The effect of expanded graphite with sodium metasilicate as lubricant at high temperature

Abstract

Lubrication is vital in enabling the key machinery contact components to operate durably and reliably at extremely high temperatures for space aviation and metal forming. In this work, the expanded graphite in sodium metasilicate solution as high temperature lubricant was evaluated using a ball-on-disc tribometer. The results suggest that the use of this mixture can remarkably reduce the friction and wear of steel counterparts by up to 85% and 60% at 850 °C, respectively, compared with unlubricated condition. These improvements result from the formed tribo-layer containing amorphous carbon and sodium carbonate melt, as well as the two different ordered crystal layers on the rubbing interface induced by high temperature, pressure and shear. The expanded graphite in sodium metasilicate solution studied here for the first time will extend its application for high temperature lubrication due to its excellent performance.

Introduction

Graphite is the most widely used solid lubricant due to the strong covalent bond within the layer and weak Van der Waals bonds between layers [1]. In humid and air environment, the lubricating performance of graphite was attributed to the beneficial effect of adsorbed moisture and gases that make it shear easily [2]. However, it loses the lubricating effectiveness at high temperatures, because of the desorption of water and possibly other gases (CO₂ in air) and oxidation [3]. Other graphite derivatives like graphite fluoride (CFx)n [4,5], graphite oxide [6], graphene [[7], [8], [9]], graphene oxide [10,11], expandable graphite and expanded graphite [12] are also potential lubricants.

Expandable graphite is produced by first treating graphite flakes with substances such as H₂O₂, KMnO₄ etc. that intercalate into the crystal structure of the graphite and then react to form compounds of graphite and the intercalant. And these expandable graphite can be transformed into expanded graphite through microwave irradiation, electrochemical method and heating [13]. Upon heating at a high temperature, the intercalants in the graphite crystal form gases, which causes the layers of the graphite to separate, and the graphite flakes to expand or exfoliate in an accordion-like fashion in the c-direction, perpendicular to the crystalline planes of the graphite [14]. Unlike other graphite derivatives, there has been limited publication reported on the tribological behaviour of expanded graphite.

Yang et al. [15] reported that the expanded graphite prepared by the microwave method greatly enhances the wear properties of nitrile-butadiene rubber in terms of friction and wear rate. Wang et al. [16] studied the effect of expanded graphite (EdG) dispersion on the tribological properties of the nitrile rubber/EdG composites, and found that EdG can decrease the friction and wear rate compared with the pure nitrile rubber. In other composites, such as NBR/CB composites, EdG could effectively decrease the friction and wear due to the presence of graphite lubricant film [17], and a better dispersion is beneficial to both the mechanical and tribological property for dry sliding. To our best knowledge, the lubricant potential of expanded graphite at high temperature has not been studied.

When the solid lubricant was used as a dry lubricating film in the fields of aeronautics, astronautics, weaponry, manufacturing, and so on, it was always mixed with a binder, which contains organic and inorganic binders [18,19]. Inorganic binders can improve significantly the thermal stability and high-temperature resistance of bonded solid lubricating coatings [18]. As one of the most common inorganic binders [6], silicate has particular advantages, such as environmental friendliness, low cost and strong adhesion to the substrate [20,21].

Here we combine the expanded graphite with sodium metasilicate as a lubricant at high temperature to exploit the lubricating properties of graphite derivatives, overcome their current restriction due to oxidation and ineffective lubrication at high temperature, and broaden their applications in tribology.