Atmospheric Plasma Spraying of High Melting Temperature Complex Perovskites for TBC Application

High melting materials have always been very attractive candidates for materials development in thermal barrier coating (TBC) applications. Among these materials, complex perovskites with Ba(Mg_1/3Ta_2/3)O₃ and La(Al_1/4Mg_1/2T_1/4)O₃ compositions have been developed and deposited in TBC systems by atmospheric plasma spraying. Spray parameters were optimized and in-flight particle temperatures were recorded using Accuraspray-g3 and DPV 2000. Plasma sprayed coatings were found to undergo non-stoichiometric decomposition of components which could have contributed to early failure of the coatings. Particle temperature diagnostics suggest that gun power of ~15 kW or lower where majority of the particles have already solidified upon impact to the substrate could probably prevent the decomposition of phases. Additionally, it has been found that the morphology of the powder feedstock plays a critical role during atmospheric plasma spraying of complex perovskites.     

New Generation Perovskite Thermal Barrier Coating Materials

Advanced ceramic materials of perovskite structure have been developed for potential application in thermal barrier coating systems, in an effort to improve the properties of the pre-existing ones like yttria-stabilized zirconia. Yb₂O₃ and Gd₂O₃ doped strontium zirconate (SrZrO₃) and barium magnesium tantalate (Ba(Mg_1/3Ta_2/3)O₃) of the ABO₃ and complex A(B′_1/3B′′_2/3)O₃ systems, respectively, have been synthesized using ball milling prior to solid state sintering. Thermal and mechanical investigations show desirable properties for high-temperature coating applications. On atmospheric plasma spraying, the newly developed thermal barrier coatings reveal promising thermal cycle lifetime up to 1350 °C.     

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.     

Lanthanum hexaaluminate—a new material for atmospheric plasma spraying of advanced thermal barrier coatings

One of the main application fields of the thermal spraying process is thermal barrier coatings (TBCs). Today, partially stabilized zirconia (YSZ or MSZ) is mainly used as a TBC material. At temperatures above 1000 °C, zirconia layers age distinctively, including phenomena shrinkage and microcrack formation. Therefore, there is a considerable interest in TBCs for higher temperature applications. In this paper, lanthanum hexaaluminate, a newly developed TBC material with long-term stability up to 1400 °C, is presented. It ages significantly more slowly at these high temperatures than commercial zirconia-based TBCs. Its composition favors the formation of platelets, which prevent a densification of the coating by postsintering. It consists of La₂O₃, Al₂O₃, and MgO. Its crystal structure corresponds to a magnetoplumbite phase. Lanthanum hexaaluminate powders were produced using two different fabrication routes, one based on salts and the other one based on oxides. To optimize the granulate, various raw materials and additives were tested. The slurry was spray dried in a laboratory spray drier and calcined at 1650 °C. Using these two powders, coatings were produced by atmospheric plasma spraying (APS). The residual stresses of the coatings were measured by the hole drilling method, and the deposition process was optimized with respect to the residual stresses in the TBC. The coatings were extensively analyzed regarding phase composition, thermal expansion, and long-term stability, as well as microstructural properties.     

Ultrafast thermal plasma physical vapor deposition of yttria-stabilized zirconia for novel thermal barrier coatings

This research aims to develop advanced thermal plasma spraying technology for the next-generation thermal barrier coatings (TBCs) with a high power hybrid plasma spraying system. By using thermal plasma physical vapor deposition (TP-PVD), various functional structured yttria-stabilized zirconia (YSZ) coatings were deposited. Parameters, such as powder feeding rate, hydrogen gas concentration, and total mass flow rate of the plasma gas, were optimized, and their influences on the evaporation of YSZ powder were investigated. Ultrafast deposition of a thick coating was achieved at a rate of over 150 μm/min. The deposited porous coating has a low thermal conductivity of 0.7W/mK and the dense coating with interlaced t′ domains possesses a high nanohardness of 27.85 GPa and a high reflectance. These characteristics show that the TP-PVD technique is a very valuable process for manufacturing novel TBCs.     

Cyclic-Oxidation Behavior of Thermal-barrier Coatings Exposed to NaCl Vapor

A Ni–24Cr–6Al–0.7Y (NiCrAlY) coating was deposited on a nickel-base superalloy by low-pressure plasma spraying, and the top coating, ZrO₂ partially stabilized with Y₂O₃ (7.5 wt%), was deposited on the NiCrAlY coating by air-plasma spraying. The cyclic-oxidation behavior of the NiCrAlY + YSZ coating exposed to NaCl vapor was investigated under atmospheric pressure at 1,050 °C, 1,100 °C and 1,150 °C. The cyclic-oxidation life of the NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The higher the temperature is, the shorter the cyclic oxidation life. The oxide scale formed at the interface between the bond coat and the ceramic layer after exposure to NaCl vapor consisted of voluminous and non-protective NiO, Al₂O₃ and NiCr₂O₄ spinel. The failure of the TBC exposed to NaCl vapor occurs within the top coat and close to the YSZ/thermal growth oxide interface. The failure mechanism has been discussed based on the experimental results and thermodynamics.     

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.     

Isothermal oxidation of physical vapor deposited partially stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilized zirconia (PSZ, 8 wt.%Y₂O₃) and a diffusion aluminide bond coat, were characterized as a function of time after oxidative isothermal heat treatment at 1373 K in air. The experimental characterizations was conducted by X-ray diffraction analysis and scanning electron microscopy (SEM) with energy-dispersive spectroscopy. During cooling to room temperature, spallation of the PSZ ceramic coatings occurred after 200 and 350 h of isothermal heat treatment. This failure was always sudden and violent, with the TBC popping from the substrate. The monoclinic phase of zirconia was first observed on the bottom surface of the PVD PSZ after 200 h of isothermal heat treatment. The failure of TBCs occurred either in the bond coat oxidation products of αAl₂O₃ and rutile TiO₂ or at the interface between the oxidation products and the diffusion aluminide bond coat or the PSZ coating.     

Proposal of self-healing coatings for nuclear fusion applications

Avoiding cracks in ceramic coatings is one of the most important problems to be solved for the thermally sprayed tritium permeation barriers in fusion reactor. In this paper, a self-healing composite coating composed of TiC + mixture (TiC/Al₂O₃) + Al₂O₃ was developed to address this problem. The coating was deposited on certain martensitic steel by plasma spraying. The morphology and phase of the coating were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) while the porosity was analyzed by using Image Pro software. The thermal shock resistance test and residual stress measurement of the coating were also performed. In the experiment, NiAl + TiC + mixture (TiC/Al₂O₃) + Al₂O₃ and mixture (TiC/Al₂O₃) + Al₂O₃ films were also fabricated and studied respectively. The results showed that the TiC + mixture (TiC/Al₂O₃) + Al₂O₃ coating exhibited the best mechanical integrity and self-healing ability among the three samples with the porosity decreased by 90% after heat-treatment under normal atmosphere. The oxidation/expansion of TiC in the coating played an important role in the sealing of pores. This self-healing coating made by thermal spraying is proposed as a good candidate for tritium permeation barrier in fusion reactors.     

预氧化处理对热障涂层热冲击性能及残余应力的影响

采用等离子喷涂工艺在镍基高温合金基体上制备了热障涂层(底层为MCrAlY,面层为ZrO2+ 8％ Y2O3),通过控制高真空烧结炉的氧分压对涂层进行预氧化处理,分析了预氧化处理对热障涂层热冲击性能和涂层应力状态的影响.结果表明,预氧化处理提高了粘接层的致密度,涂层组织变得均质化,降低了粘结层由于凸起尖角产生复杂应力的概率;有效干预热生长氧化物(TGO)的生长过程,降低了TGO的生长速度;热障涂层残余应力随热冲击次数的增加而增大,但经过预氧化处理的涂层应力增长幅度较缓慢,经过400次热冲击后的残余应力为492.5 MPa,未经过预氧化处理涂层热冲击350次后应力值为650.1 MPa.     

Hot-corrosion behavior of graded thermal barrier coatings formed by plasma-spraying process

The hot-corrosion behavior of thermal barrier coatings (TBCs) has been studied by comparing double-layer coatings and graded coatings. Two types of oxide ceramics, 2CaO·SiO₂-15mass%CaO·ZrO₂ (C₂S-15CZ) and 8 mass% Y₂O₃·ZrO₂ (8YSZ), with a bond coating of NiCrAlY, were applied to metallic substrates in this study. After hot-corrosion testing with V₂O₅-Na₂SO₄ corrosive ash for 3 h at 1273 K, the TBCs were investigated by visual inspection, a scanning electron microscope, x-ray diffraction, and electron probe microanalysis. The findings for the resulting coating of C₂S-15CZ reacted with V₂O₅ only where it was in direct contact with the corrosive ash. The affected area from the reaction was limited to the coating surface where V₂O₅ was present. The coating showed adequate hot-corrosion resistance against V₂O₅-Na₂SO₄ corrosive ash for 3 h at 1273 K. The findings for the 8YSZ coating were that Y₂O₃, the stabilizing component, particularly reacted with V₂O₅ and lost its function, which led to partial spalling of the coating. It was observed that the hot-corrosion resistance of the double-layer TBC was largely influenced by the performance of a corrosion-resistant NiCrAlY bond coat, which provided protection against corrosive components penetrating through the ceramic topcoat. Last, the graded coating degraded due to the oxidation of NiCrAlY particles that existed near the topcoat surface and affected the durability of the TBC.     

表面陶瓷层厚度对Sm2Zr2O7/8YSZ热障涂层热冲击性能的影响

稀土锆酸盐与8YSZ所组成的双陶瓷层涂层是目前热障涂层领域研究的热点,而陶瓷层厚度对其热冲击性能有着显著影响。采用有限元软件ANSYS研究了表层厚度对Sm2Zr2O7/8YSZ热障涂层淬冲击热应力的影响,并与单一Sm2Zr2O7涂层进行了比较。结果表明,在Sm2Zr2O7/8YSZ涂层的表面处具有最大的径向热冲击应力,最大轴向应力则存在于陶瓷层/金属粘结层界面处,涂层各处剪应力基本相当。涂层表面及两陶瓷层界面处的径向热应力随表层厚度的增加而减小,陶瓷层/粘结层界面处径向应力则随表层厚度增加而增大。每个界面处的轴向应力随表层厚度增加而降低,而剪应力绝对值则随表层厚度增加而增大。与单一Sm2Zr2O7涂层相比,Sm2Zr2O7/8YSZ涂层的热应力明显偏小,说明增加涂层的层数有利益改善涂层的抗热冲击性能。     

Comparison of thermal shock behaviors between plasma-sprayed nanostructured and conventional zirconia thermal barrier coatings

NiCoCrAlTaY bond coat was deposited on pure nickel substrate by low pressure plasma spraying(LPPS), and ZrO2-8%Y2O3 (mass fraction) nanostructured and ZrO2-7%Y2O3 (mass fraction) conventional thermal barrier coatings(TBCs) were deposited by air plasma spraying(APS). The thermal shock behaviors of the nanostructured and conventional TBCs were investigated by quenching the coating samples in cold water from 1 150, 1 200 and 1 250 ℃, respectively. Scanning electron microscopy(SEM) was used to examine the microstructures of the samples after thermal shock testing. Energy dispersive analysis of X-ray(EDAX) was used to analyze the interface diffusion behavior of the bond coat elements. X-ray diffractometry(XRD) was used to analyze the constituent phases of the samples. Experimental results indicate that the nanostructured TBC is superior to the conventional TBC in thermal shock performance. Both the nanostructured and conventional TBCs fail along the bond coat/substrate interface. The constituent phase of the as-sprayed conventional TBC is diffusionless-transformed tetragonal(t′). However, the constituent phase of the as-sprayed nanostructured TBC is cubic(c). There is a difference in the crystal size at the spalled surfaces of the nanostructured and conventional TBCs. The constituent phases of the spalled surfaces are mainly composed of Ni2.88Cr1.12 and oxides of bond coat elements.     

基于磁过滤技术的热障涂层表面Al2O3 覆盖层抗CMAS腐蚀性能

为了提高热障涂层（TBC）的抗沉积物（主要成分为CaO、MgO、Al₂O₃和SiO₂,简称CMAS）腐蚀性能,采用磁过滤阴极真空电弧（FCVA）技术在TBC表面上制备了致密的Al₂O₃覆盖层,比较和分析了Al₂O₃改性TBC和沉积态TBC的润湿行为和抗CMAS腐蚀性能。结果表明：使用FCVA技术制备Al₂O₃覆盖层的过程对7%（质量分数）氧化钇稳定的氧化锆（7YSZ）相的结构无明显影响,且经Al₂O₃改性的TBC综合性能均优于沉积态TBC。在1250 ℃、CMAS腐蚀条件下,Al₂O₃覆盖层有效地限制了熔融CMAS在TBC表面上的扩散行为。同时,Al₂O₃填充了7YSZ柱状晶之间的间隔并且阻碍了熔融CMAS的渗透,证明了FCVA可作为一种制备Al₂O₃涂层的新方法以提高TBC的抗CMAS腐蚀性能,且Al₂O₃涂层及其制备过程对TBC的热震性能均无消极影响。     

等离子喷涂热障涂层的热循环氧化行为研究

本文采用大气等离子喷涂在HA188合金基材上制备NiCrAlY+YSZ热障涂层，并进行了1100 oC、1120 oC和1150 oC三个温度点的高温循环氧化行为对比研究。结果表明，随着考核温度的升高，热障涂层热循环失效寿命显著下降，失效主要是由YSZ/NiCrAlY界面附近YSZ 层中裂纹形成和扩展导致。循环失效后的YSZ与制备态的相结构一样，均为非平衡四方相t"-ZrO2，未发生t"→c+m相变。在热循环过程中，YSZ/NiCrAlY界面形成的热生长氧化物层（Thermally Grown Oxide, TGO）增厚基本符合“抛物线”规律，并且YSZ中裂纹的产生和扩展与TGO的增厚直接相关。     

Plasma-sprayed duplex and graded partially stabilized zirconia thermal barrier coatings: deposition process and properties

Atmospheric plasma spraying of duplex and graded ZrO₂ (8% Y₂O₃) thermal barrier coatings (TBCs) on Inconel 617 substrate with a NiCrAlY bond coat is described in terms of a deposition process of con-trolled coating structure. Special attention is devoted to the dominant spray parameters and the injector configuration for powder feeding, which play a fundamental role in graded coating deposition with con-trolled formation of a graded metal-ceramic (GMC) intermediate zone. The results of the graded coating spraying allow: (a) suppression of step-interface effects, (b) suppression of large differences (misfit) be-tween physical and mechanical constants of the coating and those of the substrate material, and (c) favor-able intergrowth of crystallites for a microstructurally integrated structure. Sprayed TBCs were investigated and compared with regard to their thermal cycling, oxidation behavior, and mechanical properties. The influence of crystal anisotropy changes on the resulting coating structure and properties is shown. On the basis of finite element (FE) calculations, the stress distribution within thermally cycled coating systems was analyzed. It is confirmed that the graded coating structure relaxes considerably the stresses resulting from the internal constraint due to thermal expansion difference between both metallic and ce-ramic materials. This stress distribution also decreases the gradient of elastic deformation and/or resid-ual stresses between the metal bond coat and top ceramic coating, and hence leads to a better thermal cycling behavior of the graded TBC systems. However, this advantage is not practical in every case, since the rapid oxidation of the metallic lamellae causes the ceramic phase in the GMC zone to undergo tensile stresses within a short thermal exposure time. The lifetime of duplex TBC systems that are under steady-state thermal load conditions is much higher than that of graded ones.     

Furnace cyclic oxidation behavior of multicomponent low conductivity thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in advanced gas turbine engines due to their ability to further increase engine operating temperatures and reduce cooling, thus helping achieve future engine low emission, high efficiency, and improved reliability goals. Advanced multicomponent zirconia (ZrO₂)-based TBCs are being developed using an oxide defect clustering design approach to achieve the required coating low thermal conductivity and high-temperature stability. Although the new composition coatings were not yet optimized for cyclic durability, an initial durability screening of the candidate coating materials was conducted using conventional furnace cyclic oxidation tests. In this paper, furnace cyclic oxidation behavior of plasma-sprayed ZrO₂-based defect cluster TBCs was investigated at 1163°C using 45 min hot-time cycles. The ceramic coating failure mechanisms were studied using scanning electron microscopy (SEM) combined with x-ray diffraction (XRD) phase analysis after the furnace tests. The coating cyclic lifetime is also discussed in relation to coating processing, phase structures, dopant concentration, and other thermo-physical properties.     

等离子喷涂La2(Zr0.7Ce0.3)2O7热障涂层的抗热震性能

采用等离子喷涂制备了La₂(Zr_0.7Ce_0.3)₂O₇（LZ7C3）热障涂层,并对涂层的微观组织、相结构、成分、相稳定性、涂层热导率以及抗热震性能进行了研究.结果表明,LZ7C3涂层由单相烧绿石结构物质组成,高温稳定性较好;涂层的热导率较块材下降约20%,这是由于涂层具有较高的孔隙率所致:涂层在不同温度范围的热震寿命和失效机制不同,在室温至约1000℃间的热震寿命为116 cyc,涂层失效方式以片状剥落为主:在室温至1100℃间的热震寿命为53 cyc,涂层失效方式为片状剥落和层状撕裂;在室温至1200℃间的热震寿命为3 cyc,涂层失效方式以层状撕裂为主.     

Thermal properties and microstructure of a plasma sprayed wollastonite coating

Wollastonite coatings were deposited using an atmospheric plasma spraying technique. The microstructure and phase compositions of the coating before and after heat treatment were investigated using scanning electron microscopy (SEM), x-ray diffraction (XRD), and differential thermal analysis (DTA) technologies, respectively. In addition, the coefficient of thermal expansion and thermal diffusivity of the coating were also investigated. Crystalline wollastonite, glassy phase, and tridymite (SiO₂) were observed in the coating. Tridymite (SiO₂) likely reacted with other composites such as CaO and glassy phase to form crystalline wollastonite when the coating was heated at about 882 °C. During the first thermal cycle, the coefficient of thermal expansion of the coating decreased dramatically between 700 and 850 °C and the thermal diffusivity of the coating was 2.7–3.1 × 10⁻³cm²/s between 20 and 1000 °C. During the second thermal cycle, the coefficient of thermal expansion of the coating increased slightly between room temperature and 1000 °C and the thermal diffusivity of the coating increased by about 20% compared with that of the first thermal cycle. The atmospheric plasma sprayed Wollastonite coating may be used as thermal barrier coating.     

Characterization of oxide scales formed on high-velocity oxyfuel-sprayed Ni-Co-Cr-Al-Y + ReTa coatings

A high-velocity oxyfuel-sprayed 30 wt.% Ni-20 wt.% Co-30 wt.% Cr-10 wt.% Al-2 wt.% Y-4 wt.% Re-4 wt.% Ta coating was oxidized between 1000 and 1200 °C for up to 200 h in air, and the oxide scales were examined. The dense, sprayed coating consisted mainly of Cr₃Ni₂, Ni₃Al, Ni₃Ta, Ni, NiO, Al₅Y₃O₁₂, and Cr₂O₃. Intermetallics and some oxides formed during spraying. During oxidation, mainly αAl₂O₃, along with some Al₅Y₃O₁₂, CoAl₂O₄, CoCr₂O₄, Ta₂O₅, and Ta₂O_2.2 formed on the coating. The preferential oxidation of Al to form the Al-rich scales resulted in the formation of an Al-depleted region beneath the scales. Rhenium, being the most noble element, was distributed throughout the oxide scale and the coating, without forming any independent oxides.