Synergistic effects of 3D porous graphene and T161 as hybrid lubricant additives on 316 ASS surface

Highlights

• The stability of 3D porous graphene and T161 in PAO 6 base oil can be maintained for more than 3 months.

• The reduction in FC of about 4.0 %, 26.1 % and 73.1 % and the reduction in WR of about 28.9 %, 65.6 % and 97.8 % respectively under 5 N, 50 N and 100 N.

• Graphene tribofilm is evenly distributed on 316 ASS (Austenitic stainless steel) surface.

• The tribological mechanism of 3D porous graphene and T161 in sliding-induced lubrication system is revealed.

Abstract

An effective and simple method is reported to enhance the property and economics of graphene lubricants, that is, 3D porous graphene and T161 are proposed as hybrid lubricant additives. Compared with PAO 6, 3D PG-T161 can reduce the friction coefficient of 316 ASS (Austenitic stainless steel) by about 4.0%, 26.1% and 73.1% respectively under the normal load of 5 N, 50 N and 100 N, and the wear rate is reduced by about 28.9%, 65.6% and 97.8% respectively. The excellent tribological performance is attributed to the formation of uniform protective lubricating film. These findings are of great significance for enhancing the tribological properties of ASS and expanding its application range.

Introduction

ASS (Austenitic stainless steel) is widely used in various industrial parts and some specific environment fields such as cylinder liner, the selective laser melting process and the prototype fast breeder reactor (PFBR) due to its great corrosion resistance, excellent oxidation resistance and low cycle fatigue performance [1], [2], [3]. These contact components are often accompanied by sliding wear during operation and maintenance. However, the high friction coefficient and low hardness of ASS compromise their tribological properties, which will also bring challenges to the sheet metal forming and processing in tribologically difficult operations [4]. Usually these components need to be used with lubricants to avoid galling. Compared with coatings, the presence of lubricants can impede wear debris from scratching the substrate and eliminate the heat generated in the friction process [5]. Currently, in order to solve the environmental pollution problem of traditional lubricant additives, researchers have investigated different technologies to explore new additives to replace environmentally harmful additives (ZDDP), including sulfate, sulfur and phosphorus [6], [7].

Graphene shows excellent tribological properties owing to the tightly packed 2D (two-dimensional) laminated structure, high specific surface area and inherent mechanical properties [8], [9]. In the recent past, several reports have demonstrated the potential of 2D graphene as a lubricant additive, in which the characteristics of lamellar structure and surface adherence enable it to easily enter the friction interface, and properties of high strength and toughness as well as excellent thermal properties are conducive to the enhancement of the tribological performance of the lubricants [10], [11]. However, the high price and poor dispersibility of graphene limit the commercial development of graphene lubricants [12], [13]. Interestingly Rasheed et al. [14] reported that very few layers graphene displayed the disadvantages of wrinkling, fracture and poor bending stiffness, while graphene with more layers showed better tribological behaviors. Zheng et al. [15] utilized the molecular dynamics simulations to confirm that the number of graphene folds had a marked impact on the mechanical performance and the strain and compressive strength of folded graphene were much higher than planar graphene. In addition, it has been demonstrated that the graphene with more defects are easier to be dissolved in oil [16]. Consequently, from some certain perspectives, the few-layered graphene with high price may not be the best choice as a lubricant additive.

To solve the problem of graphene dispersibility in oil, researchers tried to add some surfactants or dispersants such as Span 80, but the effect was not satisfactory [17]. Besides, in most studies, the influence of the dispersant itself on the tribological properties was often ignored [18], [19]. Moreover, chemical functionalization is considered as an effective approach. For instance, Mungse et al. [20] used amide bonds to load ODA molecules on acylated rGO to obtain ODA-rGO tablets showing an excellent dispersibility. Kumari et al. [21] reported that the octadecyltriethoxysilane-functionalized h-BNNPs (h-BNNPs-ODTES) remained stable after being placed in pentaerythritol tetraoleate (polyol ester) for 10 days. The main mechanism is that graphene is modified on the edge or surface with polar groups to remove hydroxyl, carboxyl or carbonyl groups, while non-polar group (long alkyl chains) extends into the oil as lipophilic group [20], [21], [22]. The chain length of grafted molecules has a significant impact on dispersibility and the dispersibility of alkylated graphene increases with the increase of the chain length of alkyl grafted. This can be explained by the fact that the longer alkyl chains are easier to fully stretch in the medium to form a steric hindrance layer, which prevents particles from agglomerating and gravitational precipitation [22]. Furthermore, some additives with polar groups and long-chain structures can adsorb or react chemically to form dense molecular layers or thick reactive viscosity layers on polar metal surfaces, thereby reducing friction and wear [23], [24], [25]. For example, some ionic liquids (ILs) can strongly adsorb on the metal surface to form effective protective tribofilms on the sliding surface due to their inherent polarity, and thereby greatly reducing the wear and friction on the steel surface. Investigative results revealed that 1.5% urea-based functionalized imidazolium-organophosphate ionic liquid could significantly reduce the dimension of the wear spot [23]. The addition of 0.8 wt% ricinoleate anion based ionic liquid can effectively reduce the ball wear scar diameter (17–25%) and enhance the load-carrying capacity of base oil [24]. Although diverse different additives sometimes produce antagonistic effects, some ILs and nanoparticles (NPs) exhibit interesting positive synergies as additives of lubricants [26], [27], [28]. For example, N12 ILs and Mo NPs as additives exhibited an excellent anti-wear property [26]. Nasser et al. [27] proved that h-BN and phosphonium ionic liquids as additives for PAO 32 could reduce friction and wear significantly.

In this work, we chose 3D porous graphene and high molecular weight polyisobutylene succinimide (T161) as hybrid additives for PAO 6 base oil. 3D porous graphene is composed of a number of graphene nanosheets, which exhibits superior compressibility and resilience. Its price is also much lower than that of 2D graphene [29], [30], [31]. Excellent electrical conductivity and porous structure are conductive to ion diffusion and electron transport. As an excellent candidate for electrode materials, 3D porous graphene has been extensively investigated in Li-ion batteries (LIBs) and supercapacitors, but its application in tribology remains largely unexplored [29], [30], [31]. Similar in structure to some ILs, high molecular weight polyisobutylene succinimide is a high molecular weight long-chain linear polymer composed of polar end (polyvinyl polyamine block) and non-polar end (polyisobutylene block). Wu et al. [32] have tried to use it as a dispersant of MoS₂ nanoparticles in liquid paraffin, but its own contribution to tribological performance still needs to be further investigated.

Herein, we aim to comprehensively and deeply investigate the influence of 3D porous graphene and T161 (High molecular weight polyisobutylene succinimide) as PAO 6 additives on the tribological properties of 316 ASS. Furthermore, the detailed tribological mechanism was also discussed. This work can expand the use of 3D porous graphene in tribology and provide a new opportunity for the production of economic and environmentally friendly fully formulated lubricating oil.