Microstructure of Coatings on Nickel and Steel Platelets Obtained by Co-Milling with NiAl and CrB(2) Powders

Metal matrix composite coatings are developed to protect parts made from materials susceptible to wear, like nickel alloys or stainless steel. The industry-established deposition method is presently an atmospheric plasma spraying method since it allows the production of both well-adhering and thick coatings. Alternatively, similar coatings could be produced by co-milling of ceramic and alloyed powders together with metallic plates serving as substrates. It results in mechanical embedding of the powder particles into exposed metallic surfaces required coatings. The present experiment was aimed at the analysis of microstructure of such coatings obtained using NiAl and CrB(2) powders. They were loaded together with nickel and stainless steel platelets into ball mill vials and rotated at 350 rpm for up to 32 h. This helped to produce coatings of a thickness up to ~40 µm. The optical, scanning, and transmission electron microscopy observations of the coatings led to conclusion that the higher the rotation speed of vials, the wider the intermixing zone between the coating and the substrate. Simultaneously, it was established that the total thickness of the coating deposited at specified conditions is limited by the brittleness of its nanocrystalline matrix. An increase in the hardness of the substrate results in a decrease of the intermixing zone. The above results indicate that even as the method based on mechanical embedding could so far produce thinner coatings than the plasma spraying, in the former case they are characterized by a more uniform nanocrystalline matrix with homogenously distributed fine ceramic particles.