Structure and properties of Cr x N coating

In the present study, an attempt has been made to produce a coating of mixed nitride (CrN and Cr₂N) of ultrafine size on a low alloy high carbon steel by a two-stage hybrid process comprising electrodeposition of Cr (45–85 μm) followed by gas nitriding in controlled atmosphere (using a gas mixture of NH₃/Ar = 1∶3) in the temperature range 750–950°C for 2–6 h. The nitrided surface records extremely high hardness (∼19 GPa), enhanced wear resistance and residual compressive stress. Potentiodynamic polarisation test in a 3·56 wt-% NaCl solution shows an improvement in corrosion resistance due to formation of noble nitride. However, presence of brittle nitride phases and related surface roughness seems to deteriorate both tribological and corrosion properties.