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.