Experimental Investigation of Friction Fundamentals at the Microscale

摘要

Friction is a complex phenomenon which originates from the resistance to motion experienced when contacting surfaces slide against each other. Friction exists in every mechanical system and causes significant loss of energy. Yet, there is a lack of complete understanding of the nature and behavior of friction. The three main objectives of this study were: a) to experimentally investigate the friction behavior during the transition of the contact from pre-sliding into full sliding for a sphere-on-flat contact, b) to visualize and measure the adhesion and friction forces between a steel ball and a sapphire window, c) to examine methods of reducing friction and wear between two surfaces in relative motion through the use of solid submicrometer particles and dry solid-state lubricant. Two different test rigs have been developed and used to study friction and investigate the correlation between adhesion and friction forces. A micro optical friction (MOF) apparatus was designed and developed to conduct dry sliding friction experiments and to allow for in situ visualization of the contact area for a sphere-on-flat configuration with a normal load in the range of 0.1 to 2.5 N. An optical micro apparatus (OMA) was constructed and used to investigate, visualize and measure the adhesion and friction forces between a 1 mm diameter steel ball and sapphire window with a normal load in the range of 1 to 200 mN. The OMA utilizes a precision piezo z-stage actuator that can operate at extremely low velocities for careful observation of the contact evolution during contact and separation. The study then focused on improving the tribological performance of lubricating oils by using submicrometer carbon spheres as an efficient oil additive. The submicron spheres significantly reduced the friction and wear losses in the contact. This reduction is attributed to the perfectly spherical shaped and ultrasmooth surface of carbon spheres filling the gap between surfaces and acting as submicron scale ball bearings. Furthermore, a novel graphene-zinc oxide composite film was created and studied as a solid-state lubricant for friction and wear reduction under extreme load conditions. The liquid-free composite is made from a slurry of graphene, zinc oxide, and polyvinylidene difluoride spin-coated onto a stainless steel substrate. Enhanced tribological performance was measured under ambient conditions using a ball-on-disk tribometer with contact pressures up to 1.02 GPa and sliding distances up to 450 m. The graphene-rich lubricant demonstrates substantial friction and wear reduction (ca. 90 %) compared to unlubricated sliding.