Friction properties of La-doped Mg/Al layered double hydroxideand intercalated product as lubricant additives

Shuo Li, Haojing Qin, Ranfang Zuo, Zhimin Bai n

School of Materials Science and Technology, China University of Geosciences, Beijing 100083, PR China

a r t i c l e i n f o

Article history:Received 26 March 2015Received in revised form9 June 2015Accepted 12 June 2015Available online 23 June 2015

Keywords:Mg/Al/La-LDHIntercalationFriction propertiesMechanisms

a b s t r a c t

La-doped Mg/Al layered double hydroxide (LDH) was synthesized and intercalated by sodium dodecylsulfate. Friction properties of LDHs as lubricant additives were evaluated in four-ball friction and aircompressor test. The results showed that Mg/Al/La-LDH was prepared with La/Al molar ratio of 0.1. Theinterlayer spacing was expanded from 7.662 to 25.663 Å after intercalation. The intercalated productexhibited better tribological property than its precursor in the friction tests. The improved performancewas attributed to a reduction in friction between the expanded laminates, good dispersion of thenanoparticles in the medium and an effective tribofilm formed on the contact surfaces.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Nanoparticle additives for tribological applications have attra-cted considerable interest in recent decades. It has been found thatnanoparticles can enhance the friction-reducing, anti-wear andload-carrying properties of base oil significantly under boundarylubrication condition [1]. The friction properties are dramaticallyinfluenced by the structure, shape, size and chemical properties ofnanoparticles as well as experimental conditions (friction mode,normal load and speed). The lubrication mechanisms of nanopar-ticles have been described in terms of rolling friction [2,3], surfacedeposition [4,5] and chemical reaction tribofilm effects [6–11].Complex physical and chemical reactions occur simultaneouslyduring the friction and lubrication process. The functional mech-anisms of nanoparticle lubricants have been explained in previouswork [12–16].

Layered double hydroxides (LDHs) are a family of laminate-structure compounds which contain positively charged brucite-likehydroxide layers and charge-balancing exchangeable anions in theinterlayer [17]. It has been found that LDHs nanoparticles exhibitedexcellent tribological properties as lubricant additives [18,19]. In theseprevious investigations the friction reducing mechanism was attrib-uted to the formation of a tribochemical reaction film on the surface[20] or structure distortion of LDHs during a friction process [21].

It has been observed that rare earth compound as lubricantadditives can reduce friction and wear and improve lubricationproperties of base oil like other kinds of inorganic nanoparticles

[22–25]. Zhang et al. [22] studied the friction properties of La(OH)3nanocluster modified with N-containing compounds in liquid paraffin.In this study, the maximum non-seizure load, friction coefficient andwear scar size were ameliorated. It was attributed to physical adsorb-ing film as well as chemical reaction film composing of La2O3 formedon the rubbed surface. Liu et al. [25] found that mixed rare earthnanoparticles improved the tribological performance by forming aprotective film which consisted of ferrous oxides, organic acid, rareearth oxides and other compounds on the friction surface. Theconcentration of ferrous oxides on the surface was increased by thecatalytic action of rare earth elements. The formation of the oxide layerenhanced the friction-reducing effect of the lubricant.

In this work, La-doped Mg/Al layered double hydroxide (MAL–LDH) was prepared by coprecipitation at La3þ/Al3þ molar ratio of 0.1.The LDH precursor was intercalated by sodium dodecyl sulfate (SDS).The lubrication properties of the precursor and intercalated LDH wereinvestigated using a four-ball tribometer and a reciprocating aircompressor that was instrumented with thermocouples and a powermeter. The results indicated that both the products improved friction-reducing and anti-wear properties of base oil. Furthermore, theintercalated LDH (MAL–SDS–LDH) possessed better tribological per-formance than the precursor. The mechanisms were discussed.

2. Experiment

2.1. Synthesis of MAL–LDH nanoparticles

Mg(NO3)2 �6H2O, Al(NO3)3 �9H2O, La(NO3)3 �6H2O, NaOH andNa2CO3 were analytical pure without further purification.

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Tribology International

http://dx.doi.org/10.1016/j.triboint.2015.06.0120301-679X/& 2015 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ86 10 82323201; fax: þ86 10 82322974.E-mail address: zhimibai@cugb.edu.cn (Z. Bai).

Tribology International 91 (2015) 60–66

Deionized water was prepared in the laboratory. Mg/Al/La-LDHwas synthesized by coprecipitation. The nitrates were mixed anddissolved in deionized water with the Mg2þ/(Al3þþLa3þ) molarratio of 2.0 and La3þ/Al3þ of 0.1. A mixed solution of NaOH andNa2CO3 was added dropwise at a rate of 5 ml min�1 to the stirredsalt solution until the final pH value reached 10. When thetitration was completed the solution was stirred for an additional30 min at room temperature to ensure homogeneity. The slurrywas aged at 120 1C for 10 h in an autoclave. Finally the precipitatewas filtered and washed with deionized water to neutral and driedat 80 1C. Similarly, Mg/Al-LDH (MA–LDH) was also prepared withMg2þ/Al3þ molar ratio of 2.0 under the same synthesis conditionsof MAL–LDH as a comparison.

2.2. Intercalation of MAL–LDH

Sodium dodecyl sulfate [CH3(CH2)11OSO3Na] was analyticalpure without further purification. MAL–LDH was intercalated withthe agent SDS by ion exchange. The mixed powders with agent/LDH molar ratio of 2.0 were added and stirred in 500 ml deionizedwater until turned to be homogeneous slurry. Then the slurry wasadjusted and maintained pH value 4–5 with nitric acid for 3 h at80 1C. Finally the precipitate was filtered and washed with hotdeionized water and ethyl alcohol until pH value reached 7 toremove the remaining agents totally and dried at 80 1C. Theintercalated product with agent SDS was signed as MAL–SDS–LDH.

2.3. Characterization of LDHs nanoparticles

All the LDHs products were characterized by X-ray diffraction(XRD; Rigaku diffractometer, CuKα source, λ¼0.15406 nm, oper-ated at 40 kV and 100 mA, scanning rate 81 min�1 from 31 to 701(2θ)), Fourier transform infrared spectra (FT-IR; NICOLET750 FT-IRspectrometer, 4000–450 cm�1, KBr sheet). The morphology ofLDH was obtained on a scanning electron microscope (SEM;JSM-6460LV, JEOL, Japan) which was connected with an energydispersive spectroscopy (EDS) at the acceleration voltage 20.0 kV.The particle size distribution of LDH nanoparticles was measuredby a Zetasizer (Nano ZS90, Malvern).

2.4. Friction properties of LDHs nanoparticles as lubricant additives

A MS-10JR four-ball tester was utilized for the friction and wearexperiment. The balls were made of GCr15 steel (AISI 52100 steel,diameter¼12.70 mm, hardness¼64–66 HRC) and were cleanedwith ethyl alcohol and petroleum ether in an ultrasonic bathbefore testing. The diesel engine oil (CD 15W-40) was selected asbase oil which had viscosity index of 141, pour point of �27 1C,flashing point of 228 1C, boiling point of above 300 1C, viscosity of110.60 mm2/s at 40 1C and 15.02 mm2/s at 100 1C [18]. The twoLDHs products (MAL–LDH and MAL–SDS–LDH) were ground withSpan 60 powders (mass ratio of 1:1) respectively before they wereadded to the base oil to provide a concentration of 0.5 g LDH per100 ml oil [10]. The oil samples were homogenized using a highspeed mixer (10,000 rpm) and an ultrasonic bath at 80 1C tothoroughly disperse the nanoparticles. The four-ball test wasconducted under rotating speed of 1200 rpm and load of 392 Nfor 60 min at room temperature. The average friction coefficientwas calculated with the data after 15 min and average wear scardiameter of the three lower balls was determined by an opticalmicroscopy.

Reciprocating air compressors output pneumatic power byconverting the mechanical energy input of a rotating driveshaftinto reciprocating motion of a piston which produces the flow ofcompressed air. Friction between the pistons and cylinder wall aswell as churning of the crankcase lubricant affects power

consumption and the lubricating oil temperature [18]. Therefore,the test can demonstrate the tribological properties of LDHsindirectly. The air compressor test was conducted on the ModelDingwei ZB-0.11/7 compressor (rotating speed of motor 2850 rpmand test duration 20 min). The consumption and oil temperaturewere measured by a Model 8705B power meter and a JM222thermometer respectively. LDH products were added to base oilwith the same content and treatment method as four-ball frictiontest respectively.

3. Results and discussion

3.1. X-ray diffraction and morphology of LDHs

From the XRD patterns of MA–LDH and MAL–LDH (Fig. 1), it isobvious that both the LDHs possess the characteristic layeredcrystal structure diffraction peak (003), (006) and (009) of hydro-talcite. The peak at (110) represents internal laminate structure ofbrucite-like layers [17]. An analysis of the interlayer spacing basedupon the X-ray diffraction results is shown in Table 1. Theinterlayer spacing d(003) (7.635 Å) and lattice parameter a(3.046 Å) of Mg/Al-LDH correspond to the values reported inprevious work [17]. It should be mentioned that interlayer spacingd(003) rises when rare earth element lanthanum is doped to Mg/Al-LDH. It can be explained that the positive charge density ofbrucite-like laminates decreases due to the small Al3þ (ionicradius¼0.53 Å) substituted by larger La3þ (ionic radius¼1.03 Å)which weakens the electrostatic attraction effect between posi-tively charged layers and interlayer CO2�

3 anions [26,27]. Thereplacement of larger La3þ increases the size of octahedron whichboth lattice parameter a and the thickness of laminate increasecompared with Mg/Al-LDH. It also contributes to the change ofinterlayer spacing at a certain extent. Additionally, the crystallinityof MAL–LDH decreases due to the distortion of lattice caused bythe substitution of lanthanum as well (shown in the Fig. 1).

The morphology of MAL–LDH is shown in Fig. 2 in which thenanoparticles possess typical hexagonal laminate structure. Theparticle size distribution (top right corner in Fig. 2) indicates thatthe size of LDH nanoparticles mainly ranges from 150 to 200 nmwith mean disc-diameter of 185.6 nm. The EDS test resultsare shown in Table 2. The molar ratio of Mg:Al:La equalsto 1.98:1.00:0.08 which approaches the theoretical value2.20:1.00:0.10. Although the accuracy of EDS is limited, the resultprovides a good indication of the relative ratio of Mg, Al and La inthe LDH compounds.

Fig. 3 shows that MAL–LDH is intercalated with the agent ofsodium dodecyl sulfate. It is obvious that the peaks (003), (006)and (009) of MAL–SDS–LDH keep the diffraction feature and move

Fig. 1. XRD patterns of MA–LDH and MAL–LDH.

S. Li et al. / Tribology International 91 (2015) 60–66 61

to small angle integrally. The interlayer spacing d(003) is expandeddramatically from 7.662 to 25.663 Å after intercalation (Table 1). Itis considered that the thickness of LDH laminate is about 4.8 Å [17]although it increases slightly due to the lanthanum. The end-to-end length of dodecyl sulfate anion is about 20.8 Å based on itschemical structure [28]. The actual interlayer distance of MAL–SDS–LDH is approximately equal to the sum of the layer thicknessand anion length. Therefore, it can be concluded that carbonchains of dodecyl sulfate extend completely and stand vertically(Fig. 4a) or are inclined with partial overlap in the interlayer(Fig. 4b). Besides, the peak (110) of MAL–SDS–LDH arises at almostthe same angle with MAL–LDH which represents the ideal octa-hedral structure of LDH layers.

3.2. FT-IR spectroscopy of MAL–LDH and MAL–SDS–LDH

The FT-IR spectra of MAL–LDH and MAL–SDS–LDH are shownin Fig. 5. For MAL–LDH, the broad band displayed at 3468 cm�1 isassigned to O–H stretching vibration from hydroxyl in brucite-likelayers as well as absorbed and interlayer water molecular. Thebands arising at 1367 and 679 cm�1 are recognized as the v3asymmetric stretching and v4 vibration of the CO2�

3 in theinterlayer of LDH respectively. The broad vibration band at783 cm�1 is associated with the interaction of CO2�

3 with thebrucite-like layers. The metal–hydroxyl M (Mg, Al and La)–OHvibrations appear at 552 cm�1 [18,19]. For the MAL–SDS–LDH, thebasic FT-IR spectrum characteristics of LDH are maintained. Theintense absorption bands at 2920 and 2852 cm�1 are assigned tothe asymmetric and symmetric vibration of CH2 respectively dueto the presence of alkyl groups in dodecyl sulfate [29,30]. More-over, the broad band with coupled peaks at 1206 and 1176 cm�1

corresponds to the asymmetric S–O stretching vibration andthe symmetric S–O stretching band arises at 1065 cm�1 as well[31,32]. The characteristic peaks of carbonate (1369 and 674 cm�1)

appear in the spectrum as well. This is an indication that thedodecyl sulfate anions have only partially replaced the carbonateanions in the interlayer region.

3.3. Friction properties of LDHs nanoparticles as lubricant additives

A comparison of the friction coefficients of the base oil and theoil with LDH nanoparticles in the four-ball test is shown in Fig. 6.The MAL–LDH and MAL–SDS–LDH nanoparticles reduce the aver-age coefficient of friction from 0.111 to 0.093 and 0.082 respec-tively. Additionally, MAL–SDS–LDH has better friction propertythan MAL–LDH (more reduction of coefficient). The oil with MAL–SDS–LDH reaches the stable friction stage more quickly (only10 min) than the other two oil samples after a transitional process.The morphology of the four-ball wear scars is shown in Fig. 7. Thewear track of the specimen that was lubricated by the base oilshows adhesive and plowing wear. A deep and wide groove can beobserved along the sliding direction in the middle of Fig. 7A. Theanti-wear properties of the lubricating oil were improved with theaddition of MAL–LDH particles. As shown in Fig. 7B, the wear trackhas a narrow linear wear path but the areas of adhesion are highlypronounced. The MAL–SDS–LDH lubricant produced the smallestwear scar and the surface is noticeably smoother (Fig. 7C). Thegrooves and furrows shown in above images decrease due to theanti-wear effect of MAL–SDS–LDH which corresponds to themeasurements of friction coefficient. As shown in Table 3, thediameter of the wear scars produced by MAL–LDH and MAL–SDS–LDH was reduced by 12.9% and 16.5% respectively, further eviden-cing the enhanced anti-wear performance of these compounds.

The variation of power consumption and oil temperature of areciprocating air compressor was used to evaluate the tribologicalproperties of lubricating oil with LDH nanoparticles as mentionedabove. The test results are indicated in Fig. 8. The consumption ofall the three oil samples (base oil, oil with MAL–LDH and MAL–SDS–LDH) starts at a high level and then continues to fall due tothe lubrication effect of oil. Meanwhile, LDH nanoparticles in oildeposit on the surface gradually and make a difference in reducingfriction after about 5 min. Subsequently, base oil maintains in ahigh value of around 1280 W without more reduction further.However, both oil samples with LDH keep decreasing the con-sumption effectively and moreover the friction-reducing perfor-mance of intercalated product is superior to its precursor with theconsumption of MAL–SDS–LDH nanoparticles drops to lower1225 W compared with MAL–LDH (1239 W) at the end of test(Table 4). Certainly piston motion will also decide the oil tem-perature in the air compressor due to friction heat besides thechange of power consumption of motor. As shown in Fig. 8, thetemperature of all oil samples continues to rise up during the testprocess. The friction-reducing effect of LDH starts to be reflected in

Table 1The interlayer spacing d(003) and lattice parameters of LDHs.

LDHs La/Al ratio d(003) (Å) d(006) (Å) d(110) (Å) a (Å) c (Å)

MA�LDH 0 7.635 3.802 1.523 3.046 22.905MAL–LDH 0.1 7.662 3.809 1.524 3.048 22.986MAL–SDS–LDH 0.1 25.663 12.763 1.524 3.048 76.104

Fig. 2. SEM image of MAL–LDH nanoparticles (black square selected for EDS test).

Table 2The EDS test results of MAL–LDH (black square selected for test in Fig. 2).

Element Mg Al La C O Total

Mass(%) 17.80 9.97 4.10 25.79 42.34 100Atom(%) 12.36 6.24 0.50 36.24 44.66 100

Fig. 3. XRD patterns of MAL–LDH and the intercalated product MAL–SDS–LDH.

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the variation of oil temperature a few minutes later than thechange of consumption due to the lag of heat conduction whichtakes place at about 13 min. Then LDHs slow down the climbing

trend of temperature although base oil keeps its tendency all thetime. Finally the temperature of oil with MAL–SDS–LDH and MAL–LDH reaches to 66.5 and 70.1 1C respectively which is similar withthe results of consumption that MAL–SDS–LDH's friction perfor-mance is better.

From the above four-ball and air compressor test, it can beobviously concluded that LDH nanoparticles can reduce frictioncoefficient, wear scars size as well as power consumption and oiltemperature of air compressor compared with base oil. Moreover,the intercalated product MAL–SDS–LDH has better tribologicalperformance than its precursor MAL–LDH in the tests.

3.4. Discussion

It has been proved that LDHs naoparticles with layered-structure possess friction-reducing and anti-wear properties aslubricant additives in previous work. However, it is demonstratedthrough above work that LDH with different interlayer spacingsexhibit different tribological performances in base oil for the firsttime (MAL–LDH and MAL–SDS–LDH). The highly-magnified SEMimage of wear scar produced by oil with MAL–SDS–LDH nanopar-ticles is shown in Fig. 9. The double-headed arrow at the right partrepresents the sliding direction. It is obvious that there is acomplete crystal grain of LDH with hexagonal laminate crystal aswell as large amounts of wear debris (dotted square frame area)around it on the worn surface. Meanwhile, the right part of theimage (black frame area) shows smooth and flat surface. Besides,there are obvious boundaries (dotted frame areas marked as A andB) between the smooth surface and grooves at the top left corner.It is indicated that LDH nanoparticles deposit and form incon-secutive protective tribofilm on the worn surface to achieve lowfriction and wear. The lubricant mechanisms can be explained thatMAL–SDS–LDH nanoparticles which have high surface chemicalactivity deposit and absorb on the fresh rubbed surface of frictionpairs easily with base oil (Fig. 10A). There is strong electrostaticattraction between positively charged brucite-like laminates andinterlayer anions in the LDH structure. However, long chaindodecyl sulfate anions enter into the interlayer and expand theinterlayer distance of LDH dramatically which contributes toweaken the attraction. Besides, monovalent dodecyl sulfate ions

Fig. 4. The schematic diagram of MAL–LDH intercalation with sodium dodecyl sulfate.

Fig. 5. The FT-IR spectrum of MAL–LDH and MAL–SDS–LDH.

Fig. 6. The friction coefficient of base oil and oil with MAL–LDH and MAL–SDS–LDHat load of 392 N for 60 min in four-ball test.

S. Li et al. / Tribology International 91 (2015) 60–66 63

replace bivalent carbonate ions to reduce the charge number ofinterlayer anions at the same time. Thus, the electrostatic force isweakened with the increase of interlayer spacing. Adjacent lami-nates of MAL–SDS–LDH with low strength are very easier to sliderelatively under shearing force during the friction process com-pared with MAL–LDH (black frame in Fig. 10B). As a consequence,LDH crystal grains are ground to be thinner and smaller (Fig. 10B).The sliding motion can absorb and reduce part of friction forceapplied on the contact surface effectively which is the main factorto decide the tribological performance of MAL–SDS–LDH. Themechanism is similar with other layered structure solid lubricants,like graphite, molybdenum disulfide and serpentine [4,9,10,14,15].Additionally, the dodecyl sulfate organic anions are also absorbed

on the layers of LDH crystal grains when they intercalate into theinterlayer at the same time. The end of sulfate group connects withsurface of LDH and the other end of long carbon chain stretchesinto the oil medium which improves the suspension stability ofLDH nanoparticles in base oil (Fig. 11). The organic long carbonchains on the surface of LDH would decrease the friction due tothe “brush mechanism” as well as stop the agglomeration ofnanoparticles which the good dispersibility of LDH nanoparticlesis beneficial to reduce the friction and wear [33,34]. LDH nano-particles fill in the grooves and cracks and act as third bodies toprotect the rubbed surface (Fig. 10C) [10]. Under sliding, grindingand squeezing effect in the friction process, high temperature andpressure create necessary atmosphere for tribochemical reactionwhich can form a hard protective tribofilm (Fig. 9) on the contact

Fig. 7. The morphology of worn surface lubricated by base oil and oil with MAL–LDH and MAL–SDS–LDH at load of 392 N for 60 min.

Table 3The diameter of wear scars produced by base oil and oil with MAL–LDH and MAL–SDS–LDH nanoparticles.

Oil sample Average diameter (mm) Maximum diameter (mm) Reduction of average diameter compared with base oil (%)

Base oil 0.490 0.520 —

Oil with MAL–LDH 0.427 0.430 12.9Oil with MAL–SDS–LDH 0.409 0.412 16.5

Fig. 8. The variation of power consumption and oil temperature tested by base oiland oil with MAL–LDH and MAL–SDS–LDH at 2850 rpm for 20 min in aircompressor test.

Fig. 9. The highly-magnified SEM of worn surface lubricated by oil with MAL–SDS–LDH nanoparticles.

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surface with low roughness to separate the friction pairs (Fig. 10D)[10,20].

4. Conclusion

(1) Mg/Al/La layered double hydroxides were synthesized bycoprecipitation with La3þ/Al3þ molar ratio of 0.1 which hadtypical layered structure diffraction peaks without impurities.The average diameter of crystal grains was about 150 nm. Bothinterlayer spacing and lattice parameter a of Mg/Al/La-LDHturned to be higher compared with Mg/Al-LDH due to thereplacement of Al3þ by La3þ .

(2) The sodium dodecyl sulfate intercalated the MAL–LDH by ionexchange and expanded the interlayer spacing from 7.662 to25.663 Å while maintaining the inner laminate structure.Based upon X-ray diffraction analysis it can be concluded that

Fig. 10. The friction-reducing mechanisms of MAL–SDS–LDH nanoparticles as lubricant additives.

Fig. 11. The suspension state of MAL–SDS–LDH nanoparticles in oil medium.

Table 4The power consumption and oil temperature of base oil and oil with MAL–LDH and MAL–SDS–LDH at the end of air compressor test.

Air compressor test Base oil Oil with MAL–LDH Reduction compared with base oil (%) Oil with MAL–SDS–LDH Reduction compared with base oil (%)

Power consumption (W) 1284 1239 3.5 1225 4.6Oil temperature (1C) 74.9 70.1 6.4 66.5 11.2

S. Li et al. / Tribology International 91 (2015) 60–66 65

the dodecyl sulfate anions were oriented vertically or weretilted with partial overlap in the interlayer of LDH.

(3) The FT-IR spectroscopy of MAL–SDS–LDH showed the asym-metric and symmetric vibration bands of CH2 as well asasymmetric and symmetric S–O stretching vibration respec-tively which represented the infrared characteristics of dode-cyl sulfate ions.

(4) LDH nanoparticles as lubricant additives exhibited friction-reducing and anti-wear performance effectively in the four-ball and air compressor test compared with base oil. Moreover,the intercalated product MAL–SDS–LDH showed better frictionproperties than the precursor MAL–LDH in the test results offriction coefficient, wear scars, power consumption and oiltemperature.

(5) The lubrication mechanisms of MAL–SDS–LDH can be con-cluded that the expanded interlayer spacing and the decreaseof interlayer anions charge number weakened the electrostaticforce between positively charged laminates and anions whichmade layers easier to sliding relatively to absorb and decreasethe friction force applied on the rubbed surface. Long carbonchains of dodecyl sulfate ions enhanced the stability ofsuspension which was help for the friction-reducing effect.LDH nanoparticles filled in the grooves and cracks of surface. Aprotective tribofilm was formed on the rubbed surface toreduce the contact area of friction pairs.

Acknowledgments

The supporting from China Scholarship Council was particu-larly acknowledged. This research was sponsored by the NationalNatural Science Foundation of China (Grant No. 51044011) and theFundamental Research Funds for the Central Universities (Nos.2652013038 and 2652013039). Thanks for the great help of Ph.D.student Haiwei Du from University of New South Wales.

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