Abstract

Nanostructured zirconia coatings have been prepared by atmospherical plasma spraying (APS) on NiCrAlY-coated superalloy substrates. The isothermal oxidation test results indicate that the oxidation kinetics of nanostructured TBC follows a parabolic law and the oxidation resistance of the nanostructured TBC is comparable to that of the conventional TBC. The nanostructured thermal barrier coatings exhibit excellent thermal cyclic resistance and low thermal diffusivity. The failure of the nanostructured TBC occurs within the top coat and close to the YSZ/thermal growth oxide interface. The thermal diffusivity of the coating is 90% of that of conventional thermal barrier coatings, and it increases after heat treatment at 1050 °C for 34 h. The increase in the thermal diffusivity of the coating is ascribed to grain growth, the crack healing as well as sintering neck formation.

ZrO₂-3mol% Y₂O₃ coating was deposited by air plasma spraying using reconstituted nanosize particles. The microstructure was examined by SEM and TEM. The thermal conductivity was calculated based on the specific heat and thermal diffusivity detected by DSC and laser flash method. The results show that the thermal barrier zirconia coating with thermal conductivity ranging from 0.63-0.80 W/(m·K) is obtained. This can be attributed to the homogeneous distribution of micro-size pores and amounts of micro-cracks with average length of about 10 μm, to which heat flow is nearly perpendicular. The phase transition of t to m can result in the change of thermophysical properties. The grain growth and the decrease of porosity lead to increase of thermal conductiviy of as-sprayed coating.

利用大气等离子喷涂技术和纳米粉体造粒料, 制备了3mol% Y₂O₃部分稳定ZrO₂热障涂层. 利用扫描电镜(SEM)、透射电镜(TEM)研究了涂层的显微结构, 采用差示扫描量热仪(DSC)、激光脉冲法热导仪对涂层的热物理性能进行了实验研究和理论分析. 结果表明: 利用纳米粉体造粒料和等离子喷涂技术可获得导热系数为0.63~0.80W/(m·K)的热障涂层. 原因在于涂层显微结构中大量分布均匀的微小气孔和平均长度在十几微米的、垂直于热流方向的层状微裂纹. 相变可导致涂层热物性变化异常. 热处理后涂层的晶粒长大和气孔率降低可导致导热系数升高.

Abstract

An electron beam directed vapor deposition method was used to grow 7 wt.% Y₂O₃–ZrO₂ (7YSZ) coatings and the effects of substrate rotation upon the coating porosity, morphology, texture, and thermal conductivity were explored. As the rotation rate was increased, the texture changed from 〈1 1 1〉 to 〈1 0 0〉. Under stationary deposition, the coatings were composed of straight columns, while low-frequency rotation resulted in wavy columns. Increases in rotation rate resulted in a gradual straightening and narrowing of the growth columns. The pore fraction slowly decreased as the rotation rate increased. The thermal conductivity was found to be inversely related to the pore fraction. The structural and thermal conductivity alterations are a result of changes to flux shadowing associated with specimen rotation in a gas jet-entrained vapor plume. The minimum thermal conductivity at a low rotation rate is 0.8 W/(m K), well below that of conventionally deposited coatings.

Abstract

Modelling of thermal conductivity of two and three phase composite materials is used to determine the thermal conductivity of thick porous zirconia based thermal barrier coatings for use in high temperature applications. These coatings, depending on the deposition technique and process parameters exhibit different degrees of porosity. The porosity of the coating has an affect on thermal properties in completely different ways depending on the morphology and the orientation of the pores dispersed within a continuous matrix. In this work air plasma sprayed coatings have been considered. The experimental results were successfully compared to the modelled thermal conductivities. In the model the effects of porosity were taken into account considering the shape, orientation and volumetric percentage of pores. Image analysis and mercury porosimetry was used in experimental porosity determination.

Abstract

Thermal barrier coatings (TBC) are applied on cooled gas turbine components. Between the heated and cooled surfaces a thermal gradient develops, resulting for constraint components in high multiaxial stresses, which may exceed stresses due to mechanical loading. In order to investigate the damage behaviour of TBC systems under close to reality conditions, thermal mechanical fatigue tests with controlled thermal gradients (TGMF-tests) were performed on coated tubular specimens. The specimen substrate was made from directionally solidified nickel base superalloy IN100 DS, and the coating system comprised a NiCoCrAlY bond coat and a ceramic top coat from partially stabilized zirconia. The stress distribution over the specimen wall, which was generated in the course of one TGMF cycle, has been analysed by linear elastic Finite Element calculations.

In TGMF testing specific damages occurred underneath the adherent ceramic top coat, evolving into fatigue cracks, which propagated primarily in the metallic bond coat parallel to the surface. In length sections the crack shape in this stage resembles a ‘smiley’. During further cycling the crack path deviated to the interface between BC and top coat, enhancing spallation. The ‘smiley’-crack patterns, which only occurred in TGMF but not in isothermal low cycle fatigue or thermal fatigue, are discussed with respect to the cyclic local stress distribution.

Abstract

During high-temperature exposure, the microstructure of thermal barrier coatings evolves, leading to increased thermal conductivity. We describe the evolution in the thermal properties of a 7 wt.% Y₂O₃ stabilized ZrO₂ electron beam-physical vapor deposited (EB-PVD) thermal barrier coating with thermal cycling between room temperature and 1150 °C until failure. The thermal diffusivity and conductivity of the coating were evaluated non-destructively based on the analysis of its photothermal infrared emission. Although the coating density does not increase significantly with thermal cycling, the thermal diffusivity and conductivity of the coating increased substantially, particularly during the first 20 1 h cycles. The values then approach a limiting value. Complementary Raman spectroscopy suggests that the increase is accompanied by a reduction in the defect concentration in the coating and that there is also a correlation between the width of the Raman lines and the thermal conductivity.

Abstract

The microstructure of thermally grown oxide (TGO) in thermal barrier coatings (TBCs) oxidized under dry/humid atmosphere at 1100 °C has been characterized by transmission electron microscopy. A thin and continuous oxide layer is formed in the as-deposited TBCs produced by electron beam physical vapor deposition. The TGO formed in dry atmosphere consists of an outer layer of fine α-alumina, zirconia grains and an inner layer of columnar α-alumina grains. However, a small amount of spinel is observed in the TGO under humid atmosphere. The presence of water vapour promotes the formation of spinel.

Graphical abstract

The presence of water vapor promoted the formation of spinels in the TBC.

Abstract

Oxide dispersed (OD) β-NiAl coatings and OD-free β-NiAl coatings were deposited onto a Hf-containing Ni-based superalloy by electron beam physical vapor deposition (EB-PVD). Excessive enrichment of Hf was found in the TGO on the OD-free coating due to outward diffusion of Hf from the superalloy, causing accelerated TGO thickening and spalling. The OD-coating effectively prevented Hf from outward diffusion. Only small amount of Hf diffused to the coating surface and improved the TGO adherence by virtue of the reactive element effect. The OD-coating exhibited an improved oxidation resistance as compared to the OD-free coating.