On the use of in situ X-ray computed tomography for soft contact mechanics

The real contact area AR between two solids in contact is an outstanding quantity that controls the frictional and adhesive behavior of a contact interface. Most of the experimental methods to measure AR are based on the contrast in local optical properties of the interface, between in-and out-of-contact regions. Although those methods recently enabled various new insights into contact mechanics and tribology, they suffer from important limitations: they require that at least one of the two solids is optically transparent; they only provide information about the real interface, including AR, but not to the bulk deformation that is at the origin of AR. Here, we propose in situ X-ray Computed Tomography (XRCT) as an appealing alternative method to overcome those limitations. Indeed, it enables three-dimensional access to interfaces within potentially non-transparent contact pairs. We test the advantages and disadvantages of the method on the smooth contact between a smooth elastomer sphere in contact against a smooth rigid plate. Such a tribological system is chosen because the real contact area measurement can be benchmarked against standard optical results. We show that XRCT can, in addition, give unique access to the full surface deformation of the solids in contact, opening the way to deeper comparisons with existing models of adhesive spherical contacts.