Cracking in Brittle Films on Ductile Substrates

0103385雷曼尼我们目前对表面破裂现象的了解太有限，无法充分预测复杂、不均匀结构中的破裂条件。近年来在梯度结构力学建模方面的进展，以及对特定薄膜结构制备的微观和纳米尺度控制的进展，为更好地理解脆性薄膜中裂纹扩展与性能之间的关系打开了新的大门。这里提出的工作将研究表面裂纹如何依赖于1)表面结构特征，包括分级结构和相关的残余应力分布，2)衬底的弹性和塑性属性，3)膜/衬底界面的强度，以及4)膜的微结构特征。将通过沉积氮化铬(CrN和Cr2N)以及CrxCy、TixN和TixC薄膜来制备表面，这些薄膜已经在科罗拉多矿业学院进行了广泛的研究。药筒黄铜(铜-30wt.将使用锌)和镍合金作为模型衬底系统，而钢衬底将提供与技术系统的连接，并与文献中的数据进行比较。传统的层状结构和新型结构将被制造出来，它们的力学行为将通过基本断裂实验来表征。一系列全面的实验将阐明不同薄膜之间的一般行为，并将这种行为与现有的理论模型进行比较，目的是从材料行为效应中提取力学效应。这些系统固有的复杂性决定了需要用数值方法来模拟机械响应。因此，有限元分析将被用来评估残余应力状态和裂纹尖端应力场。脆性薄膜具有广泛的用途：腐蚀、辐射和热保护以及耐磨性。在高温超导体和许多微电子部件等应用中，薄膜是一种设备，其设计目的是传导电流或响应光子、电子或离子。一部电影的表现在很大程度上取决于它的结构完整性。在高温超导带中，表面裂纹会显著降低电流密度；对于涡轮发动机部件，表面裂纹可能会导致增强氧化；对于工具和模具，表面裂纹会降低耐磨性。在一些应用中，例如用于陆基发电的涡轮发动机，实际上可能需要有限的裂解量以增加薄膜的顺应性。显然，理解工程表面如何产生裂缝在技术上是极其重要的。这项拟议的工作旨在增进对具有复杂结构的硬脆薄膜断裂行为的理解，从而最终可能应用于设计新的高性能表面。

0103385ReimanisOur current understanding of surface cracking phenomena is too limited to adequately predict conditions for fracture in sophisticated, inhomogeneous structures. Recent advances in mechanics modeling of graded structures, as well as advances in the micro- and nano-scale control over the fabrication of specific film structures have opened new doors for developing a better understanding of the connection between crack growth and performance in brittle films. Work proposed here will examine how surface cracking depends on 1) surface architectural characteristics, including the gradation architecture, and the associated residual stress distributions, 2) the elastic and plastic properties of the substrate, 3) the strength of the film/substrate interface, and 4) microstructural features of the film. Surfaces will be prepared by depositing films of chromium nitride (CrN and Cr2N), which have been extensively studied at Colorado School of Mines, as well as CrxCy, TixN, and TixC. Cartridge brass (Cu-30wt. % Zn), and Ni alloys will be utilized as model substrate systems, while steel substrates will provide a connection to technological systems, offering comparison to data in the literature. Conventional layered and novel architectures will be fabricated, and their mechanical behavior will be characterized using fundamental fracture experiments. A comprehensive set of experiments will elucidate the generic behavior among the different films, and this behavior will be compared to existing theoretical models for the purpose of extracting mechanics effects from material behavior effects. The inherent complexity in these systems dictates the need for numerical methods to model the mechanical response. Thus, finite element analysis will be employed to evaluate residual stress states and crack tip stress fields. %%%Brittle films serve a wide variety of purposes: corrosion, radiation and thermal protection, and wear resistance. In applications such as high temperature superconductors and for many microelectronic components, the film is the device, engineered to conduct current or respond to photons, electrons or ions. The performance of a film depends in large part on its structural integrity. In the case of high temperature superconductor tape, a surface crack dramatically reduces the current density; in the case of turbine engine components, cracking may result in enhanced oxidation; in the case of tools and dies, surface cracking decreases the wear resistance. In some applications, such as turbine engines for land based power generation, a limited amount of cracking may actually be desired in order to increase the film compliance. Clearly, it is technologically extremely important to understand how cracking occurs in engineered surfaces. The proposed work is designed to advance understanding in the fracture behavior of hard, brittle films with complex structures so that it may ultimately be applied to design new, high performance surfaces.