Hybridization Design And Corrosion/wear Resistance Integration Of Boroxine Based Coatings In Magnesium Alloy Surfaces

As the lightest metal structural material,magnesium（Mg）alloy has the advantages of high specific strength/specific stiffness and good vibration damping,which makes it play an important role in automotive manufacturing,electronic communication and aerospace applications.However,Mg and its alloys have low standard electrode potentials,high chemical activity and loose porous surface films,which lead to their extreme susceptibility to corrosion.In order to improve the corrosion resistance of magnesium alloy,the most effective method is to construct an organic coating on its surface.However,traditional coatings have many problems,such as:poor wear resistance,unable self-healing,internal holes in the coating and other shortcomings,which can seriously affect the protection ability of magnesium alloy.Therefore,the development of protective coatings with corrosion and wear resistance and self-healing integration on the surface of Mg alloy is of great importance for the improvement of its long-term service performance.The presence of a large number of reversible covalent bonds（B₃O₃）and low dielectric constant branched chains（PDMS）in nitrogen-liganded boroxine cross-linked polydimethylsiloxane（N-B-PDMS）provides good self-healing and electrochemical corrosion resistance.Meanwhile,the hydrophobic methyl group on PDMS,which confers hydrophobicity.Polymers with good self-healing properties are usually soft,which reduces their abrasion resistance,and lose their protective ability when the coating is worn.It is found that the addition of nanoparticles and 2D materials to the polymer helps to improve the resistance to penetration and lubricity.In this paper,by using N-B-PDMS as raw material and compounding with silicon dioxide（SiO₂）nanoparticles,hexagonal boron nitride（h-BN）and graphene derivatives respectively,a series of protective coatings were successfully prepared on the surface of Mg-3.0Zn-0.5Ca alloy.The effects of the organization structure and bonding type of the coating on its corrosion/wear resistance,self-healing and hydrophobic ability were systematically studied,and the main research contents and mechanism analysis are as follows:（1）By mixing SiO₂ with N-B-PDMS and spraying it onto the surface of magnesium alloy,it prepared B@SiO₂coating.Its hydrophobic angle is 158°.The nano-rough surface formed by low surface energy N-B-PDMS and SiO₂makes it super hydrophobic and self-cleaning,leading to the formation of an air film between the coating and the corrosive medium,which can effectively prevent the corrosive medium from coming into direct contact with the coating,and the corrosion resistance is significantly improved.The wear resistance is also significantly improved due to the lubricity of SiO₂,and the coating wear rate is reduced by 70.7%compared to N-B-PDMS.The results show that the coating can provide integrated corrosion/wear resistance and hydrophobic protection for magnesium alloys.（2）In order to improve the interfacial bonding,h-BN is mixed with N-B-PDMS in different ratios and prepared onto the surface of cerium（Ce）-treated magnesium alloy to finally obtain a Ce-B-BN_xgradient coating.The gradient distribution of h-BN within the coating can block the erosion of the corrosive medium to the metal substrate layer by layer,and the corrosion current density is reduced by 4 orders of magnitude compared with that of magnesium alloy,and the corrosion resistance is significantly improved.The gradient change in h-BN triggers a change in the hardness of the coating,and the wear process facilitates the release of accumulated deformation energy to non-wear regions with the smallest wear rate compared to non-gradient coatings,indicating that Ce-B-BN_xhas the best wear resistance.In addition,the scratches on the coating surface could still be repaired within 15 min,indicating that the h-BN added protective coating still maintains good self-healing capability.（3）The N-B-PDMS@RGO/PVA（B@RGO/PVA）coating is prepared by fully mixing RGO with polyvinyl alcohol（PVA）and constructing a bionic"exoskeleton"structure on the surface of N-B-PDMS coating using spin-assisted layer-by-layer self-assembly（SA-LBL）technology.The layer-by-layer structure of RGO/PVA can effectively impede further penetration of the corrosion solution,prolong the corrosion path and enhance the corrosion resistance of the coating.The presence of RGO sheets in the bionic layer leads to a decrease in the coefficient of friction and confers excellent wear resistance to the coating.In addition,its surface scratches can be repaired within 5min and some of the original corrosion resistance is restored.