热障涂层无损检测技术研究进展

为提高发动机的涡轮前温度和热端部件服役寿命,热障涂层（TBCs）被广泛应用于燃气涡轮发动机。热障涂层具有多相、多界面和非均质特性,且其服役工况恶劣复杂。寻找一种可以表征涂层显微组织、缺陷、热物性、应力等反映涂层质量和剩余寿命的无损检测方法,对发动机的热端部件安全性和可靠性至关重要。文中综述了超声检测技术（UT）、声发射技术（AE）、红外热成像技术（IRT）、阻抗谱技术（IS）和光激发荧光压电光谱技术（PLPS）的原理以及其在热障涂层无损检测中的研究应用,并详细介绍了太赫兹时域光谱（THz-TDS）技术及其在热障涂层中的应用。最后总结了上述无损检测方法的检测能力,并对热障涂层无损检测方法进行展望。     

Vacancy-ordered yttria stabilized zirconia as a low-temperature electronic conductor achieved by laser melting

Laser melting is known to be capable in initiating thorough evolution in microstructure and bringing novel functional performance in metals. But realization of this potential in ceramics only reaches a preliminary stage that needs further investigation. Here we demonstrate zirconia, traditionally an insulative ceramic at low temperature, could be transformed into an electronic conductor with the conductivity on order of 10⁻³ S⋅cm^-1 at room temperature by a simple laser melting process without inducing metallic phases. Transmission electron microscopy and ab-initio simulation show that oversaturated oxygen vacancies, together with their ordered metastable distribution along <001 > , are introduced during this non-equilibrium process, and result in a clear defect level significantly narrowing bandgap to less than 1 eV, leading to the considerable electronic conductivity. These results identify a strategy of utilizing this non-equilibrium method in oxide ceramics to realize some unconventional performances determined by metastable structure thoroughly altered down to atomic level.     

粘结层和陶瓷层厚度对纳米结构热障涂层性能的影响

采用超音速火焰喷涂+大气等离子喷涂工艺,在K403高温合金表面制备不同层厚比的NiCrA-lY/纳米7YSZ热障涂层,研究了涂层厚度变化对热障涂层表面粗糙度、结合强度、热震性能和热循环寿命的影响规律。结果表明:当粘结层厚度一定时,随着陶瓷层厚度的增加,其表面粗糙度增加,涂层结合强度下降;当粘结层厚度为50μm时,热障涂层的抗热震性能随陶瓷层厚度增加而降低,粘结层厚度提高至100μm时,热障涂层的抗热震性能随陶瓷层厚度增加先提高,后降低,热障涂层在1100℃的热循环寿命测试结果也基本对应这一规律;当粘结层厚50μm且陶瓷层/粘结层的层厚比在(1～2)∶1的范围内,或者粘结层厚100μm且陶瓷层/粘结层的层厚比在(2～2.5)∶1范围内时,热障涂层具有较优异的性能。     

Hot corrosion behavior of NdYbZr2O7 exposed to V2O5 and Na2SO4 + V2O5 molten salts

In order to evaluate the application prospects of NdYbZr₂O₇ as a novel TBC material, NdYbZr₂O₇ ceramic was synthesized via a solid-state reaction sintering method, and its hot corrosion behavior exposed to V₂O₅ and Na₂SO₄ + V₂O₅ molten salts at 900 °C, 1000 °C, and 1100 °C was comparatively investigated. For the V₂O₅ salt, the primary corrosion products were granular (Nd,Yb)VO₄ as well as cube-like m-ZrO₂. The corrosion layer consisted of two distinct layers, one of which was Zr-rich layer and another was V-rich layer. In the case of Na₂SO₄ + V₂O₅, NaVO₃, as an intermediate product, played an important role in dissolving the NdYbZr₂O₇ ceramic. Herein, the (Nd,Yb)VO₄ exhibited a rod/plate-like morphology, which could be attributed to the synergistic effect of low driving force and low nucleation rate. Since the molten salt infiltration rate was superior to the pore filling rate throughout the hot corrosion, the thickness of corrosion layer increased with the rise of temperature. The hot corrosion mechanisms of NdYbZr₂O₇ ceramic in various molten salts were discussed based on the phase diagram, Lewis acid-base rule and chemical thermodynamics. On this basis, the NdYbZr₂O₇ coating was prepared by atmospheric plasma spray (APS) and it exhibits a higher corrosion resistance compared to YSZ coating.     

High temperature fracture toughness and residual stress in thermal barrier coatings evaluated by an in-situ indentation method

High temperature fracture toughness and residual stress are important for the evaluation of TBCs. In this paper, an in-situ high temperature indentation method was originally developed to investigate the high temperature fracture toughness and residual stress in a typical TBC, nanostructured 8?wt% yttria partially stabilized zirconia (YSZ) coating. The cracks caused by in-situ high temperature indentation tests were observed, and high temperature fracture toughness and residual stress were experimentally measured. The fracture toughness was measured to be 1.25, 0.91 and 0.75?MPa*m^1/2 at 25, 800 and 1000?°C, respectively. The residual stress was measured to be ? 131.3, ? 55.5 and ? 45.5?MPa, correspondingly. Moreover, the residual stress and fracture toughness both decrease with increasing environmental temperature. It is also found that the fracture toughness without consideration of residual stress is significantly larger than the intrinsic fracture toughness, which may result from the compressive stress state.     

纳米8YSZ粉末不同温度下烧结行为研究

目的通过关注纳米8YSZ粉末在不同温度下烧结过程中的晶粒长大行为及相结构组成变化,获得纳米8YSZ粉末的高温稳定性,防止高温烧结导致纳米8YSZ涂层性能明显衰减,致使涂层在正常服役过程中过早失效。方法采用共沉淀工艺合成低杂质含量的8YSZ纳米粉末,经过低温煅烧预处理后,在900~1200℃温度区间进行3~12 h的烧结。通过X射线衍射仪和扫描电子显微镜对纳米颗粒进行物相结构和形貌分析,根据Scherrer公式计算热处理后的颗粒平均晶粒尺寸,采用Arrhenius公式得到晶粒生长活化能,进而确定晶粒的生长机制。结果经过低温煅烧预处理后,粉末绝大多数仍然保持非晶态结构,经过高温热处理后,粉末均完成了晶态转化,相结构基本为单一四方相。温度为900~1100℃时,晶粒生长的活化能为42.638 k J/mol;温度为1100~1200℃时,晶粒生长的活化能为3.849 k J/mol。结论高温热处理后,纳米8YSZ粉末物相结构为单一四方相,可以保持高温稳定性,防止涂层性能明显衰减。温度为900~1200℃时,晶粒生长机制以表面扩散为主的聚合生长。     

A comparative investigation on the corrosion degradation of plasma sprayed YSZ and LnMgAl11O19 (Ln = Nd,Sm, Gd) coatings exposed to the molten V2O5 + Na2SO4 salt mixture at 1100 °C

Corrosive attack of the molten 50 wt. % V₂O₅ + 50 wt.% Na₂SO₄ salt mixture has been comparably studied for the APS YSZ and LnMgAl₁₁O₁₉ (LnMA, Ln = Nd, Sm, Gd) thermal barrier coatings upon a 10 h anneal at 1100 °C in air. The YSZ coating suffered from a deepest infiltration of the molten salt along its thickness direction through the open and connected pores as well as inter-lamellae microcracks. A large number of newly formed voids were widely distributed in the YSZ coating due to the corrosion degradation followed by the t, t’ to m-ZrO₂ phase transformation. While, a relative thin corrosion layer mainly consisting of α-Al₂O₃ and LnVO₄ were present for the corroded LnMA coatings. The much reduced number of open pores and connected microcracks together with the rapid chemical reaction between the molten salt and LnMA coatings, especially for the NdMA coating, preventing further infiltration of the molten salt, was beneficial to mitigate further attacks to the inner coating at the expense of sacrificing a thinner top layer. The presences of amorphous phases were thought to further accelerate the corrosion reaction and strengthen such a corrosion protection mode for all the APS LnMA coatings.     

Corrosion products evolution and hot corrosion mechanisms of REPO4 (RE= Gd,Nd, La) in the presence of V2O5 + Na2SO4 molten salt

REPO₄ (RE = Gd, Nd, La) ceramics with a monazite structure were fabricated by a chemical co-precipitation and calcination method. Hot corrosion tests were carried out in V₂O₅+Na₂SO₄ molten salt at 800 °C, 900 °C and 1000 °C for 2 h and 10 h. The temperature and heat duration had little effect on the type of corrosion products in this study. However, GdPO₄ and REPO₄ (RE = Nd, La) revealed different hot corrosion behavior. Exposed to the molten salt, GdVO₄ and Gd₄(P₂O₇)₃ formed as the corrosion products for the GdPO₄ case, while an RE(P,V)O₄ (RE = Nd, La) solid solution was generated for NdPO₄ and LaPO₄ cases. The formation of the solid solution had less damage to the original microstructure, which benefited the hot corrosion resistance of the ceramics. From the crystallographic characteristics of rare earth phosphates/vanadates and a thermodynamics perspective, the hot corrosion mechanisms of REPO₄ (RE = Gd, Nd, La) are discussed.     

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。     

Microstructure evolution and hot corrosion mechanisms of Ba2REAlO5 (RE = Yb,Er, Dy) exposed to V2O5 + Na2SO4 molten salt

Ba₂REAlO₅ (RE?=?Dy, Er, Yb) powders were synthesized by a solid state reaction method, followed by cold pressing and sintering to produce pellets for hot corrosion tests. When exposed to V₂O₅?+?Na₂SO₄ molten salt at 900?°C and 1000?°C for 4?h and 20?h, REVO₄, Ba₃(VO₄) ₂ and BaAl₂O₄ formed as corrosion products due to chemical interactions between the ceramics and the molten salt, which were temperature and time independent. After the hot corrosion tests at 1000?°C, continuous, dense reaction layers with a thickness of ～80?μm formed on the sample surfaces, which had an effective function on suppressing further penetration of the molten salt. The hot corrosion mechanisms of Ba₂REAlO₅ are proposed based on Lewis acid-base rule, phase diagrams and thermodynamics. From a thermodynamics perspective, the molten salt directly reacting with Ba₂REAlO₅ is difficult compared with Yb₂O₃, Al₂O₃ and BaO.