In situ tensile tests in SEM of sputtered CN x films deposited on Ti 6Al 4V substrate: effect of film thickness and plasma surface pretreatment

Abstract

To evaluate the mechanical behaviour of CN x thin films deposited on a Ti 6Al 4V titanium alloy by RF magnetron sputtering, in situ tensile tests in a scanning electron microscope were performed. Sputtered films were deposited on one side of Ti 6Al 4V samples using a conventional magnetron sputtering system with a graphite target and Ar/N 2 plasma. The amount of nitrogen in film was about 30 at.%. Three film thicknesses (100, 300, and 800 nm) were tested, and different surface pretreatments before deposition were applied in order to improve film adhesion: inductive plasma generated by an additional RF antenna (ionized magnetron sputtering, or IMS) and plasma immersion ion implantation (PIII). 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.