*2.2. Characterizations*

To obtain the cross-sectional microstructure and mechanical properties of the TBCs, the specimens are cold-mounted with an epoxy resin and then polished using SiC papers and finally polished with 3 and 1 μm diamond pastes in sequence. The cross-sectional microstructures of TBC samples are observed using a scanning electron microscope (SEM; Model JSM–5610, JEOL, Tokyo, Japan). The hardness of the coatings is determined using a microindentor (HM-114, Mitutoyo Corp., Kanagawa, Japan) with a Vickers tip for a load of 3 N [17]. More than 10 indentation points are measured for achieving statistical results. The sizes of hardness impression are measured using the SEM and all experiments are performed at a room temperature. The adhesive strength of the TBC samples is determined following the American society for testing and materials (ASTM) standard (ASTM C-633-01) [18].

#### *2.3. Cyclic Furnace Thermal Fatigue, Flame Thermal Fatigue, and Thermal Shock Tests*

CFTF test are performed for all samples in a specially designed programmable furnace until 1429 cycles. The surface of the sample is about 1100 ◦C and the backside of the sample is air-cooled to keep the temperature di fference of 150 ◦C. The dwell time is 60 min. Then the samples are cooled in a static air environment for 10 min. FTF tests are also performed for 1429 cycles using liquefied petroleum gas as the heating source. The top surface of sample is heated with flame of about 1100 ◦C for 5 min. Then the samples are cooled in a static air environment for 25 min. The criterion of failure in FTF test is defined as over 25% spalling of top coat. The TBC samples are cycled to the failure criterion for observing indication of failure. For TS tests, the TBC samples are heated in a mu ffle furnace till 1100 ◦C for 60 min, then the samples are directly quenched in water for 5 min. The water temperature is kept at 20–35 ◦C during the test. For failure criteria, over 50% of the spalled region in top coat is used [19–22]. In TS tests, more than five specimens are tested to achieve statistical results.

## **3. Model Details**

## *3.1. Finite Element Model*

Due to symmetry, a two-dimensional (2D) axisymmetric FE model of TBCs is developed to simulate the coating behavior in CFTF test conditions, and correlate the predictions with the experimental observations. The model consists of an YSZ coat (top), NiCrCoAlY bond coat (middle), and Inconel 718 substrate (bottom). Because the TGO layer is very thin, it is not included in the model. The dimensions of the TBCs are consistent with those described in Section 2.1. The computation is conducted using FE software ANSYS Workbench (ANSYS 19) with PLANE182 axisymmetric element type.

## *3.2. Properties of Materials Used in the FE Model*

To evaluate the thermal cyclic effect on the TBCs, the fatigue-creep behavior of the bond coat is focused on as a potential contributor leading to TBCs failure. The creep of the bond coat leads to TBC system failure and spalling due to differential stresses in the coating layers during thermal cycling. This means that life prediction of the TBC system must take into account of the fatigue-creep of the bond coat.

In this FE model, a secondary steady-state creep equation is incorporated to account for high-temperature creep, using Norton power-law creep (- = *A*σ*<sup>n</sup>*). The temperature-dependent creep constants " *A*" and "*n*" for YSZ, Inconel 718, and NiCoCrAlY bond coat made using HVOF and APS [23,24] are noted in Table 1. The materials properties of YSZ [25], bond coat [25], and substrate Inconel 718 [26] as listed in Table 2.


**Table 1.** Temperature-dependent creep constants used in the FE model [23,24].


**Table 2.** Temperature-dependent material properties used in the FE model [25,26].

## *3.3. Boundary Conditions*

A transient thermomechanical analysis is conducted with the same cycle timings and boundary conditions as CFTF test described in Section 2.3. In transient thermal analysis, a single cycle of total 4200 s, with 3600 s of constant heating at 1100 ◦C, is followed by 600 s of cooling at ambient thermal conditions of 25 ◦C. The convection coefficient between samples and the environment is assumed as 25 <sup>W</sup>/(m<sup>2</sup>·◦C). Because the TBC samples are rested on a sample holder, the bottom surface is assumed to be perfectly insulated.

## **4. Results and Discussion**

## *4.1. The Microstructure Analysis*

The cross-sectional microstructures of the TBC samples are shown in Figure 1, where Figure 1(a–1,b–1) are the microstructures of the bond coats fabricated by APS and HVOF, respectively. The top layers of all the samples are prepared by the same method using the EB-PVD method. The top coat of all the coating systems show a columnar structure with small gaps between the columns. The gaps develop from the interface between the top and bond coats to the coating surface. The fine inter columnar pores are mostly aligned to the heat flow direction. These pores help reducing the thermal conductivity of TBC systems. The interface of the TBC samples (Figure 1(a–2,b–2)) shows no cracks and a relatively flat shape. A thin TGO layer about 1–2 μm is found at the interface of each samples without thermal tests. The heat treatment for deposition of the top coat is the main reason for the formation of the thin TGO layer. The dense TGO layer can prevent oxygen diffusing into the bond coat. The bond coat prepared by APS shows several intrinsic defects, such as unmelted particles, oxides, and pores. The microstructures of bond coats that prepared by HVOF are denser and don't have oxide formation and interlayer cracks.

**Figure 1.** *Cont*.

**Figure 1.** Cross-sectional microstructures of as-prepared TBCs: APS bond coat TBC system (the overall (**a–1**) and the interface (**a–2**) microstructures), and HVOF bond coat TBC system (the overall (**b–1**) and the interface (**b–2**) microstructures).

## *4.2. Service Life of TBC Systems*

The cross-sectional microstructures of TBCs with different bond coats after FTF tests are shown in Figure 2. During cyclic thermal exposure tests, the ceramic top coat appear starting sintering phenomenon. The microstructures of the top coats are more compact and the porosity is reduced due to the densification during the sintering process.

After FTF tests, the interface microstructures are very similar to each other, compared with the as-prepared samples. There is no cracking or delamination in the interface microstructures of each TBC system, as shown in Figure 2(a–2,b–2). The relatively short heating time (5 min) and continuous high-temperature time of the interface (only 2 min) result in that the TGO layer is not fully developed after 1429 cycles. The total heating time is only 119 h. The thickness of the TGO layer is in the range of 2–3 μm without substantial increase.

**Figure 2.** Cross-sectional microstructures of TBC system after FTF tests: APS bond coat TBC system (the overall (**a–1**) and the interface (**a–2**) microstructures) and HVOF bond coat TBC system (the overall (**b–1**) and the interface (**b–2**) microstructures).

Figure 3 shows the cross-sectional microstructures of each TBCs system after CFTF tests. The APS and HVOF bond coats (Figure 3(a–1,b–1)) are delaminated at the interface between the TGO layer and the bond coat in the range of 100–380 and 210–390 cycles, respectively. The APS bond coat shows more degree of oxidation in the interbond coat after CFTF test compared with APS bond coat without any test. The oxidation of the bond coat leads to a change in the sign of stresses due to the smaller CTEs of the TGO layer. It is assumed that small cracks will be created at the peak tips [27]. After CFTF test, the HVOF bond coat shows a different oxidation behavior compared with APS bond coat. No internal oxides are found, but element segregation occurs.

**Figure 3.** Cross-sectional microstructures of TBCs after CFTF tests: TBC with APS bond coat (the overall (**a–1**) and the interface (**a–2**) microstructures) and TBC with HVOF bond coat (the overall (**b–1**) and the interface (**b–2**) microstructures).

The service lives of the all the TBC systems are summarized in Figure 4. The safe zone of the TBC systems with the HVOF bond coat is twice that with the APS bond coat, although they can reach the same ultimate lifetime. After CFTF tests, the HVOF bond coat TBCs (Figure 3(b–2)) show a diffusion zone near the interface, indicating that these elements are involved in the late reaction to form the TGO consisting of mixed oxide clusters [28]. The increase of the thickness of the TGO layer leads to TBC system failure. In addition, the TBCs with the APS bond coat show a different oxidation behavior. The outer APS bond coat near the interface of the top and bond coats shows a diffusion zone similar to the HVOF bond coat, while the inter-APS bond coat has more oxidation. The lifetime of the TBCs indicates that the TBC systems with the HVOF bond coat show a better thermal durability than those with the APS bond coat.

Figure 5 shows the cross-sectional microstructures of the TBCs after TS tests. It shows a typical mode of delamination at the interface between the bond coat and the TGO layer. During the thermal shock process, a large temperature difference is developed between the substrate and the top coat, which causes thermal or residual stresses at the interface between the TGO layer and the bond coat. When the TGO layer reaches a certain thickness in cyclic thermal fatigue, the TBC will be cracked or delaminated at the interface between the TGO layer and the bond coat, owing to the relatively low adhesive strength of the TGO layer and bond coat in the EB-PVD TBC system. In addition, the mismatch in the CTEs between the ceramics layer, TGO layer, and the bond coat leads to delamination and failure. In TS test, the lifetime of TBCs with the HVOF bond coat is obviously longer than those with the APS bond coat. The TBCs with the HVOF bond coat are delaminated in the range of 345–372 cycles, whereas the TBCs with the APS bond coats are delaminated in the range of 44–80 cycles, showing an aluminum depletion region in the HVOF bond coats similar to that after CFTF test. The results of thermal durability for the TBCs with different bond coats are summarized in Table 3 as a function of thermal exposure species.

**Figure 4.** The service lives of TBCs with different bond coats after CFTF tests.

**Figure 5.** Surface micrographs and cross-sectional microstructures of TBCs after TS tests: TBC with APS bond coat (the surface micrographs (**a–1**), cross-sectional (**a–2**) and magnified interface (**a–3**) microstructures) and TBC with HVOF bond coat (the surface micrographs (**a–1**), cross-sectional (**a–2**) and magnified interface (**a–3**) microstructures).


**Table 3.** Thermal durability of TBCs with bond coat species in various thermal exposure tests.

## *4.3. Mechanical Properties*

The hardness values of top coats before and after cyclic thermal exposure, which were measured using a Vickers indentation method, are shown in Figure 6.

**Figure 6.** Hardness values of top coats before and after cyclic thermal exposure. Indentation for hardness was conducted on the sectional planes with a load of 3 N. Open and filled marks indicate the hardness values of top coats before and after FTF tests, respectively.

The indentation tests are conducted on the samples' sectional planes. For the as-prepared samples, the hardness values of top coats, APS and HVOF bond coats, are determined to be 3.1 ± 0.3 (mean ± standard deviation) and 3.0 ± 0.3 GPa, respectively. After FTF tests, the hardness values of top coats are increased to 3.6 ± 0.4 and 3.4 ± 0.3, 3.3 ± 0.3 GPa for the APS and HVOF bond coats, respectively. The increase in the hardness values after FTF tests is due to the reduction of gaps between the adjacent columns, which is consistent with the microstructure evolution (Figure 2a,b). The microstructure evolution of the EB-PVD top coat is more advanced in CFTF and TS tests. The microstructure is densified owing to resintering in CFTF and TS tests, resulting in the disappearance of space between the columns and showing the delamination of the top coat at the interface between the TGO layer and the bond coat.

Adhesive strength is an important mechanical property for TBC systems, with a direct connection to interface stability. Therefore, the adhesive strength values of samples are measured before and after FTF test, which are shown in Figure 7. For the as-prepared samples with APS and HVOF bond coats, the adhesive strength values are determined to be 66.8 ± 5.4 and 76.9 ± 1.2 MPa, respectively. After FTF tests, the adhesive strength values of the samples with APS and HVOF bond coats are determined to be 35.7 ± 13.1 and 70.1 ± 8.9 MPa, respectively. The adhesive strength values of the samples with APS bond coats show a significant decrease, due to the TGO growth and internal APS bond coat oxidation behavior during FTF tests. The TBCs with HVOF bond coats show no obvious change after FTF test by virtue of the HVOF bond having a better oxidation resistance than APS bonds.

**Figure 7.** Adhesive strength values of TBCs before and after FTF tests.

The surface photographs and cross-sectional microstructures of the as-prepared samples after measuring the adhesive strength are shown in Figure 8. All of the as-prepared samples are completely delaminated near the interface between the jig fixture and the epoxy adhesive, indicating that the adhesive strength of the EB-PVD is sufficiently high.

**Figure 8.** Surface and cross-sectional microstructures after measuring adhesive strength for as-prepared TBCs: TBC with APS bond coat (the entire (**a–1**) and magnified (**a–2**) microstructures) and TBC with HVOF bond coat (the entire (**b–1**) and magnified (**b–2**) microstructures). Surface morphologies of each sample are inserted in each figure.

The surface and cross-sectional microstructures after measuring the adhesive strength values for the TBCs after FTF tests for 1429 cycles are shown in Figure 9.

**Figure 9.** Surface and cross-sectional microstructures after measuring adhesive strength for TBCs after 1429 cycles in FTF tests: TBC with APS bond coat (the entire (**a–1**) and magnified (**a–2**) microstructures) and TBC with HVOF bond coat (the entire (**b–1**) and magnified (**b–2**) microstructures). Surface morphologies of each sample are inserted in each figure.

The fracture microstructure of the samples with APS bond coat is entirely different from the samples with HVOF bond coat. Adhesive failure, such as cracking, fragmentation, and spallation, in the samples with APS bond coat is initiated near the interface between the TGO layer and the bond coat with a complete delamination. The samples with the HVOF bond coats are completely delaminated near the interface between the jig fixture and the epoxy adhesive, indicating that the interface stability of the sample with HVOF bond coat is better than that with APS bond coat. Therefore, the samples with the HVOF bond coats will provide a superior TBC performance in cyclic thermal exposure environments.

## *4.4. Simulated Temperature Evolution*

Under cyclic thermal conditions, the temperature response of TBCs increases in a transient manner initially and reaches a steady-state temperature after approximately 700 s. The temperature falls rapidly during the cooling phase. The temperature evolution is shown in Figure 10.

**Figure 10.** Temperature evolution in the coating.

The temperature distribution of TBCs is shown in Figure 11, during the transient heating and cooling phases. Figure 11a,b shows the results of the two bond coats. Because the thermal conductivity of TBCs is similar in both APS and HVOF, the temperature distribution is similar in both cases.

**Figure 11.** Temperature distribution in the coatings (**a**) during the heating cycle, and (**b**) during the cooling cycle.

## *4.5. Simulated Stress and Creep Strain Evolutions*

Figure 12 shows a comparison of the equivalent von Mises stress of TBCs in both cases at the end of the heating cycle, i.e., at 3600 s. In both cases, the maximum stresses are observed at the interface between the bond coat and substrate. For the APS deposited bond coat (Figure 12a), the maximum stress is about 83 MPa, and the stress difference within the bond coat is large. In comparison, the maximum stress in the HVOF deposited bond coat (Figure 12b) is only about 40 MPa, so the stress variation within the coat is less than the APS case.

 **Figure 12.** *Cont*.

**Figure 12.** Temperature distribution in the coatings (**a**) during the heating cycle and (**b**) during the cooling cycle.

A comparison of equivalent stress evolution in the YSZ layer in response to temperature in both cases is plotted in Figure 13. The results are extracted from the extreme node at the interface of YSZ and bond coat, indicated by "Max" in Figure 12.

**Figure 13.** Comparison of equivalent stress of TBC with a bond coat made using APS and HVOF.

In the case of a bond coat made using the HVOF process, the equivalent stress increases with a corresponding increasing temperature until 750 ◦C. The creep constants are defined to actuate at 750 ◦C, and thus a stress relaxation is observed once the temperature is beyond 750 ◦C. The relaxation in the case of TBCs made using APS is not as steep—it relaxes gradually until the end of the heating cycle. In addition, creep strain in the YSZ layer is plotted on the same location (Figure 14). It is evident that in the case of bond coat prepared using HVOF, the induced creep strain is significantly lower than with APS, suggesting that the HVOF bond coat has a better cyclic life.

**Figure 14.** Comparison of creep strain of TBC with a bond coat made using APS and HVOF.
