Ping Xiao

Fabrication of ceramic composite coatings using electrophoretic deposition, reaction bonding and low temperature sintering

We have developed a novel combination of electrophoretic deposition (EPD), reaction bonding and low temperature sintering techniques for the fabrication of yttria stablised zirconia (YSZ)/alumina composite coatings on Fecralloys. A mixture of ethanol and acetylacetone solvent was found to be an effective medium for YSZ and aluminium particle suspension. With the particle size of YSZ and aluminium being significantly reduced during ball milling. By using the EPD process, uniform green form coatings containing YSZ and aluminium particles were produced on Fecralloys. After oxidation of aluminium at 500°C and sintering at 1200°C, a dense and adherent YSZ/Al2O3 coating was produced. The presence of aluminium in the green form coatings not only contribute to the bonding between the coating and the metal substrate, but also compensate for the volume shrinkage of the coatings during sintering by the volume expansion arising from oxidation of aluminium to alumina.

Effect of microstructural features on the electrical properties of TiO2

Abstract The effect of microstructural features on the electrical properties of TiO2 (rutile) was studied using impedance spectroscopy in conjunction with X-ray diffraction and scanning electron microscopy. The electrical properties of TiO2 were measured at 350 °C after sintering in air at different temperatures. It was found that the electrical resistivity increased significantly after the interconnected pores closed in the TiO2 due to sintering. However, further decrease of porosity in the TiO2 had little effect on its resistivity prior to the closure of pores. On the other hand, after the sintered specimens were annealed in vacuum, the resistivity of TiO2 decreased considerably without apparent change of the microstructure in TiO2. Moreover, the resistivity, measured at room temperature, decreased after the interconnected pores closed due to sintering. These results suggest that the electrical conduction in TiO2 after sintering in air is mainly due to ionic conduction, while the conduction in the sintered TiO2 after annealing in vacuum was mainly due to electronic conduction. The closure of the open pores in the TiO2 reduced the surface ionic conduction but increased the electronic conduction route. Overall, the microstructural features of TiO2 have characteristic effects on its electrical properties.

Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

Abstract In this work, impedance spectroscopy (IS) has been used to evaluate the degradation of thermal barrier coatings (TBCs) due to oxidation at 1150 o C. The spallation of TBCs was found to be induced by the development of thermally grown oxides (TGO) produced from the oxidation of TBCs. The change in the electrical properties of the TGOs was found to be related to the change in the microstructure and microchemistry of the TGO. The resistivity of the TGO due to oxidation from 10 to 1000 h decreased rapidly, which corresponded to the increase in porosity in the TGO and the compositional change of the TGO from α-Al 2 O 3 to a mixture of α-Cr 2 O 3 and (Ni,Co)(Cr,Al) 2 O 4 spinel. The slow decrease in the resistivity of the TGO from oxidation for 1000–2000 h indicated that there was little change in the composition of the mixed oxides, although the growth rate of the TGOs was relatively fast during this oxidation period. The disappearance of α-Al 2 O 3 in the TGO caused a rapid oxide growth during oxidation and led to the spallation of TBCs.

Non-destructive evaluation of thermal barrier coatings using impedance spectroscopy

Abstract Determining the oxidation of thermal barrier coatings (TBCs) non-destructively is essential for monitoring the performance of TBCs and predicting the lifetime of TBCs in service. In this research, impedance spectroscopy was used, as a non-destructive tool, to examine the oxidation of thermal barrier coatings (TBCs). Impedance diagrams obtained from impedance measurements at 623 K, were analysed according to equivalent circuit models of the oxidised TBCs. TBCs after oxidation at 1373 K consist of yttria stablised zirconia as a top coat, an alumina layer from the oxidation and a MCrAlY alloy as a bond coat. When the oxidation time is less than 200 h, the impedance spectra of TBCs can be fitted by using an equivalent circuit model, which represents a discontinuous layer of alumina between the top coat and bond coat. When the oxidation time is between 400 and 1500 h, the impedance spectra of TBCs can be fitted by using a simple equivalent circuit model, which represents a three-layer structure consisting of a continuous alumina layer, a mixed oxide layer and the top coat. In this case, the thickness of the alumina layer can also be measured by simulating the electrical modulus spectra of TBCs. Therefore, impedance spectroscopy is a powerful tool for non-destructive evaluation of oxidation of TBCs.

Fabrication of composite coatings using a combination of electrochemical methods and reaction bonding process

The major difficulty in fabricating ceramic coatings on metal substrates using the electrophoretic deposition process (EPD) is problems caused by the volume shrinkage during the sintering of the green form ceramic coatings produced by EPD. Numerous cracks normally form in the EPD coating during sintering. In this work, we have developed the reaction bonding process to fabricate crack-free and dense ceramic coatings, where the volume shrinkage is compensated by the volume expansion due to the oxidation of aluminium in the green form coatings during sintering in air. Both EPD and electroplating were used here to produce green form coatings which contain aluminium particles and, in some cases, an intermediate nickel layer. During the subsequent heat treatment, melting and oxidation of the metals in the green form coating promote densification during sintering. By these means, relatively dense composite coatings have been fabricated on metal substrates.

Fabrication and characterisation of La0.8Sr0.2MnO3/metal interfaces for application in SOFCs

Abstract Fabrication and characterisation of La 0.8 Sr 0.2 MnO 3 (LSMO)/metal interfaces are important for the application of LSMO as the cathode and the metal as the interconnector in solid oxide fuel cells (SOFCs). Interfaces between LSMO and either Fecralloy, or Cr–5Fe–Y 2 O 3 , or Pt metal were fabricated by using screen printing, followed by sintering at 1200°C in different atmospheres. The microstructures of these LSMO/metal interfaces were examined using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and X-ray diffraction analysis. The electrical properties were characterised using impedance spectroscopy. The LSMO/Fecralloy interface fabricated in air shows a high electrical resistance (about 10 5 Ω cm 2 at 800°C) due to the formation of an interlayer consisting of alumina and the mixed oxide M 2 O 3 · n Al 2 O 3 at the interface during the fabrication process. When the interface was fabricated at 1200°C in argon or vacuum, a thicker interfacial layer was formed compared with that formed in air. In addition, the La 0.8 Sr 0.2 MnO 3 decomposed into (La 0.8 Sr 0.2 ) 2 MnO 4+ λ , (La, Sr) 2 O 3 and MnO. Annealing of the specimen at 1200°C for 5 h in air can partially reverse the decomposition. The interface resistance of the annealed sample across the interface is one order of magnitude lower than that of an interface fabricated in air. A thick Cr 2 O 3 layer was found at the LSMO/Cr–5Fe–Y 2 O 3 interface fabricated in air and a spinel phase (Mn, Cr) 3 O 4 formed a network surrounding LSMO grains due to the reaction between LSMO phase and CrO 3 vapour released from the Cr 2 O 3 layer during the fabrication. The electrical resistances of both phases are high at room temperature, but become significantly lower at 400°C. They can be negligible above 500°C. However the evaporation of CrO 3 increases the resistivity of the LSMO layer. No reaction or oxidation interlayer was found between LSMO and Pt metal producing an interface resistance of 0.52 Ω cm 2 at room temperature.

Joining alumina using an alumina/metal composite

Abstract Alumina/alumina (Al 2 O 3 /Al 2 O 3 ) joints and alumina/aluminium metal–matrix composite (Al 2 O 3 /Al–MMC) joints were fabricated using an Al 2 O 3 –Al composite as an interlayer. The Al 2 O 3 /Al 2 O 3 joining procedures involved the sintering of silica or silicate glass on the surface of the Al 2 O 3 , followed by the reactive penetration of liquid aluminium into the sintered glass to form an Al 2 O 3 –Al composite interlayer between the two Al 2 O 3 pieces. To join Al 2 O 3 to Al–MMC, after the fabrication of the Al 2 O 3 –Al composite layer on the surface of Al 2 O 3 using the combination of sintering and reactive penetration, Al 2 O 3 /composite/MMC joints were fabricated by using diffusion bonding between the Al 2 O 3 –Al composite layer and the MMC. The microstructures, phase composition and mechanical properties of the joints were examined using scanning electron microscopy (SEM), XRD analysis and shear testing, respectively. Experimental results showed that the high porosity in the sintered glass layer led to the formation of a porous Al 2 O 3 –Al composite layer between two Al 2 O 3 pieces. The addition of cordierite (2MgO·2Al 2 O 3 ·5SiO 2 ) to the pure silica lowered the melting point of the glass and led to the formation of a dense glass layer at the surface of the Al 2 O 3 . Consequently, a dense composite layer was produced. Stronger joints were fabricated with shear strengths of the Al 2 O 3 /Al 2 O 3 and Al-MMC/Al 2 O 3 joints of 105.2 and 47.6 MPa respectively.

Determining oxide growth in thermal barrier coatings (TBCs) non-destructively using impedance spectroscopy

Thermal barrier coatings (TBCs) have been applied to gas turbine blades and vanes for providing thermal protection and corrosion resistance. One of the major concerns for application of TBCs is their reliability and durability. It has been established that the oxidation of bond coat underlying yttria stablized zirconia (YSZ) is a dominant factor in controlling the failure of TBCs [1, 2]. The oxidation resistance of bond coat is attributed to the formation of a protective oxide layer at the surface of bond coat. The growth of the oxide layer increases the stress at the bond coat/YSZ interface and leads to the spallation of TBCs [3]. The thickness of the oxide layer is therefore a useful indicator of life expectancy of TBCs. For this reason the non-destructive detection of oxidation layer growth is of great importance in the application of TBCs. Impedance is a cheap and quick non-destructive testing method. It is particularly efficient in characterizing multi-layer materials when different layers have very different electrical properties [4, 5]. In this investigation, we have successfully used impedance spectroscopy in determining the alumina layer growth in TBCs. The TBCs used in this investigation consist of the top coat YSZ (8 wt% yttria stabilized zirconia) and the bond coat MCrAlY (with 38.5wt%Co-32wt%Ni21wt%Cr-8wt%Al-0.5wt%Y). The substrate is Haynes 230 alloy. The TBC samples were cut into 10 mm by 10 mm squares. The samples were oxidized at 1100 ◦C for 400–1500 h. Impedance measurements were made using a Solartron SI 1255 HF Frequency Response Analyser coupled with a 1296 Dielectric Interface (Solartron, UK.) with applied voltage of 1 volt at frequencies from 106 Hz to 10−3 Hz, with 5 data reading per decade. Electrodes were applied by painting a thin layer of silver ink on the top coat and subsequent firing at 400 ◦C for 15 min. Then impedance measurements were made at 350 ◦C. Microstructure of the TBC samples was examined using SEM coupled with energy dispersive X-ray spectroscopy (EDS). Fig. 1 shows the Nyquist plot obtained from impedance measurements of TBCs. For all the oxidized samples, the Nyquist plot is composed of two semicircles: one is a small semicircle at high frequency (HF), another is a large semicircle at low frequency (LF). The small HF semicircle can be identified clearly in an enlarged diagram of Fig. 1. By comparison with the impedance spectrum of the as-received TBC without

Characterisation of clay sintering process using impedance spectroscopy

Abstract Impedance measurements were made in situ while a clay compact was being fired at different temperatures. The measured impedance spectra consist of a high frequency (HF) semicircle arc and a low frequency (LF) tail. By employing an equivalent circuit of the clay compact to simulate the impedance spectra, we have obtained values for electrical properties and the parameters of constant phase elements (CPEs) corresponding to both bulk specimen and electrode effects. Both Arrhenius plot of specimen conductance and the dielectric loss curve demonstrate a phase transition occurring in the clay at a temperature between 900 and 950xa0°C. Such a phase transition was believed to be the formation of a liquid phase, which was also confirmed by using XRD technique and dilatometer analysis. The variation of specimen conductance as a function of time, which was obtained from isothermal impedance measurements, was found to be in agreement with the shrinkage curve. An equation relating the electrical conductance to the density of the specimen has been established, which has been verified by examining the densification of clay during sintering. Therefore, it is promising to use impedance measurements for examining the sintering of a wide range of ceramics in situ.

Nondestructive characterisation of alumina/silicon carbide nanocomposites using impedance spectroscopy

Abstract Nondestructive evaluation (NDE) of ceramic matrix composites is essential for developing reliable ceramics for industrial applications. In the research described here, impedance spectroscopy has been used to characterise Al 2 O 3 /SiC nanocomposites nondestructively. Electrical modulus spectra from impedance measurements were used to determine the content of SiC nanoparticles in Al 2 O 3 /SiC composites. Meanwhile, electrical impedance measurements have been used to characterise the oxidation of Al 2 O 3 /SiC nanocomposites. Based on the microstructural features of the nanocomposites, equivalent models were developed to calculate the capacitance of the nanocomposites and oxidised specimens. The calculated results were used (i) to examine the relationship between the composition and electrical properties of the Al 2 O 3 /SiC nanocomposites; (ii) to predict the thickness of oxide scales formed at the surface of the nanocomposites after oxidation. The comparison showed reasonable agreements between theoretical prediction and experimental results.