Elsevier

Applied Surface Science

Volume 475, 1 May 2019, Pages 847-856
Applied Surface Science

Full Length Article
Tribological performances of SiO2/graphene combinations as water-based lubricant additives for magnesium alloy rolling

https://doi.org/10.1016/j.apsusc.2019.01.062Get rights and content

Highlights

Abstract

The tribological performances and rolling lubrication properties of SiO2/graphene combinations as water-based lubricant additives were evaluated using a reciprocating ball-on-plate tribometer and two-high rolling mill. The results obtained therein revealed that dispersion of the SiO2/graphene combinations in water is conducive to the reduction of the friction coefficient and wear volume and the increase in the load-bearing capacity, which were superior to the results obtained for the graphene nanofluids or SiO2 nanofluids separately. In the best case, the addition of 0.1 wt% nano-SiO2 mixed with 0.4 wt% graphene in water reduced the friction coefficient by 48.5% and the wear volume by 79% compared with the addition of 0.5 wt% graphene. Transmission electron microscopy images of the SiO2/graphene combinations additives after friction testing showed that nano-SiO2 supports the graphene layers as pillars and prevent assembly between the graphene layers. The synergistic effect of the components considerably enhanced the SiO2/graphene combination. Additionally, the preliminary application of the SiO2/graphene combination nanofluids in magnesium alloy rolling effectively decreased the rolling force and improved the surface quality of sheets.

Introduction

Magnesium alloys have attracted significant attention owing to the increasing demand for lightweight structural components in automobile, aerospace and electronics industries [1], [2]. Currently, commercial components of magnesium alloy are mainly fabricated via die casting. However, the casting defects limit further industrial application of as-cast magnesium alloy, mainly owing to its low strength and ductility. Wrought Mg alloys, such as extruded profiles, rolled sheets and forgings, exhibit many advantageous mechanical properties compared with cast Mg alloys owing to the pronounced grain refinement without the inherited casting defects after the deformation process [3]. However, it is inevitable that the metal-forming process generates high friction between the workpiece and tool interfaces. Some part of the energy transmitted through the contact is lost owing to friction [4]. Moreover, the presence of friction may result in several effects, such as, inhomogeneous deformations, a relatively short tool life, the generation of heat, and poor surface quality of workpiece.

The application of appropriate lubricants is critical for optimizing the metal forming process. However, so far, there have been no suitable lubricants for use in the forming process of Mg alloy, even at some conditions, the lubricants used for Al alloy forming are casually used and the result is not satisfactory. Most of the lubricants for Al alloy in the market contain additives such as sulfur-, chlorine-, and phosphorous- containing organic compounds. The environmental impact of the abovementioned additives is often undesirable owing to their toxicity and non-biodegradability [5]. Moreover, the fast chemical degradation of these additives and the excessive wear during application could affect the efficiency and security of mechanical components [6]. Therefore, the search for sustainable lubricant additives with promising lubricant performances is necessary for magnesium alloy forming. To achieve the aforementioned objectives, various advanced additives such as N-containing compounds [7], borates [8], and ionic liquids [9], have been developed for magnesium alloys. Overall, the above additives play a critical role in improving lubricating behaviors during rubbing process, which could be due to the formation of several categories of tribo-films between the sliding surfaces. Although many nitrogen heterocyclic compounds exhibited acceptable anti-wear and corrosion inhibition properties, their friction-reducing behavior was limited. The friction-reducing and wear resistance effects of borate without an active element, such as nitrogen, sulfur and chlorine, are not evident for magnesium alloys. In addition, boron is an electron-deficient element and has a great affinity for oxygen borate esters susceptible to hydrolysis in the presence of moisture [10]. Although ionic liquids exhibit an impressively low friction coefficient and significant wear resistance, the complicated process and high cost pose an obstacle to their practical application [11].

In recent years, investigations on nanoparticles as lubricant additives have attracted significant attention owing to their small size, lack of harmful emissions, large surface area, and excellent chemical stability [12], [13]. Among various nanoparticles, graphene has been studied extensively as an emerging lubricant additive [14], [15], [16]. It has been documented that graphene as a lubricant additive exhibits favorable friction-reducing and anti-wear properties under various test conditions. In this respect, Berman et al. [17] reported the tribological properties of graphene-containing ethanol solution for self-mated steel tribo-pairs. The experimental measurements showed that the presence of graphene at the tribological interface decreased wear by almost four orders of magnitude and the friction coefficients by a factor of six. The lubrication mechanism is based on building a conformal protective film to avoid direct contact between the sliding contact interfaces. Eswaraiah et al. [18] synthesized ultrathin graphene and the tribological performances of the as-prepared ultrathin graphene as a lubricant additive in engine oil were evaluated using a reciprocating friction and wear tribometer. It was observed that the friction characteristics, wear resistance, and extreme pressure properties of graphene-based nanolubricants were improved by 80%, 33%, and 40%, respectively, compared with those of the base oil. The superior lubricant behaviors achieved can be ascribed to the ultimate mechanical strength of graphene and the nanobearing effect during rubbing process. Liang et al. [19] investigated the tribological properties of in-situ exfoliated graphene as a water-based lubricant additive for steel-steel contacts. Compared with pure deionized water, graphene nanofluids can offer 81.3% and 61.8% reduction in friction coefficient and wear scar diameter, respectively. Moreover, it is generally known that the SiO2 nanoparticles possess many advantages including low cost, high temperature resistance, and facile preparation. They have been widely used as additives to improve the tribological properties of some lubricants in metal-forming processes, such as machining and drilling [20], [21], [22]. These excellent lubricating performances are attributed to the rolling action of SiO2 nanoparticles between the interacting surfaces. According to the previous studies, it was anticipated that graphene and SiO2 nanoparticles would be promising lubricant additives. However, the combinations of nanoparticles are of particular significance for their application as lubricant additives because such hybrids usually provide many possibilities for further improvement of tribological performances through a synergistic effect among two or more components. So far, the tribological properties of SiO2/graphene combinations as water-based lubricant additives for magnesium alloy have rarely been described.

The primary motivation of this work is to provide effective SiO2/graphene combinations using as water-based lubricant additives for the development of magnesium alloy forming fluids. The tribological behaviors of the SiO2/graphene combinations used as water-based lubricating additives are examined using a reciprocating ball-on-plate tribometer for magnesium alloy/steel pairs. Additionally, the synergistic lubrication mechanism of the SiO2/graphene combinations used as water-based lubricant additives is assessed systematically. Further, the rolling lubricant properties of all lubricants were investigated in the cold rolling of magnesium alloy.

Section snippets

Materials

The graphene used in the present investigation was provided by Hengqiu Graphene technology Co., Ltd, Suzhou, China. The morphology of graphene was characterized using transmission electron microscopy (TEM, JEM 1200EX, Japan). As displayed in the representative TEM images (Fig. 1a), graphene is apparently transparent with the size of several micrometers and no micropores are observed on the sheet surface. Through high-resolution TEM (HRTEM) lattice imaging in Fig. 1b, the layered structure of

Influence of the SiO2/graphene mixing ratio on lubrication performance

To acquire the maximum lubrication effect from the SiO2/graphene combination additives, the choice of their optimal mixing ratio is critical. Thus, the tribological performances of the SiO2/graphene combination nanofluids with different ratios for magnesium alloy/steel contacts were examined under a load of 3 N at speed of 0.08 m/s for 0.5 h test duration. For comparison, the same friction tests were performed with pure water and individual nanofluids. The average friction coefficient and the

Conclusions

The tribological properties of the SiO2/graphene combinations in water for the AZ31 magnesium alloy/AISI 52100 bearing steel contacts and the rolling lubrication properties for AZ31 magnesium alloy were investigated in this work. The main conclusions are as follows:

  • (1)

    Compared with individual nanofluids, the graphene/SiO2 combination nanofluids presented remarkable friction-reducing and anti-wear behaviors and a high load-carrying capacity.

  • (2)

    The best tribological properties were at the ratio of SiO2

Acknowledgments

The authors are thankful for the financial support of Natural Science Foundation Project of Chongqing Science & Technology Commission (Grant No. cstc2018jcyjAX0760, cstc2016jcyjA0041) and Scientific and Technological Research Program of Fuling Science and Technology Commission (Grant No. FLKW2017ABA1013) and Research Founds for the Yangtze Normal University (Grant No. 2016XJQN31, 2017KYQD41).

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