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Graphene on a stainless steel substrate will appear as localized bulges, and these bulges will lead to large fluctuations in friction force, which is the key to influencing the lubrication efficiency of graphene. Here, the effect of normal load on the nanofriction behavior of the bulged graphene on a stainless steel substrate is investigated. In the normal load study interval from 20 to 110 nN, an increment of 10 nN of normal load can increase friction force the more the normal load increases, and the two are not linearly correlated. The friction force in the graphene bulge region consists of two parts: the first part originates from the interaction of the graphene material itself in contact with the tip, and the second part originates from the resistance due to the pile-up of graphene deformation in front of the tip. The nonlinear correlation between the normal load and friction force is mainly related to the pile-up effect of bulged graphene. In addition, the friction force increase due to the graphene bulge region needs to go through a process, which is divided into two stages. The first stage is the increase in friction force due to the tip starting to contact the bulged graphene and the contact area increasing. The second stage is the increase in friction force due to the graphene regulating its configuration and repeatedly interacting with the tip. The normal load is linearly related to the first stage but not linearly related to the second stage. The nonlinear correlation between the normal load and friction force is mainly related to the second stage of friction force rise. Therefore, the change of normal load affects the stacking effect and the ability to regulate the configuration of bulged graphene, which leads to a nonlinear increase of friction force. This contributes to the application of graphene for nanolubrication of stainless-steel components in MEMS.
Guo et al. (Wed,) studied this question.