Vacuum 86 (2012) 1174e1185
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Microstructure and indentation mechanical properties of plasma sprayed nano-bimodal and conventional ZrO2e8wt%Y2O3 thermal barrier coatings L. Wang a, b, Y. Wang a, *, X.G. Sun a, J.Q. He a, Z.Y. Pan a, C.H. Wang a a b
Laboratory of Nano Surface Engineering, Department of Materials Science, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2011 Received in revised form 21 October 2011 Accepted 30 October 2011
The nanostructured agglomerated feedstock used for plasma spraying was obtained by the nanoparticle reconstituting technique. Nanostructured and conventional ZrO2e8wt%Y2O3 (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on 45# steel substrates with the NiCrAlY as the bond-layer. The microstructure and phase composition of feedstocks and corresponding coatings were characterized. The top layer of nanostructured 8YSZ TBCs is denser and has fewer defects than that of conventional TBCs. The elastic modulus, micro-hardness and Vickers hardness of nanostructured 8YSZ TBCs exhibit bimodal distribution while the conventional 8YSZ exhibit monomodal distribution. The elastic modulus and elastic recoverability were also obtained by the nanoindentation test. The results indicate that the elastic modulus of nanostructured 8YSZ coating is lower than that of conventional 8YSZ coating, but the nanostructured 8YSZ coating has higher elastic recoverability than that of the conventional 8YSZ coating. The prediction of the average elastic modulus was established by the mixture law and weibull distribution according to the fraction of phases with different molten characteristic. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
Keywords: Thermal barrier coating Plasma spray Microstructure Indentation Weibull distribution
1. Introduction The demand to the high temperature component is becoming more urgent than ever with the increasing of the inlet and outlet temperatures of the engine. The endurable temperature of the conventional superalloy (nickel-based or cobalt-based superalloy) has reached the limit. Based on this fact, it became more and more important to further improve the service temperature of the high temperature components with the aid of the surface and coatings technology. Especially, thermal spray technology is an effective and convenient method to improve the substrate wear resistance, corrosion resistance and thermal insulation ability [1e3]. Among all the coating materials, ceramic coatings are applied more prevalent due to their unique mechanical and thermophysical properties, such as high hardness, high wear resistance and low thermal conductivity. Thermal barrier coatings (TBCs) are very important ceramic coating materials. They play an important role in protecting substrate, reducing the working temperature and increasing working efficiency of high temperature components. They are currently considered for engine applications such as
* Corresponding author. Tel.: þ86 451 86402752; fax: þ86 451 86413922. E-mail address:
[email protected] (Y. Wang).
aerospace, aircraft, marine automobiles, nuclear fusion reactors and heavy-duty utilities [4,5]. At present, typical TBCs usually with the content of 7 1wt.% Y2O3 partially stabilized ZrO2 (YSZ) are deposited on the nickel-based superalloy substrate with the MCrAlY (where M ¼ Ni and /or Co) as the metallic coating in order to protect the substrate against high temperature oxidation, corrosion and wear [6e8]. Usually, there are two common techniques to prepare the TBCs: electrical beam-physical vapor deposition (EB-PVD) and atmospheric plasma spray (APS). The ceramic layer of TBCs prepared by EB-PVD usually contains columnar grain and inner sub-grain distributed between the columnar grains, while the conventional TBCs fabricated by APS exhibits lamellar layer structure. This feature is not evident in the nanostructured TBCs. Nanoindentation is regarded as “fingerprint” of materials. It is usually adopted to characterize the mechanical properties of films and coatings such as elastic modulus and micro-hardness [9e12]. Some research work about the characterization of the mechanical properties of materials by using the nanoindentation technique has been published. Lugscheider et al. [13] have investigated the mechanical properties of EB-PVD zirconia TBCs by nanoindentation. The mechanical properties in the vertical and the horizontal direction are completely different for the EB-PVD coatings due to the different microstructure characteristic at the grain boundaries
0042-207X/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.029
L. Wang et al. / Vacuum 86 (2012) 1174e1185
and the grain. Bouzakis et al. [14] have investigated the mechanical properties of the EB-PVD ZrO2 coating by means of advanced experimental analytical procedures. Panich et al. [15] have investigated the nanoindentation process of a soft coating on a harder substrate by the finite element method. The authors discussed the relationship of the critical indentation depth with the ratio of the yield strength of the soft coating to the harder substrate and the indenter tip radius in detail. Chen et al. [16] have investigated the plastic deformation behavior and densification of TBC with a columnar microstructure as well as the column distortions caused by the impression by finite element method. The authors thought that it was also possible of exploring the deformation heterogeneities observed experimentally, such as shear bands, by embodying salient constituent properties, such as the column width, contact friction and inter-columnar friction. Finite element simulation result could provide some understanding of the plastic response of several thermal barrier coating systems when comparing with measurements. Previous research has been reported about nanostructured and conventional YSZ TBCs in the aspect of experiment, including the following topics: (1) preparation of nanostructured and conventional TBCs; (2) thermophysical characterization, such as thermal conductivity or thermal insulation capability; and (3) mechanical properties characterization such as residual stress, adhesive strength, thermal shock resistance, high temperature oxidation resistance, and lifetime prediction and so on. Little work has been reported on nanoindentation mechanical behavior of TBCs fabricated by APS, much less the nanostructured TBCs. In this paper, nanostrutured TBCs were prepared using the nanostructured agglomerated feedstock, and nanoindentation tests were performed on the topcoat of the two kinds of TBCs. As nanostrutured TBCs have proved to have attractive performance, it
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is expected to be the candidate materials for the industrial application in the future. Indentation (microindentation and nanoindentation) is a very important method to characterize the mechanical properties of nanostructured TBCs. 2. Experimental procedure 2.1. Preparation of feedstock It is very important to prepare suitable feedstock in order to fabricate the coatings. Generally, there are three factors for the feedstock which may influence the coating quality, i.e. flowability, density and size distribution. Flowability is a critical factor which may influence the deposition efficiency of the coating, the feedstock with spherical morphology often has good flowability. Size distribution is also a necessary factor which may influence the coating quality and spray efficiency, the feedstock with too large size will plug up the feedstock giving pipe. Density is also a crucial factor. It is well known that individual nanoparticles cannot be plasma sprayed because of their low mass and inability to be carried in a moving gas stream and deposited on a substrate. The most important in using nanoparticles as raw material, to generate the nanostructured coatings, is to reconstitute individual nanoparticles into sprayable agglomerates [17]. In the present work, the nanostructured agglomerated feedstock was obtained by the method of spraying granulation which includes three critical steps (Fig. 1): Firstly, the nano-sized original ZrO2/8%wt. Y2O3 composite particles with the size ranging from 8 to 18 mm were dispersed with polyvinyl alcohol (PVA) as the adhesive and deionized water as the solvent and were ball milled for long enough time (8 h) in order to form slurry with a certain flowability. Secondly, the slurry was spray dried. At last, the as spray dried powder was further sintered at 1200 C for 6 h.
Fig. 1. Manufacturing process of nanostructured sprayable feedstock. (a) Ball milling, (b) spray drying and (c) sintering heat treatment.
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2.2. Preparation of coating sample The substrates were ultrasonically degreased in anhydrous ethylene alcohol, dried in cold air and grit blasted with brown corundum by grit blasting, prior to coating deposition in order to increase the bonding strength. The obtained Ra of the substrates was between 7 mm and 13 mm. The substrates were preheated to approximate 220 C before the feedstock was given in order to reduce the residual stress. The HVOF (High Velocity Oxygen Fuel) and APS technology were used to prepare the metallic and ceramic layers, respectively. DJ-2700 HVOF (Sulzer Metco, Westbury, NY, USA) spray gun has been used in the experiment. The throat internal diameter of the nozzle is 5.5 mm. The particles were injected axially. The injected nitrogen was the carrier gas. The size distribution of the sprayed metallic-layer powder was ranging from 20 mm to 65 mm. The two ceramic 8YSZ feedstocks were thermally sprayed using an Ar/H2 APS torch (9 MB plasma gun, GH nozzle, Sulzer Metco, Westbury, NY, USA). The feedstock which was used to deposit the two YSZ coatings was given externally. The thermal spraying parameters are shown in Tables 1 and 2, respectively. The coatings were sprayed onto the 45# steel substrates with NiCrAlY as the bond coats. The thickness of the produced TBCs was 300 mm (including the bond coating and the ceramic coating). 2.3. Characterization of feedstock and corresponding coating The occurrence of different phases in the powder and coating was analyzed by the rotating anode X-ray diffractometry (XRD, D/ max2400, RIGAKU, Japan) with a CuKa source. The microstructures of the feedstock and corresponding coatings were determined by Scanning Electron Microscope (SEM, S-570, Hitachi, Tokyo, Japan). 2.4. Measurements of the pore content and adhesive strength of the TBCs The porosity of TBCs was measured by Matlab image operation program. Firstly, the original SEM image was transferred to a gray image by digital image processing technology. The digital gray image with m n pixel can be defined as a matrix array F (m n). Each element of this matrix array corresponds to a pixel f(i,j), and then the image is identified as two modes by the gray levels, and every pixel contains only one gray level. The digital image is the basis for thresholding segmentation. Suppose T reflects the thresholding of gray level. In the ceramic coat, the pore and the dense coating are considered to be the two different phases when the cracks are ignored. Then a suitable thresholding T will be selected to separate these two phases. The black portion represents the pores, and the white portion represents the 8YSZ phase. Usually, the gray level of the black is low (f(i,j) < T), and the gray level of the white is high (f(i,j) > T). Pixels labeled 1 represent pores (f(i,j) < T); pixels labeled 0 represent 8YSZ (f(i,j) > T). Then the porosity can be obtained by the number of pixels point which labeled 1 and the matrix array F (m n) according to the thresholding segmentation method (Fig. 2). 2.5. Nanoindentation test of the TBCs The elastic modulus of the cross-section for TBC was tested by the TRIBO nanoindentation instrument made by HYSITRON
Company. The maximum load was 8 mN in the experiment, loading for 10 s, unloading 10 s, without holding time. The elastic modulus, nano hardness and Vickers hardness for the ceramic coat of TBCs were taken from the average value of all measurements. Twenty measurements were conducted for a Weibull analysis of the elastic modulus, nano hardness and Vickers hardness distribution of the ceramic top coating which can be highly skewed or broadly distributed, because the ceramic coating has a nonuniform microstructural characteristic. The formula of the two-parameter Weibull distribution can be seen in reference [18]. The Weibull distribution characteristic of the mechanical properties can reflect the microstructural characteristic of the as-sprayed coatings [19e21]. 3. Results and discussion 3.1. Surface morphologies of feedstock The SEM micrographs of the nanostructured agglomerated feedstock and the conventional feedstock are shown in Fig. 3. It can be seen that nanostructured feedstock is composed of spherical/ equiaxed shape of nanostructured agglomerates with the size from 10 to 50 mm (Fig. 3(a)). When a split of a nanostructured agglomerated particle was in high magnitude, it can be seen that every nanostructured aggregate consists of smaller nanostructured aggregate (Fig. 3(b)). It can be designated as sphere package structure. It is clear that conventional feedstock exhibits polyhedral, irregular and angular shape with different sizes ranging from 10 mm to 40 mm (Fig. 3(c)), which may be caused by the fabrication using sintering and crushing technique. This will affect its flowability and decrease the deposition efficiency of the coating in the plasma spray process. 3.2. Surface and cross-section morphology of the TBCs Fig. 4 shows the surface morphologies of the two kinds of TBCs. It can be seen that the surface of both coatings is uneven and has a certain degree of roughness. A certain amount of pores and microcracks can be observed. It can be seen that the density of topcoat for conventional TBCs is significantly lower than that of the nanostructured TBCs (Fig. 5). The number of defects (pores and cracks) is higher in the conventional TBCs, and the cracks exhibit irregular and chaotic distribution characteristic. A very serious phenomenon of brittle fracture for top coating can also be seen. According to the image processing procedure, the pore content of the conventional and nanostructured TBC are 10e13% and 6e8%, respectively. The cracks in nanostructured TBCs are relatively finer, fewer without obvious growth direction, which is mainly attributed to the refined grains of nanostructured coating and the increasing of toughness. 3.3. Phase composition of feedstock and corresponding coatings The XRD patterns of feedstocks and as-sprayed coatings are shown in Fig. 6. The conventional powder was mainly composed of Y0.15Zr0.85O1.93, m-ZrO2 phase and c-ZrO2 phase (Fig. 6(a)), the corresponding coating was mainly composed of Y0.15Zr0.85O1.93, cZrO2 phase after the plasma spraying process (Fig. 6(b)). The nanostructured 8YSZ feedstock was mainly composed of t phase
Table 1 Parameters of depositing bond coat by HVOF. N2 pressure (MPa)
O2 pressure (MPa)
Air pressure (MPa)
Voltage (V)
Feeding rate (g/min)
Spraying distance (cm)
Spraying angle (deg.)
0.62
1
0.8
8e10
6.3
15
90
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Table 2 Parameters of depositing ceramic topcoat by APS. Current (A)
Voltage (V)
Primary gas flow rate (SCFHa)
Feeding rate (g/min)
Spraying distance (cm)
Spraying angle (deg.)
CPSPb
600
60
100
14
10
90
360
a b
SCFH, Standard Cubic Foot per Hour,1 SCFH ¼ 0.472 L min1. CPSP ¼ (Voltage (V) Current (A)/Primary gas flow rate (SCFH)).
(Fig. 6(c)), the corresponding coating was predicted to be that it was mainly composed of t0 phase after the plasma spraying process (Fig. 6(d)). The average grain size of phases in the nanostructured feedstock and as-sprayed nanostructured coating can be estimated based on XRD peak broadening by using Debye Scherrer formula [22e30]:
D ¼
kl kl ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Bð2qÞcos q 2 Bobs B2std cos q
(1)
where k is a constant (being taken as 0.9), l is the wavelength of the X-ray radiation, CuKa (l ¼ 1.5418 Å), D is the average grain size, and q is the Bragg angle. B(2q) is the true broadening of the diffraction line at half-maximum intensity, also known as full-width at halfmaximum (FWHM) after correction for the instrumental line broadening. Bobs is the measured FWHM of the peak and Bstd is the instrumental line broadening. XRD of a fully annealed silicon specimen was used to measure the instrumental broadening (i.e. Bstd) as it has no grain refinement, no stress (macro-stress or microstress), and no distortion. 2q ¼ 30.0569 peak was selected to calculate the average grain size, The average grain size of ZrO2 in nano-8YSZ aggregate powder and corresponding coating were 59.1 nm and 59.5 nm respectively. This is shown that the assprayed TBCs are nanostructured TBCs. According to the results obtained by Miller et al. [31] and Brandon et al. [32], this tetragonal zirconia phase which was observed in the as-sprayed coatings was mainly composed of so-called non-transformable phase t0 . The t0 phase has lower c and c/a compared with the t phase (where a, c denote the lattice parameters). The free yttria phase was not observed in the XRD pattern of the nanostructured as-sprayed zirconia coatings. Anderson et al. [33] showed that the t0 phase resulted from the high temperature cubic phase by a diffusionless transformation due to the high quenching rate about 106 K/s. The t phase is normal tetragonal phase. And the content of the Y2O3 in the t phase is lower compared with t0 phase. Fig. 7 shows the Transmission Electron Microscopy (TEM) image of the topcoat of conventional and nanostructured TBCs, it can be seen that the grains of the topcoat of conventional 8YSZ coating are in the range of 300e600 nm (Fig. 7(a)), while the grains of the topcoat of nanostructured 8YSZ TBC are in the range of 30e70 nm (Fig. 7(b)). And the corresponding EDS analysis result can be also seen (Fig. 7(c)).
3.4. Microindentation mechanical characterization for the top layer of TBCs As elastic modulus of materials has a direct-ratio relationship with the hardness; the Vickers hardness of the two coatings was measured before the nanoindentation test. Fig. 8 shows the morphology of the indentation on the two coatings. It can be seen that the imprint size of the nanostructured TBC is smaller than that of the conventional TBC. At the same time, the crack at the imprint edge also exhibit different characteristic. The crack in the front of the imprint edge of nanostructured TBC is finer than that of the conventional TBC. The mean Vickers hardness of topcoat of the nanostructured and conventional TBCs are HV910.1 7 2.3 and HV872.6 68.5, respectively. Fig. 9 just shows the probability function of Weibull plots of Vickers hardness of the two kinds coatings, it can be seen that the Vickers hardness has a certain scattering characteristic due to the nonuniform microstructure characteristic of the coating (Fig. 9a). Usually, the bigger the m, the smaller the Vickers hardness of the scattering degree. It can be seen from Fig. 9b that the conventional TBCs does not present a bimodal distribution in the Weibull plot with respect to the Vickers hardness. A mono-modal distribution is observed. As the coating has a certain defect, the Vickers hardness cannot be the same in the cross-section everywhere, the deviations may exist from the straight line, the Weibull fit. These deviations result in a higher variation coefficient of the Weibull modulus in comparison with the theoretical solutions, whereas the scale parameter is much less affected. This may attribute to the fact that on the one hand the scale parameter has a standard deviation much less than the Weibull modulus, and on the other hand it characterizes a mean Vickers hardness, which is less sensitive than the modulus, which characterizes the uniformity of the defects distribution on the cross-section and thus the distribution of Vickers hardness values. There are numerous possibilities for these deviations, for example, the nonuniform of the feedstock, the thermal spray parameter, such as the moving rate of the spray gun, feeding rate, spray angle and spray distance. The spray power also affect the molten condition of feedstock, the surface of the coating will exhibit nonuniform characteristic, edge effects or transition from volume to surface flaws. The bimodal distribution of Vickers hardness on plasma sprayed 8YSZ TBCs has been observed, because under the thermal spray parameter presented in this paper, the
Fig. 2. The procedure of the image analysis method to characterize the porosity of the as-sprayed coating. (a) Original SEM image, (b) gray level transformation image, and (c) matrix array image.
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Fig. 3. SEM micrograph of feedstock. (a) Nanostructured TBC, (b) high magnification of (a), and (c) conventional TBC.
nanostructured 8YSZ coatings are mainly composed of two phases, the molten and non-molten phases. Especially very interesting, there is a transmission zone which may indicate the semi-molten phase, but the semi-molten phase cannot be seen in the image of the cross-section of the topcoat clearly. Fig. 10(a) and (b) shows the image of the cross-section for the topcoat of nanostructured and conventional 8YSZ TBCs. Regions can be detected (as indicated on the micrographs) where nonmolten feedstock particles are localized. The molten phase can be observed in regions where a large, dense and smooth structure is found. The non-molten phase is located in regions which have a similar morphology to that of the feedstock particle and around the pores because the non-molten or partially molten feedstock particles are not sufficiently overlapped in the process of thermal spraying. The agglomerated particles are not loosely connected to each other. Both molten and non-molten phases may present
nanostructural characteristics, because the nanostructure of the feedstock is formed by the agglomeration of individual nanoparticles within a colloidal suspension. The non-molten phase in the coating may come from the non-molten feedstock particles which were retained in the coating after the thermal spraying being finished and the nanostructure in the non-molten feedstock particles have also been retained. The molten phase may also have nano-grains because the fully molten feedstock particles have enduring the nucleation and recrystallization process under the impact of plasma flame with a high heating and cooling rate (106 K/ s). This microstructural difference of nano-grains agglomeration rather than a packed structure consisting of nano-grains may produce a bimodal distribution in the mechanical properties of the as-sprayed coatings. Fig. 10(c) shows the fracture surface of the topcoat of the nanostructured 8YSZ coating. It can be seen that the structure is dense and there are some micropores in the coating.
Fig. 4. Surface morphologies of thermal barrier coatings. (a) Conventional TBC under low magnification; (b) nano TBC under low magnification; (c) conventional TBC under high magnification; and (d) nano TBC under high magnification.
L. Wang et al. / Vacuum 86 (2012) 1174e1185
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Fig. 5. The section image the as-sprayed (a) conventional 8YSZ TBC and (b) nanostructured 8YSZ TBC.
The microstructure of the coating consists of molten phase and non-molten phase. The molten particles which came from the position where the temperature of plasma flame is higher than the melting point of feedstock merge into together and rapidly directionally solidify along the direction of thermal conduction and then form columnar particles. The microstructure is alternate by molten phase and non-molten phase, which could be beneficial to the consolidation of the topcoat. Fig. 10(d) shows the fracture surface of the topcoat of the conventional 8YSZ coating. It can be seen that the topcoat of the conventional 8YSZ coating did not exhibit the bimodal phase structure characteristic. Only pores and cracks can be seen on the fracture surface.
In order to determine the fracture toughness, Vickers indentations were also performed on polished cross-sections of the coatings using a hardness tester (Fig. 11). The indentation load varied from 10 to 200 N. Both Vickers diagonals and crack lengths were measured in a profile projector using a 100 magnification. Thermally sprayed coatings have an anisotropic microstructure characteristic due to the deposition process. The splat structure is different in different directions. Cracks produced under indentation are mainly developed along the lamellae (interlamellar cracks) direction because of the laminar structure characteristic of thermal sprayed coating, and this direction is the one most prone to
Fig. 6. XRD patterns of thermal sprayed feedstocks and corresponding coatings. (a) Conventional 8YSZ feedstock; (b) conventional 8YSZ coating; (c) nanostructured 8YSZ agglomerated feedstock; and (d) nanostructured 8YSZ agglomerated coating.
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Fig. 7. TEM image of topcoat of (a) conventional 8YSZ TBCs; (b) nanostructured 8YSZ TBCs; and (c) the EDS analysis of the nanostructured 8YSZ.
decohesion. An average of 20 indentations was carried out at a given load and the total crack length was estimated by:
c ¼
d1 þ d2 l1 þ l2 þ 2 2
(2)
where d1 and d2 are the parallel and perpendicular Vickers diagonals to the coating surface and ll and l2 are the left and right crack lengths, respectively.
From the experimental data obtained for indentation load, P, and total crack length, c, the indentation model was used to compute Kc [34,35]:
Kc ¼ 0:0711 HV d1=2 ðE=HV Þ2=5 ðc=dÞ3=2
(3)
where Hv and E are the Vickers hardness and elastic modulus, respectively. This equation is only valid for a ‘Half-Penny’ crack
Fig. 8. Morphologies of indention of Vickers hardness on the topcoat of (a) nanostructured TBC and (b) conventional TBC.
L. Wang et al. / Vacuum 86 (2012) 1174e1185
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HV
HV
Prob.fun.,lnln[1/(1-F(x))]
1.0
850
875
900
925
950
975 1000 1025 1050
a
0.5 0.0
m=11.20 n=9
-0.5
Non-molten phase
-1.0 -1.5 -2.0 -2.5
m=28.29 n=11
Semi-molten phase
-3.0 -3.5 -4.0
molten phase 6.70 6.72 6.74 6.76 6.78 6.80 6.82 6.84 6.86 6.88 6.90 6.92 6.94 6.96
ln(HV)
Prob.fun.,lnln[1/(1-F(x))]
1.5
825
2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5
775 800 825 850 875 900 925 950 975 1000 1025
b
m=14.96 n=20
6.65
6.70
6.75
6.80
6.85
6.90
6.95
ln(HV)
Fig. 9. Weibull plot of Vickers hardness of (a) nanostructured 8YSZ TBCs and (b) conventional 8YSZ TBCs.
regime, which occurs when c/d 2.5, where d is the Vickers half-diagonal. The mean values obtained for Kc of nanostructured and conventional TBCs using Eq. (3) are 2.71 0.46 and 1.93 0.22 MPa m1/2, which indicates that the nanostructured TBCs has higher crack propagation resistance ability than that of the conventional TBCs.
3.5. Nanoindentation mechanical characterization for the top layer of TBCs Fig. 12 shows the Weibull plot of elastic modulus and micro-hardness of nanostructured and conventional 8YSZ TBCs, it can be seen that the elastic modulus and micro-hardness of
Fig. 10. Cross-section morphologies of topcoat of (a) nanostructured and (b) conventional 8YSZ TBCs (the blue arrow indicate the non-molten phases). Fracture surface morphologies of topcoat of (c) nanostructured and (d) conventional 8YSZ TBCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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L. Wang et al. / Vacuum 86 (2012) 1174e1185 D Zmax
Wt ¼
LdD
(4)
0
where L is the applied load, D is the penetration depth and Dmax is the maximum depth corresponding to the final load. The integration was performed along the loading curve. The area enclosed by the unloading curve and the maximum penetration depth represents the recoverable deformation energy, which is determined by D Zmax
Wrc ¼
LdD
(5)
Dr
where Dr is the residual depth when the load was decreased to zero. The integration was performed along the unloading curve. Clearly, a material with higher Wrc value should behaves more elastically and has greater capability to absorb impact energy and accommodate deformation with less damage. The ratio h which is determined by Eq. (6) is another parameter which also reflects such a capability.
Fig. 11. Schematic illustration of Vickers indentation and crack geometry.
nanostructured 8YSZ TBC exhibit bimodal distribution, while the conventional 8YSZ TBCs did not show this characteristic. The elastic modulus and micro-hardness of conventional 8YSZ TBC exhibit mono-modal distribution. As reference [36] illustrates, the total deformation energy is represented by the area enclosed by the loading curve and the maximum penetration depth, as illustrated in Fig. 13a. The deformation energy Wt is determined by the following equation:
h ¼ Wrc =Wt
(6)
Generally, the Young’s modulus of the indented material, Em, can be expressed as a function of the slope of the unloading curve (S) and the Poisson’s ratio of the indented material (ns) [37e39]:
pffiffiffi Em ¼
p 1 n2s pffiffiffiffiffi 2 Ac
pffiffiffi p 1 n2s dP pffiffiffiffiffi S ¼ dh 2 Ac
E (GPa)
E (GPa)
a
110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 1.5
b
0.5 0.0
m=4.37 n=6
-0.5 -1.0 -1.5
Semi-molten phase
m=7.16 n=14
-2.5
Non-molten phase
-3.0
Molten phase
-3.5 -4.0 4.7
4.8
4.9
5.0
5.1
5.2
5.3
5.4
Prob.fun.,lnln[1/(1-F(x))]
Prob.fun.,lnln[1/(1-F(x))]
1.0
-2.0
5.5
170 180 190 200 210 220 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 m=8.454 -2.0 n=20 -2.5 -3.0 -3.5 -4.0 -4.5 4.85 4.90 4.95 5.00 5.05 5.10 5.15 5.20 5.25 5.30 5.35 5.40
ln(Elastic modulus)
1.5
Prob.fun.,lnln[1/(1-F(x))]
1.0
14
16
18
20
HN (GPa)
22
24
26
28
30
32
34
d
Semi-molten phase
0.5
m=3.78 n=6
0.0 -0.5 -1.0
m=6.64 n=14
-1.5 -2.0
Non-molten phase
-2.5 -3.0
Molten phase
-3.5 -4.0 2.6
2.7
2.8
2.9
3.0
3.1
ln(HN)
3.2
130
140
150
160
ln(Elastic modulus)
3.3
3.4
3.5
Prob.fun.,lnln[1/(1-F(x))]
c
(7)
HN (GPa)
2.0 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 1.5 1.0 0.5 0.0 -0.5 -1.0 m=4.32 -1.5 n=20 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9
ln(HN)
Fig. 12. Weibull plot of (a) elastic modulus of nanostructured 8YSZ TBCs; (b) elastic modulus of conventional 8YSZ TBCs; (c) micro-hardness of nanostructured 8YSZ TBCs; and (d) micro-hardness of conventional 8YSZ TBCs.
L. Wang et al. / Vacuum 86 (2012) 1174e1185
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9000 top-coat of nanostructured TBC
b
top-coat of conventional TBC Load (P)
8000 7000
a
Force ( µ N)
6000 5000 loading
S=dP/dh
Wt
4000
Wrc unloading
Dmax h
Dr
3000 2000 1000 0 0
25
50
75
100
125
150
175
200
225
Displacement (nm) Fig. 13. The curve of relationship between the load and the indentation depth for the topcoat of TBC.
3.6. Prediction of effective elastic modulus of TBCs Based on the discussion above, the elastic modulus of nanostructured 8YSZ TBCs can be estimated as:
Ec ¼
3 X
fi Ei ¼ f1 E1 þ f2 E2 þ f3 E3
(8)
i¼1
where Ec is the average elastic modulus of the topcoat which reflect the overall microstructure characteristic, f1, f2, f3 indicate the fraction of molten phase, non-molten phase and semi-molten phase,
respectively. E1, E2, E3 indicate the elastic modulus of molten phase, non-molten phase and semi-molten phase, respectively. The microhardness and the Vickers hardness are the same calculation method. In addition, elastic modulus is also an important parameter to characterize the thermal and mechanical properties of TBCs. The elastic modulus will affect the residual stress due to the residual stress can be calculated as follows when the as-sprayed coating were cooled to ambient temperature from the high temperature in the process of thermal spray, The overall magnitude of residual stresses in TBC is the summation of quenching stress and thermal stress, whereas the contribution of stress caused by phase transformation is neglected [40,41]:
sresidual ¼ sq þ stc
(9)
Stresses originating from rapid contraction of sprayed splats from the deposition temperature to that of the underlying
220
Elastic modulus (Micro-hardness)/GPa
where Ac is the projected contact area of the indenter tip with the indented material. The relationship between the load and the depth for two kinds of TBCs is shown in Fig. 13b. It can be seen that the depth of the indentation for the topcoat of the nanostructured TBCs is higher than that of the conventional TBCs. The Wrc and h of nanostructured TBCs are higher than that of conventional TBCs. By using the area integral method, the h values of the nanostructured TBCs and conventional TBCs are 66.2% and 61.5%, respectively. Which means nanostructured TBCs have higher elastic recoverability than that of the conventional TBCs. The elastic modulus of nanostructured TBCs is lower than that of conventional TBCs. Twenty points from different positions of cross-section were selected at random. By computing the mean value, the elastic modulus values of nanostructured and conventional TBCs are 158.5 39.9 GPa and 171.8 24.1 GPa, the nano hardness values of nanostructured and conventional TBCs are 21.9 5.5 GPa and 26.7 7.4 GPa (Fig. 14), respectively. Generally, the higher the elastic modulus, the higher of the hardness. But it is not absolute due to different testing conditions. As the size of the hardness test indenter is larger than that of the nanoindenter, and there are a lot of defects (cracks, pores and voids) in the topcoat of the TBCs, the measurement of the hardness is the comprehensive result of the dense coating and defects. So the nanoindentation test reflects the mechanical properties of the coating itself more actually.
nanostructured 8YSZ thermal barrier coatings conventional 8YSZ thermal barrier coatings
200 180 160 140 120 100 80 60 40 20 0
Elastic modulus
Micro-hardness
Fig. 14. Elastic modulus and micro-hardness of topcoat of nanostructured and conventional TBCs.
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L. Wang et al. / Vacuum 86 (2012) 1174e1185
materials named “quenching stresses”. Analytically, the magnitude of tensile quenching stress sq can be estimated from:
sq ¼ ac ðTm Ts ÞEc
(10)
where ac, Ec, Tm, Ts are coefficient of thermal expansion, elastic modulus of coating, melting point of the sprayed material and substrate temperature, respectively. Stresses originated from differences in thermal expansion coefficients between those of underlying materials and coatings. Thermal stress at the surface of coating stc can be estimated by:
stc ¼
Ec Ec DaDT ðac as ÞDT ¼ 1 nc 1 nc
(11)
where Ec, Da, DT, and n are the elastic modulus of coating, coefficient of thermal expansion (CTE) mismatch between the substrate and coating, temperature difference upon cooling, and the Poisson’s ratio of coating, respectively. It can be seen from Eqs. (10) and (11), the residual stress will increase when the elastic modulus of the coating has improved, so controlling the elastic modulus is very important because the residual stress is often the cause of the coating failure. From another viewpoint, when the residual stress was measured, the elastic modulus can be also calculated as follows:
Ec ¼
sresidual ð1 nc Þ DaDT þ ac ðTm Ts Þð1 nc Þ
(12)
As for the 8YSZ TBCs, when the coating was exposed to high temperature, the coating will sinter and shrink, some micropores will be closed, the elastic modulus of 8YSZ TBCs will be different under different heat treatment condition, such as annealing at different temperatures, at different gas pressures, such as on the air or on the vacuum [42e46]. How the elastic modulus will be changed under different thermal condition and the relationship between the microstructure evolution and the elastic modulus will be reported separately in the future paper. 4. Conclusions In this paper, microstructure and indentation (microindentation and nanoindnetation) mechanical properties of nanostructured and conventional ZrO2e8wt%Y2O3 TBCs have been investigated systematically, some important and useful results can be summarized as follows. (1) The conventional powder was mainly composed of Y0.15Zr0.85O1.93, m-ZrO2 phase and c-ZrO2 phase, the corresponding coating was mainly composed of Y0.15Zr0.85O1.93, cZrO2 phase after the plasma spraying process. The nanostructured 8YSZ feedstock was mainly composed of t phase, the corresponding coating was predicted to be that it was mainly composed of t’ phase after the plasma spraying process. (2) The nanostructured TBCs has lower porosity than that of the conventional TBCs due to the difference of the microstructure of the spray feedstock. (3) The elastic modulus, micro-hardness and Vickers hardness of nanostructured 8YSZ thermal barrier coatings exhibit bimodal distribution due to the coating are mainly composed of molten phase and non-molten phase. While the conventional 8YSZ TBCs exhibit mono-modal distribution. (4) The topcoat of the nanostructured TBCs has higher hardness and lower elastic modulus than that of the conventional 8YSZ TBCs due to the effect of the indenter size and coating defects. But the nanostructured 8YSZ TBCs has exhibited higher elastic
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