Investigation of the coating ability of functional ceramic composites on lightweight metals by the sequential sol-gel dip coating method

The applicability of AZ91 magnesium alloy as a biodegradable implant material is limited by its high corrosion rate and surface instability; therefore, these drawbacks are commonly addressed through surface engineering approaches. In this thesis, the coatability of ceramic hybrid coatings containing hydroxyapatite (HAp) and the functional additives nano-silica (n-SiO₂) and nano-graphene (n-Gr) on AZ91 alloy was investigated using a sequential sol–gel dip-coating method. The novelty of the study lies in employing a sequential coating strategy based on low-temperature sintering after each dipping step, in contrast to the widely used multiple-dip–single-sinter approach. This strategy aimed to create a denser, more stable, and more strongly adherent sub surface to support the subsequent layer.AZ91 specimens were coated using three different sol–gel systems: a sol containing only HAp, a hybrid sol containing HAp + n SiO₂, and a hybrid sol containing HAp + n-SiO₂ + n-Gr. The dipping parameters were kept constant, while the sintering temperature (especially 460 °C and 500 °C), the number of sintering cycles (1×–2×–3×), and the atmospheric conditions (unprotected air and inert N₂) were treated as variables. After coating, surface morphology and layer continuity were examined by SEM, and elemental composition as well as coating substrate interactions were evaluated using EDS and elemental mapping analyses. The results showed that heat treatments performed at 500 °C in air promoted oxidation and localized burning of the Mg substrate, thereby deteriorating coating integrity. In contrast, sequential sintering at 460 °C under an N₂ atmosphere limited oxidation, improved the continuity of HAp and hybrid coatings, and reduced surface cracking. The SiO₂ additive contributed to a more compact and homogeneous film structure. For the HAp + n-SiO₂ + n-Gr hybrid coatings, improved continuity—particularly after 1×–2× sintering under inert atmosphere—was obtained, with a reduced substrate effect and a more uniform distribution of phases on the surface. Overall, the findings demonstrate that sequential sol–gel dip-coating combined with post-layer sintering is a promising method for AZ91 alloy in biomedical implant applications in terms of surface integrity and potential corrosion resistance.