Frictional behaviour of coated self-locking aerospace fasteners

Nuts and bolts used in aero-engines are manufactured from heat-resistant super-alloys. These materials have a high CoF, and frequently seizure occurs. In order to prevent this, a silver coating is applied to the nut threads, providing a low friction boundary at the interface. Additionally, a radial crimp is applied to the nut, in order to provide a self-locking feature preventing vibration self-loosening. In this study, the CoF of the thread contact will be investigated both during initial joint assembly, and after thermal ageing. Additionally, a finite element model will be employed to investigate the contact mechanics as a consequence of the crimp. The low CoF observed during initial assembly was found to be a consequence of shear flow of the silver coating, with an approximate doubling of this value once the coating aged. Areas of silver removal were found to be coincident with areas of high contact pressure in the joint, attributable to the crimp feature. Additionally, new alternatives coating were tested in order to identify a replacement for the electroplated silver. Through a series of analyses, similarly done for the silver coating, from a list of 19 different thin films and paints, the list is reduced to three possible films, Chromium, Titanium and Nickel-Titanium. Finally, through the FEA approach, a new self-locking design was developed, with an axial deformation instead of the radial crimp typically used. Thereafter, few nuts were manufactured, tested and compared to the elliptical, demonstrating it is a promising design with respect to the contact pressure distribution and silver removal.