Effects of Mo content on the grain size, hardness and anti-wear performance of electrodeposited nanocrystalline and amorphous Ni-Mo alloys

Ni-Mo alloys with various Mo contents are prepared by electrodeposition in sulfate-citrate acid solutions under different temperature, pH, stirring rate, current density and MoO42- concentration. Influences of Mo content on the grain size, hardness and wear resistance are studied. Results show that the increase in Mo content decreases the deposition rate of Ni-Mo alloys prepared at a constant temperature. As-prepared alloys are composed of nanocrystalline Ni(Mo) solid solution when Mo is less than 24.4 at.%. The grain size of alloys decreases with the increment of Mo content. Amorphous phase appears in deposits when Mo content exceeds 24.4-27.3 at.%. Hardness and wear resistance of alloys firstly increases with Mo content, and then it has a decline due to the formation of amorphous. Additionally, Hall-Petch (HP) relationship is found in deposits as the grain size decreases from 12.4 nm to 7.9 nm, and then the inverse HP relationship appears with the further decrease in grain size to 1.5 nm. Meanwhile, traditional Archard law is found in Ni-Mo alloys with Mo content in the range of 6.3-27.3 at.%. Additionally, HP breakdown causes a variation of wear resistance at the critical grain size of 7.9 nm.