Characterization Of Young’s Modulus And Interfacial Mechanical Properties Of Silicon Carbide Nanowires

Silicon carbide(SiC)nano materials have great application prospects in the structural and coating materials of micro/nano electromechanical systems(MEMS/NEMS)due to their excellent mechanical properties,high thermal stability and chemical stability.The mechanical properties such as elastic modulus(E)and fracture strength of SiC plays an important role in the design,sensitivity and reliability of MEMS/NEMS devices based on SiC nanowire devices.However,the study of the interfacial mechanical behaviors between two individual nanowires/nanotubes such as shearing and peeling are not noly very important for understanding of the attachement/detachement mechanism of biological adhesive system,but also very crucial to the rational design of based on one-dimensional array devices.The main contents of the thesis are as follows:(1)Young’s modulus of Fe-catalyzed SiC nanowires have been characterized over the temperature range of 300-575 K by a laserDoppler vibration method.The SiC nanowire has a single-crystalline cubic(3C)structure,grows along the[111]direction,and exhibits various crosssectional profiles including circle,rectangle,hexagon,ellipse,trapezoid,and triangle.In the test,the cross-section of each nanowire specimen is examined by scanning electron microscopy(SEM),which successfully eliminates one of the most important uncertainty sources in previous nanomechanical measurements.It is shown that,with the decrease of effective diameters from 200 to 55 nm,the room-temperature Young’s moduli of nanowires decrease from～550 GPa(the bulk value)to～460 GPa,i.e.a decrease of～16%,while the temperature coefficients of Young’s modulus increase from-47.4±1.7 ppm/K(the bulk value)to～78.1±5.9 ppm/K,i.e.an increase of～65%.(2)A new technique involving optical microscope(OM)-based nanomanipulation that permits optical observation of the shearing process between cantilevered nanowires in air have been presented.Realtime observation of the deflection shape of nanowires during shearing under controlled environmental conditions enables their shear stress evolution to be evaluated with respect to relative humidity(RH).The average static and kinetic shear strengths,τs=7.1±0.8 MPa and τ_k=5.2±1.1 MPa,are found to be insensitive to the RH within the range of 40%to 60%.When the RH increased to 74%,the shear strength values rapidly increase to τs=18.7±3.0 MPa and τ_k=14.7±1.8 MPa.When the RH decreases to 20%,a dramatic increase in the static and kinetic shear strengths is observed:τs=64.4±7.2 MPa and τ_k=49.6±5.8 MPa.Water films absorbed on the surface of nanowire within the RH range of 40-60%can lubricate their frictional interface and thus dramatically decrease the shear strength.With a high RH of above 60%,the viscous effects of a water meniscus condensed around the contact point between two shearing nanowires can significantly enhance the shear strength as well as dampen stick-slip behavior.The complete detachment of two nanowire is then controlled by the rupture of a meniscus bridge,leading to a high ultimate shear strength value of several tens of MPa.(3)A new experimental method which can visually characterize the dynamic interfacial peeling behavior between two parallel attached fibers/nanowires/nanotubes under an OM in atmospheric environment have been presented.The interfacial peeling behavior of two parallel contact SiC nanowire cantilevers was observed in real time within the range of 40-80%relative humidity.The adhesion energy between two SiC nanowires are calculated by Euler-Bernoulli beam theory.It is found that,the interfacial adhesion energy between two SiC nanowires is 0.45 J/m~2 at 40-50%relative humidity;exhibited relatively large adhesion energy of 0.64 J/m~2 at 50-65%relative humidity;Gradually reduces to 0.18 J/m~2 at 65-80%relative humdity.During the peeling process,discontinues transition from tensile peeling(S shape contact)to shear peeling(arc shape contact)is observed,and complex contact modes between nanowires,such as S-shape,arc shape intersection,arc shape tip-side,arc shape tip-tip,etc.are also examined,and verified by COMSOL multiphysics software.Ambient humidity will significantly change the contact modes between nanowires.The final separation of two parallel bonded nanowires is often determined by shear friction at medium humidity(RH<65%),normal adhesion force(capillary force)at high humidity(RH>65%).