Types Of Thermal Barrier Coatings Research Articles

Failure behavior of RE2M2O7/YSZ TBCs prepared by EB-PVD in gas thermal shock-CMAS corrosion environment

Three types of thermal barrier coatings (TBCs), YSZ, LZC/YSZ and GZO/YSZ, were deposited by EB-PVD. These coating systems were then subjected to thermal shock-CMAS corrosion testing using a high-temperature gas spray gun. The results demonstrate that the double-layer thermal barrier coatings exhibit excellent resistance to thermal shock-CMAS corrosion. Microstructural analysis revealed that the LZC/YSZ coating mitigated thermal damage owing to the formation of a sealing layer at the CMAS/LZC interface through high-temperature chemical interactions with CMAS. Compared to LZC/YSZ, the GZO/YSZ coatings showed greater resistance to CMAS-induced damage, which was associated with lower theoretical optical basicity values. This research provides valuable insights into the coupled thermal shock-CMAS corrosion failure mechanisms of TBCs on turbine blades and paves the way for new system-safe applications of TBCs.

Thermal Barrier Coatings in burner rig experiment analyzed through LAser Shock for DAmage Monitoring (LASDAM) method

This study investigates failure mechanisms in a typical thermal barrier coating (TBC) system comprising an EB-PVD columnar top coat, an aluminide bond coat, and a Ni-based single crystal superalloy substrate, simulating gas turbine operating conditions using a burner rig. TBC degradation, initiated by interfacial defects from the LASAT method, was studied during thermal gradient cycling under fast and slow cooling. In-situ optical and infrared imaging, along with ex-situ SEM cross-sectional analysis, monitored failure mechanisms. The Laser Shock for Damage Monitoring (LASDAM) technique provided insights into gradient and cooling rate impacts on columnar TBC damage. Results showed significant effects of cooling rate on delamination and localized failure at blister sites, with LASDAM revealing significant overheating at damage sites. Analysis included full-field temperature and damage assessment, emphasizing blister-driven delamination under severe thermal gradients. Discussion focused on elastic stored energy effects, noting that fast cooling induced transient conditions where reversed temperature gradients increased damage, limiting TBC lifespan.

Enhanced thermal shock resistance of gradient high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7/YSZ thermal barrier coatings

High-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 (LECO) has been verified as a potential candidate for thermal barrier coatings (TBCs). In this work, two different structured LECO coatings were constructed and the thermal shock behavior was systematically investigated. In the classical double-layered LECO coating (D-LECO), the cracks initially originated from the edge and completely exfoliated from substrate after failure. To strengthen the thermal stress resistance of the coating, a gradient multilayered LECO/YSZ coating (M-LECO) was designed with five ceramic sub-layers. The thermal shock life of M-LECO was greatly enhanced by approximately 4 times. Unlike the typical TBCs, the delamination first appeared at an annular region on the surface, located away from the center and edge. Besides, the spalling only occurred in the topmost LECO layer with a thickness loss of less than 5%. After failure, the underneath coating could still provide efficient thermal protection to substrate. The better thermal shock resistance of M-LECO was mainly attributed to the fine gradient structure of the coating and the intrinsic property of high-entropy LECO. The microhardness and fracture toughness in M-LECO were strengthened layer by layer along the thickness direction, alleviating the thermal stress accumulation in the coating and greatly improving the cracking resistance of the interface. Moreover, the high-entropy effect and sluggish diffusion effect of LECO improved its sintering resistance, thereby enhancing the thermal shock resistance of M-LECO. Such design not only fully utilizes the unique thermo-physical properties of high-entropy ceramics, but also effectively solves the problem of weak adhesion between the LECO and the substrate during thermal shock.

Driving forces in thermal barrier coatings blistering

Thermal barrier coating (TBC) systems are known to be extremely sensitive to the applied thermo-mechanical loading in terms of both microstructure evolution and of subsequent damage, leading to final failure by ceramic top-coat layer spallation. The analysis of the relationship between mechanical behavior and final failure is limited by the scatter in the experimental results. Based on a typical TBC system for columnar top-coat, this study focuses on blister evolution for pure thermal cycling. The blister is processed by LAser Shock Adhesion Test (LASAT) method. It is observed experimentally that prior to significant further interfacial debonding, the height of the blister increases with the number of applied cycles. To clarify this, a finite element analysis was developed to account for multilayer system behavior and growth strain associated to a thermal-grown oxide layer evolution. Finally, this study demonstrates that rough interfaces, elasto-viscoplastic behavior of the metallic bond-coat, and oxide growth strain are needed to model the experimentally observed evolution of the blister. Thus, the driving forces acting on bond-coat rumpling, are similar to driving forces for blister height increase and final failure of the TBC.

Comparison of Hot Corrosion Behavior of YSZ, YSZ/Mullite and YSZ/Mullite Gradient Coatings on Nickel Base Superalloy Prepared by Plasma Spray Method

Three types of thermal barrier coatings, namely NiCrAlY/YSZ double-layer, NiCrAlY/YSZ/mullite three-layer and NiCrAlY/YSZ/gradient of YSZ and mullite coatings, were deposited on a nickel-base superalloy using a plasma spray method. Hot corrosion studies of the plasma-sprayed thermal barrier coatings were conducted in 45wt% Na2SO4 + 55 wt% V2O5 molten salt at 1050 °C. Microstructure and morphology of the coatings were examined and compared by scanning electron microscopy and X-ray diffraction. The addition of mullite in thermal barrier coating results in the mullite phase reacting with corrosive salt and oxygen to form NaAlSiO4 phase. In addition to this phase, alumina (Al2O3) and Na2O phases are also formed. The presence of alumina phase (Al2O3) along with Na2O leads to formation of NaAlO2. A dense layer of NaAlO2 can increase the resistance to hot corrosion of the coating compared to YSZ coating.

Corrosiveness of CMAS and CMAS + salt (NaVO3, Na2SO4 and NaCl) to YSZ thermal barrier coating materials

Calcium-magnesium-alumina-silicate (CMAS) is found to consist with many salts especially in marine environment, where thermal barrier coatings (TBCs) are widely used. In this study, the corrosiveness of CMAS + salt (NaVO3, Na2SO4 and NaCl) to a typical TBC material of Y2O3 partially stabilized ZrO2 (YSZ) was investigated. As compared to CMAS, CMAS + salt had lower melting temperature, viscosity and crystallization ability. This led to the CMAS + salt more aggressive to YSZ at relatively low temperature, and more permeable at 1250 ℃. Accordingly, the CMAS + salt had higher corrosiveness than CMAS to TBCs, which is worth special attention.

Thermal shock resistance of thermal barrier coating with different bondcoat types and diffusion pre-coating

This paper investigates the resistance of two types of thermal barrier coatings and compares their behavior with common coatings. Coatings’ layers in the first and second target sample were fabricated as HVOF/APS/APS (two bondcoats and one topcoat) and APS/APS (one bondcoat and topcoat) with diffusion pre-coating, respectively. Also, to accurately compare the behavior of these two types of coatings with conventional coatings used in gas turbines, this paper explored the resistance of three types of coatings applied as APS/APS, HVOF/APS, and HVOF coatings against thermal shock. In order to create shock loading, five types of laboratory samples were heated under regular cycles and cooled down with water. During the experiment, the sample changes caused by thermal shock loading were investigated through visual inspections. Then, after the experiment, the SEM images were leveraged to inspect the changes. In addition, changes in the structure of coating layers and their degradation process were studied. The results show that using two bond layers increases the resistance and life of the coating against heat shock by up to 1.40 times. Among the samples with one band coat, the sample with a diffusion coating applied under the BC showed the best performance. The sample life increased by 1.25 times compared to the common APS/PAS coating.

Numerical Simulation of Initial Residual Stress in Thermal Barrier Coatings: Planar geometry model

Thermal barrier coatings (TBCs) are one of the key materials of turbines for propulsion and power generation. A typical TBCs system generally contains four layers, i.e. top ceramic coat (TC), thermally grown oxide (TGO), bond coat (BC) and substrate. The material parameters （such as Young’s modulus, Poisson’s ratio and thermal expansion coefficient） of each layer will result in the different cooling velocity of thermal barrier coatings (TBCs) system during the deposition process. Initial residual stresses in TBCs occur after cooling to the ambient, which play an important role for evaluating the failure and reliability of TBCs. In this paper, the initial residual stresses of a planar TBCs model were obtained by finite element method, the effects of cooling kinds and the thickness of TC and TGO on the fields of initial residual stresses were also considered. The results may be useful for the optimization of TBCs system design and preparation.

Modelling the stress state of a thermal barrier coating system at high temperatures

Abstract A model is proposed to assess the time- and temperaturedependent creep and oxidation behavior of typical thermal barrier coating (TBC) systems consisting of EB-PVD manufactured partially stabilized ZrO2 and a plasma-sprayed MCrAlY bond coat (BC) on a typical Ni-base sheet alloy. The creep behavior of the BC was investigated by compression tests at the NiCoCrAlY alloy PWA 1370. With the objective of describing the formation of the so-called thermally grown oxide, an oxidation kinetic for the BC alloy is presented, which predicts the oxide thickness as a function of time and temperature. Using a finite element model of the complete TBC system, the influence of time- and temperature- dependent oxide growth and deformation behavior of the components on the stress development was studied. The obtained results show a development of compressive stresses above the peaks and of tensile stresses in the valleys of the rough TBC/BC interface. With this, the delamination stress between top coat and BC, which finally leads to failure by spallations, can be assessed.

Tailoring Thermal Transport Properties by Inducing Surface Oxidation Reactions in Bulk Metal Composites.

Surface modification is used to dramatically alter the thermal properties of a bulk metallic material. Thermal barrier coatings (TBCs) are typically applied using spray deposition or laser-based techniques to create a ceramic coating on a metal substrate. In this study, an effective TBC is created directly on a metallic substrate by inducing surface chemical reactions. Aluminum-zirconium (Al-Zr) substrates are used to induce surface-limited reactions that produce a 75-80% decrease in bulk thermal conductivity and diffusivity, respectively. The substrates are cylindrical disks 12.6 mm diameter and 2 mm thickness. Thermal properties are measured using laser flash analysis (LFA) at incrementally elevated temperatures. Focused ion beam (FIB) slicing of the substrate coupled with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) show that the substrate oxidized only along the outer 20 μm of the bulk surface. The layer thickness is significantly less than typical TBCs that can range from 50 to 300 μm yet the 20 μm coating still achieves a dramatic reduction in thermal transport properties. Additionally, thermal analysis reveals a sequence of exothermic reactions starting at 439 °C that include both intermetallic (i.e., ZrAl3) and oxidation (i.e., Al2O3 and ZrO) reactions suggesting continuous surface bonding at the coating-metal interface. The onset of exothermic activity coincides with the transition in thermal properties measured using LFA. These results show that surface oxidation reactions could be used to dramatically alter the thermal transport properties of a metal substrate.

Rheological and chemical interaction between volcanic ash and thermal barrier coatings

Combustion temperatures (>1200 °C) in gas turbines cause softening and eventually melting of ingested volcanic ash particles. In the downstream section, lower temperatures lead to droplet deposition onto the thermal barrier coatings (TBCs) of the turbine blades. The intensity of interaction (wetting and chemical interaction) depends on the ash and TBC chemistry as well as the TBC structure. We have determined the spreading behavior of five different volcanic ash melts on four types of TBCs. The TBCs differed in their composition - yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) – and in their fabrication - air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). The spreading properties have been parameterized on the basis of four parameters, 1) the CaO-SiO 2 -ratio, 2) optical basicity, 3) the R b/a ratio and 4) the viscosity of the volcanic ash melts. Infiltration efficiency of the melts into the TBCs and corrosion characteristics have been determined via electron microprobe analysis of cross sections. In this study, the highest damage potential was found following interaction with basaltic melt, as its low viscosity is favorable to extensive spreading and high chemical reactivity. • Basaltic (SiO 2 -lean) silicate melts show the most extensive spreading on TBC surfaces and the most intense chemical reaction with the TBC materials • Spreading of silicate melts on APS TBCs is enhanced over EB-PVD TBCs and spreading on GZO TBCs is enhanced over YSZ TBCs • Spreading ability of silicate melts can be estimated based on a parametrization deduced from our spreading experiments

Thermal shock resistance of double-layer thermal barrier coatings

Abstract

An investigation of oxidation, hot corrosion, and thermal shock behavior of atmospheric plasma-sprayed YSZ–Al2O3 composite thermal barrier coatings

Abstract In the current study, different types of thermal barrier coatings with various mass fractions were investigated in terms of oxidation, hot corrosion, and thermal shock resistance. The thermal barrier coatings consisted of six different samples, which included the usual YSZ, Al2O3, hybrid composites with 65 wt.% YSZ and 35 wt.% Al2O3, 50 wt.% YSZ and 50 wt.% Al2O3, 35 wt.% YSZ and 65 wt.% Al2O3 and the final one, which was a double layer composite (Al2O3 and YSZ). High temperature isothermal oxidation behavior of the coatings was tested at 1050 °C, using an air furnace for 48 h, 80 h, and 120 h respectively. Hot corrosion tests were applied at 1050 °C using a 45 wt.% Na2SO4 and 55 wt.% V2O5 mixture of salts. The microstructure and phase stability of coatings were evaluated by means of scanning electron microscopy and X-ray diffraction techniques. The usual YSZ showed better hot corrosion and thermal shock resistance, while Al2O3 showed the lowest hot corrosion and oxidation resistance. Thermally grown oxide formation, thermal expansion coefficient mismatch and phase transformation in the thermal barrier coatings could be the main causes of degradation after thermal shock testing.

An investigation of oxidation, hot corrosion, and thermal shock behavior of atmospheric plasma-sprayed YSZ–Al2O3 composite thermal barrier coatings

Abstract In the current study, different types of thermal barrier coatings with various mass fractions were investigated in terms of oxidation, hot corrosion, and thermal shock resistance. The thermal barrier coatings consisted of six different samples, which included the usual YSZ, Al2O3, hybrid composites with 65 wt.% YSZ and 35 wt.% Al2O3, 50 wt.% YSZ and 50 wt.% Al2O3, 35 wt.% YSZ and 65 wt.% Al2O3 and the final one, which was a double layer composite (Al2O3 and YSZ). High temperature isothermal oxidation behavior of the coatings was tested at 1050 °C, using an air furnace for 48 h, 80 h, and 120 h respectively. Hot corrosion tests were applied at 1050 °C using a 45 wt.% Na2SO4 and 55 wt.% V2O5 mixture of salts. The microstructure and phase stability of coatings were evaluated by means of scanning electron microscopy and X-ray diffraction techniques. The usual YSZ showed better hot corrosion and thermal shock resistance, while Al2O3 showed the lowest hot corrosion and oxidation resistance. Thermally grown oxide formation, thermal expansion coefficient mismatch and phase transformation in the thermal barrier coatings could be the main causes of degradation after thermal shock testing.

The thermo‐mechanical properties and ferroelastic phase transition of RENbO4(RE = Y, La, Nd, Sm, Gd, Dy, Yb) ceramics

AbstractIn this work, RENbO4(RE = Y, La, Nd, Sm, Gd, Dy, Yb) ceramics with low density, low Young's modulus, low thermal conductivity, and high thermal expansion have been systematically investigated, the excellent thermo‐mechanical properties indicate that RENbO4ceramics possess the potential as the new generation of thermal barrier coatings (TBCs) materials. X‐ray diffraction and Raman spectroscopy phase structure identification reveal that all dense bulk specimens obtained by high‐temperature solid‐state reaction belonged to the monoclinic (m) phase with C12/c1 space group. The ferroelastic domains are detected in the specimens, revealing the ferroelastic transformation between tetragonal (t) and monoclinic (m) phases of RENbO4ceramics. The Young's modulus and hardness of the RENbO4ceramics measured by the NanoBlitz 3D nanoindentation method are discussed in details, and the lower Young's modulus (60‐170 GPa) and higher hardness (the maximum value reaches 11.48 GPa) indicating that higher resistance of RENbO4ceramics to failure and damage. Lower thermal conductivity (1.42‐2.21 W [m k]−1at 500°C‐900°C) and lower density (5.330‐7.400 g/cm3) than other typical TBCs materials give RENbO4ceramics the unique advantage of being new TBCs materials. Meanwhile, the thermal expansion coefficients of RENbO4ceramics reach 9.8‐11.6 × 10−6 k−1and are comparable or higher than other typical TBCs materials. According to the first‐order derivative of the thermal expansion rate, the temperature of the ferroelastic transformation of RENbO4ceramics can be observed.

Investigation of thermal shock resistant in three kinds thermal barrier cerium oxide coating (CeO2) with MCrAlY intermediate layer

The study aimed to compare the thermal shock behavior of three types of thermal barrier coatings (TBC) containing two and five layers. The substrate of the coatings, like as industrial samples, was selected from IN738LC superalloy. The first type was a two-layer TBC produced from CoNiCrAlY and CeO2 as a bond and top layers, respectively. The second type was a common five-layer TBC with a bond layer of CoNiCrAlY, a top layer of CeO2 and three intermediate layers composed of three mixing kinds of CeO2 + CoNiCrAlY. The third type was composed of a top layer of nano-structured CeO2, and the other four down layers were similar to the second type. To thermal shock test, the samples were kept at 1100 °C for 5 min and quenched in 20–25 °C water. The test was continued until all the samples were destructed. The sample was considered destructed when 20% of the coating surface was detached. To evaluate the microstructure of the samples, SEM and FESEM were used. Finally, the thermal shock lifetime of the five-layer TBC was 1.6 times higher than the two-layer TBC, and by making the top nano-structured layer in five-layer TBC, the lifetime was enhanced approximately 14%.

The impact of the casting thickness on the interfacial heat transfer and solidification of the casting during permanent mold casting of an A356 alloy

Many important quality indicators for components manufactured using permanent mould casting, such as the presence of shrinkage porosity, microstructure features, dimensional stability, cycle time and filling mechanisms are controlled directly or indirectly by the heat transfer mechanisms linking the casting to the mould. While interfacial heat transfer in permanent mould casting has been significantly investigated and widely reported in the literature, the geometrical dependence of heat transfer parameters has not been studied or reported in detail. Understanding this dependency is very important as the same cast component most often is constituted by different sections and geometrical variations. In this paper, experimental methods and analytical correlations have been developed and presented that enable an accurate determination of the time dependent interfacial heat flux density and heat transfer coefficient at the casting-mould interface. The variation of these parameters is investigated and analysed for three different casting sections and two types of thermal barrier coatings.

Adhesion strength studies on zirconia based pyrochlore and functionally gradient thermal barrier coatings

Abstract Thermal Barrier Coating (TBC) plays a major role in the improvement of gas turbine and engine components in terms of their service life and performance. Generally, all coatings must possess certain primary properties to perform in the intended applications. However, regardless of applications, suitable adhesion strength is one major characteristic they must have to adequately protect the basic components on which they are applied upon. In TBCs, adhesion (or Bond) strength is a parameter that helps to illustrate the resistance of the ceramic top coat against spallation either from the bond coat (and component) or within the TBC layers itself. The performance of TBCs are reliant upon the adhesion between the coating and the metal substrate and also adhesion (or cohesion) between the bond coat and the overlying ceramic top coat layer. The de-bonding of the top coat layer or the inter-metallic bond coat layers are the main reasons of the failure of the overall TBC system. Some of the prominent problems associated with coatings applications are residual stresses, micro-cracks and pores etc. These and many other factors influence the adhesion of the coatings in addition to service environment conditions and pre coating substrate preparations such as substrate cleaning, grit blasting and very importantly plasma spray parameters. In the present work, results obtained from adhesion strength measurements carried out by following the ASTM C 633 standard test method, on various types of TBCs are being shared. Thermal barrier coatings (TBCs) were synthesized with NiCrAlY bond coat deposited on SS 304L substrate by using air plasma spray and different ceramic top coats (a) commercial 8%Yttria Stabilized Zirconia (8YSZ) (b) lab synthesized plasma spray powders of (i) Lanthanum Zirconate (La2Zr2O7) (ii) Lanthanum Ceria Zirconate (La2 (Zr0.7Ce0.3)2O7) and (iii) Lanthanum cerate (La2Ce2O7). The coating depositions were carried out in different configurations i.e. two layers, three layers and gradient layers (Functionally gradient materials). The evaluation of properties includes the studies of morphology of the strength (adhesive/cohesive failure mode) tested specimen as well. General conclusions drawn from the studies on several specimen in various configurations are that cohesive failures (between the ceramic top coat layers) is the predominant mechanisms followed by few adhesive failures in bond coat coat/ceramic interface.

Influence of isothermal oxidation on microstructure of YSZ and Mullite-YSZ thermal barrier coatings

Microstructural changes in conventional yttria stabilized zirconia (YSZ) and multilayer Mullite-YSZ/YSZ (M-YSZ) thermal barrier coatings were compared based on high temperature isothermal oxidation at the temperatures of 1050, 1150 and 1250 °C. Both types of thermal barrier coatings were produced from commercially available powders utilizing atmospheric plasma spray technique. Initial M-YSZ powder mixture consisted of 29 vol. % of Mullite and 71 vol. % of YSZ. Inconel Alloy 713LC nickel-based superalloy was used as a substrate material (cylinders, 25 mm in diameter and 10 mm in thickness). The TBCs lifetime, phase stability, thickness of the thermally grown oxide layer, porosity, and changes in microstructure after high temperature testing were evaluated using light microscope, scanning electron microscope equipped with EDX analyzer, and XRD technique.

CHT/CFD Analysis of Thermal Sensitivity of a Transonic Film-Cooled Guide Vane

Thermal parameters are important variables that have great influence on life time of turbine vanes. Therefore, accurate prediction of the thermal parameters is essential. In this study, a numerical approach for conjugate heat transfer (CHT) and computational fluid dynamics (CFD) is used to investigate thermal sensitivity of a transonic guide vane which is fully film-cooled by 199 film holes. Thermal barrier coating (TBC), i.e., the typical TBC and a new one as the candidate TBC, and turbulence intensity (Tu), i.e., Tu=3.3%, 10% and 20%, are two variables used for the present study. At first the external surface temperatures of the vane material are compared. Next, the TBC surface temperatures are considered. Results show the major role of the lower thermal conductivity of TBC which results in the lower and more uniform temperature on the external surface of the vane substrate. Finally, the thermal sensitivity is presented in terms of the percentage reduction of the external surface temperatures of the vane material and the structural temperatures of the vane material at midspan, including the variations of average and maximum vane temperatures. Results show that TBC and Tu have significant effects on the external surface and structural temperatures of the vane substrate. The lower thermal conductivity of TBC leads to the higher difference between the thermal conductivity of the vane substrate and TBC, the reduction of heat transfer and the more uniform temperature within the vane structure. The results also show more effective protection for the average vane temperature from the two TBCs at higher Tus. However, Tu does not significantly affect the reduction of the maximum vane temperature even though the new TBC, which has the very low thermal conductivity, is used.