Role of Carbon Nanotube on Multi-Length Scale Tribological Properties of Al2O3-Based Thermal Barrier Coating

Introduction

The application of thermal barrier coatings (TBCs) on turbine engine components, such as combustors, high-temperature turbine (HPT) blades and nozzles is increasing in commercial and military jet engines. The insulating capability of TBC enables high operating temperatures, thereby improving efficiency, reducing emission and increasing the thrust/weight ratio of the turbine and combustors (Bikramjit & Kantesh, 2013; Hassanzadeh et al., 2018; Musalek et al., 2020; Amir Hossein Pakseresht et al., 2016). The structure of TBC is a complex system of functionally graded materials, that contains a metallic bond coat and ceramic coat over a metallic substrate (engine components) (Amir Hossein Pakseresht, 2018). The TBC aims to improve the structural stability of the metallic components which are exposed to a high-temperature or extreme condition. It mandates the utilization of high thermal insulation to the metallic components and the structural stability of these components are eventually conserved. So, thermally insulating TBC materials must possess sufficient thickness with durability to withstand at an exposed temperature of the load-bearing materials. Ultimately, it should possess good mechanical, thermal and tribological properties at high-temperature or working temperature. Among the aforementioned properties, thermal properties are the prerequisite for the selection of TBC materials. Low thermal conductivity and thermal expansion coefficient (CTE) mismatch with the substrate are the prime assets for TBCs. Low thermal conductivity will increase the temperature gradient across the coating and ultimately decreases the exposure temperature. Thus it improves the efficiency or working temperature of the engine by giving structural stability to the engine components. After several working hours of engine, the TBCs lose their properties and start to fail.

The failure is caused in two ways: (i) internally and (ii) externally induced failures. Among the internally induced failure, the formation of thermally grown oxide (TGO) and stress accumulation at the interface due to relatively high CTE mismatch with the underneath layer is the major cause for failure. Especially, high CTE mismatch between the ceramic top coat (ZrO₂ based ceramic) and TGO (continuous & thin layer of Al₂O₃) lead to premature failure of coating at the interface. But, the existence of TGO is unavoidable and it is intensely grown layer to avoid the oxidation of underneath bond coat and substrate. Thin and continuous layer of TGO will act as a barrier to the atmospheric oxygen. So, the main objective of the TBC system i.e. protection at high-temperature will be attained. On the other hand, there will be accumulation of stresses at TGO-ceramic top coat interface due to the CTE mismatch that leads to failure of the coating. So, the scientist have put efforts to increase the life span of the TBCs by tailoring the microstructure, chemistry and other properties of the interface. One of the ways to improve the life span of the TBCs is tailoring the chemistry of the top ceramic layer without compromising its prime characteristics, such as thermal insulation and thermal stability. The scientific community has studied several materials for the top layer of TBCs. After controlling several process parameters, several materials have been used in place of conventional TBCs, such as BaTiO₃, La₂Ce₂O₇, SiO₂, and Al₂O₃ reinforced composites (Ariharan et al., 2013; Hassanzadeh et al., 2018; Musalek et al., 2020; A H Pakseresht et al., 2015, 2016; Y. Wang et al., 2009). Based on the primary prerequisite properties for TBC, conventional Y₂O₃ stabilized ZrO₂ (YSZ) was replaced with Al₂O₃-YSZ based ceramic matrix composites (CMCs). It showed excellent stability against the impact of foreign particles and corrosive salt (hot corrosion) (A H Pakseresht et al., 2020; X. Wang et al., 2015).