Ternary low-friction coatings on thermoplastics by plasma spraying: Investigations on the process-structure

Thermally sprayed MoS₂/graphite/Zn coatings were developed by the atmospheric pressure plasma jet technique to investigate the process-structure-relationship of the low-friction coatings. The fine-tuning of the powder carrier gas flow to the plasma jet was supported by computational fluid dynamics and revealed the impact on the powder particles within the coating process i.e. ideal powder particle temperature for transferring the solid Zn powder into the liquid state as main requirement for a successful embedding process of the dry lubricants. MoS₂/graphite/Zn composite coatings were deposited on polyamide 4.6 (PA4.6) substrates to evaluate the adhesion/cohesion properties and tribological performance by tuning the current of the atmospheric pressure plasma jet (APPJ) from 125 A to 150 A. In particular, the plasma spraying distribution of the composite feedstock and the resulting coating architecture are strongly dependent on the plasma process setting, which are investigated by scanning electron microscopy and transmission electron microscopy in combination with an energy-dispersive x-ray spectrometer. Tribological characterisation indicates that coating composition and thickness influence the coating performance significantly. According to the 125 A plasma current setting, the low frictional compounds MoS₂ and graphite are embedded in a Zn matrix in contrast to plasma current settings of 150 A, where Zn is embedded in a MoS₂ and graphite matrix and demonstrates excellent low frictional properties.