Thermal Stability of YSZ Thick Thermal Barrier Coatings Deposited by Suspension and Atmospheric Plasma Spraying

crystals

Article Thermal Stability of YSZ Thick Thermal Barrier Coatings Deposited by Suspension and Atmospheric Plasma Spraying

Shiqian Tao 1,2, Jiasheng Yang 1, Minglong Zhai 1, Fang Shao 1, Xinghua Zhong 1, Huayu Zhao 1, Yin Zhuang 1, Jinxing Ni 1, Wei Li 2,* and Shunyan Tao 1,* 1 Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China; [email protected] (S.T.); [email protected] (J.Y.); [email protected] (M.Z.); [email protected] (F.S.); [email protected] (X.Z.); [email protected] (H.Z.); [email protected] (Y.Z.); [email protected] (J.N.) 2 School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China * Correspondence: [email protected] (W.L.); [email protected] (S.T.); Tel.: +86-21-6990-6321 (S.T.)

Received: 24 September 2020; Accepted: 25 October 2020; Published: 30 October 2020

Abstract: Two types of segmentation-crack structured yttria-stabilized zirconia (YSZ) thick thermal barrier coatings (>500 µm, TTBCs) were deposited onto the stainless steel substrates using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS) process, respectively. In this work, thermal aging behaviors, such as the microstructures, phase compositions, grain growth, and mechanical properties of APS TTBCs and SPS TTBCs, were systematically investigated. Results showed that both as-sprayed TTBCs exhibited a typical segmentation-crack structure in the through-thickness direction. APS coatings mainly comprised of larger columnar crystals, while a large number of smaller equiaxed grains existed in SPS coatings. Both of the coatings underwent tetragonal-monoclinic phase transformation after 1550 ◦C/40 h heat treatment. The poorer phase stability of SPS TTBCs may have a connection with smaller grain size. Thermal-aged APS and SPS coatings exhibited a signiﬁcant increase in H and E values with the rising of thermal aging temperature, and for the samples that thermal aged at 1550 ◦C, the H and E values increased sharply during initial stage then decreased after 80 h due to the phase decomposition. The segmented APS coatings had weak bonding between the lamellaes during thermal exposure, which caused the mean Vickers hardness value of APS TTBCs to be much lower than that of SPS TTBCs.

Keywords: atmosphere plasma spraying; suspension plasma spraying; thick thermal barrier coatings; segmentation cracks; thermal stability

1. Introduction Plasma-sprayed yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been increasingly used in aero- and land-based gas turbines to effectively protect the high-temperature components against hot oxidation and corrosion, which improves work efficiency and prolongs the serve life [1–4]. A typical TBC system consists of a superalloy substrate, an oxidation-resistant bond coat, and a thermal insulating ceramic topcoat. Considering the low thermal conductivity and high thermal expansion coefficient close to that of the metallic substrate, 6–8 wt.% YSZ is widely used as the ceramic topcoat of TBCs [5]. The thickness of traditional YSZ TBCs is generally less than 500 µm, and can only reduce the substrate surface temperature by 100–170 ◦C under service [6]. Therefore, measures should be taken to further improve the thermal insulation performance of TBCs to meet the increasing working temperature requirements of gas turbines.

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Studies have found that increasing the thickness of the ceramic layer is one of the most effective methods to improve the thermal durability of TBCs. However, the rise of coating thickness also brings undesirable influences: (1) Long-term spraying causes greater residual stress inside the coating and reduces the bonding strength between ceramic layer and substrate [7]. (2) The coating is more prone to sintering at higher temperatures, which ultimately leads to a decrease in the strain tolerance of the coating [8,9]. (3) The thermal shock resistance of TTBCs is reduced with an improved temperature gradient through the ceramic coating during long-term operations [10]. A successful method to resolve is the introduction of segmentation cracks into topcoat through varying spraying parameters, such as increasing the spraying power and shortening the standoff distance [11,12]. The segmented TTBCs have superior thermal shock resistance than that of conventional lamellar coatings, which is attributed to the fact that segmentation cracks not only restrain the propagation of horizontal cracks, but also release thermal mismatch stress formed across the ceramic layer and improve the strain tolerance of the top coatings [13,14]. Atmospheric plasma spraying (APS) and suspension plasma spraying (SPS) are two typical technologies for segmentation-crack structured TTBCs preparation. Zhai et al. [15] revealed that the thermal shock resistance of the SPS TTBCs (166 thermal cycles) is higher than that of nanostructured APS TTBCs (146 thermal cycles) and is around twice that of the traditional APS TTBCs. SPS TTBCs have exhibited excellent thermal cycle lifetimes, which contributes to the increased segmentation crack density, but no relevant researches have reported the systematic study on the thermal stability of the segmented-crack nanostructured APS and SPS TTBCs. TTBCs usually endure higher gas temperatures (>1250 ◦C). Due to long-term thermal exposure, YSZ coatings undergo severe sintering, which has a great impact on microstructure, phase composition, and mechanical properties [16]. Thompson et al. [17] and Witz et al. [18] investigated the effect of heat treatment on plasma-sprayed YSZ coatings and found that pores and microcracks healed gradually during thermal treatment, and with the increase of temperature and the extension of time, the initial metastable tetragonal (t’-ZrO2) phase partitioned into equilibrium tetragonal (t-ZrO2) and yttrium-rich cubic (c-ZrO2) due to yttrium diffusion. Upon further cooling, the newly generated t-ZrO2 transformed into monoclinic (m-ZrO2) phase. The formation of m-ZrO2 is generally accompanied by a 3–5% expansion of coating volume, and the accumulation of thermal stress may eventually lead to the YSZ TBCs peeling off [19–21]. For these reasons, it is meaningful to study the phase stability and microstructural evolution at higher temperatures and their influence on performance of the coatings. Ganvir et al. [22] and Zhao et al. [23] investigated the thermal aging behavior of SPS TTBCs, and several researches have quantified the thermal stability of segmentation-crack TBCs deposited by APS via varying spraying parameters. However, research has rarely focused on the thermal stability of nanostructured APS TTBCs, in particular, compared with SPS TTBCs, which provide theoretical and experimental basis for the preparation and engineering application of segmented-crack thick thermal barrier coatings with good thermal insulation and long service life at higher operating temperatures. In this paper, two types of segmentation-crack structured YSZ TTBCs were deposited by atmospheric plasma spraying (APS) and suspension plasma spraying (SPS) with nano-agglomerated particles and suspension, respectively. Based on the long life and high reliability of the thermal barrier coating for the protection of high-temperature components in air- and land-based gas turbines, we aimed to comparatively study the thermal stability of both coatings. The effect of thermal aging time and temperature on microstructure, phase composition, grain growth, and mechanical properties were investigated systematically. This basic research provides some useful insight for engineering application of TTBCs.

2. Experiments Procedure

2.1. Coating Preparation

The commercially available 7YSZ (ZrO2-7 wt.% Y2O3) nano-agglomerated powder and 7.5YSZ suspension were used to deposit top coatings. The nanostructured feedstock was Nanox S4007 (Inframat Crystals 2020, 10, 984 3 of 13

Corp., USA), with sizes ranging from 15 µm to 150 µm, and was sprayed by atmospheric plasma spraying (APS) process using a F4MB-XL gun (Sulzer Metco, Wohlen, Switzerland). For comparison, an aqueous suspension of 7.5YSZ submicron-particles (Jiangsu Lida Hi-tech Special Material Co., Ltd., Changshu, China) was used as feedstock, with a medium particle diameter (D50) was 0.06 µm. The fraction of solid phase in the suspension was approximately 20 vol.%, and was deposited by a suspension plasma spraying (SPS) system (Northwest Mettech Corp., North Vancouver, Canada) using the Axial III™ plasma torch with a NanoFeed™ models 350 suspension feeder. The thickness of the top coatings was about 750 µm, and the detailed spraying parameters are listed in Table1. The free-standing samples, which functioned as thermal-aging treatment, were debonded from the stainless-steel substrate due to the mismatch of thermal expansion coeﬃcients between coatings and substrate. Ten sets of substrate-free specimens were prepared for thermal-aging treatment in a high-temperature chamber furnace (HTK 20/17, ThermConcept, Bremen, Germany) at 1200 ◦C, 1300 ◦C, 1400 ◦C, 1500 ◦C, and 1600 ◦C for 24 h and at 1550 ◦C for periods of 20 h, 40 h, 60 h, 80 h, and 100 h, respectively. The samples were put into the furnace and heated at about 10 ◦C/min to the target temperature. After keeping in an air atmosphere for a selected time, the coatings were cooled in the furnace to the room temperature. All the free-standing samples were impregnated with epoxy (conductive resin for EBSD), cut to prepare the cross-sections for observation, and ﬁnally polished by routine metallographic methods. Particularly, ion beam polishing was necessary for the samples used for EBSD testing.

Table 1. The spraying parameters applied for the (yttria-stabilized zirconia) YSZ coating.

Parameters APS Coating SPS Coating

Plasma gas mixture H2 + Ar H2 + Ar + N2 Plasma gas ﬂow rate (slpm) H2: 7–9 H2: 20–25 Ar: 30–36 Ar: 170–176 N2: 48–53 Carrier gas (slpm) Ar: 3.0–3.5 14.5–15.2 Powder inject diameter (mm) 1.8 1.5 Stand-oﬀ distance (mm) 80–85 70–90 Feed rate (g/min) 30 40

2.2. Coating Characterization The microstructures of the substrate-free samples were observed by a scanning electron microscope (SEM, S-4800, HITACHI, Tokyo, Japan) operated in backscattered electron image mode. The information on phase compositions of each specimens before and after heat treated was characterized by an X-ray diffractometer (XRD, D/max 2550 V, Rigaku, Japan) with filtered Cu Kα (40 kV, 40 mA) radiation 1 within the 2θ = 10–90◦ range at a scan rate of 4◦min− . Besides a more detailed scanning within 1 the 2θ = 72–76◦ range at a slower scan rate of 0.2◦min− , the step size was 0.02◦. Phase content and distribution and grain growth behaviour were investigated by electron backscatter diffraction (EBSD, Symmetry, Oxford, UK). In particular, the scanning area was approximately 5100 µm2, and the step size was 150 nm. Using the EBSD image analysis of a certain coating area, some useful information, such as the size, distribution, orientation and grain boundary distribution of the crystal grains, and the phase contained in the area can be easily acquired. The nanoindentation test was carried out on the polished cross section of the coatings before and after thermal treatment to determine the Hardness (H) and Young’s Modulus (E), using a G-200 nanoindenter (Agilent Technologies, Oak, Ridge, USA) equipped with a Berkovich indenter. The instrument was operated in a continuous stiffness mode (CSM) at a constant nominal strain 1 rate of 0.05 s− , 2 nm harmonic displacement, and 45 Hz oscillation frequency. A Poisson’s ratio of 0.25 was used to calculate the Young’s Modulus. Indentations were performed at a 5000 nm maximum indentation depth. We selected 10 measurement points at an indentation depth of 500 nm for each sample, and took the average result as the Hardness and Young’s Modulus of the samples. Crystals 2020, 10, 984 4 of 13

Crystals 2020, 9, x FOR PEER REVIEW 4 of 12 Vickers microhardness measurements were also carried out on the polished cross-section of the coatings usingTester a(Vickers, Hardness Tukon Tester-2100B, (Vickers, Instr Tukon-2100B,on, USA) under Instron, the load USA) of 300 under g with the a dwell load of time 300 of g 1 with0 s. We a dwell took time10 measurement of 10 s. We points took 10 for measurement each sample pointsand record for eached the sample average and results recorded as Vickers the average hardness results of the as Vickerssample. hardnessAll the measurement of the sample. points All thewere measurement selected away points fromwere the edge selected of the away crack froms and the the edge coating. of the cracks and the coating. 3. Results and Discussion 3. Results and Discussion

3.1. Phase Composition The XRD patterns of samples before and after isothermal exposure at various temperatures for 24 h and at 1550 °C for various times were performed to trace the phase composition, and the relevant 24 h and at 1550 ◦C for various times were performed to trace the phase composition, and the relevant resultsresults are displayed in Figures1 1––33.. AsAs cancan bebe seen,seen, thethe mainmain phasephase ofof as-sprayedas-sprayed nano-agglomeratednano-agglomerated APS and SPS coatings was the nontransformable metastable tetragonal (t’-ZrO2) phase, and no peaks APS and SPS coatings was the nontransformable metastable tetragonal (t’-ZrO2) phase, and no peaks corresponding to monoclinic phase (m-ZrO2) were detected. The emergence of t’-ZrO2 might be corresponding to monoclinic phase (m-ZrO2) were detected. The emergence of t’-ZrO2 might be attributed to the didiffusionlessffusionless transition in the process of rapid solidificationsolidification and rapid cooling of molten YSZ particles during plasma spraying. XRD results showed that the phases presented in both coatings were not altered,altered, even after heating up 1600, inferring thatthat there was almost no equilibrium tetragonal (t-ZrO2) → monoclinic phase transformation that took place in the APS and SPS coatings. tetragonal (t-ZrO2) monoclinic phase transformation that took place in the APS and SPS coatings. As it can be seen from→ Figures 2b and 3b, the m-ZrO2 peaks at 2θ values of about 28.2°(111) and As it can be seen from Figures2b and3b, the m-ZrO 2 peaks at 2θ values of about 28.2◦(111) and 31.5°(111) occurred over a short period of time at 1550 °C, and m-ZrO2 peak intensities of the both 31.5◦(111) occurred over a short period of time at 1550 ◦C, and m-ZrO2 peak intensities of the both YSZ coatingsYSZ coatings increased increased with with prolonged prolonged aging aging time. time. This alsoThis indicatesalso indicate thatstemperature that temperature made made the most the significantmost significant contribution contribution to the extent to the of extent the phase of the transformation. phase transformation. As reported, As reported, the phase the transition phase transition of t-ZrO2 into m-ZrO2 always comes together with a continuous volume expansion up to of t-ZrO2 into m-ZrO2 always comes together with a continuous volume expansion up to 3–5%, bringing3–5%, bringing the accumulation the accumulation of the coatingof the coating compressive compressive stress, which stress, may which eventually may eve resultntually in result the YSZ in the YSZ coatings peeling off. It should be noted that the intensities of m-ZrO2 in SPS coatings were coatings peeling off. It should be noted that the intensities of m-ZrO2 in SPS coatings were higher than thathigher of inthan nanostructured that of in nanostructured APS coatings, whichAPS coatings, may denote which that may the SPSdenote coatings that the had SPS relatively coatings weaker had phaserelatively durability. weaker phase durability.

Figure 1. XRD patterns of as-sprayed coatings with 2θ between 72◦ and 76◦:(a) APS coating; (b) SPS coating. Figure 1. XRD patterns of as-sprayed coatings with 2θ between 72° and 76°: (a) APS coating; (b) SPS coating.

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FigureFigure 2.2. XRDXRD patternspatterns ofof as-sprayedas-sprayed nanostructurednanostructured APS coating and heated-treatedheated-treated APS coatings: Figure(a) Samples 2. XRD after patterns aging of for as - 24sprayed h at different nanostructured temperatures APS coating; (b) samples and heated after- treatedaging atAPS 1550 coatings: °C for (a) Samples after aging for 24 h at diﬀerent temperatures; (b) samples after aging at 1550 ◦C for (a) Samples after aging for 24 h at different temperatures; (b) samples after aging at 1550 °C for didifferentﬀerent times.times. different times.

FigureFigure 3.3. XRDXRD patternspatterns ofof as-sprayedas-sprayed SPSSPS coatingcoating andand heated-treatedheated-treated SPSSPS coatingscoatings [[2223]:]: ((aa)) SamplesSamples afterFigureafter agingaging 3. XRD forfor 2424patterns hh atat didifferent ffoferent as-sprayed temperatures;temperatures; SPS coating (b(b)) samples samples and heated after after- aging treatedaging at at SPS 1550 1550 coatings◦ °CCfor fordi different[22fferent]: (a) times. Stimesamples. after aging for 24 h at different temperatures; (b) samples after aging at 1550 °C for different times. 3.2. Microstructure 3.2. Microstructure 3.2. MicrostructureFigure4 illustrates the polished cross-section morphologies of the as-sprayed nanostructured Figure 4 illustrates the polished cross-section morphologies of the as-sprayed nanostructured APS and SPS coatings. The SEM images indicated that both of the coatings displayed a unique APS Figure and SPS 4 illustrate coatings.s theThe polishedSEM images cross - indicatesection dmorphologies that both of of the the coatings as-sprayed displayed nanostructured a unique microstructure, with some obvious vertical cracks running perpendicular to the coating surface and APSmicrostructure and SPS coatings., with some The obvious SEM imagesvertical indicatecracks runningd that both perpendicular of the coatings to the displayedcoating surface a unique and branching cracks running parallel to the coating surface. Typically, segmentation cracks were vertical microstructurebranching cracks, with running some parallel obvious to vertical the coating cracks surface. running Typically, perpendicular segmentation to the coatingcracks were surface vertical and cracks penetrating at least half of the coating thickness, which were favorable to the relaxation of residual branchingcracks penetrating cracks running at least parallel half of to the the coating coating thickness, surface. Typically, which were segmentation favorable cracksto the wererelaxation vertical of stress and the enhancement of strain tolerance of coatings. Meanwhile, the branching cracks were cracksresidual penetrating stress and theat least enhancement half of the of coating strain tolerance thickness, of whichcoatings. were Meanwhile, favorable the to branchingthe relaxation cracks of generally connected with the segmentation crack and in favor of increasing the compliance and thermal residualwere generally stress and connected the enhancement with the segmentation of strain tolerance crack andof coatings. in favor Meanwhile,of increasing the the branching compliance cracks and insulation of coatings. Segmentation crack densities (Ds) were defined by the ratio of the number of werethermal generally insulation connected of coating withs. the Segmentation segmentation crack crack densities and in favor (Ds) ofwere increasing defined the by compliancethe ratio of andthe such cracks in the entire observation section to the horizontal length of this section. The measured Ds thermalnumber insulationof such cracks of co inating the s.entire Segmentation observation crack section densities to the (Ds) horizontal 1were defined length ofby this the1 section.ratio of Thethe values of the APS and SPS YSZ coatings were about 2.5 cracks mm− and 4 cracks mm− , respectively. numbermeasured of Ds such values cracks of thein the APS entire and SPSobservation YSZ coatings section were to theabout horizontal 2.5 cracks length mm−1 of and this 4 crackssection. mm The−1, Both coatings rarely had unmolten or semi-molten particles, and the SPS coating was much denser measuredrespectively. Ds Bothvalues coatings of the APS rarely and had SPS unmolten YSZ coatings or semi were-molten about 2.5particles, cracks andmm −1the and SPS 4 cracks coating mm was−1, than the nanostructured APS coating, which may have been due to the effect of the characteristic respectively.much denser Boththan coatingsthe nanostructured rarely had unmoltenAPS coating, or semi which-molten may particles,have been and due the to SPSthe coatingeffect of was the spraying conditions. muchcharacteristic denser sprayingthan the nanostructuredconditions. APS coating, which may have been due to the effect of the characteristic spraying conditions.

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200μm 200μm

Figure 4. Cross-sectional microstructures of as-sprayed TTBCs: (a) Nanostructured APS TTBCs [15], Figure 4. Cross-sectional microstructures of as-sprayed TTBCs: (a) Nanostructured APS TTBCs [15], Figure(b) SPS 4. CrossTTBCs-sectional [24]. microstructures of as-sprayed TTBCs: (a) Nanostructured APS TTBCs [15], (b) SPS TTBCs [24]. (b) SPS TTBCs [24]. Figure 5 shows a comparison with the fracture morphologies of nanostructured APS and SPS Figure5 shows a comparison with the fracture morphologies of nanostructured APS and SPS coatingsFigure before 5 shows and a after comparison thermal treatment.with the fracture Remarkable morphologies differences of in nanostructured microstructure APS were and observed SPS coatings before and after thermal treatment. Remarkable differences in microstructure were observed coatingsfor the beforeas-sprayed and after and thermalaged coatings. treatment. As- sprayedRemarkable APS differences coating had in amicrostructure typical lamella werer structure, observed and for the as-sprayed and aged coatings. As-sprayed APS coating had a typical lamellar structure, forthe the splat as-sprayed-splat interactions and aged coatings.and numerous As-sprayed dense APScolumnar coating grains had ain typical each splat lamella werer structure, visible. Part and of and the splat-splat interactions and numerous dense columnar grains in each splat were visible. theintersplats splat-splat formed interactions a good and connection, numerous which dense was columnar probably grains due in to each the continuoussplat were visible.growth Partof some of Part of intersplats formed a good connection, which was probably due to the continuous growth intersplatscolumnar formed crystals a between good connection, the lamellaes. which In cwasontrast probably to the dueAPS to TTBCs, the continuous the SPS coating growth consisted of some of of some columnar crystals between the lamellaes. In contrast to the APS TTBCs, the SPS coating columnarsuccessive crystals equiaxial between grains, the there lamellaes. were no In longer contrast plenty to the of APS columnar TTBCs, grains the SPS, evident coating splat consisted boundaries of consisted of successive equiaxial grains, there were no longer plenty of columnar grains, evident splat successiveexisted in equiaxial the coating, grains, and there some were branching no longer cracksplenty extendof columnared along grains equiaxed, evident crystal splat boundaries edges. This boundaries existed in the coating, and some branching cracks extended along equiaxed crystal edges. existeddifference in the in coating,microstructure and some mainly branching depends cracks on the extend distinguishinged along equiaxedfeatures of crystal both APS edges. and This SPS This difference in microstructure mainly depends on the distinguishing features of both APS and SPS diffedropletsrence andin microstructure the bonding strength mainly betweendepends theon lamellas.the distinguishing As reported, features the single of both droplet APS size and of SPS APS droplets and the bonding strength between the lamellas. As reported, the single droplet size of APS dropletsYSZ coatings and the isbonding usually strength 10–100 between μm with the a lamellas. thickness A ofs reported, 0.2–3 μm, the acsinglecompanied droplet withsize of a APS dense YSZ coatings is usually 10–100 µm with a thickness of 0.2–3 µm, accompanied with a dense microcrack YSZmicrocrack coatings network, is usually while 10– the100 SPS μm YSZ with-coated a thickness single droplet of 0.2– size3 μm, range accompanieds from a few with nanometers a dense to network, while the SPS YSZ-coated single droplet size ranges from a few nanometers to a few microns microcracka few microns network, in diameter. while the For SPS both YSZ of -thecoated heat -singletreated droplet coatings, size an range apparents from increas a fewing nanometers trend in grain to in diameter. For both of the heat-treated coatings, an apparent increasing trend in grain size and a a fewsize micronsand a clear in diameter. crystal boundar For bothy ofbetween the heat grains-treated can coatings, be observed an apparent with improved increasing temperature trend in grain and clear crystal boundary between grains can be observed with improved temperature and prolonged sizeprolonged and a clear time crystal. Figure boundar 5b,e alsoy between shows grains the evidence can be observed of both promotedwith improved intersplat temperature bonding and and time. Figure5b,e also shows the evidence of both promoted intersplat bonding and microcrack healing. prolonged time. Figure 5b,e also shows the evidence of both promoted intersplat bonding and Themicrocrack grain size healing and morphologies. The grain of size APS and coatings morphologies changes were of APS much coatings more conspicuous. changes were Severe much crystal more microcrackconspicuous healing. Severe. T hecrystal grain cracking size and can morphologies be observed ofin APStwo coatings coatings after change 1550s were °C /100 much h thermal more - cracking can be observed in two coatings after 1550 ◦C/100 h thermal-aged coatings. This significant conspicuousaged coatings.. Severe This crystal significant cracking change can be inobserved microstructure in two coatings may haveafter 1550 been °C due/100 to h thermal the phase- change in microstructure may have been due to the phase transformation from t-ZrO2 to m-ZrO2 phase, agedtransformation coatings. T hisfrom significant t-ZrO2 to change m-ZrO 2 inphase microstructure, which was may accompanied have been by due vo lumeto the expansion, phase which was accompanied by volume expansion, eventually resulting in the nucleation and propagation transformationeventually result froming tin-ZrO the2 nucto leation m-ZrO and2 phase propagation, which wasof microcracks accompanied. by volume expansion, of microcracks. eventually resulting in the nucleation and propagation of microcracks. (a) (b) (c) (a) (b) (c)

10μm 10μm 10μm 10μm 10μm 10μm (d) (e) (f) (d) (e) (f)

10μm 10μm 10μm 10μm 10μm 10μm FigureFigure 5. 5. FractureFracture morphology of of free free-standing-standing specimens: specimens: (a– (ca)– Nc)anostructured Nanostructured APS APS coating, coating, (d–f) Figure(dSPS–f) SPScoating; 5. Fracture coating; (a,d (morphology)a as,d-)sprayed, as-sprayed, of(b ,freee ()b 1600,e-standing) 1600 °C /24◦C /specimens:h,24 (c h,,f) ( c1550,f) 1550 (°Ca–/100c◦) CN/ 100anostructuredh. h. APS coating, (d–f) SPS coating; (a,d) as-sprayed, (b,e) 1600 °C /24 h, (c,f) 1550 °C /100 h.

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FigureFigure6 presents 6 present thes the cross-sectional cross-sectional microstructures microstructures of nanostructuredof nanostructured APS APS and and SPS SPS coatings coatings beforebefore and and after after thermal thermal treatment. treatment. It canIt can be be seen seen that that di ffdifferenterent types types of of defects, defect suchs, such as as microcracks microcracks andand pores, pores coexisted, coexisted in in both both as-sprayed as-sprayed coatings.coatings. There was was a a significant significant difference difference in in crack crack gap gap and andlength length of ofthe the two two coatings, coatings, and and the thenanostructured nanostructured APS APS coating coating was wasmuch much coarse coarser.r. The SPS The coating SPS coatinghad larger had larger penetrating penetrating cracks cracks,, and and the the measured measured crack crack gap gap was was about about up to to 3 3µ m, μm, which which was was beneficialbeneficial in in releasing releasing the the thermal thermal mismatch mismatch stress stress during during heat heat treatment. treatment. The The higher higher Ds Ds value value was was stronglystrongly attached attached to the to substrate the substrate temperature temperature and spraying and spraying power. Both power. of the Both thermal-aged of the thermal coatings-aged hadcoatings the same had tendencies. the same tendenc Microcracksies. Microcracks surrounding surrounding in the penetration in the penetration crack regions crack began region tos heal, began andto the heal, porosity and the decreased porosity first decreased along with first the along coating with sintering, the coating then sintering, increased then after increased long-term after thermal long- exposureterm thermal due to exposure the formation due to of the m-ZrO formation2 phase, of leading m-ZrO to2 phase, accumulation leading ofto theaccumulation coating thermal of the stress, coating andthermal finally stress, gave rise and to finally the generation gave rise ofto morethe ge microcracksneration of more in the microcracks coatings. in the coatings.

(a) (b) (c)

20μm 20μm 20μm (d) (e) (f)

20μm 20μm 20μm

FigureFigure 6. Polished6. Polished cross-sectional cross-sectional morphology: morphology: (a –(ca)– Nanostructuredc) Nanostructured APS APS coating, coating, (d –(df)– SPSf) SPS coating; coating; (a,d(a), as-sprayed,d) as-sprayed, (b, e(b), 1600e) 1600◦C /°24C/24 h, (h,c,f ()c 1550,f) 1550◦C /°100C/100 h. h.

InIn the the pursuit pursuit of gaining of gaining a better a better understanding understanding of the phase of the transformation phase transformation phenomenon phenomenon during thermalduring exposure, thermal exposure Figures7, andFigur8 eshoweds 7 and the8 showed EBSD the images EBSD of images the polished of the cross-sectionspolished cross- ofsections both of theboth as-sprayed the as-sprayed coatings coatings and the and samples the samples after 1550 after◦C /1550100 h° thermalC/100 h exposure.thermal exposure. Band contrast Band contrast maps indicatedmaps indicated that there that were there two were distinct two grain distinct types grain in two types as-sprayed in two as coatings,-sprayed larger coatings columnar, larger grains columnar in theg APSrains coating in the andAPS smallercoating equiaxed and smaller grains equiaxed in the SPS grains coating, in the revealing SPS coating, the diff revealingerent microstructure the different ofmicrostructure the lamellar interface. of the lamellar The SPS interface coating. wasThe shortSPS coating of splats, was while short the of splat-splatsplats, while interactions the splat-splat in APSinteractions coating were in APS evident. coating The were as-sprayed evident. APS The coating as-sprayed had poorer APS coating compactness had poorer than compactness the SPS coating. than Thethe phase SPS maps coating. in Figure The phase7b,e and maps Figure in8 Fb,eigures exhibit 7b,e the and phase 8b, compositione exhibit the of phase each sample, composition the content of each ofsample, each phase, the thecontent pore distribution,of each phase, and the the pore size distribution, and feature ofand crystal the size grains. and Di featurefferent of colors crystal indicate grains. diffDifferenterent phase colors compositions. indicate different Red represents phase compositions. the tetragonal Red phase, represent blues represents the tetragonal the monoclinic phase, blue phase,represent blacks representsthe monoclinic the pores phase, and black unrecognized represents compositions. the pores and There unrecognized were some compositions micropores. andThere m-ZrOwere2 somein the micropores as-sprayed APSand coatings.m-ZrO2 in The the formation as-sprayed of theAPS tetragonal coatings. phaseThe formation was caused of bythe the tetragonal rapid coolingphase and was solidification caused by the of in-flightrapid cooling particles, and preventing solidification yttrium of in from-flight di ffparticles,usion. After preventing 1550 ◦C/ 100yttrium h, thefrom grains diffus grewion obviously.. After 1550 Meanwhile, °C/100 h, both the grains of the coatings grew obviously. underwent Meanwhile, phase decomposition, both of the coatings and a largeunderwent quantity phase of pores decomposition was observed., and The a large percentage quantity of of m-ZrO pores2 wasin SPS observed coating. The was percentage about 21.1%, of m- nearlyZrO2 twice in SPS than coating that of was APS about coating, 21.1%, implying nearly thattwice the than SPS that coating of APS suff eredcoating, from implying more severe that phasethe SPS transitioncoating fromsuffered t-ZrO from2 to m-ZrOmore severe2. In the phase decomposition transition process,from t-ZrO the2 increased to m-ZrO microstrain2. In the decomposition dominated theprocess, crack initiation the increased after propagation microstrain and dominated extension, the eventually crack initiation leaving after coating propagation failure. The and orientation extension, imageseventually also exhibited leaving coating that there failure. was The no preferred orientation grain images orientation, also exhibited whether that or there not the was coating no preferred was grain orientation, whether or not the coating was thermal treated. The content of monoclinic phase, tetragonal phase, and grain size can be obtained from the data calculated by EBSD, as summarized in Table 2.

Crystals 2020, 10, 984 8 of 13

Crystals 2020, 9, x FOR PEER REVIEW 8 of 12 thermal treated. The content of monoclinic phase, tetragonal phase, and grain size can be obtained Crystals 2020, 9, x FOR PEER REVIEW 8 of 12 from the data calculated by EBSD, as summarized in Table2. (a) (b) (c) (a) (b) (c)

20μm 20μm 20μm (d) 20μm (e) 20μm (f) 20μm (d) (e) (f)

20μm 20μm 20μm 20μm 20μm 20μm Figure 7. EBSD images of nanostructured APS coatings before and after heat treatment: (a,d) Band . FigureFigurecontrast 7. 7. EBSDEBSD maps, imagesimages (b,e) phaseof of nanostructured nanostructured maps, (c,f) orientation APS APS coatings coatings maps; before before (a–c) andas and-sprayed, after after heat heat (d –treatf treatment:) 1550ment°C: (/100a (,ad,d) h)B Bandand contrastcontrast maps, maps, (b (b,e,e) )phase phase maps, maps, (c (,cf,)f )orientation orientation maps; maps; (a (a––cc) )as as-sprayed,-sprayed, (d (d––f)f )1550 1550 °◦CC/100/100 h h.. (a) (b) (c) (a) (b) (c)

20μm 20μm 20μm (d) 20μm (e) 20μm (f) 20μm (d) (e) (f)

20μm 20μm 20μm 20μm 20μm 20μm Figure 8. EBSD images of SPS coatings before and after heat treatment: (a,d) Band contrast maps,

(bFigure,e) phase 8. EBSD maps, images (c,f) orientation of SPS coatings maps; before (a–c) as-sprayed, and after heat (d– ftreat) 1550ment◦C:/ 100(a,d h.) Band contrast maps, (b,e) phase maps, (c,f) orientation maps; (a–c) as-sprayed, (d–f) 1550 °C/100 h. Figure 8.Table EBSD 2. imagesThe microstructure of SPS coatings parameters before and of after coatings heat treat beforement and: ( aftera,d) B thermaland contrast treatment. maps, (b,e) phase maps, (c,f) orientation maps; (a–c) as-sprayed, (d–f) 1550 °C/100 h. Table 2. The microstructure parameters of coatings before and after thermal treatment. Samples Tetragonal Phase (%) Monoclinic Phase (%) Average Grain Size (µm) . Table 2.as-sprayed The microstructure parametersTetragonal 95.5 of coatings Monoclinic before 0.41 and after Phase thermal trAeatveragement 0.96 Grain Size APS TTBCs Samples1600 C /24 h 97.8 0.79 1.83 ◦ TetragonalPhase (%) Monoclinic(%) Phase Average Grain(μm) Size Samples1550 ◦C/100 h 82.6 9.38 1.23 as-sprayed Phase9 (%)5.5 (%)0 .41 (μm)0.96 as-sprayed 99.4 0.01 0.72 as1-sprayed600 °C/24 h 95.597.8 0.410.79 0.961.83 SPSAPS TTBCs TTBCs 1600 ◦C/24 h 95.6 2.45 1.58 155016001550 C°C/ 100°/24C/100 hh h 70.497.882.6 0.79 21.19.38 1 1.18.831.23 APS TTBCs ◦ 1550as °C-sprayed/100 h 82.699.4 9.380.01 1.230.72 as1-sprayed600 °C/24 h 99.495.6 0.012.45 0.721.58 SPSFor TTBCs a more speciﬁc description of grain change, grain size obtained by EBSD analysis was plotted 16001550 °C °/24C/100 h h 95.670.4 2.4521.1 1.581.18 inSPS Figure TTBCs9. It can be seen that the average grain sizes of the as-sprayed APS and SPS coatings were 1550 °C/100 h 70.4 21.1 1.18 For a more specific description of grain change, grain size obtained by EBSD analysis was plotted in FFigureor a more 9. It specific can be descriptionseen that the of average grain change, grain grainsizes ofsize the obtained as-sprayed by EBSD APS analysisand SPS wascoating plotteds were in Figure 9. It can be seen that the average grain sizes of the as-sprayed APS and SPS coatings were

Crystals 2020, 9, x FOR PEER REVIEW 9 of 12 about 0.96 μm and 0.72 μm, respectively. Both coatings had a remarkable increase in grain sizes after 1600 °C/24 h thermal aging. It is worth noting that grain growth was not the mutual bond of small grains, but the result of grain boundary movement. When small grains grew into large grains, giving rise to the decrease of grain boundary area, the interface free energy was reduced, and the grain size increased. The grain growth rate of the SPS coating was higher than that of APS coating, which suggests that SPS TTBCs tended to be sintered during thermal exposure. This may have close correlation with the finer crystal grains in the SPS coating, because the smaller the crystal grains, the higher the surface energy. In order to further analyze the anti-sintering performance of Crystalsnanostructured2020, 10, 984 APS and SPS coating, Equation (1) was used to calculate the activation energy9 of of 13 grain growth. Q about 0.96 µm and 0.72 µm, respectively. DT = BothDo × coatingsEXP (− had) a remarkable increase in grain sizes(1) RT after 1600 ◦C/24 h thermal aging. It is worth noting that grain growth was not the mutual bond ofwhere small DT grains, is the average but the grain result size of at grain temperature boundary (T), movement. Do is the average When smallgrain size grains at as grew-sprayed, into large Q is grains,the activation giving energy, rise to and the decreaseR is gas constant. of grain For boundary comparison, area, we the chose interface two set free samples energy (APS was coatings, reduced, andSPS coatings) the grain which size increased. were thermal The graintreated growth at 1600 rate °C/24 of theh. It SPS needs coating to be wasemphasized higher than that, that during of APS the coating,calculated which of each suggests group thatof samples, SPS TTBCs the corresponding tended to be sintered as-sprayed during coatings thermal were exposure. used as Thisreference may havesamples. close correlation with the ﬁner crystal grains in the SPS coating, because the smaller the crystal grains,Hence, the higher the calculated the surface activation energy. energy In order for to SPS further coating analyze grain the growth anti-sintering was 12.2 performance KJ/mol, which of nanostructuredwas larger than that APS of and APS SPS coating coating, (10.0 Equation KJ/mol), (1) demonstrating was used to calculatethat the SP theS coating activation grain energy growth of grainwas more growth. sensitive to temperature and sintering was more likely to occur at higher temperature, along with the apparent microstructure evolution, such asQ crack healing, pore shrinking, and grain DT = Do EXP (1) growth. For coatings treated at 1550 °C, grains grew× quickly−RT during the initial stage and slower after

20where h, and DT theis the crystal average boundary grain size was at visible. temperature Part of (T), fractured Do is the grains average and grain microcracks size at as-sprayed, appeared, Q which is the activationwas caused energy, by and the R greater is gas constant. thermal For-induced comparison, stress. we As chose previously two set samples mentioned, (APScoatings, the phase SPS coatings)transformation which werefrom thermalt-ZrO2 to treated m-ZrO at2 1600 phase◦C /promoted24 h. It needs lattice to be expansion emphasized and that, collapse, during leading the calculated to the ofaggregation each group of of vacancies samples, thein grains corresponding, causing as-sprayeda large number coatings of microcracks were used as to reference be formed. samples.

Figure 9. GrainGrain size size of of YSZ YSZ coatings coatings before before and and after after thermal thermal treat treatment:ment: ( (aa)) N Nanostructuredanostructured APS APS TBCs; TBCs; (b) SPS TBCs.TBCs. Hence, the calculated activation energy for SPS coating grain growth was 12.2 KJ/mol, which was 3.3. Mechanical Properties larger than that of APS coating (10.0 KJ/mol), demonstrating that the SPS coating grain growth was moreIt sensitive was well to known temperature that, andafter sintering high-temperature was more likelytreatment, to occur the at microstructure higher temperature,, including along pore with thestructures, apparent crack microstructure states, and grain evolution, morphology such asof crackthe YSZ healing, coatings pore, change shrinking,s significantly, and grain which growth. has Fora prominent coatings treated impact at on 1550 the ◦ mechanicalC, grains grew properties quickly of during the TTBCs. the initial Figure stages 1 and0 and slower 11 present after 20 the h, andhardness the crystal (H) and boundary Young’s was modulus visible. (E) Part of ofnanostructured fractured grains APS and and microcracks SPS coatings appeared, measured which by wasthe causednanoindentation by the greater method thermal-induced. Both thermal- stress.aged coatings As previously exhibited mentioned, similar tendency the phase under transformation long-term fromservice t-ZrO conditions.2 to m-ZrO With2 phase temperature promoted continuous lattice expansionly increas andingcollapse, up to 1600 leading °C, there to the were aggregation significant of vacanciesaugments inin grains,the H and causing E value a larges. Thermal number-treated of microcracks samples grew to be denser formed. due to sintering. Pore healing and grain growth took place in the process of heat treatment, which was conductive to the closer 3.3. Mechanical Properties

It was well known that, after high-temperature treatment, the microstructure, including pore structures, crack states, and grain morphology of the YSZ coatings, changes significantly, which has a prominent impact on the mechanical properties of the TTBCs. Figures 10 and 11 present the hardness (H) and Young’s modulus (E) of nanostructured APS and SPS coatings measured by the nanoindentation method. Both thermal-aged coatings exhibited similar tendency under long-term service conditions. With temperature continuously increasing up to 1600 ◦C, there were significant augments in the H and E values. Thermal-treated samples grew denser due to sintering. Pore healing and grain growth took place in the process of heat treatment, which was conductive to the closer bonding between Crystals 2020, 9, x FOR PEER REVIEW 10 of 12 Crystals 20202020, 109, ,x 984 FOR PEER REVIEW 10 of 12 13 bonding between splats and coherence across the grain boundary. The sintering effect from 1200– 1500bonding °C for between 24 h was splats outstanding and coherence. Then, asacross the temperaturethe grain boundary. continued The to sinteringincrease, theeffect value from varied 1200 – splats and coherence across the grain boundary. The sintering effect from 1200–1500 C for 24 h was slightly,1500 °C and for 24fluctuation h was outstanding was small.. Then,For the as samplesthe temperature thermal continuedaged at 1550 to increase,°C, the H the ◦and value E values varied outstanding. Then, as the temperature continued to increase, the value varied slightly, and fluctuation increasedslightly, andsharply fluctuation during initialwas small. stage ,For then the decreased samples afterthermal 80 h.aged The at improvement 1550 °C, the ofH theand H E andvalues E was small. For the samples thermal aged at 1550 C, the H and E values increased sharply during valueincreaseds can sharply be ascribed during to theinitial coating stage ,sintering, then decreased along◦ after with 80 the h. elevated The improvement splat-splat of bonding the H and and E initial stage, then decreased after 80 h. The improvement of the H and E values can be ascribed to the healingvalues ofcan defects be ascribed like pores to the and coating microcracks. sintering, The along phase with transformation the elevated splataccompanied-splat bonding by some and coating sintering, along with the elevated splat-splat bonding and healing of defects like pores and irreversiblehealing of microstructure defects like pores changes and resulted microcracks. in the Thedecline phase of mechanical transformation properties. accompanied by some microcracks. The phase transformation accompanied by some irreversible microstructure changes irreversible microstructure changes resulted in the decline of mechanical properties. resulted in the decline of mechanical properties.

FigureFigure 10. 10. HardnessHardness and and Young’s Young’s modulus modulus of of the the as as-sprayed-sprayed and and heat heat-treated-treated nanostructured nanostructured APS APS coatings:Figurecoatings: 10. (a () a HardnessS)amples Samples after and after thermalYoung’s thermal aging modulus aging treated treated of theat atdifferent as di-ﬀsprayederent temperature temperatures and heats-treated for for 24 24 h; nanostructured h; (b ()b thermal) thermal aging aging APS treated at 1550 °C for different times. coatings:treated at ( 1550a) Samples◦C for after diﬀerent thermal times. aging treated at different temperatures for 24 h; (b) thermal aging treated at 1550 °C for different times.

FigureFigure 11. 11. HardnessHardness and and Young’s Young’s modulus modulus of the of theas-sprayed as-sprayed and andheat heat-treated-treated SPS coatings SPS coatings [21]: ( [a23) ]: SamplesFigure(a) Samples 11.after Hardness afterthermal thermal andaging agingYoung’s treated treated modulus at different at di ffoferent the temperature as temperatures-sprayeds forand for24 heat 24h; h;(-btreated) ( bthermal) thermal SPS aging coatings aging treated treated [21]: at(a at ) 15501550Samples °◦CC for forafter different di ffthermalerent time times. agings. treated at different temperatures for 24 h; (b) thermal aging treated at 1550 °C for different times. ToTo further further verify verify the the microhardness microhardness changes of the nanostructurednanostructured APSAPS and and SPS SPS TTBCs, TTBCs, Figure Figure 12 12shows showTo variationss further variation verify ofs theof the Vickers microhardnessVickers hardness hardness of changes the of twothe of coatingstwo the coatings nanostructu with thermalwithred thermal agingAPS and treatmentaging SPS treatment TTBCs, as a function Figure as a function1of2 exposureshow ofs variationexposure temperatures temperatureof the and Vickers time. and Thehardness time. mean The hardnessof themean two valueshardness coatings of thevalue with as-sprayeds thermal of the as APSaging-spray and treatmented SPS APS coatings and as a were 919 21 HV0.3 and 950 50 HV0.3, respectively. The higher Hardness value of SPS coating SPSfunction coatings of± exposurewere 919 temperature± 21 HV0.3± and and 950 time. ± 50The HV0.3, mean respectively. hardness value Thes ofhigher the asHardness-sprayed valueAPS andof SPSSPSmay coating coatings have amay relation were have 919 with a relation± 21 smaller HV0.3 with grain and smaller sizes,950 ± grain which 50 HV0.3, sizes, raised whichrespectively. the deformation raised theThe deformation higher resistance Hardness of resistance YSZ value coatings. of of YSZSPSBoth coatings. coating of thermal-aged may Both have of thermal coatingsa relation-aged exhibited with coatings smaller a significant exhi grainbited sizes, increasea significant which in raised H increase with the the deformation in rising H with of thermalthe resistance rising aging of of thermalYSZtemperature, coatings. aging temperature, andBoth the of Vickersthermal and- hardnesstheaged Vickers coatings of hardness the exhi SPSbited coating of the a significant SPS was coating always increase was greater always in thanH greaterwith that the ofthan rising the that APS of ofthermalnanostructure the APS aging nanostructure coating.temperature, As coating. noted, and the the As Vickers hardness noted, hardnessthe of thehardness coatings of the of SPS isthe not coating coatings only related was is alwaysnot to theonly greater porosity, related than butto thethat also porofdepends osity,the APS but on nanostructurealso the bondingdepends strength oncoating. the bonding between As noted, strength splats. the hardness Thebetween improved ofsplats the bonding .coatings The improved between is not onlybonding the lamellaesrelated between to wasthe theporembodied lamellaesosity, but in was higheralso embodied depend H value.s onin After higherthe bonding thermal H value. strength aging After for bethermal 80tween h at 1550 agingsplats◦C, for. The the 80 Himprovedh at values 1550 of bonding°C the, the two H between coatingsvalues ofthereduced the lamellaes two gradually,coatings was embodiedreduced which was gradually, in causedhigher w byHhich value. phase was After decomposition. caused thermal by phase aging decomposition. for 80 h at 1550 °C, the H values of the two coatings reduced gradually, which was caused by phase decomposition.

Crystals 20202020,, 109, ,x 984 FOR PEER REVIEW 11 of 1312

Figure 12. Variation of Vickers hardness of nanostructured APS and SPS coatings with thermal aging Figure 12. Variation of Vickers hardness of nanostructured APS and SPS coatings with thermal aging treatment: (a) Samples after thermal aging treated at different temperatures for 24 h; (b) thermal aging treatment: (a) Samples after thermal aging treated at different temperatures for 24 h; (b) thermal aging treated at 1550 ◦C for different times. treated at 1550 °C for different times. 4. Conclusions 4. Conclusions In conclusion, two types of segmentation-crack yttria-stabilized zirconia (YSZ) thick thermal barrierIn coatings conclusion, (TTBCs) two types were of fabricated segmentation via atmospheric-crack yttria plasma-stabilized spraying zirconia (APS) (YSZ) and thick suspension thermal plasmabarrier coatingsspraying (TTBCs) (SPS) processes, were fabricated respectively. via atmospheric The evolution plasma of the spraying microstructures, (APS) and phase suspension stability, grainplasma growth spraying behavior, (SPS) process and mechanicales, respectively. properties The of evolution both TTBCs of the before microstructures, and after thermal phase treatment stability, weregrain investigated growth behavior and compared, and mechanical systematically. properties Based of onboth the TTBCs experimental before results,and after the thermal main conclusions treatment were drawninvestigated as follows: and compared systematically. Based on the experimental results, the main conclusions were drawn as follows: (1) Both of the coatings displayed a unique microstructure with some obvious segmentation cracks (1) Bothand of branching the coatings cracks. displayed The measured a unique Ds microstructure of the nanostructured with some APS obvious and SPS segmentation YSZ coatings cracks was and branching cracks. 1The measured Ds of1 the nanostructured APS and SPS YSZ coatings was about 2.5 cracks mm− and 4 cracks mm− , respectively. −1 −1 (2) aboutThere 2.5 were cracks two mm distinct and grain 4 cracks types mm in the, respectively. two as-sprayed coatings: Larger columnar grains in the (2) ThereAPS coatingwere two and distinct smaller grain equiaxed types grains in the in two the SPS as-sprayed coating, revealingcoatings: theLarger diff erentcolumnar microstructure grains in theof theAPS lamellar coating interface. and smaller The SPS equiaxed coating was grains short in of the splats, SPS while coating, the splat-splat revealing interactions the different in microstructurethe APS coating of werethe lamellar evident. interface. The SPS coating was short of splats, while the splat-splat interactions in the APS coating were evident. (3) The initial metastable tetragonal (t’) phase of the APS and SPS coatings underwent (3) The initial metastable tetragonal (t’) phase of the APS and SPS coatings underwent tetragonal- tetragonal-monoclinic phase transformation after 1550 ◦C/40 h heat treatment. The poorer monoclinic phase transformation after 1550 °C/40 h heat treatment. The poorer phase stability of phase stability of SPS TTBCs may have a connection with the smaller grain size. SPS TTBCs may have a connection with the smaller grain size. (4) Thermal-aged APS and SPS coatings exhibited a significant increase in H and E values accompanied (4) Thermal-aged APS and SPS coatings exhibited a significant increase in H and E values by pores healing and grain growth, and for the samples thermal aged at 1550 ◦C, the H and accompanied by pores healing and grain growth, and for the samples thermal aged at 1550 °C, E values increased sharply during the initial stage then decreased after 80 h due to the phase the H and E values increased sharply during the initial stage then decreased after 80 h due to the decomposition. The segmented APS coatings had weak bonding between the lamellaes and more phase decomposition. The segmented APS coatings had weak bonding between the lamellaes uniform pores distribution during thermal exposure, which caused the mean Vickers hardness and more uniform pores distribution during thermal exposure, which caused the mean Vickers value of APS TTBCs to be much lower than that of SPS TTBCs. hardness value of APS TTBCs to be much lower than that of SPS TTBCs.

Author Contributions: Contributions:Conceptualization, Conceptualization, S.T. S.T. (Shiqian (Shiqian Tao) Tao) and S.T. and (Shunyan S.T. (Shunyan Tao); data Tao); curation, data c S.T.uration, (Shunyan S.T. Tao),(Shunyan M.Z., Tao), X.Z., M. W.L.,Z., X.Z., F.S.,H.Z., W.L., Y.Z., F.S.,and H.Z., J.N.; Y.Z., formal and J.N.; analysis, formal S.T. analysis, (Shiqian S.T. Tao) (Shiqian and J.Y.; Tao) writing-original and J.Y.; writing draft- preparation,original draft S.T. preparation, (Shiqian Tao); S.T. writing-review (Shiqian Tao); and writing editing,-review J.Y. and and S.T. editing, (Shunyan J.Y. Tao). andAll S.T. authors (Shunyan have Tao). read andAll agreedauthors to have the publishedread and agreed version to of the the published manuscript. version of the manuscript.

Funding: TThishis research was funded by National Science and Technology Major Project (No. 2017 2017-VI-0010-0082),-VI-0010-0082), National Natural Science Foundation of China (NSFC) (No. 51701235 and No. 51971148), and Science and TechnologyNational Natural Innovation Science of Shanghai Foundation (No. of 18511108702). China (NSFC) (No. 51701235 and No. 51971148), and Science and Technology Innovation of Shanghai (No. 18511108702). Acknowledgments: The authors are grateful for the technical assistance provided by Shiqian Tao. Acknowledgments: The authors are grateful for the technical assistance provided by Shiqian Tao. Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest.

Crystals 2020, 10, 984 12 of 13

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