Study of mechanical behavior of diamond-like carbon coatings by several instrumented tribometers

Abstract

Mechanical applications of coatings obtained by physical vapor deposition (PVD) and plasma-assisted chemical vapor deposition (PA-CVD) are numerous. In many cases, the thickness of material deposited does not exceed 5 μm. This means that when the coated element is submitted to a load, the film is not thick enough to avoid transmission of a significant part of the applied stress to the substrate. In addition, during the service life of a part used for mechanical applications, the superficial film generally encounters fatigue type solicitations. These solicitations differ radically from a regularly increasing load applied by a classical scratch test apparatus. To study the mechanical behavior of thin films, specific test rigs must be developed. This paper focuses on tests performed on diamond-like carbon (DLC) films. The first test apparatus is a scanning electron microscope (SEM) in situ tribometer that was used to test DLC produced for aerospace applications. The second apparatus is a pin-on-disc type which performs a repetitive scratch test with acoustic emission monitoring. This apparatus was designed to study the fatigue behavior of hard coatings at room atmosphere with a solicitation localized in the film and with immediate detection of damage. Using the same hardness diamond indenters as on scratch test devices, tests were performed on a pin-on-disc tribometer. During these tests, the diamond undergoes almost no damage, which means that the contact geometry remains constant (as does the contact pressure). Another advantage is that the highest shear stress is located in the layer.