| Friction and wear impose serious constraints and limitations on the performance and lifetime of micro electro-mechanical systems (MEMS) and,undoubtedly, will impose even more severe constraints on the emerging technology of nano electro-mechanical systems(NEMS) and MEMS. Reducing friction between surfaces and using materials possessing superlow friction are methods that may provide desired routes to overcome these major problems. A mechanism for superlow dry friction, which arises from the structural incompatibility of two contacting solids, was first suggested by Hirano and Shinjo in 1990. In the last two decades, researchers could realize superlubricity in ambient air, at macroscale and high speed leading application of superlubricity in MEMS becomes possible. Previous investigations on superlubricity and its mechanism of mesoscopic contacts were performed without application of normal force(i.e., normal pressure). In case of considering both normal load and friction force, previous experimental methods were merely realized at nano-scale(contact between conventional or modified tip and surface) and macro-scale (pin-on-disk or ball-on-disk wear tests). Hence, in order to insight into the mechanism of superlubricity of graphite, it is really essential to set up a method that can simultaneously measure both normal force and frictional force between two single crystalline graphite surfaces, and based on this method to research effect of normal load on superlubricity of graphite. This study not only clarifies the mechanism of superlubricity of graphite, but also contributes to a promotion of manufacturing of MEMS. In addition, application of heterostructures (i.e., the contacts of two different material's surfaces) based on structural superlubricity in MEMS is an inevitable development tendency,especially in "contact recording" technique of hard disk drive (HDD)technology. However, previous researches rarely systematically investigated on tribology of contact between superlubricity of graphite and other two-dimensional material. Hence, in order to realize contact recording technology in HDD shortly, systematic probe tribological behaviour of heterostructures based on superlubricity of graphite is indeed necessary.Based on superlubricity of graphite (self-retracting graphite mesa), the effect of normal load on superlubricity of graphite and tribological behaviour of heterotructures based on superlubricity of graphite are systematically studied. Firstly, a commercial AFM is used to equip with a high numerical aperture objective lens and a special AFM tip in order to real-time observe movement of samples in experimental process and simultaneously measure both normal load and frictional force of mesoscale graphite contacts. This method, thereby, can accurately measure frictional force between single crystalline graphite surfaces under a variation of normal force in the range up to 4 orders of magnitude (20 nN to 15 ?N). Secondly,the study reveals that superlubricity of graphite is preserved with the increase of the normal load up to the maximum magnitude allowed by the experiments (1.67 MPa). The study also further measured frictional force between two superlubric graphite surfaces in different conditions of temperature, the obtained results allow one to draw an important conclusion on the contribution of contaminants to friction in the case of superlubricity of graphite. Finally,the tribological behaviour of heterostructures based on superlubricity of graphite is revealed using the simultaneous measurement of normal load and frictional force and two methods of wear characterization. Based on these systematic investigations, the main results and achievements of the present work are as follow:(1). An accurate experimental method capable of precisely simultaneously measuring normal load and frictional force at microscale is performed In the current thesis, an accurate experimental method capable of precisely measuring both normal load and lateral force at microscale has been presented using a commercial AFM and a high numerical aperture objective lens, equipped with a special AFM cantilever. The AFM cantilever is calibrated using a magnetic levitation spring system. The method gives a foundation for the further study of the thesis in the next chapter. Besides, the study can therefore provide an effective method to real-time observe movement of samples and simultaneously measure both normal load and frictional force of two flat surfaces at microscale.(2). Evolution of frictional force between two single crystalline graphite surfaces (i.e.,superlubric graphite surfaces) under the variation of normal load is revealed Frictional force between two single crystalline graphite surfaces was measured using above method with the variation of normal force in the range up to 4 orders of magnitude (20 nN to 15 ?N). Experimental results show that superlubricity of graphite is robust against the application of normal load (up to maximum pressure allowed by the experiments, 1.67 MPa).For every single selected sample the friction force remains essentially unchanged with the increase of normal force, and after unloading the graphite flakes still exhibit SRM. However,the obtained results show that different frictional force for different sample (which has the same size of 3x3 ?m2). Hence, the reasons leading to the difference of friction forces of selected samples have been also analysed and testified. Further experimental results show that almost the same magnitude of frictional force for all selected samples has been observed at high temperature because of contaminants due to water does not exist. Obviously, frictional force in case of superlubricity of graphite is mainly come from contaminants and the latter can be eliminated by heating process. The current study therefore may pave the way for manufacturing MEMS devices.(3). Tribological behaviour of heterostructures based on superlubricity of graphite(superlubric graphite surface and two-dimension material's surface sliding on each other) has been investigatedTribology of heterostructures based on superlubricity of graphite (graphite flake and diamond-like carbon) at different conditions of temperature has been probed. The obtained results show that the friction force virtually remains unchanged after a number of sliding cycles. This allow one to predict that there is virtual zero wear in the case of SRM graphite flake slides on DLC surface. The AFM topography image and Raman spectra have been further presented to confirm that there is no wear track in both SRM graphite and DLC surfaces. Hence, the current study serves as a proof-of-concept that SRM graphite/DLC heterostructure could be used as a wear-reducing material in practical application. In addition,the present work has revealed tribological behaviour of two atomically smooth surfaces(including physical and chemical surfaces) in contact. Obtained results reveal that this physical-chemical heterostructure hardly preserves ultralow friction under the condition of ambient air.All in all, results and achievements in the current thesis provide insight into the understanding of the mechanisms of mesoscale friction in mesoscopic graphite contacts as well as tribology of heterostructures based on superlubricity of graphite. Our results therefore have provided experimental bases for utilizing superlubricity of graphite/graphene in practical micro electro-mechanical systems in further. |
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