Stabilized Zirconia Thermal Barrier Research Articles

Effect of Yttria-stabilized zirconia coating on the corrosion and thermal behaviour of additive manufactured Inconel 718 alloy

The need for additively manufactured Inconel 718 alloy is increasing in high-temperature functional applications, including gas turbine engines, aviation engines, cast blades, and housings. The proposed work aims to improve the corrosion resistance and thermal characterization of additively manufactured Inconel 718 using a Yttria Stabilized Zirconia (YSZ) thermal barrier coating. YSZ is coated on the additively manufactured Inconel 718 alloy using a plasma spraying process. Using an electrochemical workstation, a corrosion test is conducted for the YSZ-coated Inconel 718 alloy and the non-coated Inconel 718 alloy. Thermal shock loading and thermal stability analysis for the YSZ-coated Inconel 718 alloy are carried out using a tubular furnace and a thermogravimetric analyser. YSZ-coated, additively manufactured Inconel 718 alloy is characterized by phases formed, microanalysis of corroded specimens, morphology, surface cracks, and elemental presence using X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Energy dispersive spectroscopy (EDS). YSZ-coated Inconel 718 alloy shows a minimum corrosion rate of 0.0000472 mm/year, which is better than the non-coated Inconel 718 alloy (5.93 mm/year). The YSZ coating on Inconel 718 alloy improved the thermal stability and thermal shock resistance better than the non-coated Inconel 718 alloy.

The crack behavior and delamination mechanisms of air plasma sprayed thermal barrier coatings under ultrasonic plasma jet at 1600 °C

In this contribution, the performance and delamination mechanisms of air plasma sprayed (APS) yttria stabilized zirconia thermal barrier coatings (YSZ TBCs) under high intensity heat flux at 1600 °C was investigated. The plasma jet was adopted as the heat source with a combination of high temperature and high-speed jet-stream erosion. It was found that the YSZ topcoat would exfoliate when the vertical crack connected to the interior (interfacial/shear) cracks to form a free edge, which turned the mixed-mode buckle delamination into a mode I cracking. The thermo-mechanical analysis indicated that energy release rate associated with the transient cooling was the driving force for the interfacial cracks, while the vertical cracks were developed by the tensile stress during cooldown. The findings obtained in this study implied that the YSZ TBCs can be used for the thermal protection of combustor in ramjet engines.

Retracted: Combined Effect of Thermal Shock and Hot Corrosion on the Failure of Yttria Stabilized Zirconia Thermal Barrier Coatings

Owing to the authors' ignorance, a SEM picture (fig.3(d) in this paper) has been used in another paper (Fig.3(b) in a Chinese Journal). After carefully thinking, the authors decided to withdraw the paper with pity. This decision has been approved by both Editorial Office of JMMR and all authors. We apologize for the inconvenience may caused to readers.
 
 Refers to original article：
 https://doi.org/10.30564/jmmr.v1i1.307
 
 Date of Retraction: July 10, 2023

A relationship of porosity and mechanical properties of spark plasma sintered scandia stabilized zirconia thermal barrier coating

Porous ceramic materials are popularly accepted as thermal barrier coatings (TBCs) in insulating gas turbine parts working at high temperatures. In this research, three different types of Scandia stabilized zirconia (ScSZ) systems, a nanometric 10 mol% ScSZ (10-ScSZ), a micrometric 8 mol% ScSZ (8-ScSZ) and a combination of these two powders (10-8-ScSZ) have been developed by Spark Plasma Sintering (SPS) process. Varying SPS parameters, for instance, temperature, pressure and dwell time were applied to develop the different volumes of porosity in the materials. Subsequently, the microstructure of the materials has been studied and mechanical properties have been evaluated. All three materials demonstrate a reduced porosity level at high sintering temperature and pressure. However, the nanometric 10-ScSZ material shows a higher reduction of porosity from 51.8% to 11.1% at 30 MPa pressure and 40.2%–8.5% at 60 MPa pressure within the temperature range of 1000–1200 °C. Besides, the 10-8-ScSZ composite exhibits substantially increased porosity in comparison to its constituent parts. The results also show that the nanometric 10-ScSZ material exhibits a greater mechanical strength including Vickers microhardness of 81 HV, flexural strength of 361 MPa and elastic modulus of 187 GPa at 5% porosity level, as compared to the other two materials. Additionally, it is observed that all the mechanical properties for all three materials consistently decrease with the increase in porosity levels. While compared with the traditional atmospheric plasma spray (APS) processed ceramic coating, the porous ScSZ coating materials exhibit a larger elastic modulus. Therefore, the porous ScSZ developed by the SPS process could be a prospective alternative thermal barrier coating (TBC).

Comparative study on failure behavior of promising CMAS-resistant plasma-sprayed thermal barrier coatings in burner rig test with/without CMAS deposition

Along with continuous progress in inlet temperature of turbine engine, calcium-magnesium-aluminum-silicate (CMAS) deposition has become one of serious challenges for traditional yttria partially stabilized zirconia thermal barrier coatings at elevated temperature. Although lots of materials with superior CMAS resistance have been proposed, there is few comparative research on performance of corresponding coatings reported especially when subjected to thermal cycling and CMAS simultaneously. To this end, some coatings were prepared in present study, and thereafter failure behavior in condition of thermal cycling and thermal cycling-CMAS was systemically investigated and compared. Experimental results showed a varied lifetime and cracking behavior in thermal cycling test and thermal cycling-CMAS test, indicating that CMAS infiltration affected failure behavior of coatings. Besides, it was numerically found that CMAS penetration would lead to a promotion of thermal stress, which increased the tendency for cracking during thermal cycling. And the phenomenon that channel crack was the precondition of delamination crack was revealed.

Effect of molten glass on degradation of stabilized zirconia thermal barrier coatings

Effect of molten glass on degradation of stabilized zirconia thermal barrier coatings

Development and Characterisation of thick non-conventional yttria stabilised zirconia thermal barrier coating systems on aluminium and superalloys by sol-gel synthesised single unit dip-coating

ABSTRACT In the present paper, the two routes of preparation of seven weight% yttria stabilised zirconia (7YSZ) composite sols are proposed under optimised conditions. Sol of route-I was made from zirconium (IV) propoxide, yttrium nitrate hexahydrate-acetylaceone-water-1-propanol solvent, and sol of route-II was made from zirconium (IV) propoxide, yttrium nitrate hexahydrate-acetylacetone-sulphuric-acid-water-1-propanol solvent-2weight% polyvinyl alcohol (2PVA). After preparation of both sol, thermal behaviour was characterised using thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) for checking the weight loss with respect to high temperature. Route-II YSZ sol-gel survived with practically no mass loss, while route-I had extensive degradation as well as considerable mass loss. In the second stages, both the sols were deposited onto as-air plasma sprayed CoNiCrAlY bond-coated substrates, i.e. superalloys IN800 and aluminium alloys AA2024-T351. The microstructure of route-I and route-II YSZ sol coating was characterised using scanning electron microscopy. Route-II deposited sol was found to be more porous, homogenised and thick thermal barrier coatings as compared to route-I sol. The presence of sulphuric acid enhanced the better processing control of sol-gel coatings with uniform surface, as confirmed by atomic force microscopy (AFM).

Process and properties of dense and porous vertically-cracked yttria stabilized zirconia thermal barrier coatings

Highly durable thermal barrier coatings (TBCs) are desired for various applications in hot sections of gas turbine engines. In this work, dense vertically-cracked (DVC) and porous vertically-cracked (PVC) yttria stabilized zirconia (YSZ) TBCs were produced using the air plasma spray process with a specially engineered powder. The effects of plasma enthalpy on the in-flight particle behavior and coating microstructures were investigated. Thermal conductivity, thermal cycling behavior, as well as, failure mechanisms of both DVC TBCs and PVC TBCs were evaluated. The thermal conductivity of PVC TBCs was found to be approximately 40%–50% lower than that of DVC TBCs due to their higher porosity. In addition, the thermal cycling life of PVC TBCs was demonstrated to be more than 1.8 times that of DVC TBCs with the same substrates and bond coats. PVC TBCs also showed excellent sintering resistance after 743 one-hour thermal cycles at 1125 °C. This work demonstrates that highly durable 8YSZ PVC TBCs with simultaneously high strain tolerance, low thermal conductivity, and high sintering resistance can be produced by the application of an engineered powder feedstock.

Segmentation crack formation dynamics during air plasma spraying of zirconia

Air Plasma Sprayed (APS) Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coatings (TBCs) is a well-established technology in the gas turbine industry. A conventional APS TBC is a layered structure with heterogenous distribution of defects (microcracking, pores, etc.) which allow it to simultaneously possess low thermal conductivity and elastic modulus compared to its bulk counterpart. However, interfacial defects can be a source of delamination failure during thermal cycling. In addition, conventional porous coatings can experience sintering during sustained exposure, which augments failure through stiffening-induced delamination. Electron Beam Physical Vapor Deposition (EB-PVD) TBC coatings, due to their dense columnar structures, are less susceptible to both sintering and delamination. This has led to the consideration of more economically-applied APS TBCs that are dense with periodic vertical segmentation cracks. Such dense vertically cracked coatings (DVCs) have been successfully developed and implemented in gas turbine engines. These microstructures are produced in-situ through control of the process conditions with high deposition temperatures. However, the mechanism of such segmentation cracks is unclear. In this study, formation dynamics of segmentation cracks were observed through in-situ beam curvature monitoring during deposition in combination with microstructural evaluations. It was observed the initial layers of the coating are dense without segmentation cracking. As subsequent layers are deposited, periodic macrocracking initiates and typically propagates through the remaining coating thickness. The in-situ in-plane coating stress is significantly reduced after segmentation cracking begins. These results are reconciled through interpretation of thin-film fracture literature, and an initial framework to interpret the experimental observations is provided.

Performance of plasma-sprayed thermal barrier coating with engineered unmelted nano-particle contents

This paper provides an insight into the effect of unmelted nano-particle content (UNC) on the thermal cycling property and thermal insulation performance of plasma sprayed nanostructured yttria stabilized zirconia thermal barrier coatings. The results suggested that the as-sprayed nanostructured coating displayed a multi-modal microstructure, including the nano-grains (30–90 nm in size), submicron grains (110–900 nm in size) and unmelted nano-particles (20–60 nm in size). These unmelted nano-particles were loosely distributed among the nano-grains or submicron-grains. When the UNC increased from 11% to 34%, the bond strength of coating decreased from 28.1 MPa to 6.3 MPa and the thermal insulation temperature of coating increased from 135 °C to 170 °C. The coating with 22% UNC showed the highest thermal cycling life at 1250 °C since the unmelted nano-particles significantly reduced the residual stress and effectively prevented the propagation of cracks. The thermal insulation temperature of coating increased with the increase of thermal cycles and reached its maximum after approximately 600 cycles due to the coalescence of cracks located at the unmelted nano-particles/crystalline region interface. The performance of nanostructured coating can be tailored by modulating the UNC.

Prediction of critical rupture of plasma-sprayed yttria stabilized zirconia thermal barrier coatings under burner rig test via finite element simulation and in-situ acoustic emission technique

In the current work, the yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been fabricated by atmospheric plasma spraying. The failure mechanism of the TBCs during burner rig test (BRT) has been investigated via finite element modeling (FEM) and in-situ acoustic emission (AE) technique systematically. During the BRT, the oxygen-propane flame was heated onto the surface of the coating samples with the dwell time 3 min, then the surface and the backside of the coating samples were cooled by the compressed air simultaneously for 3 min. This can be viewed as one thermal cycle. Then repeat the same heating and cooling process. The history and distribution of the temperature along the through-thickness direction of the coating samples has been obtained via FEM, and the residual stress at different stages has been also calculated. The FEM results have indicated that large tensile stress and compressive stress existed at the heating stage and cooling stage of each thermal cycle, respectively. The crack propagation tends to occur at the initial period of heating stage in the BRT process. The in-situ acquisition of the AE signals during the BRT has also been adopted. The crack propagation patterns have been obtained based on the analysis of AE signals. The filtering technique has been used to exclude the disturbance of the noise in the process of BRT in order to capture the actual and effective AE signals which represent the crack propagation and the deformation of the coating systems. Based on the characteristic waveform of effective AE signals, the Fast Fourier Transformation (FFT) and wavelet transformation has been adopted to analyze the key distribution range of the amplitude and frequency. The investigation results indicate that the acquired AE signals during BRT mainly include signals which came from the plastic deformation of the substrate and creep of each layers, propagation of the vertical crack and propagation of the interfacial cracks (horizontal crack). The detailed failure mechanisms of the TBCs during BRT have been clarified in the current work.

Combined effect of thermal shock and hot corrosion on the failure of yttria stabilized zirconia thermal barrier coatings

The thermal barrier coatings (TBCs) of 8wt.% yttria stabilized zirconia (8YSZ) on a GH2132 alloy were deposited by air plasma spraying. The combined effect of thermal shock and hot corrosion on the failure of TBCs was highlighted investigated. The results showed that the combined effect of hot corrosion and thermal shock was prone to induce the failure of TBCs. The main failure of TBCs under the combined effect was in the form of penetrating crack to the bondcoat/substrate interface and oxidation at the penetrating crack front.

Impermeable CeO2 overlay for the protection of plasma sprayed YSZ thermal barrier coating from molten sulfate-vanadate salts

Yttria stabilized Zirconia thermal barrier coatings with around 200 μm overlay of Al2O3 and CeO2 were deposited on Inconel substrate using atmospheric plasma spray technique. Hot corrosion tests were carried out on the thermal barrier coating in molten salt mixtures (45 wt% Na2SO4 + 55 wt% V2O5) at 1150 °C for 30 h. Although the YSZ and Al2O3/YSZ coatings were gradually degraded by the infiltration of the molten salt, the CeO2/YSZ coatings remarkably restrained the infiltration of the molten salt into the YSZ coating. For the YSZ coating without any overlay, the YSZ reacted with V2O5 to form YVO4 during exposure to the molten salt, leading to the formation of a large amount of monoclinic phase of ZrO2 due to the leaching of Y2O3 from YSZ. For the Al2O3/YSZ coating, the Al2O3 overlay reduced the rigorous infiltration of the molten salt, which caused in lesser amount of leaching of Y2O3 from YSZ. However, for the CeO2/YSZ coatings, CeO2 not only physically prevented the molten salt infiltration, but also completely reacted with V2O5 to form CeVO4 and completely restrained the infiltration of the molten salt into the YSZ coating. Additionally, the Al2O3/YSZ and CeO2/YSZ showed 17% and 52% increase in thermal shock resistance respectively compared to YSZ coating.

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.

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)

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.

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.

Phase stability and thermal conductivity of nanostructured tetragonal yttria–stabilized zirconia thermal barrier coatings deposited by air–plasma spraying

High performance thermal barrier coatings (TBCs) produced from nanocrystalline 4 mol.% Y2O3–stabilized ZrO2 (4YSZ) powder show significant thermal and mechanical property advantages compared with current benchmark products. The objective of the present study was to investigate the phase stability and thermal conductivity of the nanostructured 4YSZ coatings plasma–sprayed from the sol–spray pyrolysis powder. Characterization results indicated that the as–sprayed 4YSZ coatings were chemically very homogeneous and had a completely tetragonal–prime (t′) structure. Such a uniform composition was almost entirely consistent with that of the feedstock powder. The grain size of as–sprayed 4YSZ coatings was in the range of 35 ~ 75 nm, which resulted in an extremely low thermal conductivity of 0.8–1.2 W m–1 K–1 in the coatings. The high–temperature aging test showed that no monoclinic ZrO2 (m–ZrO2) phase present at all of the 4YSZ coatings after a heat treatment at 1473 K for 1–650 h. Furthermore, the phase composition of the 4YSZ coating kept t′–ZrO2 structure after 1500 thermal cycles at a peak surface temperature of 1373 K. The relative phase stability was attributed to the highly pure t′ phase composition and uniform constituents in the 4YSZ coatings. These results suggested that the development of high performance 4YSZ ceramic powders was benefit to improve the coating properties for TBC application.

Plasma-sprayed nanostructured YSZ thermal barrier coatings: Thermal insulation capability and adhesion strength

Abstract Adhesion strength and thermal insulation of nanostructured Yttria Stabilized Zirconia (YSZ) thermal barrier coatings (TBC) were investigated and compared with those of conventional YSZ TBCs. A Nickel based superalloy (IN-738LC) was used as the substrate with NiCrAlY bond coat, and nanostructured and conventional YSZ top coats were applied by using air plasma spray (APS). The adhesion strength of coatings was evaluated according to ASTM C633-01, and their thermal insulation capability was evaluated using a specially designed test setup at an electrical furnace. The results revealed the nanostructured YSZ coating to have a bimodal microstructure consisting of nanosized particles and microcolumnar grains. The bimodal microstructure of nanostructured coatings prevented crack propagation by splat boundaries and unmelted particles, thereby improving the bonding strength. Also, due to the presence of nano-zones in the microstructure of nano TBCs, coatings exhibited superior thermal insulation capability.

High Temperature Thermal Properties of Columnar Yttria Stabilized Zirconia Thermal Barrier Coating Performed by Suspension Plasma Spraying

Performance enhancement of gas turbines is a main issue for the aircraft industry. Over many years, a large part of the effort has been focused on the development of more insulating Thermal Barrier Coatings (TBCs). In this study, Yttria Stabilized Zirconia (YSZ) columnar structures are processed by Suspension Plasma Spraying (SPS). These structures have already demonstrated abilities to get improved thermal lifetime, similarly to standard YSZ TBCs performed by EB-PVD. Thermal diffusivity measurements coupled with differential scanning calorimetry analysis are performed from room temperature up to 1100 °C, first, on HastelloyX substrates and then, on bilayers including a SPS YSZ coating. Results show an effective thermal conductivity for YSZ performed by SPS lower than 1 W.m-1K-1 whereas EB- PVD YSZ coatings exhibit a value of 1.5 W.m-1K-1.