Y2O3 Thermal Barrier Coatings Research Articles

High-temperature cyclic oxidation of micro- and nano-ZrO2–25wt.%CeO2–2.5wt.%Y2O3 thermal barrier coatings at 1300 °C

In this study, the high-temperature cyclic oxidation resistance of micro- and nano-ZrO2–25wt.%CeO2–2.5wt.%Y2O3 (CYSZ) thermal barrier coatings (TBCs) was investigated. The TBCs were applied on IN738LC/CoNiCrAlY using the atmospheric plasma spray (APS) technique and subjected to high-temperature cyclic oxidation through 5-h cycles at 1300 °C. Various characterization techniques, including X-ray diffraction (XRD), Raman spectroscopy, nanoindentation, and field-emission scanning electron microscopy (FESEM), were used to assess the TBCs. Results indicated that annealing tetragonal zirconia at 1300 °C can generate antiphase boundary defects. The Young's modulus and hardness of nano- and micro-CYSZ TBCs decrease with the increase of antiphase boundary density. XRD and Raman results, the phase transformation behavior of tetragonal zirconia can be correlated to antiphase boundary density. Furthermore, the study showed that the nano-CSZ TBC has better oxidation resistance than the micro-CYSZ TBC at 1300 °C.

Experimental and numerical study of TGO-induced stresses of plasma-sprayed thermal barrier coating

Experimental and numerical studies were conducted in order to investigate the stress changes of ZrO2-8 wt% Y2O3 (YSZ) thermal barrier coatings (TBC) under isothermal loading. After conducting the isothermal oxidation test at a temperature of 1070 °C, the Thermally Grown Oxide (TGO) growth process and the life of samples were evaluated using scanning electron microscopy (SEM). The X-ray diffraction (XRD) approach was selected for the measurement of surface residual stress and its change in terms of TGO growth. Furthermore, a finite element model based on a real TGO profile, extracted by SEM, was executed, where TGO-induced stress was measured at the TGO/Top Coat (TC) interface direction. Employing the finite element model, following the selection of the appropriate creep strength based on experimental results, the stress evolution and distribution were assessed during time exposure at maximum temperature. It was shown that the lifetime of samples was about 240 h under isothermal loading of 1070℃. By increasing the time exposure, stresses increased and resulted in the formation of a delamination line above peak regions.

Development of deposition beam current dependent microstructure and nanomechanical properties in ZrO2-8wt%Y2O3 thermal barrier coatings produced by electron beam-physical vapor deposition technique

8 wt% yttria stabilized zirconia (8YSZ) coatings have been produced by changing the beam currents of electron beam-physical vapor deposition. The phase composition, microstructure, roughness, nanomechanical properties and residual stress were characterized and analyzed. The as-deposited 8YSZ coating was mainly composed of the metastable tetragonal phase. The enhancement of deposition beam current caused larger crystallite sizes and root mean square roughness, which confirmed by microscopic observations and atomic force microscopy technique. The nanomechanical properties were evaluated by an instrumented nanoindentation test. It was found that Young's modulus and hardness increase with the improvement of deposition beam current, which is due to the lower porosity. The thermal residual stresses generated in the deposition process was measured by two-dimension X-ray diffraction using sin2ψ technique. The measurement result indicated that residual stresses were dependent on the coating process.

Effect of Spray Distance on the Microstructure and High Temperature Oxidation Resistance of Plasma Spray-Physical Vapor Deposition 7YSZ Thermal Barrier Coating

In order to study the effect of spray distance on the structure and high temperature oxidation resistance of feather-columnar thermal barrier coatings, the feather-columnar ZrO2-7wt. % Y2O3 (7YSZ) thermal barrier coatings were prepared at spray distances of 650 mm, 950 mm, 1100 mm, 1250 mm, and 1400 mm by plasma spray-physical vapor deposition (PS-PVD) technology. The surface roughness, micro morphology, and porosity of the sprayed 7YSZ coating were analyzed by 3D surface profiler, SEM, XRD, etc., and the impedance spectrum characteristics of the 7YSZ coating were characterized by electrochemical alternating current (AC) impedance technology. In addition, the high temperature oxidation resistance test of 7YSZ coating under different spray distances was carried out at a temperature of 1000 °C to study the influence of spray distance on the high temperature oxidation resistance of 7YSZ coating. The research results show that the surface roughness and porosity of feather-columnar 7YSZ coating increased sequentially with the increase of spray distance. At the same time, The YSZ grain boundary resistance value increased exponentially as the porosity of the coating increases. Where the spray distance was in the range of 650 mm and 1250 mm, the high temperature oxidation rate constant of the coating increased with the spray distance. However, the spray distance was greater than 1250 mm, and the spray distance had no significant effect on the high temperature oxidation resistance of the coating.

Improving the Hot Corrosion Resistance of Nanostructured ZrO2-7wt.%Y2O3 Thermal Barrier Coatings Fabricated by Plasma Spraying

Conventional and nanostructured ZrO2-7wt.%Y2O3 thermal barrier coatings (TBCs) were prepared by plasma spraying technology. Microstructure and hot corrosion resistance of two TBCs in 75%Na2SO4+25%NaCl (mass fraction) mixed molten salt under 850 °C were compared. Meanwhile, hot corrosion failure mechanisms of TBCs were investigated. Results demonstrated that both conventional and nanostructured TBCs were typical lamellar stacking characteristic. However, compared with conventional TBC, the nanostructured TBC presented the base phase which is gained from recrystalization of fully melted nano-particles as well as the embedding phase of partially melted nano-particles. The nanostructured TBC has lower porosity and higher density. For both conventional and nanostructured TBCs, hot corrosion failure is caused by partial transformation of tetragonal phase (t-ZrO2) into monoclinic-phase (m-ZrO2) martensite as a response to the consumption of the stabilizer Y2O3. However, the nanostructured TBC generally has lower degree of hot corrosion and better protection to the metal bonding layer and matrix compared with conventional TBC.

Perspectives on Thermal Gradients in Porous ZrO2-7–8 wt.% Y2O3 (YSZ) Thermal Barrier Coatings (TBCs) Manufactured by Air Plasma Spray (APS)

Porous (~10–20%) ZrO2-7–8 wt.% Y2O3 (YSZ) thermal barrier coatings (TBCs) manufactured via air plasma spray (APS) and exhibiting a thickness range of ~250–500 µm, provide thermal insulation from the hot combustion gases to the metallic parts located in the hot stationary sections of gas turbine engines (e.g., combustion chambers of aerospace turbines). The objective of this paper was to measure and report the thermal gradient values in a benchmark porous (~15%) APS YSZ TBC, working within the known acceptable maximum temperature envelop conditions of a TBC/substrate system, i.e., T-ysz ~1300 °C and T-sub ~1000 °C. In order to accomplish this objective, the following steps were performed. A benchmark APS YSZ TBC exhibiting two distinct thicknesses (~260 and ~460 µm) was manufactured. In addition, a thermal gradient laser-rig was employed to generate a temperature drop (ΔT) along the coated coupon, with the target operate within the acceptable maximum temperature capabilities of this type of TBC/substrate architecture. This target was achieved, i.e., T-ysz values were not higher than ~1300 °C while the substrate temperatures did not reach values above ~1000 °C. The ΔTs for the ~260 and ~460 µm YSZ TBCs were ~280 and ~465 °C, respectively. The thermal gradient value for both YSZ TBCs was ~0.90 °C/µm, which falls within those reported in the literature for porous APS YSZ TBCs.

High temperature oxidation resistance and TGO growth mechanism of laser remelted thermal barrier coatings

In the present work, ZrO2–Y2O3 thermal barrier coatings (YSZ TBCs) and La2Zr2O7/ZrO2–Y2O3 thermal barrier coatings (LZ/YSZ TBCs) were prepared by atmospheric plasma spraying (APS) respectively, and then were surface remelted by laser remelting process. Thereupon, high temperature oxidation experiment was carried out at 1100 °C. The results showed that the compactness of these two kinds of as-sprayed TBCs was enhanced by remelting treatment. Hence, compared with the as-sprayed TBCs, the weight gains of the remelted TBCs were smaller, the thermally grown oxide (TGO) was thinner, and no obvious holes formed in the TGO. Al2O3, oxides of Cr, Co, Ni elements formed successively, eventually CS (spinel type oxides) formed in TGO of the remelted YSZ TBCs. However, due to the presence of La3+ in the remelted LZ/YSZ TBCs, the oxidation rate of Al, Cr, Co, Ni elements was slowed down, thus the formation of CS was delayed accordingly. Therefore, the high temperature oxidation resistance of the YSZ TBCs and the LZ/YSZ TBCs was improved by laser remelting treatment, and the remelted LZ/YSZ TBCs was superior to the remelted YSZ TBCs.

Effect of Bionic Unit Shapes on Solid Particle Erosion Resistance of ZrO2–7wt%Y2O3 Thermal Barrier Coatings Processed by Laser

Inspired by the coupling phenomena in biological systems, to improve the solid particle erosion resistance of Thermal Barrier Coatings (TBCs), different kinds of bionic units were made on the coating surfaces using Bionic Coupled Laser Remelting (BCLR) process. The NiCoCrAlYTa/ZrO2–7wt%Y2O3 double-layer structured TBCs were prepared by air plasma spraying. The microstructure, microhardness and phase composition of the as-sprayed and bionic specimens were examined. The solid particle erosion behaviors of bionic specimens as function of bionic unit shape were investigated. The results indicated that the bionic specimens had better erosion resistance than the as-sprayed specimen. The specimen with striation and grid bionic units had the better erosion resistance, while the dot showed the worse. The bionic units were characterized by the dense columnar crystal structure and the high hardness, which are the main reasons for improving the erosion resistance. Under the synergistic action of the shear stress and normal stress on the protrusive coating surface, the erosion failure of the as-sprayed TBCs was proved to be the fracture and spallation of the splats. By contrast, the spallation of segmented bionic unit occurred in the overlapping area between the adjacent laser irradiation, and the erosive unit surface presented the clear and deep furrows, which revealed that the erosion failure mechanism of bionic TBCs was dominated by brittle and some ductile erosion. These results showed more opportunities for bionic application in improving the solid particle erosion resistance of components in the windy and sandy environment.

Novel route of a self-healing film preparation in laser re-melted thermal barrier coatings

In this study, a novel self-healing method based on pressure sintering was employed for segmented cracks healing, the latter formed in the laser re-melted ZrO2-7wt% Y2O3 thermal barrier coatings (TBCs), by four sealed particles respectively. Full metallurgically bonded sealed films were formed in the segmented gaps for the application of nano-Al2O3+Ni-20wt% Al particles, whereas a partial healing occurred for the other particles. Also, the necessary thermodynamic analysis and formation mechanisms of the sealed films were also investigated. The cyclic oxidation results confirmed that the fully sealed nano-Al2O3 reinforced films had superior anti-oxidation resistance in comparison with the other sealed coatings.

Enhanced cyclic oxidation resistance through the self-healing of segmented cracks using nano-Al2O3/Ni-20 wt%Al particles in laser re-melted thermal barrier coatings

This paper introduces a novel and effective self-healing method of segmented cracks in laser re-melted ZrO2-7wt% Y2O3 thermal barrier coatings (TBCs). The method uses pressure sintering of nano-Al2O3/Ni-20wt%Al particles at 1150°C for 12h. After treatment to facilitate self-healing, a very dense and metallurgically bonded nano-Al2O3 reinforced Ni-matrix sealed film formed in the segmented gaps. Cyclic oxidation results indicate that dense sealed films can effectively suppress the growth of thermally grown oxides (TGO) and prevent the formation of other brittle and fragile oxides (i.e., spinels) at the top coat/bond coat (TC/BC) interface. Using the pressure-sintering effect, any aluminums contained in the sealed particles or films becomes prone to diffusion into the TC/BC interface to facilitate the formation of a thin-continuous TGO during the initial oxidation. This results in a significantly enhanced anti-oxidation effect of laser re-melted TBCs.

Impedance Analysis of 7YSZ Thermal Barrier Coatings During High-Temperature Oxidation

ZrO2-7 wt.%Y2O3 (7YSZ) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying. High-temperature oxidation of 7YSZ TBCs was accomplished at 950 °C and characterized by impedance spectroscopy and scanning electron microscopy with energy-dispersive spectrometry. The results indicated that the thermally grown oxide (TGO) mainly contained alumina. The increase of the thickness of the TGO layer appeared to follow a parabolic law. Impedance analysis demonstrated that the resistance of the TGO increased with increasing oxidation time, also following a parabolic law, and that characterization of the TGO thickness based on fitting an equivalent circuit to its measured resistance is feasible. The YSZ grain-boundary resistance increased due to increasing cracks within the coating for oxidation time less than 50 h. However, beyond 150 h, the YSZ grain-boundary resistance slightly decreased, mainly due to sintering of the coating during the oxidation process.

Influence of columnar grain microstructure on thermal shock resistance of laser re-melted ZrO2-7 wt.% Y2O3 coatings and their failure mechanism

In this study, influence of columnar grain morphology on thermal shock resistance of laser re-melted ZrO2-7wt.% Y2O3 thermal barrier coatings (TBCs) has been researched. The influence of variation in columnar grain microstructures on the fatigue mechanism of laser re-melted coatings in thermal shock experiments has been investigated. The thermal shock experiments have confirmed the positive effect of small grain size on thermal fatigue life. This is thought to have resulted from an increase in the number of grain boundaries that restrict the grain motion during deformation. Further, “void effect” could be responsible for promoting peeling of the laser re-melted coatings when spallation is less than 10% because it can cause a relatively large tensile stress between the columnar grains and the underlying splats. However, it is found to have negligible effect on the shedding of LRCs shed when the spallation percentage is 20% or more because the remaining LRCs are relatively homogeneous having less or no voids. The presence of separate columns and large intergranular clearance may have allowed better compliance of stress through free-movement of the individual columnar grains. There are many more restrictions on stress relaxation. An additional moving resistance is found to be present at the undulate intergranular interfaces for the irregular gear-like columnar grains with small inter-crystalline space and large intergranular contact area. The one-piece structure with pseudo-columnar grains was found to improve the thermal cycle lifetime slightly. It is believed to be due to its relatively lower strain tolerance as well as lower fracture toughness as compared to that of the structure with separate columnar grains.

Thermal Cyclic Resistance Behavior of Inconel 800 Super Alloy Substrate with Thermal Barrier Coatings by Plasma Spraying

Micro-structural aspects of thermal cyclic loading at maximum cyclic temperature of 11000C of ZrO2-7wt.%Y2O3 (7YSZ) thermal barrier coatings, which were deposited by air plasma spraying(APS) on a nickel based super alloys, Inconel 800 have been reported as a function of thermal cycles at 11000C. Thermal cycling test consisted of a 10minute heating and 10minute cooling. Microstructural characterization was done by scanning electron microscopy (SEM). Thermal fatigue damage was observed as spallation of YSZ coatings, which was initiated from sample corner and sustained upto 156 cycles, 30% spalling was taken as thermal fatigue life criteria for this test. But no other mode of failure except spallation was recognized throughout the cycling. All spallation was occurred in cooling time.

Air Plasma-Sprayed Yttria and Yttria-Stabilized Zirconia Thermal Barrier Coatings Subjected to Calcium-Magnesium-Alumino-Silicate (CMAS)

Yttria (Y2O3) and zirconia (ZrO2) stabilized by 8 and 20 wt.%Y2O3 thermal barrier coatings (TBCs) subjected to calcium-magnesium-alumino-silicate (CMAS) have been investigated. Free-standing Y2O3, 8 and 20 wt.%YSZ coatings covered with synthetic CMAS slurry were heated at 1300 °C in air for 24 h in order to assess the effect of Y2O3 on the corrosion resistance of the coatings subjected to CMAS. The microstructures and phase compositions of the coatings were characterized by SEM, EDS, XRD, RS, and TEM. TBCs with higher Y2O3 content exhibited better CMAS corrosion resistance. Phase transformation of ZrO2 from tetragonal (t) to monoclinic (m) occurred during the interaction of 8YSZ TBCs and CMAS, due to the depletion of Y2O3 in the coating. Some amounts of original c-ZrO2 still survived in 20YSZ TBCs along with a small amount of m-ZrO2 that appeared after reaction with CMAS. Furthermore, Y2O3 coating was found to be particularly highly effective in resisting the penetration of molten CMAS glass at high temperature (1300 °C). This may be ascribed to the formation of sealing layers composed of Y-apatite phase [based on Ca4Y6 (SiO4)6O and Y4.67(SiO4)3O] by the high-temperature chemical interactions of Y2O3 coating and CMAS glass.

Deposition and characterization of thermal barrier coatings of ZrO2–4 mol.% Y2O3–1 mol.% Gd2O3–1 mol.% Yb2O3

Multicomponent oxide-doped zirconia based thermal barrier coatings (TBCs) have been found to possess significantly lower thermal conductivity and better sintering resistance at high temperatures than current state of the art ZrO2–(7–8)wt.% Y2O3 TBCs. In this study, two TBC systems of multicomponent oxide-doped zirconia (ZrO2–4mol.% Y2O3–1mol.% Gd2O3–1mol.% Yb2O3) were produced with a three-cathode plasma generator TriplexPro-210. The first TBC system was deposited using a conventional fused and crushed spray powder. This powder was further processed by high energy ball milling to get a highly porous second TBC system. The two TBC systems were characterized in terms of their microstructures, phase compositions, mechanical properties and thermo-physical properties. Their thermal shock behavior was evaluated using thermal cyclic tests at 1150°C. The results showed that the second TBC system had a significantly lower thermal diffusivity and better thermal shock behavior compared to the first TBC system due to its highly porous microstructure.

激光重熔对等离子喷涂ZrO2-20%Y2O3热障涂层隔热性能的影响

激光重熔对等离子喷涂ZrO₂-20%Y₂O₃热障涂层隔热性能的影响

The Friction and Wear Properties of Conventional and Nanostructured ZrO<sub>2</sub>-7wt.%Y<sub>2</sub>O<sub>3</sub> Thermal Barrier Coatings Deposited on TiAl Intermetallic Alloy by Plasma Spraying

It has attracted very strong interests in enhancing the wear resistance of the TiAl intermetallic alloy. In this paper, both plasma-sprayed conventional and nanostructured ZrO2-7wt.%Y2O3 thermal barrier coatings (TBCs) were prepared on TiAl alloy, and microhardness of coating were investigated. Meanwhile, the dry sliding friction tests on the substrate, the traditional coating, and the nanostructured coating were evaluated by ball-on-disk tribometer at room temperature. The results show that the microhardness of nanostructured coating is higher than that of the conventional one. Lower friction coefficient and higher wear resistance than the original TiAl alloy is achieved in the coatings under wear test conditions. Due to the existence of nanostructured microstructures, the nanostructured coating exhibits the lowest friction coefficient, and the best wear resistance. The wear mechanism of the original TiAl substrate is severe adhesive wear. However, the wear mechanisms of the TBCs are mild ploughing and severe adhesion.

Evolution of high temperature corrosion behavior of La2(Zr0.7Ce0.3)2O7 with the addition of Y2O3 thermal barrier coatings in contacts with vanadate–sulfate salts

Thermal barrier coatings (TBCs) of La2(Zr0.7Ce0.3)2O7 with the addition of 3 wt.% Y2O3 (3Y-LZ7C3) were deposited by electron beam-physical vapor deposition (EB-PVD). The phase structure, surface and cross-sectional morphologies, hot corrosion behavior of the 3Y-LZ7C3 coatings were studied in detail. The present study investigates the hot corrosion performance of 3Y-LZ7C3 coating in the presence of molten mixture of Na2SO4+V2O5 at 1173K for 100h. X-ray diffraction (XRD) indicates that the chemical reaction between NaVO3 and 3Y-LZ7C3 produces LaVO4, YVO4, m-ZrO2 and CeO2, leaching La2O3 and Y2O3 from 3Y-LZ7C3 and causing the progressive destabilization transformation from t′ to m-ZrO2. The excess La2O3 in the as-deposited 3Y-LZ7C3 coating can largely aggravate the degradation of 3Y-LZ7C3 coating by both NaVO3 and Na2SO4 (Usually formation of the mixtures of LaVO3, (YkLa1−k)2O2SO4 and few of (YkLa1−k)2(SO4)3 (k=i or j) phases), resulting in very rapid disintegration of the coating. Phase transformation and chemical interaction are the primarily corrosive mechanisms for deterioration of 3Y-LZ7C3 coating.

Preparation and Solid Particle Erosion Behaviors of Plasma-Sprayed and Laser-Remelted ZrO<sub>2</sub>-7wt.%Y<sub>2</sub>O<sub>3</sub> Thermal Barrier Coatings

In this study, ZrO2-7wt.%Y2O3 thermal barrier coatings (TBCs) were prepared on TiAl base intermetallic alloy substrates by plasma spraying process. After that, the plasma-sprayed TBCs were laser remelted using a CO2 laser. Influences of laser remelting on the microstructure and solid particle erosion characterization of the coatings were researched. Meanwhile, the erosion damage modes of the two types of TBCs were discussed. The results show that the as-sprayed TBC has a typical lamellar stacking characteristic. The lamellar defect of the plasma-sprayed coating is erased, and the compactness of the coating is improved significantly after laser remelting. The laser-remelted coating consists of column-like crystals along the direction of the heat current. The laser-remelted coating had better erosion resistance than the as-sprayed coating. Owing to the limited bonding at the interfaces between lamellar, the spalling of the sprayed splats from the lamellar interface is mainly attributed to the erosion failure of the as-sprayed coating. In addition, crushing of brittle ceramic coating is also responsible for the erosion damage of the plasma-sprayed TBC. In contrast, cracking occurs within region near the surface of the laser-remelted layer and that erosion occurs mainly by removal of these small blocks. Moreover, the laser-remelted has evident mciro-cutting marks and shows some ductile erosion characteristic.

Investigation of Thermal Shock Behavior of Plasma-Sprayed NiCoCrAlY/YSZ Thermal Barrier Coatings

ZrO2-8wt.%Y2O3 (8YSZ) thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS) on NiCoCrAlY-coated Inconel 738LC substrates. The thermal shock behavior was investigated by quenching the samples in water with temperature of 20-25°C from 950°C. To study of failure mechanism results from thermal cycling, microstructural evaluation using scanning electron microscope (SEM), elemental analysis using energy dispersive spectroscopy (EDS) and phasic analysis using x-ray diffractometry (XRD) were done. The results revealed that failure of the TBC system was due to the spallation of ceramic top coat. Thermal mismatch stress was the major factor of TBC failure in thermal shock test.