Enhancing aesthetics, hardness and corrosion resistance of aluminum alloy via deposition of Al/Si/AlN coatings

To address the low hardness, weak corrosion resistance and color homogeneity of Al alloys for mobile phone application, reactive magnetron sputtering was employed to deposit Al/Si/AlN coatings on the Al alloy substrate. The morphology, chemical composition, structure, optical properties, hardness and corrosion resistance of the deposited coatings were characterized using FESEM, XPS, GIXRD, colorimeter, spectrophotometer, nanoindentation, and electrochemical corrosion meter, respectively. The results show that the AlN layer is of a hexagonal crystal structure with the preferential orientation of (002). Its stoichiometric formula is Al _1.2N. The porous coating surface is due to the absence of substrate bias. The Al alloy could display red, yellow, green, cyan, blue, and purple colors, as the AlN layer thickness is 283, 267, 243, 218, 202, and 184 nm, respectively. With the Al/Si/AlN coating, the hardness of the Al alloy reaches 12.0 GPa, about 7 times harder than the Al alloy; and the corrosion current density reduces to 2.09×10 ⁻⁸A/cm ², which is two orders of magnitude smaller than the Al alloy. The role of Si interlayer in developing the structural color by thin-film interference is studied systematically. The reasons for the enhanced hardness and corrosion resistance is discussed in details.