An insight into formation of nanostructured coatings on metallic substrates by planetary ball milling

• Mechanical coating of Al by Ni metal and SiC ceramic powders was investigated.
• Two positions for the substrate: top and bottom of the vial were tested and compared.
• Top position for the substrate exhibits significantly better coating characteristics.
• Different deposition mechanisms for the two positions were proposed.
• Contrary to SiC, Ni formed a proper coating with superior mechanical properties.

Recently, high-energy ball milling has been shown to be a potential room temperature method for applying a coating to substrates inside the milling vial. The experiments discussed in this paper were performed in the hope of broadening the applicability of the technique to different types of coating materials and to different substrate positions. Ni and SiC powders were separately employed as the coating material to be mechanically deposited on an Al substrate fixed at either the top end or the bottom end of the milling vial. The obtained coatings were characterized using X-ray diffraction, scanning and transmission electron microscopy. Mechanical properties of the coatings, including wear performance, cohesion and bonding strength, were also studied. Under the milling condition used in the present study, SiC coatings showed low microhardness and wear resistance irrespective of the substrate position, mostly due to an insufficient degree of consolidation. However, excellent results were obtained for Ni coatings produced under similar milling condition. It was realized that the top position for the substrate provided a remarkably better microhardness, wear resistance and adhesion strength. Ni coating on the top substrate with an average crystallite size of about 58 nm, showed a microhardness of 422 ± 12 HV that is at least 30% higher than that of a typical electroplated Ni coating. The difference in coating characteristics for the two substrate positions was attributed to different deposition mechanisms.

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