圆柱基体热障涂层制备工艺中的残余应力分析

由于基体和热障涂层各层材料的热膨胀系数不同,在温度变化时各部分变形大小也不同,从而导致涂层内部产生残余应力。本文利用过盈配合模型求解圆柱形基体上热障涂层的残余应力,计算了涂层的厚度、弹性模量和热膨胀系数对残余应力的影响,并从应力角度探讨热障涂层的剥落失效。     

热障涂层界面微区域热蠕变应力演化分析

热障涂层热循环载荷下不同界面层蠕变特性是影响界面微区域残余应力变化的关键因素,探究热障涂层蠕变与残余应力的关系有助于提高热障涂层的稳定性。以热弹塑蠕变理论为依据,采用Norton蠕变模型,建立陶瓷层、氧化层、粘接层和基体四层几何分析模型,考虑不同层蠕变和蠕变程度因素,研究热循环载荷作用下涂层界面微区域应力演化规律。结果表明,蠕变参数和蠕变层数的变化影响热障涂层界面残余应力的大小和分布,这对预测热障涂层失效具有指导意义。     

热障涂层氧化残余应力大小与演化过程测试

热障涂层是一种应用广泛的提高高温部件工作效率的技术,在通常的涂层中,粘结层起到过渡和保护基体氧化的作用,在高温环境下,粘结层会发生氧化,而氧化产生的残余应力将引起热障涂层的损伤,表现为涂层的屈曲、起裂和剥落,导致热障涂层失效。研究测试和检测涂层的残余应力,对于评估涂层的安全性和寿命是非常重要的。本文用应力引起光学频谱位移的方法测试了空气等离子喷涂热障涂层中残余应力的大小及分布,并测试了残余应力的大小随氧化时间的演化过程。     

采用数字图像体相关方法的核级石墨断裂特性研究

为研究核工程备选石墨(SNG742细粒石墨)的断裂性能，探明材料的三维断裂力学特性，对压缩双裂纹圆孔板(DCDC)试件进行压缩试验，使用X射线计算机断层扫描(XCT)技术对试件内部进行测量。采用数字图像体相关(DVC)方法对XCT扫描图像进行分析，获得了DCDC试件的三维位移场和应变场。通过对比DVC和有限元模型的x方向应变场，反演了试件的弹性模量。结果表明，当弹性模量取10.6 GPa时，有限元模拟结果与试验结果最接近，二者的最小应变误差约为0.09×10^-3。此外，利用XCT图像中裂缝与周围区域灰度差异明显的特点，定量分析了尺度上与主裂缝宽度接近的内部缺陷，整体上受拉区域的缺陷密度高于受压区域的，其平均值大约是受压区域的2.7倍。从试件的位移场得到裂缝的三维特征，分析表明：局部裂缝长度沿试件厚度方向呈现内部高、表面低的分布规律，裂缝张开位移与局部裂缝长度成正比，裂尖张开角基本保持不变，约为1.5°。最后，分析了裂缝前缘处应变，发现应变不随局部裂缝长度的变化而改变，其值始终保持在5.8×10^-3上下。本研究方法可为通过小尺寸试件获得材料的断...     

橡胶材料弹性模量数字图像相关测定法

利用数字图像相关方法测量了小应变下柔性橡胶的弹性模量.用CCD相机记录单轴压缩实验中圆柱体橡胶试样表面人工散斑图像,作为数字图像相关测量技术中的变形信息载体.分析了镜头畸变对位移测量的影响,运用数字图像相关法得出小应变范围内像胶的应力应变曲线,计算出橡胶的弹性模量.并与采用千分表所得到的结果进行了比较,两者符合较好.实...     

基于圆筒模型的热障涂层安定分析

热载荷作用下,由于热障涂层(thermal barrier coatings, TBCs) 各层材料的热不匹配以及材料参数的温度相关等因素,会使热障涂层界面区域存在复杂的应力应变场,影响系统安定性,并导致涂层开裂和剥落. 将热障涂层外凸和内凹微观界面结构简化为多层圆筒模型,借助经典机动安定定理,利用特雷斯卡(Tresca) 屈服准则和增量破坏准则处理对时间的积分问题,避免了常规安定性分析的数学规划问题,建立了热障涂层安定极限分析方法,将材料屈服强度随温度变化关系简化为双线性关系,利用补偿变换的方法简化求解过程,对典型热障涂层安定性进行了研究. 结果表明,利用基于圆筒的安定极限分析方法,能够方便求解安定极限,便于工程应用;热障涂层安定极限值明显高于弹性设计值,且界面外凸区域安定极限高于内凹区域极限值,结构首先在内凹处失效;圆筒模型基体曲率和涂层厚度越大,结构安定极限越高,分析结果与试验结果一致;所建立的热障涂层安定分析方法,对进一步研究考虑蠕变因素影响的热障涂层安定性具有重要意义.      

新型汽车结构胶剪切实验研究

利用数字图像相关方法(DICM),分别测定了准静态单向剪切拉伸试验条件下,新型汽车结构胶粘接试件和传统点焊连接试件粘接部分的剪切力学性能。实验采用了非接触测量物体应变的方法,运用CCD及其计算机图像处理系统,实时获取变形前后试件表面图像。利用数字相关软件对变形前后的图像进行分析,从而获得试件该时刻的应变。最后确定了试件拉伸过程中的力-位移曲线及应力-应变关系曲线。测试结果及分析表明:采用新型结构胶粘接试件的力学性能与点焊结构相比有明显的优势。这为该结构胶进一步改进提供了一定的实验依据。测试中的数字图像相关法是非接触测量物体应变的方法,在实际应用中有很大的意义。     

表面涂层材料的残余应力测量技术

本文采用电测盲孔法测定表面涂层材料残余应力的测量技术，在试件和表面涂层结构上进行测试．大量试验表明，此方法可靠、有效，可在工程实际中使用，为解决表面涂层“爆裂”等问题，提供了有效的测定残余应力的手段     

平头压痕试验确定热障涂层弹性模量的方法研究

本文模拟了典型热障涂层(TBC)结构在平头压痕作用下的力学响应,重点研究利用平头压痕法确定TBC涂层弹性模量的方法。研究发现k(单位压入应力下压头的压入位移)不仅与材料的性质有关,还与压头的半径有关。本文提出:通过两个不同尺寸的平压头可以同时确定陶瓷层和粘结层的弹性模量。     

热喷涂涂层中残余应力分析和检测研究进展

综述热喷涂涂层中残余应力研究进展.主要内容有:对残余应力产生的最新认识,残余应力分布实验测试技术、理论分析模型及其对热喷涂材料界面结合强度影响等领域的研究进展.最后对该领域几个学术界和工程界关注的研究方向进行了展望.      

Mechanical Characterization of Coatings Using Microbeam Bending and Digital Image Correlation Techniques

A new technique for characterizing end-supported microbeams of coating materials is presented. Microbeams are fabricated using micro-EDM machining to isolate the material under investigation from the underlying substrate. Three- and four-point bending is realized by a custom-built microspecimen testing system, and digital image correlation is employed to capture full-field strains and displacements in theses microbeams. These experiments provide the foundation for the use of finite element modeling and inverse methods to determine the mechanical properties (elastic moduli, strength, interfacial toughness) of the coatings. Here, the experimental details of the microbeam bending experiments are explained, discussed and illustrated through application to a multilayered metal/oxide/ceramic thermal barrier coating system commonly used in aero-turbines.     

热障涂层高温失效行为的正射投影-单相机三维数字图像相关实验研究

热障涂层(Thermal Barrier Coating,TBC)作为一种隔热材料,可降低发动机高温部件的表面温度,提高涡轮发动机的整体性能。涂层服役过程中,TBC陶瓷层的失效将导致高温部件中的金属基底直接暴露于恶劣的高温环境中,严重影响发动机的安全性。其高温服役环境下的力学性能研究是备受材料和力学工作者关注的问题。本文结合热障涂层高温检测的需求,实现了涂层-基底异质表面的高对比度高温散斑制作;发展了正射投影-单相机三维数字图像相关方法(正射投影-单相机3DDIC),构建了跨界面形函数,实现了涂层与基底的三维形貌及位移、应变的同时测量;基于涂层结构的高温拉伸实验结果,建立了依据涂层-基底应变方差差值的涂层失效判断方法,得到了热障涂层在室温(27℃)至1000℃的断裂极限应变。实验结果表明,正射投影-单相机3DDIC方法作为一种可靠测试方法,可以应用于涂层结构高温下的力学失效行为研究。     

Residual stress evolution regularity in thermal barrier coatings under thermal shock loading

Residual stress evolution regularity in thermal barrier ceramic coatings (TBCs) under different cycles of thermal shock loading of 1100°C was investigated by the microscopic digital image correlation (DIC) and micro-Raman spectroscopy, respectively. The obtained results showed that, as the cycle number of the thermal shock loading increases, the evolution of the residual stress undergoes three distinct stages: a sharp increase, a gradual change, and a reduction. The extension stress near the TBC surface is fast transformed to compressive one through just one thermal cycle. After different thermal shock cycles with peak temperature of 1100°C, phase transformation in TBC does not happen, whereas the generation, development, evolution of the thermally grown oxide (TGO) layer and micro-cracks are the main reasons causing the evolution regularity of the residual stress.     

IGBT模块封装中大面积基板连接的应力翘曲分析

模拟绝缘栅双极型晶体管(IGBT)模块中用烧结银对直接覆铜(DBC)基板与铜底板进行大面积连接的过程，发现大面积基板连接会产生很大的翘曲与严重的残余应力．增加铜底板厚度使其刚度增大，能有效改善基板连接的翘曲问题．基板连接的最大残余应力产生于DBC上铜层与陶瓷层连接的角点处，随DBC铜层厚度的增加而增加，随陶瓷层厚度的增加而减小，与焊层厚度关系不大．其主要原因是各层材料厚度变化对结构整体热膨胀系数的改变程度不同，而DBC铜层与陶瓷层之间热失配程度最严重．结构厚度改变对连接层应力大小影响不大，但增加底板厚度会恶化连接层的应力分布，不利于模块长期可靠性．采用DBC下沉法和反向预翘曲法可以在控制连接层残余应力的条件下有效降低IGBT模块因烧结引起的翘曲，不会降低模块的可靠性．     

热障涂层-基体体系界面高温疲劳和断裂性能的实验研究

热障涂层(TBCs)作为发动机叶片的热防护涂层,能够显著提高叶片在高温环境下的使用寿命.本文围绕TBCs-镍基高温合金基体体系的界面性能,展开了比较系统的实验研究.通过实验方法得到了等温热处理前后陶瓷层的弹性模量、硬度及陶瓷层-粘结层界面的微结构的变化.结果显示,随着等温热处理时间的增加,弹性模量及硬度先增加后降低;氧化层随等温热处理时间和温度的增加逐渐增厚.利用本文提出的多相位角界面断裂韧性试验方法,建立了以应力强度因子为表征参数的TBCs界面失效准则.在假定界面间为粘性接触的条件下,预测了界面承载能力随陶瓷层弹性模量和氧化层厚度的变化趋势.通过热循环实验研究了TBCs-基体体系的热疲劳性能及失效机理.随着热循环高温保温时间的增加,热疲劳寿命先升高后降低,失效模式由界面失效转化为界面失效与陶瓷层失效并存;体系的失效由陶瓷层及氧化层的应变能密度、陶瓷层、氧化层及界面的断裂韧性,以及它们和界面微结构缺陷的相互作用共同决定.     

A numerical and experimental investigation into residual stress in thermally sprayed coatings

This paper is concerned with an investigation into the thermal spray process and is particularly concerned with the residual stresses that arise when a steel-alloy coating is sprayed onto a copper-alloy substrate. This material combination was used recently to enhance the thermal and mechanical efficiency of the pressure die casting process. A difficulty with the spraying of steel on copper is the attainment of appreciable thickness of the coating due to debonding during the thermal spraying process. Prominent among possible causes of debonding is residual stress, which is the focus of the research presented in the paper. An investigation into the thermal spray process is performed using experimentation, simplified numerical modelling and finite element modelling. The development of residual stress for a range of process parameters, i.e. deposited layer thickness, interval of layer deposition and the number of layers in a coating (i.e. block deposition versus multilayer deposition for a desired coating thickness) is recorded. The results from the three investigation methods agreeably indicate a progressive change in average interfacial residual stress from compressive towards tensile with increase in thickness of deposited layer; and a tensile interfacial stress in a two-layer coating, which increases with increase in interval of deposition between the two layers. On the whole, the observations from the results suggest an increase in potential for coating debonding with increase in both deposited layer thickness and layer deposition interval. The results further suggest higher potential for coating debonding with block deposition compared to multilayer deposition for a desired coating thickness.     

A Novel Technique for the Determination of Surface Biaxial Stress under External Confinement Using Raman Spectroscopy

Bi-axial compressive stress induced as a result of mechanical confinement within a zirconium diboride-silicon carbide (ZrB₂-SiC) ceramic composite has been quantified using micro Raman spectroscopy and then validated using two independent experimental methods. First a relationship relating the Raman peak-shift on a confined silicon carbide (SiC) particle to magnitude of imposed confinement stress was developed by utilizing phonon deformation potentials for 3C-SiC diamond and zinc-blende crystal structures. ZrB₂-5wt%SiC samples, prepared using spark plasma sintering were subjected to different confinement pressure on the lateral surface by thermal shrink fitting metallic sleeves. The relationship between Raman peak-shift and confinement stress was then verified by comparing the measured stress in this method with that calculated from analytical expressions readily available for thick walled cylinders. The relationship was further validated independently using digital image correlation (DIC) by measuring the displacements for unknown levels of progressively increasing confinement stress induced by a shaft-collar ring on similar specimens. The Raman peak-shift relation derived for SiC phase also correctly predicted process-induced residual stresses due to a mismatch in coefficient of thermal expansion between the matrix phase and SiC particles. The derived Raman peak-shift relationship can also be generalized and can be a valuable tool to experimentally determine unknown bi-axial stress in a Raman active structure.