Cracking in Brittle Films on Ductile Substrates

0103385雷曼尼我们目前对表面裂纹现象的理解太有限，不能充分预测复杂、不均匀结构的断裂条件。 梯度结构的力学建模的最新进展，以及在特定的薄膜结构的制造的微米和纳米尺度的控制的进展，开辟了新的大门，开发一个更好地理解裂纹生长和性能之间的连接脆性薄膜。 本文提出的工作将研究表面开裂如何取决于1）表面结构特征，包括渐变结构，以及相关的残余应力分布，2）基底的弹性和塑性特性，3）膜/基底界面的强度，以及4）膜的微观结构特征。 将通过沉积氮化铬（CrN和Cr 2N）以及CrxCy、TixN和TixC薄膜来制备表面，科罗拉多矿业学院已对氮化铬和Cr 2N薄膜进行了广泛研究。 弹壳黄铜（Cu-30 wt. % Zn）和Ni合金将用作模型基底系统，而钢基底将提供与技术系统的连接，提供与文献中数据的比较。 传统的分层和新的架构将制作，其机械性能将使用基本断裂实验的特点。 一组全面的实验将阐明不同薄膜之间的一般行为，并将此行为与现有的理论模型进行比较，以便从材料行为效应中提取力学效应。 这些系统的固有复杂性决定了需要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。 %脆性薄膜具有多种用途：防腐蚀、防辐射、防热以及耐磨。 在诸如高温超导体和许多微电子元件的应用中，薄膜是设计用于传导电流或响应光子、电子或离子的器件。 薄膜的性能在很大程度上取决于其结构完整性。 在高温超导带材的情况下，表面裂纹会显著降低电流密度;在涡轮机发动机部件的情况下，裂纹可能导致氧化增强;在工具和模具的情况下，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机发动机，实际上可能需要有限量的破裂以增加膜柔量。 显然，了解工程表面如何发生开裂在技术上极其重要。 拟议的工作旨在提高对具有复杂结构的硬脆薄膜断裂行为的理解，以便最终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.