Microstructure and thermal properties of nanostructured gadolinia doped yttria-stabilized zirconia thermal barrier coatings produced by air plasma spraying

In this article, the nanostructured 2 mol% Gd₂O₃-4.5 mol% Y₂O₃-ZrO₂(2GdYSZ) coating was developed by the atmospheric plasma spraying technique. And the microstructure and thermal properties of plasma-sprayed 2GdYSZ coating were investigated. The result from the investigation indicates that the as-sprayed coating is characterized by typical microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks. The 2GdYSZ coating shows a lower resistance to destabilization of the metastable tetragonal (t′) phase compared to the yttria stabilized zirconia(YSZ). The thermal diffusivity and thermal conductivity of the nano-2GdYSZ coating at room temperature are 0.431 mm² s⁻¹ and 1.042 W/m K, respectively. Addition of gadolinia to the nano-YSZ can significantly reduce the thermal conductivity compared to the nano-YSZ and the conventional YSZ. The reduction is mainly attributed to the synergetic effect of gadolinia doping along with nanostructure.     

Bimodal microstructure toughens plasma sprayed Al2O3-8YSZ-CNT coatings

Current work pursues generating controlled bimodal microstructure by plasma spraying of micrometer-sized Al₂O₃ and nanostructured spray-dried agglomerate with reinforcement of 20 wt% of 8 mol % yttria stabilized zirconia (8YSZ) and 4 wt% carbon nanotube (CNT) as potential thermal barrier coating (TBC) on the Inconel 718 substrate. Composite coatings exhibit bimodal microstructure of: (i) fully melted and resolidified microstructured region (MR), and (ii) partially melted and solid state sintered nanostructured regions (NR). Reinforcement with 8YSZ has led to an increase in hardness from ∼12.8 GPa (for μ-Al₂O₃) to ∼13.9 GPa in MR of reinforced Al₂O₃-YSZ composite. Further, with the addition of CNT in Al₂O₃-8YSZ reinforced composite, hardness of MR has remained similar ∼13.9 GPa (8YSZ reinforced) and ∼13.5 GPa (8YSZ-CNT reinforced), which is attributed to acquiescent nature and non-metallurgical bonding of CNT with MR. Indentation fracture toughness increased from 3.4 MPam^0.5 (for μ-Al₂O₃) to a maximum of 5.4 MPam^0.5 (8YSZ- CNT reinforced) showing ∼57.7% improvement, which is due to crack termination at NR, retention of t-ZrO₂ (∼3.3 vol%) crack bridging, and CNT pull-out toughening mechanisms. Modified fractal models affirmed that the introduction of bimodal microstructure (NR) i.e., nanometer-sized- Al₂O₃, nanostructured 8YSZ and CNTs in the μ-Al₂O₃ (MR) contributes ∼44.6% and ∼72% towards fracture toughness enhancement for A8Y and A8YC coatings. An enhanced contribution of nanostructured phases in toughening microstructured Al₂O₃ matrix (in plasma sprayed A8YC coating) is established via modified fractal model affirming crack deflection and termination for potential TBC applications.     

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al₂O₃-SiO₂ (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.     

Mechanical properties and thermal conductivities of 3YSZ-toughened fully stabilized HfO2 ceramics

In this work, the 3 mol% yttria stabilized zirconia (3YSZ) composed of tetragonal phase has been introduced into the 10 mol% Er₂O₃ stabilized cubic hafnia (10ErSH) matrix to improve its fracture toughness. The effects of the addition of 3YSZ on the phase composition, microstructure, mechanical properties and thermal conductivities of the 10ErSH have been investigated. The results showed that all the 3YSZ-toughened 10ErSH samples were composed of cubic phase and a little (<10 mol%) monoclinic phase. The introduced tetragonal phase of 3YSZ fully disappeared even when the volume fraction of 3YSZ reached 50%, indicating that the phase transformation occurred during 1500 °C. The fracture toughness for the sample with 50% 3YSZ was improved by 60% compared with the pure 10ErSH ceramics owing to the sub-mico/micro hybrid structure, which changed the crack propagation mode and consumed part of the crack extension energy. Additionally, the thermal conductivity slightly decreased due to the mass and radius misfits induced by substitution atoms (Zr⁴⁺, Er³⁺ and Y³⁺). Considering the improved mechanical and thermal properties, the 3YSZ-toughened 10ErSH ceramics may be alternative TBC materials.     

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 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.     

Preparation of long-lifetime thermal barrier coatings and toughening mechanism by using hierarchy structured zirconia-based microspheres

The thermal cycling lifetime of thermal barrier coatings was doubled when deposited by electro-sprayed (ESP) microspheres instead of by commercial hollow spherical powders. It was believed that partial-molten nodules with featured microstructures inherited from the feedstock microspheres were the main contributor for prolonged thermal cycling durability due to improved fracture toughness and strain tolerance. The maximum lifetime was observed on samples with 20?30 vol.% of partial-molten microspheres. The hierarchy pores may both slow down the crack propagation by triggering multi-deflecting and promote cracking by reducing the tendency of interfacial deflection, the net effect depends on situation. The ESP coatings exhibited bimodal Weibull moduli upon indentation, which was regarded as originated from the hierarchy porous structure. Finally, the criterion was verified by micro-indentation and residual stain-stress evaluation by Raman spectroscopy.     

Comparison of hot corrosion behaviors of plasma-sprayed nanostructured and conventional YSZ thermal barrier coatings exposure to molten vanadium pentoxide and sodium sulfate

The purpose of the current study was evaluation and comparison of hot corrosion behaviors of plasma-sprayed conventional and nanostructured yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Hot corrosion studies were performed on the surface of coatings in the presence of a molten mixture of V₂O₅+Na₂SO₄ at 1000 °C for 30 h. Results indicated that the hot corrosion mechanisms of conventional and nanostructured YSZ coatings were similar. The reaction between corrosive salt and Y₂O₃ produced YVO₄, leaching Y₂O₃ from YSZ and causing the detrimental phase transformation of zirconia from tetragonal to monoclinic. The nanostructured coating, as compared to its conventional counterpart, in spite of a further reaction with the corrosive salt, showed a higher degradation resistance during the hot corrosion test due to increased compliance capabilities resulting from the presence of an extra source of porosity associated with the nano-zones.     

Comparison of thermal shock resistances of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of the current study is evaluation and comparison of thermal shock behavior of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, the nanostructured and conventional YSZ coatings were deposited by atmospheric plasma spraying (APS) on NiCoCrAlY-coated Inconel 738LC substrates. The thermal shock test was administered by quenching the samples in cold water of temperature 20–25 °C from 950 °C. In order to characterize elastic modulus of plasma-sprayed coatings, the Knoop indentation method was employed. Microstructural evaluation, elemental analysis, and phase analysis were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffractometry (XRD) respectively. The results revealed that failures of both nanostructured and conventional TBCs were due to the spallation of ceramic top coat. Thermal stresses caused by mismatch of thermal expansion coefficients between the ceramic top coat and the underlying metallic components were recognized as the major factor of TBC failure. However, the nanostructured TBC, due to bimodal unique microstructure, presented an average thermal cycling lifetime that was approximately 1.5 times higher than that of the conventional TBC.     

Comparison of corrosion behaviors and wettability of CMAS on Ta2O5-Y2O3 co-stabilized ZrO2 and YSZ thermal barrier coatings

High-temperature degradation of the plasma sprayed 16 mol% TaO_2.5 + 16 mol% YO_1.5 co-stabilized ZrO₂ (YTZ) and YSZ (7.6 wt% Y₂O₃-stabilized ZrO₂) coatings under calcium-magnesium-aluminon-silicate (CMAS) attack at 1200 °C and 1250 °C were comparatively investigated. Results indicated that the coatings were insensitive to the infiltration of CMAS after 10 h corrosion at 1200 °C. At 1250 °C, the entire YSZ cross-section completely failed and also underwent serious chemical corrosion after 3 h hot corrosion. Even after 10 h corrosion, the penetration depth of CMAS into the YTZ was only about 80 µm. For YTZ coating, the YTaO₄ stabilizer could not easily dissolve in CMAS and precipitated out of the YTZ crystal lattice owing to the strong chemical interaction between Ta⁵⁺ and Y³⁺. The wettability of CMAS on YTZ coating was worse than that on YSZ coating. Compared with YSZ coating, the YTZ coating showed better resistance to CMAS corrosion.     

Resistance of 2ZrO2·Y2O3 top coat in thermal/environmental barrier coatings to calcia‐magnesia‐aluminosilicate attack at 1500°C

Internally cooled, hollow SiC‐based ceramic matrix composites (CMCs) components that may replace metallic components in the hot section of future high‐efficiency gas‐turbine engines will require multilayered thermal/environmental barrier coatings (T/EBCs) for insulation and protection. In the T/EBC system, the thermally insulating outermost (top coat) ceramic layer must also provide resistance to attack by molten calcia‐magnesia‐aluminosilicate (CMAS) deposits. The interactions between a potential candidate for top coat made of air‐plasma‐sprayed (APS) 2ZrO₂·Y₂O₃ solid‐solution (ss) ceramic and two different CMASs (sand and fly ash) are investigated at a relevant high temperature of 1500°C. APS 2ZrO₂·Y₂O₃(ss) top coat was found to resist CMAS penetration at 1500°C for 24 hours via reaction products that block CMAS penetration pathways. In situ X‐ray diffraction (XRD) studies have identified the main reaction product to be an Ca‐Y‐Si apatite, and have helped elucidate the proposed mechanism for CMAS attack mitigation. Ex situ electron microscopy and analytical spectroscopy studies have identified the advantageous characteristics of the reaction products in helping the CMAS attack mitigation in the APS 2ZrO₂·Y₂O₃(ss) coating at 1500°C. Finally, the Y³⁺ solubility limit and transport behavior are identified as potential comparative tools for assessing the CMAS resistance ability of top‐coat ceramics.     

Electrospinning of Y2O3- and MgO-stabilized zirconia nanofibers and characterization of the evolving phase composition and morphology during thermal treatment

The current paper focuses on the fabrication of yttria and magnesia stabilized zirconia nanofibers via electrospinning from zirconyl chloride octahydrate and polyvinylpyrrolidone precursors with minor additions of yttrium nitrate hexahydrate (3 mol.%) or magnesium nitrate hexahydrate (10 mol.%). The precursor materials were dissolved in an ethanol-water mixture in a ratio of 75:25. After successful fiber preparation, the thermal decomposition behavior of the starting materials and the subsequent phase evolution at elevated temperatures were studied. Pure tetragonal zirconia nanofibers were obtained for the composition stabilized with 3 mol.% yttria when the thermal treatment was conducted with a heating rate of 10 K/min up to 1100 °C. In future research work, these tetragonal zirconia nanofibers will be used as reinforcing material in metal matrix composites based on metastable austenitic steel. The combination of the TRIP/TWIP-effect in the steel matrix with the stress-assisted tetragonal to monoclinic phase transformation in the tetragonal stabilized zirconia will lead to a composite material with outstanding mechanical properties.     

Comparison of temperature dependent deformation mechanisms of 8YSZ thermal barrier coatings prepared by air-plasma-spray and D-gun thermal spray: An in situ study

8 weight percent yttria stabilized zirconia (8YSZ) has gained widespread use in thermal barrier coatings for the hot sections of aero and power generation turbines due to its superb thermal and mechanical properties. In this study, in situ microcompression tests were conducted to evaluate the mechanical performance of 8YSZ coatings with dense vertically cracked (DVC) microstructures produced by detonation gun thermal spray to those deposited by air plasma spray (APS). At room temperature, the APS coatings showed high variability in fracture strength resulting from cracks and pores in the coating. DVC coatings, conversely, exhibited fracture strengths ranging from 3.9 to 6.6 GPa and less variability in fracture strength attributed to the relatively dense and less defective microstructure. At 500 °C, both coatings showed better consistency of fracture strength and enhanced deformability owing to deformable pores, ferroelastic domain switching, and dislocation activities.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO₄, and YSZ-20%LaPO₄ coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO₄ was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO₄ phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000 °C for 120 h. After 120 h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.     

Interplay of the phase and the chemical composition of the powder feedstock on the properties of porous 8YSZ thermal barrier coatings

Some chemical impurities enhance sintering kinetics of ceramic Thermal Barrier Coatings (TBCs) which can cause their premature failure during operation in gas turbine engine by causing reduction in coating’s strain compliance as well as faster bond-coat oxidation due to increased thermal conductivity. Certain chemical impurities are also believed to suppress resistance to tetragonal to monoclinic phase transformation in 8YSZ, which can also be an important factor regarding TBC’s performance. Most of the impurities and some of the monoclinic phase present in the powder feedstocks can survive into the as-sprayed coating. Therefore, there is a general trend towards OEMs requiring the lowest amounts of chemical impurities and the lowest amounts of monoclinic phase in the powder feedstocks. This paper presents a comprehensive investigation aimed at understanding the role and the relative importance of the chemical and phase purities of the powder feedstock for the properties and performance of thick 8YSZ TBCs.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

Effect of microstructure on the performance of Zr6Ta2O17 ceramics as thermal barrier coatings

In this study, Zr₆Ta₂O₁₇ ceramics with porous, fine-grained, and coarse-grained structures were obtained via in situ solid-state reactions, and their mechanical characteristics were examined. The significantly low thermal conductivity of dense Zr₆Ta₂O₁₇ ceramics (1.0 W m⁻¹ K⁻¹) was due to the grain boundary gap caused by superstructured grains. A calcium–magnesium–alumina–silicate (CMAS) corrosion experiment demonstrated that the formation of an interlocking structure composed of ZrO₂, CaTa₂O₆, and ZrSiO₄ prevented the penetration of CMAS impurities, thereby revealing the application potential of porous ceramics. In dense Zr₆Ta₂O₁₇ ceramics, the low-volume diffusion induced by an entropy-stable structure is conducive for corrosion resistance; however, the grain boundary is vulnerable to attacks by CMAS, which can be mitigated by the formation of a coarse crystal structure, thereby effectively improving the corrosion performance. This work provides a critical perspective on the thermal barrier coating design of A₆B₂O₁₇ (A = Zr, Hf; BNb, Ta) ceramics.     

Gradient thermal cycling behavior of a thermal barrier coating system constituted by NiCoCrAlY bond coat and pure metastable tetragonal nano-4YSZ top coat

Pure metastable tetragonal (t’) phase 4YSZ top coats with thickness of 100 and 200 μm were deposited on NiCoCrAlY-coated second generation single crystal superalloy by air plasma spray (APS). The two thermal barrier coatings were evaluated under gradient thermal cycling test using gas mixture of propane and oxygen. After flame shock test, the values of Young's modulus, hardness and degree of densification all exhibited a gradient distribution across YSZ thickness. In contrast to intensive sintering at surface of 200 μm 4YSZ coating, the TBC sample with 4YSZ layer of 100 μm underwent poor oxidation at interface of YSZ and bond coat, forming a duplex oxide scale: (Ni,Co)(Cr,Al)₂O₄ spinel over Al₂O₃, which promoted the delamination at the top-coat/bond-coat interface. The resistance against gradient thermal cycling, the phase stability of 4YSZ and the failure mechanism of the TBCs, were discussed correlating to the effects of YSZ thickness.     

Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition

Plasma spray physical vapor deposition (PS-PVD) was used to deposit yttria stabilized zirconia (YSZ) coatings with different columnar morphologies by varying the spray distance. Although similar quasi-columnar structures were formed at the spray distances of 600 mm and 1400 mm, the formation mechanisms of particles in the coatings were different. Besides, an electron beam physical vapor deposition (EB-PVD) like columnar coating out of pure vapor was deposited at a spray distance of 1000 mm and the columnar consisted of elongated nano-sized secondary columns. The hardness and Young׳s modulus of the coatings were investigated. Compared to the other two quasi-columnar structures, the EB-PVD like columnar coating exhibited higher hardness (~9.0 GPa ) and Young׳s modulus (~110.9 GPa), mainly due to its low porosity and defect.