In-Situ Agglomeration and De-agglomeration by Milling of Nano-Engineered Lubricant Particulate Composites for Cold Spray
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M. Neshastehriz, I. Smid, and A.E. Segall
(Submitted February 28, 2014; in revised form May 30, 2014) Nano-engineered self-lubricating particles comprised of hexagonal-boron-nitride powder (hBN) encapsulated in nickel have been developed for cold spray coating of aluminum components. The nickel encapsulant consists of several nano-sized layers, which are deposited on the hBN particles by electroless plating. In the cold spray deposition, the nickel becomes the matrix in which hBN acts as the lubricant. The coating demonstrated a very promising performance by reducing the coefficient of friction by almost 50% and increasing the wear resistance more than tenfold. The coatings also exhibited higher bond strength, which was directly related to the hardenability of the particles. During the encapsulation process, the hBN particles agglomerate and form large clusters. De-agglomeration has been studied through low- and high-energy ball milling to create more uniform and consistent particle sizes and to improve the cold spray deposition efficiency. The unmilled and milled particles were characterized with Scanning Electron Microscopy, Energy-Dispersive X-Ray Spectroscopy, BET, and hardness tests. It was found that in low-energy ball milling, the clusters were compacted to a noticeable extent. However, the high-energy ball milling resulted in breakup of agglomerations and destroyed the nickel encapsulant.
Keywords
agglomeration, aluminum substrate, ball milling, cold spray, electroless nickel plating, hardness testing, hexagonal-boron nitride, nano-engineered coating materials, nickel
1. Introduction Different coatings can be used to improve the wear resistance of mating surfaces of metallic components, as well as to increase the components durability. Many conventional techniques exist to apply the coating onto surfaces. Undesirably, high temperatures in most conventional coating processes can lead to melting, evaporation, and oxidation of the coating and substrate. The Cold Spray (Ref 1–9) method can be used in many situations, where conventional coating techniques are not applicable because of material degradation at coating temperatures. In the cold spray process, particles are accelerated in a gas stream to a critical velocity, by which they deform plastically after impacting the substrate and form a strong bond at the interface. In this process, the particles and the substrate undergo no or minimal microstructural changes due to the slightly raised temperatures (Ref 10, 11). Deformation and bonding are induced by the high impact M. Neshastehriz, I. Smid, and A.E. Segall, The Pennsylvania State University, State College, PA. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
velocity of up to 1000 m/s. Therefore, ductile particles, composites, polymers, and metals such as magnesium, aluminum, copper, and nickel can be deposited on substrates via this method without losing their beneficial properties (Ref 12). In a previous study, nano-engineered self-lubricating composite particles composed of micro
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