Tribological properties of attapulgite/La2O3 nanocomposite as lubricant additive for a steel/steel contact

Attapulgite is a layered silicate with good friction-reduction and self-repairing properties. In order to further improve its tribological properties, the present work mainly focuses on the preparation of attapulgite/La2O3 nanocomposite and study on its tribological behaviors. The tribological properties of mineral lubricating oil (150SN) containing attapulgite/La2O3 nanocomposite were investigated through an Optimal SRV-IV oscillating friction and wear tester. The rubbing surfaces and generated tribofilms were characterized by SEM, EDS, XPS and nanoindentation. Results indicated that the friction-reducing ability and antiwear property of the oil were both remarkably improved by attapulgite/La2O3 nanocomposite. A tribofilm mainly composed of Fe, Fe3C, FeO, Fe2O3, FeOOH, SiO, SiO2 and organic compound was formed on the rubbing surface under the lubrication of oil with attapulgite/La2O3 nanocomposite. The tribofilm possess excellent self-lubricating ability and mechanical properties, which is responsible for the reduction of friction and wear.


Introduction
Wear caused by friction is one of the main causes of materials failure. It is generally known that employing ultrane powders as lubricant additives is effective in reducing friction and wear. [1][2][3][4][5] In recent years, some researchers have reported that several layer-chained silicates, such as serpentine and attapulgite, can be used as lubricant additives. [6][7][8][9][10] Zhang et al. 6,7 investigated the friction and wear properties of surface-coated natural serpentine powders as lubricant additives on a steel-steel contact by Optimal SRV oscillating friction and wear tester. The friction coefficient and wear loss of the base oil (CD 15w-40) were both obviously decreased in the effect of serpentine powders. During the friction procedure, tribochemical reactions occurred between serpentine particles and friction surfaces, consequently an amorphous tribolm enriched in Si-O structures was formed on the rubbing surface. The tribolm was mainly composed of iron oxides, silicon oxides, graphite and organic compounds. The tribolm possess excellent lubricating ability and mechanical properties, which is responsible for the reduced friction and wear. Yu et al. 8 investigated the tribological behavior of natural serpentine mineral powders as lubricant additive through CETR UMT-2 test system. It is found that the lubricity of the base oil (500SN) was improved remarkably by serpentine mineral powders. A nanocrystalline tribolm, mainly composed of Fe 3 O 4 , FeSi, SiO 2 , AlFe and Fe-C compound (Fe 3 C), was formed on the worn surface under the lubrication of 500SN oil with serpentine powders. The tribolm possess a high surface hardness (about 8.0 GPa) and a low modulus (<240 GPa). Qi et al. 9 investigated the friction and wear behaviors of nanoscale serpentine and heat-treated serpentine as lubricating oil additives at 400 C. In the effect of two kinds of lubricant additives, selfrepairing protective layers could be well formed on the contact surfaces. During the friction procedure, tribochemical reactions and metallurgical bonding are the dominant mechanisms.
As we can see, the research on serpentine powders as lubricant additives is systematic and insightful. However, there were very few reports of attapulgite powders using as lubricant additives. Attapulgite is a kind of layer-chained silicates that possess similar chemical composition and crystal structure with serpentine. In the previous study, we found that attapulgite powders can also improve the friction-reducing ability and antiwear property of lubricating oil. With the lubrication of oil with attapulgite powders, a complex tribolm, mainly composed of FeO, Fe 2 O 3 , FeOOH, SiO x and organic compound formed on the worn surface. But the improvement effect of attapulgite powders is not ideal, especially at low load and high load. This result demonstrated that the tribolm forming ability of attapulgite powders is poor and the properties of formed tribolm are poor. In addition, the friction-reducing and antiwear mechanisms of the attapulgite are still obscure.
Rare-earth oxides possess some special properties, including adsorbability and catalytic activity. 11,12 And some researches had reported the using of La 2 O 3 in the researches eld of tribology. Xu et al. 13 have investigated the tribological properties of La 2 O 3 nanoparticles as lubricant additives in bio formulated diesel. It was found that with addition of 1.0 wt% La 2 O 3 nanoparticles, friction and corrosive wear were obviously reduced. The effect of nano-bearing was proposed as the reason for the reduction of friction and wear. Mo et al. 14 investigated the sliding friction and wear behaviors of Cu-La 2 O 3 -graphite composites against Cu-Ag alloy. The results showed that the hardness, exural strength, wear resistance and electrical resistivity of Cu-graphite composites were increased with the addition of La 2 O 3 .
In order to improve the friction-reducing and antiwear properties of attapulgite powder, La 2 O 3 were selected as repairing accelerant. In this work, the tribological behaviors of attapulgite/La 2 O 3 nanocomposite as lubricant additives were investigated using an optimal SRV-IV oscillating friction and wear tester. The microstructure, chemical composition and mechanical properties of the tribolm generated during the friction procedure were characterized. The strengthening mechanism of La 2 O 3 was discussed.

Material processing and sample preparation
Attapulgite powder was purchased from Jiangsu Jiuchuan Nanometer material Science and Technology Ltd, China. La 2 O 3 powder was purchased from Beijing DK Nano Technology Co., Ltd, China. 150SN was purchased from Qingdao Compton Technology Co., Ltd, China. Oleic acid was supplied by Aladdin Co., Ltd, China. All materials were used without any treatment.
The preparation process of oil containing attapulgite/La 2 O 3 nanocomposites is described as follows. First, 6.0 g attapulgite powder with 0.0 g, 2.0 g, 4.0 g, 6.0 g and 8.0 g La 2 O 3 powder were carefully dissolved in 100 ml ethanol containing 5.0 ml oleic acid respectively. Second, the mixed solutions were milled by in a ball crusher. The rotational speed of ball crusher is 250 rpm and the milling duration is 8 h. Third, the mixed solutions were heated in a vacuum drying oven to remove ethanol, and then the attapulgite/La 2 O 3 nanocomposites were prepared. Finally, specic amount of attapulgite/La 2 O 3 nanocomposites were added into the 150SN base oil and subsequently were treated by ultrasound for 60 minutes. Lubricants of different components shown in Table 1 were prepared.

Friction and wear tests
The tribological properties of the lubricants were investigated using an optimal SRV-IV tribo-tester with a ball-on-disk conguration as shown in ref. 15. The balls with a diameter of 10.0 mm and hardness of HV 710 were made of AISI 52100 steel. The disks with a diameter of 24.0 mm, a thickness of 8 mm and hardness of HV 710 were made of AISI 1045 steel. And the surfaces of the disks were mechanically ground and polished to a minute surface (R a z 0.2 mm).
In the friction and wear tests, the optimum addition of attapulgite/La 2 O 3 nanocomposites was investigated rstly. Subsequently, the effect of load and frequency on the tribological behaviors of the nanocomposites was investigated. Detailed test parameters were shown in Table 2. For each experimental condition, each test was carried out for three times. The average friction coefficient was calculated during the steady friction state. A MicroXAM 3D non-contact surface mapping proler was employed to characterize the rubbing surfaces and measure the volumes of the wear scar on the disks. Each wear scar was measured for three times and the average value was calculated. At last, the wear rate was calculated. The wear rate was dened as the wear volume per unit product of sliding distance and load.

Characterization
The XRD pattern of attapulgite/La 2 O 3 nanocomposite was analyzed by an Advanced D8 diffractometer. The morphology of attapulgite/La 2 O 3 nanocomposite was examined by scanning electron microscopy (Hitachi S4800) and transmission electron microscopy (Tecnai F20).
The morphologies and element distribution of the rubbing surfaces were characterized by SEM (NovaNano SEM 650) equipped with EDS (Oxford). The chemical states of some typical elements were characterized by XPS (ESCALAB 250Xi). Monochromatic AlK a X-ray radiation (1486.6 eV) was used as the excitation source of XPS. The XPS spectra were calibrated with reference to the C1s line at 284.8 eV. The nano-hardness and elastic modulus of the metal matrix and tribolms were detected by the nano-indentation tester (G200 Nano Indenter). The indentation controls displacement from 100 nm to 500 nm at a single point. And the variations of nano-hardness and elastic modulus with depth were recorded.  had been founded. The TEM image shown in Fig. 2(b) indicated that most spherical La 2 O 3 nanoparticles were absorbed on the surface of attapulgite nanobers. During the ball milling process, the agglomerated nanoparticles were scattered by mechanical force. And the graing of oleic acid molecule remarkably improved the lipophilicity of nanoparticles, effectively preventing the reunion of the nanoparticles. Consequently, the nanoparticles could gain long term stability in the oil. In the meantime, attapulgite nanobers and La 2 O 3 nanoparticles can combine together through the adsorption of attapulgite and hydrogen bonding. For the asprepared lubricants, aer being settled for 30 days, no obvious settlement was observed. In addition, with the addition of attapulgite/La 2 O 3 nanocomposite, the typical physicochemical properties of the 150SN were not changed basically, which can be seen from Table 3.     Fig. 4. For L1 and L7, the friction coefficient displayed high value coupled with much uctuation. This demonstrated that the lubricating lm of 150SN cannot afford the high pressure between the friction pairs. And the strengthening effect of the single La 2 O 3 powder is not obvious. For L2 and L4, aer a short time of running-in process, the curves kept steady to the end of tests.  nanocomposite, the tribochemical reaction between attapulgite and friction pairs were promoted by La 2 O 3 , a tribolm with higher content of O and Si was formed. Fig. 7 shows the three-dimensional morphologies of the rubbing surfaces for L1 and L4. For L1, the rubbing surface was very course, with some high peaks and deep troughs. As for L4, the rubbing surface was smooth and at, no obvious peaks and troughs appeared. This result agreed with the tribological test results well.

Nano-indentation test for the rubbing surfaces
Nano-indentation test results for the steel disk and rubbing surfaces of L2 and L4 are shown in Fig. 9 and Table 3. The hardness-displacement curves were shown in Fig. 9(a). For the steel disk, the hardness decreased gradually from 6 GPa to 4 GPa with the increase of the pressed depth. For the tribolm formed on the rubbing surface of L2, the hardness increased gradually from 7 GPa to 8 GPa. While for the tribolm formed on the rubbing surface of L4, the hardness kept stable about 10 GPa. The elastic modulus-displacement curves were shown in Fig. 9(b). For the steel disk, the elastic modulus kept stable about 250 GPa. For the tribolm formed with L2 and L4, the elastic modulus both increased gradually from 150 GPa to 230 GPa with the increase of the pressed depth. During the friction procedure, the deposited lubricating additives took tribochemical reactions with the rubbing surfaces, leading to a formation of tribolms. The thickness and compactness of the tribolm increased constantly. Thus the mechanical properties of the tribolm were improved with the increase of the pressed depth. Table 4 shows the average hardness (H), average elastic modulus (E) and H/E ratio of the steel disk and tribolms. For the steel disk, the hardness and elastic modulus was 4.30 GPa and 268.01 GPa. For the tribolm formed by L2, the hardness and the elastic modulus was 8.15 GPa and 216.21 GPa. AS for the tribolm formed by L4, the hardness increased to 10.41 GPa and the elastic modulus was 221.49 GPa. It is concluded that the tribolms possess higher hardness and a little lower elastic modulus than the AISI 1045 steel. So the tribolms possess some excellent properties of metal and ceramics meanwhile, such as high hardness, high wear resistance, good ductility and good plasticity. It is reported that the H/E ratio can directly reect the wear resistance of the materials. 19 A material with a higher H/E ratio is easier to recover from elastic deformation induced by external stress, thus fewer asperities would formed, resulting in a lower friction and wear. It can be seen from Table  4 that the H/E value of the metal disk was the lowest and that for L4 was the highest. This result further demonstrated that the tribolms generated on the rubbing surfaces by L2 and L4 possess excellent mechanical properties, especially L4.

Discussion
From the experimental results, we can see that the as-prepared attapulgite/La 2 O 3 nanocomposite displayed good friction-reducing and antiwear properties. In the effect of nanocomposite, a tribolm composed of Fe, Fe 3 C, iron oxides, silicon oxides and organic compound with high hardness was formed on the rubbing surface. The formation mechanism of the protective lm is mainly associated with the friction thermodynamics effect and unique crystal structure of attapulgite. The formation mechanism of the tribo-chemical lm was schematically depicted in Fig. 10. In the rubbing process, attapulgite/La 2 O 3 nanoparticles were adhered onto the rubbing surface. Under the action of extrusion pressure, shearing force and friction, interlayer cleavage, structural water removal and lattice distortion occurred, leading to structural instability of attapulgite, the crystallinity of attapulgite decreases and tend to disordering. In the meantime, a plenty of active oxygen atoms, free bonds (Si-O-Si, O-Si-O) and hydrogen bond released. 9,10 In the effect of high ash temperature and high shear stress between the friction pairs, such released substances took tribochemical reactions with the metallic matrix, and La 2 O 3 acted as a catalyst to accelerate the tribo-chemical reactions. 20 The main tribo-chemical reactions were as follows: (1) the active iron atoms on the rubbing surface and iron lings took reaction with active oxygen atoms and hydroxyl groups released by attapulgite, forming multiphase iron oxides and FeOOH.   became thicker and denser. Meanwhile, part tribolm were worn out. At last, dynamic equilibrium between the forming and worn of tribolm, consequently stable friction coefficient presented. The tribolm could prevent the metallic friction pairs from direct contacting, greatly relieved the adhesion effect between the friction pairs. The tribolm possess high hardness, which could bring milder distortion and better wear resistance. In addition, the tribolm possess good plasticity and ductility like metal. The inclination of forming microcracks and peeling during the friction procedure is greatly reduced. Hence, the attapulgite/La 2 O 3 nanocomposite could remarkably improve the friction-reducing and antiwear properties of 150SN base oil. In order to investigate the tribological behavior of attapulgite/La 2 O 3 nanocomposite more systematically, the effect of load and frequency on the friction coefficient and wear rate was investigated. Fig. 11 shows the effect of load on the mean friction coefficient and wear rate respectively for L2 and L4. Compared with L2, the mean friction coefficient and wear rate for L4 were lower at all test loads, especially at 60 N, 80 N and 100 N. This result may be due to following reason: the higher the load is, the higher the generated heat will be. Thus the catalysis of La 2 O 3 is more signicant. In addition, for L4, the mean friction coefficient and wear rate were both the least at 60 N. This result may be attributed to the competition between the formation and abrasion of the tribolm. With the increase of the load, more energy is supplied to impel the reaction between the nanoparticles and metallic matrix. But when the load is too high, the abrasion of the tribolm may be dominated. Fig. 12 shows the effect of frequency on the mean friction coefficient and wear rate for L2 and L4. In the presence of La 2 O 3 , the mean friction coefficient and wear rate for L2 both decreased at all test frequencies. Moreover, with the increase of the frequency, the mean friction coefficient and wear rate for L2 and L4 both decreased continuously. Under the lubrication of the lubricating oil, the lubrication regime is mixed lubrication. From the Stribeck curves, 21 it can be seen that the friction coefficient under mixed lubrication is proportional to speed and inversely proportional to load. Under the same load, the friction and wear both decreased along with the increase of frequency.
From the tribological tests, it can be concluded that La 2 O 3 nanoparticles can improve friction-reducing and antiwear properties of the oil containing attapulgite. However, the improvement effect on the friction-reducing property is not signicant. This phenomenon can be explained as follows: under the lubrication of oil containing attapulgite and oil containing attapulgite/La 2 O 3 nanocomposite, the chemical composition of the tribolms were basically the same, hence the self-lubrication properties of the formed tribolms were not obviously different. In the other hand, in the effect of La 2 O 3 Fig. 11 The effect of load on the tribological properties of lubricants. Paper nanoparticles, the thickness, uniformity and compactness of the tribolm were obviously improved, consequently the antiwear property of oil containing attapulgite was remarkably improved.

Conclusions
The tribological properties of attapulgite/La 2 O 3 nanocomposite as lubricant additive were investigated. In the effect of attapulgite/La 2 O 3 nanocomposite, the friction-reduction and antiwear properties of the oil were obviously improved. During the friction procedure, tribochemical reactions between attapulgite powders and metal matrix occurred, and La 2 O 3 can be served as catalysts to accelerate the tribochemical reactions. At last, a tribolm mainly composed of Fe, Fe 3 C, FeO, Fe 2 O 3 , FeOOH, SiO, SiO 2 and organic compound was formed on the rubbing surface. The multiphase tribolm possess excellent self-lubricating ability, antiwear property and mechanical properties, which is responsible for the reduction of friction and wear. We expect our work can offer new routes to develop self-repairing lubricant additives with high friction-reduction and antiwear properties.

Conflicts of interest
There are no conicts to declare.