**1. Introduction**

Thermal barrier coatings (TBCs) enable the engines to operate at higher temperatures without raising the base metal temperatures using cooling systems inside the hot section components and thus, enhance the operating efficiency of the engines [1]. Therefore, continued develop‐ ment of TBCs is essential to increase the inlet gas temperature further for improving the performance of gas turbines. Hence, TBCs with low thermal conductivity, phase stability, and high resistance to sintering have ever increasing demands [2]. Generally, TBCs consist of a ceramic (e.g., yttria partially stabilized zirconia) top coat and a NiCoCrAlY/PtAl-based metallic bond coat. A bond coat is deposited between the metallic substrate and the top coat to protect the metal substrate from oxidation and high temperature corrosion and assist the coupling of the ceramic top coat and the metallic substrate [3]. Two methods are generally used to deposit the ceramic top coat. These are the electron beam physical vapor deposition (EB-PVD) and the atmospheric plasma sprayed (APS) methods. TBCs with EB-PVD top coats generally provide longer thermal cycle lifetimes because of its more strain-tolerant columnar structure than those observed with APS TBCs. However, APS TBCs are widely applied due to lower thermal

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conductivity and lower processing costs [3]. Recently, various processing techniques have been developed to deposit the ceramic coatings.

The objective of this article is to present an overview of the TBC requirement, application of TBCs, degradation mechanisms for TBCs, different processing techniques used for preparation of TBCs, and their thermal properties. Recent developments in TBC material have been also described. The prospect of innovative materials as bond coat in a TBC system has been elucidated.
