Mechanism of vertical crack formation in Yb2SiO5 coatings deposited via plasma spray-physical vapor deposition

Zhang, Xiaofeng; Wang, Chao; Ye, Ruijun; Deng, Chunming; Liang, Xinghua; Deng, Ziqian;Niu, Shaopeng; Song, Jinbing; Liu, Guo; Liu, Min; Zhou, Kesong; Lu, Jian; Feng, Junli

Published in:Journal of Materiomics

Published: 01/03/2020

Document Version:Final Published version, also known as Publisher’s PDF, Publisher’s Final version or Version of Record

License:CC BY-NC-ND

Publication record in CityU Scholars:Go to record

Published version (DOI):10.1016/j.jmat.2020.01.002

Publication details:Zhang, X., Wang, C., Ye, R., Deng, C., Liang, X., Deng, Z., Niu, S., Song, J., Liu, G., Liu, M., Zhou, K., Lu, J., &Feng, J. (2020). Mechanism of vertical crack formation in Yb2SiO5 coatings deposited via plasma spray-physicalvapor deposition. Journal of Materiomics, 6(1), 102-108. https://doi.org/10.1016/j.jmat.2020.01.002

Citing this paperPlease note that where the full-text provided on CityU Scholars is the Post-print version (also known as Accepted AuthorManuscript, Peer-reviewed or Author Final version), it may differ from the Final Published version. When citing, ensure thatyou check and use the publisher's definitive version for pagination and other details.

General rightsCopyright for the publications made accessible via the CityU Scholars portal is retained by the author(s) and/or othercopyright owners and it is a condition of accessing these publications that users recognise and abide by the legalrequirements associated with these rights. Users may not further distribute the material or use it for any profit-making activityor commercial gain.Publisher permissionPermission for previously published items are in accordance with publisher's copyright policies sourced from the SHERPARoMEO database. Links to full text versions (either Published or Post-print) are only available if corresponding publishersallow open access.

Take down policyContact lbscholars@cityu.edu.hk if you believe that this document breaches copyright and provide us with details. We willremove access to the work immediately and investigate your claim.

Download date: 14/04/2022

ilable at ScienceDirect

Journal of Materiomics 6 (2020) 102e108

Contents lists ava

Journal of Materiomics

journal homepage: www.journals .e lsevier .com/journal-of-materiomics/

Mechanism of vertical crack formation in Yb2SiO5 coatings depositedvia plasma spray-physical vapor deposition

Xiaofeng Zhang a, b, *, Chao Wang a, Ruijun Ye d, Chunming Deng a, Xinghua Liang c, **,Ziqian Deng a, Shaopeng Niu a, Jinbing Song a, Guo Liu b, Min Liu a, Kesong Zhou a,Jian Lu b, Junli Feng d

a National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou, 510650, Chinab Department of Mechanical Engineering, City University of Hong Kong, Hong Kong, Chinac Guangxi Key Laboratory of Automobile Components and Vehicle Technology, Guangxi University of Science & Technology, Liuzhou, 545000, Chinad Shenzhen Customs Industrial Products Inspection Technology Center, Shenzhen, 518067, China

a r t i c l e i n f o

Article history:Received 24 October 2019Received in revised form1 December 2019Accepted 6 January 2020Available online 16 January 2020

Keywords:Environmental barrier coatingsYb2SiO5 coatingVertical crackPS-PVD

* Corresponding author. National Engineering LaboSurface Engineering Technology, Guangdong Instituthou, 510650, China.** Corresponding author.

E-mail addresses: zxf200808@126.com (X. Z(X. Liang).

Peer review under responsibility of The Chinese C

https://doi.org/10.1016/j.jmat.2020.01.0022352-8478/© 2020 The Chinese Ceramic Society. Pcreativecommons.org/licenses/by-nc-nd/4.0/).

a b s t r a c t

Plasma spray-physical vapor deposition (i.e., PS-PVD) is a promising method for obtaining advancedenvironmental barrier coatings (EBCs). The EBCs must meet some requirements in the application, inwhich the thermal cycle performance affects the service lifetime. The preparation of artificial verticalcracks in Yb2SiO5 coatings is an effective approach for meeting the requirements above because verticalcracks provide a strain tolerance. To clarify the formation mechanism of vertical cracks during the PS-PVD, the effects of coating thickness and substrate temperature on the formation of vertical crackswere investigated. In addition, the interactions of spray powder and plasma flame during coatingdeposition were also characterized by optical spectroscopy. It is indicated that vertical cracks are formeddue to a thermal expansion mismatch between Yb2SiO5 and mullite coating, transient cooling afterdeposition and the nucleation of evaporated Yb2SiO5 as well.

© 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open accessarticle under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

It is necessary for the production of turbine engines with higherthrust-to-weight ratios and lower service costs to replace super-alloys due to their temperature limits [1,2]. Ceramic matrix com-posites (CMCs) such as SiCf/SiC composites are potential candidatesfor replacing the super-alloys because of their higher operatingtemperatures (up to 1400 �C) and lower density (1/3 of nickel-based super-alloys) [2,3]. However, the CMCs suffer from oxida-tion and volatilization in high-temperature gas combustion envi-ronments during their service [3,4], and developing environmentalbarrier coatings (EBCs) is essential to overcoming these limitations.Also, the EBCs capable of effectively reflecting the thermal energy

ratory for Modern Materialse of New Materials, Guangz-

hang), 309602373@qq.com

eramic Society.

roduction and hosting by Elsevi

radiated from high-temperature combustion are expected to pro-vide the water-oxidation corrosion protection [5�7].

Ceramic EBCs with Yb2SiO5/mullite/Si coatings are widely usedto protect the CMCs, but the EBCs are subjected to cyclic thermalenvironments under working conditions such as frequent takeoffsand landings [8e10]. As a result, some residual stresses aregenerated, thus accumulating in coatings due to different co-efficients of thermal expansion (CTEs) of ceramic coatings and theCMCs substrates [11]. In order to obtain the highly durable EBCs,Yb2SiO5 top coating should be able to tolerate the resultant thermalstrain. To increase the durability of EBCs, the preparation of densevertical cracks (DVCs) is an effective approach to improve the straintolerance of coatings. The densities of some DVC EBCs are 88%greater than their theoretical density [12e15].

Atmospheric plasma spray (i.e., APS) is the most commonapproach for preparing the EBCs [16], but it is difficult to obtain theAPS DVC EBCs due to the high density limitation. The feedstockpowder in APS cannot be melted fully due to their low velocities.The APS coating is formed through the accumulation of splat layersdeposited as the plasma torch passes over the substrate [17]. Somevertical cracks in the DVC EBCs are generated via the alignment of

er B.V. This is an open access article under the CC BY-NC-ND license (http://

Fig. 1. PS-PVD facility, (aeb) overview, (c) O3CP plasma gun.

X. Zhang et al. / Journal of Materiomics 6 (2020) 102e108 103

vertical micro-cracks produced in each splat layer due to theshrinkage of the deposited splats [18]. It was found that theshrinkage strain could be absorbed or compensated by pores if thecoating density was less than ~88% of its theoretical density [18,19].A recent work reported that plasma spray-physical vapor deposi-tion (PS-PVD) also forms some vertical cracks due to the high spraypower (~180 kW) and low work pressure (~150 Pa). The feedstockpowder in plasma jets can reach supersonic speeds [20], and thefeedstock powder can be melted or even vaporized at a high tem-perature of plasma, thus depositing dense coatings readily. Basedon various coating deposition mechanisms, the vertical crack for-mation in the PS-PVD EBCs appears different from that in the APSDVC EBCs. The aim of this paper is thus to clarify the mechanism forvertical crack formation in the PS-PVD EBCs.

2. Experimental

2.1. Coating preparation

Tri-layered EBCs (Yb2SiO5/mullite/Si) were prepared by a PS-PVD system (Sulzer-Metco Multicoat) with an O3CP plasma gun.Fig. 1(a) and (b) show the PS-PVD facility, and Fig. 1(c) shows the

Table 1Parameters of EBCs prepared by PS-PVD.

Materials Current (A) Ar (NSPM) He (NSPM) H2 (NSPM) Spra

Si 1650 110 e 6 350Mullite 2600 70 50 e 1000Yb2SiO5 2400 100 20 e 1000

O3CP plasma gunwith a double feeding tube. In order to investigatethe vertical crack formation mechanism, Yb2SiO5 coatings withdifferent thicknesses (i.e., 20 mm, 50 mm, and 100 mm) at differentsubstrate temperatures (i.e., 600 �C, 800 �C, and 1000 �C) wereformed. Table 1 shows the parameters of EBCs prepared by the PS-PVD. The preparation parameters of Si and mullite coatings werefixed.

2.2. Characterizations

The plasma flame spectra before and after injection of Yb2SiO5powders during the PS-PVD were determined by a model IHR550optical spectroscope (HORIBA, Japan). In Fig. 2, a raster model was530e27-141 in the measurement range of 400e1400 nm. A filterUV-S1window (slit width: 0.02mm, exposure time: 0.3 s) was usedto filter ultraviolet rays. Specimens mounted and polished bystandard metallographic techniques were characterized by a modelGemini 300 field-emission scanning electron microscope (FE-SEM,ZEISS, Germany). In addition, the microstructure of EBCs sampleswas also analyzed by a focused ion beam (FIB, SMI3050MS2, SII,Japan) and a model Titan Themis 20 transmission electron micro-scope (TEM, FEI, USA).

y distance (mm) Work pressure Thickness (mm) Pre-heatingTemperature (�C)

4100 e 900150 e 1100150 20 1000150 50150 100150 50 600150 800150 1000

Fig. 2. Schematic diagram of the spectrometer used to characterize the PS-PVD flame.

X. Zhang et al. / Journal of Materiomics 6 (2020) 102e108104

3. Results and discussion

3.1. Effects of coating thickness and pre-heating temperature

Tensile stress will be generated in coating due to the rapid so-lidification of molten particles impacting on the substrate duringplasma spray deposition. Meanwhile, the tensile stress will beaccumulated along with the increase of coating thickness. Thus, arelationship between the thickness and microstructure of Yb2SiO5coatings was investigated. Fig. 3 shows the coating microstructuresof EBCs prepared via PS-PVD at fixed spray parameters. Clearly, thethickness of Yb2SiO5 coating has an effect on the microstructure. Siand mullite coatings were prepared on the SiCf/SiC substrate priorto the top coating deposition. In Fig. 3(a), the coating is densewithout any apparent vertical cracks or pores when the thickness is20 mm. In Fig. 3(b), some vertical cracks appear in the dense Yb2SiO5

Fig. 3. Microstructure of EBCs on SiCf/SiC substrate with different Y

Fig. 4. Microstructure of Yb2SiO5 coatings at different subst

coating when the thickness increases to 50 mm. Also, the amount ofvertical cracks increases and the cracks become wider as thecoating thickness further increases to 100 mm (see Fig. 3(c)).

Besides, the effect of pre-heating temperature on the verticalcrack formation was also investigated. Fig. 4 shows the corre-sponding results. In Fig. 4(a), there are no vertical cracks in thecross-sectional microstructure of a Yb2SiO5 coating at 600 �C. InFig. 4(b), some vertical cracks appear in the coating when thetemperature increases to 800 �C. In Fig. 4(c), the crack widths in-crease when the temperature further increases to 1000 �C.

Fig. 5(aec) show the magnified images of the cross-sections andsurface microstructures of Yb2SiO5 coatings at a pre-heating tem-perature of 1000 �C. In Fig. 5(a), a typical vertical crack in the upperportion of the Yb2SiO5 coating occurs. The same vertical crack in thebottom layer in Fig. 5(b) shows that the crack propagates throughYb2SiO5 coating but stops at the interface of Yb2SiO5/mullitecoating. In Fig. 5(c), many interconnected cracks do not propagatethrough the top coating.

3.2. Microstructural characterization of Yb2SiO5 coatings

Fig. 6 shows the results of TEM analysis on the cross-section ofYb2SiO5 coatingwith a thickness of 100 mmat 1000 �C. Fig. 6(a) is anoverview of Yb2SiO5 coating. Fig. 6 (b) shows the magnifiedmicrostructure of Yb2SiO5 coating, showing that laminar structuresconsisted of many splats with nanoparticles deposited in between.Fig. 6(ced) are the corresponding element maps of Si, Yb, and O,and Fig. 6(c) is the combined maps of Si and Yb. The results showthat the nanosized grains are mainly SiO2 phase, as shown inFig. 6(b). During the deposition of Yb2SiO5 powder, Yb2SiO5 grainsare divided into Yb2O3 and SiO2 phases in high-temperature plasmaflame (i.e., >10,000 �C) [21]. The melting points of Yb2O3 and SiO2are 2356 �C and 1650 �C, respectively, and the boiling points ofYb2O3 and SiO2 are 4070 �C and 2230 �C, correspondingly. Most ofYb2O3 grains only melt and form some splats when they impact thesubstrate due to the higher melting and boiling points [22,23]. Only

b2SiO5 coating thicknesses: (a) 20 mm, (b) 50 mm, (c) 100 mm.

rate temperatures: (a) 600 �C, (b) 800 �C, (c) 1000 �C.

Fig. 5. Microstructures of Yb2SiO5 coatings on substrate at 1000 �C, (a) upper vertical crack, (b) bottom crack, (c) surface.

X. Zhang et al. / Journal of Materiomics 6 (2020) 102e108 105

a few Yb2O3 grains evaporate and nucleate to form nano-sizedgrains [24], but most of SiO2 grains undergo these two processesdue to the lower melting and boiling points.

The interaction of Yb2SiO5 powder with plasma flame wascharacterized by optical spectrometry, as shown in Fig. 7. InFig. 7(a), the plasma flame of Ar100-He20 without injected Yb2SiO5powder appears white. The velocity and enthalpy transfer of theplasma flame (AreHe) are 700 m/s and 8000 kJ/kg, respectively,thus being capable of heating Yb2SiO5 powder [24,25]. After theinjection of Yb2SiO5 powder, the plasma flame becomes yellow (see

Fig. 6. The results of TEM analysis on the cross-section of Yb2SiO5 coating with a thickness(ced) corresponding elemental maps of Si, Yb, and O.

Fig. 7(b)). These images show that the color difference of plasmaflame is due to the decomposition and evaporation of Yb2SiO5powder. Fig. 7(c) shows the spectral lines of Fig. 7(a) of Ar (at awavelength of 810.274 nm) and He (at a wavelength of 843.167 nm)plasmas. Fig. 7(d) is the spectral line of Fig. 7(b), showing SiO2 (at awavelength of 810.274 nm) and Yb2O3 (at a wavelength of810.274 nm) vapors. In Fig. 7(d), the intensity of Yb2O3 is lower thanthat of SiO2, indicating that the amount of evaporated Yb2O3 islower than that of SiO2.

of 100 mm at 1000 �C: (a) overview, (b) magnified microstructure of Yb2SiO5 coating,

Fig. 7. Spectral analysis of Yb2SiO5 coating during PS-PVD deposition, (a) plasma flame without powder, (b) plasma with powder, (c) spectral chart of Fig.7(a), amd (d) spectral chartof Fig.7(b).

Fig. 8. Surface temperature of Yb2SiO5 coating during PS-PVD deposition measured byan infrared thermometer.

X. Zhang et al. / Journal of Materiomics 6 (2020) 102e108106

3.3. Vertical crack formation mechanism

The primary driving force for the formation of vertical cracks inYb2SiO5 coatings is in-plane tensile stress. During the deposition ofPS-PVD coatings, in-plane tensile stresses could be generated fromsome factors including the thermal expansion mismatch betweenYb2SiO5 and mullite coatings, transient cooling after deposition,and the nucleation of evaporated Yb2SiO5.

The CTE of Yb2SiO5 coating (i.e., 7.2 � 10�6 K�1) is larger thanthat of mullite coating (i.e., 6.5 � 10�6 K�1). Therefore, stresses dueto this thermal expansion mismatch are generated during coatingdeposition when the top Yb2SiO5 coating is heated and cooledduring the successive passes of the plasma torch and after the finalcooling to room temperature. In the deposition, the temperature ofthe substrate and coating increases with time. As a result, in-planetensile stresses are generated in the coating due to the greater CTEof the mullite coating. Also, the cumulative shrinkage of thedeposited splats quickly increases as the thickness of Yb2SiO5coating increases in the deposition, [24]. The residual stresses in-crease due to the thermal mismatch between coatings, resulting invertical crack formation (see Fig. 3).

Fig. 8 shows the surface temperature of Yb2SiO5 coating duringPS-PVD deposition measured by an infrared thermometer. In thisprocess, the coating surface temperature increases and then de-creases rapidly when the plasma torch passes over the coating. Atthe transient cooling stage, the coating surface firstly cools, and theresulting contraction is restricted by a relatively hotter underlayer,generating some in-plane tensile stresses [25]. The amplitude ofthe stress depends on the temperature gradient through thecoating thickness, which is determined by the heating capability of

the plasma flame and the surface cooling condition. Hotter plasmaand stronger surface cooling increase the temperature gradientthrough the coating at the transient cooling stage. Thus, a higherpre-heating temperature increases the temperature gradient, easilygenerating the vertical cracks (see Fig. 4). The surface temperatureand thickness of Yb2SiO5 coating gradually increase as the depo-sition time increases, resulting in higher in-plane tensile stressesand the vertical cracks in the thicker coatings (see Fig. 3).

Fig. 9. Schematic diagram of vertical crack formation in Yb2SiO5 coating.

X. Zhang et al. / Journal of Materiomics 6 (2020) 102e108 107

During PS-PVD of Yb2SiO5 coatings, Yb2SiO5 powder was evap-orated at a high temperature of plasma gun, and this vapor phasenucleated on the substrate. The decomposition of Yb2SiO5 wasaccompanied by a large volume shrinkage due to the phase trans-formation during transient cooling. The decomposition of SiO2resulted in an estimated volume shrinkage of 7%. The phasetransformation formed in-plane stresses along with other sourcesof tensile stress. Once the total tensile stress exceeded the tensilestrength of the coating, some vertical cracks formed on the externalsurface of the coating [15,26e28]. These cracks extended fatherinto the coating as Yb2SiO5 further decomposed during thedeposition.

Fig. 9 shows the entire vertical crack formation process inYb2SiO5 coating. Agglomerated Yb2SiO5 powder was injected intoan O3CP gun andwas then separated into its original small particlesby crashing [28,29]. Small particles of Yb2SiO5 decomposed intoYb2O3 and SiO2 phases in the plasma flame. Meanwhile, portions ofYb2O3 and SiO2 evaporated and nucleated on the substrate. In thecoating, the phase transformation occurs in parts of SiO2 nano-particles, leading to the volume change in spray process. DuringYb2SiO5 deposition, some factors including thermal expansion co-efficient mismatch, transient cooling, and nucleation of evaporatedphases generated stresses and formed vertical cracks.

4. Conclusions

Yb2SiO5 coatings with various thicknesses (i.e., 20 mm, 50 mm,and 100 mm) were deposited via PS-PVD at constant parameters,and the coatings were formed at different substrate pre-heatingtemperatures (i.e., 600 �C, 800 �C, and 1000 �C). It was indicatedthat the tensile stress derived from the thermal expansionmismatch, transient cooling after deposition, and the nucleation ofevaporated Yb2SiO5 were the main driving forces for the formationof vertical cracks in Yb2SiO5 coatings. Once the tensile stress

exceeded the tensile strength of the coating, vertical cracks formedon the top surface, and then extended towards Yb2SiO5/mullitecoating interface to generate larger vertical cracks. The high per-formance EBCs could be obtained as the thickness of Yb2SiO5coating was 50 mm and its pre-heating temperature was 800 �C.

Declaration of competing interest

We declare that we have no financial and personal relationshipswith other people or organizations that can inappropriately influ-ence our work, there is no professional or other personal interest ofany nature or kind in any product, service and/or company thatcould be construed as influencing the position presented in, or thereview of, the manuscript entitled.

Acknowledgements

We would like to acknowledge financial supports from NationalNatural Science Foundation of China (No. 51801034, No. 51771059),Guangdong Academy of Sciences Program (No. 2020GDASYL-20200104030, No. 2018GDASCX-0402, No. 2017GDASCX-0202, No.2017GDASCX-0111). Guangdong Technical Research Program (No.2017B090916002, No. 2017A070701027, No. 2014B07075007),Guangdong Natural Science Foundation (No. 2016A030312015, No.2017A030310315), and Guangzhou Technical Research Program(No. 201906010015, No. 201707010455, No. 201707010385).

References

[1] Padture N. Advanced structural ceramics in aerospace propulsion. Nat Mater2016;15:804e9.

[2] Liu B, Liu Y, Zhu C, Xiang H, Chen H, Sun L, et al. Advances on strategies forsearching for next generation thermal barrier coating materials. J Mater SciTechnol 2019;35:833e51.

[3] Mohan A, Udayakumar A, Gandhi A. High temperature oxidation behavior ofCVD b-SiC seal coated SiCf/SiC composites in static dry air and combustion

X. Zhang et al. / Journal of Materiomics 6 (2020) 102e108108

environment. Ceram Int 2017;43:9472e80.[4] Appleby M, Zhu D, Morscher G. Mechanical properties and real-time damage

evaluations of environmental barrier coated SiC/SiC CMCs subjected to tensileloading under thermal gradients. Surf Coat Technol 2015;284:318e26.

[5] Richards B, Begley M, Wadley H. Mechanisms of ytterbium monosilicate/mullite/silicon coating failure during thermal cycling in water vapor. J AmCeram Soc 2015;98:4066e75.

[6] Nasiri N, Patra N, Pezoldt M, Colas J, Lee W. Investigation of a single-layer EBCdeposited on SiC/SiC CMCs: Processing and corrosion behavior in high-temperature steam. J Eur Ceram Soc 2019;39:2703e11.

[7] Hu X, Xu F, Li K, Jin G, Xu Y, Zhao X. Water vapor corrosion behavior andfailure mechanism of plasma sprayed mullite/Lu2Si2O7-Lu2SiO5 coatings.Ceram Int 2018;44:14177e85.

[8] Lu M, Xiang H, Feng Z, Wang X, Zhou Y. Mechanical and thermal properties ofYb2SiO5: a promising material for T/EBCs applications. J Am Ceram Soc2016;99:1404e11.

[9] Zhang X, Zhou K, Liu M, Deng C, Deng C, Niu S, et al. CMAS corrosion andthermal cycle of Al-modified PS-PVD environmental barrier coating. Ceram Int2018;44:15959e64.

[10] Tian Z, Zheng L, Wang J, Yang J, Yang G, Wang J. Damge tolerance andextensive plastic deformation of b-Yb2Si2O7 from room to high temperatures.J Am Ceram Soc 2015;98:2843e51.

[11] Li B, Fan X, Okada H, Wang T. Mechanisms governing the failure modes ofdense vertically cracked thermal barrier coatings. Eng Fract Mech 2018;189:451e80.

[12] Bakan E, Marcano D, Zhou D, Sohn Y, Mauer G, Vaßen R. Yb2Si2O7 environ-mental barrier coatings deposited by various thermal spray techniques: apreliminary comparative study. J Therm Spray Technol 2017;26:1011e24.

[13] Viswanathan V, Dwivedi G, Sampath S. Multilayer, multimaterial thermalbarrier coating systems: design, synthesis, and performance assessment. J AmCeram Soc 2015;98:1769e77.

[14] Xie L, Chen D, Jordan E, Ozturk A, Wu F, Ma X, et al. Formation of verticalcracks in solution-precursor plasma-sprayed thermal barrier coatings. SurfCoat Technol 2006;201:1058e64.

[15] Richards B, Zhao H, Wadley H. Structure, composition, and defect controlduring plasma spray deposition of ytterbium silicate coatings. J Mater Sci2015;50:7939e57.

[16] Yang G, Li C, Hao S, Xing Y, Yang E, Li C. Critical bonding temperature for thesplat bonding formation during plasma spraying of ceramic materials. SurfCoat Technol 2013;235:841e7.

[17] Li C, Li C, Yang G, Wang Y. Examination of substrate surface melting-inducedsplashing during splat formation in plasma spraying. J Therm Spray Technol2006;15:717e24.

[18] Cheng B, Yang N, Zhang Q, Zhang M, Zhang Y, Chen L, et al. Sintering inducedthe failure behavior of dense vertically crack and lamellar structured TBCswith equivalent thermal insulation performance. Ceram Int 2017;43:15459e65.

[19] Lu Z, Kim M, Myoung S, Lee J, Jung Y, Kim I, et al. Thermal stability and

mechanical properties of thick thermal barrier coatings with vertical typecracks. Trans Nonferrous Met Soc China 2014;24:s29e35.

[20] Liu M, Zhang M, Zhang X, Li G, Zhang Q, Li C, et al. Transport and depositionbehaviors of vapor coating materials in plasma spray-physical vapor deposi-tion. Appl Surf Sci 2019;486:80e92.

[21] Liu M, Zhang M, Zhang Q, Yang G, Li C, Li C. Gaseous material capacity of openplasma jet in plasma spray-physical vapor deposition process. Appl Surf Sci2018;428:877e84.

[22] Zhang X, Zhou K, Deng C, Liu M, Deng Z, Deng C, et al. Gas-depositionmechanisms of 7YSZ coating based on plasma spray-physical vapor deposi-tion. J Eur Ceram Soc 2016;36:697e703.

[23] Zhang X, Zhou K, Liu M, Deng C, Deng Z, Chen B. Toughness and elasticitybehaviors in nano-structured 7 wt.% Y2O3-stabilized ZrO2 coating. Surf CoatTechnol 2015;276:316e9.

[24] Mauer G. Plasma characteristics and plasma-feedstock interaction under PS-PVD process conditions. Plasma Chem Plasma Proces 2014;34:1171e86.

[25] Zhang X, Song J, Deng Z, Wang Chao, Liu S, Liu G, et al. Interface evolution ofSi/Mullite/Yb2SiO5 PS-PVD environmental barrier coatings under high tem-perature. J Eur Ceram Soc 2019. https://doi.org/10.1016/j.jeurceramsoc.2019.10.062.

[26] Anwaar A, Wei L, Guo H, Zhang B. Plasmaepowder feedstock interactionduring plasma sprayephysical vapor deposition. J Therm Spray Technol2017;26:292e301.

[27] Kesler O, Matejicek J, Sampath S, Suresh S, Gnaeupel-Herold T, Brandc P, et al.Measurement of residual stress in plasma-sprayed metallic, ceramic andcomposite coatings. Mater Sci Eng A 1998;257:215e24.

[28] Rejda E, Socie D, Beardsley B, Rejda E, Socie D, Beardsley B. Fatigue behavior ofa plasma-sprayed 8%Y2O3-ZrO2 thermal barrier coating. Fatigue Fract EngMater 1997;20:1043e50.

[29] Xiang H, Feng Z, Zhou Y. Mechanical and thermal properties of Yb2SiO5: first-principles calculations and chemical bond theory investigations. J Mater Res2015;29:1609e19.

Dr. Xiaofeng Zhang is a senior engineer at National Engi-neering Laboratory of Modern Materials Surface Engi-neering Technology, Guangdong Institute of NewMaterials, China. He received his Ph.D. degree in MaterialsProcessing Engineering from South China University ofTechnology in 2016. Now, he works in City University ofHong Kong as senior research associate. His research in-terests focus on thermal/environmental barrier coatings.So far, he published 113 papers and received 11 patents.