Thermal cycling performances of multilayered yttria-stabilized zirconia/gadolinium zirconate thermal barrier coatings

Gadolinium zirconate (Gd₂Zr₂O₇, GZO) as an advanced thermal barrier coating (TBC) material, has lower thermal conductivity, better phase stability, sintering resistance, and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (YSZ, 6-8 wt%) at temperatures above 1200°C. However, the drawbacks of GZO, such as the low fracture toughness and the formation of deleterious interphases with thermally grown alumina have to be considered for the application as TBC. Using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS), double-layered YSZ/GZO TBCs, and triple-layered YSZ/GZO TBCs were manufactured. In thermal cycling tests, both multilayered TBCs showed a significant longer lifetime than conventional single-layered APS YSZ TBCs. The failure mechanism of TBCs in thermal cycling test was investigated. In addition, the CMAS attack resistance of both TBCs was also investigated in a modified burner rig facility. The triple-layered TBCs had an extremely long lifetime under CMAS attack. The failure mechanism of TBCs under CMAS attack and the CMAS infiltration mechanism were investigated and discussed.     

The influence of thermal barrier coating dissolution on CMAS melt viscosities

Glassy deposits, largely consisting of CaO-MgO-Al₂O₃-SiO₂ (CMAS), are a common product on thermal barrier coatings (TBCs) within gas-turbines after an interaction with airborne particles. Here, in order to facilitate the quantification and modelling of the spreading and infiltration behavior of CMAS melts onto and into TBCs we have determined the high temperature viscosities of four widely used synthetic “CMAS” melts and the influence of TBC materials (yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO)) dissolution upon them. After a dissolution of 6.5 wt% YSZ or GZO one out of four CMAS melts shows no significant change in viscosity, while the other three melts exhibit a viscosity increase at lower temperatures that continuously changes to a decrease in viscosity towards higher temperatures. The influence of the doping amount on the viscosity was investigated in detail for one CMAS melt (C₃₅M₁₀A₇S₄₈) and parametrized.     

Tailoring the EB-PVD columnar microstructure to mitigate the infiltration of CMAS in 7YSZ thermal barrier coatings

The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250 °C and 1225 °C for different time intervals. The 7YSZ coating with more ‘feathery’ features has resulted in higher CMAS resistance by at least by a factor of 2 than its less ‘feathery’ counterpart. These results are explained on the basis of a proposed physical model.     

Evolution of porosity,crack density,and CMAS penetration in thermal barrier coatings subjected to burner rig testing

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X-ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria-stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Solution combustion synthesis of calcia-magnesia-aluminosilicate powder and its interaction with yttria-stabilized zirconia and co-doped yttria-stabilized zirconia

Thermal Barrier Coatings (TBCs) play a significant role in improving the efficiency of gas turbines by increasing their operating temperatures. The TBCs in advanced turbine engines are prone to silicate particles attack while operating at high temperatures. The silicate particles impinge on the hot TBC surfaces and melt to form calcia-magnesia-aluminosilicate (CMAS) glass deposits leading to coating premature failure. Fine powder of CMAS with the composition matching the desert sand has been synthesized by solution combustion technique. The present study also demonstrates the preparation of flowable yttria-stabilized zirconia (YSZ) and cluster paired YSZ (YSZ-Ln₂O₃, Ln = Dy and Gd) powders by single-step solution combustion technique. The as-synthesized powders have been plasma sprayed and the interaction of the free standing TBCs with CMAS at high-temperatures (1200 °C, 1270 °C and 1340 °C for 24 h) has been investigated. X-ray diffraction analysis of CMAS attacked TBCs revealed a reduction in phase transformation of tetragonal to monoclinic zirconia for YSZ-Ln₂O₃ (m-ZrO₂: 44%) coatings than YSZ (m-ZrO₂: 67%). The field emission scanning electron microscopic images show improved CMAS resistance for YSZ-Ln₂O₃ coatings than YSZ coatings.     

Porosity–Property Relationships of Plasma‐Sprayed Gd2Zr2O7/YSZ Thermal Barrier Coatings

During the past decade, gadolinium zirconate (Gd₂Zr₂O₇, GZO) has attracted interest as an alternative material to partially yttria‐stabilized zirconia (YSZ) for thermal barrier coatings (TBCs). Despite the well‐known benefits of GZO, such as lower thermal conductivity and superior temperature capability compared to YSZ, processing of GZO via atmospheric plasma spraying (APS) still remains a challenge. Here, we report on APS experiments which were performed to investigate the influence of processing on GZO microstructure and lifetime of GZO/YSZ double‐layer TBCs. Different microstructures of GZO were produced and characterized in terms of porosity, stoichiometry, Young′s modulus, and their effects on the lifetime of YSZ/GZO double‐layer TBCs were discussed. Particle diagnostics were utilized for the optimization of the process parameters with respect to different microstructures of GZO and stoichiometry. It was found that both cumulative porosity of GZO and pore size distribution, which alter the Young′s modulus significantly, govern the lifetime of double layers. In addition, it was shown that the deviation in GZO stoichiometry due to gadolinia evaporation in the investigated range does not display any critical effect on lifetime.     

Erosion behavior of EB-PVD 7YSZ coatings under corrosion/erosion regime: Effect of TBC microstructure and the CMAS chemistry

Aero-engines operating in dust-laden environments often encounter a lot of dust/sand that causes a severe problem to the TBCs by means of erosion. As the turbine entry temperatures are rising, molten sand is also a big concern to the life-time of TBCs.This paper deals with the TBC behavior under the combined influence of erosion and corrosion attack. Variations in TBC morphology, CMAS infiltration time and CMAS composition and their influence on the erosion resistance at room temperature were investigated. Two different EB-PVD 7YSZ morphologies consisting of a different porosity arrangement were tested in the erosion/corrosion regime. The more ‘Feathery’ structure has a better resistance to erosion compared to a more columnar ‘Normal’ structure, which leads to less degradation of the TBC. However, under the influence of CMAS infiltration the effect was found to be reversed. In general, CMAS-infiltrated EB-PVD TBCs exhibit a higher erosion resistance than the non-infiltrated ones.     

Thermochemical Interaction of Thermal Barrier Coatings with Molten CaO–MgO–Al2O3–SiO2 (CMAS) Deposits

Thermal barrier coatings (TBCs) are increasingly susceptible to degradation by molten calcium–magnesium alumino silicate (CMAS) deposits in advanced engines that operate at higher temperatures and in environments laden with siliceous debris. This paper investigates the thermochemical aspects of the degradation phenomena using a model CMAS composition and ZrO₂–7.6%YO_1.5 (7YSZ) grown by vapor deposition on alumina substrates. The changes in microstructure and chemistry are characterized after isothermal treatments of 4 h at 1200°–1400°C. It is found that CMAS rapidly penetrates the open structure of the coating as soon as melting occurs, whereupon the original 7YSZ dissolves in the CMAS and reprecipitates with a different morphology and composition that depends on the local melt chemistry. The attack is minimal in the bulk of the coating but severe near the surface and the interface with the substrate, which is also partially dissolved by the melt. The phase evolution is discussed in terms of available thermodynamic information.     

Wetting,infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250?°C for short terms (1, 5, 30?min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (～100?μm thick) was fully penetrated by CMAS melt within 1?min. After the CMAS attack for 30?min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO₂ grains in the interaction regions.     

Isothermal Oxidation Behavior of Gd2Zr2O7/YSZ Multilayered Thermal Barrier Coatings

Efficiency of a gas turbine can be increased by increasing the operating temperature. Yttria‐stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used in gas turbine applications. However, above 1200°C, YSZ undergoes significant sintering and CMAS (calcium magnesium alumino silicate) infiltration. New ceramic materials of rare earth zirconate composition such as gadolinium zirconate (GZ) are promising candidates for thermal barrier coating applications (TBC) above 1200°C. Suspension plasma spray of single‐layer YSZ, double‐layer GZ/YSZ, and a triple‐layer TBC comprising denser GZ on top of GZ/YSZ TBC was attempted. The overall coating thickness in all three TBCs was kept the same. Isothermal oxidation performance of the three TBCs along with bare substrate and bond‐coated substrate was investigated for time intervals of 10 h, 50 h, and 100 h at 1150°C in air environment. Weight gain/loss analysis was carried out by sensitive weighing balance. Microstructural analysis was carried out using scanning electron microscopy (SEM). As‐sprayed single‐layer YSZ and double‐layer GZ/YSZ showed columnar microstructure, whereas the denser layer in the triple‐layer TBC was not columnar. Phase analysis of the top surface of as‐sprayed TBCs was carried out using XRD. Porosity measurements were made by water intrusion method. In the weight gain analysis and SEM analysis, multilayered TBCs showed lower weight gain and lower TGO thickness compared to single‐layer YSZ.     

Corrosion behavior of air plasma spraying zirconia-based thermal barrier coatings subject to Calcium–Magnesium–Aluminum-Silicate (CMAS) via burner rig test

Three different kinds of thermal barrier coatings (TBCs) — 8YSZ, 38YSZ and a dual-layered (DL) TBCs with pure Y₂O₃ on the top of 8YSZ were produced on nickel-based superalloy substrate by air plasma spraying (APS). The Calcium–Magnesium–Aluminum-Silicate (CMAS) corrosion resistance of these three kinds of coatings were researched via burner rig test at 1350 °C for different durations. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD. With the increase of Y content, TBCs exhibit better performance against CMAS corrosion. The corrosion resistance against CMAS of different TBCs in descending was 8YSZ + Y₂O₃, 38YSZ and 8YSZ, respectively. YSZ diffused from TBCs into the CMAS, and formed Y-lean ZrO₂ in TBCs because of the higher diffusion rate and solubility of Y³⁺ in CMAS than Zr⁴⁺. At the same time, 38YSZ/8YSZ + Y₂O₃ reacts with CAMS to form Ca₄Y₆(SiO₄)₆O/Y_4·67(SiO₄)₃O with dense structure, which can prevent further infiltration of CMAS. The failure of 8YSZ coatings occurred at the interface between the ceramic coating and the thermally grown oxide scale (TGO)/bond coating. During the burner rig test, the Y₂O₃ layer of the DL TBCs peeled off progressively and the 8YSZ layer exposed gradually. DL coatings keep roughly intact and did not meet the failure criteria after 3 h test. 38YSZ coating was partially ablated, the overall thickness of the coating is thinned simultaneously after 2 h. Therefore, 8YSZ + Y₂O₃ dual-layered coating is expected to be a CMAS corrosion-resistant TBC with practical properties.     

Performance of YSZ and Gd2Zr2O7/YSZ double layer thermal barrier coatings in burner rig tests

Double layer thermal barrier coatings (TBCs) consisting of a Gd₂Zr₂O₇ (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail.     

Thermal cycling behaviour of plasma sprayed lanthanum zirconate based coatings under concurrent infiltration by a molten glass concoction

Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, during aero engine operation, environmentally ingested airborne particles, which includes mineral debris, sand dust and volcanic ashes get ingested by the turbine with the intake air. As engine temperatures increase, the finer debris tends to adhere to the coating surface and form calcium magnesium alumino-silicate (CMAS) melts that penetrate the open void spaces in the coating. Upon cooling at the end of an operation cycle, the melt freezes and the infiltrated volume of the coating becomes rigid and starts to spall by losing its ability to accommodate strains arising from the thermal expansion mismatch with the underlying metal. The state-of-the-art ZrO₂-7-weight% Y₂O₃ (YSZ) coatings are susceptible to the aforementioned degradation. Rare-earth zirconates have generated substantial interest as novel thermal barrier coatings (TBC) based primarily on their intrinsically lower thermal conductivity and higher resistance to sintering than YSZ. In addition, the pyrochlore zirconates are stable as single phases at up to their melting point. La₂Zr₂O₇ (LZ) is one among such candidates. Hence, the present study focusses on the comparison of cyclic molten CMAS infiltration behaviour of the base metal Inconel 738 (BM), the bond coat NiCrAlY (BC), the duplex YSZ, the LZ coating and a five layered coated specimen with LZ as top layer. Among those coatings mentioned above, the five layer coated specimen showed excellent CMAS infiltration resistance under thermal cycling conditions.     

Laser thermal gradient testing of air plasma sprayed multilayered,multi-material thermal barrier coatings

Air plasma sprayed (APS) thermal barrier coatings (TBCs) are a widely used technology in the gas turbine industry to thermally insulate and protect underlying metallic superalloy components. These TBCs are designed to have intrinsically low thermal conductivity while also being structurally compliant to withstand cyclic thermal excursions in a turbine environment. This study examines yttria-stabilized zirconia (YSZ) TBCs of varying architecture: porous and dense vertically cracked (DVC), which were deposited onto bond-coated superalloys and tested in a novel CO₂ laser rig. Additionally, multilayered TBCs: a two-layered YSZ (dense + porous) and a multi-material YSZ/GZO TBC were evaluated using the same laser rig. Cyclic exposure under simulative thermal gradients was carried out using the laser rig to evaluate the microstructural change of these different TBCs over time. During the test, real-time calculations of the normalized thermal conductivity of the TBCs were also evaluated to elucidate information about the nature of the microstructural change in relation to the starting microstructure and composition. It was determined that porous TBCs undergo steady increases in conductivity, whereas DVC and YSZ/GZO systems experience an initial increase followed by a monotonic decrease in conductivity. Microstructural studies confirmed the difference in coating evolution due to the cycling.     

Understanding the Formation of Vertical Cracks in Solution Precursor Plasma Sprayed Yttria‐Stabilized Zirconia Coatings

Yttria‐stabilized zirconia (YSZ) deposition by the solution precursor plasma spraying (SPPS) route has been of interest for potential thermal barrier coating (TBC) applications. It has been surmised that realization of unique microstructural features like vertical cracks, nanosized pores and fine splats in the TBCs can significantly enhance coating durability and performance. However, satisfactory control over the YSZ coating microstructure has been elusive in the absence of an adequate understanding of the mechanism responsible for coating deposition in SPPS. This study demonstrates the ability to tailor microstructure of deposited YSZ coatings over a wide range, from nano‐porous coatings to a vertically cracked microstructure. Varying of precursor flow rate has been shown to dictate the pyrolysis events occurring in situ and, adopting this approach, YSZ coatings with widely varying microstructural features have been developed. The coatings have been characterized in detail and the observations correlated with in‐flight particle generation and splat formation. These studies also provide useful insights into the possible origin of vertical cracks in the coating for which a mechanism is proposed.     

Thermal insulation of CMAS (Calcium-Magnesium-Alumino-Silicates)- attacked plasma-sprayed thermal barrier coatings

The impact of the penetration of small quantities of calcium-magnesium-alumino- silicates (CMAS) glassy melt in the porous plasma-sprayed (PS) thermal barrier coatings (TBCs) is often neglected even though it might play a non-negligible role on the sintering and hence on the thermal insulation potential of TBCs. In this study, the sintering potential of small CMAS deposits (from 0.25–3 mg.cm⁻²) on freestanding yttria-stabilized zirconia (YSZ) PS TBCs annealed at 1250 °C for 1 h was investigated. The results showed a gradual in-depth sintering with increasing CMAS deposits. This sintering was concomitant with local transformations of the tetragonal YSZ and resulted in an increase in the thermal diffusivity of the coatings that reached a maximum of ∼110 % for the fully penetrated coating.     

CMAS corrosion resistance in high temperature and rainwater environment of double-layer thermal barrier coatings odified by rare earth

In order to explore the difference of CMAS corrosion resistance in high temperature and rainwater environment of single-layer and double-layer thermal barrier coatings (TBCs), and further reveal the mechanism of CMAS corrosion resistance in above environment of double-layer TBCs modified by rare earth, two TBCs were prepared by air plasma spraying, whose ceramic coating were single-layer ZrO₂–Y₂O₃ (YSZ) and double-layer La₂Zr₂O₇(LZ)/YSZ, respectively. Subsequently, CMAS corrosion resistance tests at 1200 °C and rainwater environment of two TBCs were carried out. Results demonstrate that after high temperature CMAS corrosion for the same time, due to phase transformation, the volume of YSZ ceramic coating in single-layer TBCs shrank and surface cracks formed, which would lead to coating failure. When LZ ceramic coating of double-layer TBCs reacted with CMAS, compact apatite phases and fluorite phases formed, the penetration of CMAS into ceramic coating was inhibited effectively. Raman analysis and calculation results show that both of the surface residual stress of ceramic coating in two TBCs were compressive stress, and the residual stress of ceramic coating in double-layer TBCs were smaller than that of single-layer TBCs. Atomic force microscopy of TBCs after CMAS corrosion show that surface of double-layer TBCs was more uniform and compact than that of single-layer TBCs. The electrochemical properties in simulated rainwater of two TBCs after high temperature CMAS corrosion showed that double-layer TBCs possessed higher free corrosion potential, lower corrosion current and higher polarization resistance than those of single-layer TBCs. Consequently, the presence of LZ ceramic coating effectively improved CMAS corrosion resistance in high temperature and rainwater environment of double-layer TBCs.     

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y₂O₃ partially stabilized ZrO₂ (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250 °C for 2 h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250 °C for 8 h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.     

Equimolar YO1.5 and TaO2.5 co-doped ZrO2 as a potential CMAS-resistant material for thermal barrier coatings

Calcium-magnesium-alumino-silicates (CMAS) corrosion in thermal barrier coatings (TBCs) is becoming more serious with increasing operation temperature of turbine engines. Here, we report an equimolar YO_1.5 and TaO_2.5 co-doped ZrO₂ (Zr_0.66Y_0.17Ta_0.17O₂, ZYTO) as a potential CMAS-resistant material for TBCs, which shows a significantly enhanced CMAS resistance than the conventional 17 mol% YO_1.5-stabilized ZrO₂ (17YSZ). After exposure at 1300°C for 100 hours, the CMAS infiltration depth in ZYTO bulk is ~80 μm (for a 20 mg/cm² CMAS deposition), in contrast to ~700 μm in 17YSZ bulk (50 hours). Compositional and morphological analyses on the CMAS reaction zone reveal that the excellent CMAS resistance of ZYTO originates from the uniform corrosion through grain and grain boundary, along with densification of the reaction layer. The high CMAS infiltration rate of 17YSZ is attributed to the severe dissolution and infiltration through grain boundary. The reaction mechanisms of CMAS with ZYTO and 17YSZ bulks are discussed and a strategy of enhancing the CMAS resistance is proposed for ZrO₂-based TBC materials.