In situ running-in wear assessment in multi-asperity nanotribology

Wear analysis at the micro/nanoscale appears as a great challenge for MEMS/NEMS devices. At this scale classical post mortem analysis - like profilometry or AFM assessment - often failed because (i) the error in wear assessment owing to the elastic recovery is no longer negligible at this scale and (ii) the presence of nanometric tribolayer within the contact cannot be take into account when the contact is opened afterward. So, this paper deals with an in situ wear assessment based on a triboscopic approach where the final position zf of the ball is known without opening the contact because its vertical position is assessed at every instant of the process. This triboscopic assessment considers the initial approach of the surfaces z0 and gives the wear rate by taking into account the presence of any tribolayer within the contact. It requires some corrections as (i) the tilt of the sample and (ii) the initial displacement of the surfaces, which is a function of the mechanical properties of the samples. The latter are determined by using an inverse method combining spherical nanoindentation and boundary element numerical simulations, which are both described too. Validation and application of this in situ approach to the running-in wear assessment of thin soft and hard coatings currently used in MEMS manufacturing are finally presented.