DEVELOPMENT OF FRACTURE RESISTANT, MULTILAYER FILMS FOR PRECISION BALL-BEARINGS

High density, sputtered MoS2 films are being investigated as lubricants for the next generation of spacecraft bearings. Low bearing torque signatures and long life are the primary requirements of these films. Although multilayer (alternating MoS2 with metal interlayers) films have exhibited relatively low wear rates (long lives) during laboratory testing using pin on disc-type testers, similar results have not been duplicated in actual precision ball bearings. Moreover, examination of bearing ball wear tracks on a race indicates little film remains intact even after relatively short periods of bearing testing. The reason for this early loss of film appears to be associated with substrate surface roughness. Although these films worked well when used with a polished substrate, the high hardness and poor fracture toughness typical of these wear resistant films resulted in film delamination when applied to ground bearing race surfaces. In order to develop films that resist fracture associated with film ductility and substrate roughness, a specialized test that involved cycling a ball on a flat substrate was developed. Using this test combined with a scanning electron microscopy (SEM) examination, dramatic differences in film fracture and delamination were observed between lapped and ground surfaces. Moreover, film fracture and delamination could be virtually eliminated on the ground surface by increasing the thickness of the metal layer in the film.