Elsevier

Applied Surface Science

Volume 378, 15 August 2016, Pages 308-319
Applied Surface Science

Lubrication performance and mechanisms of Mg/Al-, Zn/Al-, and Zn/Mg/Al-layered double hydroxide nanoparticles as lubricant additives

https://doi.org/10.1016/j.apsusc.2016.03.220Get rights and content

Highlights

  • Mg/Al-, Zn/Al- and Zn/Mg/Al-layered double hydroxide were synthesized.

  • Mg/Al-LDH had superior tribological performance compared to other LDHs.

  • The best thermal stability of Mg/Al-LDH was responsible for its friction property.

Abstract

Solid lubricant particles are commonly used as oil additives for low friction and wear. Mg/Al-, Zn/Al-, and Zn/Mg/Al-layered double hydroxides (LDH) were synthesized by coprecipitation method. The benefits of LDH nanoparticles are that they can be synthesized using chemical methods where size and shape can be controlled, and can be modified organically to allow dispersal in fluids. The LDH nanoparticles were characterized by X-ray diffraction, scanning electron microscope, thermogravimetry, and differential scanning calorimetry. A pin-on-disk friction and wear tester was used for evaluating the friction and wear properties of LDH nanoparticles as lubricant additives. LDH nanoparticles have friction-reducing and anti-wear properties compared to oil without LDHs. Mg/Al-LDH has the best lubrication, possibly due to better thermal stability in severe conditions.

Introduction

Traditional liquid lubricants cannot satisfy the demands of industrial and environmental requirements in severe operating conditions. For lubrication applications, solid lubricant additives have been used because of their friction-reducing, anti-wear, and load-carrying properties during boundary lubrication, such as graphite, MoS2, and polytetrafluoroethylene (PTFE) [1], [2], [3], [4]. The friction-reducing and anti-wear effects are greatly influenced by the shape, size, crystal structure, and chemical properties of nanoparticles and experimental conditions. The lubrication mechanisms involve rolling friction and physically and chemically reacted tribofilm formation [1], [3], [4], [5], [6], [7]. Due to physical and chemical effects between contact surfaces under boundary lubrication, the mechanisms also can be complex.

Graphite and MoS2 have a hexagonal layered structure. The bonds in the basal planes are relatively strong covalent bonds, whereas the forces between the adjacent layers are van der Waals forces. The key to graphite and MoS2 as solid lubricants is their layered crystalline structure. The platelets in such structure are easy to shear and transfer to the rubbed surfaces to control friction and wear. PTFE is a crystalline polymer that consists of a repeating unit: – (CF2–CF2) –. Its low-friction property is attributed to its smooth molecular profile and strong adhesion on contact surfaces to form a transfer film for easy sliding.

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clay, have a hexagonal layered crystal structure similar to graphite and MoS2, which results in easy shear of crystal platelets. The benefits of LDH nanoparticles are that they are generally synthesized through chemical methods in which the size and shape can be controlled. Large numbers of hydroxyl groups existing on the surface of LDH platelets make it possible for surface organic modification, which is beneficial for particle dispersal in lubricating oil [8], [9].

The LDHs’ crystalline structure consists of positively charged, brucite-like metal hydroxide layers and charge-balancing exchangeable anions in the interlayer (Fig. 1) [8]. The interaction between the layers and the interlayer anions is electrostatic forces. The divalent metal cations are substituted by trivalent metal cations, which generate surplus positive charges, compensated by interlayer anions [8], [9], [10]. The general formula of LDHs can be expressed as follow:[M2+1−xM3+x(OH)2]x+ (An)x/n·mH2Owhere M2+ and M3+ represent divalent and trivalent metal cations respectively, x is the molar ratio of M3+/(M2+ + M3+), An− is the exchangeable anion, and m is the amount of water molecules in the interlayer. LDHs have been broadly applied in the areas of catalysts, adsorbents, and anion exchange due to the variability of laminate cations and interlayer anions [10], [11], [12].

Furthermore, LDHs with an M2+/M3+ molar ratio of 2:1 have a fully ordered arrangement of metal cations in the brucite-like layers (Fig. 2a) [8], [13]. It is observed that hydroxyl groups and metal cations occupy the apexes and centers of octahedra, respectively. Each octahedron shares six of its edges with those of the six nearest neighboring positions to form LDH two-dimensional layers. Each M3+ cation is surrounded by six M2+ cations and the six positions around M2+ are occupied by three M3+ and three M2+, and there is no M3+−M3+ contact [8]. The long-range ordering of cations decides the charge density of positively charged layers, which has an effect on the bonding, interaction with interlayer anions and structure stability [13]. An M2+(OH)6 or M3+(OH)6 octahedron of brucite-like layers is shown in Fig. 2b.

Base oils can have reduced friction and wear from the addition of LDHs, such as Co/Al-LDHs [14], Mg/Al-LDHs [15], [16], and Zn/Mg/Al-LDHs [17]. Bai et al. [14] reported that Co/Al-LDHs nanoparticles with Co2+/Al3+ ratio of 3:1 had typical hexagonal lamella structure with an average diameter of 150–200 nm. As lubricant additives, Co/Al-LDHs reduced the friction coefficient and wear scar diameter by 49% and 33%, respectively, as compared to base oil in a four-ball friction test. Wang et al. [16] synthesized Mg/Al-LDHs with Mg2+/Al3+ ratio of 2.5:1 using raw material of magnesite. The average diameter was about 150 nm. Mg/Al-LDHs reduced the coefficient of friction by 11%. Li et al. [17] prepared Zn/Mg/Al-LDHs nanoparticles with Zn2+/Mg2+/Al3+ ratio of 1:1:1, and utilized oleic acid for surface modification. A four-ball friction test showed that lubricating oil with 0.5 wt% LDHs powders had the best lubrication performance in which coefficient of friction and wear scar diameter were reduced by 68.6% and 24.6%, respectively.

In previous research, the friction properties of different kinds of LDH nanoparticles as lubricant additives were reported. The variety of laminate metal cations and interlayer anions of LDHs generate different properties, such as shape, size, and structural behavior, which determine the friction and wear performance of nanoparticles in lubricating oil [1], [4], [6]. However, the difference in friction properties among the various LDHs has not been studied under the same experimental conditions.

In the present work, Mg/Al-LDH with Mg2+/Al3+ ratio of 2:1, Zn/Al-LDH with Zn2+/Al3+ ratio of 2:1, and Zn/Mg/Al-LDH with Zn2+/Mg2+/Al3+ ratio of 1:1:1 were studied because of their similar composition. Zn/Al-LDH was studied for the first time. The lubrication properties of the three LDHs as lubricant additives were measured by a pin-on-disk friction tester. The thermal properties of the three LDHs were also tested. The lubrication mechanism was concluded. The friction properties of conventional lubricants graphite and MoS2 were tested as a comparison to LDHs.

Section snippets

Synthesis and modification of Mg/Al, Zn/Al- and Zn/Mg/Al-layered double hydroxide nanoparticles

Mg(NO3)2·6H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O (analytical pure, >99.0%), NaOH (analytical pure, >96.0%), and Na2CO3 (analytical pure, >99.8%) were used as raw materials (Beijing Chemical Works). Deionized water was used as solvent for solutions. LDHs were synthesized by the coprecipitation method [8], [9]. The mixed alkaline solution (NaOH and Na2CO3) was added to a stirred mixed nitrates solution [M(NO3)2, (M = Mg2+ or Zn2+) and M(NO3)3, (M = Al3+)] until the final pH reached a set value as shown in

The crystal structure and micromorphology of Mg/Al-, Zn/Al- and Zn/Mg/Al-layered double hydroxide

The XRD patterns of Mg/Al-, Zn/Al- and Zn/Mg/Al-layered double hydroxide are shown in Fig. 4. All the patterns show typical layered structure hexagonal symmetry with diffraction peaks (003) and (006) that are narrow and sharp. That means there is a high crystallinity degree for all the LDHs. Additionally, peak (110) represents the integrated internal structure of brucite-like layers. The lattice parameter a (a = 2 d(110)) is the mean distance of adjacent metal cations in the layers [8], [10], [17]

Conclusion

Based on the present work, the following can be concluded:

  • (1)

    Mg/Al-, Zn/Al- and Zn/Mg/Al-LDH nanoparticles were synthesized by coprecipitation method. All the LDHs nanoparticles possessed typical hexagonal symmetry structure with high crystallinity and almost uniform size of 200 nm

  • (2)

    Graphite and MoS2 in paraffin oil had similar friction and wear properties at all friction test conditions.

  • (3)

    All the LDH nanoparticles as lubrication additives had friction-reducing and anti-wear properties as compared to

Acknowledgements

The authors acknowledge the help from PhD students Yifan Li, Shuyang Ding, and Shan Peng, and Drs. Daehyun Cho and Dave Maharaj. The first author thanks financial support from China Scholarship Council (CSC, No. 201406400003). This research was sponsored by the National Natural Science Foundation of China (No. 51044011) and the Fundamental Research Funds for the Central Universities (Nos. 2652013037, 2652013038 and 2652013039).

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