Rare earth oxides stabilized La2Zr2O7 TBCs: EB-PVD, thermal conductivity and thermal cycling life

Accepted Manuscript Rare earth oxides stabilized La2Zr2O7 TBCs: EB-PVD, thermal conductivity and thermal cycling life

Zaoyu Shen, Limin He, Zhenhua Xu, Rende Mu, Guanghong Huang PII: DOI: Reference:

S0257-8972(18)31152-6 doi:10.1016/j.surfcoat.2018.10.045 SCT 23908

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

18 July 2018 14 October 2018 16 October 2018

Please cite this article as: Zaoyu Shen, Limin He, Zhenhua Xu, Rende Mu, Guanghong Huang , Rare earth oxides stabilized La2Zr2O7 TBCs: EB-PVD, thermal conductivity and thermal cycling life. Sct (2018), doi:10.1016/j.surfcoat.2018.10.045

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ACCEPTED MANUSCRIPT Rare Earth Oxides Stabilized La2Zr2O7 TBCs: EB-PVD, Thermal Conductivity and Thermal Cycling Life Zaoyu Shen, Limin He*, Zhenhua Xu, Rende Mu and Guanghong Huang Key Laboratory of Advanced Corrosion and Protection for Aviation Materials,

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Beijing Institute of Aeronautical Materials, Aero Engine Corporation of China,

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Beijing 100095 (PR China)

Abstract: A series of rare earth oxides stabilized La2Zr2O7 coatings by electron

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beam-physical vapor deposition (EB-PVD). The complex hierarchical architectures

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are composited of feathery nanostructure and intra-columnar pores. The obtained coating shows relativity high thermal cycling lifetime (395 cycles) and thermal shock

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lifetime (2201 cycles). The coatings show an especially low thermal conductivity and

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low thermally grown oxide growth rate. The growth behavior of thermally grown oxide (TGO) layer can be fitted in a parabolic TGO growth law (0.1651 m2/h). The

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selection and optimization of rare earth oxides can be extended to design other new

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ceramic materials.

Keywords: Thermal barrier coatings; EB-PVD; Pyrochlore materials; Thermal conductivity; Thermal cycling lifetime

ACCEPTED MANUSCRIPT Graphical Abstract Hierarchical CZ-LZ coatings with a feathery nanostructure and intra-columnar pores were prepared by EB-PVD. The introduction of Ce2Zr2O7-stabilized La2Zr2O7 leads to the low thermal conductivity of CZ-LZ coating (0.525 W/mK at 1200 °C),

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nearly 70% lower than that of the previously reported commercial YSZ. The selection

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and optimization of rare earth oxides stabilized La2Zr2O7 can be extended to design

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other new ceramic materials.

ACCEPTED MANUSCRIPT 1. Introduction Thermal barrier coatings (TBCs) are double-layer coatings applied to gas turbine engines, consisting of a metallic bond coat and a ceramic top coat.1-3 The main function of bond coat is to protect the substrates from oxidation and corrosion.4,5 The

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ceramic coat has a relativity low thermal conductivity that can reduce the

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requirements for the cooling system.6-7 Up to date, the successful ceramic coat

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material is 6-8 wt% yttria stabilized zirconia (YSZ).1 The YSZ coatings are usually prepared by electron beam-physical vapor deposition (EB-PVD) and atmospheric

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plasma spraying (APS).4-9 In particular, the introduction of columnar structure by

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EB-PVD can improve strain compliance that can withstand long operation below 1150 °C.6,7 However, YSZ coatings suffer serious sintering and phase transformation,

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leading to the failure of TBCs (>1200 °C).10

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For the above reasons, many researchers have investigated new ceramic materials with low thermal conductivity and high thermal stability.11 Rare earth oxides

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stabilized ZrO2, pyrochlore materials, perovskite oxides and aluminates have been

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extensively investigated as possible successors to YSZ.12-20 Among them, the pyrochlore materials cover the most interesting investigations. In particular, La2Zr2O7 ceramic materials stand out due to their high melting point (>2000 °C) and lower thermal conductivity (1.56 W/(m·K) at 1000 °C).5,11 However, single-layer La2Zr2O7 coating shows lower thermal cycling performances than the conventional YSZ.5 Thus, La2Zr2O7 coating has been investigated by doping of foreign elements and constructing composites with other rare earth oxides. Among them, CeO2 has been

ACCEPTED MANUSCRIPT investigated due to the high thermal expansion coefficient (~13×10-6/K), enhancing the thermal expansion coefficient of complex coating.13-14 However, the investigation of rare earth oxides stabilized La2Zr2O7 only focus on the thermal performances of the single rare earth oxides, such as CeO2, Y2O3 and so on.9,16-20. The investigation of

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relationship between the hierarchical architectures and their thermal performances

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(thermal conductivity and thermal cycling life) are comparatively scarce. The

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La2Zr2O7 coatings stabilized by different rare earth oxides have a significant difference for their thermal conductivity and thermal cycling life.11 The stabilized

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effect is crucial for achieving high thermal cycling life.4-5 Thus, the selection and

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optimization of rare earth oxides stabilized La2Zr2O7 coatings is important and attractive. It has a great significance to explore the relationship between the

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hierarchical architectures of La2Zr2O7 coatings and their thermal performances.

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Herein, we report a series of rare earth oxides (CeO2, La2Ce2O7 and Ce2Zr2O7) stabilized La2Zr2O7 single-layer coatings. The complex hierarchical architectures are

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composited of feathery nanostructure and intra-columnar pores. The obtained coating

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shows relativity high thermal cycling lifetime and thermal shock lifetime. The coatings show an especially low thermal conductivity and low thermally grown oxide growth rate. The relationship between the hierarchical architectures and their thermal performances is discussed in details. 2. Experimental section In a typical procedure, ingot was synthesized by solid state reaction (1400 ℃ for 18 h). La2O3 (99.99%), CeO2 (99.99%) and ZrO2 (99.9%) were used as the starting

ACCEPTED MANUSCRIPT materials. In order to prepare the rare earth oxides (CeO2, La2Ce2O7 and Ce2Zr2O7) stabilized La2Zr2O7 coatings, we have optimized of the content of La2O3, CeO2 and ZrO2 in the ingots. For La2Zr2O7 ingot, the ingot was synthesized by La2O3 (99.99%) and ZrO2 (99.9%) as the starting materials. These ingots were densified after the cast

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formation (1500 ℃ for 18 h) and fabricated in size of Ф 68 mm125 mm. The

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Ni-based superalloy (30 mm×10 mm×1.5 mm) was used as the substrate. The

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Ni-based superalloy has a chemical composition of 12.5 Co, 6.3 W, 5.7 Ta, 6.3 Al, 2.2 Re, 5.8 Cr, 1.3 Mo, 0.02 C and Ni as balance (wt.%). The NiCoCrAlYHf bond

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coating was deposited by A-1000 Vacuum Arc Ion-Plating Unit. The NiCoCrAlYHf

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coating has a chemical composition of 12.0 Co, 20.0 Cr, 10 Al, 0.3 Y, 0.2 Hf, and Ni as balance (wt.%). The thickness of NiCoCrAlYHf coating is between 50 ± 10 m.

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Prior to deposition of ceramic coating, the samples were annealed under vacuum at

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870 ± 10 °C for 3 h in order to enhance element diffusion. Rare earth oxides stabilized La2Zr2O7 coatings were deposited by a commercial EB-PVD unit (UE-207S, ICEBT).

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During the deposition of these coatings (60 ± 10 m) by EB-PVD, the temperature of

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samples was 900 ± 50 °C. The samples were rotated at a speed of 20 rpm. The standard chamber pressure under oxygen addition during evaporation was about 510-3 mbar. The deposition rate for three coatings is about 3-4 m/min. XRD patterns were acquired on a Bruker D8 Advance equipped with Cu Kα radiation at a scan rate of 4 min-1. The X-ray photoelectron spectroscopy (XPS) was collected on a Thermo Fisher ESCALAB 250Xi with an Al Kα chromatic X-ray source (1486.60 eV). The microstructure and composition of these coatings was

ACCEPTED MANUSCRIPT studied by transmission electron microscopy (TEM, JEM-2100F) and scanning electron microscope (SEM, FEI-Quanta 600) equipped with energy dispersive spectroscopy (EDS, Oxford INCAx-sight 6427). The high angle annular dark fields (HAADF) were carried out on a field-emission scanning transmission electron

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microscope, a JEM-2100F, equipped with energy dispersive X-ray spectroscopy in the

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STEM mode. The cross-section analysis of these coatings by SEM were firstly

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embedded in transparent epoxy resin and polished by diamond pastes below 3 m. The thermal diffusivity (α) was measured using a laser-flash method (Netzsch LFA

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427). The front and back surfaces of samples were sprayed with a thin layer of carbon.

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The carbon layer was used to prevent direct transmission of the laser beam through the translucent samples. In the thermal cycling test, the samples were exposed at

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1100 ℃ for 55 min followed by air cooling for 5 min, and this whole process was

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accounted as one cycle. In addition, the samples were also exposed at 1100 ℃ for 5 min followed by air cooling for 5 min in the thermal shock test. These tests were

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repeated until 10% of the ceramic coat was delaminated, and the average cycling

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number of 5 specimens was regarded as the thermal cycling life of the coating. 3. Results and discussion Different rare earth oxides were used to stabilize La2Zr2O7 material. After thermal cycling test, the results showed that the Ce2Zr2O7 stabilized La2Zr2O7 coatings displayed the lowest thermally grown oxide (TGO) growth rate and the highest thermal cycling life (Fig. 4 and Fig. 5). Therefore, our group focused on the sample by the rest of this study.

ACCEPTED MANUSCRIPT As shown in Fig. 1a, the XRD patterns of three coatings are compared to La2Zr2O7 (pyrochlore structure, JCPDS No.17-0450) and La2Ce2O7 (disordered fluorite structure, JCPDS No. 65-7999). The phases of CeO2 stabilized La2Zr2O7 (C-LZ) are composed of CeO2 (cubic structure) and LZ (pyrochlore structure). The main phases

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of C-LZ coating are pyrochlore structure. Four weak peaks of the CeO2 are observed

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in the C-LZ coatings. The sharp and narrow peaks of the coating indicate that the

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coating has a high degree of crystallinity. For the phases of La2Ce2O7 stabilized La2Zr2O7 coatings (LC-LZ), the structure of the coating is a mixture of pyrochlore and

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disordered fluorite. The peaks at 28.64°, 33.12°, 47.46° and 56.14° can be indexed as

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(222), (400), (440) and (622) of La2Zr2O7 phases by comparison. Meanwhile, the peaks at 28.14°, 32.58°, 46.48°, and 55.18° can be indexed as (111), (200), (220) and

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(311) of La2Ce2O7 phases by comparison. For the phases of Ce2Zr2O7 stabilized

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La2Zr2O7 coatings (CZ-LZ), the peaks at 28.76°, 33.24°, 47.86° and 56.72°can be indexed as (222), (400), (440) and (622) of La2Zr2O7 phases. Three weak peaks of the

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Ce2Zr2O7 (cubic structure) are observed in the CZ-LZ coatings. Interestingly, the

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preferred orientation growth of CZ-LZ coating occurs in the (400). A disordered structure of coating is beneficial to reduce their thermal conductivities.11 XPS data of the CZ-LZ coatings is measured to investigate the surface chemical states (Fig. 1b). From Ce 3d XPS data (Fig. 1c), the binding energies for Ce 3d5/2 are well deconvoluted by three curves at approximately 822.3 eV, 888.7 eV and 898.2 eV, respectively. The spin-orbit components of the peak for Ce 3d3/2 are 900.7 eV, 907.9 eV and 916.5 eV, respectively. These results can be assigned to the chemical state of

ACCEPTED MANUSCRIPT Ce4+.21 The pure chemical state of Ce4+ might lead to high thermal cycling life in long-term operation.13-14 As shown in Fig. 1d, the binding energies for La 3d5/2 and La 3d3/2 are 833.1 eV and 838.3 eV, respectively, which can be assigned to La3+.21 From Zr 3d and O 1s XPS data (Fig. 1e and 1f), we can conclude that the chemical states of

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Zr and O are close to the reported value of Zr4+ and O2-, respectively.21 The above

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results confirm that the compounds were the formula of Ce2Zr2O7-La2Zr2O7 and were

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prepared in high purity.

As shown in Fig. 2, the top surface, cross-section and EDS of the as-deposited

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CZ-LZ coatings are investigated by SEM. The formation of the columnar

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microstructure of TBCs is connected to EB-PVD conditions. Columnar structure and inter-columnar gaps is obtained due to the vapor phase condensation and shadowing

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effect caused by the column tips and rotation during deposition (Fig. 2a). As shown in

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Fig. 2b, a pyramidal structure is observed on the top surface due to the cubic lattice of the pyrochlore compound.13 The EDX analysis reveals the coexistence of La, Zr, Ce

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and O four elements (Fig. 2c). SEM image of the cross-section clearly shows that the

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thickness of CZ-LZ coating is approximately 50-60 m (Fig. 2d). A branched columnar structure with inter-columnar gaps is clearly observed in Fig. 2d. These structures could be explained based on the melting points (Tm) of LZ (2300 ℃). These columnar structures are achieved following the relationship of 0.3Tm < Ts < 0.5Tm which are very helpful to the improvement of thermal cycling life.22 The morphology of CZ-LZ coating is similar to that of YSZ coating, but the diameter of column tip is smaller. The columnar diameters of the CZ-LZ coating are measured by different

ACCEPTED MANUSCRIPT positions (approximately 1.5-2.5 m) (Fig. 2d). As shown in Fig. 2, the CZ-LZ coating possesses a feathery microstructure that offers excellent compliance to thermal cycling. These columnar structure can release the concentration of thermal stress during thermal cycling test.1,2 Meanwhile, the columnar microstructure of the

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CZ-LZ coating can provide a high level of strain tolerance. Furthermore, the

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elongated pores and feathery structure can also provides compliance and reduce

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thermal conductivity. Thus, the columnar structure and columnar gaps structure of CZ-LZ coating have a good effect on reducing thermal conductivity and improving

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the thermal cycling life of TBCs.4,11 Furthermore, the SEM and EDS analysis data on

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the LC-LZ and C-LZ coatings is shown in Fig. 1S. The columnar structure of LC-LZ and C-LZ coatings is the typical and characteristic features of two coatings and

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similar to CZ-LZ coating due to the EB-PVD process.22

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As shown in Fig. 3, the columnar structure of CZ-LZ coating has been characterized by TEM, HAADF, HRTEM and SEAD. The obvious feathery structure

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and intra-columnar pores can be clearly observed at column tips (Fig. 3a). The careful

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observation of HAADF image clearly reveals that the spherical and elongated nanorods agglomerated to form a feathery nanostructure (Fig. 3b). The obvious feathery structure and intra-columnar pores can provide relatively high strain tolerance and compliance to thermal cycling process. Furthermore, the feathery nanostructure and intra-columnar pores have a significant effect on the reduction of thermal conductivity. The HAADF result also reveals that four elements are distributed homogeneously throughout the columnar microstructure. The HRTEM

ACCEPTED MANUSCRIPT image is detected on the feather of column (Fig. 3c). The lattice spacing of CZ-LZ coating is approximately 0.333 nm and 0.214 nm corresponding to the (222) and (422) planes for La2Zr2O7 and Ce2Zr2O7, respectively. The existence of La2Zr2O7 and Ce2Zr2O7 structures would lead to a highly disordered structure of coating which can

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reduce the thermal conductivity of the coating. Because of the similar structure and

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crystal space of La2Zr2O7 and Ce2Zr2O7, the corresponding SAED pattern indicates

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that the as-deposited coating is single-crystalline (Fig. 3d).

The thermal conductivity of CZ-LZ coating was calculated by the following

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equation.23

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λ=ρ  α  Cp

λ/λ0=1 - 4/3α

(1) (2)

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The density (ρ) was measured according to Archimedes Principle. The specific heat

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capacity (Cp) was calculated by Neumann-Kopp rule. The deviation of thermal conductivity was estimated to be ±5% due to the deviation for ρ, α, and Cp. The φ is

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the fractional porosity of coating. Up to date, investigators are most interested in

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ceramic materials with low thermal conductivities below 1 W/mK. Few ceramic materials can show an excellent thermal conductivity (about 0.5 W/mK). Our groups first successfully synthesized Ce2Zr2O7 stabilized La2Zr2O7 coating by EB-PVD. As shown in Fig. 3 a, the thermal diffusivity of CZ-LZ coating is 0.197 mm2/s at 1200 °C. The thermal conductivity of CZ-LZ coating is 0.525 W/mK at 1200 °C, nearly 70% lower than that of the previously reported commercial YSZ (Fig. 3b). The as-deposited CZ-LZ coatings can effectively reduce the heat transfer in the TBCs

ACCEPTED MANUSCRIPT system. To the best of our knowledge, our TBC with a composition of Ce2Zr2O7-La2Zr2O7 represents one of the most lowly thermal conductivity ceramic materials. As well known, the phase structure and microstructure of coatings have an

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important effect on the thermal conductivity. The phonon scattering composited of

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phonon-phonon scattering and point defect scattering can decide the TBCs’ thermal

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conductivity.24 On the one hand, phonon-phonon scattering is related to the composition and phase structure of material. The CZ-LZ coating has a composite

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structure of La2Zr2O7 and Ce2Zr2O7 pyrochlore phase. The complex phase structure

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and composition leads to the strong phonon-phonon scattering, resulting in short phonon mean free path which would lead to the low thermal conductivity.24-26 In

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addition, the intra-columnar pores, gaps and cracks would produce the relatively

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highly disordered structures, leading to the increase of the phonon-phonon scattering and the decrease of the thermal conductivity.25 On the other hand, the material can be

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regarded as the solid solution of Ce2Zr2O7 and La2Zr2O7.13 The massive point defects

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(substitution atoms and vacancies) have been introduced into the lattices of CZ-LZ coatings due to the differences of mass, size and inter-atomic force between Ce4+and La4+.25,26 The phonons would scatter by following the Rayleigh Regime. In addition, the differences of Ce4+and La4+ would further lead to mass and strain fluctuations that can scatter the heat-carrying phonons.25 Thus, the introduction of Ce2Zr2O7-stabilized La2Zr2O7 would reduce the thermal conductivity of coatings. In order to evaluate the thermal performance of TBCs system, the thermal cycling

ACCEPTED MANUSCRIPT life of C-LZ, LC-LZ and CZ-LZ coatings were measured under the same conditions. The LZ coating as a comparison is also displayed in Fig. 4c-d. The thermal cycling life of each coating is the average value of five specimens. Thermal shock life of coatings is compared in Fig. 4c. The CZ-LZ has the highest life of 2201 cycles among

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all the coatings. The thermal cyclic life of coatings is also compared in Fig. 4d.

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CZ-LZ has the longest life of 395 cycles compared with that of LZ coating (an

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average life of 47 cycles), C-LZ coating (an average life of 81 cycles) and LC-LZ coating (an average life of 101 cycles). Furthermore, the photographs of coatings

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before and after thermal cycles also exhibit in Fig. S2. Generally, both the thermal

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shock life and thermal cyclic life of CZ-LZ coating exhibits an improvement in the thermal cycling life. The stabilized effect cubic structure (Ce2Zr2O7) leading to the

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high thermal performance of CZ-LZ coatings could be attributed to the following two

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features. Firstly, the introduction of cubic structure (Ce2Zr2O7) leads to the preferred orientation growth of coating which the exposed crystal surface is the (400) lattice

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plane. The preferred orientation growth of coating might enhance the phonon-phonon

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scattering leading to the reduction of thermal conductivity. The low thermal conductivity (0.525 W/mK) can introduce thermal gradient condition in TBCs systems that can provide good thermal protection for bond coat. Thus, the selection and optimization of rare earth oxides stabilized La2Zr2O7 coatings improve the thermal cycling life and reduce the thermal conductivity. On the other side, the major advantage of EB-PVD is the columnar structure with the intra-columnar pores and gaps. The size of columns and intra-columnar pores are closely related to the thermal

ACCEPTED MANUSCRIPT cycling life of the coatings. The small size of columns, the massive intra-columnar pores and gaps can achieve the relativity high strain tolerance and pseudo-plasticity. These structures are believed to improve the strain tolerance of the coating leading to the high thermal cycling life.

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To evaluate the failure mechanism of TBCs system, the growth of the TGO layer at

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the ceramic coat-bond coat interface was investigated in details. Due to the growth of

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the TGO layer, stresses accumulate in the TBCs system, leading to the failure of the system. The growth and thickening of TGO layer are mostly accompanied by the

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delamination and cracks.2-4 Although no critical TGO thickness for rare earth oxides

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stabilized La2Zr2O7 failure exists, a slower TGO growth rate is appreciated in TBCs systems.27 In order to investigate the TGO growth of TBCs system, the TGO

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thickness versus the thermal cyclic life has been plotted for these coatings in Fig. 5a.

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The growth behavior of TGO layer has been fitted in a parabolic TGO growth law: (x-x0)2=2kpt, where the x (m) is the thickness after the thermal cycling test, x0 (m) is

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the ‘‘as-deposited’’ TGO thickness, t (h) is the thermal cycling time. The kp (m2/h) is

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the parabolic rate constant corresponding to TGO growth rate.3,19,27 To reduce the deviation in the data, TGO thicknesses have been measured at five positions in coatings. As shown in Fig. 5a, the growth rate of TGO layer is relatively fast before 50 h compared with the increase of thermal cycles. The difference of TGO thicknesses becomes more obvious with longer thermal lifetime. Only CZ-LZ coating can be fitted to a parabolic TGO growth law. Under the same conditions, the CZ-LZ coating has the lowest parabolic rate constant corresponding to the lowest TGO growth rate

ACCEPTED MANUSCRIPT (0.1651 m2/h) compared with that of LZ coating (1.204 m2/h), C-LZ coating (0.5875 m2/h) and LC-LZ coating(0.4564 m2/h). Based on the SEM results (Fig. 5b), TGO thicknesses below 10 μm could be stable in these systems. With the increase of the TGO thicknesses, the delamination and cracks continue to extend,

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leading to the failure in TBCs system.

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EDX line spectrum of the CZ-LZ coatings after 95% thermal cycling life is

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conducted to investigate the evolution of elements in coating (Fig. 4c). The result reveals that the four elements of La, Zr, Ce and O are distributed relatively

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homogeneously throughout ceramic coat. The EDX scanning profile of Al atom

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mainly occurs at the ceramic coat-bond coat interface corresponding to TGO layer. It means that the main reaction production is Al2O3 at TGO layer. The TGO thicknesses

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are measured to be approximately 10.5 m corresponding to the TGO growth curve.

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Based on discussion above, a low TGO growth rate would be the key factor for the longest thermal cycling life of the TBCs.

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4. Conclusion

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A variety of new EB-PVD TBCs with C-LZ, LC-LZ and CZ-LZ as ceramic top coat materials on NiCoCrAlYHf bond coats have been systematically investigated. The CZ-LZ coating shows obvious feathery nanostructure and intra-columnar pores. The structures of CZ-LZ coating are composed of La2Zr2O7 and Ce2Zr2O7. The introduction of Ce2Zr2O7-stabilized La2Zr2O7 leads to the strong phonon-phonon scattering and a large number of point defects reducing the thermal conductivity of ceramic material. The thermal conductivity of CZ-LZ coating is 0.525 W/mK

ACCEPTED MANUSCRIPT (1200 °C), nearly 70% lower than that of the previously reported commercial YSZ. The CZ-LZ coating possesses longer thermal cycling life and also shows lower TGO growth rate (0.1651 m2/h) than those of LZ, C-LZ and LC-LZ. Acknowledgement

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This work was supported by the National High Technology Plan of China (863 Plan)

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(2015AA034403). The authors gratefully thank Mrs. Z. Liu and Mr. C. Y. Shen for

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helpful discussion and support. References

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Fig. 1 (a) XRD patterns of rare earth oxides stabilized La2Zr2O7 coatings, XPS data of (b) whole-range spectrum, (c) Ce 3d core-level spectrum, (d) La 3d core-level

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spectrum, (e) Zr 3d core-level spectrum and (f) O 1s core-level spectrum.

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Fig. 2 surface morphology at (a) lower magnification, (b) higher magnification, (c)

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EDS analysis and (d) cross-section morphology of the as-deposited CZ-LZ coating.

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Fig. 3 (a) TEM image, (b) HADDF image, (c) HRTEM image and (d) SAED pattern

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of the as-deposited CZ-LZ coating.

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Fig. 4 (a)-(b) thermal conductivities of the as-deposited CZ-LZ coatings, (c)-(d) the

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thermal cycling life of the as-deposited LZ, C-LZ, LC-LZ, CZ-LZ coatings.

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Fig. 5 (a) TGO growth plotted against thermal cycling time for LZ, C-LZ, LC-LZ, CZ-LZ coatings, (b) cross-section morphology and (c) EDS analysis of the CZ-LZ

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coatings after 95% thermal cycling life.

ACCEPTED MANUSCRIPT Highlights A series of rare earth oxides stabilized La2Zr2O7 coatings have been prepared by EB-PVD. The microstructure of coatings is composited of feathery nanostructure and

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intra-columnar pores.

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The coatings show an low thermal conductivity and low thermally grown oxide

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growth rate.