Enhancement of hardness and corrosion resistance of Al-Si-N multilayer color coating via SiN/AlSiN/AlN compositional gradient interlayer

The color of AlN/Si/Al coating can be controlled by the thickness of the AlN layer according to the interference effect. However, the loading capacity is affected by the large hardness difference between the Si and Al layers. Corrosion resistance is relatively weak due to penetration defects in the AlN surface layer. In this work, therefore, a SiN/AlSiN/AlN interlayer is sputtered in between the Si/Al layer in the AlN/Si/Al coating as the transition layer. The chemical state, structure, morphology, color, hardness, and corrosion resistance of the as-deposited Al-Si-N coating are carefully characterized using x-ray photoelectron spectrometry, grazing incident x-ray diffraction, atomic force microscopy, scanning electron microscope, colorimeter, nanoindentation, and electrochemical corrosion meter, respectively. To evaluate the long-term corrosion resistance, the uncoated, AlN/Si/Al-coated, and AlN/Si/SiN/AlSiN/AlN/Al-coated AZ31B Mg alloys are immersed in salt solution for different durations, followed by characterization of morphology and composition. The results show that the SiN/AlSiN/AlN interlayer is of a gradient structure in both composition and hardness. The AlN crystals grow continuously from the Al bonding layer into the AlSiN layer, resulting in internal longitudinal grain boundaries. The coating surface becomes smoother with a roughness (R_q) of 12.6 nm. The color of the coating is controlled by the AlN surface layer thickness. The coating hardness increases from 6.5 to 20.6 GPa. The corrosion current density of the coating decreases from 2.02 × 10^-6 to 1.99 × 10^-8 A/cm². The coating could withstand corrosion in salt solution for at least 192h. The gradient structure of the interlayer effectively alleviates the hardness difference between the Si layer and the Al layer and inhibits the penetration of the corrosive medium from the surface. The mechanism for the enhanced corrosion resistance is explained through a model.