Evaluation of the adhesion strength in DLC film-coated systems using the film-cracking technique

A theoretical analysis of the adhesion dependence of film-cracking behavior resulting from the uniaxial tensile loading of a hard film on a ductile substrate is presented. An interface shear stress that develops due to the deformation mismatch of the film and substrate induces a normal tensile stress in the film; these stresses are analyzed as a function of external strain and crack spacing, using the shear lag theory. When the film tensile stress exceeds the film fracture strength σc, film cracking occurs at an early stage of uniaxial loading; but as the loading increases, the interface shear stress increases above the critical value τc and causes the interface failure. After that, no more film cracking occurs, since stress transfer into the film is impossible due to interface damage. Thus, the interface adhesion can be estimated in terms of the shear strength from the external strain εsat and crack spacing λsat measured at the time that film cracking stops. The test results for the diamond-like carbon (DLC)/Al system show an increase of the interface shear strength by a factor of 1.6 for 30-min Ar-plasma etching, compared with no etching. It was found that as the etching time increased, εsat increased and λat decreased, i.e. the interface shear strength increased. This strong dependence of εsat and λsat on interface adhesion indicates that these experimental parameters can be used as qualitative measures of the interface adhesion in a given system.