Lubricative Effect Of Ferroelectric Materials Induced By External Fields

The increment of device integration in Micro/Nano Electromechanical Systems(MNES)is accompanied with the growing energy loss caused by the surface forces in devices.For example,friction between magnetic heads and disks in mechanical hard disk will increase the error rate and decrease the durability of the devices.Friction could also lead to positioning errors in guide rails of precision machine tools.Thus it is of great importance to study friction in micro/nano systems.Two kinds of materials are studied in this thesis.One is BiFeO3 epitaxial thin film,which not only has excellent ferroelectric properties,being widely studied in the field of magnetoelectric control,but also shows exceptional properties of tribology under electric bias or stress.The other one is PMNPT relaxor ferroelectric single crystal,in which the morphotropic phase boundary exists.It shows extraordinary piezoresponse and tribological properties under external stress.Here we firstly apply electric bias or stress on the above two materials through the atomic force microscope(AFM)probe to regulate the triboligical characteristics of the surface,and then observe the change of friction between the sample surface and the AFM probe by friction force microscopy(FFM).Main points of innovation are as follows:1.Improve lubrication of BiFeO3 surface by applying electric field and mechanical stress.We applied a series of electric biases and compressive stresses to BiFeO3 epitaxial film through the AFM probe,reducing the friction by up to?35%and?40%under electric fields and stresses,respectively.During the process of poling scan,a large number of charges inject into the material and accumulate near the surface,which could weaken the depolarization field near the sample surface.According to the Prandtl-Tomlinson model,the friction and the Van der Waals force between the AFM probe and sample surface are positively correlated to each other,while the Van der Waals force is related to the intensity of the depolarization field.Therefore,the weakening of the depolarization field result from the accumulated charges will lead to the decrease of friction between the AFM probe and the sample surface.Besides,BiFeO3 epitaxial film is an extraordinary piezoresponse material,where crystal lattices will distort under compressive stress,accompanied by a weakened depolarization field,giving rise to the decrease of friction between the AFM probe and the sample surface2.Improve lubrication of PMNPT single crystal surface.We applied electric bias and compressive stress to PMNPT relaxor ferroelectric single crystal with morphotropic phase boundary,reducing the friction by up to?62%and?75%under electric fields and stresses,respectively.As rhombohedral phase,tetragonal phase and variations of monoclinic phase coexist in the morphotropic phase boundary,the internal energy of rhombohedral phase in PMNPT single crystal is in a metastable state and could easily undergo phase transitions after the disturbance of stress,followed by the decrease of polarization intensity along[001]direction,weakening the surface depolarization field at the same time.As a consequence,the friction between the AFM probe and the sample surface decreases.In addition,electric bias will induce enormous lattice distortion due to the excellent piezoelectric property near the morphotropic phase boundary in PMNPT,weakening the friction on sample surface just as compressive stress does.Without the effect of substrate clamping,phase transition in PMNPT single crystal under stress is more likely to occur than that in BiFeO3 epitaxial film.The lubrication degree of PMNPT single crystal is nearly twice as much as that of BiFeO3 epitaxial filmThe lubrication behavior on BiFeO3 epitaxial film and PMNPT relaxor single crystal under external fields may be helpful for improving the performance and durability of MNES.