Influences of Contact Geometric Parameters on the Fracture and Delamination Failure Behaviors of Thin Hard Films and Their Decoupling Analysis

Film fracture (cohesive failure) and interfacial delamination (adhesive failure) are two main damage modes of thin hard solid films on elastic-plastic steel substrates, and their initiation and propagation processes have complex coupling relationships. Tensile stress concentrations induced by the bending and stretching effects at film surfaces, and concentrations of tensile and shear stresses caused by the mismatch of elastoplastic deformation at the film/substrate interfaces, are the critical reasons for film fracture and interfacial delamination, respectively. Distribution and evolution behaviors of the film principal stress and the interfacial stress during spherical micro-indentations of thin hard films on elastic-plastic steel substrates were analyzed using the finite element method in this article. Influences of the contact geometric parameter t/R (the ratio of film thickness and spherical indenter radius) on the film fracture and the interfacial delamination were investigated, and the guideline of the decoupling analysis for those two failure behaviors were explored, which provided theoretical guidance for the characterizations of the cohesive and adhesive properties of film-substrate systems. Results indicated that the loading process of a micro-indentation test on a film-substrate system could be roughly divided into three main stages according to the position of the maximum plastic deformation of the substrate: film bearing stage (Stage I), film-substrate common bearing stage (Stage II), and substrate bearing stage (Stage III). The deformation states of the film were elastic smooth deformation, bending deformation, and tensile deformation, respectively. For the system with a large t/R (t/R ≥ 0.08), the main bearing stage were Stage I and Stage II, and the maximum tensile principal stress of the film was always located in the bottom bending deformation zone; For the system with a small t/R (t/R ≤ 0.01), the main bearing stage was Stage III, and the maximum tensile principal stress of the film was always located in the surface stretching deformation zone at the outer edge of the contact area; For the system with 0.02 ≤ t/R ≤ 0.067, the main bearing stage transferred from Stage II to Stage III as the indentation depth increase, and the maximum tensile principal stress of the film shifted from the bottom bending zone to the surface stretching zone at the outer edge of the contact area. The potential fracture form changed from radial cracks to ring cracks, and there was a linear corresponding relationship between the critical indentation depth as well as the critical stress of the position transferring point of the main stress (i.e. film potential fracture form) and the parameter of t/R. With the increase of t/R, the maximum normal stress at the interface both increased during the loading and unloading processes, and the possibility of I-type tensile delamination increased, while the maximum tangential stress at the interface slightly decreased. However, due to the substrate plastic deformation, maximum tangential stresses at interfaces were close to its shear yield strength of 0.6σ_ys( 1/\sqrt3 σ_ys, σ_ys is the substrate yield strength). To avoid the coupling influences of fracture and delamination, a larger t/R should be applied to evaluate the normal adhesive performance at interfaces; While to analyze the tangential adhesive properties a smaller t/R was the preference due to the relatively small impact of t/R on tangential stresses at interfaces and the high risk of film cracking caused by a larger t/R.