synthesized under different crystallization temperature.

Sample 70℃ 90℃ 110℃ 130℃ 150℃

FWHM(003)/º 0.209 0.224 0.223 0.220 0.221

D003/nm 43.38 39.53 40.05 39.93 39.56

N 19 17 18 18 18

a0/nm 0.308 0.306 0.307 0.309 0.307

Crystallinity/% 81.71 94.20 97.64 100 90.01

Page 25 of 34C

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Table 3 XRD parameters of products synthesized under different crystallization time.

Sample a(4h) b(8h) c(12h) d(16h)

FWHM(003)/º 0.252 0.232 0.221 0.220

D003/nm 37.41 39.05 39.84 39.93

N 17 17 18 18

a0/nm 0.306 0.307 0.306 0.307

Crystallinity/% 84.39 88.26 96.46 100

Page 26 of 34C

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127x69mm (300 x 300 DPI)

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289x429mm (300 x 300 DPI)

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