Phase composition and tribological performance of molybdenum nitride coatings synthesized by IBAD

Molybdenum nitride coatings were synthesized by ion-beam-assisted-deposition (IBAD). The effects of nitrogen partial pressure and bombarding ion energy on the phase formation were investigated. The coefficient of friction and wear resistance of the coatings in different environments were studied. The stoichiometric ratio of nitrogen to Mo of the molybdenum nitride phases increases with the nitrogen partial pressure, and higher bombarding ion energy was helpful to form single MoN phase. Mo2N, MoN or mixture of the two phases could be prepared by controlling of the ratio of nitrogen/argon and ion energy. The hardness, coefficient of friction, wear and oxidation resistance of MoN were superior to that of Mo2N coating. The coefficient of friction decreased with the increase of testing temperature, yet the wear resistance became lower. The change of friction in non-oxidation atmosphere was reversed to that in the open air, indicating that the low friction of MoN at elevated temperature depends on oxidation. The tribological performance of the MoO3 phase formed by oxidation annealing showed much higher coefficient of friction than that of as-deposited MoN in all the testing conditions, and the oxygen deficient oxides were implied to be the lubricious phases. (C) 2012 Elsevier B.V. All rights reserved.