title: Macroscale Superlubricity of Multilayer Polyethylenimine/Graphene Oxide Coatings in Different Gas Environments.
abstract: Friction and wear decrease the efficiency and lifetimes of mechanical devices. Solving this problem will potentially lead to a significant reduction in global energy consumption. We show that multilayer polyethylenimine/graphene oxide thin films, prepared via a highly scalable layer-by-layer (LbL) deposition technique, can be used as solid lubricants. The tribological properties are investigated in air, under vacuum, in hydrogen, and in nitrogen gas environments. In all cases the coefficient of friction (COF) significantly decreased after application of the coating, and the wear life was enhanced by increasing the film thickness. The COF was lower in dry environments than in more humid environments, in contrast to traditional graphite and diamond-like carbon films. Superlubricity (COF < 0.01) was achieved for the thickest films in dry N2. Microstructural analysis of the wear debris revealed that carbon nanoparticles were formed exclusively in dry conditions (i.e., N2, vacuum), and it is postulated that these act as rolling asperities, decreasing the contact area and the COF. Density functional theory (DFT) simulations were performed on graphene oxide sheets under pressure, showing that strong hydrogen bonding occurs in the presence of intercalated water molecules compared with weak repulsion in the absence of water. It is suggested that this mechanism prevents the separation graphene oxide layers and subsequent formation of nanostructures in humid conditions.
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Prabakaran
authors.initials: P
authors.lastname: Saravanan
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Roman
authors.initials: R
authors.lastname: Selyanchyn
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.|Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Hiroyoshi
authors.initials: H
authors.lastname: Tanaka
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Durgesh
authors.initials: D
authors.lastname: Darekar
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Aleksandar
authors.initials: A
authors.lastname: Staykov
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.|Center for Molecular Systems (CMS), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Shigenori
authors.initials: S
authors.lastname: Fujikawa
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.|Department of Mechanical Engineering, University of Sheffield , Sheffield S10 2TN, U.K.
authors.forename: Stephen Matthew
authors.initials: SM
authors.lastname: Lyth
authors.affiliation: WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University , Fukuoka 819-0395, Japan.|Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Joichi
authors.initials: J
authors.lastname: Sugimura
keywords: DFT; friction; graphene oxide; layer-by-layer; nanoparticles; polyethylenimine; solid lubricant; superlubricity; wear