| With the development of science and technology, thin films deposited on substrates play an important role in many applications such as semiconductor devices, magnetic storage media, and surface coatings. However, a common failure form in their applications is buckling of thin films, resulting in interfacial delamination and fracture. The quality of interfacial adhesion of a film/substrate system is essential to the lifetime of a device. Therefore, how to quantitatively characterize the interfacial adhesion of a thin film is the key in its practical application. A number of methods have been available for determining interfacial adhesion. However, there are some limitations on the scope of application in these methods. In consideration of these disadvantages, it is necessary to seek a new method that can be applied to evaluate the interfacial adhesion of a film-substrate system.In this paper, according to the two typical research subjects of brittle film on ductile substrate、ductile film on ductile substrate, we studied the interfacial adhesion of thermal barrier coatings (TBCs) and Ni thin film on steel substrate by a compression-induced buckling delamination test. Based on the three aspects of experiment finite element simulation and theoretical analysis, the relationships between the evolution of buckling morphology and interfacial energy release rate (ERR)^ phase angle were investigated. The achieved main contents and results are listed as follows:(1) The interfacial adhesion of TBCs was investigated by a compression-induced buckling delamination test. The samples of TBCs with artificial interface defect were prepared by the air plasma spraying (APS) method. The competition mechanism between interfacial delamination and surface fracture was studied. Furthermore, the failure mechanism of TBCs under compression was obtained. A nickel foil with a thickness of 40 μm was put in the middle of bond coatings as a through-width interface defect. Because of its unique advantage, an AE system was used to continuously detect the intrinsic damage caused by microcracking in a material under various testing conditions. Based on the characteristics of cumulative AE events and in-situ and real-time monitoring of the CCD camera, the failure mechanism of buckling delamination can be divided into the following stages:the specimen with an interface defect under compression firstly buckles from substrate, and then kinked cracks appear at the crack front of the buckling portion of coatings and lead to spallation. With further increasing strain, delamination develops and suddenly expands, which leads to the complete spallation of coatings;(2) A cohesive zone finite element model was established by using the commercial finite element analysis package (ABAQUS 6.10) to simulate the process of interfacial crack initiation、propagation and delamination. Based on the above method, the interfacial adhesion energies of TBCs were obtained. According to the simulation results, it was found that with the increase of strain, interface damage initiates when the damage criterion is satisfied and interface cracks occur. The interface crack quickly increases with further increasing the strain. The crack length versus applied strain curve based on finite element simulations is plotted together with that from experiments for comparison, It is found that the interfacial adhesion energy of TBCs is in the range of 100～130 J/m2, which fits the experimental data quite well;(3) Based on the nonlinear delamination theory, the relationships between interfacial ERR, mixed mode phase angle and interfacial crack length were obtained. It was found that the interface ERR quickly increases with the half-length of the crack, and then tends to be stable. And the critical interfacial adhesion energy of delamination is obtained as 120 J/m2. With the increase of the half-length of the crack, the interfacial ERR tends to be stable value of 150 J/m2. According to the variation of phase angle with the crack half-length, the interfacial delamination process is a mixed mode crack. As delamination begins, the critical loading phase angle is-40°, the relative proportion of mode II to mode I is equal. With the increase of the half-length of the crack, the loading phase angle varies from-40° to-85°, eventually tends to be a stable value of-90°, which implies that delamination experiences almost the pure mode II. In the finite element model, the interfacial adhesion energies obtained by finite element simulations are in the range of 100-130 J/m2; while in the nonlinear delamination theoretical analysis, the critical interfacial adhesion energy of delamination is 120 J/m2. The interfacial adhesion energy obtained by theoretical analysis is in good agreement with finite element simulations;(4) The experimental study of buckling of Ni thin film on steel substrate was investigated. The Ni thin film was prepared by the the electrodepositing technology and its grain size was in the range of nano to micro scale. It was found that the surface of Ni thin film is smooth and the grain size is uniform. The sample with size of 10 × 5 ×5 mm3 was chosen for compression-induced buckling delamination tests. According to the buckling profiles, the shape of an edge flaw can be simplified as a square. Based on the statistical analysis, the length distribution of edge flaws is about a normal distribution and in general, the size of edge flaws is in the range of 400×400 to 700×700μm2. Under the external loading, the buckles initiate from edge flaws and surface morphologies exhibit symmetric, half-penny shapes;(5) Taking advantage of a virtual crack closure technique (VCCT), a process of buckling-driven delamination was simulated by a finite element method. And the evolutions of buckling morphologies、interfacial ERRs and phase angles were investigated. We found that the interface crack firstly propagates along the straight side, and then along both of the straight side and curved front with the increase of compressive strain. The interface crack becomes a triangle rather than an original square. Here it is worth noting that, based on studies on semicircular and triangular edge flaws, the shape of an edge flaw has no obvious influence on the delaminated configuration, and the interfacial delaminated configuration does not grow in a self-similar way. Furthermore, the mode Ⅱ delamination plays a dominant role in the process with a straight side whilst the curved front experiences almost the pure mode Ⅰ, and the mode Ⅱ delamination is negligible in the buckle delamination process;(6) The theoretical model on characterization of the interfacial adhesion of Ni thin film on substrate by a compression-induced buckling delamination test was established. Based on the elastic film on rigid substrate, two and three dimentional analytical models of elastic-plastic film on rigid substrate were proposed. But the above models are still unsuitable for the current case with a half-penny buckle geometry and the elastoplasticity of film and substrate, because the deformation of substrate is not considered. However, deformation of substrate cannot be negligible and its contribution to ERR should be considered. Therefore, based on finite element simulations, a numerical model was developed to evaluate the interfacial ERR and phase angle. Firstly, dimensional analysis was done by ∏ Theorem for buckling delamination problem. A positive/reverse analysis was carried out to establish the theoretical frame of numerical model, which was used to evaluate the interfacial adhesion of three dimentional elastic-plastic thin film on elastic-plastic substrate. By fitting the results collected from finite element analysis, the explicit dimensionless forms were obtained. Based on the obtained numerical model, it was found that the interfacial ERRs with film thickness of 60 μm vary in the range of 250-315 J/m2 and their corresponding phase angles are from-41 to-66°. And the interfacial ERR and phase angle obtained by the numerical models were dependent on the dimensionless critical buckling amplitude,AdCr/h, and the crack half-length, b/h, at the onset of buckle-delamination without consideration of its evolution. Finally, we discussed the variations of interfacial ERR with phase angle. It was found that the interfacial ERR has a mode dependence. Furthermore, the interfacial ERR, Γ, shows a weak dependence on the phase angle ψin the range of ψ≤60°, but the mode dependence becomes more significant at a higher phase angle (ψ≥60°). |
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