Influence of the content of (NaPO3)6 on the properties of microarc oxidized coatings on non-valve metal-copper

Abstract

In this paper, for the first time, by adding (NaPO₃)₆ additives to the aluminate electrolyte, a MAO coating with good performance was successfully prepared on the surface of T2 copper, a non-valved metal, and the effects of (NaPO₃)₆ content on the organization, abrasion and corrosion resistance of the coatings were comprehensively and systematically investigated. Most importantly, this method of preparing MAO coating on non-valve metal surface does not need any pretreatment or high current density. The results obtained demonstrated that the incorporation of the (NaPO₃)₆ additive facilitated the direct deposition of MAO coatings on the surface of non-valve metal-T2 copper, with an increase in coating thickness and roughness observed as the (NaPO₃)₆ content rises. The compositions of the coatings included CuO, Cu₂O, α-Al₂O₃, CuAlO₂, Cu(OH)₂ and amorphous AlPO₄. The content of Cu-containing compounds in the coatings exhibited an initial increase, followed by a subsequent decrease, with the increase of (NaPO₃)₆ content. The highest percentage (14.39 at%) of elemental Cu was observed when the content of (NaPO₃)₆ was 10 g/L and it was observed that the coating exhibited the lowest average surface porosity (8.29 %) at this content level, in addition to the highest hardness (890.18 HV) and minimum wear depth (4.78 μm) and average COF (0.161) for a load of 10 N. Three different wear mechanisms (adhesion, abrasive grain and delamination wear) were found on the wear scars. The coating showed the best corrosion resistance with an i_corr of 4.0257 × 10^–6 A/cm² at a (NaPO₃)₆ content of 15 g/L, which was four orders of magnitude lower than the substrate.