界面形貌对热障涂层界面残余应力影响的数值模拟

利用有限元方法 ,对热障涂层热循环降温过程中产生的残余应力进行了数值模拟 ,研究了有尖点和无尖点正弦波形微坑以及矩形微坑对界面残余应力的影响。结果表明 ,微坑形状不同 ,界面上残余应力的大小、性质与分布也不同 ;微坑的形状变化和尖点的出现 ,将会使该处的应力发生突变 ,不利于增强界面结合强度。因此 ,从对热障涂层界面残余应力的影响角度看 ,无尖点的正弦波形微坑既优于有尖点微坑也优于矩形微坑     

Evolution of Residual Stresses in PS-PVD Thermal Barrier Coatings on Thermal Cycling

Plasma spray-physical vapor deposition (PS-PVD) is an advanced technique to fabricate quasi-columnar structured thermal barrier coatings (TBCs) with excellent thermal cyclic lifetime. In this study, PS-PVD TBCs were investigated via burner rig test. The residual stresses in both of the topcoat layer and the thermally grown oxide (TGO) scale were measured non-destructively using Raman spectroscopy and Cr³⁺ photoluminescence piezo-spectroscopy, respectively. Evolution of the microstructures and distribution of residual stresses in such kind structured TBCs before and after thermal cycling test were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly and then essentially maintained at a relatively stable state with further testing. Moreover, the pre-heating of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat.     

CMAS对热障涂层界面裂纹和残余应力的影响

随着航空发动机涡轮叶片工作温度的提升,使得一种主要由CaO,MgO,Al₂O₃和SiO₂组成的玻璃态物质(CMAS)对热障涂层的危害越来越严重,从而对热障涂层的性能和耐久性有了更高的要求。本文以电子束物理气相沉积热障涂层为研究对象,利用有限元方法研究了CMAS的渗入对界面裂纹扩展及CMAS对陶瓷层(TC)内部残余应力的影响规律。采用波长固定、振幅变化的正弦曲线表示不同粗糙度的涂层界面,同时考虑了CMAS的弹性模量变化的影响及不同界面形貌与CMAS之间的相互作用。结果表明：CMAS弹性模量的增加对界面裂纹具有抑制作用,并且TGO幅值和厚度越小,抑制作用越明显。CMAS弹性模量对TC层最大残余应力S₂₂的影响存在临界点,在临界点之前,CMAS弹性模量的变化对TC层最大残余应力的影响较大,随着CMAS弹性模量的增加,TC层最大残余应力大幅度减小;在临界点之后,TC层最大残余应力基本不受CMAS弹性模量变化的影响。这些结果对电子束物理气相沉积喷涂的热障涂层失效机理的研究具有重要意义,可以为热障涂层界面的优化提供指导。     

Milestones in Functional Titanium Dioxide Thermal Spray Coatings: A Review

Titanium dioxide has been the most investigated metal oxide due to its outstanding performance in a wide range of applications, chemical stability and low cost. Coating processes that can produce surfaces based on this material have been deeply studied. Nevertheless, the necessity of coating large areas by means of rapid manufacturing processes renders laboratory-scale techniques unsuitable, leading to a noteworthy interest from the thermal spray (TS) community in the development of significant intellectual property and a large number of scientific publications. This review unravels the relationship between titanium dioxide and TS technologies with the aim of providing detailed information related to the most significant achievements, lack of knowhow, and performance of TS TiO₂ functional coatings in photocatalytic, biomedical, and other applications. The influence of thermally activated techniques such as atmospheric plasma spray and high-velocity oxygen fuel spray on TiO₂ feedstock based on powders and suspensions is revised; the influence of spraying parameters on the microstructural and compositional changes and the final active behavior of the coating have been analyzed. Recent findings on titanium dioxide coatings deposited by cold gas spray and the capacity of this technology to prevent loss of the nanostructured anatase metastable phase are also reviewed.     

热喷涂层残余应力的来源及其失效形式

探讨了涂层残余应力的产生机理,综述了当前残余应力的典型理论模型,并分析了残余应力诱导的涂层失效行为.上述这些研究对涂层的热力学完整性原理的研究具有重要的意义.     

Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work

A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved.     

Development of Particle Interface Bonding in Thermal Spray Coatings: A Review

Thermal spray ceramic coatings deposited following the conventional routine exhibit a typical lamellar structure with a limited interface bonding ratio. The bonding between particles in the coating dominates coating properties and performance. In this review paper, the bonding formation at the interface between thin lamellae in the coating is examined. The effect of spray parameters on the bonding ratio is presented to reveal the main droplet parameters controlling bonding formation, which reveals that the temperature of the spray particle rather than its velocity dominates the bonding formation. The limitation to increase significantly the ceramic particle temperature inherent to the thermal spray process leads to the observation of a maximum bonding ratio of about 32%, while through controlling the surface temperature of the coating prior to molten droplet impact, the bonding at the lamellar interface can be significantly increased. Consequently, it is shown that with the proper selection of deposition conditions and control of the deposition temperature, the bonding ratio of ceramic deposits can be altered from a maximum of 32% for a conventional deposit to a maximum of 100%. Such wide adjustability of the lamellar bonding opens new possibilities for using thermal spray coatings in various applications requiring different microstructures and properties. The examination of recent studies shows that the bonding control makes it possible to fabricate porous deposits through surface-molten particles. Such an approach could be applied for the fabrication of porous materials, the deposition of high temperature abradable ceramic coatings, and for forming functional structured surfaces, such as a surface with super-hydrophobicity or a solid oxide fuel cell cathode interface with high specific surface area and high catalytic performance. Furthermore, complete interface bonding leads to crystalline structure control of individual splats through epitaxial grain growth.     

热喷涂涂层结合强度的冲击测量法

讨论了一种新型快速定性评价热喷涂层结合强度的冲击测试方法,采用自由落体的落锤冲击设备对涂层进行冲击试验,并采用声发射技术进行动态监测,以定性分析涂层结合强度。首先用超音速等离子喷涂技术分别制备了组织致密,性能良好的Ni Cr金属、Cr_2O_3陶瓷涂层。采用90°和120°金刚石压头、120°及Ф2 mm球形硬质合金压头试验,其中120°硬质合金压头更适合冲击法测试涂层结合强度。随着冲击能量的提高,声发射的能量计数也不断提高,但是落锤高度超过400 mm后,能量计数变化不大,说明在400 mm时涂层发生了破坏。对基体金属、Ni Cr涂层、以及有无Co Cr Al Y为结合层的Cr_2O_3陶瓷涂层进行对比测试,发现声发射能量计数峰值随着涂层结合强度的增强而降低。     

Determination of Interfacial Fracture Toughness of Thermal Spray Coatings by Indentation

Adhesion is an important and basic property for thermal spray coatings. The standard tensile test method “ISO 14916” is usually used to evaluate the adhesive strength of coatings. On the other hand, the indentation test method has some advantages to evaluate the interfacial fracture toughness as the adhesive strength, arising from the following reasons: the test procedure and the specimen preparation are easy in comparison with the typical testing method. Collaborative research has been conducted by “Committee on Standard Development” in the Japan Thermal Spray Society to establish a standard test method for evaluating interfacial fracture toughness of thermal spray coatings using a conventional Vickers indenter. This article reports the differences among collaborators in round-robin tests performed in this committee and discusses the validity of the test method and test conditions with respect to the test results and finite element analyses. Comparison among collaborators reveals that interfacial fracture toughness can be obtained with a small scattering from the indentation test under constraints found on the basis of the results.     

Advances in Thermal Spray Coatings for Gas Turbines and Energy Generation: A Review

Functional coatings are widely used in energy generation equipment in industries such as renewables, oil and gas, propulsion engines, and gas turbines. Intelligent thermal spray processing is vital in many of these areas for efficient manufacturing. Advanced thermal spray coating applications include thermal management, wear, oxidation, corrosion resistance, sealing systems, vibration and sound absorbance, and component repair. This paper reviews the current status of materials, equipment, processing, and properties’ aspects for key coatings in the energy industry, especially the developments in large-scale gas turbines. In addition to the most recent industrial advances in thermal spray technologies, future technical needs are also highlighted.     

Modeling Residual Stress Development in Thermal Spray Coatings: Current Status and Way Forward

An overview of analytical and numerical methods for prediction of residual stresses in thermal spray coatings is presented. The various sources and mechanisms underlying residual stress development in thermal spray coatings are discussed, then the various difficulties associated with experimental residual stress measurement in thermal spray coatings are highlighted. The various analytical and numerical models used for prediction of residual stresses in thermal spray coatings are thoroughly discussed. While analytical models for prediction of postdeposition thermal mismatch stresses are fully developed, analytical quenching and peening stress models still require extensive development. Various schemes for prediction of residual stresses using the finite element method are identified. The results of the various numerical and analytical models are critically analyzed, and their accuracy and validity, when compared with experiments, are discussed. Issues regarding the accuracy and applicability of the models for predicting residual stresses in thermal spray coatings are highlighted, and several suggestions for future development of the models are given.     

Role of Powder Granulometry and Substrate Topography in Adhesion Strength of Thermal Spray Coatings

APS coating is deposited with different treated surfaces to understand the effects of surface topography and particle sizes on adhesion bond strength. Grit blasting and laser surface texturing have been used to create a controlled roughness and controlled surface topography, respectively. Coating adhesion is mainly controlled by a mechanical interlocking mechanism. Fully melted Ni-Al powder fills the respected target surface with high-speed radial flow. Pores around central flattening splat are usually seen due to splash effects. Laser surface texturing has been used to study near interface coating depending on the target shape and in-contact area. Pull-off test results have revealed predominant correlation with powder, surface topography, and adhesion bond strength. Adhesion bond strength is linked to the in-contact area. So, coating adhesion might be optimized with powder granulometry. Pores near the interface would be localized zones for crack initiations and propagations. A mixed-mode failure has been reported for sharp interface (interface and inter-splats cracks) due to crack kicking out phenomena. Coating toughness near the interface is a key issue to maximize adhesion bond strength. Volume particles and topography parameters have been proposed to enhance adhesion bond strength for thermal spray process for small and large in-contact area.     

Effect of Thermal Annealing on Machining-Induced Residual Stresses in Inconel 718

Nickel-based alloys are widely employed in the manufacturing of aero-engines. These alloys are difficult to machine, and tensile residual stresses are generated during machining. These tensile residual stresses can negatively affect the performance of aero-engine components. Nevertheless, residual stresses can vary due to thermal or mechanical loading. These variations must be considered to evaluate the real influence of residual stresses on component behavior. This paper studies the effect of thermal loads on machining-induced residual stresses in the alloy Inconel 718. A ring-shaped Inconel 718 part was face-turned, and specimens were extracted from it. Specimens were exposed at 550 and 650 °C for 10 min, 1 and 10 h. Residual stresses were measured, and microstructure was observed before and after thermal exposure. Residual stress variations found after thermal exposure were the consequence of two factors: relaxation of strain bands during the early stage of exposure and diffusion-controlled creep. In addition, a modified Zener-Wert-Avrami model is proposed to predict residual stress relaxation caused by the diffusion-controlled creep. Once having fitted the modified Zener-Wert-Avrami model, the study was extended for a wider range of temperatures (400-650 °C). This analysis showed that surface residual stresses do not relax significantly at temperatures below 500 °C.     

热喷涂非晶合金涂层及其在金属腐蚀防护中的应用

对近年来国内外热喷涂非晶合金涂层材料的研究进展及其在金属腐蚀与防护领域中的应用前景进行了综述,指出热喷涂非晶合金涂层的开发与应用,对于促进金属腐蚀与防护技术的发展具有重要的意义。     

热障涂层界面微结构尺寸与残余应力关系的数值模拟

利用有限元方法,对热障涂层热循环降温过程中产生的残余应力进行了数值模拟。以无尖点正弦波形界面形貌为对象,研究了其微结构尺寸（包括微坑间距,宽度和深度）对平行和垂直于界面的残余应力最大值SMX和SMY的影响规律。结果表明,在本文所给定的尺度范围内,微结构尺寸为SMX和SMY均有影响,但对SMY的影响要更为显著;在微坑深度,宽度和间距等3个微结构尺寸参数中,微坑深度对界面残余应力的影响最大。     

残余应力对W-Cu复合材料导热性能的影响

用熔渗法制备的W-Cu复合材料在小批量生产时，发现熔渗结束后样品的热导率数据分散不稳定，受熔渗时摆放位置的影响很大。将熔渗结束后的样品退火，采取不同的方式冷却。用X射线衍射法测量其残余应力，用热脉冲法测量其热导率，研究了残余应力对材料导热性能的影响。结果表明：随冷却速度的加快，残余应力值增大，热导率降低。钨和铜的热膨胀系数相差较大，从高温冷到室温时两相收缩程度不一样，冷却速度过快时，在界面处产生残余应力，使材料的热导率降低。分析了残余应力对材料导热性能的影响及机制。     

等离子喷涂涂层中残余应力分析

分析了等离子喷涂涂层中残余应力产生的起因,建立了基于增层模型的残余应力数学模型,并进行了实验验证,为热喷涂材料界面结合强度的研究提供了依据和方法.     

电热爆炸喷涂和等离子喷涂联合法制备热障涂层的研究

采用电热爆炸喷涂和等离子喷涂联合制备热障涂层,以电热爆炸喷涂法在DZ125合金表面制备NiCoCrAlY粘结层,以等离子喷涂技术制备陶瓷顶层。利用扫描电镜(SEM)和X射线衍射(XRD)仪对所制备的粘结层进行分析,结果表明:电热爆炸喷涂的粘结层与基体结合良好,喷涂态的粘结层的相主要由Ni3Al组成。采用联合法制备的热障涂层,在喷涂态的陶瓷层、粘结层、基体3者结合良好,界面清晰。在高温热循环过程中,粘结层/陶瓷层界面间生成了连续、致密的Al2O3膜,阻碍粘结层的氧化。粘结层/TGO界面产生平行于界面的裂纹,是导致热障涂层失效的主要原因。     

Thermal Spray Coatings Engineered from Nanostructured Ceramic Agglomerated Powders for Structural,Thermal Barrier and Biomedical Applications: A Review

Thermal spray coatings produced from nanostructured ceramic agglomerated powders were tailored for different applications, some of which required almost completely opposite performance characteristics (e.g., anti-wear and abradable coatings). The influence of nanostructured materials on important areas, such as, thermal barrier coatings (TBCs) and biomedical coatings was also investigated. It was determined that by controlling the distribution and character of the semi-molten nanostructured agglomerated particles (i.e., nanozones) embedded in the coating microstructure, it was possible to engineer coatings that exhibited high toughness for anti-wear applications or highly friable for use as abradables, exhibiting abradability levels equivalent to those of metallic-based abradables. It is shown that nanozones, in addition to being very important for the mechanical behavior, may also play a key role in enhancing and controlling the bioactivity levels of biomedical coatings via biomimetism. This research demonstrates that these nanostructured coatings can be engineered to exhibit different properties and microstructures by spraying nanostructured ceramic agglomerated powders via air plasma spray (APS) or high velocity oxy-fuel (HVOF). Finally, in order to present readers with a broader view of the current achievements and future prospects in this area of research, a general overview is presented based on the main papers published on this subject in the scientific literature.