Measurement of fracture toughness and interfacial shear strength of hard and brittle Cr coating on ductile steel substrate
Measurement of fracture toughness and interfacial shear strength of hard and brittle Cr coating on ductile steel substrate
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球墨钢基体硬脆Cr镀层断裂韧性和界面剪切强度的测定
DOI：
10.1179/174329408x282587
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发表时间：
2008-09
影响因子：
2.8
通讯作者：
Ban-Quan Yang, Kun Zhang, Guang-Nan Chen
中科院分区：
文献类型：
--
作者：
Ban-Quan Yang, Kun Zhang, Guang-Nan Chen
涂层的断裂韧性和界面结合性能是决定涂层-基体系统性能和可靠性的重要参数。本文通过拉伸试验研究了中碳钢表面硬脆铬涂层的断裂韧性和界面剪切强度。在普通中碳钢基体上电镀硬脆铬镀层，通过准静态拉伸在镀层中诱导出平行裂纹。利用光学显微镜观察了涂层在加载过程中的开裂和涂层与基体的界面脱粘情况。结果表明，涂层在临界应变下开始开裂，然后随着拉伸应变的增加，涂层单位轴向距离上的裂纹数量增加。在另一个临界应变下，涂层的裂纹数达到饱和，即在该临界应变之后，每单位轴向距离上的裂纹数变为常数。根据实验结果，可以用力学模型确定脆性涂层的断裂韧性。有趣的是，即使当整个试样在基材的极端应变下完全断裂时，其基材上的涂层的界面脱粘或屈曲也完全不存在。虽然采用相同的试验方法和脆性涂层/韧性金属基体系统，但试验结果与文献中的结果不同。结果发现，这种差异可以归因于一个重要的机制，铬涂层在钢基体上具有良好的附着力，和铬涂层和钢基体之间的极限界面剪切强度已超过钢基体的最大剪切流动强度水平。这一结果也表明，延性钢基底的最大剪切流动强度水平只能作为一个下限估计的极限剪切强度的界面。极限界面剪切强度的这种估计与文献中提出的理论分析和预测是一致的。
The fracture toughness and interfacial adhesion properties of a coating on its substrate are considered to be crucial intrinsic parameters determining performance and reliability of coating-substrate system. In this work, the fracture toughness and interfacial shear strength of a hard and brittle Cr coating on a normal medium carbon steel substrate were investigated by means of a tensile test. The normal medium carbon steel substrate electroplated with a hard and brittle Cr coating was quasi-statically stretched to induce an array of parallel cracks in the coating. An optical microscope was used to observe the cracking of the coating and the interfacial decohesion between the coating and the substrate during the loading. It was found that the cracking of the coating initiated at critical strain, and then the number of the cracks of the coating per unit axial distance increased with the increase in the tensile strain. At another critical strain, the number of the cracks of the coating became saturated, i.e. the number of cracks per unit axial distance became a constant after this critical strain. Based on the experiment result, the fracture toughness of the brittle coating can be determined using a mechanical model. Interestingly, even when the whole specimen fractured completely under an extreme strain of the substrate, the interfacial decohesion or buckling of the coating on its substrate was completely absent. The test result is different from that appeared in the literature though the identical test method and the brittle coating/ductile metal substrate system are taken. It was found that this difference can be attributed to an important mechanism that the Cr coating on the steel substrate has a good adhesion, and the ultimate interfacial shear strength between the Cr coating and the steel substrate has exceeded the maximum shear flow strength level of the steel substrate. This result also indicates that the maximum shear flow strength level of the ductile steel substrate can be only taken as a lower bound estimate on the ultimate shear strength of the interface. This estimation of the ultimate interfacial shear strength is consistent with the theoretical analysis and prediction presented in the literature.
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