Characterization of isothermal and cyclic thermal damage of EB-PVD TBCs with the help of the 3D-DIC technique

The thermal barrier coatings (TBCs) are working under complex elevated temperature loading conditions. In the present work, a damage model for the isothermal and cyclic thermal loads was developed to quantify the failure process of the coatings subjected to isothermal and cyclic thermal exposures. Effects of different damage mechanisms, such as thermal exposure, thermal cycling, aluminum migration, and thermal dwell times, were experimentally and computationally studied. The digital image correlation (DIC) technique was introduced to evaluate degradation of TBCs. The complex damage can be quantified with the help of the DIC strain variations. The introduced model provides a general method to estimate the remaining life.     

Ultrasonic Plasma Spray--A New Plasma Spray Process

The method of arc- ultrasonic is introduced into plasma spray process. The process of spray ZrO2-NiCoCr AlY thermal barrier coatings (TBCs) using air plasma spray (APS) process is studied. A exciting source which can be adjusted from audio frequency to several hundred thousand Hertz is designed successfully. The ultrasonic exciting source is coupled with conventional DC spraying power supply. A few ultrasonic frequencies are selected in the testing. Several parts of the coatings with the coupling arc- ultrasonic are compared with the coatings without it. The results show: with 50 kHz and 80 kHz ultrasound, the coating qualities are improved, whereas 30 kHz has an opposite effect.     

Failure mechanism of EB-PVD thermal barrier coatings on NiAl substrate

Yttria stabilized zirconia（YSZ） was deposited on the line cut β-NiAl substrate by electron-beam physical vapour deposition（EB-PVD）, and the cyclic oxidation behaviors of thermal barrier coatings on β-NiAl substrate were investigated in 1 h thermal cycles at 1 200 ℃ in air. The results show that the samples fail after 80-100 cycles. Sub-interface cavitations in the substrate develop due to depletion of Al in forming thermally grown oxides（TGOs）. The collapse and closing up of cavities result in the ragged YSZ/TGO/substrate interface. Since the specific crack trajectories are quite sensitive to local geometry, cracks along the YSZ/TGO/substrate interfaces ultimately lead to YSZ spallation.     

Modeling of micro-crack growth during thermal shock based on microstructural images of thermal barrier coatings

Based on digital image processing theory and finite element mesh generation principle, a methodology is proposed to model the micro-crack growth of thermal barrier coatings (TBCs) during thermal shock with the aid of finite element program. Firstly, a microstructural image of plasma sprayed TBCs is transferred to digital image; secondly, a finite element grid model is generated by thresholding segmentation according to the actual microstructure; finally, based on the finite element grid model, the Tuler–Butcher failure criterion is employed to model the micro-crack growth of TBCs during thermal shock. The numerical simulation result agrees well with the experimental result, and the methodology presented in this paper is found to be effective to model the micro-crack growth.     

放电等离子烧结工艺对不导电材料整体电阻的影响

基于Si3N4纳米陶瓷放电等离子烧结(SPS)试验数据,研究了不导电材料烧结过程中烧结系统整体电阻的变化规律。结果表明:烧结温度与烧结粉体高度是影响烧结系统整体电阻的两大因素,在烧结的升温阶段,电阻随着温度的升高和粉体高度的减小逐渐减小,当粉体烧结致密后,如果烧结温度继续升高,则电阻逐渐增大。根据烧结试验结果,得到了总电阻随烧结温度与粉体初始高度的变化曲线,并用SiC陶瓷材料的SPS试验数据对所拟合公式进行了验证,证明了该公式对不导电材料在SPS烧结过程中烧结系统整体电阻变化规律的适用性。     

超高速陶瓷CBN砂轮纳米陶瓷结合剂性能实验研究

以超高速陶瓷CBN砂轮的结合剂低温高强性能要求为目标,在以化学纯原料为主的R2O+RO-B2O3-Al2O3-SiO2玻璃体系基础上,引入纳米改性剂来对陶瓷结合剂基体进行改良强化。4种成分结合剂性能的测试结果表明,纳米陶瓷结合剂的抗折强度、耐火度、浸润性与普通陶瓷结合剂相比有着显著优势。4号纳米陶瓷结合剂的抗折强度达到了83.75MPa,耐火度降至795℃,其膨胀系数也与CBN磨料更为接近,是一种更适合用于制备超高速陶瓷CBN砂轮的结合剂。     

纳米陶瓷的研究进展

纳米陶瓷是指晶界宽度、晶粒尺寸、缺陷尺寸和第二相分布都在纳米量级上的陶瓷材料,因其克服了传统陶瓷脆性较大的致命缺点,并在超塑性、铁电性能和力学性能等方面具有特殊的性能,而受到人们的广泛关注。综述了纳米陶瓷粉体的制备方法,包括物理合成与化学合成两种方式。物理合成方法消耗较大的能量且设备投资较大,因此纳米陶瓷粉体的制备以化学合成为主。在制备粉体的后期,还需通过加入分散剂和超声等方法来解决其团聚的问题。成型与烧结过程直接影响纳米陶瓷的性能。成型过程包括干法与湿法成型。干法成型简单易行,成体均匀,但容易引入杂质;而湿法成型能最大程度地减少杂质与团聚,但其工艺复杂、条件苛刻。烧结方法分无压烧结和压力烧结。在烧结过程中,如何将颗粒限制在纳米级别上成为各种技术需要攻克的难点。最后,阐述了纳米陶瓷的力学性能、铁电性能和超塑性,并简要介绍其在防护与涂层、生物医学材料和器具等方面的应用与创新。     

Investigation on growth mechanisms of columnar structured YSZ coatings in Plasma Spray-Physical Vapor Deposition (PS-PVD)

By Plasma Spray-Physical Vapor Deposition (PS-PVD), major fractions of the powder feedstock can be evaporated so that the coating builds up mainly from vapor phase. In this work, the deposition mechanisms at different PS-PVD conditions were investigated. Depending on the plasma flow conditions and the substrate temperature, the columns in the coatings possess successively pyramidal, cauliflower, and lamellar shaped tops. In addition, the different microstructures show characteristic crystallographic textures, in which different in-plane and out-of-plane orientations were observed by pole figures. Based on investigations by electron back-scatter diffraction (EBSD), the overall coating growth process can be roughly divided into three subsequent stages: equiaxed growth, competitive growth, and preferential growth. Influences of diffusion and shadowing on final coating microstructure and orientation were discussed. The formation of equiaxed grains was proposed to be caused by high nucleation rates, which are probably induced by large undercooling and super-saturation at the beginning of deposition. The preferential growth orientation was preliminarily explained based on an evolutionary selection mechanism.     

热障涂层对涡轮叶片冷却效果影响的数值研究

以带有复合冷却结构的涡轮叶片为基础模型,对有/无热障涂层保护下的叶片冷却效果进行了气热耦合数值计算,并通过改变热障涂层的厚度研究了热障涂层对叶片换热的影响规律。研究发现：带有热障涂层的叶片温度明显下降,且越靠近叶片前缘处,温度降低幅度越大;随着热障涂层厚度的增加,下降趋势逐渐变慢;与无热障涂层时叶片温度相比,叶片前缘区域温降最大,当热障涂层厚度为0.35mm时,温降高达160K,压力面和吸力面与前缘相比温降程度较低。     

Effect of Thermal Aging on Microstructure and Functional Properties of Zirconia-Base Thermal Barrier Coatings

Thermal barrier coating (TBCs) systems made of plasma sprayed zirconia are commonly used in gas turbine engines to lower metal components surface temperature and allow higher combustion temperature that results in higher fuel efficiency and environmentally cleaner emissions. Low thermal conductivity and long service life are the most important properties of these coatings. The objective of this work was to study the influence of a long-term heat treatment (i.e., 1200 °C/2000 h) on different characteristics of atmospheric plasma sprayed TBCs. Two zirconia feedstock materials were evaluated, namely, yttria partially stabilized zirconia and dysprosia partially stabilized zirconia. Several spray conditions were designed and employed to achieve different coating morphologies. Microstructure analyses revealed that the coating microstructure was significantly dependent on both operating conditions and heat treatment conditions. Significant changes in coatings porosity occurred during heat treatment. The lowest thermal conductivity was reached with the dysprosia partially stabilized zirconia material. Heat treatment affected TBCs adhesion strength as well.