Microstructure control of cold-sprayed pure iron coatings formed using mechanically milled powder

Nanocrystalline metallic materials with grain sizes of less than 100 nm exhibit high strength and unique characteristics, such as high levels of ductility and corrosion resistance. Mechanical milling techniques can be used to produce large amounts of nanocrystalline metallic powder. However, it is essential to perform a sintering treatment, which induces coarsening of the crystal grains, to form a bulk material from the mechanically milled powder. The cold spray technique is expected to effectively form a nanocrystalline metallic coating using the mechanically milled powder. This study focuses on pure iron coating, which is formed by a cold spray technique using mechanically milled pure iron powder. Pure iron powder, with crystal grains of ~100 nm, was obtained by a mechanical milling process, and was deposited onto a low-carbon steel substrate by cold spraying. It was confirmed that a dense, nanocrystalline coating with a hardness of over 300 HV can be achieved; however, the deposition efficiency of this coating was half of that of the coating formed using the as-received powder. The improvement of the deposition efficiency and properties of such coatings was attempted using powder mixtures comprising both milled and as-received powders. The results indicate that the deposition efficiency and microstructure, directly linked to the mechanical properties of the coating, can be controlled by varying the content of the as-received powder of the mixture.