Metals, such as steel and aluminium, are crucial construction materials we encounter daily. However, these versatile materials are susceptible to corrosion, leading to safety hazards, aesthetic issues, and financial losses. Therefore, a significant aspect of metal surface engineering concerns corrosion protection.
Physical Vapour Deposition (PVD) based composite coatings are commonly used on various substrates for aesthetic and protective purposes. When applied to metals, their primary technical feature is to prevent corrosion. However, the thickness of the coating does not necessarily determine its effectiveness as a physical barrier.
Proper substrate surface preparation, expert formulation of treatment and coating chemicals, appropriate application processes, and adaptation to different uses and service environments are necessary for adequate corrosion protection provided by a PVD-based coating.
Many industrial practices are available on different aspects of corrosion protective coatings. However, it is essential to understand that corrosion protection by PVD coatings is a complex issue that requires a cross-functional approach. Currently, there needs to be a unified approach highlighting the role of all disciplines involved in creating and using corrosion protection coatings for metals.
Currently, various types of thin films are employed for corrosion protection. There are several new progressive concepts:
Self-assembled monolayers and conducting polymers were very popular. However, biopolymers are exciting as a potential conductive matrix or passive protective coatings with simultaneous biocompatibility.
The use of various techniques for corrosion protection, including the combination of thin films with nanomaterials to form nanocomposites, is currently the most popular area of growth in corrosion science. Galvanic coatings are also widely used but considered conservative, and there are only a few reports on their modification in the literature. These reports primarily focus on incorporating nanoparticles and polymer micro containers into the coating.
Nanocomposite coatings have gained significant interest in recent years across various strategic industries, including automotive, aerospace, petroleum, and electronics. These coatings are engineered to provide functional surface coatings that are both cost-effective and attractive, with superior properties for applications such as anti-corrosion, antimicrobial, antifogging, and adhesion. The unique characteristics of nanocomposite coatings include enhanced mechanical strength, weight reduction, improved barrier properties, and increased heat, wear, and scratch resistance, ensuring lifelong performance. Compared to traditional anticorrosive composite coatings, the superior performance of nanocomposite coatings is mainly attributed to the improved morphology with nanoscale phase-separated domains.
Recent advancements in nanotechnology have enabled the development of innovative protective polymer nanocomposite (PNC) coatings that provide anti-corrosion, anti-fouling, and self-healing properties to surfaces. PNC coatings can be applied to surfaces using three main depositing techniques: mechanical, physical, and chemical. The automated depositing technique is relatively low-cost and can be achieved through painting, spraying, dip-coating, or spin-coating. The physical depositing technique can be conducted through sputtering, bonding, or condensation. Unlike other chemical techniques, the physical bonding technique is condensation and conventionally performed on a vacuum-like physical vapour deposition (PVD), conducted at normal pressure parameters. Chemical adhering techniques are lower in cost. However, chemical techniques require expensive and toxic precursors, including atomically layered deposition (ALD), Langmuir, and sol-gel processes. Also, PVD possesses the advantages of achieving controlled coating structure, strong adhesion, compound layers with functional gradient properties and low deposition temperatures.
Novel nanocomposite PVD coatings are preferred for magnesium alloys to improve their wear and corrosion resistance – and thereby explore the potential to extend the use of such alloys to moving parts for light-weighting of tribological components, where the possibility for cumulative weight savings is immense if critical parts can be made from magnesium. Still, the alloys cannot be used successfully due to their poor wear and corrosion behaviour under dynamic loading.
The first stage of the process involved creating a base layer that will serve as a foundation for the subsequent deposition of either a PVD ceramic nitride coating or a nitrogen-doped hard metallic coating. In the second stage, a nanocomposite coating should be deposited with enhanced tribological performance by introducing nitrogen reactive gas sequentially after the base layer preparation step. The development of the coatings consisted of two stages: first, the production of an optimized base layer for the protection of magnesium alloy, and second, the modification of the base layer by incorporating nitrogen-reactive gas to produce a complex and wear-resistant nanocomposite coating.
