Effects of gradient interlayer on residual stress and cracking in TiN thin films

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｒｅｓｉｄｕａｌｓｔｒｅｓｓｉｎｆｉｌｍｓｈａｓｉｍｐｏｒｔａｎｔｉｎｆｌｕｅｎｃｅｏｎｔｈｅｐｒｏｐｅｒｔｉｅｓｏｆａｆｉｌｍ/ｓｕｂｓｔｒａｔｅｓｙｓｔｅｍ[1].Ｐｒｅｖｉｏｕｓｒｅｓｅａｒｃｈ[2]ｏｎＴｉＣ/ＡｌｒｅｖｅａｌｅｄｔｈａｔｔｈｅｍａｇｎｉｔｕｄｅｏｆｒｅｓｉｄｕａｌｃｏｍｐｒｅｓｓｉｖｅｓｔｒｅｓｓｉｎＴｉＣｔｈｉｎｆｉｌｍｓｉｓａｓｇｒｅａｔａｓ10ＧＰａ,ｗｈｉｃｈｄｅｃｒｅａｓｅｓｒｅｍａｒｋａｂｌｙｗｉｔｈｔｈｅｉｎｃｒｅａｓｅｏｆｔｈｅｔｈ…     

激光熔覆对改善等离子涂层界面结合性能的研究

研究激光熔覆对改善等离子涂层界面的作用,基体待喷表面采用激光熔覆工艺预处理,在涂层－基体的界面处形成成分、显微硬度呈梯度分布的熔覆层,三点弯曲试验结果表明：界面梯度有显改善喷涂材料的界面结合状况,使界面承受外载茶的能力提高,并用相关的有限元程序分析了试样承载时界面处剪切应力的分布。     

Edge Effect on Crack Patterns in Thermally Sprayed Ceramic Splats

To explore the edge effect on intrasplat cracking of thermally sprayed ceramic splats, crack patterns of splats were experimentally observed and investigated through mechanical analysis. Both the polycrystalline splats and single-crystal splats showed obvious edge effects, i.e., preferential cracking orientation and differences in domain size between center fragments and edge fragments. In addition, substrate/interface delamination on the periphery was clearly observed for single-crystal splats. Mechanical analysis of edge effect was also carried out, and it was found that both singular normal stress in the substrate and huge peeling stress and shear stress at the interface were induced. Moreover, effective relief of tensile stress in splats is discussed. The good correspondence between experimental observations and mechanical analysis is elaborated. The edge effect can be used to tailor the pattern morphology and shed further light on coating structure design and optimization.     

Influence of localized cyclic substrate plastification on residual stress,load stress and cracking near the interface between hard coating and WC-Co hard metal substrate

WC-Co hard metal covered with thin ceramic coatings represents a composite structure with a supreme performance in many metalworking tool applications. The failure behaviour of this substrate-coating composite is significantly influenced by the plastic deformation of the hard metal, especially if the plastification is localized and associated with changes in the residual stress state. Today, there is no complete understanding of the effect of localized cyclic substrate plastification on the residual stress, and its interaction with the superimposed stress induced by external load. The current work sheds light on this interplay via an approach combining experimental observations and finite element simulation. To induce different levels of localized plastification, uncoated and AlCrN coated notched specimens were subjected to different levels of uniaxial cyclic compressive loads at a stress ratio R = σ_min/σ_max of minus infinity. The associated shift of residual stress towards tension at the notch root was determined in the hard metal for the same load situations experimentally via X-ray diffraction and in simulations. The applied finite element simulations included an experimentally parameterized material model to describe the cyclic deformation behaviour of the hard metal. A type of fatigue crack oriented parallel to the substrate-coating interface was observed close to a surface asperity via scanning electron microscopy in a cross section prepared by focused ion beam milling. The observed type of crack can be interpreted as an early stage of coating delamination. Simulation results indicate that the maximum applied compressive load induces tensile stresses acting perpendicular to the substrate-coating interface, when considering the surface roughness features present in the specimen. The magnitude of the observed tensile stresses, that drive the growth of the observed crack type, decreases rapidly due to the cyclic plastification of the hard metal substrate.     

Formation Mechanisms of Hot Cracks in Laser Additive Repairing Single Crystal SuperalloysEI北大核心CSCD

以第二代单晶高温合金DD432为研究对象开展激光增材修复实验，利用实验与理论计算相结合的手段，分析和研究了单晶高温合金在激光增材修复过程中的热裂纹形成机制。结果表明，单晶修复区内热裂纹在大角度晶界处形成，裂纹两侧呈现显著应力集中，裂纹源区域分布大量MC型碳化物。热裂纹的形成取决于液膜的稳定性、应力集中及碳化物析出相的共同作用。液膜稳定性取决于枝晶凝并过冷度，并与相邻晶粒间的晶界角度密切相关。基于Rappaz枝晶凝固过冷理论，计算获得DD432单晶合金形成稳定液膜的最小晶界角为2.9°，即该合金热裂纹形成的临界角；大角度晶界处的枝晶凝并过冷度为395 K，远高于晶粒内部枝晶间液膜的过冷度(29.58 K)以及小角度晶界(3.6°)处的枝晶凝并过冷度(56 K)，大角度晶界为开裂提供了稳定液膜；沉积区内部的高水平应力集中驱动了热裂纹的萌生与扩展；MC型碳化物析出相通过“钉扎作用”抑制液相补缩及弱化与基体之间结合强度等作用进一步促进了热裂纹形成。     

Effects of process variables on interfacial quality of laser cladding on aeroengine blade material GH4133

Consecutive layer of a cobalt alloy, stelliteX-40, were deposited onto a GH4133 superalloy plate. The microstructure and mechanical properties of laser cladding were examined. An inappropriate combination of laser power, scanning velocity or pre-placed powder height can result in the formation of incomplete fusion interface between the cladding layer and substrate, or reheat cracking along the coarse grain boundary in the heat-affected zone (HAZ) after ageing treatment. The SEM and energy dispersive spectroscopy (EDS) show the absence of precipitate particles within the reheating crack with similar average compositions for the crack and the substrate. The plastic deformation due to residual thermal stress induced by thermal cycle of laser cladding process is concentrated on the coarse grain boundaries within the HAZ as grains are strengthened by precipitating particles during the aging process. This concentrated plastic deformation is mainly responsible for reheat cracking. The laser cladding layer with integrated melt interface and without reheat cracking can be obtained by optimizing laser power, scanning velocity and powder thickness. The experimental results of microhardness show the homogenous property distribution within the material after the ageing treatment.     

铝合金表面激光熔覆铜合金层中的裂纹及其有限元分析

采用激光熔覆方法在铝合金表面得到了铜合金熔覆层,试验观察到在熔覆层与基体表面界区域易于产生宏观裂纹,在熔覆层顶层易于产生微观裂纹。利用MSC／NASTRAN有限元分析软件对熔覆层横断面内平面残余热应力进行了模拟分析,得出了其残余热应力分布,解释了涂层中的裂纹现象。     

裂解炉管焊接残余应力场中裂纹扩展行为的数值分析

针对裂解炉管对接焊接热影响区中常见的环向与斜裂纹在焊接残余应力的作用下的扩展行为进行数值预测.采用基于VNA算法的有限单元交互法(FEAM),结合有限元方法研究焊接残余拉应力对炉管裂纹扩展的影响,通过研究裂纹尖端区域的应力强度因子的变化来判断裂纹的准静态扩展倾向.结果表明,在多道焊焊接残余应力的影响下,在炉管焊缝热影响区内表面的环向裂纹有沿环向变长的倾向,而斜裂纹则同时有变长、变深的倾向.     

新型耐磨钢NM400斜Y形坡口焊接抗裂性分析

采用斜Y形坡口焊接抗裂性试验对NM400钢的焊接抗裂性进行了研究,通过测定其表面裂纹率、断面裂纹率及根部裂纹率,评定NM400的抗裂性能,并分析焊前预热对材料抗裂性的影响.同时,应用热弹塑性有限元法对斜Y形坡口试样进行了焊接过程的有限元数值模拟计算,得出横向焊接残余应力的分布情况及峰值位置.结果表明,常温下NM400的根部裂纹率较高,预热150℃时各裂纹率均降至0.因此,焊前预热能够提高NM400的焊接抗裂性.另斜Y形坡口根部产生较大的应力集中,局部横向残余应力超过材料的强度,是致使其产生裂纹的力学因素.     

电弧增材制造钛合金界面处残余应力及其影响研究

采用电弧增材制造方法制备了含增材/基材界面钛合金板,采用轮廓法测量了其残余应力分布。建立了模拟紧凑拉伸(C(T))试样加工和裂纹扩展过程中残余应力发展的有限元模型,缺口状态C(T)试样内残余应力分布与轮廓法测试结果吻合良好。采用该模型讨论了试样内残余应力随裂纹扩展的变化规律及对裂纹扩展的影响。试验和数值分析结果表明:2种类型试样缺口状态的残余应力分布有很大差别,A类试样(缺口位于基材)残余压应力区域靠近缺口根部,C类试样(缺口位于增材)残余压应力区域远离缺口根部;A类试样内残余应力随裂纹扩展迅速释放,残余应力引起的应力强度因子较小;C类试样内残余应力随裂纹扩展变化较小,残余应力引起的应力强度因子较高,降低了疲劳裂纹扩展寿命。     

Crack formation and its prevention in PVD films on epoxy coatings

Severe cracking was found to occur in PVD titanium films on epoxy powder coatings. After all baking treatments, the epoxy coating had smooth, crack-free surfaces and the cracking of both the titanium film and the epoxy only took place as a result of physical vapour deposition. Tensile cracks were observed in the titanium film and not the compressive cracks expected from the conventional two-layered theoretical model. An alternative model has been developed for the prediction of thermal stress in a three-layered film-epoxy-substrate system. The model is consistent with the experimental trials and showed that cracking originated from thermal stresses developed in the titanium-epoxy-aluminum system due to the PVD process. Tensile instability and cracking were initiated where pores intersected the film-coating interface. The results showed that crack formation could be prevented by increasing the baking temperature to 210 °C. This critical temperature activates full crosslinking in the epoxy structure and raises its strength sufficiently to avoid tensile instability due to residual stress. Crack-free and high-gloss sputtered titanium films could therefore be produced on organic coatings. This offers the potential of a combined in-line PVD-powder coating technology as an alternative to electroplating.     

Effect of a laser pre-quenched steel substrate surface on the crack driving force in a coating–steel substrate system

A mechanical model of a coating/laser pre-quenched steel substrate specimen with a crack oriented perpendicular to the interface between the coating and the hardened layer is developed to quantify the effects of the residual stress and hardness gradient on the crack driving force in terms of the J-integral. It is assumed that the crack tip is in the middle of the hardened layer of the pre-quenched steel substrate. Using a composite double cantilever beam model, analytical solutions can be derived, and these can be used to quantify the effects of the residual stress and the hardness gradient resulting from the pre-quenched steel substrate surface on the crack driving force. A numerical example is presented to investigate how the residual compressive stress, the coefficient linking microhardness and yield strength and the Young’s modulus ratio of the hardened layer to the coating influence the crack driving force for a given crack length.     

In situ observation of crack initiation and propagation of the charged magnesium alloy under cyclic wet-dry conditions

The process of crack initiation and propagation of the charged MgAl9Zn1 alloy in an atmosphere of water vapor under cyclic wet-dry conditions was in situ observed using environmental scanning electron microscope (ESEM). The results showed that crack initiated at scratches after 6 wet-dry cycles, indicating that residual stress caused by scratches promoted crack initiation. With the increase of wetting time, the fracture interval of the cracking β phase enhanced and the crack length increased, implying that hydrogen produced by corrosion reaction participated in the cracking process.     

中强低合金钢焊缝的人字裂纹

采用多层埋弧焊方法,研究了低合金钢焊缝人字裂纹。该裂纹的表面形态是:裂纹走向与焊缝纵向成45°角,并由焊缝柱晶界小段裂纹与穿柱晶小段裂纹连接成阶梯扩展途径,裂纹断口为韧窝或准解理。离子探针分析证实,锰、硫、磷、镍等元素与人字裂纹无直接联系;硅在断裂过程中析出:氢的增加有助于裂纹的产生。对人字裂纹的产生条件和形成机理作了探讨。提出人字裂纹断裂模型,屈服强度低于500N/mm~2的焊缝金属,在较大的塑性应变消耗和残余应力作用下,可能产生人字裂纹。     

加氢反应器T接管结构优化

T接管是加氢反应器氢气和蒸气混合气的进口,裂纹出现较多,焊接残余应力是主要原因之一.利用有限元法,对加氢反应器T接管与球形封头焊接残余应力进行计算分析.从焊接残余应力的角度,优化了T接管的连接方式.结果表明,当T接管轴线沿球形封头半径方向时,在T接管内壁以及焊缝根部,产生了较大的残余应力,对应力腐蚀开裂影响很大.因此,...     

ZG0Cr14Ni5Mo2Cu上位锁支臂断裂原因分析

某型号飞机电缆装配前进行例行检查时,发现ZG0Cr14Ni5Mo2Cu上位锁支臂开裂。通过现场调查、对断裂件的材料成分分析、金相组织观察、断口的宏微观观察、硬度检测以及设计结构、受力及现行工艺等综合分析,以确定该上位锁支臂断裂的性质及产生原因。结果表明:上位锁支臂的断裂为过大的装配应力、校形残余应力与氢共同作用下的氢致脆断造成的。此案例分析中发现了隔板设计结构、校形的残余应力问题,为此提出了合理的改进建议。     

真空退火对TC4钛合金表面Ti/TiN多层复合涂层性能的影响

金属基体材料表面硬质膜层在服役过程中,残余应力在膜基界面以及膜层内部界面之间的积聚会导致膜层发生界面剥落失效。以TC4钛合金基体表面Ti/TiN多层复合膜层为研究对象,探讨真空退火对复合膜层结构及性能的影响,并表征退火前后复合膜层的界面划痕失效以及抗粒子冲蚀性能。结果表明,真空退火促进了膜层内部以及膜基界面两侧原子的热扩散,使得界面结构特征明显弱化。界面状态的改变使得复合膜层的表面显微硬度降低以及膜基结合强度提高。在划痕载荷作用下,复合膜层抵抗裂纹沿界面扩展的能力得到增强。真空退火有助于提高膜层的强韧性匹配,可有效抵抗小角度冲蚀粒子的犁削以及大角度粒子冲蚀下的疲劳,因此Ti/TiN多层复合膜层表现出较好的抗冲蚀性能。     

Edge effects in thin film delamination

Thin films bonded to a substrate often sustain large in-plane residual stresses that are transferred to the film via shear stresses on the interface near their edges. These edge zones play a significant role in film delamination. A new method is introduced to analyze both the residual stress distribution in a film near its edge and the energy release rate and mode mix for an interface delamination crack emerging from, or converging upon, an edge. Two two-dimensional configurations are considered: (a) a film whose edge lies in the interior of the substrate and (b) a film whose edge is aligned with the edge of the substrate (i.e. the film/substrate geometry is a quarter-plane). There are significant differences between the two cases. For the former, (a), the energy release rate approaches the steady-state, limiting rate for a long interface crack when the crack has extended less than one film thickness. By contrast, the energy release rate in case (b) remains far below the steady-state rate until the crack extends to ten or more film thicknesses from the edge. In case (b), the edge effect provides a significant protection against edge delamination, whereas in case (a) it does not. Elastic mismatch between the film and the substrate is significant in case (b), but not in case (a). A second set of behaviors is investigated wherein the interface crack approaches the edge of the film from the interior. For both types of edges, the energy release rate drops well below the steady-state rate at remaining ligament lengths that are very large compared to the film thickness, approaching zero as the delamination converges on the edge. Analytic features which account for the various behaviors will be highlighted, and practical implications for thin film delamination will be discussed.     

Intergranular stress corrosion cracking of copper in nitrite solutions

Semi-hard tubes of deoxidized high phosphorous copper with different levels of tangential residual stresses have been exposed to nitrite solutions in a laboratory heating circuit. After characterization of investigated materials influence of temperature, location of heating, concentration of solution, electrochemical potential, and atmosphere on stress corrosion cracking susceptibility of those copper tubes has been investigated. Threshold stress for crack initiation has been determined. Maximum duration of experiments was 1 month. Breakthrough time of tubes has been measured as criterion for susceptibility to SCC.To vary stress level in a wider range constant load tests on tubes with different degrees of cold working (annealed, semi-hard, hard) were done by mounting them in a steel frame.Stress corrosion cracks were always intergranular. A tenorite layer covered surface of cracked copper tubes. A reaction scheme for ammonia formation is presented. Necessary conditions for formation of stress corrosion are shown and critically discussed.Results show that intergranular cracking takes place at much lower stress levels below yield strength when compared to literature data on transgranular cracking above yield strength. For transgranular cracking cross slipping and cleavage formation as cracking mechanism is confirmed while for intergranular cracking chemical dissolution of grain boundaries plays a more important role.     

The corrosion fatigue and stress corrosion cracking of α-brass in ammoniacal solutions

A study of the corrosion fatigue and stress corrosion cracking under a constant strain-rate in 70 Cu?30 Zn brass as a function of pH has been made. Correlation between the two has been discussed. Corrosion fatigue was very dependent on pH both with respect to the lifetime and to the nature of cracks as well as those of stress corrosion cracking. A minimum in fatigue life and a large fatigue crack growth rate were observed in neutral tarnishing solutions of pH 6.5–7.0. The fatigue crack propagation rate in neutral tarnishing solution was about twice as great as that in an acidic non-tarnishing solution. Fractographic examination on the fracture surfaces of corrosion fatigue and stress corrosion cracking showed that intergranular cracking predominated both in neutral tarnishing and acidic non-tarnishing solutions, and transgranular cracking predominated in alkaline solution. It is concluded that the tarnish film is not a primary factor controlling the cracking mode in the acidic to neutral pH range and that intergranular cracking may result from the localized dissolution of alloy at grain boundaries.