Suspension High Velocity Oxy Fuel (SHVOF) spray of graphene nanoplatelets for low-friction wear protective films

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, in fact, they have found successful use in forming nanocomposites, improving fracture toughness, providing a wear-protective tribofilm, or acting as solid lubricant at the contacting interface. Among various deposition techniques, suspension high velocity oxy-fuel spray (SHVOF) is characterised by a high-velocity jet and a low temperature compared to plasma, which is beneficial for heat sensitive materials like GNPs. However, even at moderate flame power axial injection would expose GNPs to excessive heat throughout the combustion chamber, partially deteriorating them. In this work, we explore radial injection of GNPs as a suspension in HVOF as a means of reducing the extent of interaction between the feedstock and the heat of the flame, while still allowing a high momentum transfer. Feedstock injection parameters, such as injection angle and position were explored. Raman spectroscopy indicated a low amount of GNPs deterioration and ball-on-flat wear tests showed a reduction of the friction coefficient of 70% for 500 cycles. The deposited layer of well-preserved GNPs protects the underlying substrate by allowing low-friction dry sliding.