Preparation of DCPD Coating on Magnesium Alloy and Its Interface Bonding Mechanism

To improve the bonding force of the CaHPO4.2H2O (DCPD) coating on magnesium alloys, the work aims to propose an approach for synthesizing calcium phosphate coating on the surface of magnesium alloy via electroplating. The morphology, microstructure, and interface bonding of the calcium phosphate coating were characterized by a combination of different characterization techniques (SEM, XRD and XPS) and molecular dynamics simulation (MD). The formation of lotus-shape-like calcium phosphate coating, namely its main component, was CaHPO4.2H2O. MD simulation was used to analyze the interfacial morphology, radial distribution function (RDF), the potential of mean force (PMF), total energy, interface bonding energy, and relative concentration. The interface bonding energy, mutual bonding site, change in interface structure, and micro bonding mechanism between Mg and DCPD coating were further investigated. The main conclusions included three aspects: (1) Among the four common crystal planes of the DCPD coating, namely layer (010), layer (–120), layer (11–1), and layer (111), the layer (–120) had the strongest interface bonding force with Mg (001), which was 39.09 kcal/mol; (2) The main components of CaHPO4.2H2O could be simply divided into Ca2+, HPO42– and H2O. Among them, the relative contents of HPO42–and H2O in the interface layer were higher, indicating that the groups acting as "rivet groups" in the two-phase interface layer were HPO42– and H2O groups. The bonding sites were mainly effective interactions between O and Mg atoms. In other words, HPO42– and H2O groups could form Mg-HPO42– and Mg-H2O dipole pairs with Mg through electrostatic interaction and Van der Waals force; (3) The coordination number of Mg-HPO42– and Mg-H2O dipole pairs were 0.75 and 1.16, respectively, and their molar ratio was close to 1:1. Thus, one Mg atom on the DCPD/Mg interface was closely bound with one HPO42– or one H2O. The coordination number of Mg-H2O dipole pair might be larger due to its higher concentration in the DCPD coating; (4) Compared with Mg-HPO42– dipole pairs, there were more H2O molecules closely bound to Mg atom, forming Mg-H2O dipole pairs, and its dissociation energy (4.23 kJ/mol) was also higher than that of Mg-HPO42– (2.85 kJ/mol). The reason for its higher dissociation energy might be due to the volume of H2O smaller than that of HPO42-, which was easy to rotate in the lattice, thus forming a more stable bonding with Mg. Furthermore, based on the above research, two feasible schemes are proposed to improve the interface bonding energy of DCPD coating and magnesium alloy. The magnesium alloy surface is pretreated before electroplating. Specifically, the magnesium alloy is immersed in NH4H2PO4 solution for a period, so that the H2O and HPO42–groups can better bond with the Mg substrate, thus improving the bonding force of the Mg substrate and DCPD coating interface. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.