Role of the nitrogen ratio on mechanical properties and wear resistance of CrN/Fe functionally graded coating produced by double glow plasma alloying

The hard ceramic coating under the external load tends to show cracking and spallation due to the mismatch of deformation between coating and substrate. To moderate the property difference, a CrN/Fe gradient coating was fabricated by double glow plasma alloying at different Ar/N-2 ratios. The thermodynamically favorable reaction between Cr and N ensured the spatially gradient structure of CrN and (Fe, Cr) phase without Fe nitrides. The outmost CrN phase possessed over-stoichiometry of N atoms, leading to the N1s spectrum shifted to the lower binding energy. The increased N-2 ratio made the nitride content increase from 53.4, 73.6 to 74.0 %. Various volume gradients of nitride/metal contributed to different mechanical behavior of coatings, such as the enhanced hardness (16.3 to 21.5 GPa) and Young's Modulus. Two indexes, H/E and H-3/E-2, showed an increasing trend with the increasing N-2, which meant better toughness and adequate elastic abilities to suppress radial cracks. Compressive residual stress in these coating ranging from -1.06, -1.16 to -1.44 GPa, also restrained the radial cracks and accelerated the formation of circumferential cracks. Introducing nitride-metal transitional structure lowered the probability of cracks and spallation in the wear test.