Zirconia Thermal Barrier Coatings Research Articles

Effect of Particle In-Flight Behavior on the Microstructure and Fracture Toughness of YSZ TBCs Prepared by Plasma Spraying

The present study aims to elaborate particle in-flight behavior during plasma spraying and its significance in determining the microstructure and mechanical properties of plasma sprayed yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The as-sprayed YSZ coatings were characterized in terms of defects (such as pores, unmelted particles and cracks) and fracture toughness. The results showed that, due to the higher temperature and velocity of in-flight particles in a supersonic atmospheric plasma spraying (SAPS) compared to that of atmospheric plasma spraying (APS), denser coatings were formed leading to a better fracture toughness. The percentage of defects of the microstructure was similar to the temperature and velocity of particles in-flight during plasma spraying. Furthermore, the structural defects had a strong effect on its mechanical behavior. The total defect percentage and fracture toughness in SAPS-TBCs spanned 6.9 ± 0.17%–13.26 ± 0.22% and 2.52 ± 0.06 MPa m1/2–1.78 ± 0.19 MPa m1/2; and 11.11 ± 0.36%–17.15 ± 0.67% and 2.13 ± 0.08 MPa m1/2–1.4 ± 0.12 MPa m1/2 in APS-TBCs.

HARDNESS AND FRATURE TOUGHNE OF PLASMA SPRAYED ZIRCONIA-CERIA-YTTRIA THERMAL BARRIER COATINGS

Several studies have been made during the recent years to evaluate the plasma sprayedyttria partially stabilized zirconia thermal barrier coatings but less attention wereundertaken on ceria-yttria stabilized zirconia. In this study, zirconia- 20 wt% ceria-3.6wt% yttria thermal barrier coatings were produced by air plasma spraying deposited onIN 738 LC superalloy substrate. The objective of this study is to analysis the distributionof porosity, hardness and fracture toughness on the deposited coatings. Under optimumplasma spraying conditions, it is dangerous to consider the mean values of hardness andfracture toughness. The distributions of hardness and fracture toughness are highlyscattered. The Weibull distribution is the main parameter to describe the plasma sprayedcoatings hardness and fracture toughness.

Preparation of Dense Al<sub>2</sub>O<sub>3</sub> Film on 8YSZ Coating by Sol-Gel Method

The 8 wt.% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are widely used to protect hot superalloy components of gas turbine and jet engine. However, Cracks and pores in the 8YSZ coating would accelerate the oxidation of the bondcoat alloy and the corrosion of 8YSZ topcoat by molten salts and finally result in permanent failure of the TBCs. Hence, in this work, a sol-gel spin-coating method was employed to prepare a dense α-Al2O3 film on the surface of air-plasma-sprayed 8YSZ coatings, in order to block the paths of cracks and pores while keeping the stress tolerance of 8YSZ coatings. The influences of the polyvinylpyrrolidone (PVP) and alumina content of the sol on the microstructure of the α-Al2O3 film were investigated. It is found that a sol with PVP addition amount of 50 mg/mL and alumina solid content of 10% let to the achievement of a compact α-Al2O3 film. The positive results in this work would inspire more efforts to prepare dense films on TBCs and therefore improve their performance under service conditions.

Temperature induced structure degradation of yttria-stabilized zirconia thermal barrier coatings

The structural aspects of yttria-stabilized zirconia (YSZ) thermal barrier coatings grown on a polycrystalline alumina substrate have been studied as a function of annealing time at 1250 °C with the use of electron microscopy, X-ray diffraction, and Raman spectroscopy. Upon the high temperature exposure, the feathery morphology of the columnar grains of the as-deposited coatings smoothens out with the concurrent development of surface undulations and grain coarsening. It is shown that the grain coarsening results in significant voiding along the merged grain boundaries and inside YSZ grains. The nucleation and growth of the high density of voids is related to the clustering of vacancies supplied from the merged gaps between columnar grains and subgrains within the columnar grains. The continued development of the voids leads to the formation and propagation of large cracks within the columnar grains, which weakens the overall mechanical integrity of the coating.

A multilayer structure shear lag model applied in the tensile fracture characteristics of supersonic plasma sprayed thermal barrier coating systems based on digital image correlation

We developed a multilayer structure shear lag model used in analyzing tensile fracture behavior of supersonic plasma sprayed 8 wt% yttria-stabilized zirconia thermal barrier coatings (TBCs). The closed-form solutions of plane and shear stresses were deduced by using the shear lag model. Dynamic strain fields, cracking nucleation and propagation features of TBCs during tensions were in-situ monitored by non-contact digital image correlation (DIC) measurement system. The evolutions of plane and shear stresses in TBCs were estimated by the above closed-form solutions. Using in-situ measurement results of DIC, we further obtained the fracture strengths, interface shear strengths, fracture energy and fracture toughness of TBCs, and summarized the whole tensile fracture mechanisms of TBCs. This shear lag model with the aid of DIC is useful for measuring the mechanical properties of other novel multiple-layer structure brittle coating/ductile substrate systems.

Hot corrosion resistance of air plasma sprayed ceramic Sm2SrAl2O7 (SSA) thermal barrier coatings in simulated gas turbine environments

Samarium strontium aluminate (Sm2SrAl2O7-SSA) and Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were developed on NiCrAlY bond coated Inconel 718 superalloy substrate using air plasma spray process. The hot corrosion study was conducted in simulated gas turbine environments (molten mixtures of 50 wt% Na2SO4 + 50 wt% V2O5 and 90 wt% Na2SO4 + 5 wt% V2O5 + 5 wt% NaCl) for two different temperatures of 700 and 900 °C. A developed SSA TBCs showed about 8% and 22% lower lifetime at 700 and 900 °C, respectively than YSZ TBCs in 50 wt% Na2SO4 + 50 wt% V2O5 (vanadate). The hot corrosion life of SSA TBCs being found about 13% and 39% lower than YSZ TBCs in 90 wt% Na2SO4 + 5 wt% V2O5 + 5 wt% NaCl (chloride) at 700 and 900 °C, respectively. X-ray diffraction results showed the formation of SmVO4, SrV2O6, and SrSO4 as a major hot corrosion product in 50 wt% Na2SO4 + 50 wt% V2O5 and 90 wt% Na2SO4 + 5 wt% V2O5 + 5 wt% NaCl environments respectively for SSA TBCs. Similarly, YSZ TBCs also showed YVO4 as hot corrosion product in vanadate and chloride environments. Both the TBCs suffer a more severe hot corrosion attack in chloride environment at 900 °C. The leaching of Sr2+ and Y3+ ions from SSA and YSZ respectively play a vital role in the destabilization of coating in vanadate and chloride environments at 700 and 900 °C. In both SSA and YSZ TBCs, the leaching of ion has significantly low influence as compared to attack by chloride ions at the bond coat-top coat interface in the presence of chloride environment. The hot corrosion resistance of SSA TBCs was improved three times higher in the presence of MgO and NiO inhibitor in vanadate environment at 900 °C mainly due to the formation of a stable Ni3V2O8 phase at the surface.

Primerjava med mikrostrukturami in oksidacijskimi obnašanji z itrijem in magnezijem stabiliziranih cirkonskih prevlek za termično zaščito (TBC)

Primerjava med mikrostrukturami in oksidacijskimi obnašanji z itrijem in magnezijem stabiliziranih cirkonskih prevlek za termično zaščito (TBC)

Structure-property relationship and design of plasma-sprayed La2Ce2O7/8YSZ composite coatings for gas turbine blades

Novel lanthanum-cerium oxide/8 wt% yttria partially stabilized zirconia (LC/8YSZ) thermal barrier coatings (TBCs) were deposited by supersonic atmospheric plasma spraying. The thermal insulation temperature and thermal shock resistance of LC/8YSZ double-ceramic-layer TBCs (DCL-TBCs) were quantitatively evaluated by a burner rig test. The results showed that the thermal insulation temperature increased with the increase of LC layer thickness in DCL-TBCs. When the thickness ratio between LC layer and 8YSZ layer was close to 1:1, the DCL-TBCs had the highest thermal shock resistance. LC/8YSZ thickness ratio significantly affected the energy release rate and the stress induced by thermal gradient or sintering. The sintering stress was found to be the main reason that caused the delamination of LC layer, however, the stress induced by thermal gradient resulted in the spallation of YSZ layer.

In situ transmission electron microscopy of high-temperature degradation of yttria-stabilized zirconia thermal barrier coatings

The initial stage of degradation process of yttria partially stabilized zirconia thermal barrier coatings during heating at 1000–1450K were observed in situ by transmission electron microscopy. The observation directly demonstrated that the formation of several kinds of oxides and resulting void growth started above 1000K in the bond-coat layer near the interface with the thermally grown oxide layer, leading to the development of voids over the entire interface due to void coalescence. Such development of voids corresponds to the generation and propagation of thermally grown cracks, i.e., to the initial spallation of the coating.

Effect of Isothermal Heat Treatment on Cyclic Life of Yitria Stabilized Zirconia and Gadolinium Zirconate Thermal Barrier Coating

The effect of isothermal heat treatment on the thermal cyclic life of yttria stabilized zirconia (YSZ) and gadolinium zirconate (GZ) thermal barrier coatings (TBCs) have been investigated. The purpose of the isothermal heat treatment is to simulate thermal exposure on the TBC in the gas turbine. Duplex YSZ TBC with YSZ as a topcoat and NiCrAly as a bondcoat and five-layered GZ TBC with GZ as the topcoat (GZ / 50 wt.%GZ+50 wt.%YSZ / YSZ / 50 wt.%YSZ+50 wt.% NiCrAly / NiCrAlY) were plasma sprayed onto Nimonic 263 substrates to a total thickness of 900µm using F4-MB gun. The isothermal heat treatment was performed at a temperature of 1100°C for 100 hours with a heating rate of 250°C/hour in an air furnace. The thermal cyclic tests were performed with the following cycle in an air furnace: heat up from 1000°C to 1200°C within 1 hour, dwell at 1200°C for 5 minutes, water quench and oven dry. The cycle is repeated until 20% spallation of the coating. Result indicates that the isothermally heat treated five-layered GZ TBC has a better cyclic life than the duplex YSZ TBC and as-sprayed GZ TBC.

Damage evolution and failure mechanism of thermal barrier coatings under Vickers indentation by using acoustic emission technique

Acoustic emission (AE) technique was adopted to monitor the damage evolution of air plasma- sprayed (APS) yttria-partially-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) during instrumented indentation testing, and then the failure mechanisms were investigated by cluster analysis and wavelet transform methods. The results of cluster analysis showed that there were three classes associated with distinct failure types for the 8YSZ coatings under Vickers indentation. Based on wavelet transform, these three clusters could be clearly distinguished from their dominant frequency bands, which were concentrated on levels A5 (0–156.25 kHz), D5 (156.25–312.5 kHz) and D4 (312.5–625 kHz), respectively. Thus, the failure mechanism of 8YSZ coatings under Vickers indentation could be clarified by the distribution of different failure types in indentation depth. To sum up, as indentation load increases, the 8YSZ coatings can accommodate the indenter by elastic or little plastic deformation, microcracks propagation and then debonding at the splat boundaries. By comparing the distribution of AE signals induced by different failure types in indentation depth for samples with different thermal exposure time, it can be inferred that thermal exposure treatment can accelerate the degradation of APS 8YSZ TBCs.

Thermal properties and phase stability of Yttria-Stabilized Zirconia (YSZ) coating deposited by Air Plasma Spray onto a Ni-base superalloy

Thermal properties and microstructure characterization of Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coatings (TBCs) deposited by Air Plasma Spray (APS) onto a Ni-base superalloy (Inconel 625) were studied. Two separate sets of tests were performed. The first one consisted in Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) performed over free-standing TC samples detached from TBCs. The second one included the analysis of the cross section of samples heat treated at 1100°C with holding times of 600, 1000, and 1700h. The TC porosity was analyzed for different heat treatment conditions so that inter-lamellar, intra-lamellar and globular pores, as well as cracks, were identified and quantified independently. An initial porosity reduction related to inter-lamellar and intra-lamellar pores, as well as cracks, was observed during the first 600h of heat treatment, due to sintering. However, porosity continually increased during heat treatment from 600 to 1700h driven by volumetric changes associated to phase transformations. During this period, yttrium diffused from the metastable tetragonal phase favoring the transformation to cubic phase while monoclinic phase transformed after cooling from the yttrium-depleted tetragonal phase. Energy absorption curves and the variation of heat capacity with temperature were also determined and correlated to microstructural changes.

Effect of spraying power on microstructure and property of nanostructured YSZ thermal barrier coatings

Nanostructured yttria stabilized zirconia (n-YSZ) thermal barrier coatings were fabricated by atmospheric plasma spraying (APS) at different spraying powers. The microstructures, phase stability and mechanical properties of the n-YSZ coatings were examined by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers indentation, respectively. The adhesion strength of coatings was evaluated according to ASTM C633-01 standard. Thermal cyclic oxidation method was carried out to study the effect of spraying power on thermal shock resistance of the coatings. Results showed that the n-YSZ coatings had a bimodal microstructure consisting of well melted splats and partially melted nanostructured areas. Both the porosity and the content of nanostructure in n-YSZ coatings decreased with increasing of the spraying power. After heat treatment at 1573 K, the porosity of the coating decreased and the hardness increased noticeably compared with that of the as-sprayed coating, indicating the sintering effect of the nanostructured coating. However, porosity of the coating deposited by the lowest spraying power (22 kW) still retain above 10% after annealing at 1573 K for 24 h, showing higher sintering resistant ability. Although phase structures certainly changed during heat treatment, phase compositions were more stable for the coatings deposited by the relatively low spraying powers. The average bond strength and thermal cycling life of the coating prepared by the lowest spraying power (22 kW) were about 29 MPa and 292 cycles, respectively, which were approximately twice as much as those of the coating deposited by the highest spraying power (42 kW). The best thermal shock resistance obtained for the n-YSZ coatings prepared at 22 kW can be partially attributed to its high content of nanozones, which play an important role in reducing the oxygen diffusion, releasing the thermal stress and interrupting crack propagation.

Strontium Zirconate TBC Sprayed by a High Feed-Rate Water-Stabilized Plasma Torch

A novel thermal barrier coating (TBC) material, strontium zirconate SrZrO3, was sprayed by a high feed-rate water-stabilized plasma torch WSP 500. Stainless steel coupons were used as substrates. Coatings with a thickness of about 1.2 mm were produced, whereas the substrates were preheated over 450 °C. The torch worked at 150 kW power and was able to spray SrZrO3 with a high spray rate over 10 kg per hour. Microstructure and microhardness, phase composition, adhesion, thermal conductivity and thermal expansion were evaluated. The coating has low thermal conductivity under 1 W/m K in the interval from room temperature up to 1200 °C. Its crystallite size is slightly over 400 nm and thermal expansion 12.3 µm K−1 in the similar temperature range.

Hot corrosion behavior of double ceramic layered CaZrO3/Yttria‐stabilized zirconia coatings

AbstractA novel double ceramic layered (DCL) CaZrO3/Yttria‐stabilized zirconia (YSZ) thermal barrier coatings (TBCs) was designed for improved service life against sulfate vanadate hot corrosion as compared with that of YSZ single layered coating. The hot corrosion behavior of DCL CaZrO3/YSZ coatings was studied at 950°C after dry spreading 50%Na2SO4+50%V2O5 mixture onto a coated surface. The CaZrO3 as the topmost layer in DCL CaZrO3/YSZ coatings, served as a sacrificial layer during sulfate vanadate hot corrosion protecting the underneath YSZ coating. The corrosion reactions in this case were sluggish due to the initial formation of low melting point meta‐calcium vanadate (CaV2O6) that isothermally transformed to higher melting point calcium vanadates having higher calcia (CaO) content. The corrosion reaction products sealed the top surface, impeding the oxygen movement and eventually retarded the thermally grown oxide (TGO) growth. The sulfate vanadate hot corrosion life of the DCL CaZrO3/YSZ coatings was observed to be more than double as compared with single ceramic layered YSZ coatings.

Impact of a Yttria-Stabilized Zirconia Thermal Barrier Coating on HCCI Engine Combustion, Emissions, and Efficiency

In-cylinder surface temperature has significant impacts on the thermo-kinetics governing the homogeneous charge compression ignition (HCCI) process. Thermal barrier coatings (TBCs) enable selective manipulation of combustion chamber surface temperature profiles throughout a fired cycle. In this way, TBCs enable a dynamic surface temperature swing, which prevents charge heating during intake while minimizing heat rejection during combustion. This preserves volumetric efficiency while fostering more complete combustion and reducing emissions. This study investigates the effect of a yttria-stabilized zirconia (YSZ) coating on low temperature combustion (LTC), efficiency, and emissions. This is an initial step in a systematic effort to engineer coatings best suited for LTC concepts. A YSZ coating was applied to the top of the aluminum piston using a powder air plasma spray (APS) process; final thickness of the YSZ was approximately 150 μm. The coated piston was subsequently evaluated in the single-cylinder HCCI engine with exhaust re-induction. Engine tests indicated significant advancement of the autoignition point and reduced combustion durations with the YSZ coating. Hydrocarbon and carbon monoxide emissions were reduced, thereby increasing combustion efficiency. The combination of higher combustion efficiency and decreased heat loss during combustion produced tangible improvements in thermal efficiency. When the effects of combustion advance were removed, the overall improvements in emissions and efficiency were lower, but still significant. Overall, the results encourage continued efforts to devise novel coatings for LTC.

Relationship between cracks and microstructures in APS YSZ coatings at elevated temperatures

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. Thermally grown oxide (TGO) was formed between the YSZ layer and the bond coat during heat treatment of the YSZ TBCs. The generation and extension behavior of cracks before and after heat treatment were characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) combined with 3D reconstruction technology. The characterization results give a direct evidence that the generation and extension of cracks is closely related to other microstructures such as columnar grains, TGO morphology, TGO composition, and TGO orientation. For example, large interlaminar cracks in as-sprayed YSZ coatings often appeared along with columnar grains, which can be explained by their similar formation conditions. The formation of new cracks such as vertical cracks, plane cracks and other cracks along the TGO grain boundary after heat treatment often resulted from TGO growth stresses. Therefore, the generation and extension of cracks can be controlled by regulating the formation of columnar grains and TGO.

Calcium-rich CMAS corrosion induced microstructure development of thermal barrier coatings

Ca50Mg10Al10Si30 (CMAS) corrosion induced microstructure evolution and composition development in yttria-stabilized zirconia (YSZ) thermal barrier coatings prepared by electron-beam physical vapor deposition (EB-PVD), have been investigated by means of analytical electron microcopy, Raman spectroscopy and X-ray diffraction. The corroded YSZ still mainly consists of t-ZrO2 and no harmful monoclinic zirconia formation occurs; however, the feathery and columnar morphology of the as-deposited YSZ coating transforms into an equiaxed grain structure with dramatic grain coarsening and significant CMAS infiltration. The CMAS corrosion induces dissolution of YSZ into the molten CMAS, spinel MgAl2O4 formation both in the intergranular gaps and inside YSZ grains, and anorthite (CaAl2Si2O8) precipitates in the intergranular gaps with reduced Mg and Ca content in the infiltrated amorphous CMAS. These results show that CMAS with a high content of calcium can induce significant grain coarsening and microstructure evolution that has a detrimental effect on primary function of the YSZ coating.

Inverse Analysis of In-Cylinder Gas-Wall Boundary Conditions: Investigation of a Yttria-Stabilized Zirconia Thermal Barrier Coating for Homogeneous Charge Compression Ignition

Thermal barrier coatings (TBCs) applied to in-cylinder surfaces of a low temperature combustion (LTC) engine provide an opportunity for enhanced efficiency via two mechanisms: (i) positive impact on thermodynamic cycle efficiency due to combustion/expansion heat loss reduction, and (ii) enhanced combustion efficiency. Heat released during combustion increases the temperature gradient within the TBC layer, elevating surface temperature over combustion-relevant crank angles. Thorough characterization of this dynamic temperature “swing” at the TBC–gas interface is required to ensure accurate determination of heat transfer and the associated impact(s) on engine performance, emissions, and efficiencies. This paper employs an inverse heat conduction solver based on the sequential function specification method (SFSM) to estimate TBC surface temperature and heat flux profiles using sub-TBC temperature measurements. The authors first assess the robustness of the solution methodology ex situ, utilizing an inert, quiescent environment and a known heat flux boundary condition. The inverse solver is extended in situ to evaluate surface thermal phenomena within a TBC-treated single-cylinder, gasoline-fueled, homogeneous charge compression ignition (HCCI) engine. The resultant analysis provides crank angle resolved TBC surface temperature and heat flux profiles over a host of operational conditions. Insight derived from this work may be correlated with TBC thermophysical properties to determine the impact(s) of material selection on engine performance, emissions, heat transfer, and efficiencies. These efforts will guide next-generation TBC design.

Investigation possibilities of increase physical and mechanical properties of plasma powder thermal barrier coatings ZrO2-7% Y2O3 by predrecrystallization heat treatment

This article discusses the problem of increasing the physical and mechanical properties of thermal barrier plasma sprayed coatings from the powder ZrO2-7% Y2O3, which are widely used to protect against corrosion and to reduce gas temperature of gas turbine engine blades surface. Analyzed the current methods provide enhanced properties of these coatings and revealed the necessity of the use predrecrystallization heat treatment to solve this problem. Yttria-stabilized zirconia thermal barrier coatings were produced by atmospheric plasma spraying. These coatings were subjected to heat treatment in air at 1200 °C and 1300 °C from 5, 10, 15, 20, 25 and 30 min. The structural characteristics of powders and coatings were evaluated using optical microscope.It is found that the optimum heat treatment conditions of thermal barrier plasma sprayed coatings from the powder ZrO2-7% Y2O that enhances hardness by 13% and reduced the thermal conductivity of the ceramic layer is 15% compared with the state after deposition by changes substructure is heated to a temperature of 1300 ° C, aged for 15 minutes and cooling in air. The phase analyses of coatings were conducted by X-ray method. The phase analysis of the ceramic layer after heat treatment, showed the following phase content,%: monoclinic - 5; tetragonal - 25; cubic - 70. It is shown that the rapid cooling prevents phase transformation: with increasing cooling rate during heat treatment with 0.02 ° C/sec to 34 ° C/sec tetragonal phase content increased to 40% and the content of monoclinic - not fixed.