Graded and Single-Layer NiAl/WC-Co Coatings Processed by Plasma Transferred Arc Directed Energy Deposition

Processing NiAl aluminides coatings using in situ synthesis by plasma transferred arc directed energy deposition (PTA-DED), although allowing to process sound coatings, causes a large dilution with steel substrates. However, when processing multilayer coatings, the interaction with substrate is progressively reduced as layers are deposited, having important impacts in the microstructure and mechanical properties. In this study, Ni and Al elemental powder mixtures, without and with 10 and 30 wt.% WC-Co additions, were deposited using PTA-DED, in order to obtain single and multilayer coatings. The effects of the austenitic steel substrate are mitigated by the deposition of a multilayers, which causes the NiAl/WC-Co coating properties to predominate and be compared to single-layer coatings. Results show that changes in hardness and wear performance of the coatings can be accounted for by competing phenomena: the hardening effects of iron in solid solution in the β-NiAl phase; the amount of hard tungsten carbides and the volume fraction of the ductile austenitic γ-FeNi phase. Multilayers graded coatings were successfully obtained with smother composition gradient, favoring the distribution of carbides and a reduced amount of γ-FeNi phase, enhancing hardness in over 19% and wear rate in over 85%, compared to the corresponding single-layer coatings.