A critical review on functionally graded coatings: Methods, properties, and challenges

Surface coating on metal substrates has remained a difficult challenge for researchers due to the conflicting requirements for different properties. In recent years, due to their mechanical, thermal, electrical, and tribological properties in many advanced engineering applications, functionally graded coatings (FGCs) have become fascinating materials for researchers worldwide to obtain coatings with specific requirements. FGCs are a novel type of traditional composites in which phases are not equally distributed to form a smooth gradient structure; thus, gradient coatings have shown a new research path. Therefore, this paper critically reviews the different FGC processing techniques by describing the fundamental problems and potential of these processes based on existing literature. Additionally, this research presents the most important potential applications and future challenges for gradient coatings. Consequently, the results expected from the paper are considered a guideline that enables the interested in this field to make informed decisions.