Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

Round 1

Reviewer 1 Report

Paper shows the TBC composite coating concept and the details regarding its manufacturing process and properties. Paper content is worth publishing while it is well prepared. My comments on the paper:

1.     The introduction well displays the idea of work and shows the drawbacks of knowledge.

3.      Repeated phrase, L182 and L200: a porosity of 13~15% and a topcoat thickness of about 200 µm

4.      From line 310: HV300 phrase should be explained. Values of hardness should be clarified. It is not clear wheatear the 300gf load has been employed. Then the hardness results should be written as follows: 607HV0.3 (according to ISO standard).

5.      Please improve this phrase and clarify the meaning of dots. “Under the combined effect of primary thermal cycling (cracking and splat delamination, 396
…) and secondary high-velocity gas impact (impact fatigue, erosive material removal, …), 397
no doubt the TBCs’ failure mechanism was unique and the TBCs’”

6.      Improve the formatting of the caption of figure 10 (different font sizes)

7. Conclusions are accurately stated.

Author Response

1 The introduction well displays the idea of work and shows the drawbacks of knowledge

2 L195 - improve the typo "onto Haynes 188 disk substrates. ace bonding

Yes, there is a type, and the typo sentence is deleted and corrected

3. Repeated phrase, L182 and L200: a porosity of 13~15% and a topcoat thickness of about 200 µm

Yes, thank you for pointing it out. The repeated sentence in line 200 is deleted

4 From line 310: HV300 phrase should be explained. Values of hardness should be clarified. It is not clear wheatear the 300gf load has been employed. Then the hardness results should be written as follows: 607HV0.3 (according to ISO standard).

As the reviewer suggested, HV300 is reformatted as HV0.3 and a note is added “(load: 300 grams) in line 310.

5 Please improve this phrase and clarify the meaning of dots. “Under the combined effect of primary thermal cycling (cracking and splat delamination, 396
…) and secondary high-velocity gas impact (impact fatigue, erosive material removal, …), 397
no doubt the TBCs’ failure mechanism was unique and the TBCs’”

The sentence is modified to clarify the main factors leading to the coating damages under the thermal and dynamic energized conditions

6 Improve the formatting of the caption of figure 10 (different font sizes

Yes, the font size is re-formatted

7 Conclusions are accurately stated

Thanks, and some of the statements are improved in lines 645, 649, 655

Reviewer 2 Report

This study explores a path toward durable thermal barrier coatings with improved multi-properties as to be capable for extreme environment applications. The advantages of the exemplary TBCs with dual phase and tri-phase structures over single phase TBCs were demonstrated in the variety of property tests. The samples fabricated are carefully analyzed and property tests are elaborately performed. If a few questions in the paper are resolved, it can be accepted for publication.

1.       In Figure 8, why does the thermal conductivity begin to increase above 1000°C?

2.       In thermal shock tests, why does the spallation occur from the edges toward the center?

3.       The high-velocity gas stream is causing significant damage to the surface coating. Can you quantify the characteristics of the gas stream. For example, the velocity of the gas stream.

4.       In solid particle erosion test, why is the erosion rate maximum at an impact angle of 60°?

The reason is not clear from the SEM image alone.

5.       In Figure 12, it is difficult to distinguish between groove, crater, and scar.

Minor point,

Line 350: 27% may be correct for 17%.

Author Response

1. In Figure 8, why does the thermal conductivity begin to increase above 1000°C?

A good question. A paragraph is added to give explanation, As studied by Schlichting et al., in J. Mater. Sci., Vol. 36 (2001), p 3003-3010: The dependence of thermal conductivity of YSZ on temperature can be explained by a theory considering the effect of scattering of phonons by point defects (oxygen vacancies and solute) and by the “hopping” of oxygen vacancies. It also considers an increase in the effective thermal conductivity at high temperatures due to radiation. The Maxwell theory also is used to analyze the behavior of thermal conductivity of the porous at high temperatures. For the porous coating, some pores will be annihilated and result in the increase in k value above 1000C.

2. In thermal shock tests, why does the spallation occur from the edges toward the center?

A paragraph is added to give explanation Line 368: It is generally observed for disk-type specimens due to the so-called edge effect. The sharp edge can cause localized concentration of thermal stress during thermal shock test.

3. The high-velocity gas stream is causing significant damage to the surface coating. Can you quantify the characteristics of the gas stream? For example, the velocity of the gas stream.

Per some literatures, the flame velocity can be in the range 150-250 m/s.

4. In solid particle erosion test, why is the erosion rate maximum at an impact angle of 60°? The reason is not clear from the SEM image alone.

A paragraph is added to give explanation Line 456: The maximum erosion rates can be explained by two mechanisms, (1) Alloy material: ductile erosion mechanism at lower impact angle, and (2) Ceramic material: brittle erosion mechanism at high angle. The coating is a ZrO2-based ceramic well known with high toughness as well as containing certain porosity, and therefore as a composite material behalf with combined ductile/brittle property at a middle angle.

5. In Figure 12, it is difficult to distinguish between groove, crater, and scar.

 The notes and arrows in the SEM images can be helpful to identify the morphologies of grove, crater and scar in terms of orientation, shape and size.

• Minor point, Line 350: 27% may be correct for 17%.

Thanks, the value is corrected to 17%.