In-situ formation of Ni-Al intermetallics-coated graphite/Al composite in a cold-sprayed coating and its high temperature tribological behaviors

In this study, cold spraying (CS) was used to deposit a mixture of nickel-coated graphite and 40 vol.% Al powder (Ni-Gr/Al) on a steel substrate aiming to effectively preserve a certain volume fraction of graphite in the deposited Ni-Gr/Al composite coating. The microstructure of the as-sprayed coating and the effect of post-spray heat-treatment (PSHT) temperatures on the in-situ formation of Ni-Al intermetallic phases in coating were studied. The tribological behaviors of the as-sprayed coating and the PSHTed coating under 450 °C were tested at 25 °C, while the as-sprayed coating was tested at 450 °C for comparison. As a result, the Ni-Gr particles showed a homogenous distribution in the coating. The multilayer Ni-Al intermetallics-coated graphite/Al composite coating was achieved in situ after the PSHT of 450 °C, where the graphite did decompose at 550 °C leaving big pores in the coating. The coefficients of friction (COF) of the CSed coating and the PSHTed coating were measured at 450 °C as well as 25 °C, which showed a similar tendency, much higher than that of the CSed coating tested at 25 °C. The lubrication phase (graphite) improved the formation of a graphite film during sliding friction and decreased the COF, while the hard Ni-Al intermetallic phases contributed to the increase of COF.