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首页> 外文学位 >An Improved Approach to Fracture Toughness Assessment of Brittle Coating on Ductile Substrate Systems under Indentation.
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An Improved Approach to Fracture Toughness Assessment of Brittle Coating on Ductile Substrate Systems under Indentation.

机译：压痕下韧性基体系统脆性涂层断裂韧性评估的改进方法。

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摘要

Fracture toughness is an important material property that determines the structural integrity of a component with pre-existing or service-generated flaws. In the present research, an indentation-based method and the associated fracture mechanics model are proposed for fracture toughness assessment of brittle coating/ductile substrate systems.;A series of indentation tests are conducted on a WC/10Co/4Cr coating/1080 low carbon steel substrate specimen, which is a brittle coating on a ductile substrate system, in order to obtain the required information for validating the proposed fracture toughness model. It is found that radial/median cracks are generated beneath the indenter within the coating layer in an elliptical shape and stopped at the interface under the indentation loads being used in the present experiments. Optical microscopy is employed to examine the surface (radial) crack lengths and the focused ion beam (FIB) technique is applied to dissecting the coating/substrate specimen and thereby allows for measuring the crack penetration depth; thus the full crack profile is determined. These tests have also verified the non-linear relationship between c3/2 (where c is a radial crack length) and the applied indentation load P, which is exhibited in the proposed fracture toughness model.;The fracture toughness of the tested WC/10Co/4Cr coating/1080 steel substrate system is determined in terms of the proposed fracture toughness model, utilizing the experimental results. The fracture toughness value for this coating/substrate system is then compared to the known fracture toughness value of a similar brittle coating/ductile substrate system (WC/12Co coating/1020 low carbon steel substrate) and is found to be comparable. The proposed fracture toughness model best describes the experimental observation of cracking/fracture behavior of brittle coating/ductile substrate systems under indentation, compared with other existing models, which can be utilized to assess the fracture toughness of these coating/substrate systems. The developed approach makes a significant improvement in the existing fracture mechanics methods for fracture toughness assessment of brittle coating/ductile substrate systems.;The proposed models consider well-developed radial/median cracks generated under sharp indentation, despite that the crack formation process may have gone through crack initiation and propagation phases. For generality, the geometry of a well-developed crack is assumed to be semi-elliptical in shape. The driving force of the crack is considered to stem from the residual plastic zone expansion under the indenter, as well as the far-field Boussinesq (elastic) stress. Three well-defined configurations are studied. For the first configuration, a crack with a depth of less than 7% of the coating thickness is considered. In this case, the problem is treated as the one for the monolithic material with the coating material properties. For the second configuration, a crack that runs deeper than 7% of the coating thickness but is still within the coating layer is analyzed. In this case, the composite hardness is introduced into the analysis to account for the influence of the substrate material properties; and furthermore, an interface correction factor is proposed to take into account the presence of the coating/substrate interface and its influence on the stress intensity factor of the well-developed elliptical cracks. For the third configuration, a crack penetrating into the substrate is considered. In this case, based on the condition of deformation compatibility across the coating/substrate interface, the bulk modulus for the coating/substrate system is introduced into the analysis.
机译：断裂韧性是一种重要的材料性能，它决定具有预先存在或服务产生的缺陷的组件的结构完整性。在本研究中，提出了一种基于压痕的方法和相关的断裂力学模型，用于脆性涂层/韧性基体系统的断裂韧性评估。;在WC / 10Co / 4Cr涂层/ 1080低碳钢上进行了一系列压痕测试钢基底样品，它是在韧性基底系统上的脆性涂层，目的是获得所需的信息以验证所提出的断裂韧性模型。发现在本实验中所使用的压痕载荷下，在涂层内的压头下方以椭圆形产生了径向/中部裂纹，并在界面处停止。采用光学显微镜检查表面（径向）裂纹的长度，并采用聚焦离子束（FIB）技术对涂层/基体样品进行解剖，从而测量裂纹的渗透深度；因此确定了完整的裂纹轮廓。这些测试还验证了c3 / 2（其中c是径向裂纹长度）与所施加的压痕载荷P之间的非线性关系，该关系在建议的断裂韧性模型中表现出来。;被测WC / 10Co的断裂韧性利用实验结果，根据建议的断裂韧性模型确定了/ 4Cr涂层/ 1080钢基材体系。然后将该涂层/基体系统的断裂韧性值与类似的脆性涂层/延性基体系统（WC / 12Co涂层/ 1020低碳钢基体）的已知断裂韧性值进行比较，发现具有可比性。与其他现有模型相比，所提出的断裂韧性模型最能描述压痕下脆性涂层/延性基体系统的开裂/断裂行为的实验观察结果，而其他现有模型可用于评估这些涂层/基体系统的断裂韧性。所开发的方法大大改进了用于脆性涂层/韧性基体系统断裂韧性评估的现有断裂力学方法。;尽管裂纹形成过程可能具有经历了裂纹萌生和扩展阶段。为了普遍起见，假设发达的裂纹的几何形状为半椭圆形。裂纹的驱动力被认为是由于压头下残余的塑性区扩展以及远场的Boussinesq（弹性）应力引起的。研究了三种定义明确的配置。对于第一种配置，考虑的裂纹深度小于涂层厚度的7％。在这种情况下，该问题被视为具有涂层材料特性的整体材料的问题。对于第二种配置，分析的裂纹深于涂层厚度的7％，但仍在涂层内。在这种情况下，将复合硬度引入到分析中，以考虑基材材料性能的影响。此外，提出了一种界面校正因子，以考虑到涂层/基体界面的存在及其对发达的椭圆形裂纹的应力强度因子的影响。对于第三构造，考虑到穿透到基板中的裂缝。在这种情况下，基于跨涂层/基材界面的变形相容性的条件，将涂层/基材系统的体积模量引入分析中。

著录项

  • 作者

    Demidova, Natalia V.;

  • 作者单位

    Carleton University (Canada).;

  • 授予单位 Carleton University (Canada).;
  • 学科 Engineering Aerospace.;Engineering Mechanical.;Engineering Materials Science.;Engineering Biomedical.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 161 p.
  • 总页数 161
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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