Solid/Self-Lubrication Mechanisms of an Additively Manufactured Ni–Ti–C Metal Matrix Composite

A Ni–TiC–C metal matrix composite (MMC) has been processed using laser engineered net shaping. This additive manufacturing technique produced a Ni–3Ti–20C (at.%) composition with in situ formation of homogeneously distributed eutectic and primary TiC and graphite precipitates throughout the Ni matrix. The MMC exhibits a low steady-state friction coefficient of ~0.1 in dry sliding conditions. Novel insights into solid/self-lubrication mechanisms, with implications to other metal matrix-graphite particle composites, were revealed with focused ion beam serial sectioning inside worn surfaces. It was determined that lowering of the friction coefficient was a result of the formation of a low interfacial shear strength disordered carbon tribofilm that extruded to the surface through refined Ni grain boundaries. This was supported by finite element analysis that showed the evolution of subsurface stress states and precipitate motion during repeated sliding frictional contact.