高温热处理对等离子体喷涂的纳米氧化锆热障涂层的影响

热障涂层已广泛用于燃气发动机燃烧室等高温零部件上。纳米热障涂层韧性改善,厚度可以增加,能够提高零部件使用温度和使用寿命。首先利用低压等离子体在镍基体上喷涂制备NiCoCrAlYTa金属中间结合层和大气等离子体喷涂制备Y_2O_3部分稳定的ZrO_2纳米陶瓷面层,然后将纳米氧化锆热障涂层样品在大气中于1050～1250℃温度范围内煅烧处理2～20h。通过扫描电镜和X射线衍射仪分析纳米氧化锆热障涂层高温煅烧前后的组织结构变化和相组成变化,并与常规微米氧化锆热障涂层进行比较。研究结果表明:经高温煅烧后,纳米氧化锆热障涂层中晶粒大小和在陶瓷面层/金属中间结合层界面上形成的TGO的厚度随煅烧温度升高和时间的延长而增大;纳米热障涂层中TGO的增长速度比常规微米热障涂层快;纳米热障涂层经高温煅烧空气中冷却后,主要由四方相组成;与常规微米热障涂层的相组成比较,纳米氧化锆热障涂层中的四方相为低稳定剂四方相。     

溶液注入热等离子体中直接制备纳米结构热障涂层

采用一种新的方法,将Y2O3稳定的ZrO2前驱体溶液雾化注入直流等离子体中直接制备热障涂层.扫描电镜、透射电镜、X射线衍射分析、激光闪烁法分别观察分析了涂层的显微组织、纳米晶粒、相结构和热导率,排水法、冷热冲击法分别检测了涂层密度和热循环性能.实验结果显示等离子体喷涂液体制备出来的热障涂层不具有层状组织,晶粒尺寸小于100nm,组成相为四方相,硬度为350左右,热导率在1.2～1.5 W/m·K范围,涂层中存在16%左右的孔隙率,具有比常规微米结构热障涂层更优越的热循环性能.分析结果表明液体注入热等离子体中的雾化沉积既区别于物理化学气相沉积,又区别于粉末注入热等离子体中的熔化沉积,属于表面工程技术下的交叉领域.     

高温合金表面激光熔敷热障涂层组织结构与氧化性能

采用5KW连续CO2激光器对Ni基高温合金上二次重熔NiCoCrAlY和ZrO2陶瓷层进行了研究。结果表明，激光快速熔化和凝固获得定向外延生长，紧密堆积的柱状晶氧化锆陶瓷层。NiCoCrAlY结合层与柱状晶之间存在氧化铝层，保证了柱状晶与NiCoCrAlY层的联结。扫描电镜和电子探针联合分析发现，氧化锆层与NiCoCrAlY结合层，结合层与基体间均为冶金结合，采用化学改性氧化锆消除了涂层裂纹。高温氧化性能测试得出激光重熔试样氧化动力学近似地遵守物线速度方程，在1200℃，空气下激光熔敷TBCs抗氧化性明显高于等离子喷涂TBC。     

超音速等离子喷涂制备纳米结构梯度功能热障涂层

开发了国内外独创的“双通道、双温区“超音速等离子喷涂的新工艺,分别将高熔点陶瓷粉末和低熔点合金粉末送入喷枪内孔等离子弧的不同温区,保证两种粉末都能在各自的熔点附近获得充分熔融,再均匀混合后以超音速喷射到基体形成涂层,克服了传统的“金属/陶瓷混合法“喷涂过程中高熔点陶瓷熔化不足,低熔点合金过熔、氧化、脆化的难题;同时,由于粒子超音速飞行,在射流中停留时间缩短,晶粒来不及长大,从而使得制备出的CeO_2-Y_2O_3部分稳定的ZrO_2陶瓷涂层仍保留了纳米晶尺寸;由此获得的CeO_2-Y_2O_3/NiCoCrAlY纳米结构梯度功能热障涂层中,陶瓷与金属组份呈连续梯度分布,组织均匀、致密,涂层的韧性、抗氧化性和抗热冲击性能都有明显提高。     

等离子喷涂氧化锆纳米涂层显微结构研究

利用大气等离子喷涂（APS）技术,在不锈钢基体上制备了氧化锆纳米结构涂层.运用XRD、SEM与TEM等分析手段对喷涂用粉末原料和涂层的显微结构、物相组成进行了观察和确定.实验结果表明,纳米氧化锆粉末经喷雾造粒后的颗粒粒径主要分布在15～40μm之间,流动性好,适合于等离子喷涂用.等离子喷涂氧化锆纳米涂层颗粒分布在60～120nm之间,晶粒发育良好.涂层物相由四方和立方相氧化锆所组成.氧化锆纳米涂层的气孔率约为7％,结合强度为45MPa。     

等离子喷涂纳米结构与传统结构热障涂层的残余应力对比研究

热障涂层的残余应力是影响其服役寿研究不多.在45钢基体上,用超音速火焰喷涂NiCocrAlY打底层,再用大气等离子喷涂ZrO2-8%(质量分数)Y2O3(8YSZ)工作层,制备了纳米结构与传统结构2种类型的热障涂层(TBC).采用SEM、XRD对这2种涂层的粉末及涂层进行了组织结构分析,用纳米压痕仪测得了2种涂层的弹性模量.用X射线衍射应力测试仪测得了2种涂层的表层残余应力.结果表明:喷涂工艺参数相同条件下,对于打底层及工作层的厚度均相同的2种涂层,纳米结构热障涂层的表层残余应力比传统结构热障涂层约低24.7%;相同打底层的纳米结构热障涂层表层残余应力随着陶瓷层厚度增加而增加,陶瓷层厚度在240 um以下时,表层为残余压应力;厚度超过300 um时,表层为残余拉应力.最后提出了相应的物理力学模型.     

纳米结构热障涂层研究进展

本文主要综述了目前制备纳米结构热障涂层的三种方法,即大气等离子喷涂、溶液前驱体等离子喷涂和悬浮液等离子喷涂,并对这三种方法的工艺过程、制备原理、涂层的微观结构特征和研究现状进行了归纳。最后,总结了纳米结构热障涂层研究目前存在的问题,并对其发展趋势进行了展望。     

等离子喷涂纳米与非纳米氧化锆涂层的性能比较

在确定纳米氧化锆等离子喷涂工艺的基础上,成功地制备了多种涂层试件,并与非纳米氧化锆涂层的性能进行了比较.结果表明,纳米氧化锆制备的涂层的结合强度、耐磨蚀性和韧性优于非纳米氧化锆涂层,而两者的热导率大体相当,都随着温度的升高而增大.     

等离子喷涂纳米陶瓷涂层的显微组织及结构分析

热障涂层由绝热陶瓷层和金属底层组成,是目前保证高温合金在火箭发动机热端部位正常工作的主要技术.应用等离子喷涂方法成功备了纳米氧化锆陶瓷涂层,并对其微观组织形貌及相结构等进行观察分析,探讨了纳米陶瓷涂层的沉积机理.研究表明,涂层中的孔隙主要为长条形和近球形,无贯穿性孔洞;纳米陶瓷涂层裂纹较为细小,无明显的方向性;纳米粉末和涂层均为四方相(t-ZrO2)氧化锆组成的熔融或者部分熔融状态的纳米结构.     

一种热障涂层的形貌和相结构特征研究

研究了一种热障涂层的形貌和相结构组成。溅射NiCrAlY粘结层由γ-Ni相和β-NiAl相组成，是柱状结构，其晶粒度小，属于微晶，有利于形成α-Al2O3膜。溅射NiCrAlY粘结层经真空处理后，发生γ和γ’的转变，以及β-NiAl和脱溶。EB-PVD TBC 具有开放性的柱状结构，并且存在大量的t'-ZrO2相，使涂层性能大大优化。     

Phase composition and properties of plasma-sprayed zirconia thermal barrier coatings

The use of X-ray diffraction combined with TEM analysis has been used to study the crystalline structure and change in phase composition of zirconia coatings containing 6–12 wt% Y₂O₃. The optimum composition for maximum durability, observed for coatings within this composition range, is believed to be related to the microstructure developed on rapid cooling and to the volume fractions of t′, c and m phases formed during the evolution of the coating. The amount of these phases present in commercial thermal barrier coatings has been determined using X-ray diffraction and the mechanisms of toughening deduced from TEM examination of the sections of the coatings. The results obtained are discussed in relation to the degree of toughness and hence the thermal shock resistance which is a major factor in determining service life.     

Fracture of plasma-sprayed thick thermal barrier coatings under multiaxial stress states

Failure behaviour of free‐standing plasma‐sprayed coatings was investigated under combined axial and shear loading. Thin‐walled tubular specimens were loaded with various combinations of tension/compression and torsion. This allows the failure surface to be established for loading situations where the two principal stresses are of opposite signs. Specimens failed in one of the two modes, a tensile failure perpendicular to the maximum principal stress or a compression shear failure through the thickness. Failure data were adequately described by the maximum principal stress theory. Stress–strain curves fall within a single scatter band depending on the failure mode. In situ deformation tests showed that the mechanism was microcrack closing and sliding in compression and microcrack opening, coalescence and the development of new microcracks in tension.     

Corrosion of thermal barrier coatings by vanadium and sulfur compounds

Abstract

Hot corrosion studies of two plasma-sprayed coatings, yttria-stabilized zirconia and calcium silicate, were undertaken in order to compare the performance of these materials for use as thermal barrier coatings in high-temperature combustion environments. The coatings were tested in contact with vanadium pentoxide at 1,000°C and, also, under conditions in which they were exposed to sulfur-containing compounds at 900°C or 1,000°C. The samples were subsequently characterized by scanning electron microscopy and X-ray diffraction analysis to identify the effects of these tests on the microstructure and composition. The results indicate that reactions with V₂O₅ lead to a disruptive phase transformation in zirconia that rapidly degrades the coating. For calcium silicate, the reactions with V₂O₅ appear to be more limited and less disruptive so that the coating is much more slowly degraded by the vanadium compounds. Exposure to SO_x and sulfate salts at high temperature caused rapid degradation of the calcium silicate coatings through a reaction involving the formation of CaSO₄. Under similar conditions, the yttria-stabilized zirconia coatings experienced much less attack.     

Determination and analysis of crack growth resistance in plasma-sprayed thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) are increasingly applied to enhance the performance of advanced gas turbine engines. However, the delamination cracks initiated in these coatings limit their applications. In this research, a sandwiched four-point bend specimen is used to evaluate the crack growth resistance in plasma-sprayed TBCs. Well controlled, stable and measurable crack extension is obtained. A rising crack growth resistance curve is found. The steady state strain energy release rate is obtained to be about 170 J/m². The delamination crack evolution behavior is in situ observed and simulated by the finite element analysis based on a crack bridging model.     

High-growth rate YSZ thermal barrier coatings deposited by MOCVD demonstrate high thermal cycling lifetime

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC) were prepared by metalorganic chemical vapor deposition (MOCVD) using Y(OButⁿ)₃, Zr(OButⁿ)₄ precursors and O₂ carrier gas. A thermodynamic analysis guided experiments by optimizing elemental molar (n) stoichiometric ratios for the (Zr-Y-O-C-H system). This analysis showed single-phase YSZ was favored at 950 °C, 1 kPa, n_O/(n_Y + n_Zr) > 30, n_Y/(n_Y + n_Zr) = 0.06-0.10 (fixed n_C, n_H). Experimental YSZ growth had multiple phases (fcc, monoclinic), had a relatively high growth rate (43 μm/h, 1005 °C), had an Arrhenius dependence (845-950 °C, E_a = 53.8 ± 7.9 kJ/mol), had columnar grains (SEM analysis), and had a coating through-thickness n_Y/(n_Y + n_Zr) = 0.04 (EPMA analysis). Doubling the inlet yttrium precursor mole fraction resulted in fcc YSZ growth with a coating through-thickness n_Y/(n_Y + n_Zr) = 0.07. Hot-insertion thermal cycling of YSZ coatings on FeCrAlY bond coats showed >1000 h lifetime, matching current standards for EB-PVD YSZ coatings.     

Measurement of thermal diffusivity and anisotropy of plasma-sprayed coatings

The thermal diffusivity of free-standing tungsten and zirconia plasma-sprayed coatings was measured in the directions parallel and perpendicular to their surface. The parallel thermal diffusivity was evaluated by a double-sensing Laplace-transform technique and compared to the perpendicular values obtained by the (lash technique. Ratios between the parallel and the perpendicular thermal diffusivity values were in the range of 1.1 to 1.5 for zirconia and 4 to 6 for tungsten. The results are discussed in terms of the coating thickness and microstructure.On leave from Laboratoire d'Énergetique et de Méchanique Théorique et Appliquée, B.P. 160, 54504 Vandoeuvre les Nancy, France.     

Thermal conductivity of zirconia‐based ceramics for thermal barrier coating

Lowering the thermal conductivity of thermal barrier coatings used to protect blade and vane airfoils represents an important challenge for gas turbine designers and manufacturers. Dense zirconia‐based materials have been prepared by solid state reaction methods to determine their thermal properties up to 1000 °C. Partially stabilised zirconias having a thermal conductivity 40 % lower than the thermal conductivity of the most widely used system (ZrO₂‐8wt.%Y₂O₃) have been obtained.     

Composite ceramics thermal barrier coatings of yttria stabilized zirconia for aero-engines

Composite ceramics thermal barrier coatings(TBCs) are widely used in the aero-engines field due to their excellent thermal insulation, which improves the service life and durability of the inherent hot components. The most typical, successful and widely used TBCs material is yttria stabilized zirconia(YSZ). In this paper, fabrication methods, coating structures, materials, failure mechanism and major challenges of YSZ TBCs are introduced and reviewed. The research tendency is put forward as well. This review provides a good understanding of the YSZ TBCs and inspires researchers to discover versatile ideas to improve the TBCs systems.     

Mechanical properties and fracture behavior of interfacial alumina scales on plasma-sprayed thermal barrier coatings

Abstract

The mechanical properties and fracture behavior of Y-doped Al₂O₃ scales were investigated by furnace thermal cycling (to 1,150°C) of plasma-sprayed thermal barrier coatings (TBCs) with vacuum plasma-sprayed (VPS) or air plasma-sprayed (APS) Ni–22Cr–10Al–1Y bond coatings. No significant alterations in Al₂O₃ hardness or Young’s modulus (as measured by mechanical properties microprobe) were detected as a function of bond coat type, exposure time, or number of thermal cycles. The interfacial Al₂O₃ scales on VPS NiCrAlY exhibited progressive increases in localized fracture, buckling, and delamination during thermal cycling. The concentration of arrayed lenticular voids in the columnar Al₂O₃ grain boundaries significantly increased during cyclic oxidation (as compared to isothermal oxidation), but only in scales which formed on convex surfaces, suggesting internal void growth was stress-related. The amount and frequency of scale damage was higher on convex surfaces with a relatively large radius of curvature as compared to convex surfaces with a very small radius of curvature. Although the thermo-mechanical fracture resistance of Al₂O₃ scales on APS NiCrAlY was superior to scales on VPS NiCrAlY, TBC lifetimes on VPS NiCrAlY were greater by a factor of 2. Apparently, severe interfacial scale damage did not rapidly degrade the adherence of the ceramic top coatings.     

The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract

Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.