In situ tensile tests in SEM of sputtered CNx films deposited on Ti6Al4V substrate: effect of film thickness and plasma surface pretreatment

A very low tensile strain rate was chosen (0.05 mm/min) in order to follow in situ the deformation and damaging phenomena that could appear during the test. The films revealed a brittle behaviour: straight lines of fracture perpendicular to the loading axis systematically appeared. However, the total strain amplitude at which the earlier traces of damage in the film were detected greatly differed according to its thickness, or to the surface pretreatment. The characteristics of the fracture process were also highly influenced by these parameters: length and distribution of cracks, average spacing between cracks at saturation, relationship with the local plasticity activity in the substrate, etc. Blistering and peeling off the film quasi-systematically followed the early stage of cracking often for the highest strain amplitudes. Finally, it was shown that the fracture resistance is better for thinnest films and that the nitriding of the upper surface layers of the substrate by PIII improves the adherence of films.