Abstract
Graphene has become a promising candidate to protect surfaces against friction due to its strength and lubricating ability. In this study, graphene nano platelets (GNP) thin films have been deposited onto stainless steel substrates by axially injecting GNP suspension through high velocity oxy fuel thermal spray gun. The tribological performance of the films under dry sliding wear was investigated through unlubricated ball on disc sliding wear test against a sintered alumina counter body ball under 5 N load. The understanding of the behaviour of the GNPs under sliding wear will be useful for improving the performance of graphene-based coatings which are in demand for wear resistant applications. A film was deposited showing significant improvements in friction with coefficient of friction value reduced by 7 times compared to uncoated stainless steel, even for a discontinuous film. A morphological analysis shows sliding wear led to change in particle shape from angular flakes into randomly oriented circles. Interatomic bonding and structural analysis performed reveals oxidation defect formations during wear test. Structural degradation and oxidation of GNPs during the process led to formation of amorphous carbon from graphene. Amorphous carbon formation reduces the lubricating ability and strength of the film, leading to failure.
Highlights
Graphene is an allotrope of carbon characterized by a 2D network of sp2 hybridised carbon atoms bonded to each other in a hexagonal lattice
Studies performed on elastic properties and intrinsic strength of pristine graphene via atomistic simulations and atomic force microscopy (AFM) showed that the in plane Young’s modulus is ≈ 1 TPa and the tensile strength value is higher than 100 GPa [2,3,4]
A study performed on chemical vapour deposition (CVD) grown graphene shows that the elastic stiffness is the same as pristine graphene and strength is only reduced by approximately 20 GPa [5]
Summary
Graphene is an allotrope of carbon characterized by a 2D network of sp hybridised carbon atoms bonded to each other in a hexagonal lattice. According to the first study, the main reason behind the improved wear performance is the improved localised mechanical properties and fracture toughness rather than a tribofilm formation Silicon nitride is another ceramic which was investigated in both lubricated (isooctane) and unlubricated conditions. There was not any mechanical or chemical bonding between the graphene and the applied surface, results showed promising improvements in coefficient of friction (down to around 0.2) and wear performance [35,36]. HVOF thermal spray technique is widely used in industry, as it leads to the production of coatings with lower porosity, higher bonding strength and hardness in shorter period of time It is a fast and effective method to deposit GNPs over extended surfaces. Once all the mechanisms responsible for failure and the main reason for improved tribological performance are understood, GNPs can be engineered to improve the service life or performance of GNP films and GNP incorporated composites
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