In this manuscript, the crack initiation and propagation of the TBC coating deposited using plasma spraying under mechanical bending experimentation has been monitored. TBC coating system have a microstructure with pores and cracks that provide thermal isolation and strain tolerance but can act as stress concentration points leading to deterioration. Real-time three-point bending and scanning electron microscopy were used to study crack formation and growth of air plasma spraying TBC coatings. Roughness of the coatings' free surface promotes crack initiation, while pores and splats contribute to crack branching and path deflection, respectively. Crack branching negatively affects TBC lifetime, while path deflection improves durability by dissipating energy required for crack propagation.

After a careful peer-reviewing process, I must inform you that, the subject of this paper is interesting and can be considered for publication in Coating journal after a MAJOR REVISION. I believe that the paper contains relevant information for the scientific community focused on recent advances in characterizations of high temperature TBC coating system. I believe that the results are informative for thermal spray society and subject of TBC failure study but must be well organized and improved in the next revision(s). Therefore, there are some questions about this submission and some revisions are necessary for this work. The major/minor issues are indicated as follows:

1. Section 1; The introduction could benefit from more background information on TBCs and their applications in gas turbine engines as well as other industrial applications.

2. Section 2.3: The authors should provide more details on the experimental setup and methodology used for the real-time three-point bending and SEM analysis.

3. It would be helpful to provide more information on the specific TBC material feedstock as well as MCrAlY bond coat used in the study. For this case, morphological, particle size distribution as well as other informatic data can be provided in section 2.1.

4. The authors should explain how the digital imaging correlation (DIC) analysis was performed and provide more details and discussion on the results obtained on DIC for different cases of the coatings.

5. The discussion on the role of roughness in crack initiation could be expanded upon with more theoretical background and supporting evidence.

6. The authors should provide more details on the mechanisms behind crack branching and path deflection in TBCs.

7. The implications of the findings for improving TBC performance and extending service lifetime should be further discussed.

8. The authors should provide more context on the limitations of the study and potential future research directions.

9. It would be helpful to include more information on the significance of TBCs in the broader field of materials science and engineering.

10. The authors should consider including more visual aids, such as diagrams or graphs, to enhance the clarity of their findings.

11. The discussion on fractography analysis could benefit from more detailed explanations and examples.

12. The conclusion could be strengthened by summarizing the main findings and their implications more clearly.

13. Section 3.4: It would be helpful to include more information on the specific types of microstructural defects that can contribute to stress concentration points and affect crack propagation in TBCs.

14. Please provide spraying parameters used for both cases of MCrAlY (NiCoCrAlY) bond coat and TBC top coat using separate tables.

15. Macro-scaled scale bars must be used for Fig. 1.

16. TBC / BC must be highlighted in all cross-sectional images, For example Fig. 2b.

17. Please provide major findings in the “Conclusions” section with a bullet-point style.

18. There is no description of the future plans for research in the first part of the “Conclusions” section. This should be completed in this section.

19. Recently published references are beneficial for this work. Please check and use new references focused on your work.

20. Also, please double-check and revise the reference list according to the MDPI / Coating journal requirements.

Extensive grammar / English revision is required for this submission.

For example, only for the first part of section 1: "Thermal barrier coatings (TBCs) are considered to be .... remarkable resolution and length scales [12–15].", The following correction must be made:

- "Beside" should be "in addition to."

- "Working temperature limits" should be "working-temperature limits" with a hyphen added.

- "During in-service working conditions" should be "During in-service working conditions," with a comma added after "conditions."

- "Etc." should be spelled out as "etcetera" or "and so on."

- "Air plasma spray TBC system" should be "Air plasma spray TBC system," with a capital letter added to "system."

- "The outer layer (namely top coat (TC))" should be "The outer layer, namely top coat (TC)," with a comma added after "layer."

- "TC layer provides thermal insulation" should be "The TC layer provides thermal insulation," with a capital letter added to "the."

- "While the second layer (so-called bond coat (BC))" should be "While the second layer, so-called bond coat (BC)," with a comma added after "layer."

- "M being Ni, Co or both" should be "M being Ni, Co, or both," with commas added after "Ni" and "Co."

- "Therefore, many factors can contribute to the failure of the TBCs including varied thermomechanical loading conditions, its microstructure (existing pores and cracks), and the developed residual stresses due to thermal expansion mismatch between TBC layers [6]." should be "Therefore, many factors can contribute to the failure of TBCs, including varied thermomechanical loading conditions, their microstructure (existing pores and cracks), and the developed residual stresses due to thermal expansion mismatch between TBC layers [6]." with a comma added after "TBCs."

- "Understanding of TBCs failure behavior" should be "Understanding TBCs' failure behavior," with an apostrophe added after "TBCs."

- "The tradition or ex-situ fractographic analysis" should be "The traditional or ex-situ fractographic analysis," with an "a" added to "tradition."

- "Cannot" should be capitalized as "Cannot."

- "Contrary" should be "In contrast," for clarity.

- "Real-time testing is relatively new approach" should be "Real-time testing is a relatively new approach," with an "a" added before "relatively."

- "Remarkable resolution and length scales" should be "remarkable resolution and length scale

Author Response

Dear Reviewer,

The authors greatly appreciate the effort and time that the reviewer has devoted to providing us with such valuable comments and suggestions on our manuscript. All changes made in the manuscript are highlighted in yellow. Below are specific replies to reviewer #1 comments.

Section 1; The introduction could benefit from more background information on TBCs and their applications in gas turbine engines as well as other industrial applications.

Thanks for your valuable comment, more information regarding the TBCs applications has been added in the revised manuscript.

Introduction section, page 1&2.

Thermal barrier coatings (TBCs) are considered to be of the most significant applications of thermal spraying, which are widely exploited in industrial components and aero gas turbines to protect underlaying substrate material from oxidation and corrosion, beside extending the working temperature limits of the substrate material [1,2]. For instance, TBC broadly utilized to protect the hottest sections of gas turbine engines (e.g., turbine blades and vanes) [3–5], internal combustion engines components (e.g., combustion chambers) [2,6] and several parts subjected to sever working conditions in other industries [5]. In gas turbine engines, the utilization of TBCs in addition to internal cooling of the underlying superalloy components, attains a temperature reduction of up to 300 °K in the surface of the superalloy component [7]. Hence, turbine engines can operate at temperatures beyond the melting temperature of the superalloy, thus increasing the energy efficiency and performance of engines. Furthermore, TBCs are widely used to provide insulation of the combustion chamber components of Diesel engine (referred to as low heat rejection (LHR) engines) [2]. TBCs can reduce the heat transfer between the gases in the cylinder and the cylinder wall promoting advantages including increased combustion temperature, fuel efficiency, thermal fatigue protection of the underlying metallic surfaces and possible reduction of engine emissions [2,6].

Section 2.3: The authors should provide more details on the experimental setup and methodology used for the real-time three-point bending and SEM analysis.

Many thanks for your valuable comment. More clarification about the SEM imaging has been added to the revised manuscript.

Section 2.3, page 5.

During the conducted in-situ tests, the field emission FEG-SEM was exploited to capture high quality images of the progressive failure of the APS coating using SE and BSE modes. The scanning parameters used during both imaging modes (SE and BSE) were, an acceleration voltage of 10 KeV, a working distance of approximately 20 mm, aperture size of 30 µm. SEM images were recorded at different magnifications (e.g., 50x, 100x, 150x, 300x, and 1000x) during the in-situ bending experimentation to help in the subsequent fracture analysis. The recorded sequence of images for the APS coating during the bending experimentation were properly aligned to generate illustrative videos of the coating failure (See Supplementary files).

Furthermore, the details about the in-situ bending experimentation including the used testing profile, in-situ stage specifications, and the attained methodology are provided in the revised manuscript as follow:

Section 2.4, page 5.

Three-point bending tests were performed inside SEM equipped with Micromecha Proxima testing stage (Schwerte, Germany). The stage has a 3 KN load cell and built-in linear encoder to provide measurements for the mechanically applied forces and displacement, respectively (see Figure 1 (b)). The crosshead movement of the in-situ stage was achieved using a drive screw located on the left-hand side, as illustrated in Figure 1 (b).The 3PB test was performed using the displacement control mode with cross head speed of 2 µm/sec and interrupted loading mode, thereby at defined displacement (50 µm), testing was paused, and the progressive coating fracture was recorded with acquisition of high-resolution SEM images.

It would be helpful to provide more information on the specific TBC material feedstock as well as MCrAlY bond coat used in the study. For this case, morphological, particle size distribution as well as other informatic data can be provided in section 2.1.

Thanks for your valuable comment, more information regarding the material feed stock for the TC and BC layers has been added in the “Coating Deposition and Material Properties” section of the revised manuscript. Also, the authors would like to clarify that the powders used are commercially available, and any interested party may contact the manufacturer to obtain the material or its specifications.

Section 2.1, page 4.

The bond coat was applied using an F4-MB type plasma torch and a commercial grade NiCoCrAlY powder (Amperit 410.860, Höganäs, Sweden) with approximately 23% Cobalt, 17% Chromium,12.5% Aluminum, 0.45% Yttrium and the balance Nickel. The powder was manufactured by gas atomization and had a particle size distribution D₁₀: 45µm, D₉₀: 90µm. The coating was applied to a thickness approximately 190µm in 8 passes of the plasma torch.

The ceramic topcoat was also applied using an F4-MB type plasma torch to a thickness of approximately 350µm. The powder utilized for the ceramic layer was a commercial grade yttria stabilized zirconia having approximately 7 weight percent yttrium oxide (Amperit 831.007, Höganäs, Sweden). The powder was produced from a plasma spherodisation process and had a particle size distribution D₁₀: 16µm, D₉₀: 90µm. The plasma spherodised powder is sometimes known as homogenized oven spheroidized powder (HOSP). This manufacturing process produces powder particles that are often hollow shells or they may contain untreated fine particles inside the shell structure. This powder morphology has the benefit of producing many lamellar porosity features that provide high thermal insulation, with some sacrifice in terms of thermal shock stability [28]. For both the bond coat and top coat, proprietary process parameters were used to deposit the coatings and the samples can be considered examples of current state-of-art thermal barrier coatings used industrially, similar examples can be found in previously published work [29].

The authors should explain how the digital imaging correlation (DIC) analysis was performed and provide more details and discussion on the results obtained on DIC for different cases of the coatings.

Thanks for your valuable comment, more details regarding the performed DIC analysis have been added in section 2.4. Also, more discussion has been included in the corresponding results section.

Section 2.4, page 7.

The Ncorr vers. 1.2 was employed to correlate the recorded SEM images [37]. First, a region of interest (ROI) was defined in the reference undeformed image, which divided into small subsets of pixels (subsets typically overlap). The parameters used for the carried out DIC analysis were subset size of 30 pixels with subset spacing of 2 pixels. Hence, the displacement calculations were conducted every two pixels of the image. The subsets used to calculate the displacements are in a circular shape. Different iterations were conducted to select the appropriate DIC parameters (i.e., subset size and spacing). For instance, the subset size was selected to comprise enough regional variation of features, enabling successful tracking of subset locations while avoiding noisy displacement data. Whereas subset spacing can help to attain good resolution results in the DIC analysis. After the displacements are computed at each predefined pixel spacing, the deformation fields can be obtained.

Section 3.1, page 11&12.

As mentioned before, the DIC strain prediction (e_xx) at onset of crack initiation does not match well with the theoretical calculations. Hence, to ensure the reliability of the conducted DIC analysis, a virtual extensometer was used to verify the DIC measurements. Virtual extensometer technique is quite similar to the conventional methods of strain measurement at macroscopic level. In this technique, the displacement between two points at each image were chosen to calculate strain [18]. Thus, the change in the horizontal extension (x-direction) between two points defined within the TC layer at nearly 75 µm from its free surface (see Figure 5 (a)) was measured at different displacements up to coating’ failure to calculate the developed strain. Furthermore, the strain in x-direction (e_xx) along the trace of the imposed virtual extensometer was extracted from the DIC software, averaged and compared to the corresponding values obtained from the virtual extensometer at different bending displacement as shown in Figure 5 (b). There is acceptable agreement between the strain calculations from the virtual extensometer and the averaged DIC measurements at different stage displacement. Moreover, the evolution of strain values attains an almost linear behavior that corresponds to the increase in bending displacement, which confirmed by both DIC and virtual extensometer measurements. This trend may be attributed to the inherent brittleness of the coating material. The DIC strain (e_xx) distribution along the trace of the virtual extensometer at different bending displacement are shown in Figure 5 (c). It can be noticed that as the bending displacement increases, strain concentration take place in the cracking area that can be recognized as peaks, Figure 5 (c). Whereas the regions surrounding the cracks are characterized by relatively low strain values. Furthermore, the strain distribution along the extensometer line reveals an uneven pattern, which may be attributed to the heterogeneous microstructure of the APS coatings. These microstructural features may cause differing strain accommodations.

Section 3.2, page 15.

In order to verify the obtained DIC measurements for sample #2, a virtual extensometer was employed to measure the change in the horizontal displacement between two points defined in the TC layer at approximately 90 µm from its free surface at different displacement and up to the failure of the APS coating, see Figure 9 (a). The strain in x-direction (e_xx) computed through the virtual extensometer is compared to the averaged strain values obtained from the DIC e_xx-map along the extensometer line at different bending displacement, Figure 9 (b). The strain values calculated from the virtual extensometer acceptably agreed with those obtained from averaging the DIC measurements at different stage displacement as shown in Figure 9 (b). The strain measurements exhibited a linearity with the progression of the bending load up to failure. Figure 9 (c) shows the distribution of strain in x-direction (e_xx) obtained from the DIC analysis along the virtual extensometer line at different bending displacement. The strain distribution shows the concentration of strain within the cracking area, indicated by the central peak, whereas a relatively lower strain values are observed in other regions as shown in Figure 9 (c), which matches with the DIC strain fields introduced in the prior section. Moreover, the DIC strain measurements in Figure 9 (c) shows an uneven distribution, which may be related to the varied microstructural features of the coatings, thereby accommodating the strain differently.

Section 3.3, page 20.

The verification process for the conducted DIC analysis of sample #3 was achieved through comparing the strain values computed through a virtual extensometer imposed in the TC layer at approximately 100 µm from its free surface (see Figure 13 (a)) and the averaged DIC measurements along the extensometer line. Figure 13 (b) shows acceptable match between the strain (e_xx) calculated from the virtual extensometer and the averaged strain values obtained from the DIC strain field in x-direction at different bending displacement (up to failure of coating). Again, the strain evolution under the different bending displacement up to failure showed nearly a linear trend as can be seen in Figure 13 (b). Figure 13 (c) shows the distribution of strain in x-direction (e_xx) obtained from the DIC analysis along the extensometer line at different bending displacement, which confirms the strain localization at the cracking regions (two peaks at extreme ends of the graph). Again, the distribution of e_xx-strain along the extensometer line (Figure 13 (c)) exhibited an uneven pattern, that could be attributed to the microstructure heterogeneity of the APS coatings.

The discussion on the role of roughness in crack initiation could be expanded upon with more theoretical background and supporting evidence.

Thanks for your valuable comment, more discussion concern crack initiation behavior supported with relevant literature has been added in the revised version.

Section 3.1, page 8.

Surface roughness creates local stress concentrations at critical points or irregularities (e.g., off-peaks or valley regions) on the coating's surface and interfaces [39]. These stress concentrations can be the driving force behind the crack initiation and propagation withing the coating system. For instance, one of the most concerns that affect TBCs’ lifetime is the interfacial adhesion between the coating layers. Many researchers studied the effect of surface roughness at the interfaces between TBCs layers (e.g., surface roughness of BC and TGO) to quantify the stress distribution along the surface roughness using numerical modelling procedures [39–41]. It was demonstrated that rougher surfaces or interfaces led to easier crack initiation in addition to that stress localization was detected to occur at the off-peak or valley regions promoting micro-crack formation. These observations match well with the detect crack initiation behavior for the APS coatings during the in-situ bending experimentation.

The authors should provide more details on the mechanisms behind crack branching and path deflection in TBCs.

Thanks for your valuable comment, more discussion concern mechanisms behind crack branching and path deflection based on literature has been added in section 3.1.

Section 3.1, page 9.

In order to provide further insights into the potential mechanisms responsible for crack propagation, branching and deflection observed in the APS coatings, the following discussion is presented. Good understanding of the crack initiation and propagation mechanism inside TBCs is important to prolong its lifetime and performance. The crack takes place when the energy available for crack growth is sufficient to surpasses the resistance of the material [45]. Yoffe [46] made the assumption that the crack growth occurs along the direction normal to the induced maximum stress, and the crack propagation process is stable provided that the propagation velocity is lower than a critical value. Once the crack velocity exceeds this critical value, the propagation process becomes unstable. At this point, the stress state at the crack tip changes, and the hoop stress in the vicinity of the crack tip will have a maximum angle of approximately 60° from the crack propagation direction, which may result in crack branching. Another possible explanation for the crack branching phenomena was made by Ravi-Chandar and Knauss [47], who suggested that exist microcracks in front of the main crack can lead to branching as a result of the growth and coalescence of the microcracks. Crack branching phenomena led to increased cracking area and hence accelerating the deterioration of the coating system. Crack branching is particularly common in brittle materials that can occur symmetrically or asymmetrically [45]. As mentioned before and based on the in-situ observations, crack propagation occurred in the APS coating along the direction normal to the induced maximum tensile stresses, while asymmetrical crack branching took place due to the existence of microcracks and pores in front of the main crack. The crack branching mechanism is complicated phenomena as it dependent on may factors including crack tip speed, stress intensity factor and its rate [45]. Therefore, numerical simulations are usually exploited to study crack branching. On the other hand, altering the path of crack propagation is a favorable mechanism to enhance the toughness of brittle materials (e.g., ceramics). Crack deflection dissipate or absorbs the fracture energy needed for crack propagation and improves the fracture toughness of the coatings [42,43]. Various factors can affect the crack propagation, such as changes in material properties, interfaces, stress gradients, and the presence of inclusions or defects. The conducted in-situ observations showed that crack deflection occurred at the splat boundaries.

The implications of the findings for improving TBC performance and extending service lifetime should be further discussed.

Many thanks for your comment further discussion were added in the revised manuscript regarding how the cracking behavior can affect the performance APS-TBC coating and its implications (e.g., introduction, results and conclusion sections).

The authors should provide more context on the limitations of the study and potential future research directions.

Thanks for your valuable comment, this has been elaborated in the conclusion section.

Conclusion section, page 24.

The current research work concern studying the cracking behavior of the as-deposited APS coatings without being subjected to any heat treatment or oxidation procedures to imitate in-service working conditions. The provided observation may differ when the coatings are subjected to thermal loading procedures, such as heat treatment and thermal cycling. In the future work, in-situ bending experimentation will be conducted on heat treated coatings to provide deep insights into the effect of heat treatment on the cracking behavior of the coating system. Heat treatment processes can significantly alter the microstructure, mechanical properties, and residual stresses within the coating, which, in turn, can influence its response to external mechanical loading.

It would be helpful to include more information on the significance of TBCs in the broader field of materials science and engineering.

Many thanks for your comment more information was added in the revised manuscript in the introduction section, page 1,2.

The authors should consider including more visual aids, such as diagrams or graphs, to enhance the clarity of their findings.

Many thanks for your comment, more figures have been added to the revised manuscript (i.e., Figure 5, 9 and 13). The revised manuscript contains fifteen figures (including SEM images, charts and schematic representations) to illustrate and describe the outcomes of the current research, that should be sufficient to provide the intended explanations.

The discussion on fractography analysis could benefit from more detailed explanations and examples.

Many thanks for your comment more discussion supported with examples from literature were added in the revised manuscript in fractography analysis section.

Section 3.4, page 22.

The splat cracking mechanism has been detected by other researchers [8,27]. For instance, the splat cracking along with interlinking of the pre-exist cracks were the dominant mechanisms for crack propagation for different coatings (i.e., APS and HVOF-sprayed Al₂O₃-40ZrO₂ coatings) under in-situ bending driven failure experimentation [27]. Mušálek et al. [8] conducted In-situ bending tests on free-standing plasma sprayed ceramic (alumina) and metallic (stainless steel 316L) coatings to observe its fracture behavior. For the ceramic coatings, splat cracking was defined as a prime failure mechanism. Also, splat debonding was detected in the regions weakened by pores. While for the bond coat layer, the fracture mechanisms for crack propagation are assumed to be mainly due to cracking of oxides inclusions along with splat debonding and cracking as seen in Figure 12 (b). Splat debonding can take place in coatings at regions where the splat bonds were weakened by pores and oxide stringers [8,44]. For instance, Mušálek et al. [9] introduced fractography analysis of failed APS-TBC coating under bending experimentation and showed that loose bonding between splats structure in the BC layer permitted crack propagation along the splat-splat interface. The oxides or voids (pores and microcracks) at the spalts interface were assumed to deteriorate the splats bonding strength.

The conclusion could be strengthened by summarizing the main findings and their implications more clearly.

Many thanks for your comment this have been addressed in the revised manuscript.

Conclusion section, page 23.

Crack initiation can take place at locations of high stress concentrations (e.g., valleys) that are not necessarily at the theoretical point of maximum stress. This behavior can be attributed to the coatings’ high surface roughness, resulting in locally weakened regions (i.e., valleys) where cracks can originate.
Crack branching occurred when the path of crack propagation came along with pre-existing microstructural defects (e.g., pores and micro cracks), thereby affecting the durability of the TBC system and facilitate its failure. The increased cracking area promote oxygen infiltration through the TC layer leading to accelerated growth of the TGO layer beyond critical thick-ness, causing coating delamination. While the splat microstructure of the APS-TBC helped in deviating the crack propagation path which can help for enhanced fracture toughness of the coatings through dissipating energy required for crack propagation.
Based on the fractography analysis, it can be inferred that the dominant crack propagation mechanism in the top coat is splat cracking. On the oth-er hand, for the bond coat, crack propagation occurs through cracking of the lamellar oxide stringers, along with splat cracking and debonding.
DIC measurements can provide effective method to track the crack initiation and propagation process in the APS coatings. Developed strain maps showed higher strain levels within the cracking region, indicating localized deformation and stress concentration.

Section 3.4: It would be helpful to include more information on the specific types of microstructural defects that can contribute to stress concentration points and affect crack propagation in TBCs.

Many thanks for your comment more information have been added in the revised manuscript.

Section 3.1, page 8&9.

Section 3.4, page 22.

Please provide spraying parameters used for both cases of MCrAlY (NiCoCrAlY) bond coat and TBC top coat using separate tables.

Thanks for your valuable comment, more information has been added in the “Coating Deposition and Material Properties” section of the revised manuscript (page 4). The authors would like to clarify that the deposition process parameters used are proprietary to a commercial coating manufacturer and are considered confidential. In the current research work, the authors are not looking at comparing the impact of plasma condition changes, and the main aim is to provide more understanding to the cracking behavior of the studied coatings and to assign fracture features to its microstructure. Hence, there is little impact on the scientific validity of the work.

Macro-scaled scale bars must be used for Fig. 1.

Many thanks for your comment. Figure 1 (a) contains the schematic representation of the specimen and the bending rig, which showing their dimensions in mm (units mentioned in the figure caption). Figure 1 (b) shows image of the bending rig inside SEM chamber, highlighting the different components. Therefore, there is no need to add scale bar since the dimensions, units and components are stated clearly. Scale bars have been used in all the other figures when relevant.

TBC / BC must be highlighted in all cross-sectional images, For example Fig. 2b.

Many thanks for your comment. The authors would like to confirm that the abbreviations for the coating’ layers were provided at each cross-sectional figure (i.e., first subfigure). Repeating the same abbreviations for each subfigure is a redundant information and can affect the clarity of the figures. For instance, putting unnecessary data on subfigures can hide regions of cracks propagation. Therefore, the first subfigure of each figure contains and highlights all the needed information.

Please provide major findings in the “Conclusions” section with a bullet-point style.

Many thanks for your comment this have been addressed in the revised manuscript.

Conclusion section, page 23.

Crack initiation can take place at locations of high stress concentrations (e.g., valleys) that are not necessarily at the theoretical point of maximum stress. This behavior can be attributed to the coatings’ high surface roughness, resulting in locally weakened regions (i.e., valleys) where cracks can originate.
Crack branching occurred when the path of crack propagation came along with pre-existing microstructural defects (e.g., pores and micro cracks), thereby affecting the durability of the TBC system and facilitate its failure. The increased cracking area promote oxygen infiltration through the TC layer leading to accelerated growth of the TGO layer beyond critical thick-ness, causing coating delamination. While the splat microstructure of the APS-TBC helped in deviating the crack propagation path which can help for enhanced fracture toughness of the coatings through dissipating energy required for crack propagation.
Based on the fractography analysis, it can be inferred that the dominant crack propagation mechanism in the top coat is splat cracking. On the oth-er hand, for the bond coat, crack propagation occurs through cracking of the lamellar oxide stringers, along with splat cracking and debonding.
DIC measurements can provide effective method to track the crack initiation and propagation process in the APS coatings. Developed strain maps showed higher strain levels within the cracking region, indicating localized deformation and stress concentration.

There is no description of the future plans for research in the first part of the “Conclusions” section. This should be completed in this section.

More clarification on the intended future work have been added in the revised manuscript.

Conclusion section, page 24.

In the future work, in-situ bending experimentation will be conducted on heat treated coatings to provide deep insights into the effect of heat treatment on the cracking behavior of the coating system. Heat treatment processes can significantly alter the microstructure, mechanical properties, and residual stresses within the coating, which, in turn, can influence its response to external mechanical loading.

Recently published references are beneficial for this work. Please check and use new references focused on your work.

This has been addressed in the revised manuscript. More recent references focused on the current research work were added (more than 10 references have been added) as follow:

Mondal, K.; Nuñez III, L.; Downey, C.M.; van Rooyen, I.J. Thermal Barrier Coatings Overview: Design, Manufacturing, and Applications in High-Temperature Industries. Ind. Eng. Chem. Res. 2021, 60, 6061–6077.
Ramalingam, S.; Murugesan, E.; Rajendran, S.; Ganesan, P. Application of Thermal Barrier Coating for Improving the Suitability of Annona Biodiesel in a Diesel Engine. Therm. Sci. 2016, 20, 973–979.
Blaber, J.; Adair, B.; Antoniou, A. Ncorr: Open-Source 2D Digital Image Correlation Matlab Software. Exp. Mech. 2015, 55, 1105–1122, doi:10.1007/s11340-015-0009-1.
Xu, H.; Guo, H.; Gong, S. 16 - Thermal Barrier Coatings. In Woodhead Publishing Series in Metals and Surface Engineering; Gao, W., Li, Z.B.T.-D. in H.T.C. and P. of M., Eds.; Woodhead Publishing, 2008; pp. 476–491 ISBN 978-1-84569-219-3.
Song, J.; Qi, H.; Shi, D.; Yang, X.; Li, S. Effect of Non-Uniform Growth of TGO Layer on Cracking Behaviors in Thermal Barrier Coatings: A Numerical Study. Surf. Coatings Technol. 2019, 370, 113–124.
Jiang, J.; Wang, W.; Zhao, X.; Liu, Y.; Cao, Z.; Xiao, P. Numerical Analyses of the Residual Stress and Top Coat Cracking Behavior in Thermal Barrier Coatings under Cyclic Thermal Loading. Eng. Fract. Mech. 2018, 196, 191–205.
Yoffe, E.H. LXXV. The Moving Griffith Crack. London, Edinburgh, Dublin Philos. Mag. J. Sci. 1951, 42, 739–750.
Sun, Y.; Edwards, M.G.; Chen, B.; Li, C. A State-of-the-Art Review of Crack Branching. Eng. Fract. Mech. 2021, 257, 108036.
Li, S.; Qi, H.; Song, J.; Yang, X.; Che, C. Effect of Bond-Coat Surface Roughness on Failure Mechanism and Lifetime of Air Plasma Spraying Thermal Barrier Coatings. Sci. China Technol. Sci. 2019, 62, 989–995.
Ravi-Chandar, K.; Knauss, W.G. An Experimental Investigation into Dynamic Fracture: III. On Steady-State Crack Propagation and Crack Branching. Int. J. Fract. 1984, 26, 141–154.
Bertrand, G.; Bertrand, P.; Roy, P.; Rio, C.; Mevrel, R. Low Conductivity Plasma Sprayed Thermal Barrier Coating Using Hollow Psz Spheres: Correlation between Thermophysical Properties and Microstructure. Surf. Coatings Technol. 2008, 202, 1994–2001.
Curry, N.; Markocsan, N.; Li, X.-H.; Tricoire, A.; Dorfman, M. Next Generation Thermal Barrier Coatings for the Gas Turbine Industry. J. Therm. spray Technol. 2011, 20, 108–115.
Vaßen, R.; Kaßner, H.; Stuke, A.; Hauler, F.; Hathiramani, D.; Stöver, D. Advanced Thermal Spray Technologies for Applications in Energy Systems. Surf. coatings Technol. 2008, 202, 4432–4437.

Also, please double-check and revise the reference list according to the MDPI / Coating journal requirements.

Thanks for your comment this has been checked.

Comments on the Quality of English Language

Extensive grammar / English revision is required for this submission.

Many thanks for your comment the English style of the manuscript have been revised thoroughly.

- "Beside" should be "in addition to."