Fractural Study Of Interfacial Adhesion Measurement With Interfacial Toughness For Coating/Substrate System

Interfacial adhesion is one of the most important mechanical properties for coating/substrate system. In the past, a number of techniques were developed and applied to different coating / substrate systems. However, none of them can produce results under all conditions and each technique has intrinsic limitations. Among those techniques, energy evaluation system is based on well-known fracture theory and can be applied to study the bond strength of thermal spray coating in porous nature on the metallic substrate. Compared to those techniques that evaluate the adhesion with critical load needed to detach the coating, the energy evaluation system has advantages of clear analysis model and application in mixed-mode loading conditions. Under the support of national science foundation (No. 50601018) and shanghai Nano-foundation (No. 0359nm005), this work systemically studies the interfacial adhesion measurement of a MoB-CoCr coating/2Cr13 steel system. The main results include:The elastic modulus of the coating is required for fractural analysis of interfacial toughness by energy evaluation system. There are lots of techniques can be used to determine the modulus. Bit different results are often obtained by different techniques for one coating. In order to obtain reasonable result, the microstructure and performance of the coating should be considered in the selection of the measurement. The work studies the applications of the nanoindentation, bending and buckling techniques for bulk materials (2Cr13 steel and Al), films ((Ti,Al)N film and Ti film) and coating (MoB-CoCr thermal spray coating). For those samples with large thickness and homogenous mechanical properties such as 2Cr13 steel and Al bulk samples, the satisfied results that agree with the results of standard tensile test can be achieved by nanoindentation and bending techniques. However the results of the nanoindentation only reflect the mechanical properties of small volume and are sensitive to the microstructure in the local indentation region. If the indentation depth is relatively large compared to coating thickness, the results are also influenced by the mechanical properties of substrate materials. For (Ti,Al)N film, reliable results without influence of the substrate can be obtained when the indentation depth is smaller than one tenth of the coating thickness. In large indentation depth condition, the determined elastic modulus is the combined modulus of coating and substrate. The results of bending test reflect the integral properties of the overall sample structure, because whole sample experiences deformation in the bending test. For thermal spray coating in porous nature, bending technique is more appropriate to determine its elastic modulus if the integral mechanical properties of the sample are the objective. The modulus of the thermal spray coating determined by bending technique is smaller than that determined by nanoindentation, because the porous nature of the thermal spray coating. For the film of rather small thickness, buckling technique exhibits excellent performance. In the work, the buckling technique successfully determined the elastic modulus of Ti film, which was 100nm in thickness and covered on 107 vulcanized rubber, as 127 GPa.Two new techniques were designed for studying the interfacial adhesion of MoB-CoCr coating on 2Cr13 steel. One is based on three-point bending test, which uses single-faced coating samples. During the bending, the crack is induced by the tensile stresses in the surface of coating, followed by extending to the interface. The other is based on four-point bending test, which uses sandwich structured samples. A notch is prefabricated in one substrate covered coating, and the bottom of the notch is near the interface. During the bending, the normal stress in the interface is arisen by the stress concentration in the notch region, and results in the initiation and extension of interfacial crack. Using finite element analysis (FEA), the cracking processes in the bending tests are simulated for calculating the stress field at the crack tip and the consumed energy. With the data extracted by the simulation, the critical energy release rate and phase angle are determined for evaluating the interfacial adhesion. Compared to other reported techniques, the cracking ways and mechanical analysis models of those two techniques are different. Especially the dissipated energy by plastic deformation in steel is considered in the calculation. Therefore our designed techniques are more suitable for the thermal spray coating/metallic substrate system. Using those two techniques, a MoB-CoCr coating/2Cr13 steel substrate system was studied. The energy release rates and phase angle are respectively determined as 73 J/m~2 and 36.8 o by 3PB test, 76 J/m~2 and 37.1 o by 4PB testDue to the mismatch physical performance between the coating and substrate such as thermal expansion coefficient, the residual stresses always exist in the coating/substrate system. The presence of the residual stresses influence the cracking and interfacial toughness. The cohesive element FEA simulation shows that the residual stresses influence the load needed to induce the initiation and extension of the crack, the length and deflection of the delaminated coating, and the energy consumed in the cracking. Therefore the fracture analysis for evaluating the interfacial adhesion based on those mechanical quantities is also influenced by the residual stresses.A new close-form solution on energy release rate was obtained for analyzing the effects of the residual stress. Compared to other reported solutions, our work exhibits two features. One is the loading spot is a circular region with certain radius. Another is the extension of the crack is under the interactive action of the external force and residual stresses. Therefore our solution has wider range of application than the point loading model and is more suitable for studying the condition that the coating is in presence of residual stresses than free residual stresses model. Numerical analysis using the solution and FEA model shows that the influences of the residual stresses on the interfacial toughness present non-linear features and the degree to which the residual stresses influence the energy release rate varies with the geometry of the loading spot and interfacial crack.