Plasma electrolytic oxidation technology for the development of high-performance coatings on cast Al-Si alloys

The transport industry currently represents one of the leading sectors globally. However, considering that most of the current vehicles are powered by fossil fuels, this industry is also one of the most polluting. One of the most effective strategies for the reduction of fuel consumption and pollutant emissions is the development of components manufactured from lighter materials, among which aluminium alloys stand out significantly. Among the wide range of aluminium alloys, cast aluminium-silicon (Al-Si) alloys are particularly interesting, constituting the most widely used non-ferrous materials in the automotive industry. Nevertheless, their low wear and corrosion resistance has severely restricted their use in several applications. Surface modification of Al-Si alloys through the development of coatings has proven to be the most effective method for the improvement of tribological properties even under extreme conditions.

Plasma electrolytic oxidation (PEO) is an advanced electrochemical technology that has revolutionised the surface modification of several metallic materials, including aluminium, titanium, and magnesium. Due to the interesting properties of PEO coatings, PEO technology has been postulated as one of the most promising techniques for increasing the durability and improving the performance of different aluminium components, while at the same time being an environmentally friendly technology. However, and despite having provided promising results compared to anodising, the development of high-performance PEO coatings on cast Al-Si alloys is still a challenge nowadays.

This work has focused on the development of high-performance multifunctional PEO coatings on cast Al-Si alloys.

To achieve this aim, novel water-based alkaline electrolytes, specifically formulated for this type of metallic substrates, have been developed, while at the same time the influence of the most relevant electrical process parameters has been investigated. Therefore, by means of a multivariable optimisation of the process parameters and the application of different polarization regimes, the development of coatings with high thickness, density, homogeneity, and hardness, as well as with aesthetic functionality, on complex secondary and hypereutectic cast Al-Si alloys has been successfully achieved. Furthermore, the novel PEO coatings have led to an excellent improvement in the tribological performance of the cast Al-Si alloys, as well as to an enhancement in the corrosion and tribocorrosion resistance, and the dielectric features. Moreover, specific tribological tests have been carried out to study the feasibility of PEO coatings grown on cast Al-Si alloys for engine liners and brake system applications. At this stage, the performance provided by the PEO coatings has been compared with that obtained with cast iron, a material traditionally used in these applications, having achieved competitive results at a tribological level, also leading to a notable lightweighting of the components. Lastly, an analysis of the scaling of PEO technology at industrial level has been carried out, considering technical, economic, and environmental factors.

The present PhD thesis provides novel perspectives into the development of multifunctional PEO coatings on cast Al-Si alloys. The findings derived from this study have revealed that PEO technology not only represents an enhanced alternative to anodizing, in terms of efficiency and sustainability, but also that the application of PEO coatings in the transport sector would contribute to the development of high-performance components and the improvement of the environmental sustainability.