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Al layered double hydroxides as lubricant additives
Accepted Manuscript Full Length Article Friction performance and mechanisms of calcined products of Mg/Al layered double hydroxides as lubricant additives Shuo Li, Lingling Ren, Zhimin Bai PII: DOI: Reference:
S0169-4332(18)33093-9 https://doi.org/10.1016/j.apsusc.2018.11.025 APSUSC 40870
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
31 July 2018 21 October 2018 4 November 2018
Please cite this article as: S. Li, L. Ren, Z. Bai, Friction performance and mechanisms of calcined products of Mg/ Al layered double hydroxides as lubricant additives, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.11.025
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Friction performance and mechanisms of calcined products of Mg/Al layered double hydroxides as lubricant additives Shuo Li1, Lingling Ren1, Zhimin Bai2 1
Division of Nano Metrology and Materials Measurement, National Institute of Metrology, Beijing, 100029 P.R. China
2
School of Materials Science and Technology, China University of Geosciences, Beijing, 100083 P.R. China
Abstract Mg/Al layered double hydroxides (LDHs) and the calcined products were prepared and the powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray fluorescence (XRF) and Fourier transform infrared spectrometer (FT-IR). The friction properties of LDHs and the calcined products were tested and the rubbed areas were analyzed by scanning electron microscope, energy disperse spectroscopy (EDS) and auger electron spectrometer (AES). The results showed that the diameter of LDHs was about 100~175 nm and the formula was Mg0.69Al0.31(OH)2(CO3)0.15·0.16H2O. All the calcined products had the laminated structure. The anti-wear properties of 1000 oC calcined products were superior to the LDHs, which was attributed to the supporting and mechanical polishing effects of the hard spinels. The distribution laws of elements of worn area in the horizontal and
Corresponding author. Tel.: +86 10 82323201; fax: +86 10 82322974.
Email address: [email protected] (Z. Bai). 1
vertical direction were concluded. The tribological performance of calcined products can be attributed to the “multi-layer self-lubrication mechanisms”. The tribofilms consisted of “carbon-enriched layer”, “oxygen- enriched layer” and “transition layer” and combined strongly with the metal substrate. Key words: Mg/Al layered double hydroxides; Calcined products; Elements distribution laws; Multi-layer self-lubrication mechanisms 1. Introduction In the past years, layered double hydroxides (LDHs) nanoparticles have been proved to possess excellent friction-reducing and anti-wear properties and protect the friction pairs effectively as lubricant solid additives under boundary lubrication condition, due to the layered crystal structure, nanoscale and high chemical activity. Bai [1] found Co/Al-CO3 layered double hydroxides can improve friction performance of lubricating oil through testing the coefficient of friction and wear of rubbed surface for the first time. Mg/Al, Zn/Al, Zn/Mg/Al, Mg/Al/Ce layered double hydroxides and the related intercalation products with different crystal size and synthesis methods were studied afterwards, which proved the friction properties of LDHs further [2-11]. Under the high surface flash temperature and contact pressure during friction process, not only do the organic compounds of lubricating oil decompose, but the phase transitions of solid additives happen, which the new crystal phases could change friction behaviors of original particles [12]. In the work, the calcined products of Mg/Al layered double hydroxides in different temperature were prepared and the
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friction properties of the products as lubricant additives were tested. The results showed that the calcined products in high temperature possessed better anti-wear properties, compared with Mg/Al LDHs. The distribution laws of elements of rubbed surface were analyzed and the “multi-layer self-lubrication mechanisms” were put forward for the tribological performance of the calcined products. 2. Experiment 2.1. Preparation of the calcined products of Mg/Al LDHs Mg/Al LDHs were synthesized by the coprecipitation method in the paper [11] with Mg2+/Al3+ molar ratio of 2:1. The dried LDHs nanoparticles were calcined under 150 oC, 250 oC, 350 oC, 500 oC, 600 oC, 700 oC, 850 oC and 1000 oC in N2 atmosphere for 3 h respectively. All the particles were modified by oleic acid for good dispersibility in lubricating oil [5]. Finally, the modified LDHs and calcined products were filtered and washed until pH reached 7 and dried at 80 oC for 12 h. 2.2. Characterization of LDHs and calcined products Mg/Al LDHs and calcined products were characterized by X-ray diffraction (XRD) with Rigaku diffractometer (CuKα source, λ=0.15406 nm, scanning range from 3° to 70° (2θ)) for crystal structure. The content of elements of LDHs was obtained by wavelength dispersive X-ray fluorescence (WD-XRF, Rigaku ZSX PrimusⅡ). The particle size distribution of LDHs was obtained by Zetasizer (Nano ZS90, Malvern). The surface properties of particles were recorded on a NICOLET750 Fourier transform infrared (FT-IR) spectrometer (4000-450 cm-1, KBr sheet). The morphology of particles was observed on a scanning electron microscope (SEM,
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JSM-6460LV, JEOL). 2.3. Tribological properties of LDHs and calcined products The friction properties were tested on a MS-10JR four-ball friction machine. The GCr15 steel balls with diameter of 12.70 mm and hardness of 64~66 HRC were used and cleaned by petroleum ether and ethyl alcohol for 10 min in an ultrasonic cleaner before tests. The LDHs and calcined products were added to lubricating oil (Table 1) with the same amount of 0.5g powders per 100 ml oil respectively. Then the oil samples were stirred under high speed agitator (10,000 rpm) and ultrasound for good dispersibility respectively. Each test was conducted under the rotating speed of 1200 rpm and load of 392 N for 30 min at room temperature. The average coefficient of friction was calculated with the data of stable stage. The average wear scar diameter of 3 lower balls was obtained by an optical microscopy. 2.4. Surface analysis of worn areas The morphology of worn areas was observed on a scanning electron microscope (SEM, JSM-6460LV, JEOL). The elements distribution laws in friction surface were analyzed by energy disperse spectroscopy (EDS) and auger electron spectrometer (AES, PHI-710, Argon ion sputtering rate 13 nm/min). 3. Results and discussion 3.1. X-ray diffraction and morphology of Mg/Al LDHs and the calcined products The XRD pattern and crystal structure of synthetic Mg/Al LDHs were shown in Fig. 1. From the XRD pattern, the LDHs showed layered structure diffraction peaks of hexagonal symmetry phase. The characteristic peaks (003), (006), (009) and (110) of
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Mg/Al LDHs were narrow and sharp and base line was flat, which showed the high crystallinity of LDHs powders. The peak (110) represented the perfect internal structure of brucite-like layers. Based on the Bragg equation λ=2d(hkl)sinθ, interlayer spacing d(hkl) and lattice parameter a=2 d(110) and c=3 d(003) were calculated (Table 2), which corresponded to the values of Mg/Al LDHs with CO32- reported in previous work [13-15]. The chemical formula of Mg/Al LDHs could be calculated as Mg0.69Al0.31(OH)2(CO3)0.15·0.16H2O based on the mass fraction of elements of Mg/Al LDHs from the results of XRF (Table 3). The molar ratio of elements in the formula was close to the original proportion of LDHs preparation. From the SEM image (Fig. 2), it is obvious that LDHs powders showed typical hexagonal layered structure and the grain size of particles was homogeneous and distributed in the range of 100~175 nm mostly. The average size and thickness of powders were about 145 nm and 30 nm, respectively. Under high temperature, the crystal structure of Mg/Al LDHs changed and new phase might appear. The XRD patterns of the products of LDHs under different calcination temperature were shown in Fig. 3. The diffraction peak (003) shifted gradually to high angle from 150 oC to 250 oC, which showed the decrease of interlayer spacing along with the rising of temperature, compared to Mg/Al LDHs (Table 4). The peak (003) disappeared and crystallinity decreased obviously as the temperature rose to 350 oC. However, the position of peak (110) was always stable until 350 oC and the lattice parameter a of calcined products of LDHs at 150~350 oC had the same value (Table 4), which showed LDHs kept the layered structure. The
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result fit with Yang’s report [16], however, other research showed the layered structure collapsed below 330 oC [17-19], which was attributed to the structure stability of LDHs due to different preparation methods and calcination process. The peak (110) disappeared and the layered structure collapsed and Mg/Al layered double oxides (LDO) generated as temperature rose to 500~700 oC. The Mg/Al spinel phase MgAl2O4 appeared in the calcined products until 850 oC, although the crystallinity was low. The calcination temperature rose to 1000 oC further, the diffraction peaks of periclase and Mg/Al spinel were sharp, which represented the high crystallinity of calcined products at 1000 oC [20, 21]. The morphology of products of LDHs at different calcination temperature (350 o
C, 600 oC, 850 oC and 1000 oC) was shown in Fig. 4. The crystal grains of calcined
products at 350 oC kept the whole hexagonal layered structure of LDHs powders. At 600
o
C, the thickness of calcined products decreased obviously although the
hexagonal morphology remained, compared to LDHs. The products possessed irregular laminar structure and the edge of crystal grains broke mostly and small particles distributed on the grains after calcined at 1000 oC. It was notable that all the calcined products possessed laminar structure similar to the LDHs, which was crucial for the friction properties. 3.2. FT-IR spectroscopy of modified Mg/Al LDHs and the calcined products The FT-IR spectra of modified Mg/Al LDHs and the calcined products with oleic acid were shown in Fig. 5. The peaks at 2925 and 2854 cm-1 in Mg/Al LDHs spectrum were assigned to the asymmetric and symmetric stretching vibration of C-H groups
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from carbon chains of oleic acid molecules, respectively. It is obvious that the calcined products had the same vibration peaks of C-H groups, which showed oleic acids modified the products successfully. Moreover, Mg/Al LDHs and calcined products possessed the vibration peak of carboxylate at 1565 cm-1, but the vibration peak of -COOH at 1710 cm-1 disappeared. It is shown that the LDHs and calcined products were modified chemically by oleic acid with monolayer molecules on the surface of powders [5]. The intense and wide band at 3454 cm-1 in Mg/Al LDHs spectrum corresponded to O-H stretching vibration of hydroxyl groups of brucite-like layers and absorbed and interlayer water molecules. The calcined products of Mg/Al LDHs also showed similar vibration, which indicated the existence of hydroxy groups in the products. However, the intensity decreased gradually with the increasing of calcination temperature, which was caused by the removal of water molecules and hydroxyl groups. The vibration peak v3 of interlaminar CO32- of LDHs was at 1360 cm-1. The state of CO32- changed above 350 oC, which the peak of calcined products disappeared and new peak arose at 1416 cm-1. The peak at 675 cm-1 corresponded to asymmetric bending vibration v4 of CO32- of LDHs and the absorbed peak of interaction between interlayer CO32- and laminates was at 554 cm-1. Both peaks disappeared above 250 oC. Moreover, the peak led by vibration of metal cations M-OH in brucite-like layers (449 cm-1) shifted above 250 oC, compared with LDHs. It is concluded that the junction state of cations changed at high temperature [4, 5, 15, 16]. 3.3. Friction properties of Mg/Al LDHs and the calcined products as lubricant
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additives Based on the crystal structures (XRD results) and surface morphology (SEM results), the friction properties of Mg/Al LDHs and the calcined products at 250 oC, 350 oC, 600 oC and 1000 oC were tested by four-ball friction machine. The friction tests results were shown in Fig. 6 and Table 5. All the powders (LDHs and calcined products) as lubricant additives showed friction-reducing and anti-wear properties at different levels, compared with lubricating oil, which had the highest average coefficient of friction (0.126) and diameter of wear scar (0.453 mm). Mg/Al LDHs reduced coefficient of friction and wear scar like other kinds of LDHs, which had been proved previously. It is remarkable that the calcined products at 350 o
C and 600 oC possessed better friction-reducing properties with reduction of
coefficient about 9%, compared with LDHs. It can be concluded that the removal of carbonate ions in the interlayer weakened the interactions between laminates, including the electrostatic force and hydrogen bond. Therefore, the laminates were easy to slide relatively under friction shearing force, which played roles in reducing friction coefficient. The most direct mode to evaluate the friction properties of lubricating oil is the size of wear scar on the friction pairs. The calcined product at 1000 oC had the best anti-wear properties with reduction of diameter of wear scar 18.76%, compared to LDHs and other calcined products. The phase included periclase MgO and spinel MgAl2O4. The product kept the laminate structure and small particles generated at the edge of crystal grains. The hard spinels MgAl2O4 (Mohs hardness 8), which were the
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in situ products of phase transition of LDHs at high temperature, played a role of particle-supported effect between metal pairs under friction process. Moreover, the hard particles can repair the microcracks and polish the rubbed surface. The size of 1000 oC calcined products was close to the LDHs so that the size had limited effects on the friction properties. The surface morphology of rubbed areas lubricated by base oil and oil with calcined products at 1000 oC were shown in Fig. 7. From the SEM picture, severe wear occurred on the surface lubricated by base oil, which had bigger size of wear scar and wide and deep grooves and furrows that was a typical abrasive wear. However, there were only shallow worn traces on the surface lubricated by oil with hard calcined products and the rubbed area was smaller and smoother, which indicated the excellent anti-wear properties. 3.4. The distribution of elements on the surface lubricated by oil with 1000 oC calcined products of Mg/Al LDHs The SEM image and horizontal distribution of main elements of the whole round worn area with base oil by energy disperse spectroscopy (EDS) was shown in Fig. 8. It is obvious that the surface was worn severely and amounts of deep and wide wear scar appeared. The main elements of Zn, Fe, O and C were detected, which the Zn came from the ZDDP. The Zn and O distributed intensely and the content of Fe was less in the worn area compared with non-worn surface, although the C had no obvious content comparison. The carbonization products of base oil (black frame A) were found on the worn surface, which demonstrated the cracking reactions of organic compounds of lubricating oil in the friction contact area. The oil with 1000 oC
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calcined products of Mg/Al LDHs had better anti-wear properties than base oil. Therefore, it had smaller rubbed area and milder abrasion compared with surface with base oil (Fig. 9). It had similar distribution laws of main elements Zn, Fe, C and O in the area, which had small amounts of Mg from the calcined products. The decomposition products of oil were also found on the surface. The black particle A that had only C may be the graphite, and the particle B with carbon and zinc may be generated by decomposition of ZDDP. The SEM image and horizontal distribution of main elements of surface with base oil by energy disperse spectroscopy (EDS) was shown in Fig. 10. The main elements included Zn, Fe, O, C, S and P, which the S and P derived from base oil. The rubbed surface was worn severely and had wide and deep grooves (yellow dotted line frames), in which O and Zn distributed less and C deposited more along the direction of friction compared with flat area. However, the smooth area had more O and Zn and less Fe and C than the grooves (white dotted line frames). The surface lubricated by oil with 1000 oC calcined products of Mg/Al LDHs had similar distribution laws of elements (Fig. 11). The Zn, Mg and O distributed intensively in the smooth area (yellow dotted line frames) and Fe and C deposited less. Similarly, C deposited more in the grooves (white dotted line frames). Mg distributed less in surface furrows (green dotted line frames). It is considerable that Al was not detected from the surface lubricated by 1000 oC calcined products of Mg/Al LDHs (Fig. 11). Nevertheless, Zhao’s research by pin-on-disc friction tester showed the element of Al appeared on the surface lubricated
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by oil with Mg/Al LDHs. The chemical state of Al was similar to that in the α-Al2O3 tested by the X-ray absorption near-edge structure (XANES) spectra, which the state of Al distinguished from that in the LDHs and the coordination environment of part of Al changed to tetrahedral structure [6]. The Co/Al LDHs also had similar results [9]. The difference can be concluded that the Al3+ had high bond strength with O2- in the spinel MgAl2O4. Thus, the Al3+ was hard to separate from the spinel [15]. The four-ball friction test had severe condition with load of 392 N and rotating speed of 1200 rpm. The metal surface on the tiny contact area (point contact) was exfoliated easily under high pressure and speed so that the Al3+ was hard to exist on the contact point. However, the conditions of pin-on-disc friction test in Zhao’s research were milder with load of 220 N and frequency of 25Hz including larger contact area, compared to the four-ball test. The bivalent metal cations Mg2+ were easier to exchange with same valence Fe2+ in the metal substrate under friction process, compared with trivalent metal cations Al3+ so that Al3+ was hard to integrate itself into the contact surface [1]. It can be concluded that the surface lubricated by base oil and oil with calcined products of LDHs had similar elements distribution laws along the sliding direction which the Zn, O (or Mg) distributed intensively in the smooth and flat area and C deposited more in the grooves. The comparison of relative atomic content of Mg, Zn, Fe, C and O in the whole worn area lubricated by base oil and oil with 1000 oC calcined products of Mg/Al LDHs through energy disperse spectroscopy was shown in Table 6. The depth of EDS
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detecting can reach from dozens of nanometers to hundreds of nanometers and the contents of elements represented the average values. It is remarkable that the content of Zn, Fe and C had obvious difference after the addition of calcined products. It can be concluded that the calcined products increased the content of Zn and Fe but decreased the C content. The deposition of more ZDDP and the Fe played key roles in reducing friction and abrasion. The vertical depth distribution laws of elements obtained through auger electron spectroscopy (AES) in the surface lubricated by oil with 1000 oC calcined products of Mg/Al LDHs was shown in Fig. 12, which concentrated on the change of relative content of elements of Mg, Zn, Fe, C and O with sputtering time in the vertical direction. Both the elements of O and C had high and close content (above 40%) in the top layer of rubbed surface (42.5% C and 41.8% O). Besides, there were about 7.2% Fe, 5.4% Zn and 3.1% Mg in the top. The “oxygen-enriched layer” represented the top oxidized highly and the rich carbon showed that there were amounts of graphite and organics with carbon, which had self-lubricating effect in friction process. The high molar ratio of O/Fe indicated the top had more oxides with high valence state iron. The existence of Zn and Mg demonstrated that the ZDDP and calcined products transferred to the rubbed surface and participated in the formation of tribofilms. After 0.5 min of sputtering time, the content of O and Fe increased further. However, the content of C declined rapidly, which showed the “carbon-enriched layer” was pretty thin with thickness of less than 10 nm based on the sputtering velocity of 13 nm/min.
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As time went on, the content of Fe rose quickly, although the O had a fast decrease. The carbon kept the low level till the end. The ratio of Fe : O was close to 1:1 except the O bonded with metal elements of Zn and O at 2 min of sputtering. It showed that the “oxygen-enriched layer” ended nearly and the high valence iron oxides decreased and the content low valence iron oxides even iron increased gradually with deeper sputtering. It is remarkable that the content of Zn and Mg kept stable before the 2~2.5 min, which showed Zn and Mg distributed uniformly in the “oxygen-enriched layer”. As the depth of sputtering deepened further, the content of Fe kept increasing to about 70% (9.5 min) and the O decreased to only 7% and both the Zn and Mg declined to less than 2%. It demonstrated that the tribofilms had approached to the lower boundary even the metal substrate of steel balls. The thickness of tribofilms was about 120 nm at least calculated by the sputtering velocity. 3.5. The tribological mechanisms of 1000 oC calcined products Mg/Al LDHs as lubricant additives The micro/nano sized solid particles as lubricant additives showed excellent friction-reducing and anti-wear properties. Due to the complex physical and chemical reactions in friction process, there have not been enough convincing and uniform evidences to prove the tribological mechanisms of micro/nano particles so far. The explanations about the mechanisms can be concluded as follows: (1) “The ball bearing effect” applies to spherical particles generally. The particles support the friction pairs and transform the sliding to rolling friction with lower coefficient of friction to protect the rubbing surface, like polytetrafluoroethylene (PTFE), Hollow
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nanoparticles WS2 and nanosized diamonds [22-26]; (2) The particles deposit on the worn surface to form a tribofilm with low coefficient of friction physically, which can prevent the friction pairs from contacting directly, like metal and graphite [27-29]. It can be marked as “physical deposition theory”; (3) The particles with high activity react with fresh rubbing surface physically and chemically to form “tribochemical reaction film” to protect the friction pairs, like metal sulfide, oxides, borates, rare earth compounds and mineral powders [30-38]. The theory is accepted by amounts of researches. In the past few years, various kinds of LDHs powders and intercalated products as lubrication additives have been studied and showed excellent friction-reducing and anti-wear properties [1-11]. There were few enough evidences to explain the tribological mechanisms of LDHs. Zhao studied the formation mechanisms of tribofilms of Mg/Al LDHs intensively. The individual Mg/Al LDHs could improve the friction-reducing properties of base oil and the anti-wear performance was enhanced as both LDHs and ZDDP were added in base oil. The XANES results showed that the coordination structure of Mg and Al in discontinuous tribofilms (containing Mg, Al, C, O, Zn, P and S) changed, which indicated the new phases were generated, like α-Al2O3. LDHs promoted the formation of polyphosphate with medium chain and the oxidation of sulfide in the near surface, which weakened the wear. Through the friction tests, the morphology of worn surface, the distribution of surface elements in horizontal and vertical direction, the tribological mechanisms of
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1000 oC calcined products of Mg/Al LDHs as lubricant additives can be concluded as the “self-repairing protective tribofilms theory” [8, 10, 11]. The structure of tribofilms in the worn surface of metal substrate was analyzed in detail for the first time and can be divided into three layers based on the content of C and O roughly which had no obvious boundary between neighbouring layers (Fig. 13). The top layer with thickness of only several nanometers, called as “carbon-enriched layer”, contained the elements of Mg, Zn, Fe, C, O, S and P except the Al. The atomic content of C was highest, up to 50%. The content of O was about 30%~40% and Mg, Zn, Fe were slight relatively in the carbon-enriched layer. There were carbonization products of organics in lubricating oil on the surface of the top layer besides graphite with self-lubricating effect. The C mainly existed in the grooves and furrows and smooth and flat areas contained small amounts of C. Based on the EDS tests, the surface with calcined products had less C compared with that with base oil, which demonstrated the calcined products can reduce effectively the wear with less grooves or furrows. Besides the element of C, the “carbon-enriched layer” also had high content of O, which the iron mainly existed in the form of high valent oxides. The content of O increased rapidly with extension of sputtering time and the “oxygen-enriched layer” appeared in the tribofilms. The O reached the peak (close to 60 at%) in the area of adjacent carbon-enriched layer then decreased gradually. It is remarkable that the content of Mg and Zn kept stable in the vertical direction of the layer, which showed calcined products participated in the formation of tribofilms
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continuously and stably. The content of C declined rapidly and then stayed at a level in the layer. However, the Fe increased inversely and low valent iron oxides increased. The oxygen-enriched layer almost ended as the content of Fe was close to the O. The thickness of the layer was about 20~30 nm. After the “carbon-enriched layer” and “oxygen-enriched layer”, the content of O decreased continuously and was exceeded by the Fe. The “transition layer” appeared with more low valent iron oxides even the elements of iron. Meanwhile, the Mg and Zn declined gradually. The protective tribofilms approached the bottom and reached the metal substrate. Mg, Zn and Fe distributed uniformly in the transition layer, which proved that the tribofilms and substrate had no obvious boundary and combined each other strongly. The tribofilms generated by calcined products was about 120 nm. Based on the above AES results, the main elements had obvious distribution laws in the tribofilms with multi-layer structure. Thus, the “multi-layer self-lubrication mechanisms” can be concluded as the reasons of the friction properties of the calcined products. The Mg/Al LDHs and calcined products had similar laminated structure, which was key to the friction properties. The Mg/Al LDHs as lubricant additives participated in the complex tribochemical and physical reactions during the friction process to form the protective films on the rubbed surface. Thus, it can be concluded that the friction mechanisms of Mg/Al LDHs and calcined products were related closely and the “multi-layer self-lubrication mechanisms” of calcined products were also applicable to the Mg/Al LDHs or other LDHs. The 1000 oC calcined products of Mg/Al LDHs had the phase of periclase MgO
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and Mg/Al spinel MgAl2O4. The spherical shaped Zn/Al spinels ZnAl2O4 powders as lubricant additives showed friction-reducing and anti-wear properties and the mechanisms can be attributed to the “micro-bearing effect” of spherical hard spinels [25]. The hard in-suit phase transition products of LDHs had laminated structure. Therefore, the reason which the 1000 oC calcined products had better anti-wear properties compared with the LDHs was mainly attributed to supporting (surface microbulges of rubbed area) and mechanical polishing effects of the hard spinels to reduce the abrasion finally (Fig. 14). Moreover, it was shown that amounts of small spherical particles generated at the edge or surface of crystal grains of calcined products. Thus, the “ball bearing effects” were also possible in the friction process. The friction behaviors of Mg/Al LDHs can be concluded that LDHs deposited on the worn surface under high temperature and pressure, and then the phase change of nanoparticles occurred, which the process can absorb the power. Meanwhile, the results above proved the phase change products (calcined products) had also the friction properties. After complex reaction process, the LDHs and phase change products participated in the formation of “self-repairing protective tribofilms”. Moreover, the phase change products might have the supporting, mechanical polishing or ball bearing effects. 4. Conclusion (1) The Mg/Al-CO3-LDHs were synthesized with diameter of about 100~175 nm. The formula was Mg0.69 Al0.31(OH)2(CO3)0.15·0.16H2O. The 350 oC calcined products of LDHs still kept the diffraction peak (110) and all the calcined products possessed
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the laminated structure. The 1000 oC calcined products were periclase MgO and Mg/Al spinel MgAl2O4 and the anti-wear properties were superior to the LDHs, which was attributed to the supporting and mechanical polishing effects of the hard spinels. (2) The Mg, Zn and O distributed intensively in the smooth surface of worn area with 1000 oC calcined products of LDHs and C distributed more in the grooves and furrows. Compared to the area with base oil, the area with 1000 oC calcined products contained more Zn and Fe and less C (EDS results). The absence of Al can be concluded that Al3+ was hard to separate from the spinel and the friction conditions of four-ball tests were severe. The trivalent cations Al3+ were hard to exchange with bivalent Fe2+ of substrate. (3) The tribological mechanisms of 1000 oC calcined products of LDHs can be attributed to the “multi-layer self-lubrication mechanisms”. The tribofilms (about 120 nm) consisted of three layers, “carbon-enriched layer”, “oxygen-enriched layer” and “transition layer” from the surface to the bottom. There were no obvious boundaries between neighbouring layers and the tribofilms combined strongly with the metal substrate. Acknowledgement This research was sponsored by the National Key Research Plan (Project No. 2017YFB0310703). References [1] Z.M. Bai, Z.Y. Wang, T.G. Zhang, F. Fu, N. Yang, Synthesis and characterization
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serpentine mineral powders as lubricant additive, Wear 297 (2013) 802-810. [36] Q. Xue, W. Liu, Z. Zhang, Friction and wear properties of a surface-modified TiO2 nanoparticle as an additive in liquid paraffin, Wear 213 (1997) 29-32. [37] Z. Zhang, L. Yu, W. Liu, Q. Xue, The effect of LaF3 nanocluster modified with succinimide on the lubricating performance of liquid paraffin for steel-on-steel system, Tribol. Int. 34 (2001) 83-88. [38] J.X. Dong, Z.S.Hu, A study of the anti-wear and friction-reducing properties of the lubricant additive, nanometer zinc borate, Tribol. Int. 31 (1998) 219-223.
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Table 1 Typical properties of base oil Table 2 The interlayer spacing and lattice parameters of Mg/Al-CO3-LDHs Table 3 X-ray fluorescence results of Mg/Al-CO3-LDHs Table 4 The interlayer spacing and lattice parameters of calcined products of Mg/Al-LDHs Table 5 The average friction coefficient and wear scar diameter of base oil and oil with LDHs and calcined products (250 oC, 350 oC, 600 oC and 1000 oC) Table 6 The comparison of relative atomic content of elements (EDS results) in the worn surface lubricated by base oil and oil with 1000 oC calcined products of Mg/Al LDHs
Table 1 Typical properties of base oil Base oil
Kinematic viscosity (mm2/s) 40 oC 100 oC
diesel engine oil (CD 15W-40)
110.60
Open Viscosity flash-point index (oC)
15.02
141
228
Pour point (oC)
Boiling point (oC)
-27
>300
Table 2 The interlayer spacing and lattice parameters of Mg/Al-CO3-LDHs
Mg/Al-CO3-LDHs
d(003)
d(006)
d(009)
d(110)
a
c
7.55
3.76
2.57
1.52
3.04
22.65
Table 3 X-ray fluorescence results of Mg/Al-CO3-LDHs Elements
Mg
Al
C
O
Mass fraction (wt%)
23.41
11.78
2.53
58.80
24
Table 4 The interlayer spacing and lattice parameters of calcined products of Mg/Al-LDHs Calcination Temperature
d(003)
d(006)
d(009)
d(110)
a
c
Mg/Al LDHs
7.55
3.76
2.57
1.52
3.04
22.65
150 oC
7.46
3.76
2.58
1.52
3.04
22.38
250 oC
6.75
─
─
1.52
3.04
20.25
─
─
─
1.52
3.04
─
o
350 C
Table 5 The average friction coefficient and wear scar diameter of base oil and oil with LDHs and calcined products (250 oC, 350 oC, 600 oC and 1000 oC) Average
Reduction
wear scar
Reduction
friction
compared to
diameter
compared to
coefficient
base oil
(mm)
base oil
Base oil
0.126
─
0.453
─
Mg/Al LDHs
0.120
4.76%
0.416
8.89%
250 oC
Oil samples
0.121
3.97%
0.422
6.84%
o
0.115
8.73%
0.393
13.25%
o
0.114
9.52%
0.398
12.14%
0.119
5.55%
0.368
18.76%
350 C 600 C o
1000 C
Table 6 The comparison of relative atomic content of elements (EDS results) in the worn surface lubricated by base oil and oil with 1000 oC calcined products of Mg/Al LDHs Elements
Mg
Zn
Fe
C
O
Surface (base oil, at%)
─
2.1
43.9
37.9
16.1
1.2
3.2
53.3
28.6
13.6
Surface (oil with calcined products, at%)
25
Figure caption Fig. 1 The XRD pattern and crystal structure of Mg/Al LDHs Fig. 2 The SEM image and size distribution of Mg/Al LDHs Fig. 3 The XRD patterns of calcined products of LDHs (150 oC, 250 oC, 350 oC, 500 o
C, 600 oC, 700 oC, 850 oC and 1000 oC)
Fig. 4 The morphology of calcined products of LDHs (350 oC, 600 oC, 850 oC and 1000 oC) Fig. 5 The FT-IR of calcined products of LDHs modified with oleic acid Fig. 6 The friction coefficient of base oil and oil with LDHs and calcined products (250 oC, 350 oC, 600 oC and 1000 oC) under 392 N and 1200 rpm for 30 min Fig. 7 The SEM image of surface lubricated by base oil and oil with 1000 oC calcined products of LDHs under 392 N and 1200 rpm for 30 min Fig. 8 The horizontal distribution of main elements in the whole round worn area lubricated by base oil Fig. 9 The horizontal distribution of main elements in the whole round worn area lubricated by oil with 1000 oC calcined products of LDHs Fig. 10 The horizontal distribution of main elements in the worn surface lubricated by base oil Fig. 11 The horizontal distribution of main elements in the worn surface lubricated by oil with 1000 oC calcined products of LDHs Fig. 12 The vertical depth distribution of main elements in the worn surface lubricated by oil with 1000 oC calcined products of LDHs (Area 1 selected)
26
Fig. 13 The multi-layer structure of self-repairing protective tribofilms Fig. 14 The supporting effect of 1000 oC calcined products of LDHs between worn surfaces
27
Highlights
The anti-wear property of calcined products was superior to Mg/Al LDHs.
The horizontal and vertical elements distribution in worn surface was analyzed.
Multi-layer self-lubrication mechanisms of calcined products were proposed.
Supporting and polishing contributed to anti-wear property of calcined products.
28
Graphical abstract
29
S0169-4332(18)33093-9 https://doi.org/10.1016/j.apsusc.2018.11.025 APSUSC 40870
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
31 July 2018 21 October 2018 4 November 2018
Please cite this article as: S. Li, L. Ren, Z. Bai, Friction performance and mechanisms of calcined products of Mg/ Al layered double hydroxides as lubricant additives, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.11.025
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Friction performance and mechanisms of calcined products of Mg/Al layered double hydroxides as lubricant additives Shuo Li1, Lingling Ren1, Zhimin Bai2 1
Division of Nano Metrology and Materials Measurement, National Institute of Metrology, Beijing, 100029 P.R. China
2
School of Materials Science and Technology, China University of Geosciences, Beijing, 100083 P.R. China
Abstract Mg/Al layered double hydroxides (LDHs) and the calcined products were prepared and the powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray fluorescence (XRF) and Fourier transform infrared spectrometer (FT-IR). The friction properties of LDHs and the calcined products were tested and the rubbed areas were analyzed by scanning electron microscope, energy disperse spectroscopy (EDS) and auger electron spectrometer (AES). The results showed that the diameter of LDHs was about 100~175 nm and the formula was Mg0.69Al0.31(OH)2(CO3)0.15·0.16H2O. All the calcined products had the laminated structure. The anti-wear properties of 1000 oC calcined products were superior to the LDHs, which was attributed to the supporting and mechanical polishing effects of the hard spinels. The distribution laws of elements of worn area in the horizontal and
Corresponding author. Tel.: +86 10 82323201; fax: +86 10 82322974.
Email address: [email protected] (Z. Bai). 1
vertical direction were concluded. The tribological performance of calcined products can be attributed to the “multi-layer self-lubrication mechanisms”. The tribofilms consisted of “carbon-enriched layer”, “oxygen- enriched layer” and “transition layer” and combined strongly with the metal substrate. Key words: Mg/Al layered double hydroxides; Calcined products; Elements distribution laws; Multi-layer self-lubrication mechanisms 1. Introduction In the past years, layered double hydroxides (LDHs) nanoparticles have been proved to possess excellent friction-reducing and anti-wear properties and protect the friction pairs effectively as lubricant solid additives under boundary lubrication condition, due to the layered crystal structure, nanoscale and high chemical activity. Bai [1] found Co/Al-CO3 layered double hydroxides can improve friction performance of lubricating oil through testing the coefficient of friction and wear of rubbed surface for the first time. Mg/Al, Zn/Al, Zn/Mg/Al, Mg/Al/Ce layered double hydroxides and the related intercalation products with different crystal size and synthesis methods were studied afterwards, which proved the friction properties of LDHs further [2-11]. Under the high surface flash temperature and contact pressure during friction process, not only do the organic compounds of lubricating oil decompose, but the phase transitions of solid additives happen, which the new crystal phases could change friction behaviors of original particles [12]. In the work, the calcined products of Mg/Al layered double hydroxides in different temperature were prepared and the
2
friction properties of the products as lubricant additives were tested. The results showed that the calcined products in high temperature possessed better anti-wear properties, compared with Mg/Al LDHs. The distribution laws of elements of rubbed surface were analyzed and the “multi-layer self-lubrication mechanisms” were put forward for the tribological performance of the calcined products. 2. Experiment 2.1. Preparation of the calcined products of Mg/Al LDHs Mg/Al LDHs were synthesized by the coprecipitation method in the paper [11] with Mg2+/Al3+ molar ratio of 2:1. The dried LDHs nanoparticles were calcined under 150 oC, 250 oC, 350 oC, 500 oC, 600 oC, 700 oC, 850 oC and 1000 oC in N2 atmosphere for 3 h respectively. All the particles were modified by oleic acid for good dispersibility in lubricating oil [5]. Finally, the modified LDHs and calcined products were filtered and washed until pH reached 7 and dried at 80 oC for 12 h. 2.2. Characterization of LDHs and calcined products Mg/Al LDHs and calcined products were characterized by X-ray diffraction (XRD) with Rigaku diffractometer (CuKα source, λ=0.15406 nm, scanning range from 3° to 70° (2θ)) for crystal structure. The content of elements of LDHs was obtained by wavelength dispersive X-ray fluorescence (WD-XRF, Rigaku ZSX PrimusⅡ). The particle size distribution of LDHs was obtained by Zetasizer (Nano ZS90, Malvern). The surface properties of particles were recorded on a NICOLET750 Fourier transform infrared (FT-IR) spectrometer (4000-450 cm-1, KBr sheet). The morphology of particles was observed on a scanning electron microscope (SEM,
3
JSM-6460LV, JEOL). 2.3. Tribological properties of LDHs and calcined products The friction properties were tested on a MS-10JR four-ball friction machine. The GCr15 steel balls with diameter of 12.70 mm and hardness of 64~66 HRC were used and cleaned by petroleum ether and ethyl alcohol for 10 min in an ultrasonic cleaner before tests. The LDHs and calcined products were added to lubricating oil (Table 1) with the same amount of 0.5g powders per 100 ml oil respectively. Then the oil samples were stirred under high speed agitator (10,000 rpm) and ultrasound for good dispersibility respectively. Each test was conducted under the rotating speed of 1200 rpm and load of 392 N for 30 min at room temperature. The average coefficient of friction was calculated with the data of stable stage. The average wear scar diameter of 3 lower balls was obtained by an optical microscopy. 2.4. Surface analysis of worn areas The morphology of worn areas was observed on a scanning electron microscope (SEM, JSM-6460LV, JEOL). The elements distribution laws in friction surface were analyzed by energy disperse spectroscopy (EDS) and auger electron spectrometer (AES, PHI-710, Argon ion sputtering rate 13 nm/min). 3. Results and discussion 3.1. X-ray diffraction and morphology of Mg/Al LDHs and the calcined products The XRD pattern and crystal structure of synthetic Mg/Al LDHs were shown in Fig. 1. From the XRD pattern, the LDHs showed layered structure diffraction peaks of hexagonal symmetry phase. The characteristic peaks (003), (006), (009) and (110) of
4
Mg/Al LDHs were narrow and sharp and base line was flat, which showed the high crystallinity of LDHs powders. The peak (110) represented the perfect internal structure of brucite-like layers. Based on the Bragg equation λ=2d(hkl)sinθ, interlayer spacing d(hkl) and lattice parameter a=2 d(110) and c=3 d(003) were calculated (Table 2), which corresponded to the values of Mg/Al LDHs with CO32- reported in previous work [13-15]. The chemical formula of Mg/Al LDHs could be calculated as Mg0.69Al0.31(OH)2(CO3)0.15·0.16H2O based on the mass fraction of elements of Mg/Al LDHs from the results of XRF (Table 3). The molar ratio of elements in the formula was close to the original proportion of LDHs preparation. From the SEM image (Fig. 2), it is obvious that LDHs powders showed typical hexagonal layered structure and the grain size of particles was homogeneous and distributed in the range of 100~175 nm mostly. The average size and thickness of powders were about 145 nm and 30 nm, respectively. Under high temperature, the crystal structure of Mg/Al LDHs changed and new phase might appear. The XRD patterns of the products of LDHs under different calcination temperature were shown in Fig. 3. The diffraction peak (003) shifted gradually to high angle from 150 oC to 250 oC, which showed the decrease of interlayer spacing along with the rising of temperature, compared to Mg/Al LDHs (Table 4). The peak (003) disappeared and crystallinity decreased obviously as the temperature rose to 350 oC. However, the position of peak (110) was always stable until 350 oC and the lattice parameter a of calcined products of LDHs at 150~350 oC had the same value (Table 4), which showed LDHs kept the layered structure. The
5
result fit with Yang’s report [16], however, other research showed the layered structure collapsed below 330 oC [17-19], which was attributed to the structure stability of LDHs due to different preparation methods and calcination process. The peak (110) disappeared and the layered structure collapsed and Mg/Al layered double oxides (LDO) generated as temperature rose to 500~700 oC. The Mg/Al spinel phase MgAl2O4 appeared in the calcined products until 850 oC, although the crystallinity was low. The calcination temperature rose to 1000 oC further, the diffraction peaks of periclase and Mg/Al spinel were sharp, which represented the high crystallinity of calcined products at 1000 oC [20, 21]. The morphology of products of LDHs at different calcination temperature (350 o
C, 600 oC, 850 oC and 1000 oC) was shown in Fig. 4. The crystal grains of calcined
products at 350 oC kept the whole hexagonal layered structure of LDHs powders. At 600
o
C, the thickness of calcined products decreased obviously although the
hexagonal morphology remained, compared to LDHs. The products possessed irregular laminar structure and the edge of crystal grains broke mostly and small particles distributed on the grains after calcined at 1000 oC. It was notable that all the calcined products possessed laminar structure similar to the LDHs, which was crucial for the friction properties. 3.2. FT-IR spectroscopy of modified Mg/Al LDHs and the calcined products The FT-IR spectra of modified Mg/Al LDHs and the calcined products with oleic acid were shown in Fig. 5. The peaks at 2925 and 2854 cm-1 in Mg/Al LDHs spectrum were assigned to the asymmetric and symmetric stretching vibration of C-H groups
6
from carbon chains of oleic acid molecules, respectively. It is obvious that the calcined products had the same vibration peaks of C-H groups, which showed oleic acids modified the products successfully. Moreover, Mg/Al LDHs and calcined products possessed the vibration peak of carboxylate at 1565 cm-1, but the vibration peak of -COOH at 1710 cm-1 disappeared. It is shown that the LDHs and calcined products were modified chemically by oleic acid with monolayer molecules on the surface of powders [5]. The intense and wide band at 3454 cm-1 in Mg/Al LDHs spectrum corresponded to O-H stretching vibration of hydroxyl groups of brucite-like layers and absorbed and interlayer water molecules. The calcined products of Mg/Al LDHs also showed similar vibration, which indicated the existence of hydroxy groups in the products. However, the intensity decreased gradually with the increasing of calcination temperature, which was caused by the removal of water molecules and hydroxyl groups. The vibration peak v3 of interlaminar CO32- of LDHs was at 1360 cm-1. The state of CO32- changed above 350 oC, which the peak of calcined products disappeared and new peak arose at 1416 cm-1. The peak at 675 cm-1 corresponded to asymmetric bending vibration v4 of CO32- of LDHs and the absorbed peak of interaction between interlayer CO32- and laminates was at 554 cm-1. Both peaks disappeared above 250 oC. Moreover, the peak led by vibration of metal cations M-OH in brucite-like layers (449 cm-1) shifted above 250 oC, compared with LDHs. It is concluded that the junction state of cations changed at high temperature [4, 5, 15, 16]. 3.3. Friction properties of Mg/Al LDHs and the calcined products as lubricant
7
additives Based on the crystal structures (XRD results) and surface morphology (SEM results), the friction properties of Mg/Al LDHs and the calcined products at 250 oC, 350 oC, 600 oC and 1000 oC were tested by four-ball friction machine. The friction tests results were shown in Fig. 6 and Table 5. All the powders (LDHs and calcined products) as lubricant additives showed friction-reducing and anti-wear properties at different levels, compared with lubricating oil, which had the highest average coefficient of friction (0.126) and diameter of wear scar (0.453 mm). Mg/Al LDHs reduced coefficient of friction and wear scar like other kinds of LDHs, which had been proved previously. It is remarkable that the calcined products at 350 o
C and 600 oC possessed better friction-reducing properties with reduction of
coefficient about 9%, compared with LDHs. It can be concluded that the removal of carbonate ions in the interlayer weakened the interactions between laminates, including the electrostatic force and hydrogen bond. Therefore, the laminates were easy to slide relatively under friction shearing force, which played roles in reducing friction coefficient. The most direct mode to evaluate the friction properties of lubricating oil is the size of wear scar on the friction pairs. The calcined product at 1000 oC had the best anti-wear properties with reduction of diameter of wear scar 18.76%, compared to LDHs and other calcined products. The phase included periclase MgO and spinel MgAl2O4. The product kept the laminate structure and small particles generated at the edge of crystal grains. The hard spinels MgAl2O4 (Mohs hardness 8), which were the
8
in situ products of phase transition of LDHs at high temperature, played a role of particle-supported effect between metal pairs under friction process. Moreover, the hard particles can repair the microcracks and polish the rubbed surface. The size of 1000 oC calcined products was close to the LDHs so that the size had limited effects on the friction properties. The surface morphology of rubbed areas lubricated by base oil and oil with calcined products at 1000 oC were shown in Fig. 7. From the SEM picture, severe wear occurred on the surface lubricated by base oil, which had bigger size of wear scar and wide and deep grooves and furrows that was a typical abrasive wear. However, there were only shallow worn traces on the surface lubricated by oil with hard calcined products and the rubbed area was smaller and smoother, which indicated the excellent anti-wear properties. 3.4. The distribution of elements on the surface lubricated by oil with 1000 oC calcined products of Mg/Al LDHs The SEM image and horizontal distribution of main elements of the whole round worn area with base oil by energy disperse spectroscopy (EDS) was shown in Fig. 8. It is obvious that the surface was worn severely and amounts of deep and wide wear scar appeared. The main elements of Zn, Fe, O and C were detected, which the Zn came from the ZDDP. The Zn and O distributed intensely and the content of Fe was less in the worn area compared with non-worn surface, although the C had no obvious content comparison. The carbonization products of base oil (black frame A) were found on the worn surface, which demonstrated the cracking reactions of organic compounds of lubricating oil in the friction contact area. The oil with 1000 oC
9
calcined products of Mg/Al LDHs had better anti-wear properties than base oil. Therefore, it had smaller rubbed area and milder abrasion compared with surface with base oil (Fig. 9). It had similar distribution laws of main elements Zn, Fe, C and O in the area, which had small amounts of Mg from the calcined products. The decomposition products of oil were also found on the surface. The black particle A that had only C may be the graphite, and the particle B with carbon and zinc may be generated by decomposition of ZDDP. The SEM image and horizontal distribution of main elements of surface with base oil by energy disperse spectroscopy (EDS) was shown in Fig. 10. The main elements included Zn, Fe, O, C, S and P, which the S and P derived from base oil. The rubbed surface was worn severely and had wide and deep grooves (yellow dotted line frames), in which O and Zn distributed less and C deposited more along the direction of friction compared with flat area. However, the smooth area had more O and Zn and less Fe and C than the grooves (white dotted line frames). The surface lubricated by oil with 1000 oC calcined products of Mg/Al LDHs had similar distribution laws of elements (Fig. 11). The Zn, Mg and O distributed intensively in the smooth area (yellow dotted line frames) and Fe and C deposited less. Similarly, C deposited more in the grooves (white dotted line frames). Mg distributed less in surface furrows (green dotted line frames). It is considerable that Al was not detected from the surface lubricated by 1000 oC calcined products of Mg/Al LDHs (Fig. 11). Nevertheless, Zhao’s research by pin-on-disc friction tester showed the element of Al appeared on the surface lubricated
10
by oil with Mg/Al LDHs. The chemical state of Al was similar to that in the α-Al2O3 tested by the X-ray absorption near-edge structure (XANES) spectra, which the state of Al distinguished from that in the LDHs and the coordination environment of part of Al changed to tetrahedral structure [6]. The Co/Al LDHs also had similar results [9]. The difference can be concluded that the Al3+ had high bond strength with O2- in the spinel MgAl2O4. Thus, the Al3+ was hard to separate from the spinel [15]. The four-ball friction test had severe condition with load of 392 N and rotating speed of 1200 rpm. The metal surface on the tiny contact area (point contact) was exfoliated easily under high pressure and speed so that the Al3+ was hard to exist on the contact point. However, the conditions of pin-on-disc friction test in Zhao’s research were milder with load of 220 N and frequency of 25Hz including larger contact area, compared to the four-ball test. The bivalent metal cations Mg2+ were easier to exchange with same valence Fe2+ in the metal substrate under friction process, compared with trivalent metal cations Al3+ so that Al3+ was hard to integrate itself into the contact surface [1]. It can be concluded that the surface lubricated by base oil and oil with calcined products of LDHs had similar elements distribution laws along the sliding direction which the Zn, O (or Mg) distributed intensively in the smooth and flat area and C deposited more in the grooves. The comparison of relative atomic content of Mg, Zn, Fe, C and O in the whole worn area lubricated by base oil and oil with 1000 oC calcined products of Mg/Al LDHs through energy disperse spectroscopy was shown in Table 6. The depth of EDS
11
detecting can reach from dozens of nanometers to hundreds of nanometers and the contents of elements represented the average values. It is remarkable that the content of Zn, Fe and C had obvious difference after the addition of calcined products. It can be concluded that the calcined products increased the content of Zn and Fe but decreased the C content. The deposition of more ZDDP and the Fe played key roles in reducing friction and abrasion. The vertical depth distribution laws of elements obtained through auger electron spectroscopy (AES) in the surface lubricated by oil with 1000 oC calcined products of Mg/Al LDHs was shown in Fig. 12, which concentrated on the change of relative content of elements of Mg, Zn, Fe, C and O with sputtering time in the vertical direction. Both the elements of O and C had high and close content (above 40%) in the top layer of rubbed surface (42.5% C and 41.8% O). Besides, there were about 7.2% Fe, 5.4% Zn and 3.1% Mg in the top. The “oxygen-enriched layer” represented the top oxidized highly and the rich carbon showed that there were amounts of graphite and organics with carbon, which had self-lubricating effect in friction process. The high molar ratio of O/Fe indicated the top had more oxides with high valence state iron. The existence of Zn and Mg demonstrated that the ZDDP and calcined products transferred to the rubbed surface and participated in the formation of tribofilms. After 0.5 min of sputtering time, the content of O and Fe increased further. However, the content of C declined rapidly, which showed the “carbon-enriched layer” was pretty thin with thickness of less than 10 nm based on the sputtering velocity of 13 nm/min.
12
As time went on, the content of Fe rose quickly, although the O had a fast decrease. The carbon kept the low level till the end. The ratio of Fe : O was close to 1:1 except the O bonded with metal elements of Zn and O at 2 min of sputtering. It showed that the “oxygen-enriched layer” ended nearly and the high valence iron oxides decreased and the content low valence iron oxides even iron increased gradually with deeper sputtering. It is remarkable that the content of Zn and Mg kept stable before the 2~2.5 min, which showed Zn and Mg distributed uniformly in the “oxygen-enriched layer”. As the depth of sputtering deepened further, the content of Fe kept increasing to about 70% (9.5 min) and the O decreased to only 7% and both the Zn and Mg declined to less than 2%. It demonstrated that the tribofilms had approached to the lower boundary even the metal substrate of steel balls. The thickness of tribofilms was about 120 nm at least calculated by the sputtering velocity. 3.5. The tribological mechanisms of 1000 oC calcined products Mg/Al LDHs as lubricant additives The micro/nano sized solid particles as lubricant additives showed excellent friction-reducing and anti-wear properties. Due to the complex physical and chemical reactions in friction process, there have not been enough convincing and uniform evidences to prove the tribological mechanisms of micro/nano particles so far. The explanations about the mechanisms can be concluded as follows: (1) “The ball bearing effect” applies to spherical particles generally. The particles support the friction pairs and transform the sliding to rolling friction with lower coefficient of friction to protect the rubbing surface, like polytetrafluoroethylene (PTFE), Hollow
13
nanoparticles WS2 and nanosized diamonds [22-26]; (2) The particles deposit on the worn surface to form a tribofilm with low coefficient of friction physically, which can prevent the friction pairs from contacting directly, like metal and graphite [27-29]. It can be marked as “physical deposition theory”; (3) The particles with high activity react with fresh rubbing surface physically and chemically to form “tribochemical reaction film” to protect the friction pairs, like metal sulfide, oxides, borates, rare earth compounds and mineral powders [30-38]. The theory is accepted by amounts of researches. In the past few years, various kinds of LDHs powders and intercalated products as lubrication additives have been studied and showed excellent friction-reducing and anti-wear properties [1-11]. There were few enough evidences to explain the tribological mechanisms of LDHs. Zhao studied the formation mechanisms of tribofilms of Mg/Al LDHs intensively. The individual Mg/Al LDHs could improve the friction-reducing properties of base oil and the anti-wear performance was enhanced as both LDHs and ZDDP were added in base oil. The XANES results showed that the coordination structure of Mg and Al in discontinuous tribofilms (containing Mg, Al, C, O, Zn, P and S) changed, which indicated the new phases were generated, like α-Al2O3. LDHs promoted the formation of polyphosphate with medium chain and the oxidation of sulfide in the near surface, which weakened the wear. Through the friction tests, the morphology of worn surface, the distribution of surface elements in horizontal and vertical direction, the tribological mechanisms of
14
1000 oC calcined products of Mg/Al LDHs as lubricant additives can be concluded as the “self-repairing protective tribofilms theory” [8, 10, 11]. The structure of tribofilms in the worn surface of metal substrate was analyzed in detail for the first time and can be divided into three layers based on the content of C and O roughly which had no obvious boundary between neighbouring layers (Fig. 13). The top layer with thickness of only several nanometers, called as “carbon-enriched layer”, contained the elements of Mg, Zn, Fe, C, O, S and P except the Al. The atomic content of C was highest, up to 50%. The content of O was about 30%~40% and Mg, Zn, Fe were slight relatively in the carbon-enriched layer. There were carbonization products of organics in lubricating oil on the surface of the top layer besides graphite with self-lubricating effect. The C mainly existed in the grooves and furrows and smooth and flat areas contained small amounts of C. Based on the EDS tests, the surface with calcined products had less C compared with that with base oil, which demonstrated the calcined products can reduce effectively the wear with less grooves or furrows. Besides the element of C, the “carbon-enriched layer” also had high content of O, which the iron mainly existed in the form of high valent oxides. The content of O increased rapidly with extension of sputtering time and the “oxygen-enriched layer” appeared in the tribofilms. The O reached the peak (close to 60 at%) in the area of adjacent carbon-enriched layer then decreased gradually. It is remarkable that the content of Mg and Zn kept stable in the vertical direction of the layer, which showed calcined products participated in the formation of tribofilms
15
continuously and stably. The content of C declined rapidly and then stayed at a level in the layer. However, the Fe increased inversely and low valent iron oxides increased. The oxygen-enriched layer almost ended as the content of Fe was close to the O. The thickness of the layer was about 20~30 nm. After the “carbon-enriched layer” and “oxygen-enriched layer”, the content of O decreased continuously and was exceeded by the Fe. The “transition layer” appeared with more low valent iron oxides even the elements of iron. Meanwhile, the Mg and Zn declined gradually. The protective tribofilms approached the bottom and reached the metal substrate. Mg, Zn and Fe distributed uniformly in the transition layer, which proved that the tribofilms and substrate had no obvious boundary and combined each other strongly. The tribofilms generated by calcined products was about 120 nm. Based on the above AES results, the main elements had obvious distribution laws in the tribofilms with multi-layer structure. Thus, the “multi-layer self-lubrication mechanisms” can be concluded as the reasons of the friction properties of the calcined products. The Mg/Al LDHs and calcined products had similar laminated structure, which was key to the friction properties. The Mg/Al LDHs as lubricant additives participated in the complex tribochemical and physical reactions during the friction process to form the protective films on the rubbed surface. Thus, it can be concluded that the friction mechanisms of Mg/Al LDHs and calcined products were related closely and the “multi-layer self-lubrication mechanisms” of calcined products were also applicable to the Mg/Al LDHs or other LDHs. The 1000 oC calcined products of Mg/Al LDHs had the phase of periclase MgO
16
and Mg/Al spinel MgAl2O4. The spherical shaped Zn/Al spinels ZnAl2O4 powders as lubricant additives showed friction-reducing and anti-wear properties and the mechanisms can be attributed to the “micro-bearing effect” of spherical hard spinels [25]. The hard in-suit phase transition products of LDHs had laminated structure. Therefore, the reason which the 1000 oC calcined products had better anti-wear properties compared with the LDHs was mainly attributed to supporting (surface microbulges of rubbed area) and mechanical polishing effects of the hard spinels to reduce the abrasion finally (Fig. 14). Moreover, it was shown that amounts of small spherical particles generated at the edge or surface of crystal grains of calcined products. Thus, the “ball bearing effects” were also possible in the friction process. The friction behaviors of Mg/Al LDHs can be concluded that LDHs deposited on the worn surface under high temperature and pressure, and then the phase change of nanoparticles occurred, which the process can absorb the power. Meanwhile, the results above proved the phase change products (calcined products) had also the friction properties. After complex reaction process, the LDHs and phase change products participated in the formation of “self-repairing protective tribofilms”. Moreover, the phase change products might have the supporting, mechanical polishing or ball bearing effects. 4. Conclusion (1) The Mg/Al-CO3-LDHs were synthesized with diameter of about 100~175 nm. The formula was Mg0.69 Al0.31(OH)2(CO3)0.15·0.16H2O. The 350 oC calcined products of LDHs still kept the diffraction peak (110) and all the calcined products possessed
17
the laminated structure. The 1000 oC calcined products were periclase MgO and Mg/Al spinel MgAl2O4 and the anti-wear properties were superior to the LDHs, which was attributed to the supporting and mechanical polishing effects of the hard spinels. (2) The Mg, Zn and O distributed intensively in the smooth surface of worn area with 1000 oC calcined products of LDHs and C distributed more in the grooves and furrows. Compared to the area with base oil, the area with 1000 oC calcined products contained more Zn and Fe and less C (EDS results). The absence of Al can be concluded that Al3+ was hard to separate from the spinel and the friction conditions of four-ball tests were severe. The trivalent cations Al3+ were hard to exchange with bivalent Fe2+ of substrate. (3) The tribological mechanisms of 1000 oC calcined products of LDHs can be attributed to the “multi-layer self-lubrication mechanisms”. The tribofilms (about 120 nm) consisted of three layers, “carbon-enriched layer”, “oxygen-enriched layer” and “transition layer” from the surface to the bottom. There were no obvious boundaries between neighbouring layers and the tribofilms combined strongly with the metal substrate. Acknowledgement This research was sponsored by the National Key Research Plan (Project No. 2017YFB0310703). References [1] Z.M. Bai, Z.Y. Wang, T.G. Zhang, F. Fu, N. Yang, Synthesis and characterization
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of Co-Al-CO3 layered double-metal hydroxides and assessment of their friction performances, Appl. Clay Sci. 59-60 (2012) 36-41. [2] D. Zhao, Z. Bai, F. Zhao, Preparation of Mg/Al-LDHs intercalated with dodecanoic acid and investigation of its antiwear ability, Mater. Res. Bull. 47 (2012) 3670-3675. [3] Z.M. Bai, Z.Y. Wang, T.G. Zhang, F. Fu, N. Yang, Characterization and friction performances of Co-Al-layered double-metal hydroxides synthesized in the presence of dodecylsulfate, Appl. Clay Sci. 75-76 (2013) 22-27. [4] X. Wang, Z. Bai, D. Zhao, F. Zhao, Friction behavior of Mg-Al-CO3 layered double hydroxide prepared by magnesite, Appl. Surf. Sci. 277 (2013) 134-138. [5] S. Li, Z. Bai, D. Zhao, Characterization and friction performance of Zn/Mg/Al-CO3 layered double hydroxides, Appl. Surf. Sci. 284 (2013) 7-12. [6] D. Zhao, T-K. Sham, M. Kasrai, Z. Bai, F. Zhao, Tribological properties of Mg/Al-CO3 layered double hydroxide as additive in base oil, Tribol.-Mater., Surf. & Inter. 8 (2014) 222-234. [7] Z. Bai, S. Li, Z. Wang, T. Zhang, F. Fu, N. Yang, J. Gao, Synthesis of Co-Al-layered double-metal hydroxides and the friction performance, J. Chin. Ceram. Soc. 1 (2014) 70-77. [8] S. Li, H. Qin, R. Zuo, Z. Bai, Tribological performance of Mg/Al/Ce layered double hydroxides nanoparticles and intercalated products as lubricant additives, Appl. Surf. Sci. 353 (2015) 643-650. [9] D. Zhao, M. Kasrai, T-K. Sham, Z. Bai, F. Zhao, S. Li, Preparation of platy Co/Al
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Table 1 Typical properties of base oil Table 2 The interlayer spacing and lattice parameters of Mg/Al-CO3-LDHs Table 3 X-ray fluorescence results of Mg/Al-CO3-LDHs Table 4 The interlayer spacing and lattice parameters of calcined products of Mg/Al-LDHs Table 5 The average friction coefficient and wear scar diameter of base oil and oil with LDHs and calcined products (250 oC, 350 oC, 600 oC and 1000 oC) Table 6 The comparison of relative atomic content of elements (EDS results) in the worn surface lubricated by base oil and oil with 1000 oC calcined products of Mg/Al LDHs
Table 1 Typical properties of base oil Base oil
Kinematic viscosity (mm2/s) 40 oC 100 oC
diesel engine oil (CD 15W-40)
110.60
Open Viscosity flash-point index (oC)
15.02
141
228
Pour point (oC)
Boiling point (oC)
-27
>300
Table 2 The interlayer spacing and lattice parameters of Mg/Al-CO3-LDHs
Mg/Al-CO3-LDHs
d(003)
d(006)
d(009)
d(110)
a
c
7.55
3.76
2.57
1.52
3.04
22.65
Table 3 X-ray fluorescence results of Mg/Al-CO3-LDHs Elements
Mg
Al
C
O
Mass fraction (wt%)
23.41
11.78
2.53
58.80
24
Table 4 The interlayer spacing and lattice parameters of calcined products of Mg/Al-LDHs Calcination Temperature
d(003)
d(006)
d(009)
d(110)
a
c
Mg/Al LDHs
7.55
3.76
2.57
1.52
3.04
22.65
150 oC
7.46
3.76
2.58
1.52
3.04
22.38
250 oC
6.75
─
─
1.52
3.04
20.25
─
─
─
1.52
3.04
─
o
350 C
Table 5 The average friction coefficient and wear scar diameter of base oil and oil with LDHs and calcined products (250 oC, 350 oC, 600 oC and 1000 oC) Average
Reduction
wear scar
Reduction
friction
compared to
diameter
compared to
coefficient
base oil
(mm)
base oil
Base oil
0.126
─
0.453
─
Mg/Al LDHs
0.120
4.76%
0.416
8.89%
250 oC
Oil samples
0.121
3.97%
0.422
6.84%
o
0.115
8.73%
0.393
13.25%
o
0.114
9.52%
0.398
12.14%
0.119
5.55%
0.368
18.76%
350 C 600 C o
1000 C
Table 6 The comparison of relative atomic content of elements (EDS results) in the worn surface lubricated by base oil and oil with 1000 oC calcined products of Mg/Al LDHs Elements
Mg
Zn
Fe
C
O
Surface (base oil, at%)
─
2.1
43.9
37.9
16.1
1.2
3.2
53.3
28.6
13.6
Surface (oil with calcined products, at%)
25
Figure caption Fig. 1 The XRD pattern and crystal structure of Mg/Al LDHs Fig. 2 The SEM image and size distribution of Mg/Al LDHs Fig. 3 The XRD patterns of calcined products of LDHs (150 oC, 250 oC, 350 oC, 500 o
C, 600 oC, 700 oC, 850 oC and 1000 oC)
Fig. 4 The morphology of calcined products of LDHs (350 oC, 600 oC, 850 oC and 1000 oC) Fig. 5 The FT-IR of calcined products of LDHs modified with oleic acid Fig. 6 The friction coefficient of base oil and oil with LDHs and calcined products (250 oC, 350 oC, 600 oC and 1000 oC) under 392 N and 1200 rpm for 30 min Fig. 7 The SEM image of surface lubricated by base oil and oil with 1000 oC calcined products of LDHs under 392 N and 1200 rpm for 30 min Fig. 8 The horizontal distribution of main elements in the whole round worn area lubricated by base oil Fig. 9 The horizontal distribution of main elements in the whole round worn area lubricated by oil with 1000 oC calcined products of LDHs Fig. 10 The horizontal distribution of main elements in the worn surface lubricated by base oil Fig. 11 The horizontal distribution of main elements in the worn surface lubricated by oil with 1000 oC calcined products of LDHs Fig. 12 The vertical depth distribution of main elements in the worn surface lubricated by oil with 1000 oC calcined products of LDHs (Area 1 selected)
26
Fig. 13 The multi-layer structure of self-repairing protective tribofilms Fig. 14 The supporting effect of 1000 oC calcined products of LDHs between worn surfaces
27
Highlights
The anti-wear property of calcined products was superior to Mg/Al LDHs.
The horizontal and vertical elements distribution in worn surface was analyzed.
Multi-layer self-lubrication mechanisms of calcined products were proposed.
Supporting and polishing contributed to anti-wear property of calcined products.
28
Graphical abstract
29