Submicron friction mechanics at ambient and cryogenic temperatures

Mechanism interface mechanics play an important role in the static and dynamic dimensional stability of deployable optical instruments. Friction mechanics in deployment mechanisms has been found to be a source of kinematic indeterminacy allowing elastic energy to be stored throughout the structure. At submicron scales, microslip mechanics allow this behavior to persist well below the classical Coulomb friction limit. This paper presents the design of a cryogenic tribometer for measuring this behavior in candidate mechanism interfaces in both room temperature and cryogenic environments. Room temperature results are presented and compared to a proposed generalized microslip model form. This model form is intended to allow the parametric characterization of microslip behavior caused by smooth nonconforming contact as well as roughness-induced microslip. Spherical ball-on-flat interface geometries were used with two unlubricated material combinations: 440C stainless steel ball on a 440C stainless steel flat and a silicon nitride ball on a 440C stainless steel flat. Consistent parameters were identified for the generalized microslip model from steady cyclic shear responses for both of these interface cases. While these parameters exhibited a measurable sensitivity to normal preload levels, the model form appears to provide the necessary level of robustness. Non-ideal transient shear phenomena including rate dependence were also observed but should play only a secondary role in future modeling efforts.

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