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Effect of β-Mg17Al12 phase on the surface interactions and morphology of hydroxyapatite coating deposited on AZ91 alloy

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Abstract

Coating morphology is one of the challenging parameters that significantly control the surface properties of magnesium alloys. The surface reactions on the AZ91 Mg alloy substrate and the formation of an intermediate film significantly affect the morphology of electrodeposited hydroxyapatite coating. Hence, this study aims to provide a comprehensive investigation of the effect of heat treatment times on the morphology, volume fraction, and distribution of secondary phases present in the AZ91 Mg alloy, and then the effect of these changes on the morphology of the subsequent hydroxyapatite coating. In this regard, different morphologies of hydroxyapatite coating were developed by changing the ratio of the cathode (β-phase) to anode (α-phase) area in AZ91 alloy as a controlling factor in the intensity of micro-galvanic reactions on the substrate surface as well as the formation of the next intermediate film by performing processes such as heat treatment at different times. Scanning electron microscope images of the coatings and cross-sectional regions have shown considerable changes in the morphology of coatings as a result of the change in the uniformity and thickness of Mg(OH)2 intermediate film. The homogeneity of the coatings changes as well as the substrate-coating interactions by changing the heating time. After 8 h heat treatment, the formation of a thin and uniform Mg(OH)2 intermediate film leads to a homogeneous and fine microstructure of hydroxyapatite coating by controlling the current density and release of Mg2 + ions. In addition, the thermally stable Al8Mn5 intermetallic compound can provide conditions to form a thick film of Mg(OH)2 after the complete dissolution of β-phase which leads to the formation of a coarse morphology of coating. Overall, there is a meaningful relationship between the nucleation and growth of the coating and surface reactions of the substrates.

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