Microstructure of Coatings on Nickel and Steel Platelets Obtained by Co-Milling with NiAl and CrB2 Powders

Abstract

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 CrB2 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.

Deposition of NiAl/Al3Ni2(CrB2) Coatings from Ni, Al and CrB2 Powders Using Mechanical Synthesis in Planetary Ball Mill

“…This rapidly progressing process was soon after that slowed down and stopped. This phenomenon was caused by changes taking place during ball milling of the powder, e.g., refined particles tended to form unyielding spherical agglomerates which pressed into the coatings that were being built and which were easily cracked and chipped off [17]. Therefore, it was practically impossible to cover the substrate with more than several tens of micrometers on average of the NiAl intermetallic or composite with such a matrix.…”

Effect of Heating on the Microstructure of NiAl + CrB2 Coatings Deposited by Mechanical Embedding in a Ball Mill

“…The newly developed method, relying on the mechanical embedding (ME) of NiAl and CrB 2 powders into the metallic substrates during their co-milling, seems to have great potential. It secures the formation of a uniform nano-crystalline microstructure even if the porosity remains at the level characteristic for the PS ones (Szlezynger et al, 2019).…”

“…Eventually, the amount of removed material approaches the level of that added in other places, which practically stops the deposition process. The obtained coatings are of several tens of micrometers thick at most (Szlezynger et al, 2019). Therefore, to resume the building up of coatings with ME, its significant softening by applying precisely controlled heat treatment is needed.…”