ZrO2/NiCoCrAIY功能梯度涂层残余应力分析

采用有限元方法研究了等离子ZrO2/NiCoCrAlY功能梯度涂层形成过程中的热力学行为以及残余应力的分布.结果表明,由于涂层与基体的热膨胀系数不匹配等原因,在界面等区域存在严重的应力集中.涂层与基体厚度比、中间层以及喷涂过程的冷却速率对残余应力水平有很大影响.涂层内部关键区域的残余应力水平,随着涂层厚度增加而增加;50%ZrO2+50%NiCoCrAlY中间层对降低涂层内部的应力水平是有利的,但并不能消除涂层内部的应力集中;对于瞬态分析而言,涂层内部的应力随着冷却速度的增加而增加.     

(Ti,Al)N涂层应力沿层深分布的调整及大厚度涂层的制备

利用电弧离子镀技术在不锈钢基体上制备了（Ti,Al）N涂层,研究了N₂分压改变对涂层残余应力沿层深分布及相关力学性能的影响.结果表明,低N₂分压下,（Ti,Al）N涂层残余应力沿层深分布较均匀,随N₂分压的增加,涂层应力沿层深呈"钟罩型"分布,且全膜厚的应力值也明显增大;通过对涂层生长结构及微观成分分析,初步探讨了应力分布机理.随N₂分压的增加,涂层硬度会显著增加,而膜/基结合力则大幅下降;采用改变N₂分压工艺制备（Ti,Al）N涂层,可有效调整涂层残余应力沿层深分布趋势,改善其力学性能,并可成功制备厚度在130μm以上的硬质涂层.     

Thermal modeling of various thermal barrier coatings in a high heat flux rocket engine

One- and two-dimensional thermal models were developed to predict the thermal response of tubes with and without thermal barrier coatings (TBCs) tested for short durations in a H₂/O₂ rocket engine. Temperatures were predicted using median thermophysical property data for traditional air plasma sprayed ZrO₂–Y₂O₃ TBCs, as well as air plasma sprayed and low pressure plasma sprayed ZrO₂–Y₂O₃/NiCrAlY cermet coatings. Good agreement was observed between predicted and measured metal temperatures. It was also shown that the variation in the reported values of the thermal conductivity of plasma sprayed ZrO₂–Y₂O₃ coatings can result in temperature differences of up to 180°C at the ceramic/metal interface. In contrast, accounting for the presence of the bond coat or radiation from the ceramic layer had only a small effect on substrate temperatures (<20°C). The thermal models were also used to show that for the short duration test conditions of this study, a 100 μm thick ZrO₂–Y₂O₃ coating would provide a metal temperature benefit of approximately 300°C over an uncoated tube while a 200 μm thick coating would provide a benefit greater than 500°C. The difference in the thermal response between tubes and rods was also predicted and used to explain the previously-observed increased life of TBCs on rods over that on tubes.     

Microcrack formation in plasma sprayed thermal barrier coatings

Air plasma sprayed ZrO₂–8wt%Y₂O₃ thermal barrier coatings were deposited under tightly controlled conditions. The lengths and orientations of the horizontal cracks and vertical cracks in these coatings were characterized in detail, and process/structure maps of the crack distribution as a function of particle and substrate states were constructed. A fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. The model also showed that surface roughness plays a key role in determining the magnitude of maximum stresses, and that only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration.     

Sm2Ce2O7/YSZ功能梯度热障涂层的残余热应力

目的研究基体材质、厚度及半径对Sm_2Ce_2O_7/YSZ功能梯度热障涂层残余热应力的影响。方法采用ANSYS10.0软件中Plane13单元,通过直接耦合计算,系统分析了不同基体条件下,Sm_2Ce_2O_7/YSZ功能梯度热障涂层的残余热应力。结果在Sm_2Ce_2O_7/YSZ功能梯度热障涂层中存在较大的残余热应力。涂层残余热应力随时间的增加而逐渐降低,900 s后基本维持稳定。涂层径向热应力从中心处到试样边缘逐渐递减。2Cr13对应的涂层应力最小。金属基体厚度在2~10 mm范围内,径向热应力虽然增加,但变化幅度不大。当基体厚度为20 mm时,涂层径向热应力则显著增加。金属基体半径对涂层的最大剪切应力并不产生影响,轴向热应力随基体半径的增加而逐渐降低,径向热应力随半径增加到一定值后趋于稳定。结论 2Cr13钢基体对应的涂层具有最小热应力,基体厚度为10 mm时比较合适,基体半径对涂层轴向热应力的影响最明显。     

ZrO2纳米颗粒对PEO涂层微观结构及热物理性能的影响

目的 提高PEO涂层的热物理性能.方法 以活塞主流材料——高硅铝合金(ZL109)为基体,在硅酸盐系电解液中,添加不同浓度的ZrO2纳米颗粒,制备一系列ZrO2/Al2O3复合PEO涂层,并通过涡流测厚仪、扫描电子显微镜(SEM)、能谱仪(EDS)及X射线衍射仪(XRD)分析ZrO2纳米颗粒对涂层生长、微观形貌、元素组...     

Formation of high hardness zirconia coatings by gas tunnel type plasma spraying

The high hardness zirconia (ZrO₂) coatings could be obtained at an atmospheric pressure by using a gas tunnel type plasma spraying. The characteristics of these high hardness ZrO₂ coatings were investigated. The Vickers hardness of the ZrO₂ coating at a short spraying distance was very high; a high hardness of more than H_v=1200 was achieved at the surface side of the coating. The microstructure of the obtained high hardness ZrO₂ coating was also investigated by the microscopic method. And the characteristics of the high hardness ZrO₂ coating was discussed in comparison with that of the coating formed by the conventional type plasma spraying. It was clarified that the ZrO₂ coating of the gas tunnel type was not only much harder but also less porous than that of the conventional type.     

铜基体上超音速火焰喷涂镍基涂层残余应力分析

采用X射线应力分析技术和曲率法测试了在铜基体上喷制不同成分、厚度的镍基涂层以及改变基体厚度喷制的涂层的残余应力。结果表明,超音速火焰喷涂的镍基涂层残余应力均为压应力,涂层成分、厚度的变化对残余应力均有明显的影响,添加一定量的WC、控制涂层和基体厚度,可获得较大的残余压应力。     

Effects of plasma-sprayed NiCrAl/ZrO2 intermediate on the combination ability of coatings

A NiCrAl/ZrO₂ composite coating was deposited on the surface of metal carrier FeCrAl alloy by a plasma-spray technique. After static-state oxidation at 800°C, the transitions in structure and composition of the coating was analyzed by XRD, SEM and EDX. The results showed that the surface phases of the as-sprayed coating were mainly composed of Ni and ZrO₂. When the oxidation time was extended from 8 to 50 h, NiO crystallites were formed and these grew coarse on the coating surface, and alloy elements were diffused between the NiCrAl/ZrO₂ coating and the FeCrAl substrate. With the pretreatment, an intermediate coating was prepared with a coarse and porous structure, high cohesive strength and high heat resistance. These developed properties could provide high geometric surface area for a catalytic γ-Al₂O₃ washcoat, and enhance the adhesive strength between ceramic washcoat and metal substrate so as to extend the lifetime of the washcoat.     

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Microscopic observation of degradation behavior in yttria and ceria stabilized zirconia thermal barrier coatings under hot corrosion

In this study, the effects of yttria stabilizer replacement with ceria on hot corrosion properties of thermal barrier coatings (TBCs) have been investigated in microscopic respects. Yttria stabilized zirconia (YSZ; ZrO₂–8 wt.%Y₂O₃) and ceria stabilized zirconia (CSZ; ZrO₂–25 wt.%CeO₂–2.5 wt.%Y₂O₃) thermal barrier coatings (TBCs) were fabricated to have similar microstructures, followed by hot corrosion tests with a NaVO₃ salt at 900 °C. Very dense YSZ TBCs with porosities below 4% were also fabricated in order to investigate the effects of coating defects on hot corrosion behavior. Results revealed that CSZ TBCs were better resistant to hot corrosion environment than YSZ TBCs in terms of phase stability and overall damage by the salt. From studies on microscopic failure behavior, it was found that YSZ and CSZ TBCs showed a different degradation behavior and that coating defects, such as pores, microcracks and splat boundaries, play important roles as effective paths for the salt in hot corrosion. From the present results, a possible microscopic degradation mechanism and effects of coating microstructures for zirconia-based TBCs under hot corrosion are discussed.     

Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al₂O₃ coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al₂O₃ coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al₂O₃ coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 μm and the substrate temperature of 500 °C. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness.     

Thermal stress analysis of HVOF sprayed WC–Co/NiAl multilayer coatings on stainless steel substrate using finite element methods

The scope of this study is to find out the effects of thermal cycling on the coating-substrate system of WC–Co coatings by finite element modeling. With this regard, WC–Co/NiAl coating layers were successfully deposited on 316 L stainless steel substrates by using a HVOF technique and microstructural observations were carried out using SEM. The SEM study revealed that the coating was very dense with very low oxide content and had a very good contact with the substrate, indicating a very good bonding to the substrate. Thermal cycling tests were performed at the temperature range of 373 and 873 °K without external load. In finite element modeling (FEM), thermal residual stresses, developed during and after thermal cycling, were determined by using ANSYS software package. It was found that the stress distributions were obtained in the WC–Co/NiAl architectured coating systems during heating and cooling steps because of the different thermal and mechanical properties of the coating layers and substrates. According to thermal analysis results, the calculated tensile stresses were higher than the compressive stresses and also thermal stress components for x-direction were bigger than for y-direction.     

可计及温度与层状结构影响的超高温陶瓷基复合材料涂层残余热应力理论表征模型

目的创建可计及温度与层状结构共同影响的超高温陶瓷基复合材料涂层与基体层因热不匹配导致的残余热应力的理论表征模型。方法基于经典的层合板理论与超高温陶瓷基复合材料热物理性能参数对温度的敏感性研究,引入温度和层状结构对涂层与基体层所受残余热应力的影响,形成各层残余热应力温度相关性的理论表征方法,并以ZrB_2-SiC复合材料涂层为例,利用该理论方法系统地研究了各种控制机制对残余热应力的影响及其随温度的演化规律。结果超高温陶瓷基复合材料涂层与基体层所受的残余热应力随着温度的变化而变化,涂层热膨胀系数与基体层热膨胀系数差别越大,变化幅度越大。当涂层材料热膨胀系数大于基体层材料热膨胀系数时,涂层材料遭受残余拉应力,基体层材料遭受残余压应力;随着涂层厚度的增加,涂层所受拉应力减小,而基体层所受压应力增大;当涂层材料热膨胀系数小于基体层材料热膨胀系数时,涂层材料遭受残余压应力,基体层材料遭受残余拉应力;随着涂层厚度的增加,涂层所受压应力减小,而基体层所受拉应力增大。低温下,各层所受残余热应力对层厚与每层材料组成的变化比较敏感,随着温度的升高,敏感性降低。结论对于涂层材料,应设计涂层材料的热膨胀系数小于基体层材料的热膨胀系数,使涂层遭受残余压应力,这不仅能够降低材料表面产生裂纹的危险,同时可以抑制表面已有缺陷的扩展。同时应当设计相对较小的涂层厚度,以增大涂层所受的残余压应力,降低基体层所受的残余拉应力,有效提高整体材料在不同温度下的强度性能。     

Cross-Sectional Residual Stresses in Thermal Spray Coatings Measured by Moiré Interferometry and Nanoindentation Technique

A plasma-sprayed thermal barrier coating (TBC) was deposited on a stainless steel substrate. The residual stresses were firstly measured by moiré interferometry combined with a cutting relaxation method. The fringe patterns in the cross-section of the specimen clearly demonstrate the deformation caused by the residual stress in thermal spray coatings. However, restricted by the sensitivity of moiré interferometry, there are few fringes in the top coat, and large errors may exist in evaluating the residual stress in the top coat. Then, the nanoindentation technique was used to estimate the residual stresses across the coating thickness. The stress/depth profile shows that the process-induced stresses after thermal spray are compressive in the top coat and a tendency to a more compressive state toward the interface. In addition, the stress gradient in the substrate is nonlinear, and tensile and compressive stresses appear simultaneously for self-equilibrium in the cross-section.     

X-ray diffraction measurement of residual stress in WC-Co thermally sprayed coatings onto metal substrates

Investigation of the residual stresses and microstructural properties associated with HVOF thermal spray coating of WC-17 wt% Co of same thickness on three substrates with coefficients of thermal expansion different to that of WC. The residual stresses were measured by X-ray diffraction sin²ψ techniques using CoKα radiation. The results indicated residual stresses that have different natures for the as-sprayed coatings despite using the same powder as feedstock. The magnitudes of the stresses in the as-sprayed condition are low.     

高速电弧喷涂过程涂层表面残余应力预测

目的提出一种由连续叠加热失配应力和曲率淬火应力组成的新型涂层表面残余应力预测模型,用于预测高速电弧喷涂成形过程涂层表面残余应力值。方法基于高速电弧喷涂层逐层叠加成形基本假说,利用力和力矩平衡原理,分别建立了涂层逐层叠加热失配应力模型和淬火应力模型,将两模型组合后,得出喷涂过程涂层表面残余应力数值模型。结果通过与X射线衍射仪测得的不同厚度FeAlCrBSiNb涂层表面残余应力值进行比较,发现喷涂一道次时,涂层表面残余应力值为涂层形成过程中最大表面残余应力值。喷涂层厚度为500μm前后,涂层表面残余应力变化规律不同,即理论预测值与实际值随喷涂厚度的增加先减少,当喷涂厚度沉积至500μm后,理论预测值趋于有增有降的波动平稳,而实际测量值在厚度增加至1500μm后逐渐下降。结论高速电弧喷涂过程涂层表面残余应力预测模型,可以较为准确地预测涂层形成过程中表面最大残余应力值和涂层厚度小于500μm时的涂层表面残余应力值,揭示出高速电弧喷涂层实际成形过程表面残余应力的分布规律,即随着喷涂层沉积厚度的增加,残余应力先减少而后沉积至500μm后略微增大。     

Strength and residual stresses of functionally graded Al2O3/ZrO2 discs prepared by electrophoretic deposition

Biaxial strength testing of functionally graded Al₂O₃/ZrO₂ discs revealed that the strength of such discs, prepared by electrophoretic deposition, was almost doubled from 288 MPa for pure Al₂O₃ to 513 MPa for the graded discs; this was due to the compressive surface residual thermal stresses in the Al₂O₃ surface layer caused by the graded compositional profile. The surface compressive stress measured by means of X-ray diffraction was compared with the analytically calculated stress distribution in the graded component.     

The effects of ceria on the mechanical properties and thermal shock resistance of thermal sprayed NiAl intermetallic coatings

The effects of ceria addition to thermal sprayed NiAl intermetallic coatings were investigated through micro-indentation, thermal shock testing and microstructural analysis techniques (scanning electron microscopy with energy-dispersive X-ray spectrometry and X-ray diffraction analysis). It has been found that the addition of CeO₂ to NiAl coatings reduces the tendency of brittle peeling during thermal spraying. This reduction in peeling is presumably due to the improved wetting of the substrate by the molten coating material, which leads to better coating adhesion. The addition of CeO₂ resulted in higher coating hardness and elastic modulus. The coatings containing CeO₂ also exhibited significant increases in thermal shock resistance compared with that of the pure NiAl coating. The NiAl coating containing 2 wt.%CeO₂ had the highest hardness, elastic modulus and thermal shock resistance of the four NiAl-based coatings tested. The possible mechanisms responsible for the improvement of the properties upon addition of CeO₂ are addressed.     

Effects of coating thickness and residual stresses on the bond strength of ASTM C633-79 thermal spray coating test specimens

Wire-arc-sprayed nickel-aluminum is widely used in the aircraft industry for dimensional restoration of worn parts and as a bond coat for thermal barrier coatings and other top coats. Some repair applications require thick coatings, which often result in lower bond strength. A mechanism being investigated to ex-plain this decrease in bond strength is the free edge effect, which includes both coating residual stresses and coating thickness. The layer-removal method was used to determine experimentally the residual stresses in wire-arc-sprayed nickel-aluminum coatings of different thicknesses. Bond strength evalu-ations were performed using an improved ASTM C 633-79 test specimen. Finite-element analysis and fracture mechanics were used to investigate the effects of coating thickness and residual stress state on coating bond strength.