RESUMO
Nanoscale friction under different electronic states and the corresponding friction controlling methods are both scientifically interesting and technologically important. However, friction measurements under electrical modulation are severely hampered by electrostatic forces induced by the charge-trapping effect. Therefore, in this study, we developed a new modulation method free from the charge-trapping effect through electron beam radiation; this method successfully modulated the friction between few-layer MoS2 and the silicon tip on atomic force microscopy. Friction on monolayer MoS2 increased under electron beam radiation. Strong correlations between the accelerating voltage, beam current, and friction force were found, and constant adhesion force demonstrate that the influence of static electricity was eliminated in this method. Excited electron states caused by electron injection could be possible mechanisms for friction modulation. However, the electron beam radiation had a negligible influence on the friction of bilayer MoS2. This study is the first of its kind, revealing the effect of electron beam radiation and electronic states on friction, which is important for the development of tribological theories and nanoelectromechanical systems, and offers a new electrical modulation method for friction tuning.
RESUMO
Nanoscale friction on two-dimensional (2D) materials is closely associated with their mechanical, electronic and photonic properties, which can be modulated through changing thickness. Here, we investigated the thickness dependent friction on few-layer MoS2, WS2, and WSe2 using atomic force microscope at ambient condition and found two different behavior. When a sharp tip was used, the regular behavior of decreasing friction with increasing thickness was reproduced. However, when a pre-worn and flat-ended tip was used, we observed an abnormal trend: on WS2 and WSe2, friction increased monotonically with thickness, while for MoS2, friction decreased from monolayer to bilayer and then subsequently increased with thickness. As suggested by the density functional theory calculation, we hypothesize that the overall frictional behavior is a competition between the puckering effect and the intrinsic energy corrugation within the compressive region. By varying the relative strength of the puckering effect via changing the tip shape, the dependence of friction on sample thickness can be tuned. Our results also suggest a potential means to measure intrinsic frictional properties of 2D materials with minimum impact from puckering.
RESUMO
It is known that the anterior cruciate ligament (ACL) plays a role in providing joint stabilities under tibial varus/valgus torques and the material behavior of the ACL has changed with ageing. However, the effect of this variation of the ACL material property on joint kinematics and biomechanics under tibial varus/valgus torques has still not been clarified.In this paper, three finite element (FE) models of an intact tibiofemoral joint were reconstructed with different ACL material properties, corresponding to the ACL on the younger, middle and older ages, respectively. The joint kinematics, the stress distribution and resultant force of the ACL were obtained under a tibial varus or valgus torque load. It was found that the variation in the ACL material property would result in great changes in some joint displacements (i.e., the tibial anterior translation and external rotation). The maximal stress value in the ACL had also altered while the stress distribution did not varied obviously. The great change in the tibial anterior translation illustrated that ACL played an important role against varus/valgus torques by controlling the coupled tibial anterior translation//external rotation rather than the corresponding varus/valgus rotation.