The role of transient oxides during deposition and thermal cycling of thermal barrier coatings

Abstract

High temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC) and a partially yttria stabilised zirconia (PYSZ) thermal barrier coating (TBC), were thermally cycled to failure for three different pre-oxidation treatments performed for 1 h at 1373 K and a partial oxygen pressure (pO₂) of 20 kPa, 100 Pa and 0.1 Pa, respectively. These pre-treatments resulted in the formation of different thermally grown oxide (TGO) layers prior to TBC deposition with respect to the presence of the transient oxides NiAl₂O₄, θ-Al₂O₃, and Y₃Al₅O₁₂ at the TGO surface. The TGO microstructures after TBC deposition and thermal cycling were investigated with a variety of analytical techniques and compared with those after pre-oxidation. For all pre-oxidation treatments, a double-layered TGO developed on the BC during thermal cycling. The TGO adjacent to the TBC consisted of small Zr-rich oxide crystallites embedded in an Al₂O₃ matrix when the TGO surface after pre-oxidation comprised of Y₃Al₅O₁₂ plus α-Al₂O₃. When the TGO surface constituted of θ-Al₂O₃, the Zr-rich oxide crystallites were embedded in a NiAl₂O₄ spinel layer after thermal cycling. Zr was absent in the oxide layer when the TGO surface prior to TBC deposition was composed of NiAl₂O₄ spinel. The TGO contiguous to the BC consisted in all cases of α-Al₂O₃ with Y₃Al₅O₁₂ crystallites. The roughness of the α-Al₂O₃/BC interface increased for a higher density of Y-rich oxide protrusions (i.e. pegs) along this interface.     

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.     

Impedance spectroscopy analysis of complex perovskite Ho(Ni1/2Zr1/2)O3

The complex perovskite oxide Holmium nickel zirconate, Ho(Ni_1/2Zr_1/2)O₃(HNZ) is synthesized by solid-state reaction technique. The X-ray diffraction pattern of the sample at room temperature shows a monoclinic phase. The microstructure analysis is done by scanning electron microscope. Alternating current impedance spectroscopy is applied to investigate the electrical properties of HNZ in a temperature range from 313 K to 723 K and in a frequency range from 100 Hz to 1 MHz. The complex impedance plane plots show that the relaxation mechanism in HNZ is due to grain boundary effect. Scaling behavior of imaginary part of impedance indicates that relaxation in HNZ describes the same mechanism at various temperatures. The frequency dependent conductivity data are fitted to the universal power law. The frequency dependent electrical data are also analyzed in the framework of electric modulus formalism.     

Microstructure modifications and modulated piezoelectric responses in PLZT/Al2O3 composites

Piezoelectric ceramics for acoustic applications have been prepared by mixing the piezoelectric phase Pb_1−1.5xLa_x□_x/2(Ti_1−yZr_y)O₃ (PLZT) with variable fractions of Al₂O₃. The samples are in form of pellets and polarized at high temperature. After thermal treatment, X-ray diffraction and scanning electron microscopy have been used to determine the phase and morphological modifications. The morphotropic PLZT initial phase disproportionates into modified PLZT and ZrO₂ phases. Using electrical impedance spectroscopy, the resonance frequencies of the composite system have been determined and analyzed. As the Al₂O₃ volume fractions increase, the resonance frequency and the amplitude of the electrical response both decrease. An interpretation of the role of Al₂O₃ additions is proposed in terms of phase and microstructure modifications. Using LRC electrical equivalent circuits, the impedance variations close to the resonance frequency are modeled: the increase of the resistance R (electrical losses) and of the capacitance C are respectively correlated to the change in microstructure and in nature of PLZT initial phase.     

Effects of oxide thickness, Al2O3 interlayer and interface asperity on residual stresses in thermal barrier coatings

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al₂O₃ interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in TBC during cooling process were interpreted using concentric-circle model. The results were coupled with finite element method. The influences of interface asperity and interface topography on the distribution of residual stresses normal to interfaces in TBC were also discussed.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

Dielectric,humidity behavior and conductivity mechanism of Mn<Subscript>0.2</Subscript>Ni<Subscript>0.3</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrite prepared by co-precipitation method

Mn–Ni–Zn ferrite with the chemical formula of Mn_0.2Ni_0.3Zn_0.5Fe₂O₄ was prepared by co-precipitation method. The X-ray diffraction (XRD) results show that the prepared sample crystallizes in the cubic spinel structure with the space group of Fm3m. The morphological analysis of the sample was investigated by scanning electron microscopy (SEM). The dielectric properties of Mn_0.2Ni_0.3Zn_0.5Fe₂O₄ ferrite were studied in a frequency range from 20 Hz to 10 MHz and at a temperature range from 293 to 733 K. The dielectric constant decreases with the increasing frequency for all the temperature values chosen. The AC conductivity mechanism was found the small polaron type of conductivity, and in addition to that, the DC conductivity can be explained by Arrhenius type conductivity. According to the dielectric results, relaxation process fits Cole–Cole model. Finally, the effect of the relative humidity upon the impedance of the sample was discussed for a frequency range between 20 Hz and 10 MHz. It is found that the impedance values decrease almost linearly with the increasing % RH (relative humidity) values at low frequencies, while the impedance of the sample is independent of % RH at high frequencies.     

Decimeter-wave impedance measurement of Y- and Tl-based superconductors with and without an external magnetic field

The real part of low-temperature surface impedance of samples of high-T _c superconductors in the YBa₂ Cu₃O_6+x ceramic system was studied by the method of registration of the ownQ-factor of coaxial shortringed cylindrical resonator constructed entirely from examined material. For ceramic samples of high-T _c superconductors in the Tl₂Ba₂Ca₂Cu₃O system the same measurements were made with the help of coaxial Al resonator. The wavelength of radio frequency field equals 70cm. The dependence of the impedance of Y-based superconductors on weak magnetic field at helium temperature is close to the square law. The magnitude of impedance at temperature of liquid helium for Y-based superconductors composes 0,04 ohm, for Tl-based superconductors −0,01 ohm respectively which correlates with the statement of the presence of giant resonant absorption of radio frequency field power.     

Impedance spectroscopy study of polycrystalline Bi6Fe2Ti3O18

The electrical properties of polycrystalline Bi₆Fe₂Ti₃O₁₈ are investigated by impedance spectroscopy in the temperature range 30–550°C. The imaginary part of impedance as a function of frequency shows Debye like relaxation. Impedance data are presented in the Nyquist plot which is used to identify an equivalent circuit and the fundamental circuit parameters are determined at different temperatures. The grain and grain-boundary contributions are estimated. The results of bulk a.c. conductivity as a function of temperature and frequency are presented. The activation energies for the a.c. conductivity are calculated. The polaron hopping frequencies are estimated from the a.c. conductivity data.     

High oxygen ion conduction in some Bi2O3-Y2O3 (Er2O3) solid solutions

Highly densified bodies (~ 98 % theoretical density) of the Bi₂O₃-Y₂O₃(Er₂O₃) systems containing 30 moles % Y₂O₃ and 20 moles % Er₂O₃ respectively were prepared from both mixed oxides and coprecipitated oxalates. DC ionic conductivity and impedance complex plane analysis on sintered samples were performed. Under oxygen partial pressure ranging from 1 to 10 Pa was found that, samples containing 30 moles % yttria showed a pure ionic conductivity up to an oxygen partial pressure of 10⁻²⁰ Pa. Samples containing 20 moles % erbia, extended its ionic conductivity up to an oxygen partial pressure of 10⁻²² Pa. The impedance analysis shows the presence of only one semicircle at low tempertures and it is attributed to bulk conductivity contribution. The conductivity was higher for the Bi₂O₃-Er₂O₃ sintered solid electrolytes, in such a case, a conductivity as high as 1.38 ohm cm at 700ºC was obtained. The activation enthalpy for the conduction process was calculated in the temperature range from 200º to 700ºC in all the cases. Microstructural development in the sintered sample was also studied.     

Impedance spectroscopic characteristics of Bi2Fe2W3O15 ceramics

Bi₂Fe₂W₃O₁₅ was prepared in the polycrystalline form using a standard solid-state reaction technique in order to study its dielectric and electrical properties. The formation of a single-phase compound was confirmed by preliminary X-ray structural studies of the material. Studies of electrical properties (impedance, modulus and conductivity) of the compound over a wide range of temperature and frequency provide many interesting results. The impedance and modulus parameters were calculated using complex plane formalism, and suitable equivalent circuits have been proposed for different temperature and frequency regions. The nature of variation of ac conductivity with frequency at different temperatures obeys the Jonscher’s universal power law. The temperature-dependence of dc conductivity pattern follows the Arrhenius behavior.     

Effect of simultaneous substitution of magnesium and niobium on dielectric properties and phase transition temperature of bismuth sodium barium titanate ceramics

(Bi_1/2 Na_1/2)_0.94Ba_0.06Ti_1?x (Mg_1/3Nb_2/3) _x O₃ ceramic samples with x = 0.0, 0.01, 0.05, 0.15, 0.20 were synthesized by solid state method. Microstructure, dielectric properties, impedance and conductivity of the ceramics were studied. Phase formation was confirmed by X-ray diffraction. Co-doping of the ceramics with Mg and Nb at x = 0.01 raised the dielectric constant from 6510 to 8225 at the frequency of 1 KHz. Further increase in (Mg_1/3Nb_2/3)⁴⁺ concentration up to 0.15 increased the transition temperature from 275 °C to 339 °C and lowered the dielectric constant. The ac impedance measurements showed a linear response with frequency at lower temperature indicating insulating behavior and a single semicircular arc with spike at higher temperature.     

Effect of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> on the dynamics of Li<Superscript>+</Superscript> ions in Li<Subscript>2</Subscript>O·P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Lithium ion transport has been studied in bismuth lithium phosphate glasses in the frequency range 20 Hz–1 MHz and in the temperature range 423–573 K using impedance spectroscopy. The addition of Bi₂O₃ in Li₂O·P₂O₅ glass is related to the modification of the glass structure and facilitates the Li⁺ ions migration. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency are extracted from the impedance spectra. Conductivity of the present glass system is found to be ionic in nature. The electrical response of the glasses has been studied using both conductivity and electric modulus formalisms. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates the temperature independence of the dynamic processes for ions in these glasses. Nearly identical values of activation energy for dc conduction and for conductivity relaxation time indicate that the ions overcome same energy barrier while conducting and relaxing.     

Analytical electron microscopy of the mixed zone in NiCoCrAlY-based EB-PVD thermal barrier coatings: as-coated condition versus late stages of TBC lifetime

Abstract

The microstructural evolution of the alumina-zirconia mixed zone in a NiCoCrAlY-based electron beam physical vapor deposited (EB-PVD) yttria partially stabilized zirconia (Y-PSZ) thermal barrier coating (TBC) system from the as-coated condition into the advanced stages of TBC lifetime is monitored by analytical transmission electron microscopy (TEM). In the as-coated condition yttria-rich islands at the thermally-grown oxide (TGO)/TBC interface locally impede zirconia uptake of the scale during TBC deposition and give rise to the formation of an “off-plane” alumina-zirconia mixed zone textured perpendicular to the TGO/TBC interface. During prolonged isothermal/cyclic oxidation an increased chromium diffusion through the TGO scale turns the mixed zone into a reaction zone introducing a morphological instability of the mixed zone/TBC interface due to solutioning of the bottom TBC layer.

This microstructural pattern is corroborated by a triple-stage growth model for the mixed zone during three successive stages in TBC lifetime: (i) during TBC deposition, the thickness of the mixed zone increases due to predominant outward aluminum diffusion and uptake of zirconia. No columnar alumina zone (CAZ) has formed at this stage, (ii) upon completion of the transition alumina-to-corundum phase transformation the thickness of the mixed zone remains constant while the change in diffusion mechanism for an inward oxygen diffusion process now initiates parabolic growth of the columnar alumina sublayer of the TGO scale, (iii) in the late stage of TBC lifetime an marked outward chromium diffusion from the bond coat causes the mixed zone to resume growth due to TBC destabilization and the formation of a (Al, Cr)₂O₃ mixed oxide matrix phase.

A transient YCrO₃ phase is proposed for driving the destabilization of yttria-rich sections of the bottom TBC layer.     

Dielectric and electrical properties of 0.5(BiGd0.05Fe0.95O3)-0.5(PbZrO3) composite

The 0.5(BiGd_0.05Fe_0.95O₃)-0.5(PbZrO₃) composite was synthesized by means of a high temperature solid-state reaction technique using high purity ingredients. Preliminary X-ray structural analysis confirms the formation of the composite. The dielectric constant and loss tangent have been studied. The impedance parameters have been measured using an impedance analyzer in a wide range of frequency (10²–10⁶ Hz) at different temperatures. The Nyquist plot suggests the contribution of bulk effect only and the bulk resistance decreases with a rise in temperature. Electrical impedance confirms the presence of grain effect and hopping mechanism in the electrical transport of the material. The dc conductivity increases with a rise of temperature. The frequency variation of ac conductivity shows that the compound obeys Jonscher’s universal power law and from Jonscher’s power law fit confirms the Small Polaron (SP) tunneling effect. Temperature dependence of dc and ac conductivity indicates that electrical conduction in the material is a thermally activated process.     

Structural and electrical properties of Ba(Fe<Subscript>1/3</Subscript>Nb<Subscript>1/3</Subscript>Ta<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> perovskite

Ba(Fe_1/3Nb_1/3Ta_1/3)O₃ (BFNT) perovskite compound (phase purity>99%) was synthesized by conventional ceramic preparation method. XRD, microstructure, impedance spectroscopy and ac conductivity properties were analyzed in this study. BFNT compound has a cubic crystal structure, having grain size of 0.31 μm. This compound has shown normal ferroelectric behaviour but not obeying Curie-Weiss law. The impedance and electrical studies have been performed as a function of frequency and temperature. Impedance as a function of frequency revealed single relaxation process. The impedance spectroscopic plots exhibit the major response due to grains with partial contribution from the grain boundary and negligible electrode effect. Complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. Also, the centre of semicircle lies below the real axis indicating non-Debye type relaxation. Relaxation time was calculated from Z″_max of Cole–Cole plots. It is observed that conduction is due to hopping of charge carriers. Activation energies were computed from the Arrhenius plots of the sample.     

Broadband Microwave Measurements of Overdoped Y0.9Ca0.1Ba2Cu3O7−δ Films Using Corbino Geometry

The surface impedance of Y_0.9Ca_0.1Ba₂Cu₃O_7?δ thin films was measured using the Corbino spectroscopy method. This special geometry, in which the sample dimensions are well defined by a ring pattern, is ideal for broadband high frequency reflection measurements. Using the complex reflected signal, S ₁₁, measured by a vector network analyzer, one can find the surface impedance of the thin film, from which the complex conductivity can be deduced. In the current work we present the three-standard approach for calibration of the Corbino method and demonstrate the benefits of this approach in measuring superconducting Y_0.9Ca_0.1Ba₂Cu₃O_7?δ thin films up to 20 GHz and down to 6 K. For the data analysis the well-known generalized two-fluid model was implemented, taking into account a film thickness which is much smaller than the normal state skin depth and superconducting penetration depth.     

Dielectric properties and high-temperature dielectric relaxation of tungsten-bronze structure ceramics Ba2GdFeNbTa3O15

Ba₂GdFeNbTa₃O₁₅ was synthesized by a standard solid-state technique, which adopts a tetragonal filled tungsten bronze structure and exhibits paraelectric nature at room temperature. The dielectric temperature coefficient of Ba₂GdFeNbTa₃O₁₅ at 1 MHz is 147 ppm °C_.⁻¹ AC impedance plots were used as tools to analyze the electrical behavior of the sample as a function of frequency at different temperatures. The conduction is a thermally activated process with activation energy ~1.30 eV. The frequency-dependent maxima in the imaginary part of impedance are found to obey an Arrhenius law with activation energy ~1.34 eV. Such a value of activation energy suggests the existence of a relaxation mechanism (a conductive process), which may be interpreted by an ion hopping between neighboring sites within the crystalline lattice.     

An impedance spectroscopy study of oxide films formed during high temperature oxidation of an austenitic stainless steel

Impedance spectroscopy has been used to study an oxide film formed on an AISI304 austentic stainless steel by oxidation at 800°C for 200 h. Impedance spectra of the oxide film clearly showed two semicircles, which correspond to two independent oxide layers present in the oxide film. The oxide film was also examined by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Although it was difficult to identify the two phases in the oxide film using SEM, XRD showed the presence of Cr₂O₃ and MnCr₂O₄. In addition, surface analysis of the oxide film using XPS showed the presence of MnCr₂O₄. By comparing the relaxation frequencies of Cr₂O₃ and MnCr₂O₄ with those of the two semicircles, it is identified that the semicircles in the impedance spectra correspond to Cr₂O₃ and MnCr₂O₄. Furthermore, the electrical properties of both Cr₂O₃ and MnCr₂O₄ in the oxide film have been determined by impedance measurements, indicating that the chromia layer is apparently thicker than the spinel layer.     

Complex impedance analysis of Sr(NO3)2 and Sr(NO3)2:γ-Al2O3 dispersed solid electrolyte systems

Complex impedance spectroscopic studies were carried out on Sr(NO₃)₂ and Sr(NO₃)₂:-Al₂O₃ dispersed solid electrolyte systems (DSES) in the temperature range 300 to 560° C and the frequency range 100 Hz to 1 MHz. Mole percentage of the dispersoid -Al₂O₃ was varied from 17.2 to 34.2. The d.c. conductivity estimated from the Cole-Cole plots was found to increase with the mole percentage of the dispersoid. Dissipation was found to vary in a manner similar to the variation of conductivity with temperature. A.c. conductivity was found to be frequency dependent in the extrinsic region, but frequency independent in the intrinsic region. The enhanced conductivity in DSES was attributed to the formation of a space-charge layer between the host material and the dispersoid.