Coating failure analysis based on microfracture beneath point indentation

硬质涂层被越来越多地用于改善先进工程部件的摩擦学性能和耐腐蚀性，例如燃气轮机叶片和燃烧室。通常，涂层失效有两种模式：(1)界面失效或分层，导致涂层脱落；(2)涂层断裂。无论哪种方式，涂层都会失去其保护功能，因此可能会显著降低部件的使用寿命。虽然许多研究都集中在涂层/基材界面的分层上，但本文的研究是基于点压痕微断裂的机理来研究涂层的断裂。点压痕技术最初是用来表征材料的硬度或抗屈服性的。后来，它也被发现是表征脆性材料断裂韧性的一种有效手段。由于大多数涂层材料是脆性的，因此该方法有望应用于涂层强度的评估。在本研究中，将对不同厚度和材料的涂层进行压痕试验，结合扫描电子显微镜(SEM)的观察来研究它们的开裂行为。提出了一种分析涂层点压痕微断裂机理的方法。考虑了涂层厚度和界面对裂纹形态的影响。解析解包括两个步骤：(1)根据受法向点载荷作用的弹性半空间的解来确定涂层中加载压头下的应力场，因为在三个主法向应力中，拉伸应力场将预先确定弹性加载涂层中脆性断裂的最终历史；(2)根据施加的载荷来量化微裂纹的规模，从而推导出以压头载荷和特征裂纹尺寸表示的应力强度因子的表达式。这项研究的最终目标是开发一种基于压痕的方法，通过测量裂纹长度和几何尺寸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.