Investigations on Graphene Platelets as Dry Lubricant and as Grease Additive for Sliding Contacts and Rolling Bearing Application

Abstract

In recent years, graphene-based lubrication was in the focus of nano- and microtribological studies. While the sliding properties of graphene based dry lubrication were previously investigated on the nano- and micro-scale, few studies can be found in the literature for the application of graphene as an additive to oil and grease in rolling contacts. In order to apply graphene platelets as dry lubricants and as grease additives in machine elements, tests were carried out on a rolling bearing test rig under typical load conditions. For these investigations, multilayer graphene platelets of varied staple thickness were functionalized on angular contact ball bearing surfaces as a dry lubricant, which forms a thin film. In addition, bearings were lubricated with grease containing graphene platelets. In this case, a small ratio of graphene was dispersed with grease. The graphene platelets were divided into three groups of different thickness: 2 nm, 6–8 nm, and 11–15 nm. Additionally, the tests were compared to graphite nanoparticles (spheres with a size of 3–4 nm) as dry lubricant and graphite-containing grease. The experimental studies were carried out under oscillating motion. The respective load in the tribological contact was 1.5 GPa. During the tests, the pivoting angle was measured by utilizing a rotary encoder. In addition, the friction torque was recorded under a frequency of 0.2 Hz. As the balls’ velocity at the reversal point is zero, the lubrication conditions are critical. The dry lubricated bearings were compared to grease lubricated bearings. Additionally, the frictional properties of the respective greases were investigated by applying a sliding tribometer. In this case, a ball rotates against three contact planes, which causes a tribological contact under a contact pressure of 1 GPa. It was shown that applying graphene as a dry lubricant and as a grease additive under rolling contact conditions reduces friction significantly.

Layered 2D Nanomaterials to Tailor Friction and Wear in Machine Elements—A Review

“…Numerous studies have assessed the ability of 2D materials to improve friction, wear, and service life in rolling element bearings, [137] such as axial [29,[138][139][140] and/or radial bearings [141][142][143][144][145][146] as well as with [29,[141][142][143][144][145][146] and without retainer elements or cages. [138][139][140]147] The availability of fully developed industrial-scale PVD processes for deposition of the MoS 2 coatings resulted in MoS 2 being the most studied material among others.…”

“…[138][139][140]147] The availability of fully developed industrial-scale PVD processes for deposition of the MoS 2 coatings resulted in MoS 2 being the most studied material among others. [138][139][140][141][142][143][144][145] Yet, some application-oriented studies on WS 2 , [148] graphene, [146] and MXenes [29] have been performed as well. Apart from coatings for the highly loaded primary contacts between rolling elements and the raceways of bearing rings/washers, [29,[138][139][140][141][142][143]146] some studies addressed secondary contacts with lower loads but higher slip and sliding velocities such as the contact between the rolling elements and cage pockets with 2D nanomaterials as inserts or reinforcement phases of composites.…”

“…[138][139][140][141][142][143][144][145] Yet, some application-oriented studies on WS 2 , [148] graphene, [146] and MXenes [29] have been performed as well. Apart from coatings for the highly loaded primary contacts between rolling elements and the raceways of bearing rings/washers, [29,[138][139][140][141][142][143]146] some studies addressed secondary contacts with lower loads but higher slip and sliding velocities such as the contact between the rolling elements and cage pockets with 2D nanomaterials as inserts or reinforcement phases of composites. [141,144,145,147,149] Depending on the targeted application, the evaluated testing conditions ranged from atmospheric [29,[138][139][140]146,147] to vacuum [140][141][142][143][144]149] and nitrogen environments.…”

Employment of Micro- and Nano-WS2 Structures to Enhance the Tribological Properties of Copper Matrix Composites

“…Guo et al [41] have performed wear tests in a four-ball assembly on a polyalphaolefin-2 (PAO-2) oil enriched with multi-layered graphene, measuring a 78%-reduction of the friction coefficient and a beneficial reduction of wear rate. Pape et al [42] have employed 1 wt % of graphene platelets with different thicknesses (2, 6-8, 11-15 nm) as dry lubricants and as additives in greases for rolling contacts operating under reciprocating motion. They have concluded that the excellent wear resistance and friction coefficient reduction are associated to the platelets covering the surface asperities and generating favorable sliding planes, although thickness seemed to not influence the performances.…”

Delamination of multilayer graphene stacks from its substrate through wrinkle formation under high pressures

“…Combination of mechanical and wetting methods could then ap-pear as possible new paths for van der Waals 2D systems detachment and transfer to other substrates [69]. Our results may also contribute to better understand the response of graphene based coatings in different environments submitted to mechanical constraints as in their use in tribological contacts [70][71][72].…”