Fracture Characteristics and Damage Evolution of Coating Systems Under Four-Point Bending

The fracture and damage behaviors of ceramic coating/alloy substrate systems under four-point bending were investigated using a scanning electron microscope to observe insitu tests. Both the thin and thick coatings fractured by tensile instability at the pure bending sections, and multiple transverse cracks that were vertical to the interface occurred in the coatings. The average crack spacing was greater for the thick coatings than for the thin ones. A catastrophic failure model was developed to explain the damage evolution behavior of the coatings. The damage was found to increase sharply near the failure point

纳米固体热导行为的尺寸效应及损伤失效特征

纳米固体指至少一维尺寸在纳米尺度的固体如纳米颗粒、纳米线、纳米薄膜、以及微结构是纳米尺度的宏观材料,由于固体或微结构的尺度远远小于传统块体材料,纳米固体的许多物理和力学性能都呈现出不同于块体的尺度效应,如纳米线热导率降低、纳米薄膜弹性模量增加等。这里首先针对纳米固体热导行为的尺寸效应展开系列研究,发展了一个同时考虑声子限域和表界面效应的跨尺度模型,有效表征了纳米线、微纳米薄膜以及纳米结构涂层直径、厚度及微结构尺度依赖的热导率,模型预测与实验结果或分子动力学模拟结果一致,为设计开发新型热电材料、高效热障涂层等相关能源应用提供了理论依据。并进一步针对纳米结构陶瓷涂层在热震、弯曲等载荷下的损伤失效行为展开了实验和模拟研究,表征了纳米结构涂层损伤演化的幂率规律和尺寸依赖特征

热障涂层相关金属/陶瓷界面失效的原子尺度研究

热障涂层体系层裂往往发生在热生长氧化铝层与镍基合金粘结层之间,为了理解这一金属/陶瓷界面失效的物理机制,人们针对相关金属/陶瓷界面的断裂问题开展了大量原子尺度模拟研究[1-2]。针对Ni/Al_2O_3界面的第一原理计算显示,一定量的杂质稀土铪元素可提高该界面的黏附性能及分离功[3]。但第一原理方法受计算规模所限,不易刻划较大尺度界面的失效行为。分子动力学方法是一种选择,其中界面势函数是一难点,对系列金属/氧化镁(氧化铝),经第一原理反演得到的对势势参数已被证明可以有效刻划界面黏附性能[4-5]。因此本工作基于对势针对典型的Ag/MgO和Ni/Al_2O_3的界面失效开展了系列原子尺度研究。分子动力学模拟结果发现金属/陶瓷界面剪切本构呈周期准脆行为,剪切一个周期内界面剪应力逐渐增加达到剪切强度值后突然下降,界面剪切位移由之前缓慢连续增加突跳至下一个平衡位置,呈阶越形式,理想界面剪切是原子断键机制;对含有位错的缺陷界面,界面强度和周期位移都减小,界面位移从理想界面的阶越形式变得相对连续化,界面剪切是位错滑移主导。对理想界面拉伸情况,随厚度增加断裂灾变特征变得显著;含位错界面的强度也比理想界面降低,灾变特征弱化。通过分析相关的能量机制,揭示了微观界面失效行为的缺陷和厚度效应;结合原子结合能和位错滑移力的温度效应理论分析,以期理解宏观高温界面失效机制

Failure characterization of solid structures based on an equivalence of cohesive zone model

Cohesive zone models have been widely used to model interface crack initiation and propagation both in single-material media and bi-material systems. For single-material media with cohesive elements inserted into interface among segments, in order to ensure that the introduction of interface cohesive zone models does not affect the mechanical properties of single-material media before the softening stage of cohesive zone models, a selection criterion of stiffness of cohesive elements is proposed theoretically firstly based on the properties' equivalence. Taking the softening stage into account, the mechanical responses of the overall stress-strain relationship of single-material media, for the cases of stable increase of strain and snap-back instability of strain, are both obtained, and the related energy mechanism are investigated. For bi-material systems with cohesive elements at interface between two materials, the thickness-dependent failure characteristics of systems in uniaxial tension are found, which is attributed to the difference of the releasing rate of elastic strain energy in the materials with different thicknesses. Furthermore, as a more complex application of cohesive elements, based on the selection criterion proposed, failure behaviors of the ceramic coating/substrate systems under three-point bending are modeled by finite element method and inserting cohesive elements into the coating segments and the coating/substrate interface simultaneously. The simulation results indicate the transition of dominated failure mode from coating cracking to interface delamination with increasing coating thickness, and show faster damage of thick coating systems, agreeing with experimental results. The effects of interface strength and toughness of cohesive elements on failure are also revealed. These results can provide guidance for the application of cohesive elements, and help us better understand the overall failure behaviors of interface systems. (C) 2019 Elsevier Ltd. All rights reserved

Atomistic simulation study on the shear behavior of Ag/MgO interface

Metal-oxide composites with advanced mechanical properties play an important role in many practical applications, and failure of the metal-oxide interface is directly related to service life of related structures. In order to understand interface failure mechanism, study on atomic-scale separation of metal-oxide interface is significant. In this work, shear behaviors of Ag/MgO (0 0 1) coherent interface and semi-coherent interfaces are studied by employing molecular mechanics method, and some interesting size and defect effects are found. The simulation results show that interface shear stress and displacement appear periodic characteristics with loading. For coherent interface, the interface shear stress and displacement both increase first in each period, then the shear stress drops abruptly after reaching ideal shear strength, and the shear displacement jumps by a unit cell length. The shear strength keeps a constant for all periods. Atomistic simulations of interface systems with different thicknesses show size-independent shear strength and intrinsic interface adhesive energy, but needed loading displacement for the first jump of interface displacement is larger for the thicker systems due to the larger energy consumed by bulk materials. For both 1D and 2D semi-coherent interfaces with dislocations, the shear strength is more than one order of magnitude lower than the ideal shear strength, and the interface displacement changes more continuously with decreased period, which is attributed to different shear mechanism related to dislocation gliding. Comparing 2D semi-coherent interface with 1D case, the shear strength and energy barrier of dislocation motion are both higher due to pinning effect of dislocation intersections