Properties of polydimethylsiloxane hydrophobic modified duplex microarc oxidation/diamond-like carbon coatings on AZ31B Mg alloy

Abstract A reliable, high-performance coating procedure was developed using PDMS to modify a duplex MAO/DLC coating on an AZ31B Mg alloy. First, the duplex MAO/DLC coating was fabricated via a combined MAO and unbalanced magnetron sputter process. Subsequently, a PDMS solution was used to modify the MAO/DLC coating via a conventional dip-coating method. The surface characteristics, bond strength, hardness, tribological behaviour, and corrosion resistance of the coated samples were evaluated via SEM, CA, Raman spectroscopy, friction and wear behaviour, polarisation curve, and NSS tests. The PDMS modification reduced the HIT of MAO/DLC coating from 15.96 to 8.34 GPa; this is ascribed to the penetration of PDMS, which has good rheological properties to form a viscoelastic Si-based organic polymer layer on the MAO/DLC coating. However, the PDMS-modified MAO/DLC coating was denser, hydrophobic, and had higher bond strength compared with MAO- and MAO/DLC-coated samples. Moreover, the PDMS modification reduced the COF and wear rate of the duplex MAO/DLC coating. This indicates that the PDMS improved the tribological behaviour owing to the transferred Si oxide that originated from the Si-O network of the PDMS, as well as the low graphitisation of the DLC layer during sliding. Furthermore, the corrosion current density of the MAO/DLC-coated sample modified by PDMS for 10 min decreased by two order of magnitude compared with that of the MAO/DLC-coated sample but by five orders of magnitude compared with that of the bare substrate. The NSS tests proved that the PDMS layer slowed the corrosion of the Mg alloy under long-term service, enhancing the corrosion protection efficiency. The results are attributed to the high bond strength and lubricant MAO/DLC layer, and the dual role of sealing and hydrophobicity of PDMS. Therefore, PDMS modification is promising for the fabrication of protective materials for Mg alloys that require corrosion and wear resistance.