Fretting Behavior of Cold-Sprayed Cu and Cu-MoS₂ Coatings

Cu and Cu-MoS₂ coatings were fabricated by cold gas dynamic spray and the fretting wear performance of the two coatings was compared. A mixture (95 wt.% Cu + 5 wt.% MoS₂) was used as feedstock for the composite coating. Coatings were sprayed with identical gas flow conditions on the substrates preheated to approximately 170°C. The cross section of the coatings was analyzed by scanning electron microscope (SEM) and MoS₂ concentration was measured, as well as coating microhardness. Fretting tests were carried out under gross slip conditions in ambient environment. SEM observation on wear scars and counterspheres revealed the development of third bodies, by which the sliding was accommodated. For the Cu-MoS₂ coating, solid lubrication effects in the form of friction drops occurred in early cycles (< 5k), but eventually (> 5k) the coating's friction behavior was similar to the pure Cu coating. Third body morphology and wear of the two coatings were distinctly different, which could largely be attributed to the hardness reduction of the Cu-MoS₂ composite due to poorly bonded interfaces induced by the effect of MoS₂ during particle impact and coating formation.