High-Performance Smart Ceramic Coatings on Light Alloys

The application of surface treatments is a common practice to maximize the working life of light alloys used in transport. However, traditional passive coatings only act as a physical barrier against the environment and, once damaged, no longer provide protection. From this limitation, the concept of smart coatings was born, which are defined as materials capable of interacting with the environment by responding selectively to certain triggers, such as mechanical fracture, time, temperature or pH variations. Depending on the incorporated functionality, different properties can be achieved, e.g. specific corrosion inhibition, self-healing or damage indication.

The development of smart coatings is primarily a domain of organic-based materials since the high chemical reactivity of polymers promotes the incorporation of new functionalities. However, the applicability of such materials is limited to mild environments and medium loads. The scientific question proposed in this project is whether providing smart functionalities is feasible in ceramic-based coatings that have excellent performance in extremely corrosive and abrasive environments. The active functionalisation of ceramic coatings is quite challenging given the rigid and inert nature of inorganic matrices compared to organic materials.

The scientific approach proposed in this project to achieve the active functionalisation of ceramic coatings to tackle corrosion damage, is based on the incorporation of pH-sensitive nanocontainers loaded with corrosion inhibitors. These are released into the media when detecting pH changes arisen from electrochemical activity associated with corrosion initiation. The corrosion inhibitors will act locally inhibiting corrosion propagation, while the ceramic matrix will provide wear and abrasion protection simultaneously. This project focuses on the active and dynamic behaviour of the materials rather than in the passive properties of the coatings.

Two deposition techniques are used to manufacture the ceramic coatings, namely Plasma Electrolytic Oxidation (PEO) and Aerosol Deposition (AD) method. PEO is an electrolytic surface modification technique that derives from anodising. The interest for this technique arises from the current need to replace carcinogenic electrolytes used in anodising restricted by the EU (REACH). PEO is environmentally friendly and the resulting coatings are considerably superior in terms of corrosion and wear resistance to those obtained by conventional electrolytic techniques. However, PEO requires a high-energy consumption and this limits its applicability to niche components with excellent performance. Therefore, in order expand the range of applicability and increase the potential industry impact of this research, a more cost-effective and readily available technique is also proposed: the AD method-an extremely versatile room temperature spray coating technology. The deposition principle employed in AD is based on the generation of an aerosol using submicron powder particles from the precursor, which are sprayed at high velocity towards the substrate. It results in coatings with excellent high temperature and abrasion-resistant properties.

To achieve the active functionalisation of the ceramic coatings, two manufacturing processes are proposed. The single-step process will prove more challenging, but on the other hand, it would save processing times leading to reduced economical costs. Moreover, the single-step process is expected to produce a uniform functionalisation throughout the entire thickness of the coating, which would improve the active protection properties. The double-step process requires longer times, but it is more accessible.

Successful results would increase the lifetime of Al and Mg-based components used in the transport industry that would have a direct impact on energy efficiency and would contribute to the sustainable consumption of resources.