The turbine hot zones have employed a wide range of thermal barrier coatings in recent years. The coatings mainly decrease the heat transfer rate into the substrate materials (turbine blades and chambers) through application of materials with low thermal conductivity [1, 2]. The design of turbine blades, particularly those used in gas-powered electricity turbines, encounters two significant technical challenges: providing favourable mechanical properties and chemical stability at high temperatures [3, 4]. While simultaneous improvement in both of the mentioned properties seems to be impossible, strategies have been developed to achieve the best combination of outcomes. Most commonly, Ni-based superalloys such as IN738LC are used as the substrate to satisfy the high-temperature mechanical properties conditions. The improved chemical stability at high temperatures can be also obtained by depositing TBCs on the surface of the superalloy [4]. TBCs encompass two main layers; an intermediate MCrAlY layer (bond coat) in which M represents Ni or Co elements and the main ceramic layer of Yttria-stabilized zirconia (YSZ) or Ceria-stabilized zirconia (CSZ). These layers are commonly applied through thermal spray methods, such as atmospheric plasma spray [5]. The oxygen diffusion in both the bond coat and TBC layers improves at the service temperature as a result of intrinsic porous structure of the plasma-sprayed ceramic layers. This phenomenon leads to the formation of an oxide layer between the bond coat and ceramic layer called the TGO layer [6, 7]. The formation of the TGO layer, on one hand, increases the oxidation resistance of the coating due to the synthesis of the α-alumina layer, and on the other hand, destroys the ceramic TBC layer due to the tensions induced by the thickening alumina layer. Hence, strategies should be planned to control the thickness of the addressed TGO layer and, consequently, the service time of the TBC coatings [6, 8, 9]. Pure zirconia is found in three different allotropes: monoclinic, tetragonal, and cubic. It also includes two phase transformations upon heating up, which should be carefully addressed [5]. A 3–5% volume change is associated with the monoclinic/tetragonal phase transformation, which can cause ZrO2-based materials to be fractured and destructed. However, the dissolution of some metallic oxides such as Y2O3, MgO, and CaO has been used as an effective strategy to stabilize the tetragonal ZrO2 at room temperature. The product is known as stabilized zirconia [5, 10, 11]. Besides its wide range of applications, this material is also used as TBC. Although numerous compounds and materials have been used as the TBCs, ceria-stabilized zirconia (CSZ) is considered the most promising candidate due to its unique properties including low thermal conductivity (1–2 W/mK), high coefficient of thermal expansion (11.8×10− 6 K− 1), and proper thermal shock resistance [12]. TBCs can also enhance the hot-corrosion and oxidation resistance of the turbine components in addition to their primary role, manifested as the enhanced service life of the high-temperature systems under extreme conditions. The oxidation behaviour of MCrAlY coatings has been widely explored.
The resistance of the TBCs in oxidative and corrosive environments, as well as the thermal cycles, has been dramatically improved, due to the recent development of nano-structures [13–16]. Hence, many of the current research works are focused on the synthesis of the nanostructured TBCs using nano-powders through plasma spray methods [17–20].
Thanks to their unique combination of properties (low conductivity, high CTE, and good mechanical behaviour), nano-structured zirconia-based TBCs have attracted the attention of researchers since the late 1990s. Such promising outcomes are mainly obtained through the structural modifications of TBCs. In particular, the microstructure and properties of plasma-sprayed TBCs are highly dependent on the characteristics of the feeding powders. It has been confirmed that the use of nano-structured powders can improve the precipitation rate, bonding strength, and even the hardness of the coatings. However, nano-powders cannot be directly used in the plasma spray process. Hence, agglomeration techniques such as spray drying have been developed to produce 30–100 µm sized feeding particles [21–23]. The thermal stability of the nanostructured TBCs is 2–4 times greater than that of the conventional TBCs, which can be attributed to the higher fracture toughness (FT) of the nano-structured TBCs [24–26]. The higher FT is itself assigned to the higher density and more homogenous distribution of the micro-cracks and nano-porosities in the structure of nano-grained TBCs [23].
The tensions on TBCs may be induced by two leading causes, including the mismatch between the CTEs of the substrate and ceramic layer and the formation and growth of the TGO layer through the progression of the oxidation phenomenon. However, further oxidation of the intermediate layer and consequent increment of the thermal tensions at the interface (due to mismatched CTEs) can lead to early destruction and decreased lifetime of the TBCs [27]. In this regard, alumina is mainly selected to inhibit oxygen diffusion into the mentioned layer to control the growth rate. Functionally graded TBCs involving a composition gradient from thermally stable ceramics to toughened metals have also been suggested as another solution to enhance the lifetime of the TBCs [28, 29]. The oxidation of the intermediate layer and the subsequent TGO layer formation, as well as the diffusion of the corrosive agents (which promotes the phase transformation and delamination of the upper layer), are known as the main phenomena involved in the destruction of TBCs during thermal cycles. Alumina has been recommended by several researchers as an effective additive for improving the performance of TBCs. Al2O3 postponds the formation of the TGO layer and phase transformation in zirconia by inhibiting the diffusion of oxygen and other destructive atoms to the interface [27, 30–32].
Improving the strength and reliability of the materials through vulnerability management strategies has been recently addressed by several researchers. In this way, materials with the ability of internal modifications to treat the service damage, i.e., self-healing materials are of particular interest. Natural tissues such as bones and skin are well-known self-healing materials. Despite the significantly lower mechanical properties of the mentioned tissues compared to those of the syntactic metallic and polymer materials, the self-healing properties of the natural materials have prolonged their lifetime. However, the bio-inspired self-healing mechanisms cannot be precisely used for syntactic materials due to several differences between the microstructure and chemical compositions of these two types of materials. All in all, self-healing syntactic materials have been developed based on vulnerability management [33]. Induction of the self-healing behaviour in TBCs depends on the treating agent materials. However, these agents should provide three main parameters as follows:
(i) High melting point (higher than the melting point of TBCs, > 1000°C) and adjusting the CTE with the CTE of TBC components, (ii) High oxidation affinity and the ability to form liquid-phase to fill the cracks in TBC, and (iii) The high reactivity of the liquid phase toward the TBC materials to form the load-bearing phases within the cracks [34]. Hence, the materials with higher fracture toughness or higher self-healing ability are at the recentre of attention to enhance the lifetime and reliability of the TBCs.
A self-healing TBC was synthesized by Chen et al. [35] by the APS method. This system included a YSZ layer at the top surface and an interfacial Al-MoSi2 self-healing particle between the upper and intermediate layers. The properties of the synthesized TBC were analyzed through different thermal cycles, and the in-situ formation of the Al2O3 shell was investigated. The results of this study confirmed the potential of MoSi2 particles as a promising dopant for self-healing TBCs. Song et al. [36] used Si particles as the crack-healing agent for advanced self-healing TBCs. In another work, Nozahik et al. [37] developed a new type of TBCs via the spark plasma sintering (SPS) method. They produced a YSZ ceramic coating doped with MoSi2-Al2O3 particles as treating agents. They observed a localized crack healing process due to the formation of SiO2 and the consequent synthesis of zircon (ZrSiO4) through the applied thermal cycles. Pin et al. [38] investigated the thermal cycle and oxidation resistance of a TiC-doped TBC and found that the formation of TiO2 due to titanium carbide oxidation can lead to crack healing in TBC.
A thick SiO2 layer formed at high temperatures is the cause for the extreme oxidation resistance of bulk MoSi2. This feature has limited the application of this material in the coatings. Most of the previous studies in this area are focused on mullite-based coatings, achieved via the combination of MoSi2 and aluminium. This study aimed to improve TBC lifetime and oxidation resistance by synthesizing a self-healing MoSi2-based layer. A comparative analysis of the formation and propagation of a crack in the TGO layer and the composite coatings’ oxidation behaviour was conducted in the first stage. Then the self-healing mechanisms were investigated. According to the above-mentioned literature review, the oxidation behaviour of the intermediate NiCrAlY layers plays a crucial role in prolonging the lifetime of the TBCs. Despite this, as far as the authors are aware, there have been no comprehensive studies investigating the impacts of MAC composite layers on atmospheric plasma sprayed (APS) coatings’ oxidation behaviour. The stability of MoSi2-self-healing TBC was compared to the conventional CSZ using a hot corrosion test in our previous research [39]. Hence, this study is dedicated to the impacts of functionally graded composite layers reinforced with self-healing MoSi2 compound on the oxidation behaviour of MCrAlY bond coats.