等离子喷涂-物理气相沉积7YSZ热障涂层高温氧化过程中的阻抗谱分析

采用等离子喷涂-物理气相沉积(PS-PVD)在预处理的粘结层表面制备了柱状结构的7YSZ热障涂层,并在大气环境下测试了该涂层在950℃的静态高温氧化性能。利用透射电镜(TEM)、扫描电子显微镜(SEM)及能谱仪(EDS)等对热障涂层进行了表征,并采用阻抗谱分析研究了该涂层在高温氧化过程中的结构演变过程。结果表明,7YSZ热障涂层是由二次柱状晶及其纳米间隙、柱状枝晶间孔隙和分布在枝晶上的微纳米固态颗粒组合形成。阻抗分析表明,热生长氧化物(TGO)层在高温氧化150 h后氧空位含量减少,致密度增加。在高温氧化过程中,二次柱状晶的内部结构没有发生明显改变。此外,氧化过程中YSZ层内形成的烧结收缩裂纹是导致YSZ晶界电容值减小、电阻值增加的主要原因。     

一种长寿命热障涂层——LaMgAl_(11)O_(19)/YSZ双陶瓷层介绍

本文通过固相反应合成LaMgAl11O19热障涂层新材料,用喷雾干燥的方法制备粉末,用等离子喷涂方法制备LaMgAl11O19/YSZ双陶瓷层热障涂层,通过热循环实验测试了涂层的热循环性能,并分析了涂层失效机理。研究结果表明LaMgAl11O19/YSZ双陶瓷层热循环寿命比8YSZ的热循环寿命长得多,是一种非常有应用前景的热障涂层新材料。     

晶界“背脊”形貌对热障涂层热冲击寿命的影响

采用化学气相沉积和电子束物理气相沉积工艺在镍基单晶高温合金基体上制备了(Ni,Pt)Al/YSZ(Yttira stabilized zirconia,YSZ)热障涂层(Thermal barrier coatings,TBCs),研究了TBCs涂层在1100℃的抗热冲击性能。结果表明,陶瓷涂层的剥落位置主要出现在热生长氧化物(Thermally grown oxide,TGO)层/粘结层的界面或者TGO层内部。试验过程中,粘结层表面晶粒晶界"背脊"形貌共存、TGO层下方孔洞形成、陶瓷层内纵向裂纹延伸和TGO层内残余应力释放均是导致TBCs涂层过早剥落失效的主要原因。     

YZrHf热障涂层的制备及热震性能分析

目的 研究喷涂态YZrHf热障涂层的微观组织及其抵抗高温热冲击的性能,探讨高温条件下热生长氧化物(TGO)对陶瓷层的影响。方法 采用大气等离子喷涂(APS)技术制备厚度约为300μm的YZrHf热障涂层,并将涂层在950℃下保温15 min后进行水冷循环热震实验,直至涂层剥落失效,使用SEM、EDS、X射线衍射仪对制备态及热震实验后的热障涂层微观组织进行分析。结果 涂层表面粗糙不平且分布有十几到几十微米长度的网状裂纹,这些相互贯通的裂纹为氧气的进入提供了通道。经过101次循环热震实验后,涂层部分区域剥落失效,SEM结果显示,在陶瓷层/黏结层界面处、黏结层内部均出现了热生长氧化物,且在陶瓷层中分布有横向、纵向的贯通性裂纹,而在TGO生长区域,也出现了一些小裂纹,但涂层并未剥落。经测定分析可知,TGO的主要成分为Al₂O₃、Cr₂O₃、NiO以及尖晶石氧化物组成的混合物(CSN)。结论 热震实验后TGO层中Al元素贫化,Ni、Cr等元素向界面处迁移参与反应,同时尖晶石氧化物以α-Al₂     

一种新型CMAS耦合条件下热障涂层热循环实验方法

提出一种高温度梯度、燃气加热和CMAS(CaO-MgO-Al2O3-SiO2)沉积条件下热障涂层热循环实验方法,并对1200℃下CMAS沉积物对等离子喷涂热障涂层过早失效的影响因素进行讨论和分析。结果表明:无CMAS耦合条件下,热障涂层热循环寿命为573次;CMAS耦合条件下,热障涂层热循环寿命降低至70次。CMAS渗入会导致陶瓷层表层产生致密层和横向微裂纹增多。CMAS耦合条件下,热障涂层的失效以陶瓷层逐层剥离为主。     

LaMeAl11O19/YSZ热障涂层热力学性能和热循环寿命

LaMeAl₁₁O₁₉陶瓷具有独特的晶体结构, 优异的热力学性能, 低热导率, 高温相稳定性等特点, 是一类非常有应用前景的热障涂层(TBC)材料。本研究通过大气等离子喷涂(APS)制备了LaMeAl₁₁O₁₉/YSZ (Me=Mg, Cu, Zn)双陶瓷层热障涂层。通过对涂层进行火焰热循环测试并结合扫描电子显微镜、X射线衍射仪等分析技术对涂层进行失效分析。结果表明, LaMgAl₁₁O₁₉ (LMA)、LaZnAl₁₁O₁₉ (LZA)和LaCuAl₁₁O₁₉ (LCA)粉末在等离子喷涂过程中发生了分解, 导致三种涂层中磁铅石相含量的差异, 从而影响三种涂层的热循环寿命。由于LaMeAl₁₁O₁₉层与YSZ层的热膨胀系数不匹配以及非晶相重结晶产生的体积收缩, LaMeAl₁₁O₁₉层从YSZ层上剥落。YSZ层暴露在高温下, 加速了烧结和TGO的生长, 又促进了YSZ层剥落。低温下, LaMeAl₁₁O₁₉的热导率随着Me原子序数增加而降低; 高温下, 与LMA和LZA相比, LCA涂层红外发射率最高(0.88, 600 ℃), 削弱了光子传导对热导率的贡献, 导致热导率降低, LCA在高温红外辐射涂层中具有潜在的应用价值。     

DZ466合金热障涂层CoCrAlY黏结层1050℃氧化行为

采用电子束物理气相沉积(EB-PVD)法在一种新型定向合金DZ466试样上沉积CoCrAlY黏结层和Y2O3部分稳定的ZrO2(YSZ)陶瓷层,对试样进行1050℃循环氧化实验并研究其氧化行为。采用X射线衍射仪、扫描电镜以及电子探针对涂层进行显微组织分析。结果表明:在1050℃氧化1500h(热循环31次)后,热障涂层未出现脱落现象。沉积态CoCrAlY黏结层主要由β-CoAl相和γ-Co固溶体相组成;1050℃氧化后,在黏结层与陶瓷层界面生成热生长氧化物(TGO)层,黏结层逐渐发生退化,β-CoAl相逐渐转化为γ-CoNi固溶体;氧化1200h后,TGO/黏结层界面出现由活性元素效应导致的氧化物栓;TGO层皱曲行为导致TGO/陶瓷层界面出现微裂纹,并且该微裂纹沿界面横向扩展。TGO的厚度增长模式符合分段抛物线规律,初期氧化速率常数约为6.1×10-14cm2/s,氧化400h后,氧化速率常数减小,为3.5×10-14cm2/s。     

镀铝表面改性7YSZ纳米热障涂层热震性能分析

为提高7YSZ纳米热障涂层的热震性能,实验中采用超音速火焰喷涂(HVOF)在涡轮叶片模拟工件上制备了粘结层NiCrCrAlYTa,再使用大气等离子喷涂(APS)在粘结层上制备了7YSZ纳米陶瓷层。采用磁控溅射在7YSZ热障涂层样品表面镀铝,并在不同压力下(200、250、300 Pa)对镀铝样品进行热处理表面改性。对喷涂态样品和镀铝改性后样品进行水淬热震实验,1050℃保温10 min+水冷5 min为一个热循环,观察热障涂层镀铝改性前后样品在水淬热循环过程中形貌和结构演变。实验结果表明,镀铝改性后样品表面存在铝薄膜蒸发、凝固后形成的疏松纳米Al晶粒表层以及由Al和ZrO_2原位反应形成的致密α-Al_2O_3底层。在镀铝样品热处理过程中,随着压力升高,疏松层致密度逐渐增加。不同热处理压力下镀铝表面改性后样品经过73次水淬热循环后剥落面积均小于喷涂态样品,显示出良好的抗热震性。     

晶粒尺度对热障涂层高温循环氧化寿命的影响

采用化学气相沉积和电子束物理气相沉积工艺在镍基单晶高温合金基体上制备了(Ni,Pt)Al/YSZ(Yttira stabilized zirconia,YSZ)热障涂层(Thermal barrier coatings,TBCs),研究了TBCs涂层在1100℃的循环氧化性能。结果表明,陶瓷涂层的剥落位置主要出现在热生长氧化物(Thermally grown oxide,TGO)层内部、TGO层/粘结层的界面或者粘结层表面下方几个微米处。试验过程中,粘结层表面晶粒尺度不规则分布、晶粒晶界"背脊"形貌残留、TGO层下方孔洞形成、陶瓷层内纵向裂纹贯穿延伸和TGO层内残余应力释放速率过快均是可能导致TBCs涂层过早剥落失效的主要原因。     

涂覆热障涂层构件的热梯度机械疲劳行为研究

研究了应变幅、预氧化及高温保载时间对涂覆热障涂层高温合金样品的热梯度机械疲劳性能的影响。结果表明,随应变幅增大,样品疲劳寿命降低。随着预氧化及高温保载时间的增加,样品的氧化损伤增大,疲劳寿命也不断降低。试验过程中,粘结层氧化形成的热生长氧化物层(TGO层)破裂而萌生裂纹,裂纹沿粘结层/TGO层界面扩展而形成分层裂纹,分层裂纹与陶瓷层内贯穿裂纹连接导致陶瓷层剥落而失效。考虑到热障涂层内最大应力及氧化损伤,建立了一个涂覆热障涂层高温合金样品的热梯度机械疲劳寿命预测模型。     

Chronological evaluation of interfacial damage in TBC due to thermal cycling

A two layer electron beam-physical vapor deposited (EV-PVD) thermal barrier coating (TBC) on a single crystal superalloy (René N5) substrate was characterized prior to and after thermal cycling at 2, 18, 25, 44, 50, 75, 100, 110, 150, and 175 cycles in between 200 C-1177 C. Optical microscopy, scanning electron microscopy, and thermal wave imaging techniques were used to characterize the interfacial damage. Pt-Al was used as bond coat and 8 wt % YSZ was used as outer top layer. Interfacial cracking was observed even at two thermal cycles. Thermally grown oxide (TGO) layer increased with the number of thermal cycles. After numerous cycles over 100, interfacial separation was observed to be higher at the middle than at the edges of the sample. This observation is consistent with buckling induced delamination—a possible mechanism for spallation.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

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.     

TGO对热障涂层失效的作用分析

热生长氧化物(TGO)的形成与长大是热障涂层失效的根本原因。先在IC10高温合金基体上超音速火焰喷涂(HVOF)NiCoCrAlTaY粘结层(BC层),再等离子喷涂二元稀土氧化物稳定氧化锆Sc2O3-Y2O3-ZrO2,喷涂样在1 100℃恒温氧化,利用扫描电镜(SEM)、能谱仪对断面形貌、成分进行分析,讨论了TGO的形成机理及其与热障涂层失效的关系。结果表明:随着恒温氧化时间增加,TGO层底部的Al含量下降,上部、中间弥散颗粒及底部的Ni含量均增加,上部、中间弥散颗粒中Cr含量均减少;喷涂样氧化140 h后,TGO层由靠近陶瓷层的富(Cr,Al)2O3层、弥散其间的富Ni颗粒和靠近BC层的Al2O3层组成;TGO的生长速度先由Al与O2化学反应速度决定,接着受BC层金属元素扩散速度影响,最后由化学反应速度和扩散速度共同控制;减少TGO中的有害氧化物含量以降低涂层内的应力,可有效提高涂层的使用寿命。     

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.     

镀铝表面改性7YSZ纳米热障涂层热震性能分

为提高7YSZ纳米热障涂层的热震性能, 实验中采用超音速火焰喷涂(HVOF)在涡轮叶片模拟工件上制备了粘结层NiCrCrAlYTa, 再使用大气等离子喷涂(APS)在粘结层上制备了7YSZ纳米陶瓷层。采用磁控溅射在7YSZ热障涂层样品表面镀铝, 并在不同压力下(200、250、300 Pa)对镀铝样品进行热处理表面改性。对喷涂态样品和镀铝改性后样品进行水淬热震实验, 1050℃保温10 min+水冷5 min为一个热循环, 观察热障涂层镀铝改性前后样品在水淬热循环过程中形貌和结构演变。实验结果表明, 镀铝改性后样品表面存在铝薄膜蒸发、凝固后形成的疏松纳米Al晶粒表层以及由Al和ZrO₂原位反应形成的致密α-Al₂O₃底层。在镀铝样品热处理过程中, 随着压力升高, 疏松层致密度逐渐增加。不同热处理压力下镀铝表面改性后样品经过73次水淬热循环后剥落面积均小于喷涂态样品, 显示出良好的抗热震性。     

Oxide layer development under thermal cycling and its role on damage evolution and spallation in TBC system

The nature and cause of failure of thermal barrier coatings (TBCs) consisting of physical vapor deposited (PVD) yttria stabilized zirconia (YSZ, 8 wt.% Y₂O₃) and a diffusion aluminide bond coat (Pt-Al) were investigated after oxidative thermal cycling and isothermal heat treatment at 1177 °C in air. Experiments were conducted for 45 and 10-minute hold times and for isothermal condition for disk specimens with and without TBC. It is found that microcracks starts in the oxide scales at the bond coat grain boundary protrusions. Total number of thermal cycles affect the density of microcracks within the TGO layer. Evidence is presented that higher density of microcracks in the 10-min hold-time experiments tend to separate the TBC from the TGO layer via extensive coating micro-decohesion and promotes 'complete' TBC separation as opposed to traditional 'partial' spallation of TBC from the substrate as in the 45-min hold-time and isothermal experiments.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

An impedance spectroscopy study of high-temperature oxidation of thermal barrier coatings

Oxidation of air-plasma-sprayed (APS) thermal barrier coatings (TBCs) was carried out in air at 950 °C and was investigated using impedance spectroscopy coupled with scanning electron microscopy and X-ray diffraction. After oxidation for between 500 and 3000 h, a continuous alumina scale was formed at the bond coat/top coat interface in the TBCs, and was evaluated using impedance spectroscopy. The impedance spectra of the oxidised TBCs showed that there were four relaxation processes, which were attributed to the yttria-stabilised zirconia (YSZ) bulk of the TBCs, the thermally grown alumina scale, the YSZ grain boundary, and the metal electrode effect. Impedance analysis showed that the resistivity of the alumina scale increased with increasing oxidation time, demonstrating the thermally grown oxide (TGO) growth. The resistance of the YSZ grain boundaries also increased after oxidation for 2000 h, suggesting the composition change at grain boundaries after a long-term oxidation.