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Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings.

Abstract Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The micro-structure of the nanostructured coalings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.

Keywords Air plasma spray, Thermal barrier coatings, Microstructure. Thermal conductivity

Introduction

In the absence of significant high temperature structural materials development over the last 20 years, thermal barrier coatings (TBCs) have played an increasingly important role in enhancing gas turbine engine durability and performance. Typical TBCs consist of a dual-layer system comprising a ceramic coating and a MCrAlY (M = Ni and/or Co) oxidation-resistant metallic bond coat. The ceramic material almost universally used has been yttria partially stabilized zirconia (YPSZ) because of its low thermal conductivity, high thermal expansion coefficient, and the proper phase stability at high temperatures. The use of a conventional ceramic coating deposited by either air plasma spraying (APS) or electron beam physical vapor deposition (EB-PVD), together with an internal cooling of underlying superalloy components, could sustain a thermal reduction of up to 300[degress]C on the surface of the superalloy, and hence improve the efficiency and performance of the engines. (1-3)

Recently, there is considerable interest in developing TBCs with even lower thermal conductivity to provide further improvements in engine performance. Research on nanostructured TBCs by thermal spraying has been an active field because of its profound potentiality. In the as-sprayed nanostructured YPSZ, there exist two forms of nanophase (4): one evolving from the nonmolten particles, and the other resulting from the molten part of the nanostructured feedstocks due to the rapid cooling rate and nucleation rates of molten thermal spray particles. Those nanophases can provide enhanced mechanical properties, including hardness. strength, and toughness. Moreover, a refined agglomerate morphology of the nanostructured TBC increases the strain accommodation ability. Thus, the elastic modulus is lower and irreversible deformation occurs by particle sliding and microcracking that is distributed over the volume of the deformation zone. (5-9) In addition, a large quantity of preexisting microcracks and fine pores with homogeneous distribution, as well as the fine grain structure in the nanostructured TBCs, make a contribution to a better thermal shock resistance. (10), (11)

In this work, zirconia-based nanostructured TBCs were deposited by air plasma spraying with reconstituted nanostructured powders. The microstructure and phase composition of the nanostructured ceramic coatings arc investigated. Thermophysical properties, such as thermal diffusivity and thermal conductivity, are also discussed.

Experimental details

Materials and sample preparation

Titanium alloy (Ti-6.6Al-3.61Mo-l.69Zr-0.28Si in wt%) was used as the substrate material. The substrates were machined into disks that were 80 mm in diameter and 3 mm in thickness. After finely polished to 400 mesh abrasive, the disks were grit blasted with alumina, ultrasonically cleaned in anhydrous ethylene alcohol, and dried in cold air prior to coating deposition. Commercial 8 wt% YPSZ nanoscale feedstocks (DDN, Cug-nano, Hubei, China) were reconstituted into micron-sized spherical granules for the ceramic coatings. NiCrAlY metal powders (HHNiCrAlY-9, CAAMS, Beijing, China), with particle sizes ranging from 10 to 100 [micro]m, were used for the bond coat.

NiCrAlY bond coat was fabricated onto the substrates by air plasma spraying (DH80, CAAMS, Beijing, China), and then the nanostructured zirconia coating was deposited onto the bond coat using the same plasma spray system. In order to preserve a part of the powder's nanostructure and achieve the necessary physical condition for cohesion and adhesion, the spraying parameters have been optimized to partially melt the powder particles during plasma spraying. The Spraying parameters are listed in Table 1.
Table 1: Summary of the spraying parameters for the nanostructured TBCs

                                          NiCrAIY  YPSZ

Current, A                                 480     500
Voltage, V                                 65      70
Primary gas
  Ar, I [min.sup.1]                        36.7    -
  [N.sub.2], I [min.sup.-1]                -       36.7
Secondary gas, [H.sub.2], I [min.sup.-1]   3-5     3-5
Carrier gas, Ar, I [min.sup.-1]            3.5     3
Powder feeding rate, g [min.sup.-1]        54.4    10.4
Spray distance, mm                         100     70


Microstructural charaeterization

The microstructures of the powders and the as-sprayed nanostructured zirconia coatings were characterized with a transmission electron microscope (TEM. Model JEM-2010. Tokyo. Japan) and a field emission scanning electron microscope (FESEM, Model FEI Sirion 200, USA). Their phase compositions were examined In using a X-ray diffractometer (XRD, model D8, Bruker, Germany) operating with Cu[K.sub.x], radiation ([lambda] = 1.54056 [Angstrom]) at 40 kV and 30 [micro]A. The XRD technique was also used to estimate the mean grain size by using the Scherrer equation:

B(2[theta]) = 0.9[lambda]/D cos[theta] (1)

where B(2[theta]) is the true broadening of the diffraction line measured at half-maximum intensity, which is also known as "full-width at half-maximum" (FWHM); D is the mean dimension of grain; [lambda] is the wavelength of the X-ray radiation: and [theta] is the Bragg angle. The correction for instrumental broadening was taken into consideration in the measurement of peak broadening by comparing the widths at half maximum intensity of X-ray reflection between the sample and the single crystalline Si standard, and then the Cauchy correction was used to remove the instrumental broadening to obtain the true crystal broadening

[B.sub.P.sup.2](2[theta]) = [B.sub.h.sup.2](2[theta]) - [B.sub.h.sup.f(2[theta]) (2)

where [B.sub.p] (2[theta]) is the true half maximum width. [B.sub.h] (2[theta]) and [B.sub.f] (2[theta]) are the half maximum widths of the sample and the single crystalline Si standard, respectively. The applicability of this method is confirmed by comparing results from TEM and XRD techniques according to Refs. 4, 12, 13.

Free-standing ceramic specimens were produced by removing substrates from coatings with a hydrochloric acid solution. Pore size distribution in the free standing specimens was determined by a mercury intrusion porosimeter (MIP, Poresizer9320, Micromeritics instrument corporation. USA). The surfaces of the specimen were finely polished before measurement to avoid the effect of surface roughness.

Thermophysical properties

The as-sprayed nanostructured zirconia coating with disk shape (10.3 mm in diameter and 1.3 mm in thickness) was selected for thermal diffusivity [alpha] (T) measurement. The laser-flash diffusivity method was used to explore the thermal diffusivity in an argon atmosphere within the temperature range of 200-700[degrees]C. Prior to the measurement, the specimen's front and back surfaces were coated with a thin layer of carbon to prevent transmission of laser beams through the translucent specimen and increase the absorption of laser pulses. The thermal diffusivity measurement for the specimen was conducted three times at each temperature. Measurement of specific heat C[rho] (T) as a function of temperature (in the range of 100-700[degrees]C) was performed using a differential scanning calorimeter (DSC-2. Perkin Elmer. Wilmington. USA) at a heat rate of 10 K/min in air. Bulk density [rho] of the specimen was measured by the Archimedes' technique and is given by

[rho] = (W/[I - S]) x [rho][H.sub.2]O (3)

where [rho] [H.sub.2]O is the density of water at ambient temperature; W, S and I are the weight of dry sample, wet sample submerged in water, and wet sample in air, respectively. The thermal conductivity. K, was then calculated using the following equation:

K(T) = [[alpha](T)] [rho] [C[rho](T)] (4)

Results and discussion

Phase determination via X-ray diffraction

The X-ray diffraction patterns of the reconstituted YPSZ powders and the as-sprayed nanostructured zirconia coating are presented in Fig. 1, respectively. The typical m(111) and m(111) peaks of monoclinic phase can be observed obviously in the XRD spectrum of the reconstituted powders, which indicates that the powders exhibit the presence of not only the tetragonal phase and the cubic phase, but also the monoclinic phase. The volume percentage of the monoclinic phase can be estimated by the following formula (14-17):

[FIGURE 1 OMITTED]

m(%) [m(111) + m(111)]/[t(111) + w(111) + m(111)] x 100% (5)

where the terms m(l11), t(111), and so on, represent the intensities of corresponding diffraction peaks in the XRD pattern, respectively. The calculated volume fraction of the monoclinic phase in the reconstituted powders is approximately 29%. However, there is no existence of the monoclinic zirconia in the as-sprayed nanostructured zirconia coating according to the XRD spectrum. This result shows that the as-sprayed coating is wholly composed of a nonequilibrium tetragonal phase.

The comparison of XRD results between the as-sprayed coating and the powders releases that phase transformations have taken place during the plasma spraying process. The monoclinic phase disappears after plasma spraying, as can be explained by the rapid cooling rate, which is nearly in the order of [10.sup.6]-[10.sup.7] K/s in the thermal sprayed particles. (4), (18) With the help of yttria. this cooling produces the nonequilibrium tetragonal zirconia from the cubic phase via a diffusionless phase transformation and prevents the transformation of zirconia from the tetragonal phase to the monoclinic phase. The detrimental tetragonal to monoclinic phase transformation is related to large stresses (volume expansion of ~4%) and limits the lifetime of the TBCs.Therefore, mechanical properties and lifetime are optimized when the maximum amount of the non-equilibrium tetragonal t' phase can be obtained. The penetration distance of the X-ray beam into the sample can reach several micrometers depending on the sample characteristics. (4), (6) Therefore, a thin surface layer of the as-sprayed nanoslruetured zirconia coating only consists of the nonequilibrium tetragonal phase due to the quenching of the droplets after impact on the substrate from high temperatures.

The grain size of the reconstituted powders is calculated using the Scherrer equation with several different peaks. The average value is taken as the mean grain size. The mean grain size is 28 [+ or -] 5 nm for the powders. With respect to the as-sprayed nanostructured zirconia coating, the (222) peak in the XRD pattern is selected for mean grain size determination according to Ref. 4. The calculated mean grain size for the coating is 42 nm. The mean grain size of the coating increases after plasma spraying. Grain size difference between the powders and the coaling reaches 33%. It may be explained that the thermal excursion in the plasma flow helps the nanoscale grains grow up and/or the phase transformation from the melt makes new. bigger grains. The Scherrer equation is deduced under the assumption that only a small grain size is responsible for peak broadening.(12) Strain effects, which may influence the peak broadening, are not taken into account. In the thermal spray, the occurrence of the residual stress effects is evident.(19) Therefore, the use of the Scherrer equation to measure the mean grain size of the as-sprayed coating may be biased by stress effects.

Microstructure analysis

The morphologies of the nanoscale YPSZ particles and the reconstituted powders are presented in Fig. 2. It can be seen that the sizes of nanoparticles are comparatively homogeneous in the range of 20-40 nm. as shown in Fig. 2a. As discussed in the previous section, the calculated mean grain size of the powders by Scherrer equation is 28 nm, which approximately agrees with the mid-value of the particle size range. Moreover, it reveals that most of the nanoparticles may be considered single crystals since the observed particle sizes in magnitude are consistent with the calculated mean grain size. The reconstituted powders are spherical with particle sizes in the range of 20-70 [micro]m, as shown in Fig. 2b. The spherical shape can improve the flowability of the reconstituted powders during plasma spraying. The magnified surface morphology of a reconstituted granule is shown in Fig. 2c. It can be seen that the reconstituted granule is composed of many smaller particles.

[FIGURE 2 OMITTED]

The images of fractured cross sections of the as-sprayed nanostructured zirconia coatings are shown in Fig. 3. It can be observed in Fig. 3a that different microstructures coexisl in the coating. Laminar layers with columnar grains are surrounded with the preserved nanostructure of starting powders and some micron-sized equiaxed grains. Figure 3b shows that the initial nanostrueture of powder is retained in the coating. It can be seen that the nonmolten particles are loosely bonded among each other, and the nano-particle sizes in the coaling are below 100 nm. The preserved nanostrueture is in the form of an agglomerate because of the poor flattening ability of the unmelted powders. Figure 3c presents the columnar grains highly bonded among each other and similar to those in a conventional counterpart, evolved from the fully molten and flattened powders. (20) Figure 3d shows that the equiaxed grains with diameters of about 1-2 [micro]m within a splat are also present in the nanostructured coatings, and a very thin layer of columnar grains at the periphery of the splat can be seen as well.

[FIGURE 3 OMITTED]

As we know, the temperatures in plasma flow decrease rapidly from the core to the periphery. (21) In addition, the particle sizes of reconstituted powders are widely distributed from 20 to 70 [micro]m. Consequently, the temperature for each particle is different during plasma spraying, which may lead to the particles in plasma flow presenting different melting stales. Some coarse particles may stay at low temperatures due to their large mass and volume, and tend to keep the nanostructure of the starting powders intact.

Some moderate particles, whose temperatures may approximately stay at the melting point during plasma spraying, partially melt with some nonmolten solid parts or a nonmolten thin core. The subsequent impact on the substrate makes the nonmolten core or the nonmolten parts in the melt broken into pieces and homogenously dispersed in the flattened melt. They can then serve as the nuclei in crystallization and increase the nucleation rates. Moreover, high cooling rates give rise to high nucleation rates, and if the rate of heal loss by conduction through the substrate or splat interface is greater than the rate of heat librated by crystallization, the liquid temperature continues to decrease and crystallization is controlled by continued nucleation to give an equiaxed grain microstructure. (22) Thus, the micron-sized equiaxed grains can come into being inside a splat when there is enough time for nucleation growth. In the meantime, because of rapid cooling rate and heterogeneous nucleation at the surface of the molten splat, the columnar grains are formed at the peripheral layer of the splat.

Some small particles at relatively high temperatures are fully melted during plasma spraying. Large contact areas are obtained for good flowability. which promotes their thermal conduction. Rapid heterogeneous nucleation occurs at the cooler boundaries of the flattened droplets in a large undercooling. At the same time, an increase in temperature of the droplets, produced by the release of the heat of fusion during rapid crystal growth, would tend to suppress further nucleation because of the large temperature dependence of the nucleation rate. Therefore, crystals grow from the interface into the droplet (i.e., in the opposite direction to the heat flow), forming the columnar grain microstructure. (4), (18) Moreover, solidification occurred prior to the impact of subsequent splats in a discontinued manner, resulting in the lamellar structure.

It has been reported that the nanostructured zirconia coatings deposited by plasma spraying are mainly composed of column grains and nanosized equiaxed grains. (15), (16), (23) For example. Chen et al. noticed that the nanostruetured zirconia coalings exhibited a lamellar structure with columnar grains and loose microstructure resulting from the nonmolten starting powders." However, there are no reports on the micron-sized equiaxed grains in the nanostruetured zirconia coatings. In this case, the nanostructured zirconia coatings are comprised of the initial nanostructure of the powders, the micronsized equiaxed grains within splats. and laminar structure with columnar grains. Microstructure plays an important role in the properties of materials. The distinct microstructure character of the nanostructured zirconia coatings presents an opportunity for TBC microstructural control in a materials engineering endeavor to control their properties and failure mechanisms.

Porosity

The measured surface-connected porosity of the freestanding nanostructured zirconia coating is 13% and the median pore diameter (volume) is 0.3524 [micro]m. Figure 4a shows the cumulative porosity distribution of the coating measured by mercury intrusion porosimetry. Figure 4b illustrates the compositions of porosity that are based on the result of the porosity measurement in Fig. 4a. The result reveals a bimodal pore size distribution, similar to that in the conventional zirconia coatings. (24), (25) Micropores contribute a larger percentage of the total porosity than macro-pores. The tine micropores, with diameters smaller than I [mu]m. include intrasplat microcracks. intersplat gaps resulted from poor adhesion, and small voids originated from the unmelted particles. A spot of macropores or large defects, with pore sizes of even up to 100 [mu]m, are also present in the coating, which could be mainly attributed to macrocracks such as branching cracks, inter-lamellar pores due to improper adhesion, and globular pores as a result of gas entrapment. The level of coaling porosity is significantly connected with the spray conditions such as powder sizes, spray distance, spray angle, etc. To some extent, the unmelted powders in the nanostructured coaling help increase the number of micropores.

[FIGURE 4 OMITTED]

Thermal properties

Figure 5 presents the results of thermal diffusivity. specific heat measurement, and the calculated thermal conductivity of the as-sprayed nanostructured zirconia coating for various temperatures from 200 to 700[degrees]C, respectively. The specific heat is of nearly linear temperature dependence--i.e [C.sub.P] [infinity] = T--and goes up with the increase in temperature, which is plotted in Fig. 5a. The specific heal values from 100 to 700[degrees]C for the coaling span a range of 0.479-0.919 J/mK. which can be titled as the following equation in this temperature range:

[FIGURE 5 OMITTED]

[C.sub.p] = 0.40 + 7.33 x [10.sup.-4] T - 2.12 x [10.sup.-12][T.sup.2] (6)

The thermal diffusivity of the nanostructured zirconia coating decreases slightly with increasing temperatures as seen in Fig. 5a. The values of thermal diffusivity for the coaling in the measured temperatures are in the range of (2.51-1.95) x [10.sup.-3] [cm.sup.2]/s, which shows an inverse temperature dependence--i.e., [lambda] [[infinity] [T.sup.-1]. This suggests a dominant phonon conduction behavior like most polycrystalline materials.

The calculated values of thermal conductivity are obtained according to equation (4). as shown in Fig. 5b. It can be seen that the thermal conductivity of the nanostructured zirconia coating increases slightly as the temperature goes up in the range of 200-700[degrees]C. The value of thermal conductivity is 0.623 W/mK at 200[degrees]C, and increases to 0.806 W/mK at 700[degrees]C. An explanation for this may be that at high temperatures, healing of the fine pores or microcracks occurs. (26) and when this happens, the degree of contact between splats increases and thereby causes an increase in thermal conductivity.

Compared with the previous study (27) and the published data of thermal conductivity for conventional plasma-sprayed YPSZ coatings, (28), (29) the nanostructured zirconia coating exhibits a lower value of thermal conductivity. The theory of heat conduction by lattice waves in solids can be used to understand how the conductivity is influenced by point defects, grain boundaries, and extended imperfections. According to the micromechanism of thermal conduction, the thermal conduction of ceramics is mainly the result of phonon impacting. The thermal conductivity of phonon. k. can be expressed as

[kappa] = [1/3] [C.sub.v][bar.V][bar.l] (7)

where [C.sub.v] is the specific heat per unit volume, v is the speed of phonon. and l is the mean free path of phonon. [C.sub.v] is almost constant when it is above the Debye temperature. Phonon speed v is related to elastic modulus and density, which are relatively stable with the temperature changes. Therefore, the value of v may generally be taken as a constant as well. Then, the thermal conductivity is dominantly controlled by the mean free path of the phonon. according to equation (7).

It is known that lattice imperfections can scatter phonons and reduce the mean free path. (30) Oxygen vacancies (the typical point defects in YPSZ), which result from the need for charge neutrality during the substitution of [Zr.sup.4+] cations with [Y.sup.3+] cations, scatter phonons and reduce the thermal conduction. The concentration of oxygen vacancy is basically related to the content of yttria. (31) Since the nanostructured coatings have the same yttria content as the conventional ones, we can deduce that the thermal conductivity difference between the nanostructured zirconia coating and the conventional one is not primarily caused by those point defects.

The effect of grain boundary and other imperfections in the nanostructured coating should be responsible for the difference in thermal conductivity. According to the work in Ref. 27, the micropores in the nanostructured coatings are smaller in size and more homogeneously distributed with the porosity at the same level, and the splats in the nanostructured coatings are thinner, as compared to the conventional coatings. The smaller the micropores are. the more interfaces are produced, and the effect on the phonon scattering is strengthened. resulting in the reduction of thermal conductivity. Those thinner splats provide more interfaces between splats in the nanostructured coating as compared to the conventional one at the same thickness. Those interfaces are perpendicular to the heat flow direction, and effectively reduce the thermal conduction with stronger interfacial thermal resistance.

Furthermore, as discussed in the previous section, there exists the original nanostructure. the columnar grains, and some equiaxed grains in the nanostructured zirconia coatings. The preserved nanostructure could offer much more in terms of grain boundaries. Meanwhile, there are a number of micropores in the loose nanostructure. The grain boundaries and micropores help phonon scattering. Moreover, weak binding of the nanostructure leads to low shear modulus, and tends to reduce the intrinsic conductivity. (30) Unlike the columnar grains that are parallel to heat flow. the equiaxed grains in the nanostructured coating also have an effect on the thermal resistance because of their zigzag grain boundaries. Consequently, all those contribute to the greater reduction of thermal conductivity in the nanostructured zirconia coating.

Conclusion

Nanostructured zirconia coatings deposited by air plasma spraying have been investigated. The results of X-ray diffraction reveal that the nanostructured zirconia coatings are composed of the nonequilibrium tetragonal phase, whereas the reconstituted nanostructured powders show the presence of the monoclinic phase and the cubic phase.

Microstructure analysis shows that the microstructure of the as-sprayed nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains are also exhibited in the nanostructured coatings in this case. The nanostructured coating with 13% porosity shows a bimodal pore size distribution, the dominative pores are the micropores with diameter smaller than 1 [mu]m. and the others are the macropores. The grain boundaries and small micropores in the nanostructured coating have great effect on the phonon scattering, which leads to a lower thermal conductivity in the nanostructured coatings.

Acknowledgments The first author is grateful to Mr. Y. Lai in the instrumental analysis center at Shanghai Jiao Tong University for the help with sample examination.

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H. Zhou [??]. F. Li, .J. Wang, B.-d. Sun

State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University. Shanghai 200240, P. R. China

e-mail: zhou7210@yahoo.com.cn
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Author:Zhou, Hong; Li, Fei; Wang, Jun; Sun, Bao-de
Publication:JCT Research
Date:Sep 1, 2009
Words:4929
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