Microscopic observation of degradation behavior in yttria and ceria stabilized zirconia thermal barrier coatings under hot corrosion

In this study, the effects of yttria stabilizer replacement with ceria on hot corrosion properties of thermal barrier coatings (TBCs) have been investigated in microscopic respects. Yttria stabilized zirconia (YSZ; ZrO₂–8 wt.%Y₂O₃) and ceria stabilized zirconia (CSZ; ZrO₂–25 wt.%CeO₂–2.5 wt.%Y₂O₃) thermal barrier coatings (TBCs) were fabricated to have similar microstructures, followed by hot corrosion tests with a NaVO₃ salt at 900 °C. Very dense YSZ TBCs with porosities below 4% were also fabricated in order to investigate the effects of coating defects on hot corrosion behavior. Results revealed that CSZ TBCs were better resistant to hot corrosion environment than YSZ TBCs in terms of phase stability and overall damage by the salt. From studies on microscopic failure behavior, it was found that YSZ and CSZ TBCs showed a different degradation behavior and that coating defects, such as pores, microcracks and splat boundaries, play important roles as effective paths for the salt in hot corrosion. From the present results, a possible microscopic degradation mechanism and effects of coating microstructures for zirconia-based TBCs under hot corrosion are discussed.     

Novel thermal barrier coatings that are resistant to high-temperature attack by glassy deposits

Airborne sand particles that deposit on thermal barrier coatings (TBCs) in gas-turbine engines melt and form calcium–magnesium–aluminosilicate (CMAS) glass, which attacks the TBCs. A new approach for mitigating CMAS attack on TBCs is presented, where up to 20 mol.% Al₂O₃ and 5 mol.% TiO₂ in the form of a solid solution is incorporated into Y₂O₃-stabilized ZrO₂ (YSZ) TBCs. The fabrication of such TBCs with engineered chemistries is made possible by the solution-precursor plasma spray (SPPS) process, which is uniquely suited for depositing coatings of metastable ceramics with extended solid-solubilities. Here, the TBC serves as a reservoir of Al and Ti solutes, which are incorporated into the molten CMAS glass that is in contact with the TBC. This results in the crystallization of the CMAS glass and the attendant arrest of the penetrating CMAS front. This approach could also be used to mitigate attack by other types of foreign deposits (salt, ash, and contaminants) on TBCs.     

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Effect of Y203 on microstructure and oxidation of γ-Ni＋γ′-Ni3Al coatings transformed from electrodeposited Ni-Al films at 1 000℃

The electrodeposited Y2O3-dispersed γ-Ni＋γ-Ni3Al coatings on Ni substrates were developed by the conversion of electrodeposited Ni-Al-Y2O3 films with dispersed AI microparticles in Ni matrix into Ni3Al by vacuum annealing at 800 ℃ for 3 h. For comparison, Y2O3-free γ-Ni＋γ＇-Ni3Al coatings with a similar AI content were also prepared by vacuum annealing the electrodeposited microparticle-dispersed composite coatings of Ni-AI under the same condition. SEM and TEM characterizations show that the electrodeposited Y2O3-dispersed γ＋γ＇ coatings exhibit finer grains, a more homogeneous distribution of γ＇, and a narrowed γ＇ phase spacing compared with the electrodeposited Y2O3-free γ-γ＇ coatings. The oxidation at 1 000 ~C shows that the addition of Y2O3 significantly improves the oxidation resistance of the electrodeposited γ＋γ＇coatings. The effect of Y2O3 particles on the microstructure and oxidation behavior of the electrodeposited γ＋y＇ coatings was discussed in detail.     

Microcrack formation in plasma sprayed thermal barrier coatings

Air plasma sprayed ZrO₂–8wt%Y₂O₃ thermal barrier coatings were deposited under tightly controlled conditions. The lengths and orientations of the horizontal cracks and vertical cracks in these coatings were characterized in detail, and process/structure maps of the crack distribution as a function of particle and substrate states were constructed. A fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. The model also showed that surface roughness plays a key role in determining the magnitude of maximum stresses, and that only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration.     

Preparation and properties of a γ-Al2O3 washcoat deposited on a ceramic honeycomb

Washcoat deposited on ceramic honeycomb was prepared using pseudoboehmite, the CeO₂–ZrO₂–La₂O₃ solid solution, pore enlarger and other additives. The microstructures and surface performances of washcoat/honeycomb were investigated by SEM, BET surface area, XRD, ultrasonic vibration and hot shock simulation. The results show that the performance and loading of washcoat are affected obviously by the properties of slurry gel, such as the apparent viscosity, solid content, particle size and its distribution. When the apparent viscosity of slurry is lower, the gel with a narrow particle size distribution and finer particles can be obtained, with which the coating having an excellent performance can be prepared. Adding a small quantity of the CeO₂–ZrO₂–La₂O₃ solid solution can promote the thermal stability of washcoat, such as, after calcined at 1000 °C for 5 h the sample exhibits mainly the γ-Al₂O₃ phases and the θ-Al₂O₃ -Al₂O₃ and κ-Al₂O₃ phases have not been detected in the XRD spectra. It is found also that the washcoat prepared has excellent properties of the vibration-resistant, heat-resistant and its BET surface area reaches 50 m²/g.     

Mechanical loss of cubic zirconia

Cubic zirconia can be stabilized by doping with lower valent oxides such as Y₂O₃ or CaO. Oxygen vacancies are then created as charge compensating defects. Mechanical loss measurements were performed in the temperature range 300–1600 K using both free decaying (3 Hz, 3 kHz) and forced vibrations (10⁻²–10 Hz). The influence of Y₂O₃ (10–24 mol%) and CaO (10–16 mol%) on the loss spectra was studied. The low temperature spectra (<1000 K) show a composite loss maximum which can be decomposed into two peaks, I and I_A (I′, I_A′). Both peaks, with activation enthalpy values between 1 and 2 eV, rely on local jumps of oxygen vacancies which are trapped by dopant cations. Submaxima I (I′) are assigned to defect pairs of oxygen vacancies and dopant ions (Y or Ca) forming elastic (electric) dipoles. These are oriented parallel to 111 (trigonal symmetry) with the vacancies on nearest neighbor sites. Maxima IA (I_A′) are assigned to relaxation of vacancies within larger clusters and interaction effects (Y or Ca). The high temperature loss spectra (≥1000 K) show various relaxation phenomena. In Y₂O₃ stabilized ZrO₂ we observe around 1400 K a loss peak with frequency independent temperature position which is assigned to a structural phase transition. ZrO₂–CaO shows a high temperature peak of Debye type (H=4.0±0.5 eV) which is related to local diffusional jumps of cations via vacancies.     

Porous AlN ceramic substrates by reaction sintering

A novel approach was undertaken in producing porous AlN microelectronics tapes with high thermal conductivity and low dielectric constant. This method essentially utilised polymer micro-spherical powders that were used as a sacrificial mould to introduce controlled porosity into the green tapes during pyrolysis. The Al₂O₃-rich porous green tapes were then reaction sintered at 1680 °C for 12 h to achieve porous AlN tapes. This work builds upon the previously developed novel reaction sintering process that densified and converted Al₂O₃-rich tapes (Al₂O₃–20 wt.% AlN–5 wt.% Y₂O₃) to AlN tapes at a relatively low sintering temperature of 1680 °C. The sintering behaviour of the porous tapes was investigated, and the effects of the microspheres particle size and volume addition were studied. The microspheres successfully contributed to the significant reduction of tape density by porosity, and this contributed to lowering its dielectric constant. Dielectric constant of the AlN tapes were reduced to about 6.8–7.7 whilst thermal conductivity values were reasonable at about 46–60 W/m K. Coefficient of thermal expansion (CTE) values showed a linear trend according to phase composition, with the porous AlN tapes exhibiting CTE values of (4.4–4.8)×10⁻⁶ °C⁻¹, showing good CTE compatibility with silicon, at 4.0×10⁻⁶ °C⁻¹. The added porosity did not significantly affect the CTE values.     

Formation of Al–Y oxide during processing of γ-TiAl

While processing Y₂O₃ dispersed γ-TiAl, Y₂O₃ particles which dissolved during hot isostatic pressing (HIP’ing) were found to precipitate during the heat treatment in the form of a mixed Al–Y oxide. To understand the chemical reaction that occurs between Y₂O₃ and γ-TiAl during the heat treatment cycle, a powder mixture comprising of γ-TiAl and 10 wt.% Y₂O₃ was mechanically alloyed (MA’d) for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 °C prior to analyzing it using X-ray diffraction technique. The present study clearly demonstrates that aluminum in the combined form either as γ-TiAl or Al₂O₃ reacts in a similar manner with Y₂O₃ when milled and heat treated at 1150 °C. In either case there is formation of Al₂Y₄O₉ (2Y₂O₃.Al₂O₃).     

Pore morphology in sintered ZrO2–8 mol% Y2O3 ceramic: a small-angle neutron scattering investigation

Pore morphology and pore size distribution in yttria-stabilized zirconia (ZrO₂–8 mol% Y₂O₃) have been investigated, for two sintering temperatures, namely 1200 and 1270 °C, using small-angle neutron scattering. The results show that the reduction in the porosity, at 1270 °C compared to that at 1200 °C, occurs by the elimination of the pores at the lower end of the pore size distribution. In addition, the polydispersity is also lower at 1270 °C and the nature of the distribution is altered significantly near the smaller radius range. The average pore size shifts towards the higher radius range. The specific surface area of the pores is also diminished at 1270 °C because of the elimination of the finer pores.     

Development of moderate temperature CVD Al2O3 coatings

Chemically vapor deposited Al₂O₃ coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al₂O₃ coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al₂O₃. To overcome this limitation, alternate systems to deposit Al₂O₃ at moderate temperatures have been investigated. Systems using NO–H₂, H₂O₂, NO₂–H₂ and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al₂O₃ in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates.     

Achieving tensile superplasticity in 8 mol% Y2O3 cubic stabilized ZrO2 through the addition of intergranular silica

The addition of 5 wt.% SiO₂, a viscous second phase, to 8 mol% Y₂O₃ cubic stabilized ZrO₂ (8Y-CSZ) made superplastic 8Y-CSZ. This material had a fine grain size of 0.4 μm and exhibited deformations in tension as large as 520% at 1430 °C with a strain rate of 1.0 × 10⁻⁴ s⁻¹.     

Synthesis and characterization of metal oxide multilayers obtained via MOCVD as protective coatings of graphite against oxidation

Zirconia (ZrO₂) titania (TiO₂) and alumina (Al₂O₃) thin films were deposited on a graphite substrate both in mono- and in multi-layer systems, using the metal organic chemical vapor deposition technique, to test their practical qualities as protective coatings against oxidation at high temperatures. The depositions were performed using a hot wall reactor at reduced pressure (0.6 Torr) in the temperature range 350–500 °C, using, as precursors, (η⁵-C₅H₅)₂Zr(CH₂C(CH₃)₃)₂, Ti(OCH(CH₃)₂)₄, and (CH₃CH₂)₂Al(OCCH₃CHCCH₃O), respectively. Surface and topographical analysis of the deposits using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy techniques as well as thermogravimetric measurements (TG and DTA) in an oxygen flux of mono- and multi-layer systems are reported and examined.     

The effect of pack-aluminisation on the microstructure of MCrAlY and the performance of thermal barrier coatings

Plasma spraying and pack-aluminising processes were combined and applied to the nickel-base superalloy Mar-M247 to improve its cyclic oxidation resistance. The performance tests of duplex ZrO₂-8 wt.%Y₂O₃/MCrAlY thermal barrier coatings (TBCs) were conducted at 1050 °C, 1075 °C, 1100 °C, 1150 °C and 1200 °C. The results of the experiments in this study showed that TBC specimens with the aluminised MCrAlY bond coat exhibited higher cyclic lives (except for the Ni-22Cr-10Al-1Y bond coat), at all the temperatures tested, than specimens on which the bond coat was not aluminised. The microstructures of the Co-29Cr-6Al-1Y, Co-32Ni-21Cr-8Al-0.5Y and Ni-22Cr-10Al-1Y bond coats with or without aluminising treatment were examined in detail using a scanning electron microscope equipped with an electron probe microanalyzer.     

The hot corrosion resistance of 20 mol% YTaO4 stabilized tetragonal zirconia and 14 mol% Ta2O5 stabilized orthorhombic zirconia for thermal barrier coating applications

Zirconia stabilized with 3.2–4.2 mol% (6–8 wt.%) yttria (3–4YSZ), the current material of choice for thermal barrier coating applications, is susceptible to hot corrosion by acidic oxides such as vanadia in the 700–900 °C range. The current study is a preliminary examination of the hot corrosion resistance to NaVO₃–V₂O₅ mixtures in the above temperature range of two alternative materials: a tetragonal zirconia co-doped with 10 mol% yttria+10 mol% tantala (20YTaO₄Z) and an orthorhombic zirconia doped with 14 mol% tantala (14TZ). Results show that the 20YTaO₄SZ is resistant to destabilization by NaVO_3, but is attacked at higher V₂O₅ activities, resulting in the formation of YVO₄ and orthorhombic zirconia. Studies on the 14TZ itself then indicated that it is substantially more resistant than the YSZ to attack by environments more acidic (specifically V₂O₅ rich) than pure NaVO₃. However, it is less suitable than either 20YTaO₄SZ or 3–4YSZ for environments that are more basic. A comparison of the resistance of the 14TZ, the 3–4YSZ and the 20YTaO₄Z shows that the 20YTaO₄SZ is more resistant to acidic oxides than the YSZ and more resistant to the basic oxides than the 14TZ.     

Thermoelectric properties of Fe0.98Co0.02Si2 with ZrO2 and rare-earth oxide dispersion by mechanical alloying

The n-type Co-doped β-FeSi₂ (Fe_0.98Co_0.02Si₂) with dispersion of several oxides, such as ZrO₂ or several rare-earth oxides (Y₂O₃, Nd₂O₃, Sm₂O₃ and Gd₂O₃), was synthesized by mechanical alloying and subsequent hot pressing. The effects of these oxide dispersions on the thermoelectric properties of Fe_0.98Co_0.02Si₂ were investigated. ZrO₂ was decomposed in the β phase, and the ZrSi and -FeSi phases, which are metallic phases, were formed in the samples with ZrO₂ addition. The Seebeck coefficient and the electrical resistivity were significantly decreased with increasing amount of ZrO₂, indicating that a part of the Zr atoms was substituted for Fe atoms in the β phase. In the case of the samples with rare-earth oxide addition, a decomposition of a large amount of these added oxides did not occur. However, the rare-earth oxide addition caused a slight increase in the amount of the phase. The Seebeck coefficient was significantly enhanced by the rare-earth oxide addition especially in the low temperature range. These facts indicated that a small amount of rare-earth oxides was decomposed in the β phase, and rare-earth elements were substituted for Fe atoms as a p-type dopant, resulting in the decrease in the carrier concentration. The rare-earth oxide addition was also effective in reducing the thermal conductivity.     

Red photoluminescence properties and crystal structure of sodium rare earth oxyborate

The subsolidus phase relations of the system Y₂O₃–Na₂O–B₂O₃ are reported. There are seven binary compounds and two ternary compounds in this system. A new ternary compound Na₂Y₂B₂O₇ is identified. The structure has been determined for the compound Na₂Y₂B₂O₇ from powder X-ray diffraction. The lattice constants of P2₁/c for the compound Na₂Y₂B₂O₇ are a=10.5993(1) Å, b=6.2311(1) Å, c=10.2247(1) Å, β=117.756(1)° and z=4. The structure can be described as being made up of isolated BO₃ triangles and YO₈ polyhedra. The photoluminescence properties of Eu ion-doped Na₂Y₂B₂O₇ and Na₃Y(BO₃)₂ show strong red-emission of the ⁵D₀–⁷F₂ transitions at 611 and 615 nm, respectively. The results of emission spectra are in good agreement with the crystallographic study. The relationship between Eu ion content and emission intensity is analyzed too.     

Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.     

Effects of plasma-sprayed NiCrAl/ZrO2 intermediate on the combination ability of coatings

A NiCrAl/ZrO₂ composite coating was deposited on the surface of metal carrier FeCrAl alloy by a plasma-spray technique. After static-state oxidation at 800°C, the transitions in structure and composition of the coating was analyzed by XRD, SEM and EDX. The results showed that the surface phases of the as-sprayed coating were mainly composed of Ni and ZrO₂. When the oxidation time was extended from 8 to 50 h, NiO crystallites were formed and these grew coarse on the coating surface, and alloy elements were diffused between the NiCrAl/ZrO₂ coating and the FeCrAl substrate. With the pretreatment, an intermediate coating was prepared with a coarse and porous structure, high cohesive strength and high heat resistance. These developed properties could provide high geometric surface area for a catalytic γ-Al₂O₃ washcoat, and enhance the adhesive strength between ceramic washcoat and metal substrate so as to extend the lifetime of the washcoat.