Friction and wear behavior of nanocrystalline nickel in air and vacuum

A bulk and dense nanocrystalline (NC) Ni with a mean grain size of 26 nm and a microhardness of 370382 HV were fabricated by an electro-deposition technique. The microstructure and microhardness of the NC Ni samples were examined. The friction and wear behavior of the NC Ni samples under dry condition both in air and in vacuum were investigated in comparison with a conventional coarse-grained (CG) Ni with a mean grain size of 2.2 μm and a microhardness of 8090 HV using a pin-on-disk type tribometer. The results show that the NC Ni sample possesses lower friction and higher wear resistance both in air and in vacuum, compared with the CG Ni sample. An important and interesting result is that for the both samples tested, the friction coefficients in vacuum are higher than those in air, while the wear loss occurs reverse case. In the air atmosphere, the NC Ni exhibits a mild wear, and avoids the severe wear that happens on the CG Ni, which is mainly attributed to which is mainly attributed to formation of a mechanical mixed layer on the worn surface of the harder NC Ni. In the vacuum atmosphere, compared with CG Ni, NC Ni shows a significant decrease in the plastic deformation and adhesive wear.