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.     

Thermal cycling behavior of nanostructured 8YSZ，SZ/8YSZ and 8CSZ/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The nanostructured single-ceramic-layer (SCL) 8YSZ thermal barrier coatings (TBCs), double-ceramic-layer (DCL) Sm₂Zr₂O₇ (SZ)/8YSZ and SZ doped with 8 wt% CeO₂ nanoscale particles (8CSZ)/8YSZ TBCs were fabricated by atmospheric plasma spraying (APS) on nickel-based superalloy substrates with NiCoCrAlY as the bond coating. The thermal cycling behavior of the three as-sprayed TBCs was investigated systematically at 1000 ℃ and 1200 ℃. The results reveal that the thermal cycling lifetime of the nanostructured DCL 8CSZ/8YSZ TBCs is the longest among them, which is largely due to the fact that the intermediate layer buffer effect of the DCL structure, more porosity and improvement of thermal expansion coefficient from doping CeO₂ nanoparticles can relieve thermal stress to a great extent at elevated temperature. The failure mechanism of the nanostructured TBCs has been discussed in detail.     

Preparation and characterization of YSZ abradable sealing coating through mixed solution precursor plasma spraying

Yttria-stabilized zirconia (YSZ) ceramic matrix abradable sealing coatings were prepared by plasma spraying of a blend of YSZ solution precursor with YSZ nano-particles. The microstructure and phase compositions of the prepared abradable sealing coatings were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In addition, the mechanical, high-temperature oxidation, and tribological properties of the coatings were systematically investigated. The results show that addition of YSZ nano-particles increased porosity and bond strength and decreased the hardness of the coating. The optimum performance value was achieved by addition of 5?g nano-particles into the coating. The coatings maintained excellent thermal stability through a ten-cycle thermal shock test at 1150?°C. The 8YSZ-5 coating had an improved oxidation constant of 5.540?×?10^?4 and exhibited remarkable oxidation kinetics at 1150?°C. The friction coefficient of the mixed solution precursor coating was remarkably decreased compared with a traditional ceramic matrix abradable sealing coating. The results indicate that mixed solution precursor plasma spraying increased abradable sealing coating application performance.     

Failure of plasma sprayed nano-zirconia-based thermal barrier coatings exposed to molten CaO–MgO–Al2O3–SiO2 deposits

Recently, nanostructured thermal barrier coatings have received considerable attention because of some superior properties in comparison with their conventional counterpart. In this study, nanostructured 8 wt% yttria-stabilized zirconia (n-YSZ) coatings were deposited by atmospheric plasma spraying, and the degradation behavior caused by molten calcium-magnesium-aluminon-silicate (CMAS) attack was investigated. Results showed that the thermo-chemical reaction product between CMAS and YSZ (both powders and coatings) is different with the change of CMAS content. At low CMAS concentration, a cubic phase is generated by the diffusion of Ca into YSZ grains. As compared to the conventional YSZ, less C-ZrO₂ is detected for n-YSZ. When CMAS reaches a certain concentration (eg 15 mg/cm²), disruptive phase transformation from tetragonal to monoclinic will occur and the reaction is more readily for n-YSZ. Two different chemical reaction mechanisms governing the CMAS content effect were proposed. It should be noted that the nanozone in the coatings plays an important role in the CMAS degradation process, which enhances CMAS infiltration rate and accelerates the chemical reaction, leading to a poor CMAS resistance of the nanostructured coating than that of the conventional counterpart.     

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

About 6-8 wt% yttria-stabilized zirconia (YSZ) is the industry standard material for thermal barrier coatings (TBC). However, it cannot meet the long-term requirements for advanced engines due to the phase transformation and sintering issues above 1200°C. In this study, we have developed a magnetoplumbite-type SrAl₁₂O₁₉ coating fabricated by atmospheric plasma spray, which shows potential capability to be operated above 1200°C. SrAl₁₂O₁₉ coating exhibits large concentrations of cracks and pores (~26% porosity) after 1000 hours heat treatment at 1300°C, while the total porosity of YSZ coatings progressively decreases from the initial value of ~18% to ~5%. Due to the contribution of porous microstructure, an ultralow thermal conductivity (~1.36 W m⁻¹ K⁻¹) can be maintained for SrAl₁₂O₁₉ coating even after 1000 hours aging at 1300°C, which is far lower than that of the YSZ coating (~1.98 W m⁻¹ K⁻¹). In thermal cyclic fatigue test, the SrAl₁₂O₁₉/YSZ double-ceramic-layer coating undertakes a thermal cycling lifetime of ~512 cycles, which is not only much longer than its single-layer counterpart (~163 cycles), but also superior to that of YSZ coating (~392 cycles). These preliminary results suggest that SrAl₁₂O₁₉ might be a promising alternative TBC material to YSZ for applications above 1200°C.     

Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.     

Morphology and structure of YSZ powders: Comparison between xerogel and aerogel

Yttria-stabilized zirconia (YSZ) fine powders were prepared via sol–gel route in order to shape thermal barrier coatings (TBCs) from these powders. The main objective is to develop new undirectional coatings to allow best thermo-mechanical accommodations compared to conventional processes. To reach this aim, powders have to be able first to be highly dispersed into a sol (non-agglomeration, high specific surface area, etc.) and second to crystallize in the required metastable phase t′. Two routes have been used to dry gels: the conventional way which consists of simple evaporation of the solvent is compared to drying in supercritical conditions. Both YSZ powders after calcination at 950 °C of xerogel (Ex-xero-YSZ powder) and aerogel (Ex-aero-YSZ powder) crystallize in the tetragonal form. N₂ adsorption/desorption analysis of the Ex-xero-YSZ powder indicates an S_w of 2.8 m²/g. For the Ex-aero-YSZ powder, the S_w (26 m²/g) is much higher than of Ex-xero-YSZ, leading to a better sintering capability. This high S_w is correlated to the small crystallite size (26 nm) and the alveolar morphology of Ex-aero-YSZ powders compared to Ex-xero-powder (49 nm). By reducing particles size and increasing the S_w of the powders, supercritical drying appears as a promising way to prepare stable slurries or loaded sols from fine YSZ particles for TBC applications. Indeed, after preparing nanometric powders, they are dispersed into a sol before shaping on superalloys substrates. After thermal treatment at 950 °C for 2 h which corresponds to the working temperature of TBC, the final aim will be to prepare ceramic YSZ coatings.     

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.     

Thermal shock performance and failure behavior of Zr6Ta2O17-8YSZ double-ceramic-layer thermal barrier coatings prepared by atmospheric plasma spraying

Zr₆Ta₂O₁₇ has higher fracture toughness, better phase stability, thermal insulation performance and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (8 YSZ, 7–8 wt%) at temperatures above 1200 °C. However, the thermal expansion coefficients between Zr₆Ta₂O₁₇ coating and bond coating do not match well. A double-ceramic-layer design is applied to alleviate the thermal stress mismatch. The Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer thermal barrier coatings (TBCs) are prepared by atmospheric plasma spraying (APS). During the thermal shock test, Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer TBCs exhibit a better thermal shock resistance than 8 YSZ and Zr₆Ta₂O₁₇ single-layer TBCs. The thermal shock performance and failure mechanism of TBCs in the thermal shock test are investigated and discussed in detail.     

Thermal conductivity of plasma sprayed Sm2Zr2O7 coatings

Rare-earth zirconates with a pyrochlore structure have been developed for potential application in thermal barrier coating systems to further improve the performance and durability of gas turbines. The Sm₂Zr₂O₇ (abbreviated as SZ) powder was synthesized by solid state reaction and then deposited by air plasma spraying. The phase stability, microstructure and thermal conductivity of SZ and 8 wt% Y₂O₃ stabilized zirconia (8YSZ) coatings were investigated. The X-ray diffraction results indicated that the crystal structure of the as-sprayed SZ coatings was defect-fluorite, and after heat treating at 1200 °C for 50 h, it started to transform to pyrochlore, and the content of pyrochlore increased with increase in temperature of the heat treatment. The thermal conductivities of SZ coatings were significantly lower than those of 8YSZ coatings before and after heat treatments, which increased considerably after heat treatments compared to the as-sprayed states for both coatings due to sintering effects.     

YSZ/MoS2 self-lubricating coating fabricated by thermal spraying and hydrothermal reaction

Thermal sprayed ceramic coatings have extensively been used in components to protect them against friction and wear. However, the poor lubricating ability severely limits their application. Herein, yttria-stabilized zirconia (YSZ)/MoS₂ composite coatings were successfully fabricated on steel substrate with the combination of thermal spraying technology and hydrothermal reaction. Results show that the synthetic MoS₂ powders are composed of numbers of ultra-thin sheets (about 7 ~ 8?nm), and the sheet has obvious lamellar structure. After vacuum impregnation and hydrothermal reaction, numbers of MoS₂ powders, look like flowers, generate inside the plasma sprayed YSZ coating. Moreover, the growing point of the MoS₂ flower is the intrinsic micro-pores of YSZ coating. The friction and wear tests under high vacuum environment indicate that the composite coating has an extremely long lifetime (>?100,000 cycles) and possesses a low friction coefficient less than 0.1, which is lower by about 0.15 times than that of YSZ coating. Meanwhile, the composite shows an extremely low wear rate (2.30?×?10^?7 mm³ N^?1 m^?1) and causes slight wear damage to the counterpart. The excellent lubricant and wear-resistant ability are attributed to the formation of MoS₂ transfer films and the ultra-smooth of the worn surfaces of hybrid coatings.     

Interaction of CMAS on thermal sprayed ytterbium disilicate environmental barrier coatings: A story of porosity

Molten calcium magnesium alumina-silicates (CMAS) represent a challenge for the current generation of rare earth silicates environmental barrier coatings (EBCs). Their interaction with ytterbium disilicate (Yb₂Si₂O₇) free-standing coatings deposited using thermal spraying technique has been studied to further understand the reaction mechanisms. Three coatings, deposited with different porosity levels and thickness, representing traditional EBCs (<3% porosity and ～350 μm thickness) and abradable coatings (～20% porosity and 500–1000 μm thickness) were exposed to CMAS at 1350 °C. The results show that higher porosity levels facilitates CMAS infiltration in the first hour of exposure, in combination with infiltration through the inter-splat boundaries. Preferential dissolution of ytterbium monosilicate (Yb₂SiO₅) takes place, forming a 10–15 μm Ca₂Yb₈(SiO₄)₆O₂ apatite layer as the reaction product, producing a network of fine porosity (<10 μm) as the inter-splat boundary material is consumed. After exposure for 48 h, CMAS has completely infiltrated all three coatings, with apatite crystals present across the coatings, up to a depth of ～550 μm. Despite the extensive CMAS infiltration and apatite formation, no damage could be observed in any of the coatings, providing a promising first step for environmental barrier abradable coatings.     

Effect of suspension stability on bonding strength and electrochemical behavior of electrophoretically deposited HA–YSZ nanostructured composite coatings

In the present study, HA–YSZ nanostructured composites were deposited on Ti–6Al–4 V substrates by electrophoretic deposition of suspensions containing 0, 10, 20 and 40 wt% YSZ. The stability of each suspension was determined by applying response surface methodology, DLVO theory and zeta potential measurement for different YSZ contents and dispersant concentrations. The maximum zeta potential and electromobility of suspended particles was obtained for the suspension with 20 wt% YSZ. The electrophoretic deposition of HA–YSZ nanostructured composites was carried out at a constant voltage of 20 V for 120 s. The deposition kinetics was studied based on a mass-charge correlating approach under ranges of voltage (20–60 V), time (30–300 s) and wt% YSZ (0–40). The as–deposited and sintered HA–YSZ coatings were characterized by SEM, XRD, DSC–TG and FT–IR analyses. The micro-scratch behavior of coated samples indicated the highest critical contact pressures of crack initiation, P_c1 = 4.50 GPa, crack delamination, P_c2 = 5.14 GPa and fracture toughness, K_IC = 0.622 MPa m^1/2 for HA-20 wt% YSZ sample. The results of potentiodynamic polarization measurements showed that the implementation of 20 wt% YSZ could efficiently decrease the corrosion current density and corrosion rate of coated samples, while corrosion potential and linear polarization resistance were increased.     

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.     

Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

Influence of Y2O3 on the microstructure and tribological properties of Ti-based wear-resistant laser-clad layers on TC4 alloy

Multi-track Ti-based wear-resistant composite coatings were fabricated on TC4 alloy surfaces using laser-clad TC4 + Ni45 + Co–WC mixed powders with different Y₂O₃ contents (0, 1, and 3 wt%). The microstructure, microhardness, and tribological properties of the coatings were characterised using X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, electron probe X-ray micro analyser, microhardness tester, and friction and wear testing apparatus. The results showed that the number of cracks on the coating surfaces gradually decreased with the addition of Y₂O₃ and that residual Co–WC powders existed in the coating subsurfaces. The phase composition of the coatings with different Y₂O₃ contents remained unchanged and was mainly composed of reinforcing phases of TiC, TiB₂, Ti₂Ni, and matrix α-Ti. With the addition of Y₂O₃, the coating microstructure was remarkably refined, the direction characteristic of the TiC dendrites obviously weakened, and the nucleation rate significantly increased. When the added Y₂O₃ was 3 wt%, a large amount of TiB₂–TiC-dependent growth composite phases precipitated in the coating. The two-dimensional lattice misfit between (0001)_TiB2 and (111)_TiC was 0.912%, which indicated that TiB₂ and TiC formed a coherent interface. When the amount of Y₂O₃ was increased, the microhardness of the coatings gradually decreased, and the wear volume of the coatings first increased and then decreased. Under the effect of the TiB₂–TiC composite phases, the wear resistance of the 3 wt% Y₂O₃ coating was optimal. The 3 wt% Y₂O₃ coating friction coefficient was the lowest, and the wear mechanism was abrasive wear.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

A simple non-destructive method to indicate the spallation and damage degree of the double-ceramic-layer thermal barrier coating of La2(Zr0.7Ce0.3)2O7 and 8YSZ:Eu

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and Eu³⁺-doped zirconia, which was partially stabilised by 8 wt% yttria (8YSZ:Eu), were prepared by atmospheric plasma spraying. A thermal cycling test was carried out. The 8YSZ:Eu sublayer exposed during thermal cycling could produce visible luminescence under ultraviolet (UV) illumination, providing an indication of the spallation and damage degree of the coating. The result shows that the application of a Eu³⁺-doped luminescence sublayer can be a very simple and useful non-destructive technique to indicate the spallation and damage degree of DCL coatings.     

Hot Corrosion of Stabilized Zirconia Thermal Barrier Coatings and the Role of Mg Inhibitor

The 3–4 mol% yttria‐stabilized zirconia (YSZ) is widely used as a material for thermal barrier coating; however, the corrosive constituents present in fuel typically result in mechanical disintegration of YSZ coatings. The 3–4 mol% YSZ coatings with respective porosity of ~3% and ~22% have been undertaken with the objective to compare the hot corrosion behavior in air and sulfur‐rich atmospheres. The coatings are kept in contact with V₂O₅ + MgO powder mixture at 750°C for different dwell times of 24 and 76 h. The samples kept in air have shown intact YSZ layer for both the coatings, whereas a delamination of YSZ layer is observed for high porosity sample kept in sulfur‐rich atmosphere. XRD patterns of all the samples treated in sulfur‐rich atmosphere have indicated a phase transformation in YSZ from tetragonal to monoclinic. However, no such phase transformation has been found for samples treated in air. The V₂O₅‐induced hot corrosion attack on YSZ coating in air has been successfully inhibited by MgO, which forms a thermally stable Mg₃V₂O₈ compound. However, in sulfur‐rich atmosphere, MgO is partially consumed to form sulfates, which allows certain fraction of V₂O₅ to react with Y₂O₃ causing the degradation of top coat.     

The effect of ZrO2 alloying on the microstructures and thermal properties of DyTaO4 for high-temperature application

High fracture toughness of 8 YSZ (8 wt% yttria-stabilized zirconia) is linked to its ferroelastic toughening mechanism. In this work, the similar ferroelastic domain is detected in monoclinic Dy_1−xTa_1−xZr_2xO₄ ceramics, which derives from the ferroelastic transformation between the high-temperature tetragonal (t) and low-temperature monoclinic (m) phase. The lowest thermal conductivity of Dy_1−xTa_1−xZr_2xO₄ ceramics is reduced by 30% compared with 8 YSZ, and the largest thermal expansion coefficients (TECs) is up to 11 × 10⁻⁶ K⁻¹ at 1200°C, which is comparable to that of 8 YSZ. Notably, the systematic investigations containing phase, microstructure, thermophysical properties of Dy_1−xTa_1−xZr_2xO₄ ceramics will provide guidance for its high-temperature application, especially as thermal barrier coatings.