热喷涂涂层和基体中残余应力预报与控制研究

分析研究了热喷涂涂层和基体中残余应力产生的原因并发展建立了相应的理论模型。依据该模型,不但可以计算骤冷过程和冷却过程在涂层/基体结构内引发的残余应力,而且首次分析了沉积过程中,基体/涂层因喷射冲击(即喷涂粒子高速撞击基体及形成的涂层表面)而产生的残余应力,以及因基体/涂层热膨胀系数不匹配产生的残余应力。理论计算结果与实验结果基本吻合。通过理论计算,可以预报涂层/基体中残余应力的大小,并根据需要控制涂层/基体中残余应力的分布。对即将进行的热喷涂工艺具有一定的指导意义。     

等离子涂层孔隙研究进展

总结了等离子涂层孔隙形成原因,孔隙率的测试,孔隙率对涂层弹性模量、热导率、残余应力的影响;介绍了喷涂工艺参数对孔隙率影响及涂层形成过程的数值模拟.目前主要以涂层的孔隙率作为研究重点,但孔隙率仅代表了涂层的密实程度,应进一步研究等离子涂层中孔隙的形状、大小及其对涂层性能的影响以及影响涂层孔隙结构的主要工艺参数.     

等离子体喷涂涂层细观结构与工艺相关性研究

采用改进的颗粒沉积模型和一种新建议的循环算法，利用数值方法模拟了等离子体喷涂中涂层的生长过程及涂层的细观结构．数值模拟，主要包括了陶瓷液滴的高速变形与凝固、涂层材料的堆积、涂层中细观空洞的形成与温度场的迭代计算等过程．研究结果表明，涂层中孔隙率的分布与一些关键工艺参数和基底表面状态等有关，液态陶瓷颗粒的直径和飞行速度的加大会引起涂层内孔隙率的增加，而基体温度和表面粗程度的升高则有利于提高涂层的致密度．本文的研究结果将有助于定量或半定量地优化选取工艺参数以便获得所需的涂层结构和改善涂层的力学性能．     

等离子喷涂纳米结构与传统结构热障涂层的残余应力对比研究

热障涂层的残余应力是影响其服役寿研究不多.在45钢基体上,用超音速火焰喷涂NiCocrAlY打底层,再用大气等离子喷涂ZrO2-8%(质量分数)Y2O3(8YSZ)工作层,制备了纳米结构与传统结构2种类型的热障涂层(TBC).采用SEM、XRD对这2种涂层的粉末及涂层进行了组织结构分析,用纳米压痕仪测得了2种涂层的弹性模量.用X射线衍射应力测试仪测得了2种涂层的表层残余应力.结果表明:喷涂工艺参数相同条件下,对于打底层及工作层的厚度均相同的2种涂层,纳米结构热障涂层的表层残余应力比传统结构热障涂层约低24.7%;相同打底层的纳米结构热障涂层表层残余应力随着陶瓷层厚度增加而增加,陶瓷层厚度在240 um以下时,表层为残余压应力;厚度超过300 um时,表层为残余拉应力.最后提出了相应的物理力学模型.     

内嵌弹性区和结合力模型在薄膜/基底界面断裂中的应用

薄膜/基底结构是微电子学和材料科学中广泛应用的典型结构。由于加工工艺中材料力学、热学性能失配等原因导致的薄膜中出现的残余应力,是界面裂纹的萌生和扩展的重要原因。采用三参数(Γ₀, /σ_y,t)的修正的断裂过程区结合力模型,讨论了在塑性氛围下裂尖解理断裂的过程,裂尖应力分布,裂尖形貌和表征裂纹尖端断裂过程区特征参数对断裂过程的影响,并应用到均质金属薄膜/陶瓷基底结构中残余应力导致界面裂纹起裂和扩展的全过程分析中。     

含FGM的涂层结构中热残余应力的分析与优化

本文利用有限元方法和优化理论,对含ＦＧＭ（Ｆｕｎｃｔｉｏｎａｌｌｙ　Ｇｒａｄｅｄ　Ｍａｔｅｒｉａｌｓ）层的热喷涂构件中的残余应力进行了数值分析,并获得了ＦＧＭ内各组份体积含量分布的最优化形式和参数ｐ。同时,我们也研究了喷涂构件的几何形状、涂层及基底的材料性能对于ｐ的影响规 律。在本文的分析中,考虑了基底材料和ＦＧＭ的塑性变形以及其性能对于温度的依赖。本文 的工作将有利于含ＦＧＭ层的热喷涂构件的设计与生产。     

脉冲偏压对(Ti,Al)N涂层应力分布及其力学性能的影响

利用电弧离子镀技术在不锈钢基体上制备了(Ti,Al)N涂层,研究了脉冲偏压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,(Ti,Al)N涂层残余应力沿层深呈"钟罩型"分布,且随脉冲偏压的增大应力值明显增加;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。随脉冲偏压的增加,涂层硬度会显著增加,而膜/基结合力则先增加后减小;采用改变脉冲偏压的工艺制备(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。     

Y-TZP/LZAS微晶玻璃功能梯度涂层残余应力的有限元分析

设计了Y-TZP/LZAS微晶玻璃功能梯度涂层,使用有限元软件分析了成分分布指数、梯度层数目和梯度层厚度等参数对涂层/基体界面残余热应力的影响。结果表明:功能梯度材料的最佳成分梯度指数为m=1;涂层最佳层数为3-5层;涂层最佳厚度为1-1.5 mm;涂层表层主要分布为径向压应力;在涂层/基体界面的边缘区域应力集中较为严重;涂层/基体界面处的径向应力、轴向应力和剪切应力与成分分布指数、梯度层数目和梯度层厚度有密切的关系。用涂搪法制备了梯度涂层,用X射线衍射法(XRD)测试了涂层表面残余应力,验证了有限元结果的准确性。     

7075-T651铝合金喷丸工艺的仿真与试验研究

为探究喷丸工艺参数对材料残余应力分布的影响,以7075-T651铝合金材料为研究对象,基于铝合金材料特性和试验条件,利用ANSYS/LS-DYNA建立喷丸有限元模型,获得了不同喷射距离、喷射压力和弹丸直径下残余应力的分布情况。结果表明：增大喷射压力和弹丸直径可以显著提高最大残余压应力和残余压应力影响深度,且喷射距离应保持在适当的范围内,最后将应力计算值和实测值进行对比,最大偏差为(10.0±12.5) MPa,偏差率不超过6.9%。分析认为通过有限元模型能够准确地获得残余应力的分布情况,有助于开展喷丸工艺参数对残余应力分布规律的影响研究。     

热喷涂轨迹对涂层结构及性能影响的研究进展

热喷涂技术是表面工程中一门重要的学科,是重要的表面防护和强化技术之一。针对热喷涂轨迹对涂层结构及性能的影响,综述了热喷涂过程中喷枪移动路径、移动速度、倾斜角度以及喷涂距离等工艺参数对平面和曲面涂层厚度均匀性、孔隙率、应力分布、沉积效率等指标的作用;将基体分为平面和曲面两种类型,归纳了喷涂轨迹参数对涂层结构和性能的影响规律。最后,对热喷涂轨迹规划的未来研究重点做出了展望。     

Modeling development of residual stresses in thermal spray coatings

The diameter, velocity and temperature of stainless steel and tungsten carbide cobalt particles applied onto stainless steel substrates using a high velocity oxy-fuel (HVOF) torch were measured. The microstructure of the coatings produced was examined using a scanning electron microscope and coating thickness, porosity and roughness measured. Using the experimental spray parameters as inputs to a 3-D stochastic model we simulated coating formation. Measured values of coating thickness and porosity agreed well with predicted values while calculated surface roughness was somewhat higher than that observed in experiments. An object oriented finite element code (OOF) developed at the National Institute of Standards and Technology was used to calculate residual stresses in the coating. The model uses an adaptive meshing technique to discretize the coating microstructure into a mesh suitable for finite element analysis. To define the coating geometry we used either micrographs of coating cross-sections or computer generated images of coatings. Similar values of residual stress are obtained in either case. High stresses are present at the interface between the coating and substrate. The magnitude of stresses increases significantly with coating thickness. Stresses are relieved by voids such as pores or cracks in the coating. Residual stresses increase with coating temperature and can be decreased by preheating the substrate.     

MANUFACTURE AND PROPERTIES OF PLASMA SPRAYED Cr2O3

Low porosity (some %) chromia coatings have been plasma sprayed onto several substrates by atmospherical and vacuum plasma spraying. Different phases appear in the sprayed product; metallic Cr and the metastable tetragonal Cr₃O₄ phase have been identified. A post annealing treatment completely removes these phases. This behaviour can be correlated with differences between the coefficients of thermal expansion comparing the as-sprayed with the annealed material. Hot Isostatic Pressing improves the interface bonding between coating and substrate and reduces porosity as can be seen from the results of metallographical investigations. If aluminium is present in the substrate alloy, chemical interactions have been observed. First results from measurements of residual stresses in coatings show the influence of coefficients of expansion and substrate temperature during spraying. Gastightness of the sprayed chromia could not yet be established.     

碳钢热喷涂镍沉积凝固过程热力耦合场的数值模拟及温度应力场的分布规律

为了进一步弄清热喷涂残余应力产生的机理、预测与控制技术,以碳钢表面热喷涂镍为例,对镍沉积凝固冷却过程进行了合理的假设和简化。利用ANSYS平台,采用热力耦合计算,模拟了涂层冷却过程,获得了温度场和应力场分布规律,分析了涂层上某些特征点的温度和应力随时间变化的情况,并探讨了基体预热温度和涂层厚度对涂层/基体界面应力的影响。结果表明,涂层温度沿中心向边缘逐渐降低,随时间延长呈降低趋势;基体温度随时间的变化呈先增加后降低趋势;涂层主要承受残余压应力作用,随基体温度升高,残余应力呈减小趋势,随涂层厚度增加,残余应力呈增大趋势。     

Study of the delamination of diamond coatings under thermal stress

The influence of the thickness of CVD diamond coatings on the adhesion to a substrate, after cooling down from deposition temperature to room temperature, has been studied experimentally and theoretically. Diamond layers have been deposited at 850°C on W substrates by microwave plasma enhanced CVD. Cooling down of the substrate-diamond coating system to room temperature induces thermal stresses, due to different thermal expansion coefficients of coating and substrate. For thick diamond coatings a total and sudden delamination could be observed as a consequence of these stresses. On the contrary thin coatings, produced under identical circumstances, adhered well. These phenomena have been modelled and explained by the use of an energetic criterion for the delamination of a two-layer system under thermal stress. From the model a critical thickness of the coating can be calculated. Above this critical thickness, delamination will suddenly occur. The calculations also predict that for intermediate coating thicknesses delamination can easily be induced by external causes.     

Evaluation of residual stresses in PVD-coatings. Part 2

The values of residual stresses in PVD-coatings of titanium, aluminum and niobium nitrides are determined by the flexible specimen method. An experimental-and-calculation procedure for separating the structural and thermal components of residual stresses is proposed that allows one to determine the thermal expansion coefficients of the coating materials and a general level of residual stresses in coatings on a substrate of any material. The dependence of the residual stress on the roughness of the substrate surface is established. The possibility of adjusting the level of residual stresses in coatings by the discrete surface topography formation is shown.     

Modeling of evolution of growing coating composition

This paper discusses the role of coupling effects between transfer processes, chemical reactions and deformations in the evolution of phase composition of a growing coating and in the formation of a transition zone between coating and substrate. The proposed model of the coating growth is based on the equations of irreversible thermodynamics and accounts for coupling physical and chemical phenomena: diffusion, thermal diffusion, chemical reactions, elastic fields in the diffusion zone and stresses induced by diffusion. The transfer coefficients and kinetic parameters are calculated using classical theories or identified from experiments. As a result, we developed a theory that has direct practical applications and can be specified for a variety of technological and experimental situations. The formulated problem is solved numerically. The phase composition of the coating and residual stresses in the diffusion zone are obtained as a function of time under various technological conditions. The results can be used for prognosis and technology optimization.     

Effect of Differential Thermal Expansion Coefficient on Stresses Generated in Coating

Two different spraying powders [81FVNS and 80% (443)+20% (105)] were plasma sprayed on two types of substrates (AISI 316 and Ti) in order to verify the effect of differential thermal expansion coefficient (DTEC) between coating and substrate on the final residual stresses generated in the coating. Modulus of elasticity (E) was evaluated for the substrates and coatings (as a composite beam). Free-standing coatings were used to measure the thermal expansion coefficient, as a function of temperature, of the actual coatings with all defects, impurities and typical lamella structure. The results show that the residual stresses seems to be less dependent on the linear thermal expansion coefficients mismatching between substrate and coating, reflecting the importance of the other sources of stresses. i.e. quenching or deposition stresses induced during spraying. The suitable heat treatment condition of the coated samples may release some or most of the residual stresses. However in some cases the sign of the stress was completely changed as a result of neglecting the quenching stresses.     

Effect of Spraying Condition and Material Properties on the Residual Stress in Plasma Spraying

The thermomechanical behavior and the distribution of residual stresses due to thermal spraying of NiCoCrAlY coating were studied by thermomechanical finite element analysis. The effects of phase transformation due to solidifying process of coating particles, thickness and material properties of coating on the residual stresses were discussed. Results showed that residual stress decreases little with the stress relaxation due to the phase transformation. For the substrates with the same thickness, the residual stress increases with the increase in coating thickness. The state of residual stresses relates to the material properties of coating and substrate closely. The stress-induced failure model of coating is also discussed.     

Prediction of residual stresses in thermally sprayed steel coatings considering the phase transformation effect

The present study is aimed to propose an elastoplastic bilayer model for prediction of residual stresses in thermally sprayed coatings, in which the effect caused by martensite phase transformation for steel coating materials was taken into account. Closed-form solutions of the curvature and stresses within the substrate and coating are obtained for the plastically deformed structures. Applications of the model for prediction of the twin-wire electric arc sprayed high carbon steel coatings were investigated subsequently. In the application case that a thin coating layer deposited with varying temperatures, the martensite phase transformation has a significant effect on the residual stress, e.g. a lower deposition temperature leads to more amount of martensite transformation and then to a lower level of final stress. The model was also used to predict the stress distribution of high carbon steel coating after quenching heat treatment, and the calculation results were compared with the X-ray residual stress measurements. It is found that the residual stresses on the coating surface obtained from the analytical model are closed to that obtained from the experiments.