Finite Element Analysis Of Failure Of Hard Coatings/substrate Systems In Static Indentation Test And Cyclic Indentation Test

Thin hard coatings have been widely used to improve the surface performances of substrates and the service life of components because of their excellent mechanical properties.However,the coatings always have different mechanical and physical properties from the substrates.When the coating/substrate system is subjected to enough external loads,the stress mismatch between the coating and the substrate will lead to the fracture and detachment of the coating,losing the protection for the substrate.This thesis systematically studies coating damage under two radius spherical indenters(1.5 mm,0.2 mm)in the static indentation test and the interfacial fatigue damage in cyclic indentation test via the Finite Element Method(FEM).The obtainded results of this thesis have certain significance for the indentation test to test the mechanical properties of coating/substrate system and provide a theoretical reference for the design of coating/substrate system.The main contents of this thesis are as follows:(1)Firstly,the coating fracture caused by a 1.5 mm spherical indenter in the static indentation test was studied.By analyzing the magnitude and location of the maximum principal stress on the coating surface and symmetry axis,the difference of the critical load for the loading-induced crack and unloading-induced crack was revealed.After that,the variation of the maximum principal stress on the coating surface and the interfacial stress after totally loaded/unloaded were studied by inserting pre-cracks with different lengths on the coating surface.At last,the extended finite element method was utilized for verification,and the results are consistent with the FEM results.(2)Then,the coating fracture caused by a 0.2 mm spherical indenter in the static indentation test was studied.By analyzing the magnitude and location of the maximum principal stress on the coating surface and symmetry axis,the difference of the critical load for the loading-induced crack and unloading-induced crack was revealed.After that,the shape of radial crack was determined by the stress intensity factor,and the difference of interfacial stress after totally loaded/unloaded was studied by inserting pre-cracks with different lengths;At last,the extended finite element method was utilized for verification,and the results are also consistent with the FEM results.(3)Finally,the mechanism of the interfacial fatigue failure of the coating/substrate system in the cyclic indentation test was revealed.The user subroutine to define a material’s mechanical behavior of the cohesive fatigue element based on the S-N curve was developed by the FORTRAN program.Then the user subroutine was implanted into ABAQUS software to numerically obtain the initiation and propagation of the interfacial fatigue crack in the cyclic indentation test.The consistence of the simulation results and experimental results verified the user subroutine well.Furthermore,the effects of the friction coefficient between the indenter and the coating and the coating thickness on the interfacial fatigue life were investigated.