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Article

Degradation of HVOF-MCrAlY + APS-Nanostructured YSZ Thermal Barrier Coatings

by
Weijie R. Chen
1,*,
Chao Li
2,
Yuxian Cheng
3,*,
Hongying Li
4,5,
Xiao Zhang
1 and
Lu Wang
1
1
Institute of Coating Technology for Hydrogen Gas Turbines, Liaoning Academy of Materials, Shenyang 110167, China
2
Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
3
R&D Center, AECC Shenyang Liming Aero-Engine Co., Ltd., Shenyang 110043, China
4
School of Power and Energy, Northwestern Polytechnical University, Xi’an 710129, China
5
AECC Aero Science and Technology Co., Ltd., Chengdu 610500, China
*
Authors to whom correspondence should be addressed.
Coatings 2025, 15(8), 871; https://doi.org/10.3390/coatings15080871
Submission received: 15 June 2025 / Revised: 6 July 2025 / Accepted: 21 July 2025 / Published: 24 July 2025
(This article belongs to the Special Issue Thermal Barrier Coatings: Materials, Processes, Properties and Applications)
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Abstract

The degradation process of HVOF-MCrAlY + APS-nanostructured YSZ (APS-nYSZ) thermal barrier coatings, produced using gas turbine OEM-approved MCrAlY powders, is investigated by studying the TGO growth and crack propagation behaviors in a thermal cycling environment. The TGO growth yields a parabolic mechanism on the surfaces of all HVOF-MCrAlYs, and the growth rate increases with the aluminum content in the “classical” MCrAlYs. The APS-nYSZ layer comprises micro-structured YSZ (mYSZ) and nanostructured YSZ (nYSZ) zones. Both mYSZ/mYSZ and mYSZ/nYSZ interfaces appear to be crack nucleation sites, resulting in crack propagation and consequent crack coalescence within the APS-nYSZ layer in the APS-nYSZ/HVOF-MCrAlY vicinity. Crack propagation in the TBCs can be characterized as a steady-state crack propagation stage, where crack length has a nearly linear relationship with TGO thickness, and an accelerating crack propagation stage, which is apparently a result of the coalescence of neighboring cracks. All TBCs fail in the same way as APS-/HVOF-MCrAlY + APS-conventional YSZ analogs, but the difference in thermal cycling lives is not substantial, although the HVOF-low Al-NiCrAlY encounters chemical failure in the early stage of thermal cycling.

1. Introduction

Thermally sprayed MCrAlY (M = Ni/Co) + YSZ (ZrO2 + 8 wt.% Y2O3) thermal barrier coatings (TBCs) are widely used by gas turbine original equipment manufacturing (OEM) companies and maintenance, repair, and overhaul (MRO) companies on the surfaces of hot section components, enabling the gas turbines to operate at higher temperatures, thereby improving the turbine efficiency and extending service life [1,2,3,4,5,6,7]. While the YSZ topcoat provides thermal protection to the underlying superalloy component, which is usually produced by air plasma spray (APS), the γ-Ni + β-NiAl MCrAlY bond coat provides the adhesion between the YSZ layer and superalloy substrate as well as oxidation and hot corrosion protection, which is usually produced by APS, vacuum plasma spray (VPS)/low pressure plasma spray (LPPS), or high-velocity oxy-fuel (HVOF) spray.
While the TBCs with VPS/LPPS-MCrAlY bond coats exhibit remarkably improved durability over those produced using the APS-MCrAlY counterparts, thanks to the significantly reduced oxidation of the MCrAlY feedstock particles during spraying process in the vacuum/low pressure environment [8,9] the VPS/LPPS process is unfavorably costly, therefore VPS/LPPS-MCrAlY + APS-YSZ TBCs can usually be found on high-end hot section components. As HVOF-MCrAlY has a sound, uniform microstructure and homogenous composition similar to VPS-/LPPS-MCrAlY, and the production cost of HVOF-MCrAlY is close to APS-MCrAlY, it is therefore considered a potential replacement for VPS-/LPPS-MCrAlY [10].
However, the HVOF-MCrAlY/APS-YSZ interface is fairly smooth, which is prone to cracking during service in thermal cycling conditions due to the strain at this interface arising from the thermal expansion mismatch between the MCrAlY and the thermally grown oxide (TGO) scale developed on the MCrAlY surface at elevated temperatures [11], and also from the volume shrinkage of the MCrAlY on cooling via the B2 → L10 martensitic transformation [12].
TBCs with APS-nanostructured YSZ are of interest to some research institutions [13,14,15,16,17,18,19,20,21,22,23,24] for potential gas turbine applications, owing to their better thermal shock resistance over those with conventional APS-microstructured YSZ [13,14,17]; however, APS-nanostructured YSZs have not found their widespread applications in the gas turbine OEM and MRO market, compared with their APS-microstructured YSZ counterparts. This work aimed to study the TGO growth and crack propagation behaviors of the HVOF-MCrAlY + APS-nanostructured YSZ TBCs in a thermal cycling environment to envisage the feasibility for applications in the gas turbine industry.

2. Materials and Methods

The TBC samples consisted of a MCrAlY bond coat and a nanostructured YSZ topcoat. The bond coat was deposited to a thickness of 50–100 μm by HVOF technique (Tafa JP-5000, Praxair, Concord, NH, USA), with powders of Ni-22Cr-10Al-1Y (wt.%), Co-32Ni-21Cr-8Al-0.5Y (wt.%) and Ni-22Co-17Cr-12Al-0.5Y-0.5Hf-0.4Si (wt.%) (Amdry 9624, Diamalloy 4700 and Amdry 386-2.5, Oerlikon Metco, Westbury, NY, USA), and Ni-25Cr-5Al-0.5Y (TS-02B, Institute of Metals Research, Shenyang, China) onto ϕ25 mm 5–6 mm-thick Hastelloy X disks. The particle sizes of MCrAlY feedstock powders are shown in Table 1. On top of the HVOF-MCrAlYs, the topcoat was deposited to a thickness of 100–150 μm by APS technique (F4, Oerlikon Metco, Westbury, NY, USA), with powders of nanostructured ZrO2-8 wt.% Y2O3 (Zhaoxin Chemicals, Zibo, China). The particle size of agglomerated nanostructured YSZ powders is −90 +37 μm. The bond coat and topcoat were produced for a gas turbine OEM specification, using the spraying parameters developed for depositing nanostructured TBC. The sprayed samples were heat-treated in a vacuum furnace at <2 × 10−3 Pa and 1100 °C for 2 h.
A one-hour furnace cycling test (FCT) was carried out in rapid-heating bottom-loading thermal cycling furnaces (CM 1610BL, CM Furnaces, Bloomfield, NJ, USA) between ambient temperature and 1150 °C. Each cycle contained 50 min of heating up + holding and 10 min fan cooling, using a heating up time of approximately 10 min. The volumetric flow of the cooling fan was 14,100 L/min (500 cubic feet per minute (CFM)), where the sample temperature could be reduced to below 200 °C at the end of each cycle. All samples were examined every 20 cycles, and the FCT life was determined to be the number of cycles before >20% surface spallation occurred by visual inspection.
FCT samples were cross-sectioned after the completion of a predetermined number of thermal cycles, mounted using epoxy, and mechanically polished. The specimens were then examined using a scanning electron microscope (SEM, EM-30PLUS, COXEM, Daejeon, Republic of Korea) equipped with an energy-dispersive spectrometer (EDS, AZtecOne, Oxford Instruments, High Wycombe, UK). A total of 50–120 SEM micrographs were taken from the cross-section of each specimen for the measurement of TGO thickness and crack length. The average TGO thickness, δTGO, was determined to be ΣAi/ΣLi, where Ai is the TGO area and Li is the TGO length of section i of nearly parallel TGOs, with ΣLi/δTGO = 350–1300. The average crack length, aaverage, was determined to be the average value of the longest 10 cracks in each specimen based on the measurement of SEM micrographs.

3. Results and Discussion

The HVOF-MCrAlY + APS-nanostructured YSZ (APS-nYSZ) had a thin and nearly continuous Al2O3-TGO layer at the APS-nYSZ/HVOF-MCrAlY interface (Figure 1) after the vacuum heat treatment, with an interface roughness Ra > 8 μm (Table 2). While the HVOF-high Al-NiCrAlY contained some Al2O3 particles between the semi-molten NiCrAlY feedstock particles (Figure 1a), which was probably due to oxidation of the fine high Al-NiCrAlY powders during the spraying process, the other three bond coats appeared to be dense and uniform, although some voids could occasionally be seen at the MCrAlY splat boundaries. While the APS-nYSZ contained less than 30% nanostructured YSZ (nYSZ) zones, in the micro-structured YSZ (mYSZ) regions some globular pores and crack-like discontinuities, which were mYSZ/mYSZ interfaces, could also be seen. These were produced by the cooling of molten particles to room temperature.
After 10 cycles, in addition to the thickening of Al2O3-TGO, a small amount of blocky (Cr,Al)2O3 + (Ni,Co)(Al,Cr)2O4 + (Ni,Co)O (CSN) mixed oxides could be observed between the APS-nYSZ and Al2O3-TGO (Figure 2). SEM-EDS analysis revealed that these mixed oxides contained 10–50 wt.% Ni + Co and 20–60 wt.% Al + Cr. Cracks started to form in all four TBCs as the result of separations of both mYSZ/mYSZ and mYSZ/nYSZ interfaces. In the meantime, cracking at the APS-nYSZ/TGO interface could be seen (Figure 2d).
As the FCT proceeded, cracks propagated and some of adjacent cracks started to join together (Figure 3). While most cracks propagated nearly parallel to the APS-nYSZ/TGO interface, some cracks developed at various angles to the APS-nYSZ/Al2O3-TGO interface, leading to the formation of regional crack networks (Figure 3a,c). Some (Cr,Al)2O3 and (Ni,Co)(Cr,Al)2O4 (CS) mixed oxides developed at the Al2O3-TGO/HVOF-low Al-NiCrAlY interface after 100 cycles (Figure 3b) as the result of “chemical failure”, i.e., the aluminum content in the MCrAlY near the Al2O3-TGO/MCrAlY interface had dropped to below a certain level such that the continuous growth of the Al2O3-TGO became impossible, due to the aluminum depletion from the MCrAlY bond coat [25,26,27,28]. The “chemical failure” did not occur in the other three TBCs. While a limited amount of CSN mixed oxides presented in the HVOF-high Al-NiCrAlY + APS-nYSZ and HVOF-NiCoCrAlYHfSi + APS-nYSZ, a small amount of CSNs did appear at the APS-nYSZ/Al2O3-TGO interface in the HVOF-low Al-NiCrAlY + APS-nYSZ and HVOF-CoNiCrAlY + APS-nYSZ, due to the lower aluminum content in the HVOF-low Al-NiCrAlY and HVOF-CoNiCrAlY bond coats. However, the presence of limited CSN mixed oxides did not appear to significantly influence the crack propagation in the TBCs. In addition, void formation occurred within the Al2O3-TGO.
With a further increase in thermal cycles, a number of extended cracks presented in the APS-nYSZ layer (Figure 4). While some cracks nucleated on the surface of APS-nYSZ, most of the cracks originated in the vicinity of the APS-nYSZ/HVOF-MCrAlY interface due to the increased stresses in the TBCs associated with TGO growth [29,30,31]. It was anticipated that the nYSZ zones might act as crack arresters [16] since the decreased stress intensity at the crack tip and the internal compressive stress of the unmelted nanoparticles would change the crack propagation path [32]; however, this feature was not observed. The growth of CSN mixed oxides was barely noticeable, implying that they were formed at the very beginning of thermal cycling, probably due to the very low aluminum content in some areas on the MCrAlY surface as the result of microstructural non-uniformity. The limited presence of isolated CSNs appeared to not have an appreciable impact on the crack propagation (Figure 4b–d). In addition, other than in the HVOF-low Al-NiCrAlY + APS-nYSZ, “chemical failure” was not observed in the other three TBCs after 300 cycles. Moreover, cracking at the Al2O3-TGO/HVOF-MCrAlY interface [33,34] was scarcely observed.
Interestingly, breakdowns and bend-overs of the Al2O3-TGO scale were frequently presented (Figure 5), which revealed the MCrAlY underneath. This TGO scale appeared to be a thin Al2O3 foil pulled out and bent-over from the gap between YSZ and MCrAlY, with a thickness of somewhat less than 0.5 μm after 300 cycles at 1150 °C (Figure 5b), which was probably resulted from the mechanical polishing of SEM specimens. However, it is difficult to understand how a thin Al2O3 foil could be bent, since the Al2O3 is rigid and brittle at ambient temperature, although it can be crept at above 1100 °C. This occurrence suggests that this Al2O3-TGO scale is not the traditional one documented in previous researches. The microstructure details of such scale and how it was formed are not clear, which deserves further investigation. This type of scale was not included in the TGO thickness measurement.
All TBCs failed within the YSZ layers in the APS-nYSZ/HVOF-MCrAlY vicinity (Figure 6). This failure mode is primarily attributed to crack propagation and coalescence via mYSZ/mYSZ and mYSZ/nYSZ interface separation and cracking associated with the TGO growth. This is similar to the APS-/VPS-/LPPS-/HVOF-MCrAlY + APS-conventional YSZ TBCs [33,34,35,36,37] but is quite different from some TBCs with APS-nanostructured YSZ in thermal shock conditions [14,17]. Sintering of the APS-nYSZ was not observed until the TBC failed, showing that the test temperature in this study could only result in a high TGO growth rate, giving rise to the build-up of a high stress level in the YSZ/MCrAlY vicinity, thereby leading to increased crack nucleation and propagation via mYSZ/mYSZ and mYSZ/nYSZ interface separations. The increased CSN growth appeared to be associated with the cracks across the Al2O3-TGO layer, which led to the preferred oxidation of the Al-depleted MCrAlY underneath (Figure 6a,c).
As mentioned in the Materials and Methods section, the average TGO thickness, δTGO, was determined to be ΣAi/ΣLi, where Ai is the TGO area and Li is the TGO length of section i of nearly parallel TGOs, with ΣLi/δTGO = 350–1300, and in most cases ΣLi/δTGO > 1000. Regression analysis shows that the TGO growth in the four HVOF-MCrAlY + APS-nYSZ TBCs yields the following parabolic growth mechanism (Figure 7):
δ − δ 0 = k t n
where δ is the average TGO thickness at time t, δ0 is the average TGO thickness prior to the FCT, t is the exposure time at 1150 °C, and k and n are constants. The holding time for each thermal cycle is counted as 40 min, since the heating + holding time is 50 min for each cycle with a heating up time of approximately 10 min. The sequence of TGO growth rate is HVOF-high Al-NiCrAlY (10 wt.% Al) > HVOF-CoNiCrAlY (8 wt.% Al) > HVOF-NiCoCrAlYHfSi (12 wt.% Al) > HVOF-low Al-NiCrAlY (6 wt.% Al). Considering that Hf may slow down the TGO growth [38], it might be anticipated that TGO growth rate increases with the aluminum content in the “classical” MCrAlYs (without addition of other reactive or refractory element): the higher the Al content in the MCrAlY then the faster the Al amount accumulated at the Al2O3-TGO/MCrAlY interface. In addition, the n value is between 1/4 and 1/2 (Table 3), which is very close to previous studies [39,40,41]. This may deserve further investigation.
.
The crack length increases with thermal cycles in the early stage of the FCT (Figure 8); however, while an accelerated crack growth can be observed after ~150 cycles in the HVOF-high Al-NiCrAlY + APS-nYSZ (Figure 8a), the crack propagation in other three TBCs remains somewhat stable. When replotting the crack length as a function of the TGO thickness (Figure 8b), all four TBCs exhibit a steady-state crack propagation in the early stage of thermal cycling, with a nearly linear relationship between the aaverage and δTGO, which may be primarily governed by the increased stress associated with the TGO thickening. An accelerating crack propagation takes place when the TGO thickness exceeds a certain value, which may be attributed to the coalescence of adjacent cracks in extended thermal cycling. It may be worth noticing that the ratio of Δa/ΔδTGO decreases with increased aluminum content for “classical” MCrAlYs in the steady-state crack propagation stage (Table 4), which is in the reverse order of the TGO growth rate (Table 3). The transition from the steady-state crack propagation to accelerating crack propagation takes place at δstd-acc ≈ 4–4.5 μm in the HVOF-low Al-NiCrAlY + APS-nYSZ, and at δstd-acc ≈ 5–5.5 μm in the other three TBCs. Moreover, the critical TGO thickness for failure, δcrit, is between 5 and 8 μm (Table 4), which is fairly close to those reported for EB-PVD thermal barrier coatings [42].
All TBCs fail at between 320 and 380 cycles (Figure 9). The HVOF-high Al-NiCrAlY + APS-nYSZ has an average FCT life of 322 cycles, which is the shortest among the four TBCs, whereas the HVOF-NiCoCrAlYHfSi + APS-nYSZ has an average FCT life of 376 cycles, which is the longest. Although the thickness of the APS-nYSZ layers in the four TBCs are not the same, all of them meet the requirement of OEM specification and therefore, the influence on the FCT life by the difference in such APS-nYSZ thickness could be considered minimal. This is quite different from a previous study where the HVOF-high Al-MCrAlY + APS-traditional YSZ had a much shorter FCT life [43], which is possibly due to the rough APS-nYSZ/HVOF-NiCoCrAlYHfSi interface (Table 2) in this study, although the HVOF-NiCoCrAlYHfSi has a high aluminum content and a larger amount of β-NiAl phase. In addition, the FCT life does not appear to have a relationship with either δcrit or δstd-acc (Table 4), implying that it is uncertain if the remaining life assessment of the TBCs could be accomplished by predicting the TGO thickness, which deserves further investigation.
Nevertheless, the difference in the FCT lives is less than 20%, suggesting that changing the MCrAlY chemistry may not have a significant impact on the HVOF-MCrAlY + APS-nYSZ TBC durability if the surface roughness of HVOF-MCrAlYs can be improved. A recent report shows that the APS-nanostructured YSZ only exhibits a marginally enhanced FCT life with respect to its conventional counterpart [44]; therefore, it is more likely that the durability of the APS-nYSZ plays a major role in governing the FCT life of the HVOF-MCrAlY + APS-nanostructured YSZ TBC. Thus, further investigation will be necessary before the APS-nanostructured YSZ finds its momentous applications in the gas turbine industry.

4. Conclusions

TGO growth in the four HVOF-MCrAlY + APS-nYSZ TBCs yield a parabolic growth mechanism, δ − δ 0 = k t n . The sequence of TGO growth rate is HVOF-high Al-NiCrAlY > HVOF-CoNiCrAlY > HVOF-NiCoCrAlYHfSi > HVOF-low Al-NiCrAlY.
Both mYSZ/mYSZ and mYSZ/nYSZ interfaces serve as crack nucleation sites, which may result in the formation of local crack networks in the APS-nYSZ layer. Crack propagation in the TBCs can be characterized as a steady-state crack propagation stage associated with plain crack propagation where the ratio of Δa/ΔδTGO is virtually a constant and an accelerating crack propagation stage associated with crack coalescence.
HVOF-MCrAlY + APS-nYSZ TBCs exhibit a failure mode similar to the conventional APS-/VPS-/LPPS-/HVOF MCrAlY + APS-mYSZ counterparts, which is mainly due to crack propagation and coalescence in the YSZ layer near the YSZ/MCrAlY interface.
The HVOF-high Al-NiCrAlY + APS-nYSZ has the shortest FCT life while the HVOF-NiCoCrAlYHfSi + APS-nYSZ has the longest; however, the difference in the FCT lives of the four TBCs is not appreciable.

Author Contributions

Conceptualization, W.R.C. and L.W.; methodology, W.R.C.; formal analysis, W.R.C. and C.L.; investigation, W.R.C. and C.L.; resources, Y.C., H.L. and L.W.; data curation, W.R.C.; writing—original draft preparation, W.R.C.; writing—review and editing, W.R.C. and X.Z.; project administration, W.R.C., Y.C. and H.L.; funding acquisition, C.L., Y.C. and L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China under grant number 2021YFB3702305, National Natural Science Foundation of China under grant number 52401093, and the Liaoning Provincial Department of Science and Technology under grant numbers 2024JH1/11700039 and IC24ZXK300.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No additional data is available. The data presented in this study are available upon request from the corresponding author due to institutional policies.

Acknowledgments

The authors are grateful to Mengnie Li and Zheng Gong of Kunming University of Science and Technology for the assistance in the SEM work.

Conflicts of Interest

Authors Yuxian Cheng and Hongying Li were employed by the companies AECC Shenyang Liming Aero-Engine Co., Ltd and AECC Aero Science and Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. The thin, nearly continuous Al2O3-TGO layers formed after vacuum heat treatment in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ.
Figure 1. The thin, nearly continuous Al2O3-TGO layers formed after vacuum heat treatment in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ.
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Figure 2. Crack nucleation via mYSZ/mYSZ and mYSZ/nYSZ interface separation in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ. A total of 10 cycles at 1150 °C.
Figure 2. Crack nucleation via mYSZ/mYSZ and mYSZ/nYSZ interface separation in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ. A total of 10 cycles at 1150 °C.
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Figure 3. TGO growth and crack propagation in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ after 100 cycles at 1150 °C. Note the regional crack networks in APS-nYSZ (a,c), and “chemical failure” in HVOF-low Al-NiCrAlY (b).
Figure 3. TGO growth and crack propagation in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ after 100 cycles at 1150 °C. Note the regional crack networks in APS-nYSZ (a,c), and “chemical failure” in HVOF-low Al-NiCrAlY (b).
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Figure 4. TGO growth and crack propagation in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ, after 300 cycles at 1150 °C. Note that the “chemical failure” only occurred in the HVOF-low Al-NiCrAlY (b).
Figure 4. TGO growth and crack propagation in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ, after 300 cycles at 1150 °C. Note that the “chemical failure” only occurred in the HVOF-low Al-NiCrAlY (b).
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Figure 5. (a) Breakdown and (b) bend-over of the Al2O3-TGO scale.
Figure 5. (a) Breakdown and (b) bend-over of the Al2O3-TGO scale.
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Figure 6. Failure of TBCs via crack propagation and coalescence in the APS-nYSZ near the YSZ/MCrAlY interface in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ.
Figure 6. Failure of TBCs via crack propagation and coalescence in the APS-nYSZ near the YSZ/MCrAlY interface in (a) HVOF-high Al-NiCrAlY + APS-nYSZ, (b) HVOF-low Al-NiCrAlY + APS-nYSZ, (c) HVOF-CoNiCrAlY + APS-nYSZ, and (d) HVOF-NiCoCrAlYHfSi + APS-nYSZ.
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Figure 7. Average thickness of nearly parallel TGO sections, δTGO, as a function of time at 1150 °C. The holding time is counted as 40 min for each thermal cycle.
Figure 7. Average thickness of nearly parallel TGO sections, δTGO, as a function of time at 1150 °C. The holding time is counted as 40 min for each thermal cycle.
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Figure 8. The average crack length of the longest 10 cracks, aaverage, in the HVOF-MCrAlY + APS-nYSZ TBCs as a function of the (a) thermal cycle and (b) TGO thickness.
Figure 8. The average crack length of the longest 10 cracks, aaverage, in the HVOF-MCrAlY + APS-nYSZ TBCs as a function of the (a) thermal cycle and (b) TGO thickness.
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Figure 9. The difference in FCT lives of the four HVOF-MCrAlY + APS-nYSZs is not substantial.
Figure 9. The difference in FCT lives of the four HVOF-MCrAlY + APS-nYSZs is not substantial.
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Table 1. Chemistry and particle size of MCrAlY powders.
Table 1. Chemistry and particle size of MCrAlY powders.
MCrAlYProduct and ManufactureChemistry (wt.%)Powder Size (µm)
NiCoCrAlYHfSi
High Al-NiCrAlYOM Amdry 9624Bal 21–239–110.8–1.2 −37 + 11
Low Al-NiCrAlYIMR TS-02BBal 24–264–60.3–0.7 −45 + 20
CoNiCrAlYOM Diamalloy 470029–35Bal18–245–110.1–0.8 −45 + 15
NiCoCrAlYHfSiOM Amdry 386-2.5Bal19–2614–2111–140.2–0.80.1–0.50.1–0.7−63 + 22
Table 2. Chemistry and surface roughness of MCrAlY coatings after vacuum heat treatment.
Table 2. Chemistry and surface roughness of MCrAlY coatings after vacuum heat treatment.
Bond CoatChemistry (wt.%)β-NiAl (%)Ra (µm)
NiCoCrAlYHfSi
HVOF-high Al-NiCrAlYBal 20.88 ± 0.479.30 ± 0.520.41 ± 0.19 30.80 ± 6.238.63 ± 1.79
HVOF-low Al-NiCrAlYBal 25.37 ± 0.275.65 ± 0.190.40 ± 0.15 09.42 ± 1.34
HVOF-CoNiCrAlY31.73 ± 0.44Bal21.59 ± 0.207.59 ± 0.150.35 ± 0.15 39.41 ± 6.469.03 ± 1.94
HVOF-NiCoCrAlYHfSiBal17.75 ± 0.7215.25 ± 0.5510.44 ± 0.270.57 ± 0.270.14 ± 0.250.39 ± 0.0768.29 ± 4.199.46 ± 1.47
Table 3. TGO growth in HVOF-MCrAlY + APS-nYSZ TBCs at 1150 °C, δ − δ 0 = k t n .
Table 3. TGO growth in HVOF-MCrAlY + APS-nYSZ TBCs at 1150 °C, δ − δ 0 = k t n .
Bond CoatChemistry (wt.%)kn
NiCoCrAlYHfSi
HVOF-high Al-NiCrAlYBal 20.88 ± 0.479.30 ± 0.520.41 ± 0.19 0.8280.392
HVOF-low Al-NiCrAlYBal 25.37 ± 0.275.65 ± 0.190.40 ± 0.15 1.0510.284
HVOF-CoNiCrAlY31.73 ± 0.44Bal21.59 ± 0.207.59 ± 0.150.35 ± 0.15 0.8910.367
HVOF-NiCoCrAlYHfSiBal17.75 ± 0.7215.25 ± 0.5510.44 ± 0.270.57 ± 0.270.14 ± 0.250.39 ± 0.070.8950.348
Table 4. The ratio of Δa/ΔδTGO in the steady-state crack propagation stage, TGO thickness in the transition stage from the steady-state crack propagation to accelerating crack propagation (δstd-acc), and the critical TGO thickness for failure (δcrit).
Table 4. The ratio of Δa/ΔδTGO in the steady-state crack propagation stage, TGO thickness in the transition stage from the steady-state crack propagation to accelerating crack propagation (δstd-acc), and the critical TGO thickness for failure (δcrit).
Bond CoatChemistry (wt.%)Δa/ΔδTGO (μm/μm)δstd-acc (μm)δcrit (μm)
NiCoCrAlYHfSi
HVOF-high Al-NiCrAlYBal 20.88 ± 0.479.30 ± 0.520.41 ± 0.19 33.6685~5.57.51 ± 0.94
HVOF-low Al-NiCrAlYBal 25.37 ± 0.275.65 ± 0.190.40 ± 0.15 83.5014~4.55.63 ± 0.84
HVOF-CoNiCrAlY31.73 ± 0.44Bal21.59 ± 0.207.59 ± 0.150.35 ± 0.15 72.6285~5.57.45 ± 0.75
HVOF-NiCoCrAlYHfSiBal17.75 ± 0.7215.25 ± 0.5510.44 ± 0.270.57 ± 0.270.14 ± 0.250.39 ± 0.0751.5625~5.56.57 ± 0.63
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Chen, W.R.; Li, C.; Cheng, Y.; Li, H.; Zhang, X.; Wang, L. Degradation of HVOF-MCrAlY + APS-Nanostructured YSZ Thermal Barrier Coatings. Coatings 2025, 15, 871. https://doi.org/10.3390/coatings15080871

AMA Style

Chen WR, Li C, Cheng Y, Li H, Zhang X, Wang L. Degradation of HVOF-MCrAlY + APS-Nanostructured YSZ Thermal Barrier Coatings. Coatings. 2025; 15(8):871. https://doi.org/10.3390/coatings15080871

Chicago/Turabian Style

Chen, Weijie R., Chao Li, Yuxian Cheng, Hongying Li, Xiao Zhang, and Lu Wang. 2025. "Degradation of HVOF-MCrAlY + APS-Nanostructured YSZ Thermal Barrier Coatings" Coatings 15, no. 8: 871. https://doi.org/10.3390/coatings15080871

APA Style

Chen, W. R., Li, C., Cheng, Y., Li, H., Zhang, X., & Wang, L. (2025). Degradation of HVOF-MCrAlY + APS-Nanostructured YSZ Thermal Barrier Coatings. Coatings, 15(8), 871. https://doi.org/10.3390/coatings15080871

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