High-Temperature Cyclic Oxidation of Single and Dual-Layer CSZ: ZrO2-25 wt% CeO2-2.5 wt% Y2O3/MAC: MoSi2 + Al2O3 + CSZ Self-healing TBCs at 1100 °C

High-temperature oxidation has been regarded as a major challenge in the degradation of thermal barrier coatings (TBCs). The present study introduces a new TBC (CSZ: ZrO2-25 wt% CeO2-2.5 wt% Y2O3/MAC: MoSi2 + Al2O3 + CSZ) with enhanced resistance to high-temperature oxidation compared to the conventional CSZ TBC. Conventional single-layer CSZ TBC and dual-layer CSZ/MAC self-healing TBC were deposited using an atmospheric plasma spray (APS) technique on IN738LC substrates with a NiCrAlY bond coat. The high-temperature cyclic oxidation testing was performed in air at 1100 °C with 4 h in each cycle. Phase and microstructural investigations of the coatings by XRD and FESEM/EDS methods before and after the high-temperature cyclic oxidation testing indicated the better performance of CSZ/MAC self-healing TBC (relative to conventional CSZ TBC) in preventing the diffusion of oxygen. The microstructural analysis indicated that the growth rate of TGO layer was considerably slower for dual-layer CSZ/MAC self-healing TBC due to reduced oxygen infiltration and crack propagation and, therefore, has a better high-temperature performance.


Introduction
Thermal barrier coatings (TBCs) are complex systems that are utilized to enhance the performance of hot-section components in gas turbines.These coatings consist of two layers: (i) a metallic bond coat and (ii) a ceramic top coat, which are deposited on a metallic substrate (usually nickel-and cobalt-based superalloys) using thermal spray techniques.The metallic bond coat is typically an alloy mixture known as MCrAlY (where "M" can be nickel, cobalt, or a combination of these elements).Today, the most commonly employed material for the ceramic top coat in TBCs is 1 3 High Temperature Corrosion of Materials (2023) 100:305-320 stabilized zirconia with a non-transformable tetragonal phase and a composition of ZrO 2 -8Y 2 O 3 (YSZ) or ZrO 2 -25 wt% CeO 2 -2.5 wt% Y 2 O 3 (CSZ) [1][2][3][4].
Several studies have indicated that coating with CSZ is more effective than YSZ coatings in resisting hot corrosive environments.This finding is supported by the fact that CeO 2 is more resistant to chemical attacks by salts, such as sulphates and vanadates.Although cerium oxide has a higher coefficient of thermal expansion than YSZ, its thermal conductivity is lower.Therefore, adding ceria to YSZ coatings is expected to enhance the thermal cycle lifetime of YSZ TBCs.Moreover, a composite of TBC with Al 2 O 3 has been found to improve the mechanical properties of coatings and prevent high-temperature oxidation or hot corrosion [2][3][4].
While using CSZ or CSZ + Al 2 O 3 as the top coat in TBCs has several advantages, such as hot corrosion resistance [3], high-temperature oxidation resistance [4], and adequate mechanical and thermomechanical properties [2], it also has some disadvantages, including insufficient phase stability at temperatures above 1200 °C or after more than 10,000 h of cyclic exposure, and accelerated sintering at elevated temperatures [2].In recent years, enhancing the lifetime of TBCs has become a crucial research area.Using new ceramic materials as top coats to increase the lifetime of conventional TBCs is a significant approach in this field [1,5,6].
One promising candidate for TBCs is rare earth-doped zirconia (RE-ZrO 2 ), which has been the subject of extensive research [7].RE-ZrO 2 has a higher phase stability than conventional TBCs at high temperatures and thermal cyclic exposure, as well as improved sintering resistance [8][9][10][11][12].Some researchers have also investigated the use of other ceramic materials, such as alumina and mullite, as top coats in TBCs [13].Alumina-based TBCs have shown high oxidation resistance and low thermal conductivity, while mullite-based TBCs have demonstrated good thermal shock resistance.However, these materials are still in the early stages of development and require further investigation [13].
In addition to developing new ceramic materials, researchers have also explored the use of novel TBC designs and structures to enhance the performance and durability of TBCs.For example, dual-layer TBCs, where two different ceramic materials are used as top coats, have been proposed to improve the lifetime of TBCs [14,15].
Another approach is to introduce self-healing mechanisms into TBCs, where the coatings can heal themselves when they are damaged.Self-healing TBCs can be achieved by incorporating healing agents, such as glass or ceramic particles, into the coating structure.When the coating is damaged, the healing agents are released and fill the cracks or pores in the coating, restoring the coating's properties.Overall, the development of new ceramic materials and novel TBC designs and structures, as well as the incorporation of self-healing mechanisms, are promising approaches for improving the performance and durability of TBCs in gas turbines [16][17][18][19].
Recently, Soltani et al. [17] have studied CSZ: ZrO 2 -25 wt% CeO 2 -2.5 wt% Y 2 O 3 / MAC: MoSi 2 + Al 2 O 3 + CSZ self-healing TBCs in which the MAC layer reduces the diffusion rate of corrosive salts within the coatings.It seems that a layered composite of TBC with MAC improves the high-temperature oxidation.In this study, the high-temperature cyclic oxidation of the dual-layer CSZ/MAC TBC is studied in comparison with conventional CSZ TBC.The TBCs were deposited by APS method and the stability of the coatings was determined through performing the oxidation experiments at 1100 °C.

Experimental Procedures
Disc-shaped samples of nickel-based superalloy (IN738LC) were prepared with a diameter of 25 mm and a thickness of 5 mm.The substrate constituents are listed in Table 1.
The bond and top coat powders employed in the process are of micron size and have commercial codes: Amdry 962 (NiCrAlY) for the bond coat, Metco 205NS (CSZ), molybdenum disilicide (MoSi 2 , 99.5% purity; with an average size of 30-50 nm), and alumina (Al 2 O 3 , 99.5% purity; with an average size of 30-50 nm) for the top coat, respectively.Different powders including MoSi 2 powder (6.25 wt%, d MoSi2 = 6.26 g/cm 3 ), CSZ powder (56.25 wt%, d CSZ = 6.2 g/cm 3 ) and Al 2 O 3 powder (37.5 wt%, d Al2O3 = 3.9 g/cm 3 ) were mixed at a rotation speed of 150 rpm for 6 h in a ball mill using an alumina cup and balls to create a homogenous MAC powder.To make the MAC powder suitable for the APS method, it was granulated by spray drying.The results of the granulation of MAC powder have been presented in previous studies [17].
The TBCs were applied onto the IN738LC substrate using an APS method equipped with a Metco F4 gun.Prior to the application of coatings, alumina particles with a mesh size of 80-50 were blasted onto the substrate surfaces at a pressure of 50-40 psi to remove surface oxides.After washing at a temperature range of 65-90 °C, CoNiCrAlY/CSZ (Fig. 1a) and CoNiCrAlY/CSZ/MAC (Fig. 1b) TBCs were deposited using the parameters specified in Table 2.
The self-healing effect of the CSZ/MAC TBC system was studied using a heat treatment process.Two samples were heated at 720 °C in an air electric furnace for 5  High Temperature Corrosion of Materials (2023) 100:305-320 and 10 h and then cooled in the furnace.The self-healing effect in the improvement of the oxidation resistance of the CSZ/MAC TBC system was investigated through scanning electron microscopy equipped with an energy-dispersive spectrometer.
For high-temperature oxidation tests, the samples were placed in an electric furnace with an air atmosphere at 1100 °C for 200 h with 4-h cycles followed by furnace cooling.To avoid thermal shock effects, the samples were not removed until complete cooling had occurred.The total holding time of the samples at the peak temperature was 24, 48, 96, 148, and 200 h to examine the growth rate of the TGO layer.The reported data on variations in TGO growth rate represent the average of results obtained from five tested samples.For instance, to measure the thickness of the TGO layer at times of 24, 48, 96, 148, and 200 h, 25 samples were tested for one type of TBC.
Before and after the high-temperature cyclic oxidation test, a field emission scanning electron microscopy equipped with EDS analysis (FESEM/EDS) as well as X-ray diffraction analysis (X'pert, XRD, Cu, Philips, K, 40 kV, 30 mA, step size: 0.05°) was employed to investigate the structural and phase characteristics of the TBCs.

Results and Discussion
Figure 2 presents FESEM images of the cross section of single-layer CSZ and duallayer CSZ/MAC TBCs applied on IN738LC/NiCrAlY substrate using the APS method.The single-layer TBC (Fig. 2a) consists of a bond coat with an approximate thickness of ~ 150 μm and a top coat with an approximate thickness of ~ 350 μm.The dual-layer TBC (Fig. 2b) consists of a bond coat with an approximate thickness of ~ 150 μm and two top coats with approximate thicknesses of ~ 200 (CSZ) and ~ 150 μm (MAC).
The microstructure analysis of the cross section of single-layer CSZ and duallayer CSZ/MAC TBCs (Fig. 2) indicates the presence of pores and micro-cracks mostly in the ceramic layers of CSZ and MAC.The presence of pores in the microstructure of TBCs is one of the common features of coatings produced by the APS technique [5,11,20].The results presented in Fig. 2 indicate an acceptable performance for the APS technique and satisfactory homogeneity in the coating layers with appropriate distribution of porosity.Furthermore, image processing results indicate that the NiCrAlY, CSZ, and MAC layers have approximate porosities of 3-5%, 11-15%, and 8-10%, respectively.Figure 3 shows the XRD patterns of the as-sprayed CSZ and CSZ/MAC top coats.Furthermore, all peaks in the conventional CSZ are consistent with the tetragonal phase of the ZrO 2 .The ZrO 2 , Al 2 O 3 , and MoSi 2 peaks are related to the top MAC coating.
Figure 4 indicates that the surface of the MAC layer contains "Si" and "Mo" elements.Since "Mo" and "Si" elements are located beneath the thick oxide layer and as a result of the high content of "Y" and "Zr" elements on the surface, the peaks related to these elements are weak.Therefore, the coating contains only zirconia's non-transformable tetragonal phase (Fig. 3).
Figure 5 shows FESEM images of the cross section and surface of dual-layer CSZ/MAC TBC after 10 h of oxidation at 720 °C.Also, Fig. 6 presented a high magnification of Fig. 5.
A decline in porosity was observed in the CSZ/MAC TBC structure after annealing, as shown in Fig. 5. Following self-healing treatment, MAC porosity was reduced.The sealing effect can be assigned to volume expansion due to oxidation [21].The FESEM images of the MAC coating and CSZ interface at higher magnifications are presented in Fig. 6.
The oxidation reactions of the self-healing agent healed the pores and cracks in the CSZ/MAC TBC coatings as oxygen passed through the self-healing coating.The volume expansion ratio of a self-healing material can be defined as the ratio of the volume increase of the healing agent upon reaction to the initial volume of  where "V", "M", and "ρ" are volume, molar quantity, and density, respectively.As a result, MoSi 2 can expand by 13% during heat treatment due to SiO 2 formation and the filling of pores in the MAC layer.
The effects of filling and healing are presented in Fig. 6a and b at the microstructure level.The porosity in MAC coating before and after self-healing heat treatment was measured by image processing as 9.88% and 4.18%, suggesting a (1) 57.69% decline in the porosity of MAC coating.Therefore, the pores and cracks were filled and sealed after heat treatment at 720 °C due to the self-healing effects.As can be seen in Fig. 6b, no pores and micro-cracks were formed as a result of stresses that may be induced by volume expansion after self-healing heat treatment at the MAC coating and the MAC-CSZ interface as it was well bonded.Hence, the volume expansion did not destroy coating interfaces which can be due to the self-healing at temperatures above 720 °C.These results confirmed the efficient filling and sealing of pores for the MAC coating.
During the oxidation test at 1100 °C, failure of CSZ and CSZ/MAC TBC systems occurred after 59-60 and 104-105 cycles, respectively.The addition of the MAC overlayer on top of the CSZ coating resulted in a significant improvement in oxidation.The FESEM image of the cross section of conventional CSZ and CSZ/MAC TBCs after the oxidation test at 1100 °C for 200 h is shown in Fig. 7.The TGO layer is created at the interface between NiCrAlY bond coatings and CSZ coatings due to the diffusion of oxygen via the top coatings during exposure to high temperatures.Figure 7 shows the cracks, pores in the CSZ layer, TGO formed at the NiCrAlY/ CSZ interface, and oxide layer formation inside the bond coat layer.
The oxidized areas in single-layer conventional CSZ and dual-layer CSZ/MAC TBCs were measured by the image analysis to be about 38% and 21%, respectively.Figure 7 also depicts severe crack formation at the interface of the bond coat and CSZ layer, while the crack formation was not detectable for CSZ/MAC coating.Figure 8 shows the adjoining areas of the NiCrAlY-CSZ interface as well as the EDS analysis of the A box indicated in Fig. 8a and c for CSZ and CSZ/MAC TBCs, respectively.According to Fig. 8b and d, the TGO contains different elements including the "Al" and another metallic elements such as "Cr" and "Ni".
The MAC overlayer on top of the dual-layer CSZ/MAC TBC is vital in decreasing oxygen diffusion through the ceramic layer.Figure 9 shows the changes in TGO layer thickness with exposure time for single-layer conventional CSZ and dual-layer CSZ/MAC TBCs.The TGO growth curve of the TBCs is similar.A comparison, however, showed that the TGO grew more slowly in dual-layer CSZ/ MAC TBC.Because of the greater tendency for oxygen diffusion inward and "Al" diffusion outward at the beginning of oxidation, TGO growth was quicker up to ⁓ 20 h.Considering the reactivity between "Al" and oxygen, they tend to form Al 2 O 3 [23].The formation of primary TGO layers will suppress diffusions and lower growth rates.According to Fig. 9, a steady growth rate is observed for longer oxidation cycles (up to 37 cycles).Oxidation begins with the formation of non-protective oxide layers, with diffusion limiting the growth of TGO afterwards [23].In comparison with single-layer conventional CSZ TBC, the duallayer CSZ/MAC TBC showed a lower TGO growth rate.As a result, low growth rates of TGO improved dual-layer CSZ/MAC TBC spallation resistance.
The concept of stress intensity helps in a better understanding of the spallation mechanism.Eq. 2 was proposed by Evans et al. [24] to evaluate the stress intensity factor (K) at the crack tip in a TBC for the cracks arising close to the TGO: where "K" is the stress intensity factor, "v" shows the Poisson's ratio, "R" is the individual grain radius at the interface, "a" is crack length, "c" denotes a constant, "h" represents TGO thickness, "E" is Young's modulus of the TBC, and "m" stands for the volume change ratio.Moreover, the average TGO thickness "h" was determined by Chen et al. [25] as follows: (2) where A TGO is the TGO area cross section, L TGO-BC is the length of the TGO-BC interface cross section, P TGO is the TGO percentage, and A BC represents the BC area.A comparison of Eqs. ( 2) and (3) shows that the stress intensity factor (K) is directly proportional to the TGO percentage (P TGO ).Hence, the following expression can be written [22]: where "K CSZ " and "K MAC " are the intensity factors of the CSZ and CSZ/MAC TBCs, respectively, and "P TOG-CSZ " and "P TGO-MAC " are the TGO percentages of the CSZ and CSZ/MAC TBCs, respectively.TBCs organized under identical circumstances and the same properties differ only in their TGO percentage after a specific period of high-temperature oxidation.It is, therefore, clear from this comparison that CSZ/ MAC TBCs have a higher resistance level than CSZ TBCs.
Figure 10 shows polished cross section of the MAC layer in CSZ/MAC TBCs after 216 h of cyclic oxidation at 1100 °C.As can be seen from Fig. 10b, some remnant MoSi 2 particles are present in the coating after long exposure to high temperatures.This confirms the proper thermal stability of the healing particles to guarantee the healing ability of the TBCs after long high-temperature oxidation.A ZrSiO 4 layer (grey phase) is shown in Fig. 10d.Despite the diffusion coefficient of oxygen in ZrSiO 4 being several orders of magnitude smaller than the diffusion coefficient of oxygen in SiO 2 , this layer can act as a diffusion barrier against oxygen to improve the oxidation resistance [26][27][28].In the dual-layer CSZ/MAC TBC, MoSi 2 particles were oxidized at high temperatures and formed SiO 2 particles [29].This results in the reaction between particles and the ZrO 2 in the CSZ, producing ZrSiO 4 , according to Eq. 6.Additionally, researchers [28] report that ZrSiO 4 can be formed when annealing YPSZ/MoSi 2 samples or mixtures of MoSi 2 and YPSZ.Two parts of the oxidation process seem to be involved: (i) Mo 5 Si 3 and SiO 2 formation and (ii) ZrSiO 4 formation at high temperatures.In TBCs, the CSZ/MAC coatings are used to fill and seal the pre-existing pores and cracks after high-temperature oxidation.Molten salt cannot penetrate the coating due to this chemical reaction.Additionally, oxygen in ZrSiO 4 holds less correlation to stabilized zirconia in terms of diffusion rate [30].This type of coating has more resistance to thermal cycles because ZrSiO 4 reduces oxygen diffusion.
EDS analysis was conducted for the "point" and "area", as shown in Fig. 10c and  d

Conclusion
A self-healing TBC is created by a mixture of MoSi 2 , Al 2 O 3 and CSZ powders applied on NiCrAlY-coated IN738LC base metal by APS technique.Its healing behaviour was also assessed.The oxidation resistance of single-layer conventional CSZ and dual-layer CSZ/MAC TBCs was evaluated by performing the oxidation experiments at 1100 °C.Microstructural investigations were carried out before and after the oxidation tests.FESEM imaging was used to define the TGO layers' thickness.Furthermore, the self-healing effect of the dual-layer CSZ/MAC TBC system was investigated using a self-healing process performed at 720 °C for 5 and 10 h.The results revealed the better oxidation resistance of dual-layer CSZ/MAC TBC when compared to single-layer CSZ TBC as a result of the prevention of oxygen diffusion through the MAC coating layer, suggesting the practicability and the proper efficiency of the dual-layer CSZ/MAC TBC.The crack formation and spallation were reduced in the dual-layer CSZ/MAC TBC due to its smaller TGO growth rate than that of single-layer CSZ TBC.It is noteworthy that the self-healing agent effectively prevented the crack growth in the MAC coating.Author's Contribution AK was involved in supervision, writing-original draft and conceptualization.PS contributed to investigation and data analysis.MB was involved in review and editing.
Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.

Fig. 1
Fig. 1 Schematic presentation of the as-sprayed coating system: a single-layer CSZ, and b dual-layer CSZ/MAC coatings

Fig. 2 3 Fig. 3
Fig. 2 FESEM images of as-sprayed TBCs cross section of a single-layer CSZ and b dual-layer CSZ/ MAC

Fig. 4 a
Fig. 4 a FESEM image of the surface of CSZ/MAC TBC and b EDS analysis of "I" area as shown in (a)

Fig. 5 3
Fig. 5 FESEM images of TBCs cross section and surface of dual-layer CSZ/MAC a, c before and b, d after 10 h at 720 °C

Fig. 6
Fig. 6 FESEM images of the cross section of CSZ/MAC TBC a before and b after self-healing treatment for 10 h

Fig. 7 3
Fig. 7 FESEM images of TBCs after 200 h oxidation test at 1100 °C: a the cross section of single-layer conventional CSZ and b the cross section of dual-layer CSZ/MAC

Fig. 8
Fig. 8 FESEM images of TBCs after 200 h oxidation test at 1100 °C: a adjacent areas of the interface of NiCrAlY-CSZ layer of single-layer conventional CSZ, b EDS analysis of A box as shown in Fig., c adjacent areas of the interface of NiCrAlY-CSZ layer of CSZ/MAC, and d EDS analysis of A box as shown in (c)

Fig. 9 3
Fig. 9 Variation of TGO layer thickness versus the oxidation time for conventional CSZ and CSZ/MAC TBCs , and the results are presented in Fig. 11.It is clear that the MoSi 2 particles in the top coat (marked by an arrow) are oxidized, forming SiO 2 and ZrSiO 4 .Figures 10 and 11 demonstrate the MoSi 2 decomposition and oxide formation of SiO 2 and ZrSiO 4 , which results in sealing and filling of the micro-cracks and pores in the MAC coating.

Fig. 10 FESEM
Fig. 10 FESEM image of MAC layer for the CSZ/MAC TBC sample a before, b-d after cyclic oxidation at 1100 °C for 216 h

Fig. 11 3
Fig.11EDS analysis of point and area of MAC layer as shown in Fig.10c and d, respectively

Table 1
Chemical composition of the IN738LC substrate used in this study