Yttria-stabilized Zirconia Thermal Barrier Coatings Research Articles

High temperature infiltration behavior and reaction characteristics of Colima volcanic ashes on gadolinium zirconate coatings deposited by atmospheric plasma spraying

AbstractVolcanic ashes are considered a serious threat to the aircraft industry. At high temperatures, they inflict severe thermochemical damage to the typical 7 wt.% yttria-stabilized zirconia thermal barrier coatings that protect the aircraft turbine. There is a need to evaluate alternative materials with excellent resistance to infiltration of molten siliceous particles, such as gadolinium zirconate. In this work, free-standing thermal barrier coatings of gadolinium zirconate were manufactured by atmospheric plasma spraying, varying deposit parameters to obtain different relative densities to evaluate the infiltration of molten Colima volcanic ashes for 1 h and 10 h at 1250 °C. The infiltration depth and the reaction products resulting from each interaction, were studied by different characterization techniques. In general, the coatings show high resistance to the infiltration of the volcanic ashes, reaching infiltration depths between 50 µm and 80 µm after 10 h of infiltration time. In this sense, gadolinium zirconate coatings are excellent candidates against the infiltration of Colima volcanic ashes compared to the typical yttria-stabilized zirconia-based thermal barrier coatings.

High-temperature oxidation behaviours of rare earth-based pyrochlore structured A2B2O7/yttria-stabilized zirconia thermal barrier coatings

Addressing the challenge of cost-effectively synthesizing stable thermal barrier coating materials has been a significant hurdle. This study describes the bulk synthesis of stannate-based pyrochlore compounds, specifically erbium tin oxide (Er2Sn2O7) and yttrium tin oxide (Y2Sn2O7). These synthesized pyrochlore oxides and commercially available yttria-stabilized zirconia (YSZ) were coated onto nickel super alloy. The coated specimens were subjected to thermal oxidation kinetics studies at 900 ± 5°C for 200 h, and alterations in physico-chemical characteristics, modifications in surface morphology, and variations in the composition of thermal barrier coating (TBCs) before and after exposure to oxidation conditions were evaluated. Oxidation kinetics results demonstrate that Er2Sn2O7 pyrochlore oxide has better performance than other Y2Sn2O7 and commercial YSZ TBC materials in high-temperature thermal barrier systems.

Molten salt corrosion behavior of laser remelted PS-PVD YSZ thermal barrier coatings

Molten salt from the service environment leaches Y2O3 from yttria-stabilized zirconia (YSZ), threatening the security of thermal barrier coatings (TBCs). In this study, we compared the molten salt corrosion resistances of YSZ coatings prepared via plasma-spraying physical vapor deposition (PS-PVD) before and after laser remelting. At 1000 °C, the molten salt reacted with YSZ particulates in the PS-PVD YSZ coating and infiltrated the gaps between quasi-columnar crystals, resulting in phase transition of the coating and column breakage. A dense glazed layer was formed on the YSZ coating surface after remelting by the low-energy-density laser, which prohibited the penetration of molten salt and improved the hot corrosion resistance of the YSZ coating. By contrast, the failure of the high-energy-density laser-treated YSZ coating was accelerated due to the significant interfacial stress from the thick glazed layer and volume expansion caused by the corrosion products. Our investigation revealed the structural evolution of the PS-PVD YSZ coating under hot corrosion conditions, emphasized the role of laser remelting, and could serve as a guideline for TBCs modification.

Improvement of CMAS resistance of laser glazed and nano-modified YSZ thermal barrier coatings

The present investigation used atmospheric plasma spray (APS) as a method for the production of thermal barrier coatings composed of Yttria-stabilized zirconia (YSZ). A CO2 laser was used to remove surface cracks from the YSZ coatings' surface. Several parameter variations were applied to generate laser-glazed tracks on the coating surface, resulting in an impure surface microstructure with excellent surface properties. Scanning electron microscopy was utilized to analyse the tracks' microstructure. The optimal laser parameters were determined to be 77.7 W laser power, 215 mm laser distance, and, 120 mm s−1 scanning speed based on microstructural characteristics. Once the laser parameters had been optimized, a whole coating surface was laser glazed. In order to ascertain the impact of laser glazing, stability analysis, fracture toughness, and hardness evaluations were conducted on both unglazed and laser-glazed coatings. The growth of a network of cracks was seen on the surface. The surface roughness exhibited a reduction of about tenfold in comparison to the unglazed YSZ. The presence of the monoclinic ZrO2 phase within the coating structure was successfully eradicated. The laser glazing process resulted in enhancements to both the fracture toughness and surface hardness of the coating. Despite the fact that the gaps and imperfections on the coating surfaces were repaired during laser glazing, some cracked structures nonetheless appeared. Nano YSZ was deposited on laser-glazed surfaces by the electrophoretic method in order to eliminate cracks from laser-glazed surfaces. The experimental results obtained from conducting calcium-magnesium-alumina-silicate (CMAS) and hot corrosion tests on laser-glazed coatings indicate that the use of nano YSZ filler leads to a significant reduction in salt penetration during both glassy melt and hot corrosion processes.

A New Al2 O3 -Protected EB-PVD TBC with Superior CMAS Resistance.

Calcium-magnesium-aluminium-silicate (CMAS) attack is a longstanding challenge for yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) particularly at higher engine operating temperature. Here, a novel microstructural design is reported for YSZ TBCs to mitigate CMAS attack. The design is based on a drip coating method that creates a thin layer of nanoporous Al2 O3 around YSZ columnar grains produced by electron beam physical vapor deposition (EB-PVD). The nanoporous Al2 O3 enables fast crystallization of CMAS melt close to the TBC surface, in the inter-columnar gaps, and on the column walls, thereby suppressing CMAS infiltration and preventing further degradation of the TBCs due to CMAS attack. Indentation and three-point beam bending tests indicate that the highly porous Al2 O3 only slightly stiffens the TBC but offers superior resistance against sintering in long-term thermal exposure by reducing the intercolumnar contact. This work offers a new pathway for designing novel TBC architecture with excellent CMAS resistance, strain tolerance, and sintering resistance, which also points out new insight for assembly nanoporous ceramic in traditional ceramic structure for integrated functions.

A Direct Laser Sintering Approach for the Electrophoretic Deposition Overlay of Yttria-Stabilized Zirconia on the Surface of a Thermal Barrier Coating System

The laser sintering process and modification of yttria-stabilized zirconia (YSZ) coatings subjected to electrophoretic deposition (EPD) on YSZ air-plasma-sprayed (APS) thermal barrier coatings (TBCs) were investigated. A Ni-based superalloy was plasma-sprayed using yttria-stabilized zirconia (YSZ) to create a thermal barrier coating with a 400 μm thickness. The electrophoretic deposition (EPD) technique was used to deposit the nanopowder of YSZ on the surface of YSZ TBCs. In this study, a technology based on the direct sintering of a green EPD layer using a laser beam was employed. The best conditions for the deposition overlay of the YSZ coating using a DC current were obtained with an applied voltage of 40 V, deposition time of 5 min, and suspension concentration of 10 g/L. Iodine was added to the solutions as a stabilizing agent. To overcome the problems of high sintering temperatures, laser sintering was adopted as a new approach. The microstructures of all the specimens were studied using field emission scanning electron microscopy (FESEM) with energy-dispersive X-ray spectroscopy (EDS) analysis. Surface roughness was investigated using atomic force microscopy (AFM) analysis and the central line average (CLA). The direct laser sintering (DLS) process for the EPD overlay on the surface of the TBCs caused a reduction in surface roughness and porosity, and improvements in the microstructural and mechanical properties of the surface coatings were observed.

Thermal cycling behavior and failure mechanism of Yb2O3-doped yttria-stabilized zirconia thermal barrier coatings

Thermal cycling behavior and failure mechanism of Yb2O3-doped yttria-stabilized zirconia thermal barrier coatings

Next Generation Air Plasma Spray Porous Thermal Barrier Coatings for Gas Turbine Combustors

Abstract Modern gas turbine combustors rely on thermal barrier coatings (TBC) in addition to sophisticated cooling strategies to survive high temperature environments. The need for lower emissions either driven by International Civil Aviation Organization (ICAO) regulations or by customer's desire for green engines often necessitates an increased air budget for improved mixing, thus resulting in less available cooling flow. Even with state-of-the-art cooling schemes to offset reduced cooling air allocation, combustors can benefit from better TBC performance to mitigate durability challenges. This paper showcases the next generation air plasma spray (APS) porous TBC developed by Honeywell as well as discusses tests performed and results gathered to assess its suitability for combustor application. A configured test specimen was utilized to evaluate suitability of a new coating system for combustor application. The configured specimen captures the material system, geometric features, manufacturing, and assembly process associated with combustor fabrication and thus provides a realistic environment for performance assessment. The configured specimen test results indicate the new APS porous TBC showed better performance than the standard 6–8% Yttria stabilized zirconia (YSZ) porous TBC. The new TBC with its lower conductivity at higher temperature increased thickness capability, and superior stability at substantially higher temperature than 6–8% YSZ TBC enables hot section components to survive hotter engine cycles.

Structure and durability evaluation of blast furnace slag coatings and thermal barrier coatings (TBCs) under high temperature conditions

A high temperature aggressive environment is a great challenge for the long lifetime of thermal barrier coatings (TBCs). In this study, the hot corrosion and isothermal oxidation behavior of blast furnace slag (BFS) and yttria stabilized zirconia (YSZ) coated TBC systems were systematically investigated under high temperature conditions. This study aims to analyze the evaluation of microstructural properties of thermally sprayed CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings before and after high temperature oxidation and hot corrosion tests. For this purpose, the coatings were isothermally oxidized at 900 °C for 8, 24, 50, and 75 h. Hot corrosion behavior of CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings deposited on Inconel 718 superalloy substrates were studied at 900 °C in a Na2SO4 and V2O5 molten salt environment. CoNiCrAlY/BFS and CoNiCrAlY/YSZ TBC systems exposed to Na2SO4+ V2O5 molten salt at 900 °C investigate the hot corrosion behavior. The microstructure of coatings, formation of thermally grown oxide (TGO) layer, and degradation of coatings during isothermal oxidation and hot corrosion conditions were investigated. The oxidation and hot corrosion mechanisms were discussed. BFS, could be a potential alternative for the YSZ ceramic top coating in TBCs. The TBC system with BFS can be preferred for oxidation damage due to its low cost. YSZ and BFS TBCs are highly resistant to Na2SO4 and V2O5 molten salt environment.

Stresses within rare-earth doped yttria-stabilized zirconia thermal barrier coatings from in-situ synchrotron X-ray diffraction at high temperatures

There is a growing interest for smart coatings that can be integrated into turbine engines for in-situ temperature measurements or health monitoring. The addition of rare-earth dopants into standard thermal barrier ceramic top coat materials is used to obtain luminescent coatings that enable spectral measurements, for real-time temperature or health monitoring. The thermomechanical performance and durability of such novel coating compositions in extreme environments still remains to be evaluated. Consequently, the ability to manufacture sensor coatings which present suitable thermal properties needs to be demonstrated. For this study, highly luminescent erbium and europium doped yttria-stabilized zirconia and state-of-the-art yttria-stabilized zirconia coatings manufactured by air plasma spray were characterized to determine the effects of the embedded rare-earth dopants on coating internal strain and stress and to quantify and compare their high temperature response using synchrotron X-ray diffraction. In-situ depth-resolved strain measurements were performed at 15 μm intervals along the depth of the coatings to evaluate materials response at key locations, specifically at layer interfaces. In-plane stress was calculated for the coatings and a finite element model was implemented to supplement the results and enable further predictions. The results show that the sensor coatings that were manufactured in this work revealed only minor variations in the strain response of sensor coatings under a typical thermal cycle and especially at temperatures closer to that of gas turbine operating conditions, compared to state-of-the-art coatings. This work demonstrates the viability of manufacturing rare-earth doped yttria-stabilized zirconia coatings that provide beneficial spectroscopic monitoring capabilities while having minimal impact on the thermomechanical response of the thermal barrier coatings.

Machine Learning-Assisted Design of Yttria-Stabilized Zirconia Thermal Barrier Coatings with High Bonding Strength.

As a high-quality thermal barrier coating material, yttria-stabilized zirconia (YSZ) can effectively reduce the temperature of the collective materials to be used on the surface of gas turbine hot-end components. The bonding strength between YSZ and the substrate is also one of the most important factors for the applications. Herein, the Gaussian mixture model (GMM) and support vector regression (SVR) were used to construct a machine learning model between YSZ coating bonding strength and atmospheric plasma spraying (APS) process parameters. First, GMM was used to expand the original 8 data points to 400 with the R value of leave-one-out cross-validation improved from 0.690 to 0.990. Then, the specific effects of APS process parameters were explored through Shapley additive explanations and sensitivity analysis. Principal component analysis was used to explain the constructed model and obtain the optimized area with a high bonding strength. After experimental validation, the results showed that under the APS process parameters of a current of 617 A, a voltage of 65 V, a H2 flow of 3 L min–1, and a thickness of 200 μm, the bonding strength increased by more than 19% to 55.5 MPa compared with the original maximum value of 46.6 MPa, indicating that the constructed GMM–SVR model can accurately predict the bonding strength of YSZ coating.

Relationship between microstructure and bonding strength of yttria-stabilized zirconia thermal barrier coatings

Yttria-stabilized zirconia coatings were prepared by atmospheric plasma spraying, and the bonding strength of the coatings was determined by tensile testing. The microstructure of the coating, including grains and pores, was characterized using scanning electron microscopy and electron backscatter diffraction. The results show that the higher the proportion of columnar grains, the higher the bonding strength. The large misorientation at the small grains leads to large stress, which is easy to fracture in the tensile process, so the existence of small grains leads to poor bonding strength. By using image processing, pores were classified as holes, cracks, and microcracks, and their proportions were calculated. It was found that both holes and transverse and vertical cracks in pores linearly affected the bonding strength; the influence of holes was the greatest, and the influence of microcracks was the smallest. In conclusion, the smaller the porosity, the greater the bonding strength of the coating.

Solution precursor thermal spraying of gadolinium zirconate for thermal barrier coating

Gadolinium zirconate (GZ) is an attractive material for thermal barrier coatings (TBCs). However, a single layer GZ coating has poor thermal cycling life compared to Yttria Stabilized Zirconia (YSZ). In this study, Solution Precursor High Velocity Oxy-Fuel (SP-HVOF) thermal spray was used to produce a double layer GZ/YSZ TBC and compared the thermal cycling performance with the single layer YSZ TBC. The temperature behaviour of the solution precursor GZ was studied, and single splat tests were carried out to obtain an optimised spray parameter. In thermal cycling tests, the single-layer YSZ reached 20 % failure at 85 ± 5 cycles, whereas the double-layer GZ/YSZ was at 70 ± 15 cycles. The single-layer failed at the topcoat/TGO interface, whereas the double-layer failed at GZ/YSZ interface and topcoat/TGO interface. Moreover, Gd diffusion occurred near the GZ/YSZ interface, resulting in porosities in the GZ layer.

Tailoring microstructure of double-layered thermal barrier coatings deposited by suspension plasma spray for enhanced durability

Gadolinium zirconate (GZ)-based TBCs comprising GZ as the top layer and yttria stabilized zirconia (YSZ) as the base layer, are attractive double-layered thermal barrier coatings (TBCs) for high temperature gas turbine engine application. This work attempts to understand the influence of individual layer microstructure on the durability of GZ/YSZ double-layered TBCs processed by suspension plasma spray (SPS). Two different spray parameters were chosen to obtain a combination of three microstructurally distinct GZ/YSZ double-layered TBCs i.e. GZ porous (P)/YSZ porous (P), GZ dense (D)/YSZ porous (P) and GZ dense (D)/YSZ dense (D). Thermal diffusivity of the as-deposited coatings was measured using Laser Flash Analysis (LFA) technique and the thermal conductivity of the TBCs was calculated. The GZ/YSZ double-layered TBC specimens were subjected to two different durability tests, i.e. thermal cyclic fatigue (TCF) and burner rig test (BRT). Sintering behavior of the individual layer TBC microstructures was evaluated by comparing the porosity evolution in as-deposited and TCF tested TBCs. Fracture toughness measurements performed on each layer of the double-layered TBCs were correlated with the durability results. Thermal cycling results amply demonstrate that the individual layer microstructure of GZ/YSZ double-layered TBC influenced its durability. Detailed failure analysis of the TCF and BRT failed specimens revealed similar failure modes for GZ (P)/YSZ (P), GZ (D)/YSZ (P) and GZ (D/YSZ (D) TBCs under identical thermal cyclic test conditions. However, failure modes differed when subjected to different thermal cyclic test conditions (TCF and BRT) and the probable causes are discussed. Findings from this work provide key insights on designing durable GZ/YSZ double-layered TBCs.

A case study on CMAS corrosion of an in-situ microstructure modification YSZ TBCs fabricated by EB-PVD

At elevated temperature, the inter-columnar pores of electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) may act as infiltration channels for the molten calcium–magnesium–alumino-silicate (CMAS) and, therefore, makes it susceptible to be attacked by CMAS. To enhance the CMAS corrosion resistance of EB-PVD TBCs, a microstructure modification yttria-stabilized zirconia (YSZ) top coat consisting of a low porosity fibrous structure layer on the conventional columnar structure layer was fabricated by in-situ parameter modification. The anti-CMAS corrosion performance of the in-situ modified YSZ coating was evaluated at 1250 °C and the evolution of the microstructure was investigated in detail. The results show that the inter-columnar pores were sealed by the low-porosity and smooth fibrous structure layer. Due to the porosity and the size of the pores of the fibrous structure layer are dramatically reduced, the penetration rate of CMAS is significantly lower in the in-situ microstructure modification coating than in the conventional columnar structure coating, and the interaction between CMAS and YSZ coating was also reduced.

Influence of pore characteristics of air plasma sprayed thermal barrier coatings on calcia-magnesia-alumino-silicate (CMAS) attack behavior

The pore characteristics of air plasma sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs) were tailored by Al2O3-sol impregnation, and then its influence on CMAS attack behavior was investigated. Cross-sectional inspection revealed that the pore-modifier Al2O3 can reduce the width of the infiltration path, increase tortuosity, or even block the path by forming connection arrays along with the path. CMAS attack test showed that the pore-tailored TBCs had a better CMAS infiltration resistance. This study implies that tailoring the pore characteristics of TBCs by impregnation may be a potential way to mitigate CMAS damage.

Thermochemical interaction of biofuel impurities with yttria-stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs) protect superalloy components from the extreme heat in gas turbines. Airborne contaminants that interact with TBCs during flight, such as CMAS particulate where C is calcia, M is magnesia, A is alumina, and S is silica, degrade these ceramic coatings. Biofuels can directly introduce some or all these same impurities into the gas turbine based on their starting biomass, how they were harvested, and the synthesizing process. Because the impurities are unreacted until combustion, new combinations of these impurities can form harmful deposits with compositions such as S, C–S, C-A, C-A-S, and C-M-S. In this study, we applied combinations of precursors for C, M, A, and S impurities to the surface of a TBC with subsequent ablation to ~1400 °C. Observations revealed that the TBCs delaminated by two failure mechanisms dependent on the impurities applied. In the first case, impurity combination(s) of S, C-A, and C–S interacted thermochemically with the topcoat, ultimately causing the spallation of the TBC. The second failure mechanism was characterized by molten impurities infiltrating the topcoat and inducing the propagation of cracks in the proximity of the bondcoat through a thermomechanical interaction with the TBC. This occurred for impurity combinations of C-A-S, C-M-S, and C-M-A-S. Regardless of the interaction type, different combinations of impurities common to biofuels can accelerate failure and reduce lifetimes of TBCs.

High-temperature interactions between Yttria-stabilized zirconia thermal barrier coatings and Na-Rich calcia-magnesia-aluminosilicate deposits

We have evaluated the effectiveness of optical basicity, a chemical model, to predict and categorize the reaction behavior between calcia-magnesia-aluminosilicate (CMAS) deposits and ZrO2-based thermal barrier coatings (TBCs), which are used to insulate and protect metallic components in gas-turbine engines. The attack behavior of two CMAS compositions (Na-lean and Na-rich) with 7 wt% Y2O3-partially-stabilized ZrO2 (7YSZ) and 2ZrO2·Y2O3(ss) free-standing TBCs at 1340 °C were evaluated and compared to previous studies. The behavior of Y3+ in the reaction is correlated with the optical basicity of the CMAS; more basic Na-rich CMAS melt resulted in lower Y-solubility and higher Y-content in the reprecipitated ZrO2 grains than observed in the highly acidic CMAS attack for both tested TBCs. This behavior is consistent with previous studies of basic and acidic melts, and suggests that less acidic CMASs pose a unique danger to ZrO2-based TBCs that rely on Y-rich secondary phases for CMAS mitigation.

Elevated Temperature Erosion of Plasma Sprayed Thermal Barrier Coating

Yttria stabilized zirconia (YSZ) is used extensively as a ceramic top coat of thermal barrier coating (TBC) system in land-based gas turbines and aero engines. TBC enables modern engines to operate at significantly higher gas temperatures. TBCs are subjected to solid particle erosion at elevated temperature, especially during the operation of aero engines in sandy terrain. Thus, the objective of the present investigation is to evaluate the impact erosion response of these coatings as a function of various eroding conditions. Toward that purpose, YSZ TBC was deposited on Ni base alloy using air plasma spraying technique. The microstructural features of the coating are evaluated, and coated samples are subjected to erosion test employing a high temperature air jet erosion testing machine at various temperatures, different impact velocities, and at different impact angles. The results show a brittle erosion response of TBC at all temperatures. The erosion rate of TBC is generally independent of temperature under the normal impact, and it increases with temperature under oblique impact at high impact velocity. Intersplat adhesion governs the erosion mechanism.

Rheological and chemical interaction between volcanic ash and thermal barrier coatings

Combustion temperatures (>1200 °C) in gas turbines cause softening and eventually melting of ingested volcanic ash particles. In the downstream section, lower temperatures lead to droplet deposition onto the thermal barrier coatings (TBCs) of the turbine blades. The intensity of interaction (wetting and chemical interaction) depends on the ash and TBC chemistry as well as the TBC structure. We have determined the spreading behavior of five different volcanic ash melts on four types of TBCs. The TBCs differed in their composition - yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) – and in their fabrication - air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). The spreading properties have been parameterized on the basis of four parameters, 1) the CaO-SiO 2 -ratio, 2) optical basicity, 3) the R b/a ratio and 4) the viscosity of the volcanic ash melts. Infiltration efficiency of the melts into the TBCs and corrosion characteristics have been determined via electron microprobe analysis of cross sections. In this study, the highest damage potential was found following interaction with basaltic melt, as its low viscosity is favorable to extensive spreading and high chemical reactivity. • Basaltic (SiO 2 -lean) silicate melts show the most extensive spreading on TBC surfaces and the most intense chemical reaction with the TBC materials • Spreading of silicate melts on APS TBCs is enhanced over EB-PVD TBCs and spreading on GZO TBCs is enhanced over YSZ TBCs • Spreading ability of silicate melts can be estimated based on a parametrization deduced from our spreading experiments