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Gadolinium zirconate (GZ) is a promising candidate for next-generation thermal barrier coating (TBC) materials. Its corrosion resistance against calcium-magnesium-alumino-silicate (CMAS) needs to be further increased for enhancing its in-service life. As the Gd element plays an important role in the CMAS resistance, three GZ coatings (GZ-0.75, GZ-1.0, and GZ-1.2) with different Gd/Zr atomic ratios are designed and deposited by laser enhanced chemical vapor deposition (LCVD) in this work. It is found that the generated Gd-apatite in GZ-1.2 would block micro-cracks inside the column structure and the inter-columnar gap more efficiently. Thus, the CMAS penetration rate (5.2 μm/h) of GZ-1.2 decreases over 27% comparing with GZ-1.0 and GZ-0.75, which is even lower than the Gd2Zr2O7 coatings fabricated by electron-beam physical vapor depositions (EB-PVDs). This work provides a feasible way to adjust the coating’s corrosion resistance and may guide the development of future coating for long in-service life.

The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15�� distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks., Journal of thermal spray technology;Vol. 30. 2021, pp 1015���1027

Platinum-modified-βNiAl is a bond coat (BC) material for thermal barrier coatings (TBCs) applications applied on aero-engine hardware to reduce their surface temperatures. However, it is desirable to minimize its production and material costs by the low-cost alternatives of similar performance. As, it has been acknowledged that the small concentration of the reactive elements (REs), such as Zr, Hf, and Y, could tremendously enhance the oxide adhesion even in some cases better than Pt modified counterparts. The present study aims to design and fabricate the Zr-modified-βNiAl bond coat on CMSX-4 superalloy using an aluminizing method. Moreover, the study focuses on the development of a systematic understanding of underlying mechanisms behind the beneficial effects of REs. Initially, three sets of BCs were prepared: Zr-free βNiAl (undoped), Al and Zr co-deposited in a single-step process (1SP), and Zr and Al, which were individually deposited in two processing steps (2SP): zirconizing and aluminizing. Such three sets of BCs help to understand the processing, as well as Zr and Al effects on scale adhesion. In particular, 1SP/2SP BCs showed uniformity of Zr in the form of precipitates and networks that caused hardness enhancement. All BCs were isothermally oxidized at 1150oC for 100 hours wherein 2SP revealed the best spallation resistance, microstructural stability and its Zr-oxide pegs were extended to substrates. In addition to the Zr effect, BC Al content was found to affect the oxide adhesion equally. Under identical Zr contents (of 1SP and 2SP = 1at %), the higher Al showed the better spallation resistance while lower Al caused the inverse effect of Zr owing to its reactive nature that is termed as over doping. Moreover, it has been established that over-doping either local or into entire BC, accelerates the Al depletion that destabilizes the βNiAl into γ’-Ni3Al phase. An extensive discussion is presented in the light of observed results.

The high-temperature oxidation and solid particle erosion of thermal barrier coating (TBC) system which consists of a 8 wt.% yttria-partially stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited on Inconel 718 substrate via air plasma spraying (APS) process are studied experimentally. Isothermal oxidation tests of the APS-TBCs are conducted at 1050, 1100 and 1150 °C in air for up to 1970 h. Solid particle erosion tests are also performed on both as-deposited and heat-treated APS-TBC specimens at selected particle impingement angles and velocities in room temperature. The scanning electron microscopy (SEM) analyses of the cross sections of the APS-TBC specimens after the oxidation tests show the formation of thermally grown oxide (TGO) scale due to the oxidation of CoNiCrAlY bond coat, and the oxidation kinetics of TGO growth is described by the parabolic rate equation. The failure of the APS-TBC system under isothermal oxidation is associated with the spallation of the top coat through propagation and coalescence of cracks along the coating interface, which is affected by the TGO growth. The solid particle erosion rate of the top coat is found to increase with impingement angle and reach the maximum erosion rate at normal impingement of particles. The erosion resistance of the APS-TBC is observed to increase after the APS-TBC specimen has been exposed at 1100 and 1150 °C for 72 h, probably due to the sintering effect on the top coat, which results in reduction of top coat porosity.

To improve the hot section metallic parts durability in advanced gas-turbine operating in power generation and aircraft, thermal barrier coating (TBCs) are extensively utilized to increase their lifetime. The reason for applying coatings on these components is the improvement of their physical properties, mechanical properties, and outer look. The self-repairing ability of materials is very promising due to expanding the service time of materials and it is also beneficial in terms of human safety and financial aspects. In this review article, structure, properties, limitations, and the modification approaches of TBCs were studied. In addition, self-healing agents for TBCs including SiC, MoSi2, TiC were introduced, which release their oxide by reaction with air and O2 that are able to heal the pores/cracks in the coatings. In this regard, their coating methods, mechanism, and applications in TBCs were reviewed.

HfO2 alloying effect has been applied to optimize thermal insulation performance of HoTaO4 ceramics. X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy are employed to decide the crystal structure. Scanning electronic microscopy is utilized to detect the influence of HfO2 alloying effect on microstructure. Current paper indicates that the same numbers of Ta5+ and Ho3+ ions of HoTaO4 are substituted by Hf4+ cations, and it is defined as alloying effect. No crystal structural transition is introduced by HfO2 alloying effect, and circular pores are produced in HoTaO4. HfO2 alloying effect is efficient in decreasing thermal conductivity of HoTaO4 and it is contributed to the differences of ionic radius and atomic weight between Hf4+ ions and host cations (Ta5+ and Ho3+). The least experimental thermal conductivity is 0.8 W·K−1·m−1 at 900 °C, which is detected in 6 and 9 mol%-HfO2 HoTaO4 ceramics. The results imply that HfO2–HoTaO4 ceramics are promising thermal barrier coatings (TBCs) due to their extraordinary thermal insulation performance.

High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Y2O3) and ZrO2 are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO2 are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE1/5)2Zr2O7 have been formed after heated at 1000°C. The (5RE1/5)2Zr2O7 are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE1/5)2Zr2O7 high-entropy ceramics are below 1 W·m–1·K–1 in the temperature range of 300–1200°C. The (5RE1/5)2Zr2O7 can be potential thermal barrier coating materials.

In this work, we present the effect of the amount of CMAS infiltration into YSZ of Thermal barrier coatings (TBC) on the magnitude of residual stresses. The (TBC) were deposited by thermal spraying of CoNiCrAlY (Bond Coat-BC) and YSZ (Top Coat-TC) powders. The deposition of the BC was through the high velocity oxygen fuel (HVOF) system. The TC was deposited via an atmospheric plasma-spraying gun (APS). The TBCs were heat treated at 1250 °C, with a CMAS attack at a concentration of 10 mg/cm².The attack exposure was for 2 and 4 hours respectively. In this evaluation, the measured parameter was the magnitude of the residual stress state in Yttria Stabilized Zirconia (YSZ). The residual stress profiles were obtained using the Modified Layer Removal Method for Duplex Coatings (MRCMRB) and the Noda equations. An increase of 26.446 MPa was determined for 2 hours of thermal treatment and 30.743 MPa for 4 hours. Resumen Este trabajo muestra el efecto de la cantidad infiltrada de CMAS en el YSZ del TBC sobre la magnitud de los residuales. El recubrimiento de barrera térmica (TBC) fue fabricado mediante rociado térmico de dos capas, CoNiCrAlY y zirconia estabilizada con itria (YSZ). La capa metálica de enlace (BC) fue depositada mediante una pistola de rociado a alta velocidad por combustión de oxigeno (HVOF), mientras que la cerámica (TC) mediante rociado por plasma atmosférico (APS). Los TBC’s fueron tratados térmicamente con una temperatura de 1250 °C, con un ataque de CMAS con concentración de 10 mg/cm², durante 2 y 4 horas, respectivamente, con el fin de evaluar el efecto de la cantidad infiltrada de CMAS sobre la magnitud del estado de esfuerzos residuales del TC (YSZ). El estado de esfuerzos del recubrimiento fue determinado mediante el método de remoción de capa modificada para recubrimiento bicapa (MRCMRB) y las ecuaciones de Noda. Se determinó un incremento de 26,446 MPa durante 2 horas de tratamiento térmico y 30,743 MPa durante 4 horas.

Thermal barrier coatings (TBCs) prolong the durability of gas turbine engine components and enable them to operate at high temperature. Several degradation mechanisms limit the durability of TBCs during their service. Since the atmospheric plasma spray (APS) processed 7&ndash, 8 wt.% yttria stabilized zirconia (YSZ) TBCs widely utilized for gas turbine applications are susceptible to erosion damage, this work aims to evaluate the influence of their porosity levels on erosion behavior. Eight different APS TBCs were produced from 3 different spray powders with porosity ranging from 14% to 24%. The as-deposited TBCs were examined by SEM analysis. A licensed software was used to quantify the different microstructural features. Mechanical properties of the as-deposited TBCs were evaluated using micro-indentation technique. The as-deposited TBCs were subjected to erosion tests at different angles of erodent impact and their erosion performance was evaluated. Based on the results, microstructure-mechanical property-erosion performance was correlated. Findings from this work provide new insights into the microstructural features desired for improved erosion performance of APS deposited YSZ TBCs.

This work presents the results of structure, microstructure, and chemical composition investigations performed on ZrO2-Sm2O3 layers synthesized by MOCVD (metal-organic chemical vapor deposition) using Zr(tmhd)4 and Sm(tmhd)3 as reactants on quartz glass substrate. The molar percentage of Sm(tmhd)3 used to obtain the layers at both 500 and 550 &deg, C was 14% and 22.75%, respectively. Synthesis parameters were selected so that the value of the extended criterion Grx/Rex2 (Gr&mdash, Grashof number, Re&mdash, Reynolds number, x&mdash, the distance from the gas inflow point to the CVD (MOCVD) reactor) could be maintained below 0.01. It was determined from XRD (X-ray diffraction) analyses that the layers deposited at 500 &deg, C contained small amounts of a crystalline phase and the layers obtained at 550 &deg, C contained greater amounts of the crystalline phase (solid solution). SEM (scanning electron microscope) observations have also shown that the crystalline phase is present in the layers synthesized at 500 &deg, C, as well as at 550 &deg, C. EDS (energy dispersive spectroscopy) studies have indicated that molar content of Sm2O3 in the crystalline phase is lower in comparison to the amount present in the respective amorphous phase. The larger the Sm2O3 content in the layer, the higher the growth rate.

Termal bariyer kaplamalar (TBCs), yüksek sıcaklık koşullarında metalik altlık malzemeleri termal ve korozif koruma sağlamaktadır. TBC sistemleri, iş parçalarının sıcak bölüm komponentlerinde oksidasyon ve sıcak korozyon hasarlara maruz kalmaktadır. Özellikle sıcak korozyon, oksidasyona nazaran daha erken yıpratıcı hasarlara neden olmaktadır. Bu çalışmada, NiCoCrAlY tozları, yüksek hız oksi yakıt (HVOF) tekniği kullanılarak 316L paslanmaz çelik altlık malzeme üzerine püskürtülmüştür. Kalsiya ile stabilize zirkonya (ZrO2-5% CaO, CSZ), bağ kaplaması üretilmiş altlıklar üzerine üst kaplama malzemesi olarak atmosferik plazma sprey tekniği (APS) ile biriktirilmiştir. Üretilen TBC sistemi, 900 ° C' de % 50 V2O5-50 Na2SO4 karışımı ile 25 saatlik çevrimli sıcak korozyon testlerine tabi tutulmuştur. Sıcak korozyon testlerinin sonunda TBC sistemlerinde korozyon etkisi ve faz analizi stereo mikroskop, taramalı elektron mikroskobu (SEM), SEM - Enerji Dağılımlı X-Ray (EDX), SEM Haritalama ve XRD analizi kullanılarak incelenmiştir., Thermal barrier coatings (TBCs) provide thermal and corrosive protection to metallic substrate materials under high temperature conditions. TBC systems applied to hot part components of workpieces are exposed to oxidation and hot corrosion damage. Hot corrosion, in particular, causes destructive damage earlier than oxidation. In this study, NiCoCrAlY powders were sprayed on 316L stainless steel substrate material using High Velocity Oxygen Fuel (HVOF) technique. Calcia-stabilized zirconia (ZrO2-5% CaO, CSZ) was deposited as a top coating material by the atmospheric plasma spray technique (APS) on the substrate with the bond coat. The produced TBC system was subjected to 25 hours cyclic hot corrosion tests with a mixture of 50% V2O5- 50% Na2SO4 at 900 °C. At the end of the hot corrosion tests, as a result of the reaction of CaO with hot corrosion salts, phase transformation has occurred in the top coating. It has been found that the molten corrosion salts penetrate through the top coating, but have not yet seriously damaged the coating under these test conditions.

Functionally graded materials (FGMs) are engineered materials that are inhomogeneous and can be purposefully processed to obtain discrete or continuously varying compositions/microstructures over a definable geometrical length. FGMs can be used in a number of applications, such as aircrafts, combustion nozzles, gas turbines, energy conversion cells, biomaterials, etc. The objective of this paper is to review the new developments in production processes and their prospects in the creation of next-generation FGMs. Traditionally, four potential methods were used for fabrications of FGMs, i.e. powder metallurgy, chemical vapor deposition, self-propagating high-temperature synthesis, and plasma spraying. Some of the recently developed methods are the cast-decant-cast process, friction stir processing, and laser-engineered net shaping, which are usually cost-effective and used to make a quiet change in properties. An effective production method for conversion of the concept of gradient into practice is still a challenge for the research community. In this paper, research works toward meeting these challenges will be highlighted, and the future scopes of investigation in this area will be explored.

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.

Thermal barrier coatings (TBC) are multilayered systems comprising a metallic oxidation protection layer or so-called bond coat, a ceramic topcoat, and a thermally grown aluminum oxide developing at the interface. The coating systems fail typically by delamination cracking near this interface, which has a complex three-dimensional morphology influencing the crack path. This study combines laser shock adhesion test to introduce an artificial interfacial crack, known in size and location, and focused ion beam coupled with scanning electron microscopy 3D serial sectioning tomography. Methods for proper segmentation of cracks and adjacent materials and quantitative evaluation of the complex crack system are proposed and applied for analyzing the crack tip region. Finally, derived from the three-dimensional segmentation, a finite element model has been achieved and used for thermal analysis highlighting the crucial role of local damage on thermal conductivity of the TBC.

The effective high-temperature operation limit of a ZrO2-7-8 wt.%Y2O3 (YSZ) thermal barrier coating (TBC) manufactured via air plasma spray (APS) is considered to be ~1300 °C. This is related to the metastable tetragonal t′-phase formed during the rapid quenching of the YSZ particles during spraying. The t′-phase transforms into the equilibrium tetragonal and cubic phases at temperatures ≥~1300 °C, which can lead to the formation of the monoclinic phase of YSZ upon cooling to room temperature. This formation of the monoclinic phase is accompanied by a volume expansion that leads to TBC failure due to extensive micro-cracking. To further investigate this limitation, an APS YSZ TBC was sprayed on a CMSX-4 substrate. By using a thermal (laser) gradient cyclic testing, a temperature gradient was generated across the TBC/substrate system. The YSZ T-front and substrate backside T-back temperature levels were ~1500 and ~1000 °C, respectively. In cycle conditions (5-min or 1-h hot and 2-min cool), no TBC failure has been observed. This behavior was partially attributed to the unexpected absence of the monoclinic phase of the YSZ in the cycled coatings. Although preliminary, these results are promising regarding increasing the effective high-temperature operational limits of APS YSZ TBCs.

Polymer matrix composites (PMC), despite their many advantages, have limited use at elevated temperatures. To expand the scope of their uses, it becomes necessary to use thermal barrier coatings (TBC). In addition to elevated temperatures, composite structures, and thus TBC barriers, can be exposed to damage from impacts of foreign objects. Therefore, before using the thermal barrier in practice, knowledge about its behavior under high-speed loads is necessary. The paper presents results for samples with the PMC/TBC system subjected to dynamic compression using a split Hopkinson pressure bar (SHPB). The substrate was made of CFRP (carbon reinforced polymer) with epoxy matrix and twill fabric. TBC was made of ceramic mat saturated by commercial hardener from Vitcas company. The tests were carried out at ambient temperature and elevated temperature&mdash, 55 &deg, C and 90 &deg, C. Tests at ambient temperature were carried out for three pressure levels: 1, 1.5, and 2 bar. Only the pressure of 1 bar was used for the elevated temperature. Studies have shown that the limit load is 1 bar for ambient temperature. At 1.5 bar, cracks occurred in the TBC structure. Increased temperature also adversely affects the TBC barrier strength and it is damaged at a pressure of 1 bar.

BaLa2Ti3O10 ceramics for thermal barrier coating (TBC) applications were fabricated, and exposed to V2O5 and Na2SO4 + V2O5 molten salts at 900 &deg, C to investigate the hot corrosion behavior. After 4 h corrosion tests, the main reaction products resulting from V2O5 salt corrosion were LaVO4, TiO2, and Ba3V4O13, whereas those due to Na2SO4 + V2O5 corrosion consisted of LaVO4, TiO2, BaSO4 and some Ba3V4O13. The structures of reaction layers on the surfaces depended on the corrosion medium. In V2O5 salt, the layer was dense and had a thickness of 8&ndash, 10 &mu, m. While in Na2SO4 + V2O5 salt, it had a ~15 &mu, m porous structure and a dense, thin band at the bottom. Beneath the dense layer or the band, no obvious molten salt was found. The mechanisms by which the reaction layer forms were discussed.

En este trabajo, las transformaciones de fase en Recubrimientos de Barrera Térmica (TBC) constituidos por ZrO 2 – 8 wt.% Y2O3 (zirconia - 8 wt.% ytrria) fueron estudiados a través de Difracción de Rayos X (XRD) y refinamiento Rietveld. Las muestras de TBC fueron depositadas mediante aspersión por plasma atmosférico sobre un sustrato tipo Inconel 625 y fueron tratadas térmicamente con dos condiciones diferentes: en la primera se utilizó una temperatura de 1100oC con tiempos de exposición entre 1 hora y 1000 horas; en la segunda las muestras fueron sometidas a temperaturas entre 700oC y 1100o durante 50 horas. De acuerdo a los resultados obtenidos mediante refinamiento Rietveld el contenido de fase cúbica en el recubrimiento (TC) se incrementa con el tiempo y la temperatura, desde 7.3 wt.% hasta 15.7 wt.% después de 1000 horas a 1100oC. La fase cúbica en grandes cantidades es indeseable debido a que presenta inferiores propiedades mecánicas cuando se compara con la fase tetragonal. Después de 800 horas de exposición a alta temperatura, el contenido de Y2O3 en la fase tetragonal se reduce hasta 6.6 wt.% y una fracción de la fase tetragonal transforma a monoclínica durante el enfriamiento. La fase monoclínica alcanza 18.0 wt.% después de 1000 horas. Esta fase es también indeseable porque además de tener una mayor conductividad térmica, la transformación de tetragonal a monoclínica viene acompañada de un cambio volumétrico de alrededor de 5% que promueve la formación y propagación de grietas, las cuales comprometen la integridad del recubrimiento.

Thermal barrier coating (TBC) is a critical material in the aerospace domain to increase the lifetime of gas turbine components subjected to thermal load. The properties of TBC are strongly related to the growth of thermally grown oxide (TGO) whose main constituent is Al2O3. However, the oxidation of Cr and Co can affect the growth of TGO, which is not studied sufficiently. In this paper, high-temperature exposure at 1000 °C was performed to investigate the effect of Cr and Co oxides on TGO growth. The morphology and composition analysis of the interface between the ceramic top coat and the bond coat (TC/BC) were investigated by using scanning electron microscopy (SEM) and the energy dispersion spectrum (EDS). The thermodynamics and kinetics of oxidation were analyzed. The results indicated that the oxidation kinetics basically followed the sub-parabolic law with exposure time. Additionally, the major factor affecting the formation of oxides was the diffusion rate at the initial stage of exposure, then oxides depended on thermodynamics, and the oxidation was influenced by both of them in the last stage. The major elements to be oxidized were different at different stages. Moreover, the replacement reaction of Cr2O3 and the phase conversion of Al2O3 resulted in thickness variations of the TGO and Al-depleted zone during high-temperature exposure.

A computer-controlled laser test rig (using a CO2 laser) offers an interesting alternative to traditional flame-based thermal gradient rigs in evaluating thermal barrier coatings (TBCs). The temperature gradient between the top and back surfaces of a TBC system can be controlled based on the laser power and a forced air back-face cooling system, enabling the temperature history of complete aircraft missions to be simulated. An air plasma spray (APS) deposited TBC was tested and, based on experimental data available in the literature, the temperature gradients across the TBC system (ZrO2-Y2O3 YSZ top coat / CoNiCrAlY bond coat / Inconel 625 substrate) and their respective frequencies during air-to- Air combat missions of fighter jets were replicated. The missions included (i) idle/taxi on the runway, (ii) take-off and climbing, (iii) cruise trajectory to rendezvous zone, (iv) air-to- Air combat maneuvering, (v) cruise trajectory back to runway and (vi) idle/taxi after landing. The results show that the TBC thermal gradient experimental data in turbine engines can be replicated in the laser gradient rig, leading to an important tool to better engineer TBCs., International Thermal Spray Conference and Exposition, May 11-14,2015, Long Beach, CA, USA

The delamination crack growth in a plasma sprayed ZrO 2 thermal barrier coating system during thermal cycling was analysed by thermography. A simplified model system was used where a Fe–Cr–Al–Y alloy (Fecralloy Eisen-Chrom™) substrate with relative low yield stress and creep strength simulates the bond coat. The delamination crack area was measured as a function of cycle number and the growth mechanisms were observed and quantified by infrared pulse thermography. Based on this, an empirical model was created to describe the delamination crack growth. In contrast to most of the usual models, the present model calculates delamination crack area (instead of length), accounts for formation of multiple cracks (instead of one), for crack initiation at different times (instead simultaneous initiation), for the positions of the cracks on the sample and to each other as well as for the statistic spatial distribution of the crack initiation sites. The key features of delamination crack growth are met properly by the model approach. Although only one set of parameters was used, the model captures temporal discontinuities of the crack area increase and therefore scatter of delaminated area at a given time.