Conventional Thermal Barrier Coatings Research Articles

Microstructural Characterization and Thermal Barrier Effects of Conventional and Nanostructured ZrO<sub>2</sub>-7wt.%Y<sub>2</sub>O<sub>3</sub> Thermal Barrier Coatings Deposited by Plasma Spraying

In this study, conventional and nanostructured MCrAlY/ZrO2-7wt.%Y2O3 double-layer thermal barrier coatings (TBCs) were fabricated on TiAl base intermetallic alloy substrates by the plasma spraying technique. The microstructural characterization and thermal insulation capability of the two types of coatings were comparatively researched. The results show that the conventional ceramic coating has a typical lamellar stacking characteristic. However, the nanostructured coating exhibits a bimodal microstructure, which is composed of both fully melted regions and partially melted regions (remained nanoparticles). The nanostructured TBCs has higher thermal barrier effect than the traditional one. The temperature drops of the nanostructured TBCs at 1100 °C increases 53% compared with that of the conventional TBCs.

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.

Effect of physical vapor deposited Al2O3 film on TGO growth in YSZ/CoNiCrAlY coatings

Thermal barrier coatings (TBCs) comprising of yttria stabilized zirconia (YSZ) ceramic top coat and CoNiCrAlY metallic bond coat have been widely used in gas turbines. However, the developed oxides layer in the interface of the top and bond coats during thermal exposure of the TBCs always results in the destruction of the system. In order to restrain the growth of oxides layer and improve the thermal shock resistance of TBCs, a thin Al2O3 film was pre-deposited on CoNiCrAlY bond coat by physical vapor deposition (PVD) technology. After thermal exposure, morphologies and phase compositions of the thermal growth oxides (TGO) layer in the conventional and pre-deposited Al2O3 film TBCs were examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). The residual stresses in the coatings were analyzed using micro-Raman spectroscopy (LabRam-1B). It was found that TGO layer formed in the conventional TBCs was mainly composed of Al2O3, (Cr,Al)2O3+(Co,Ni)(Cr,Al)2O4+NiO (CSN), and (Cr,Al)2O3+(Co,Ni)(Cr,Al)2O4 (CS), while in the treated TBCs, the formed TGO layer appeared more uniform and compact. The CSN and CS clusters, which are normally considered as a weakness for TBCs, were greatly limited. The residual stresses in the TBCs after thermal shock were also reduced by the deposition of Al2O3 film.

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.

Microstructure and indentation mechanical properties of plasma sprayed nano-bimodal and conventional ZrO2–8wt%Y2O3 thermal barrier coatings

The nanostructured agglomerated feedstock used for plasma spraying was obtained by the nanoparticle reconstituting technique. Nanostructured and conventional ZrO2–8wt%Y2O3 (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on 45# steel substrates with the NiCrAlY as the bond-layer. The microstructure and phase composition of feedstocks and corresponding coatings were characterized. The top layer of nanostructured 8YSZ TBCs is denser and has fewer defects than that of conventional TBCs. The elastic modulus, micro-hardness and Vickers hardness of nanostructured 8YSZ TBCs exhibit bimodal distribution while the conventional 8YSZ exhibit mono-modal distribution. The elastic modulus and elastic recoverability were also obtained by the nanoindentation test. The results indicate that the elastic modulus of nanostructured 8YSZ coating is lower than that of conventional 8YSZ coating, but the nanostructured 8YSZ coating has higher elastic recoverability than that of the conventional 8YSZ coating. The prediction of the average elastic modulus was established by the mixture law and weibull distribution according to the fraction of phases with different molten characteristic.

A novel structure design towards extremely low thermal conductivity for thermal barrier coatings – Experimental and mathematical study

In this paper, a novel structure design towards extremely low thermal conductivity for thermal barrier coatings (TBCs) has been proposed. The structure design includes layer structure design and the defects configuration design. Firstly, an experimental investigation has been performed. The La2Zr2O7 (LZ) which is one of the zirconate-based TBCs materials was chosen as the investigated material in the layer structure design. Nanostructured ZrO2–8wt%Y2O3 (8YSZ), conventional 8YSZ TBCs and double-ceramic-layer (DCL) La2Zr2O7/nanostructured 8YSZ (LZ/n-8YSZ) were prepared by atmospheric plasma spraying (APS) on 45#steel substrates with the NiCrAlY as the bond-coat, respectively. The insulation temperatures of the three coatings were measured by oxyacetylene flame flash method and the effective thermal conductivities of the as-sprayed coatings have been obtained. Secondly, the effect of defects configuration on the effective thermal conductivity has been discussed systematically based on the strict mathematical calculation combined with finite element simulation. Based on the experimental and mathematical studies, an ideal coating structure was proposed for further decreasing the effective thermal conductivity of the TBCs. The effective thermal conductivity of the DCL LZ/n-8YSZ TBCs based on the actual morphology was calculated using computational micro-mechanics (CMM) method.

Mitigation of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings

Abstract Ceramic thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, in the case of syngas-fired engines, fly ash particulate impurities that may be present in syngas can melt on the hotter TBC surfaces and form glassy deposits. These deposits can penetrate the TBCs leading to their failure. In experiments using lignite fly ash to simulate these conditions we show that conventional TBCs of composition 93 wt% ZrO 2 + 7 wt% Y 2 O 3 (7YSZ) fabricated using the air plasma spray (APS) process are completely destroyed by the molten fly ash. The molten fly ash is found to penetrate the full thickness of the TBC. The mechanisms by which this occurs appear to be similar to those observed in degradation of 7YSZ TBCs by molten calcium–magnesium–alumino–silicate (CMAS) sand and by molten volcanic ash in aircraft engines. In contrast, APS TBCs of Gd 2 Zr 2 O 7 composition are highly resistant to attack by molten lignite fly ash under identical conditions, where the molten ash penetrates ∼25% of TBC thickness. This damage mitigation appears to be due to the formation of an impervious, stable crystalline layer at the fly ash/Gd 2 Zr 2 O 7 TBC interface arresting the penetrating molten-fly-ash front.

Thermal properties of plasma-sprayed thermal barrier coating with bimodal structure

Nanostructured zirconia coatings have been prepared by atmospherical plasma spraying (APS) on NiCrAlY-coated superalloy substrates. The isothermal oxidation test results indicate that the oxidation kinetics of nanostructured TBC follows a parabolic law and the oxidation resistance of the nanostructured TBC is comparable to that of the conventional TBC. The nanostructured thermal barrier coatings exhibit excellent thermal cyclic resistance and low thermal diffusivity. The failure of the nanostructured TBC occurs within the top coat and close to the YSZ/thermal growth oxide interface. The thermal diffusivity of the coating is 90% of that of conventional thermal barrier coatings, and it increases after heat treatment at 1050°C for 34h. The increase in the thermal diffusivity of the coating is ascribed to grain growth, the crack healing as well as sintering neck formation.

Influence of heat treatment on nanocrystalline zirconia powder and plasma-sprayed thermal barrier coatings

Nanostructured zirconia top coat was deposited by air plasma spray and NiCoCrAlTaY bond coat was deposited on Ni substrate by low pressure plasma spray. Nanostructured and conventional thermal barrier coatings were heat-treated at temperature varying from 1050 to 1 250 °C for 2-20 h. The results show that obvious grain growth was found in both nanostructured and conventional thermal barrier coatings (TBCs) after high temperature heat treatment. Monoclinic/tetragonal phases were transformed into cubic phase in the agglomerated nano-powder after calcination. The cubic phase content increased with increasing calcination temperature. Calcination of the powder made the yttria distributed on the surface of the nanocrystalline particles dissolve in zirconia when grains grew. Different from the phase constituent of the as-sprayed conventional TBC which consisted of diffusionless-transformed tetragonal, the as-sprayed nanostructured TBC consisted of cubic phase.

Microstructure and Thermal Properties of Plasma Sprayed Thermal Barrier Coatings from Nanostructured YSZ

Nanostructured yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were produced by atmospheric plasma spraying. The microstructure of the sprayed coating was characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM). The nano-coating had a higher porosity of ~25% than the conventional coating, which is mainly attributed to the large amount of intersplat gaps in the nano-coating. The thermal conductivity for the nano-coating was in the level of 0.8-1.1 W/m·K, about 40% lower than that for the conventional coating, indicating a better thermal insulation performance. The nano-TBC exhibited a thermal cycling lifetime of more than 500 cycles, whereas spallation failure of the conventional TBC occurred within 200 cycles. Accelerated sintering could be one of the reasons for the failure of the nano-TBC.

Comparison of thermal shock behaviors between plasma-sprayed nanostructured and conventional zirconia thermal barrier coatings

NiCoCrAlTaY bond coat was deposited on pure nickel substrate by low pressure plasma spraying(LPPS), and ZrO 2-8%Y 2O 3 (mass fraction) nanostructured and ZrO 2-7%Y 2O 3 (mass fraction) conventional thermal barrier coatings(TBCs) were deposited by air plasma spraying(APS). The thermal shock behaviors of the nanostructured and conventional TBCs were investigated by quenching the coating samples in cold water from 1 150, 1 200 and 1 250 °C, respectively. Scanning electron microscopy(SEM) was used to examine the microstructures of the samples after thermal shock testing. Energy dispersive analysis of X-ray(EDAX) was used to analyze the interface diffusion behavior of the bond coat elements. X-ray diffractometry(XRD) was used to analyze the constituent phases of the samples. Experimental results indicate that the nanostructured TBC is superior to the conventional TBC in thermal shock performance. Both the nanostructured and conventional TBCs fail along the bond coat/substrate interface. The constituent phase of the as-sprayed conventional TBC is diffusionless-transformed tetragonal( t′). However, the constituent phase of the as-sprayed nanostructured TBC is cubic( c). There is a difference in the crystal size at the spalled surfaces of the nanostructured and conventional TBCs. The constituent phases of the spalled surfaces are mainly composed of Ni 2.88Cr 1.12 and oxides of bond coat elements.

Thermal Failure of Nanostructured Thermal Barrier Coatings with Cold-Sprayed Nanostructured NiCrAlY Bond Coat

Nanostructured thermal barrier coatings (TBCs) were deposited by plasma spraying using agglomerated nanostructured YSZ powder on Inconel 738 substrate with cold-sprayed nanostructured NiCrAlY powder as bond coat. The isothermal oxidation and thermal cycling tests were applied to examine failure modes of plasma-sprayed nanostructured TBCs. For comparison, the TBC consisting of conventional microstructure YSZ and conventional NiCrAlY bond coat was also deposited and subjected to the thermal shock test. The results showed that nanostructured YSZ coating contained two kinds of microstructures; nanosized zirconia particles embedded in the matrix and microcolumnar grain structures of zirconia similar to those of conventional YSZ. Although, after thermal cyclic test, a continuous, uniform thermally grown oxide (TGO) was formed, cracks were observed at the interface between TGO/BC or TGO/YSZ after thermal cyclic test. However, the failure of nanostructured and conventional TBCs mainly occurred through spalling of YSZ. Compared with conventional TBCs, nanostructured TBCs exhibited better thermal shock resistance.

Glass–ceramics as oxidation resistant bond coat in thermal barrier coating system

Thermal barrier coating (TBC) system consisting of yttria stabilized zirconia (YSZ) top coat, glass–ceramic bond coat and nickel base superalloy substrate was subjected to static oxidation test at 1200 °C for 500 h in air. Oxidation resistance of this TBC system was compared with the conventional TBC system under identical heat treatment condition. Both the TBC systems were characterized by SEM as well as EDX analysis. No TGO layer was found between the bond coat and the top coat in the case of glass–ceramic bonded TBC system while the conventional TBC system exhibited a TGO layer of about 16 μm thickness at the bond coat-top coat interface region.

Lowering the sintering temperature for EPD coatings by applying reaction bonding

Electrophoretic deposition (EPD) allows the fabrication of ceramic coatings, for example of thermal barrier coatings for turbine engines, at lower cost and higher speed than most other deposition techniques. The commonly used sintering temperatures of more than 1200 °C however may damage the metal parts and also lead to high production costs. This study suggests a powder mixture of zirconia and zirconium nitride to lower the sintering temperature, where during sintering in air at 1000 °C a homogenous, porous zirconia coating was obtained. The thermal conductivity determined by laser flash experiments proved to be significantly lower than that of conventional thermal barrier coatings.

Phase stability of thermal barrier oxides: A comparative study of Y and Yb additions

Abstract The maximum operating temperature of conventional thermal barrier coatings based on yttria-stabilized zirconia is ultimately limited by de-stabilization of the “non-transformable” t phase, rendering it susceptible to the monoclinic transformation upon cooling. Investigations into alternative thermal barrier oxide compositions suggest that Yb offers superior t stability compared with Y, Sc and larger rare earth cations at the same concentration. The present study sheds light on this behavior by comparing the microstructure evolution of specimens with 7.6 and 11.4 % MO1.5 (M = Y or Yb) heat treated at 1450 °C for times up to 512 h. X-ray diffractometry and transmission electron microscopy revealed that the onset of partitioning occurs at short times but then the compositions of the phases evolve slowly over time until sufficient stabilizer is depleted from the t phase to render it transformable. Substitution of Yb for Y delays the onset of monoclinic formation. Differences in the transformation behavior of the Y and Yb rich phases on cooling provide new insight and suggest refinements to the current thermodynamic models for the binary ZrO2 – MO1.5 systems are needed.

Microstructures and Properties of Thermal Barrier Coatings Plasma-Sprayed by Nanostructured Zirconia

Abstract In gas turbine engine, the study of thermal barrier coatings (TBCs) has always been paid more attention because it can effectively reduce metal interface temperatures, reduce oxidation and extend life. In this paper, the microstructures and properties of TBCs plasma-sprayed by nanostructured zirconia were investigated. The TBCs had higher adhesive strength, better thermal shock resistance and micro-cracking resistance to compare with that of conventional TBCs of magnesia and yttria stabilized zirconia, which was due to closed packed structures with a small amount of laminar structures and cavities. The increase of quantities of thermal growth oxide (TGO) was mainly responsible for deteriorating the properties of TBCs.

Fundamental Coating Development Study to Improve the Isothermal Oxidation Resistance and Thermal Cycle Durability of Thermal Barrier Coatings

Thermal barrier coatings(TBCs) are used in high temperature gas turbines to reduce the surface temperature of cooled metal parts such as turbine blades[1]. TBC consist of a bondcoat (e.g. MCrAlY where M is Co, Ni, CoNi, etc.) and a partially stabilized zirconia ceramic topcoat. Usually, the MCrAlY bondcoat is applied by LPPS (low pressure plasma spray) or HVOF(high velocity oxi-fuel spray). The topcoat is applied by APS (atmospheric plasma splay) or EB-PVD (electron beam-physical vapor deposition). High temperature oxidation properties, thermal barrier properties and durability of TBC are very important to increase the reliability in high temperature service. In this study, new TBC has been investigated. The new TBC consists of a two-layered bondcoat (LPPS-MCrAlY plus dense PVD overlay MCrAlY) and the EB-PVD type YSZ columnar structure topcoat. As a result of evaluation tests, it was confirmed that the new TBC had better oxidation properties and durability than a conventional TBC system.

Comparison of Microstructure and Oxidation Behavior of CoNiCrAlY Bond Coatings Prepared by Different Thermal Spray Processes

To protect various gas turbine components against high temperature in the hot sections of power generation plants and aircraft engines, thermal barrier coatings (TBCs) have been developed and widely used. Conventional TBCs consist of a MCrAlY bond coating for oxidation resistance and a ceramic top coating for thermal insulation. High quality coatings of MCrAlYs have been produced mostly by low pressure plasma spraying but other more economical processes are also used depending on the operating conditions of the component to be coated. In this study, CoNiCrAlY powders were deposited on Inconel 718 substrate with 3 types spraying system, i.e., low pressure plasma spraying, high velocity oxy-fuel spraying, and atmosphere plasma spraying. The specimens without top ceramic coating were isothermally tested for up to 100 hrs in air at 1373 K and mass gain of the coatings was measured. Microstructure of the coating cross sections and the surface oxides were observed with SEM. Moreover, phase changes during the oxidation test were investigated with calculated phase diagrams for the CoNiCrAlY alloy. [doi:10.2320/matertrans.47.1638]

Thermal cycling life and thermal diffusivity of a plasma-sprayed nanostructured thermal barrier coating

Abstract The nanostructured thermal barrier coatings exhibited excellent thermal cyclic resistance and low thermal diffusivity. The thermal cyclic life of the coating is about 600 h for 1-h cycles at 1050 °C. The thermal diffusivity of the coating is 90% of that of conventional thermal barrier coatings.

縦割れ遮熱コーティング材の機能性評価

Recently, metal temperature of combustor and blade in a land-based gas turbine is increasing with inlet gas temperature, in order to improve the thermal efficiency. Thus, thermal barrier coating (TBC) technology is applied to the surface of these hot parts for reducing the metal temperature. Advanced TBC materials and its processing technology such as functionally graded TBC and electron beam deposited TBC had been developed for these demands. But, application of these developed TBCs to the actual turbine blade was found to be difficult, because of a high cost of the processing machine and so on.In this study, advanced TBC with vertical crack in the top coating layer (vertical-crack induced TBC) was tried to fabricate by using a combination of conventional thermal spray technology and the special heat treatment. Then, the material properties, such as thermal stress variation of the TBC during thermal cycling, bonding strength between the top coating and bond coating and the interface oxidation process, were examined. Through some material evaluations for the vertical-crack induced TBC, it was found that the thermal stress in the top coating, which is caused during thermal cycling, is very lower than one of a conventional TBC. The bonding strength of the vertical-crack induced TBC is also higher, and then its TBC interface oxidation thickness is very thin. The vertical-crack induced TBC, which was evaluated here, was found to be superior to the conventional TBC.