Microstructure of NiAl + 15 wt.% CrB₂ nanocrystalline composite coatings obtained through co-milling of NiAl and CrB₂ powders

Maciej Szlezynger, Jerzy Morgiel, Łukasz Rogal, Olena Poliarus, Paweł Kurtyka

Quarterly No. 3, 2018 pages 149-155

DOI:

keywords: NiAl, NiAl CrB₂ composite coatings, mechanical alloying, TEM

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abstract The development of a new deposition method allowing to obtain thick composite coatings is of both scientifically and practical importance. The one presented in this paper is based on a negative side effect taking place during the mechanical synthesis of alloys, i.e. sticking of milled material to the surfaces of both the vial and balls. The experiment covered the comilling of NiAl (~45 μm) with 15 wt.% CrB₂ (~40 μm) powders together with nickel platelets used as the substrates and steel balls. The above processing performed at 200 rpm resulted in a steady increase in the thickness of the rubbed-in buildup on the platelet surfaces allowing coatings of 4, 11, 22 and 33 μm to be produced after 4, 8, 16 and 32 hours. The OM, XRD and TEM investigations showed that such coatings are characterized by a gradient microstructure with heavily dislocated but coarser grains near the substrate and a more porous inner part formed with rounded well fused agglomerates of greatly refined crystallites. The CrB₂ were only slightly larger than the NiAl ones and were distributed quite uniformly. Most of the coating was found well fused with the substrate, but occasional voids and porosity at the substrate/coating interface were also noted. It is worth noting that applying the proposed method allowed the authors to produce a thick, gradient and mostly nano-crystalline NiAl and CrB₂ composite coating.