Study On Charactering The Interface Cohesion Between WC-10Ni Cemented Carbide Coating And Substrate By Inverse Finite Element Method

Interfacial adhesion is one of the most important mechanical properties for coating/substrate system.A variety of techniques have been developed,but none of these can be applied to all different coting/substrate systems.A new method combining a simple indentation test and an inverse finite element analysis were developed and applied to measure the interfacial properties of the WC cemented carbide coating prepared by HVOF,followed by a preliminary study on the influencing factors.The inverse FEM simulating interface indentation test is suitable for evaluating the thick coating with high interfacial bonding strength.Compared to the conventional press-in method,it can effectively obtain the interfacial strength from the viewpoint of stress.In order to adjust the O/F values and spray angles of the HVOF spraying process,nine groups of representative WC-10 Ni coatings with different thicknesses and elastic modulus were prepared on the 17-4PH substrate.Critical load of coating-substrate cracking was determined through interface indentation method according to previous research findings.The three-dimensional finite element model was built to simulate the indentation test using ANSYS commercial software.To model the failure process,cohesive zone model was applied at the interface of the coating and substrate.The experimentally determined load and initial guessed cohesion strength were used as an input for the finite element model.Adjust the guess value of cohesion strength until that the results of the FEM fit well with the experiment data.During the press-in process,a great stress concentration occurs in the indentation area.The stress changes abruptly at the interface between the coating and the substrate.The conclusion is drawn from the cohesive strength determined by the inverse finite element method and the experimentally determined interface fracture toughness: the cohesion parameter can more accurately characterize the connection strength of the coating and substrate.For the as-annealed WC-10 Ni coatings,the cohesion strength and critical load share the consistent trend.The maximum critical load is 45.15 N corresponding to the maximum cohesion strength is 72.5 MPa;the minimum critical load is 21.43 N corresponding to the minimum cohesion strength--6.18 MPa.It indicates that the dominant factor of the cohesion between the coating and the substrate without residual stress is the critical load.Excluding the effect of residual stress of the coating,the interfacial cohesion strength of 300-μm-thick coating,6.18~20.4 MPa,is smaller than that of 200-μm-thick coating,35.25~72.5 MPa,for the same elastic modulus.For the same 17-4PH substrate,the critical load decreases with the increase of elastic modulus of WC-10 Ni coating,so does the cohesion strength.But the interfacial fracture toughness shows a different trend.The introduction of residual tensile stress in the coating generally leads to the increase of cohesion strength between the coating and the substrate,and the larger the residual tensile stress,the easier the interface cracking.The key to inverse finite element method and indentation test is to obtain the critical load of interface cracking.Results indicate that it can accurately receive the interfacial cohesion strength based on the known critical load and mechanical performance.