Effect of Thermochemical Treatment on The Structure, Hardness and Wear Behavior of Electrolss Ni-P Coated 4140 Steel

In the present stady, thermochemical treatment in H2/NH3 atmosphere was used as a post-treatment for electroless Ni-P coatings on the AISI 4140 steel substrates. High phosphorus (9%) coatings with thicknesses of 2, 24 and 48 m were applied and the effects of the thermochemical treatment on the morphology, structural changes, roughness, hardness and wear resistance of coatings were studied by SEM, EDS, XRD, profilometry, and microhardness tester. Wear test was used to evaluate wear characteristics of coatings. The wear behaviour of the thermochemical treated/Ni-P coated samples was assessed by comparison with thermochemical treated/uncoated (nitrided) samples. The results showed that effect of thermochemical treatment varies with the coating thickness. In addition, it was shown that a multicomponent coating containing phosphide, nitride and intermetallic phases as well as diffusion region can be developed in the thin (2 m) electroless Ni-P coated steel by thermochemical treatment. This sample showed better wear resistance than 24 m Ni-P coated steel under higher load. This behavior was ascribedto nitride phases formed at the surface as well as a nitrogen diffusion zone at the subsurface of thin Ni-P coated steel