Coating failure analysis based on microfracture beneath point indentation

硬质涂层越来越多地用于改善先进工程部件（例如燃气涡轮机叶片和燃烧室）的摩擦学性能和耐腐蚀性。通常，涂层失效以两种模式发生：（1）界面失效或脱层，使得涂层从基材剥落;和（2）涂层断裂。无论哪种方式，涂层都失去了其保护功能，因此部件寿命可能会显著降低。虽然许多研究都集中在涂层/基体界面脱层，拟议的研究是调查涂层断裂的基础上的力学点压痕微断裂。点压痕技术最初是为了表征材料的硬度或抗屈服性而开发的。后来，它也被发现是一种有效的手段来表征脆性材料的断裂韧性。由于大多数涂层材料是脆性的，预计这种方法可以应用于涂层强度评估。本研究将对不同厚度、不同材质的涂层进行压痕试验，结合扫描电子显微镜（SEM）观察，研究涂层的开裂行为。将发展一种涂层点压痕微断裂力学的分析方法。涂层厚度和界面对裂纹形态的影响将受到关注。分析解决方案将涉及两个步骤：（1）根据弹性半空间受法向点载荷的解确定涂层中加载压头下的应力场，因为在三个主要法向应力中，拉应力场将预先确定弹性加载涂层中脆性断裂的极限历史，和（2）根据所施加的载荷来量化微裂纹的尺度，从而推导出根据压头载荷和特征裂纹尺寸的应力强度因子的表达式。本研究的最终目标是发展一种基于压痕的方法，通过测量裂纹长度和几何形状来评估涂层的断裂韧性。

Hard coatings are increasingly used to improve the tribological performance and corrosion resistance of advanced engineering components, for example, gas turbine blades and combustion chambers. Generally, coating failures occur in two modes: (1) interfacial failure or delamination such that the coating layer spalls off the substrate; and (2) coating fracture. In either way, the coating loses its function of protection, thus the component life may be reduced significantly. While many studies have focused on coating/substrate interfacial delamination, the proposed research is to investigate coating fracture based on the mechanics of point-indentation microfracture. The point-indentation technique was originally developed to characterize a material's hardness or resistance to yield. Later, it was also found to be an effective means to characterize fracture toughness of brittle materials. Since most coating materials are brittle, it is expected that this approach can be applied for coating strength evaluation. In this research, indentation tests will be conducted on various coatings (varying thickness and materials) to investigate their cracking behaviors, combining with the Scanning Electron Microscope (SEM) observation. An analytical approach to the mechanics of point-indentation microfracture of coatings will be developed. The effects of coating thickness and the interface on the cracking pattern will be concerned. Two steps will be involved in the analytical solution: (1) determine the stress field beneath the loaded indenter in a coating in terms of the solution of an elastic half-space subjected to a normal point load, because among the three principal normal stresses, the tensile stress field will predetermine the ultimate history of brittle fracture in an elastically loaded coating, and (2) quantify the scale of the micro-cracking in terms of the applied load and thus derive an expression for the stress intensity factor in terms of the indenter load and characteristic crack dimension. The ultimate goal of this research will develop an indentation based method to evaluate the fracture toughness of coatings through crack length and geometry measurements.