Bilayer Coating Systems: Functional Interlayers and Top Layers for Enhanced Performance.

This review article summarizes the most widely used and effective technologies for producing protective and functional bilayer coatings. Particular attention is given to methods such as electroplating and electroless metallization, chemical vapor deposition, thermal spray and vacuum arc deposition, conversion treatments, laser modification, and organic layer deposition. Bilayer architectures are highlighted for their ability to overcome the limitations of single-layer coatings by combining complementary functionalities, resulting in enhanced adhesion, improved corrosion resistance through pore sealing or superhydrophobic surface states, and increased wear and crack resistance. This article is intended for researchers, materials scientists, and engineers engaged in surface engineering, corrosion protection, and advanced manufacturing, providing them with a clear understanding of the mechanisms, advantages, and practical applications of bilayer coatings. By synthesizing recent developments, comparative analyses, and performance data, the review enables readers to make informed decisions about the selection, design, and implementation of bilayer coatings for diverse industrial applications, ranging from aerospace and automotive components to medical devices and energy systems.