Carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) have attracted significant attention as advanced nanomaterial reinforcements for improving the structural and functional performance of ceramic coatings due to their exceptional mechanical strength, thermal stability, and chemical inertness. Among various ceramic matrices, alumina (Al2O3) remains one of the most widely used coating materials for wear, corrosion, and thermal protection applications. However, its inherent brittleness and susceptibility to crack propagation limit its performance in demanding environments. This review presents a comprehensive and critical analysis of CNT- and GNP-reinforced plasma-sprayed Al2O3 hybrid nanocomposite coatings. The emphasis has been given on the underlying process–microstructure–property relationships and metallurgical mechanisms governing their performance. The review systematically examines the influence of these reinforcements on particle melting behavior, splat formation, rapid solidification, and interfacial bonding during plasma spraying. Particular attention is given to splat-level phenomena, where CNTs and GNPs play a critical role in improving splat cohesion, reducing intersplat porosity, and inhibiting crack initiation and propagation through crack-bridging and load-transfer mechanisms. The presence of hybrid CNT–GNP reinforcements has been shown to significantly improve coating hardness, fracture toughness, adhesion strength, wear resistance, and corrosion performance compared to monolithic alumina coatings. The synergistic effect of combining one-dimensional CNTs and two-dimensional GNPs enhances reinforcement efficiency by providing multidirectional load transfer and improved stress distribution within the coating microstructure. Furthermore, the review briefly evaluates the effects of reinforcement dispersion techniques, feedstock preparation, and processing routes on the resulting coating microstructure and properties. This review provides fundamental insights into reinforcement mechanisms and microstructure evolution in CNT- and GNP-reinforced ceramic coatings and establishes a framework for the design and optimization of high-performance hybrid nanocomposite coatings for advanced engineering applications.
Pandey et al. (Wed,) studied this question.