High-performance Thermal Barrier Coatings Research Articles

Enhanced thermal performance of CeYSZ coating on Ni-based high temperature alloy with a unique Y-doped Al2O3 barrier layer via cathodic plasma electrolytic deposition

Cathodic plasma electrolytic deposition (CPED) has shown considerable potential in producing high-performance thermal barrier coatings (TBCs) for components operating in extreme thermal environments, such as gas turbines and aviation engines. However, the dendritic and porous structures greatly limited the use of CPED-coatings in some harsh environments. In this study, one Y-doped Al2O3 (Y/A) barrier layer was prefabricated before the deposition of CeO2 and Y2O3 stabilized ZrO2 (CeYSZ) TBCs using CPED. The results suggested that the prefabricated Y/A barrier layer could attenuate the plasma bombardment and promote the uniform discharge, resulting in the formation of a thinner gas film on the cathode surface, which significantly avoided the dendritic defects and improved the density of the CeYSZ coating from 6.788 g/cm³ to 6.978 g/cm³. Consequently, the thermal insulation temperature of Y/A-CeYSZ coatings increased by 83.7% than that of CeYSZ coating, and the thermal cycling performance at 1300 °C demonstrated a 52.5 % improvement. This study provided a valuable method for the development of novel CPED TBCs.

Wide-velocity range high-energy plasma sprayed yttria-stabilized zirconia thermal barrier coating — Part I: Splashing splat formation and micro-adhesive property

Regulation on the disked-shape splat is generally regarded as the traditional way to enhance micro-adhesive properties of the splat/substrate interface. In this paper, a conception of impact-induced splashing splat is proposed in order to improve these micro-adhesive properties. Three contrasting types of splashing splat were studied, utilizing a wide-velocity high-energy plasma spraying method. The experimental results suggested that the needle-shaped splat formed by the impingement of supersonic particles possessed a greater micro-adhesive strength on an metallic substrate, as much as twice that of its subsonic counterpart. The threshold velocity was 572 m/s for supersonic adhesion. In relation to air film formation and interlocking reconstruction, the solidification texture and micro-adhesive at the central and peripheral regions was also investigated. The formation process of splashing splat was divided into three stages: particle deformation, edge fragmentation and impingement. This results will provide guideline for tailoring the high-performance thermal barrier coatings.

Effect of sintering temperature on phase evolution, microstructure, and mechanical properties of La2Ce2O7/40 wt.% YSZ composite ceramics

In this work, La2Ce2O7/40 wt% YSZ (LC40Z) mixed powders were used to fabricate composite bulk ceramics with their potential use as a material for high-performance thermal barrier coatings (TBCs). For this purpose, the effect of sintering temperature on the chemical reactivity of LC and YSZ was investigated via hot-pressing at the temperatures of 1300, 1400 and 1500 °C. X-ray diffraction analysis (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) were utilized to characterize the phase and microstructure evolution in both LC40Z powder mixtures and composite bulk ceramics. Results showed that solid-solution reactions occurred between the YSZ and LC during hot-pressing of the LC40Z powder mixtures, indicated by a new phase of La2Ce0·2Zr1·8O7 (LCZ) observed in the XRD patterns. The detailed analysis of Raman spectra confirmed the gradual transition from fluorite LC to pyrochlore LCZ structure, which was demonstrated by the appearance of characteristic pyrochlore bands. All composites densified by hot-pressing exhibited a high relative density above 95%. The average grain size of the LC40Z composites increased significantly with increasing sintering temperature, while gradual pore-healing was observed. The associated mechanical properties of LC40Z ceramics were also reported. The Vickers hardness values increased with increasing sintering temperature, which is consistent with the microstructure evolution and relative density variations. The highest hardness, with a value of 10.99 ± 0.23 GPa, was achieved for the composite hot-pressed at 1500 °C. The fracture toughness results showed the same dependence on sintering temperature. The fracture toughness increased from 1.97 ± 0.15 to 2.4 ± 0.14 MPa m1/2, indicating that the mechanical properties of the LC40Z composites can be tailored by changing the sintering temperature during hot-pressing.

Tuning the secondary reaction zone between PtAl coating and nickel based single crystal superalloy during high-temperature oxidation through regulating aluminizing time

Microstructure degradation caused by interdiffusion between coating and superalloy is an important factor affecting the performance of thermal barrier coatings. Tuning the interdiffusion microstructure of PtAl Coatings/Superalloys interface is the key to preparing high performance thermal barrier coatings. In this paper, two β-(Ni, Pt)Al coatings with different compositions and thicknesses were first deposited on the same nickel-based single-crystal superalloy by regulating the aluminizing time. After the isothermal oxidation at 1100 °C, a typical two-layer structure forms in the two different coating/superalloy specimens but with different interdiffusion microstructures. It was found that a noticeable secondary reaction zone (SRZ) with topologically close-packed (TCP) precipitates develop in the specimen with 6 h aluminizing after oxidation for 36 h and its thickness continuously increase. By contrast, no SRZ forms in the specimen with 4.5 h aluminizing after oxidation for even 100 h, but only the microstructure transformation of the interdiffusion zone (IDZ) occurred. That is mainly due to the existence of an incomplete transformed sublayer in the as-prepared specimen with a short aluminizing time, which may affect the subsequent interdiffusion of Al, Ni, and refractory elements. It is highly expected that regulating the aluminizing time is an effective strategy to tune the interdiffusion structure between coating and substrate, which should be helpful for designing the coating/substrate system with better performance.

Microstructure and high-temperature sintering properties of APS coatings prepared from spherical thin-walled hollow-shell powders

High-performance thermal barrier coatings (TBCs) made of 4 mol.% Y2O3–stabilized ZrO2 (4YSZ) powder with a spherical thin-walled hollow-shell (STHS) structure exhibited a special microstructure different from the conventional lamellar structure of air plasma-sprayed (APS) coatings. The as-sprayed STHS APS coatings had a completely tetragonal prime (t′) structure and non-lamellated closed-cell structure with high porosity, which resulted in relatively low thermal conductivity (∼1.0 W m−1 K−1) and high Vicker's hardness (∼6 GPa). The influences of high-temperature aging on the microstructure stability, phase stability, and sintering capability were investigated after long-time heat treatment at different temperatures. The characterization results indicated that the pore content was basically constant, and it was less than 0.5% for sintered linear shrinkage of the STHS coatings after heat treatment at 1500 °C for 100 h. Furthermore, no spalling appeared in the STHS APS coating with the t′ phase structure after 101 thermal cycles of the water-quenching method at 1050 °C, and no monoclinic ZrO2 (m-ZrO2) phase was present in all of the STHS coatings after aging at 1200 °C for 1–1100 h. The excellent anti-sintering properties and phase stability of the STHS coatings are attributed to the closed-pore microstructure and the highly pure t′ phase composition with uniform distribution of ions, respectively. The results suggested that the non-lamellated closed-cell microstructure is beneficial for improving the coating properties, and the results also provide guidelines for microstructure design of TBCs using a feedstock powder.

Modified hydrothermal treatment route for high-yield preparation of nanosized ZrO2

Nanosized ZrO2 particles are applied in high-performance thermal barrier coatings and catalyst carriers. To synthesize nanosized zirconia, precipitation from aqueous solutions followed by hydrothermal treatment is widely conducted. In this work, a modified hydrothermal treatment route is described for high-yield fabrication of well-dispersible nanosized t–ZrO2. Zirconium oxychloride and sodium hydroxide were used as the precursor and precipitant, respectively. N, N-bis(2-hydroxyethyl) glycine (bicine) was used as surface stabilizer to inhibit the early agglomeration of nuclei, and ultrasonication was used to enhance the dispersion of ZrO2 nanocrystals. The hydrothermal treatment was optimized for reaction temperature, time, fill fraction, and solid content. The synthesized zirconia was characterized using X-ray diffraction, dynamic light scattering, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The yield of zirconia increased to 134 g/L after hydrothermal. Tetragonal ZrO2 obtained with hydrothermal treatment at 200 °C for 8 h at a fill fraction of 80% has a good dispersibility, with an average particle size of 20 nm and a narrow size distribution.

Structure evolution, thermal properties and sintering resistance of promising thermal barrier coating material La2(Zr0.75Ce0.25)2O7

Rare-earth doped zirconates are promising candidate materials for high-performance thermal barrier coatings (TBCs). The phase and microstructure stability is an important issue for the materials that must be clarified, which is related to the long-term stable work of TBCs at high temperatures. In this work, La2(Zr0.75Ce0.25)2O7 (LCZ) ceramic coatings prepared by atmospheric plasma spraying present a metastable fluorite phase, which can transform into stable pyrochlore under high-temperature annealing. The detailed structure evolution of the ceramic coatings is characterized systematically by SEM, XRD and Raman. The associated thermal properties of LCZ ceramics were also reported. Results show that LCZ ceramic has an ultralow thermal conductivity (0.65 W/m·K, 1200 °C), which is only 1/3 of that of yttria-stabilized zirconia (YSZ). The thermal expansion coefficients of LCZ ceramic increase from 9.68 × 10-6 K-1 to 10.7 × 10-6 K-1 (300 - 1500 °C), which are relatively larger than those of La2Zr2O7. Besides, Long-term sintering demonstrates that LCZ ceramic coating has preferable sintering resistance at 1500 °C, which is desirable for TBC applications.

Plasma spray–physical vapor deposition toward advanced thermal barrier coatings: a review

Plasma spray–physical vapor deposition (PS–PVD) is a unique technology that enables highly tailorable functional films and coatings with various rare metal elements to be processed. This technology bridges the gap between conventional thermal spray and vapor deposition and provides a variety of coating microstructures composed of vapor, liquid, and solid deposition units. The PS–PVD technique serves a broad range of applications in the fields of thermal barrier coatings (TBCs), environmental barrier coatings (EBCs), oxygen permeable films, and electrode films. It also represents the development direction of high-performance TBC/EBC preparation technologies. With the PS–PVD technique, the composition of the deposition unit determines the microstructure of the coating and its performance. When coating materials are injected into a nozzle and transported into the plasma jet, the deposition unit generated by a coating material is affected by the plasma jet characteristics. However, there is no direct in situ measurement method of material transfer and deposition processes in the PS–PVD plasma jet, because of the extreme conditions of PS–PVD such as a low operating pressure of ~ 100 Pa, temperatures of thousands of degrees, and a thin and high-velocity jet. Despite the difficulties, the transport and transformation behaviors of the deposition units were also researched by optical emission spectroscopy, observation of the coating microstructure and other methods. This paper reviews the progress of PS–PVD technologies considering the preparation of advanced thermal barrier coatings from the perspective of the transport and transformation behaviors of the deposition units. The development prospects of new high-performance TBCs using the PS–PVD technique are also discussed.

Phase formation, stability and heat capacity of ternary TiO2–CeO2–ZrO2 solid solutions

Tetragonal zirconia based solid solutions are important materials for thermal barrier coatings (TBCs). The introduction of the third component to binary zirconia systems is regarded as a promising way to design novel high performance TBCs. The work reports the investigation of solid solution formation in ternary TiO2–CeO2–ZrO2 system in the range 600–1100 °C obtained by using co-precipitation from mixed aqueous solution and freeze-drying. Phase evolution and stability of solid solutions were investigated using DSC, qualitative and quantitative XRD analysis with Rietveld refinement technique. The factors affecting the kinetics of phase formation i.e. powders dispersity, agglomerate strength, crystallite size, Ce3+/Ce4+ ratio were studied using PSD, XRD and ESCA techniques. The heat capacity dependence on temperature was measured experimentally for the first time for 5TiO2–9CeO2–86ZrO2 composition via calorimetry technique using DSC. The obtained data significantly contributes to the schematic phase diagram suggested in the literature and allows to choose the optimal synthesis and processing conditions of ternary system compositions for their use for novel TBCs fabrication.

Architecture designs for extending thermal cycling lifetime of suspension plasma sprayed thermal barrier coatings

Suspension plasma spraying (SPS) as a relatively new spraying technology has great potential on depositing high performance thermal barrier coatings (TBCs). In some cases, however, columnar SPS TBCs show premature failure in thermal cycling test. To explain the reasons of such failure, a failure mechanism for columnar SPS TBCs was proposed in this work. The premature failure of TBCs might be related to the radial stresses in the vicinity of top coat/bond coat interface. These radial stresses were introduced by the thermal misfit and the roughness of bond coat. According to this mechanism, two architecture designs of SPS TBCs were applied to improve the thermal cycling lifetime. One was a double layered top coat design with a lamellar atmospheric plasma sprayed (APS) sub-layer and a columnar SPS top-layer. The other one was a low roughness bond coat design with a columnar SPS top coat deposited on a low roughness bond coat which was grinded before the spraying. With both designs, lifetimes of SPS TBCs were significantly extended. Especially, a lifetime even better than conventional APS TBCs was achieved with the double layered design.

Preparation of Yttria-Stabilized Zirconia Hollow Sphere with Reduced Shell Thickness by Controlling Ambient Temperature during Plasma Process

Yttria-stabilized zirconia (YSZ) hollow sphere (HS) powder is a novel potential feedstock material for the plasma spraying of next generation advanced thermal barrier coatings with low thermal conductivity and high sintering resistibility. In this study, YSZ HS powders were prepared by plasma treatment with/without a heat preservation zone around the flying path of the particles during plasma flame. The results of the scanning electron microscopy of YSZ HS powders showed that HS prepared with a heat preservation zone during the plasma process exhibited a regular spherical morphology and a homogeneous thin shell structure. Due to the sufficient heating of the shell regions, the HS powder presented a well densified shell structure. Furthermore, the mechanism of formation of the HS powder with reduced shell thickness was also discussed based on the analysis of the evolution of the powder structure. This kind of hollow sphere powder with a very thin shell structure provides a new alternative feedstock material for the development of next generation high performance thermal barrier coatings.

Hollow ceramic microspheres prepared by combining electro-spraying with non-solvent induced phase separation method: A promising feedstock for thermal barrier coatings

Hollow microspheres were prepared by a combination method of electro-spraying and non-solvent induced phase separation, using a slurry of polymer with Y2O3-stabilized ZrO2 powder (YSZ). Subsequent sintering was carried out to obtain porous hollow YSZ microspheres with proper strength aiming to be thermal spraying feedstock. The ESNP particle size and distribution strongly depended on the applied voltage of the electro-spraying process and the jet nozzle diameter. The YSZ microspheres always consisted of a thin spongy shell and highly porous interior structure. The microsphere size decreased from 1000 to 100μm with increasing voltage (10–20kV). The cone-jet mode of electrospray reached steady as the nozzle diameter varied from 0.5 to 0.8mm. Compared with the coatings made from commercial hollow spherical powder, the coatings prepared by ESNP powder had enhanced resistance to sintering and exhibited a longer life than commercial spray powder. It has demonstrated that the ESNP method provides a versatile way to manipulate the microstructure of ceramic microspheres, and could been used to fabricate high performance thermal barrier coatings by plasma spraying.

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.

Dependence of scale thickness on the breaking behavior of the initial oxide on plasma spray bond coat surface during vacuum pre-treatment

The thermally grown oxide (TGO) on the thermal spray bond coat surface was one of the most important factors which would influence the lifetime of thermal barrier coatings (TBCs). Pre-diffusion treatment (high temperature vacuum treatment) plays an important role in the growth of the TGO. Results show that the initial thin oxide scale, formed during deposition process, on the as-sprayed bond coating surface has broken and shrunk to discontinuous oxide particles through the elements diffusion during the pre-diffusion. Two kinds of bond coats with different initial oxide scale thicknesses were subjected to the same pre-diffusion. The two pre-diffused bond coats show different results of the average distance between the individual oxide particles. In this study, a three dimensional model with thermal grooving theory was developed to explore the essential condition for the scale breaking and explain the differences of these results. This research can provide reference for the preparation optimization and pre-treatment optimization of bond coat towards high performance TBCs.

Microstructure and mechanical properties of plasma spraying coatings from YSZ feedstocks comprising nano- and submicron-sized particles

Atmospheric plasma spraying (APS) is an attractive technique to obtain nanostructured coatings due to its versatility, simplicity and relatively low cost. However, nanoparticles cannot be fed into the plasma using conventional feeding systems, due to their low mass and poor flowability, and must be adequately reconstituted into sprayable micrometric agglomerates.In this work, nanostructured and submicron/nanostructured powders of yttria-stabilised zirconia (YSZ) were deposited using APS, with a view to obtaining high-performance thermal barrier coatings (TBC). All powders were reconstituted by spray-drying from different solid loading suspensions, followed by a thermal treatment of the spray-dried granules to reduce agglomerate porosity and enhance powder sinterability. The reconstituted granules were characterised by XRD, SEM, pore sizing and flowability evaluation.The reconstituted feedstocks were successfully deposited onto metallic substrates by APS. A metallic bond coat was sprayed between the substrate and the ceramic layer. The coating microstructure, characterised by SEM, was formed by partially melted zones, which retained the initial powder microstructure, embedded in a fully melted matrix which acts as a binder. It was shown that feedstock characteristics which in turn are very dependent of starting suspension characteristics, in particular agglomerate density and primary particle size, impact on coating microstructure (porosity and amount of partially-melted areas). For this reason mechanical properties of coatings are also strongly affected by feeding powder characteristics.

Melting Refining Mechanisms in Supersonic Atmospheric Plasma Spraying

In recent years, Yttria-stabilized zirconia based thermal barrier coatings (TBCs) are deposited by newly-developed high-efficiency supersonic atmospheric plasma spraying (SAPS) technology. The final microstructure of the plasma-sprayed coatings is strongly dependent on the size distribution of spray particles. It has been corroborated through experiments that there is a special phenomenon of particle melting refining in SAPS, as compared with the conventional atmospheric plasma spraying (APS). This phenomenon greatly affects the final particle size and distribution, which has not been explained reasonably up to now. Therefore, it is necessary to investigate the melting refining behavior of in-flight particles to control the particle size and to analyze the coating properties. In this paper, the breakup of particle is presented to characterize the phenomenon of particle melting refining, and the peak of size distribution becomes bigger with increasing the spray distances, which is explained by collision-coalescence. Furthermore, based on the maximum entropy formalism, the particle-size distribution is calculated and the result is in good accordance with the plasma spraying experiment results, which verifies the mechanism analysis presented in this paper. This work could provide more efficient applications of the SAPS technology in high-performance TBCs.

Isothermal Oxidation Behavior of NiCoCrAlTaY Coating Deposited by High Velocity Air-Fuel Spraying

The performance of thermal barrier coatings is influenced by the high temperature oxidation behavior of the bond coat. In this paper, NiCoCrAlTaY bond coat was deposited by high velocity air-fuel (HVAF) spraying, and the microstructure and surface morphology of the bond coat before and after oxidation were examined to aim at developing high performance thermal barrier coatings. Results showed that the HVAF sprayed NiCoCrAlTaY coating presented a dense microstructure and some partially melted particles with a near spherical morphology were deposited on the coating surface. A uniform α-Al2O3 scale was formed on the HVAF sprayed MCrAlY coating surface after the pre-oxidation treatment in an argon atmosphere. A small fraction of nodular-shaped mixed oxides was formed when the MCrAlY coating was oxidized for 100 h at 1000 °C. The amount of the mixed oxides increased less significantly after 200 h oxidation. A homogeneous α-Al2O3 oxide scale was maintained over the large particles on the bond coat surface after 200 h oxidation at 1000 °C in air. A model is proposed to explain the formation of nodular-shaped mixed oxides.

Enhancement of TBC (Thermal Barrier Coatings) Characteristics by Gas Tunnel Type Plasma Spraying

Zirconia (ZrO2) coating formed by plasma spray method is widely used industrially as a thermal barrier coating (TBC). Presently, there are some problems such as spallation and cracks inside the coating. As one solution given by the development of new spaying processing, the gas tunnel type plasma spraying is one of excellent method to enhance the TBC performances. The zirconia-alumina (ZrO2-Al2O3) composite coating formed by this method has a high hardness layer at the surface side of the coating, which shows the graded functionality of hardness, and is superior as a TBC. In this paper, the performance of such high hardness ZrO2-Al2O3 composite coating was investigated and the merit as TBC was clarified. The Vickers hardness of the high hardness layer near the coating surface increased by the thermal process of high energy plasma, which corresponded to the result that the coating became denser. Also, the effect of alumina mixing was discussed about the microstructure of this composite coating. The combination of high hardness of Al2O3 with the low thermal conductivity of ZrO2 resulted to the development of high performance TBC. The transverse thermal conductivity of such ZrO2-Al2O3 composite coatings was proved to be much smaller than that in the longitudinal direction.

The Potential for the Improvement of High Performance Thermal Barrier Coatings

The growing market for industrial gas turbines has led to an increased demand for large, cost-effective units of high efficiency, A critical issue in the development of such units is the durability of hot section components, especially first stage blades and vanes to which thermal barrier coatings (TBCs) are applied. This paper introduces the current state of the art in the production of TBCs, and discusses their various degradation mechanisms. Special emphasis is placed on the potential for the development of a new generation of TBC systems through the control of microstructure and porosity, as well as through new processing techniques.

The effect of grain size, porosity and yttria content on the thermal conductivity of nanocrystalline zirconia

High-performance thermal barrier coatings (TBCs) made of 4 mol.% Y2O3–stabilized ZrO2 (4YSZ) powder with a spherical thin-walled hollow-shell (STHS) structure exhibited a special microstructure different from the conventional lamellar structure of air plasma-sprayed (APS) coatings. The as-sprayed STHS APS coatings had a completely tetragonal prime (t′) structure and non-lamellated closed-cell structure with high porosity, which resulted in relatively low thermal conductivity (∼1.0 W m−1 K−1) and high Vicker's hardness (∼6 GPa). The influences of high-temperature aging on the microstructure stability, phase stability, and sintering capability were investigated after long-time heat treatment at different temperatures. The characterization results indicated that the pore content was basically constant, and it was less than 0.5% for sintered linear shrinkage of the STHS coatings after heat treatment at 1500 °C for 100 h. Furthermore, no spalling appeared in the STHS APS coating with the t′ phase structure after 101 thermal cycles of the water-quenching method at 1050 °C, and no monoclinic ZrO2 (m-ZrO2) phase was present in all of the STHS coatings after aging at 1200 °C for 1–1100 h. The excellent anti-sintering properties and phase stability of the STHS coatings are attributed to the closed-pore microstructure and the highly pure t′ phase composition with uniform distribution of ions, respectively. The results suggested that the non-lamellated closed-cell microstructure is beneficial for improving the coating properties, and the results also provide guidelines for microstructure design of TBCs using a feedstock powder.