Past and present of functionally graded coatings: Advancements and future challenges

Advanced engineering applications necessitate developing a new class of surface coatings that protect the surface components in these applications while also meeting the conflicting requirements for the necessary properties to improve the surface performance of many parts in biomedical, aerospace, electronics, automotive, marine, oil and gas pipelines, as well as electronics fields. However, surface coatings on many constituents have remained a difficult task for researchers due to the synchronous requirement for opposing characteristics in the same coating. In recent years, functionally graded coatings (FGCs) have been widely used to protect surfaces through achieving specific characteristics from one location to another to be suitable for different operating conditions. Deposition of FGCs with gradient behavior and high accuracy on the surface of substrate material is a versatile and successful approach for enhancing characteristics of the substrate material, primarily to increase its functionality and life span. Therefore, this review paper presents a detailed analysis of the extensively used production techniques to develop FGCs based on the process state during deposition. Also, the microstructure features, mechanical properties, wear resistance, and corrosion behavior of the produced FGCs have been explored in detail. Furthermore, the current review paper focuses on understanding FGC classifications and applications, as well as research gaps and future directions for these coatings. The findings of this paper are expected to give researchers and designers in this field a variety of options for permanent surfaces protection via following a well-organized review methodology.