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Numerical Simulation for Failure Mechanism of Hard Thin Films/Ductile Substrates System under Contact Load by Using the Cohesive Model

Author: ZhouYingQiang
Tutor: ChaiGuoZhong
School: Zhejiang University of Technology
Course: Mechanical Design and Theory
Keywords: indentation cohesive model residual stress film fracture interface delamination
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Type: Master's thesis
Year: 2011
Downloads: 10
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Abstract


Due to their high hardness, high wear resistance and contact fatigue resistance properties, the hard films have been widely used in cutting tools, hard disk protection, and mold fabrication and so on. However, the hard thin films are often subjected to high loading in the engineering applications, which lead to membrane rupture or film/substrate interface delamination, or even result in the shedding of membrane and affect their service cycle.Great efforts have been given to hard films deposited on ductile substrates by considering the simplex fraction such as thin-film ring crack, film buckling and delamination at the film-substrate interface by numerical analysis. However, several fracture modes might appear under real contact load. Furthermore, the residual stress in the films will affect the fracture form of film. In this paper, numerical simulation for failure mechanism of hard thin films/ductile substrates system is carried out by the cohesive model and finite element method. The critical indentation depth and indentation load are obtained under alterable residual stress when the breakage occurs, indicating the breakage mechanism induced by indentation. The failure mechanism map for the interface between the hard thin films and the ductile substrates is indicated when the residual stress is considered. This provides guidelines for engineering applications and the measurement of bonding behavior at the interface by using indentation. Main research efforts are shown as follows:1. The Cohesive Zone Model (CZM) is adopted for the likely fracture interface between thin films and ductile substrates, and the user element library (UEL) with axial symmetry is written based on the ABAQUS software. The pressing process of spherical indenter into thin films/ductile substrates system is simulated in order to verify the accuracy of the model.2. For strong thin films with a weak film/substrate interface, the effect of residual stress and indenter size on the indentation-induced interface delamination of film/substrate interface is analyzed and the fracture mechanism of film/substrate interface with different cohesion intensity is obtained.3. For weak thin films with a weak film/substrate interface, the effect of residual stress and indenter size on the indentation-induced circumferential cracks of the hard thin films on ductile substrates is analyzed and the fracture mechanism of thin film with different cohesion energy is obtained.4. For usual thin films/ductile substrates system, the failure mechanism map at different effect factors is supported and compared with the experiment results by using the cohesive zone model and considering the film fracture and film/substrate interface delamination.

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