Mechanical Properties of LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript> and Thermal Cycling Behaviors of Plasma-Sprayed LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript>/YSZ Thermal Barrier Coatings

Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi₂Al₉O₁₉ (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the Young’s modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m^1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO₂ and Al₂O₃. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.     

Thermo-physical and thermal cycling properties of plasma-sprayed BaLa2Ti3O10 coating as potential thermal barrier materials

Increased turbine inlet temperature in advanced turbines has promoted the development of thermal barrier coating (TBC) materials with high-temperature capability. In this paper, BaLa₂Ti₃O₁₀ (BLT) was produced by solid-state reaction of BaCO₃, TiO₂ and La₂O₃ at 1500 °C for 48 h. BLT showed phase stability between room temperature and 1400 °C. BLT revealed a linearly increasing thermal expansion coefficient with increasing temperature up to 1200 °C and the coefficients of thermal expansion (CTEs) are in the range of 1 × 10^− 5–12.5 × 10^− 6 K^− 1, which are comparable to those of 7YSZ. BLT coatings with stoichiometric composition were produced by atmospheric plasma spraying. The coating contained segmentation cracks and had a porosity of around 13%. The microhardness for the BLT coating is 3.9–4.5 GPa. The thermo-physical properties of the sprayed coating were investigated. The thermal conductivity at 1200 °C is about 0.7 W/mK, exhibiting a very promising potential in improving the thermal insulation property of TBC. Thermal cycling result showed that the BLT TBC had a lifetime of more than 1100 cycles of about 200 h at 1100 °C. The failure of the coating occurred by cracking at the thermally grown oxide (TGO) layer due to severe oxidation of bond coat. Based on the above merits, BLT could be considered as a promising material for TBC applications.     

Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems

New LaMgAl₁₁O₁₉ (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis.     

Mechanical Properties of Layered La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Thermal Barrier Coatings

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La₂Zr₂O₇ and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La₂Zr₂O₇ + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La₂Zr₂O₇ TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La₂Zr₂O₇. The erosion rate of La₂Zr₂O₇ TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La₂Zr₂O₇. DCL porous 8YSZ + La₂Zr₂O₇ had a lower erosion rate than other SCL and DCL La₂Zr₂O₇ coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.     

不同结构等离子喷涂热障涂层的性能研究

通过等离子喷涂,制备了常规层状结构、垂直裂纹结构的氧化钇部分稳定氧化锆(YSZ)和铈酸镧(LCO)热障涂层,分析了涂层的微观形貌及性能。结果表明:对于YSZ涂层,垂直裂纹结构的结合强度是层状结构的1.8倍;LCO涂层的微观结构对喷涂工艺比较敏感,适当降低喷涂功率有助于提高扁平化粒子间的结合,缓解涂层中横向裂纹的产生,促进垂直裂纹形成;与层状结构的LCO/YSZ双层涂层相比,垂直裂纹结构的LCO/YSZ双层涂层的结合强度和抗热震性能未明显提高。     

Application of Plasma-Sprayed Complex Perovskites as Thermal Barrier Coatings

In an effort to improve the performance of heat engines at high temperatures, advanced surface coatings have been developed from complex perovskites. Materials of Ba(Mg_1/3Ta_2/3)O₃ and La(Al_1/4Mg_1/2Ta_1/4)O₃ composition were synthesized and applied as ceramic topcoats of thermal barrier coating (TBC) systems by atmospheric plasma spraying (APS) in single layer and in double-layer combination with conventional yttria stabilized zirconia (YSZ). Microstructural and phase analyses reveal that plasma spraying of complex perovskites is accompanied with the formation of vertical crack networks and secondary oxide phases which influence the failure mechanism of the TBCs. The low value of fracture toughness for the complex perovskites and the thermally grown oxide at the topcoat-bondcoat interface of the TBCs are, however, the major factors which lead to the coating failure on thermal cycling at about 1250 °C.     

La2O3-modified YSZ coatings: High-temperature stability and improved thermal barrier properties

Lanthana precursor was coated on yttria-stabilized-zirconia (YSZ) powders by wet chemical infiltration, and was introduced to the crystalline structure and grain boundaries of YSZ after plasma spraying of thermal barrier coatings (TBCs). The microstructural stability and thermal barrier properties of this new kind of TBCs were studied under different annealing conditions. It demonstrates that the La₂O₃ surface coating restrains grain growth of YSZ during both deposition and post-annealing processes, compared to a TBC obtained from commercially available unmodified YSZ powders. According to the composition analysis, lanthana partially dissolved in the zirconia matrix after heat treatment. The thermal diffusivity of YSZ coating significantly decreased after lanthana modification, typically from 0.354 mm² s^− 1 for an unmodified sample to 0.243 mm² s^− 1, reflecting a decrease of 31%. Even after annealed at 1200 °C for 50 h, the thermal diffusivity of modified coatings still shows a reduction of 25% than unmodified samples.     

Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions

Plasma sprayed thermal barrier coatings (TBCs) are applied to gas turbine components for providing thermal insulation and oxidation resistance. The TBC systems currently in use on superalloy substates typically consists of a metallic MCrAlY based bond coat and an insulating Y₂O₃ partially stabilized ZrO₂ as a ceramic top coat (ZrO₂ 7–8 wt.% Y₂O₃). The oxidation of bond coat underlying yttria stabilized zirconia (YSZ) is a significant factor in controlling the failure of TBCs. The oxidation of bond coat induces to the formation of a thermally grown oxide (TGO) layer at the bond coat/YSZ interface. The thickening of the TGO layer increases the stresses and leads to the spallation of TBCs. If the TGO were composed of a continuous scale of Al₂O₃, it would act as a diffusion barrier to suppress the formation of other detrimental mixed oxides during the extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability. The TBC layers are usually coated onto the superalloy substrate using the APS (Atmospheric plasma spray) process because of economic and practical considerations. As well as, HVOF (High velocity oxygen fuel) bond coat provides a good microstructure and better adhesion compared with the APS process. Therefore, there is a need to understand the cycling oxidation characteristic and failure mode in TBC systems having bond coat prepared using different processes. In the present investigation, the growth of TGO layers was studied to evaluate the cyclic oxidation behavior of YSZ/Al₂O₃ composite TBC systems with APS-NiCrAlY and HVOF-NiCrAlY bond coats. Interface morphology is significantly effective factor in occurrence of the oxide layer. Oxide layer thickening rate is slower in APS bond coated TBCs than HVOF bond coated systems under thermal cycle conditions at 1200 °C. The YSZ/Al₂O₃ particle composite systems with APS bond coat have a higher thermal cycle life time than with the HVOF bond coating.     

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H₂), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H₂ as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.     

Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

Microstructure and Thermal Properties of Plasma Sprayed Thermal Barrier Coatings from Nanostructured YSZ

Nanostructured yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were produced by atmospheric plasma spraying. The microstructure of the sprayed coating was characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM). The nano-coating had a higher porosity of ~25% than the conventional coating, which is mainly attributed to the large amount of intersplat gaps in the nano-coating. The thermal conductivity for the nano-coating was in the level of 0.8-1.1 W/m·K, about 40% lower than that for the conventional coating, indicating a better thermal insulation performance. The nano-TBC exhibited a thermal cycling lifetime of more than 500 cycles, whereas spallation failure of the conventional TBC occurred within 200 cycles. Accelerated sintering could be one of the reasons for the failure of the nano-TBC.     

Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ7C3 coating, and even longer than that of the single layer YSZ coating. The superior sintering-resistance of LZ7C3 coating, the similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and thermally grown oxide (TGO) layer, and the unique growth modes of columns within DCL coating are all very helpful to the prolongation of thermal cycling life of DCL coating. The failure of DCL coating is mainly a result of the reduction-oxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of t′-phase in YSZ coating and the outward diffusion of Cr alloying element into LZ7C3 coating.     

Microstructural analysis of YSZ and YSZ/Al2O3 plasma sprayed thermal barrier coatings after high temperature oxidation

Air plasma sprayed TBCs usually include lamellar structure with high interconnected porosities which transfer oxygen from YSZ layer towards bond coat and cause TGO growth and internal oxidation of bond coat.The growth of thermally grown oxide (TGO) at the interface of bond coat and ceramic layer and internal oxidation of bond coat are considered as the main destructive factors in thermal barrier coatings.Oxidation phenomena of two types of plasma sprayed TBC were evaluated: (a) usual YSZ (yttria stabilized zirconia), (b) layer composite of (YSZ/Al₂O₃) which Al₂O₃ is as a top coat over YSZ coating. Oxidation tests were carried out on these coatings at 1100°C for 22, 42 and 100h. Microstructure studies by SEM demonstrated the growth of TGO underneath usual YSZ coating is higher than for YSZ/Al₂O₃ coating. Also cracking was observed in usual YSZ coating at the YSZ/bond coat interface. In addition severe internal oxidation of the bond coat occurred for usual YSZ coating and micro-XRD analysis revealed the formation of the oxides such as NiCr₂O₄, NiCrO₃ and NiCrO₄ which are accompanied with rapid volume increase, but internal oxidation of the bond coat for YSZ/Al₂O₃ coating was lower and the mentioned oxides were not detected.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.     

Neodymium-cerium oxide as new thermal barrier coating material

Neodymium-cerium oxide (Nd₂Ce₂O₇) was proposed as a new thermal barrier coating material in this work. Monolithic Nd₂Ce₂O₇ powder was prepared by the solid-state reaction at 1400 °C. The phase composition, thermal stability and thermophysical properties of Nd₂Ce₂O₇ were investigated. Nd₂Ce₂O₇ with fluorite structure was thermally stable in the temperature range of interest for TBC applications. The results indicated that the thermal expansion coefficient (TEC) of Nd₂Ce₂O₇ was higher than that of YSZ (6-8 wt.% Y₂O₃ + ZrO₂) and even more interesting was the TEC change as a function of temperature paralleling that of the superalloy bond coat. Moreover, the thermal conductivity of Nd₂Ce₂O₇ is 30% lower than that of YSZ, which was discussed based on the theory of heat conduction. Thermal barrier coating of Nd₂Ce₂O₇ was produced by atmospheric plasma spraying (APS) using the spray-dried powder. The thermal cycling was performed with a gas burner test facility to examine the thermal stability of the as-prepared coating.     

Microstructure and Thermal Cycling Behavior of Air Plasma-Sprayed YSZ/LaMgAl11O19 Composite Coatings

Yttria-stabilized zirconia (YSZ) based composite coatings with the addition of LaMgAl₁₁O₁₉ (LaMA) as the secondary phase, were prepared by air plasma spraying in order to improve the performances of the traditional YSZ coating. Results indicate that the newly developed composite coating shows increased vertical crack density with the enhancement of the LaMA content during thermal cycling process, which results in increased strain tolerance and service lifetime. However, such composite coatings about 200 ??m thick, exhibit inferior thermal cycling lifetimes with respect to the typical YSZ coating for surface temperatures above 1400 °C. The presence of amorphous LaMA phase in the composite coating results in increased thermal conductivity and a relative thin top coat leading to a reduced thermal insulation efficiency. These are believed to be responsible for the premature degradation of bond coat and final top coat spallation failure. Such an investigation gives useful guidelines to develop advanced composite coatings based on YSZ/LaMA systems.     

Thermal Failure of Nanostructured Thermal Barrier Coatings with Cold-Sprayed Nanostructured NiCrAlY Bond Coat

Nanostructured thermal barrier coatings (TBCs) were deposited by plasma spraying using agglomerated nanostructured YSZ powder on Inconel 738 substrate with cold-sprayed nanostructured NiCrAlY powder as bond coat. The isothermal oxidation and thermal cycling tests were applied to examine failure modes of plasma-sprayed nanostructured TBCs. For comparison, the TBC consisting of conventional microstructure YSZ and conventional NiCrAlY bond coat was also deposited and subjected to the thermal shock test. The results showed that nanostructured YSZ coating contained two kinds of microstructures; nanosized zirconia particles embedded in the matrix and microcolumnar grain structures of zirconia similar to those of conventional YSZ. Although, after thermal cyclic test, a continuous, uniform thermally grown oxide (TGO) was formed, cracks were observed at the interface between TGO/BC or TGO/YSZ after thermal cyclic test. However, the failure of nanostructured and conventional TBCs mainly occurred through spalling of YSZ. Compared with conventional TBCs, nanostructured TBCs exhibited better thermal shock resistance.     

Thermal cyclic behavior of air plasma sprayed thermal barrier coatings sprayed on stainless steel substrates

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The coating comprised of 93 wt.% ZrO₂ and 7 wt.% Y₂O₃ (YSZ); CoNiCrAlY bond coat; and AISI 316L stainless steels substrate. Thermal cyclic lives of the TBC were determined as a function of bond coat surface roughness, thickness of the coating and the final deposition temperature. Two types of thermal shock tests were performed over the specimens, firstly holding of specimens at 1020 °C for 5 min and then water quenching. The other test consisted of holding of specimens at the same temperature for 4 min and then forced air quenching. In both of the cases the samples were directly pushed into the furnace at 1020 °C. It was observed that the final deposition temperature has great impact over the thermal shock life. The results were more prominent in forced air quenching tests, where the lives of the TBCs were observed more than 500 cycles (at 10% spalling). It was noticed that with increase of TBC's thickness the thermal shock life of the specimens significantly decreased. Further, the bond coat surface roughness varied by employing intermediate grit blasting just after the bond coat spray. It was observed that with decrease in bond coat roughness, the thermal shock life decreased slightly. The results are discussed in terms of residual stresses, determined by hole drill method.     

Monoclinic zirconia distributions in plasma-sprayed thermal barrier coatings

Phase composition in an air plasma-sprayed Y₂O₃-stabilized ZrO₂ (YSZ) top coating of a thermal barrier coating (TBC) system was characterized. Both the bulk phase content and localized pockets of monoclinic zirconia were measured with Raman spectroscopy. The starting powder consisted of ∼15 vol.% monoclinic zirconia, which decreased to ∼2 vol.% in the as-sprayed coating. Monoclinic zirconia was concentrated in porous pockets that were evenly distributed throughout the TBC. The pockets resulted from the presence of unmelted granules in the starting powder. The potential effect of the distributed monoclinic pockets on TBC performance is discussed.     

Failure of physical vapor deposition/plasma-sprayed thermal barrier coatings during thermal cycling

ZrO₂-7 wt.% Y₂O₃ plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.