Stabilized Zirconia Thermal Barrier Coatings Research Articles

Hot Corrosion of Lanthanum Zirconate and Partially Stabilized Zirconia Thermal Barrier Coatings

The hot corrosion resistance of lanthanum zirconate and 8wt.% yttria-stabilized zirconia coatings produced by thermal spraying for use as thermal barriers on industrial gas turbines or in aerospace applications was evaluated. The two ceramic oxide coatings were exposed for various periods of time at temperatures up to 1000°C to vanadium- and sulfur-containing compounds, species often produced during the combustion of typical fuels used in these applications. Changes in the coatings were studied using a scanning electron microscope to observe the microstructure and x-ray diffraction techniques to analyze the phase composition. The results showed different behaviors for the two materials: the zirconia-based coating being rapidly degraded by the vanadium compounds and resistant to attack by the sulfur materials while the lanthanum zirconate was less damaged by exposure to vanadia but severely attacked in the presence of sulfur-containing species.

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; ZrO 2–8 wt.%Y 2O 3) and ceria stabilized zirconia (CSZ; ZrO 2–25 wt.%CeO 2–2.5 wt.%Y 2O 3) thermal barrier coatings (TBCs) were fabricated to have similar microstructures, followed by hot corrosion tests with a NaVO 3 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.

Modification of ceramic thermal spray deposit microstructures implementing in situ laser remelting

Yttria partially stabilized zirconia thermal barrier coatings (TBCs) are widely used to protect components of gas turbines against deterioration at high temperatures. Air plasma spray and laser irradiation processes are combined to improve properties of TBCs. An in situ laser remelting plasma-spraying technique, implementing in particular a diode laser, was performed. Results show that laser treatment may induce: (1) no phase transition: the metastable tetragonal phase still remains the predominant phase after laser treatment; (2) a decrease of the pore network connectivity level for a laser energy density ranging from 1.7 to 1.9 J mm −2; (3) the growth of fine dendritic structures (due to the rapid solidification), which could be interesting with regard to the improvement of the thermo-mechanical properties of the coating.

The influence of platinum on the failure of EBPVD YSZ TBCs on NiCoCrAlY bond coats

The failure of electron beam physical vapor deposition yttria stabilized zirconia thermal barrier coatings (TBCs) on NiCoCrAlY bond coats is described. It is shown that defects in the as-processed bond coats are responsible for TBC failures. By using a thin platinum layer deposited upon the NiCoCrAlY bond coat, it is shown that the lives of TBCs on such bond coats can be significantly extended since the as processed defects are removed.

Hot Corrosion of Protective Coatings

Improvement in efficiencies of gas turbine engines requires a significant increase of gas inlet temperatures. This results in an increased service temperature for blade materials and consequently in enhanced oxidation and hot corrosion attack of the blade coatings, which are usually of MCrAlY type where M is Ni, Co or NiCo. This type of coating can provide protection against oxidation and hot corrosion and act as a bond coat for thermal barrier coating systems. In both cases slow growth rates and optimum adherence of the alumina scales forming on the MCrAlY coatings during high temperature exposure are significant for component life. The above mentioned properties for the alumina scales strongly depend on the coating base composition as well as on the presence of minor alloying elements. In the present paper the performance of existing superalloys during hot corrosion is briefly described followed by the results obtained on hot corrosion of MCRAlY type coatings explaining the effect of trace elements on the life of coatings in the presence of NaCl and vanadium containing environments. Optimum thickness to improve the life of superalloys with NiCoCrAlY as a bond coat and yttria stabilized zirconia thermal barrier coatings has been identified. Based on the results, an electrochemical mechanism is proposed and shows that hot corrosion of protective coatings is an electrochemical phenomenon. Hence electrochemical techniques appear to be quite useful in evaluating the coatings for hot corrosion resistance.

Effects of Alloy Composition on the Performance of Yttria Stabilized Zirconia—Thermal Barrier Coatings

Thermal barrier coatings (TBCs) provide an alloy surface temperature reduction when applied to turbine component surfaces. Thermal barrier coatings can be used as a tool for the designer to augment the power and/or enhance the efficiency of gas turbine engines. TBCs have been used successfully in the aerospace industry for many years, with only limited use for industrial gas turbine applications. Industrial gas turbines operate for substantially longer cycles and time between overhauls, and thus endurance becomes a critical factor. There are many factors that affect the life of a TBC including the composition and microstructure of the base alloy and bond coating. Alloys such as Mar-M 247, CMSX-4, and CMSX-10 are materials used for high temperature turbine environments, and usually require protective and/or thermal barrier coatings for increased performance. Elements such as hafnium, rhenium, and yttrium have shown considerable improvements in the strength of these alloys. However, these elements may result in varying effects on the coatability and environmental performance of these alloys. This paper discusses the effects of these elements on the performance of thermal barrier coatings. [S0742-4795(00)02603-X]

Phase Transformation in EB‐PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing

Electron‐beam physical‐vapor‐deposited thermal barrier coatings consisting of ZrO2 stabilized by 7 wt% Y2O3 were investigated in regard to phase transformation after annealing. Free‐standing ceramic layers were heat‐treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X‐ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y2O3 content in the phases were calculated. At the start of transformation, small grains of a low‐Y2O3t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y2O3, an m phase, and a c phase formed after cooling, with the Y2O3 contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma‐sprayed coatings.

Microstructural Characterization of Yttria-Doped Zirconia Coatings with Electron Microprobe Wavelength Dispersive Compositional Mapping

ABSTRACTThe use of digital electron microprobe x-ray compositional mapping with wavelength dispersive spectrometers to understand the microstructure of yttria stabilized zirconia thermal barrier coatings is described. Data from quantification of element x-ray maps can be utilized to infer what phase or phases are present. Analysis of a plasma-sprayed coating prepared from a fused and crushed feedstock is compared to an annealed specimen of the same material.

Evaluation of CaO–CeO 2–partially stabilized zirconia thermal barrier coatings

Plasma sprayable powders were prepared from ZrO 2–CaO–CeO 2 system using an organic binder and coated onto stainless steel substrates previously coated by a bond coat (Ni 22Cr 20Al 1.0Y) using plasma spraying. The coatings exhibited good thermal barrier characteristics and excellent resistance to thermal shock at 1000°C under simulated laboratory conditions (90 half hour cycles without failure) and at 1200°C under accelerated burner rig test conditions (500 2 min cycles without failure). No destabilization of cubic/tetragonal ZrO 2 phase fraction occured either during the long hours (45 h cumulative) or the large number of thermal shock tests. Growth of a distinct SiO 2 rich region within the ceramic was observed in the specimens thermal shock cycled at 1000°C apart from mild oxidation of the bond coat. The specimens tested at 1200°C had a glassy appearance on the top surface and exhibited severe oxidation of the bond coat at the ceramic–bond coat interface. The glassy appearance of the surface is due to the formation of a liquid silicate layer attributable to the impurity phase present in commercial grade ZrO 2 powder. These observations are supported by SEM analysis and quantitative EDAX data.

Plasma-sprayed duplex and graded partially stabilized zirconia thermal barrier coatings: deposition process and properties

Atmospheric plasma spraying of duplex and graded ZrO2 (8% Y2O3) thermal barrier coatings (TBCs) on Inconel 617 substrate with a NiCrAlY bond coat is described in terms of a deposition process of con-trolled coating structure. Special attention is devoted to the dominant spray parameters and the injector configuration for powder feeding, which play a fundamental role in graded coating deposition with con-trolled formation of a graded metal-ceramic (GMC) intermediate zone. The results of the graded coating spraying allow: (a) suppression of step-interface effects, (b) suppression of large differences (misfit) be-tween physical and mechanical constants of the coating and those of the substrate material, and (c) favor-able intergrowth of crystallites for a microstructurally integrated structure. Sprayed TBCs were investigated and compared with regard to their thermal cycling, oxidation behavior, and mechanical properties.

Influence of phase stability on the residual stress in partially stabilized zirconia TBC produced by plasma spray

Thermal barrier coatings (TBCs) of yttria-partially-stabilized zirconia were plasma sprayed on aluminium substrates under controlled atmosphere and temperature conditions (ATCS process) at 45°C and 155°C. Residual stress and phase composition were deeply influenced by substrate temperature and atmosphere during deposition. Spraying in Ar atmosphere promoted the formation of highly disordered and under-stoichiometric ceramics with unstable phase composition. Reducing atmosphere and low substrate temperature caused the retention of excess oxygen vacancies which stabilize a high density cubic phase (ρ ≈ 6.16 g cm −3 ), which transformed to the non-transformable tetragonal phase (ρ ≈ 6.05 g cm −3) after low temperature ageing. Phase composition and residual stress field in TBCs were studied by two different techniques, X-ray diffraction (XRD) and coating length change after detaching from the substrate (CD); the two techniques gave complementary results, since the former measured surface strain, whereas the latter gave information on average bulk strain. The average in-plane stress was compressive, with a positive gradient leading to a low surface tensile stress; at higher deposition temperatures coatings were increasingly in compression. During ageing and heat treatment, the volume expansion associated with the cubic to tetragonal transformation led to a considerable increase of the compressive stress, demonstrating that high residual stresses can develop after phase transformations.

Influence of phase stability on the residual stress in partially stabilized zirconia TBC produced by plasma spray

Thermal barrier coatings (TBCs) of yttria-partially-stabilized zirconia were plasma sprayed on aluminium substrates under controlled atmosphere and temperature conditions (ATCS process) at 45°C and 155°C. Residual stress and phase composition were deeply influenced by substrate temperature and atmosphere during deposition. Spraying in Ar atmosphere promoted the formation of highly disordered and under-stoichiometric ceramics with unstable phase composition. Reducing atmosphere and low substrate temperature caused the retention of excess oxygen vacancies which stabilize a high density cubic phase (ρ ≈ 6.16 g cm −3 ), which transformed to the non-transformable tetragonal phase (ρ ≈ 6.05 g cm −3 ) after low temperature ageing. Phase composition and residual stress field in TBCs were studied by two different techniques, X-ray diffraction (XRD) and coating length change after detaching from the substrate (CD); the two techniques gave complementary results, since the former measured surface strain, whereas the latter gave information on average bulk strain. The average in-plane stress was compressive, with a positive gradient leading to a low surface tensile stress; at higher deposition temperatures coatings were increasingly in compression. During ageing and heat treatment, the volume expansion associated with the cubic to tetragonal transformation led to a considerable increase of the compressive stress, demonstrating that high residual stresses can develop after phase transformations.

Thermal stability of stabilized zirconia thermal barrier coatings prepared by atmosphere- and temperature-controlled spraying

Yttria-stabilized zirconia coatings were deposited by atmosphere- and temperature-controlled spraying (ATCS) on IN 100 superalloy substrates for thermal engine applications. The whole deposition process was performed in an Ar atmosphere, adopting a new liquid Ar device that cooled the samples during the spraying down to 60°C. Coatings prepared by ATCS were well adherent with a dense, microcracked microstructure. However, as found previously, as-sprayed zirconia coatings were understoichiometric and highly disordered; the presence of excess oxygen vacancies and high defect density led to an increased stability of the tetragonal (t) and cubic (c) polymorphs with respect to the monoclinic (m) one. Under such conditions a low temperature annealing was sufficient to cause remarkable phase changes. It was found that under the most effective cooling conditions the coating was made of a mixture of t- and c-phases, the m-phase being totally absent. The crystal structure and thermal stability of the present zirconia polymorphs were studied by X-ray diffraction full-pattern analysis, differential scanning calorimetry and thermogravimetric analysis.

Microstructural development in physical vapour-deposited partially stabilized zirconia thermal barrier coatings

The effects of processing parameters of physical vapour deposition on the microstructure of partially stabilized zirconia (PSZ) thermal barrier coatings have been experimentally investigated. Emphasis has been placed on the crystallographic texture of the PSZ coatings and the microstructure of the top surface of the PSZ coatings as well as the metal-ceramic interface. The variations in the deposition chamber temperature, substrate thickness, substrate rotation and vapour incidence angle resulted in the observation of significant differences in the crystallographic texture and microstructure of the PSZ coatings.

Microstructural Investigations of Plasma-Sprayed Yttria Partially Stabilized Zirconia TBC (In Relation to Thermomechanical Resistance and High-Temperature Oxidation Mechanisms)

This study deals with microstructural investigations of plasma-sprayed yttria partially stabilized zirconia thermal barrier coatings, performed by classical and analytical transmission electron microscopy. The aim of the study was to determine eventual relationships between coating microstructure and toughness. The ceramic/metal interface, which plays an important role during TBC thermomechanical solicitation, has also been studied. In the 6–8 wt. percent Y2O3 range, the metastable tetragonal t′ phase is observed, showing special faulted microstructural features, such as grain twinning and antiphase boundary planes. Moreover, after high-temperature annealing in air, a very fine and stable precipitation of the equilibrium cubic phase appears. It is believed that these microstructural elements could act as crack deviation sites and enhance the coatings’ intrinsic toughness. Microstructural investigations of the alumina scales grown during high-temperature annealing reveal yttrium segregation at oxide grain boundaries as well as significant quantities of zirconium inside the alumina grains. The oxide growth seems to be dominated by a classical grain boundary oxygen diffusion mechanism. The presence of zirconium inside the alumina grains suggests that Al2O3 also partially forms by chemical reduction of ZrO2 by Al.

Characterization of Partially Stabilized Zirconia Thermal Barrier Coatings

Characterization of Partially Stabilized Zirconia Thermal Barrier Coatings

Thermal properties of ceria and yttria partially stabilized zirconia thermal barrier coatings

Coatings consisting of zirconia alloyed with 12, 15, 20 and 25 wt.% ceria or 4, 6, 8, 10 and 12 wt.% yttria were prepared using the plasma spray process. For each of these coatings measurements were made of thermal expansion parallel to the substrate and of thermal conductivity perpendicular to the substrate. These results are discussed in conjunction with the phase stability of the specimens, as determined by X-ray diffraction. Coatings containing 25 wt.% ceria or greater than 8 wt.% yttria were found to have stable thermal properties. Coatings containing less of these additions exhibited a thermal expansion hysterisis which was most pronounced at the lowest concentrations. The temperature at which the hysterisis occured was found to reduce with increasing additions of alloying agent. This behaviour was also detected for ZrO 2−CeO 2 as a change in thermal diffusivity during thermal cycling and is attributed to a reversible tetragonal-monoclinic transformation.

Characterization and thermal shock testing of yttria-stabilized zirconia coatings

Industrially manufactured partially stabilized zirconia thermal barrier coatings were characterized and tested. Optical microscopy tests were employed for qualitative and quantitative characterization of the coating structures as sprayed and after testing. Particular emphasis was given to the porosity of the ceramic coatings with regard to the size, shape and distribution of the pores by use of an interactive image processing system. In addition, the nature and quantity of phases in the ceramic coatings were determined using X-ray diffraction analysis. Thermal shock resistance was detected by burner rig tests.

Neutron and X-ray diffraction of plasma-sprayed zirconia-yttria thermal barrier coatings

ZrO 2−7.8mol.%YO 1.5, a fused powder, and ZrO 2−8.7mol.%YO 1.5, a pre-reacted powder, were plasma sprayed onto steel substrates. Neutron diffraction and X-ray diffraction of the as-received powder, the powder plasma sprayed into water, as-sprayed coatings and coatings heat treated for 10 and 100 h were carried out to study phase transformations and ordering of the oxygen ions on the oxygen sublattice. The as-received fused powder has a much lower monoclinic percentage than does the pre-reacted powder, this resulting in a much lower monoclinic percentage in the coating. Heat treatment increases the percentages of the cubic and monoclinic phases, while decreasing the tetragonal content. An ordered tetragonal phase is detected by the presence of extra neutron diffraction peaks. These phase transformations and ordering will result in volume changes. The implications of these transformations on the performance of partially stabilized zirconia thermal barrier coatings is discussed.