Radial Injection in Suspension High Velocity Oxy-Fuel (S-HVOF) Thermal Spray of Graphene Nanoplatelets for Tribology

Abstract

Friction is a major issue in energy efficiency of any apparatus composed of moving mechanical parts, affecting durability and reliability. Graphene nanoplatelets (GNPs) are good candidates for reducing friction and wear, and suspension high velocity oxy-fuel (SHVOF) thermal spray is a promising technique for their scalable and fast deposition, but it can expose them to excessive heat. In this work, we explore radial injection of GNPs in SHVOF thermal spray as a means of reducing their interaction with the hot flame, while still allowing a high momentum transfer and effective deposition. Feedstock injection parameters, such as flowrate, injection angle and position, were studied using high-speed imaging and particles temperature and velocity monitoring at different flame powers using Accuraspray 4.0.Unlubricated ball-on-flat sliding wear tests against an alumina counterbody ball showed a friction coefficient reduction up to a factor 10 compared to the bare substrate, down to 0.07. The deposited layer of GNPs protects the underlying substrate by allowing low-friction dry sliding. A Transmission Electron Microscopy study showed GNPs preserved crystallinity after spray, and became amorphised and wrinkled upon wear. This study focused on GNPs but is relevant to other heat-and oxidation-sensitive materials such as polymers, nitrides and 2D materials. The unprecedented mechanical [1] and tribological [2] properties of graphene have attracted a wide range of interests in using it as a solid lubricant [3]. The lubricating effect favors a lowering of the coefficient of friction and a delay in damaging the lubricated surfaces. This can effectively improve the durability of moving mechanical parts, by reducing localized heating and subsequent wear. Some small-scale deposition techniques [4, 5] can be hardly employed for covering large areas with a considerable amount of graphene. Simple techniques like drop casting and airbrush spray [6] would allow the deposition;however, they do not provide a good bonding with the substrate. Conversely, spray techniques such as supersonic cold spray [7] proved suitable for large scale graphene coverage and enhanced the bonding with the substrate due to the high velocity at impact. An even better graphene-substrate adhesion could be reached with thermal spray, as it gives not only high kinetic energy but also a higher amount of heat to the particles. In particular, Suspension High Velocity Oxy Fuel (S-HVOF) thermal spray [8] is a good candidate for this task. This relatively new technique allows the injection and spray of suspension in HVOF instead of powders, thus allowing to handle finer particles, down to the micro-to-nano scale. Overall, this technique provides high acceleration and has a relatively low flame temperature compared to plasma spray. Graphene is known to be stable at high temperatures in an inert environment, however in air it starts degrading at around 250 °C, and a consistent mass loss can occur at around 500 °C, according to thermogravimetric analyses (TGA) [9]. The use ...

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