Fabrication Of ZrO₂/Ni Functionally Graded Materials Based On Components Distinct Difference In Magnetic Susceptibilities

Functionally graded materials(FGMs)are a class of special composites with continuously changed mechanical,physical and chemical properties,which have normally been fabricated by means of vapor deposition,powder metallurgy,plasma spraying,self-propagation high temperature synthesis,electro deposition,etc.Based on the distinct difference in magnetic susceptibility between components,a new approach was developed to prepare FGMs.When a gradient magnetic field was applied on the suspension comprised of both magnetic and nonmagnetic particles,the magnetic ones were driven towards high field regions,thus a composition gradient along the field direction was formed.After subsequent drying and sintering process,FGMs were obtained.ZrO₂/Ni FGM with a continuously changing composition has been successfully fabricated using this method.Uniform and gradient magnetic fields were designed to meet the need of experiments.The viscosity variations of suspension composed of both Ni and ZrO₂ particles within and without magnetic fields were investigated.The effects of influencing factors on microstructures of ZrO₂/Ni FGMs were studied systematically.The distributions of particles under both uniform and gradient magnetic fields were studied using two-dimensional Monte Carlo simulations.The effects of field strength,field gradient,magnetic interaction and concentration of magnetic and nonmagnetic particles on distribution of particles were investigated.Flux meter,rotational viscometer,scanning electron microscope(SEM),energy dispersive spectrometer(EDS)and optical microscope(OM)were employed.The results are summarized as follows:The field dependence of viscosity of suspensions composed of both Ni and ZrO₂ particles was obtained.With increasing PVP content,the viscosity increased,then reached the maximum value at 1.8 wt.%,and finally decreased.However,the relative viscosity decreased as PVP content increased in the presence of magnetic field.Moreover,the viscosity increased rapidly as the solid content increased.When a magnetic field was applied on the suspension,the viscosity of suspension increased dramatically.Furthermore,the viscosity increased as a function of field strength and Ni content,which can be attributed mainly to the magnetization of Ni particles.Ni particles aggregated to form chain-like clusters under magnetic fields.The size and number of clusters increased as field strength and Ni content increased,thus the viscosity of suspension increased.A composition gradient of Ni was formed along the field direction in ZrO₂/Ni FGMs prepared under gradient magnetic fields.As the solid content in suspension increased,the density of FGMs increased,but the composition gradient decreased.With increasing field gradient from 0.5 to 3.0 T/m,composition gradient of Ni increased,but the microhardness along thickness direction decreased.Moreover,the migration of magnetic particles under gradient magnetic fields was not an instantaneous process but a time-dependent one.The composition gradient increased with the prolongation of holding time,and remained nearly unchanged after 15 s.However,the composition gradient decreased as Ni concentration increased from 4 to 15 wt.%,which can be attributed mainly to the formation of Ni clusters.A schematic model was proposed to explain the distribution of particles in gradient magnetic fields.Through the two-dimensional Monte Carlo simulations,the distributions of particles under uniform and gradient magnetic fields in the suspension comprised of both magnetic particles (MPs)and nonmagnetic particles(NPs)were studied.Chain-like MP clusters were formed along the field direction.With the prolongation of computing time,clusters grew gradually, and finally kept stable.The size of clusters increased with increasing interaction between MPs. However,the distribution of particles remained unchanged as the field strength varied.As the concentration of MPs increased,the distance between MPs decreased and the interaction increased,resulting in the growth of clusters.A gradient distribution of MPs was formed along the field gradient direction under gradient magnetic fields.The composition gradient increased with increasing the field gradient,however,decreased as the interaction between MPs increased,accompanied by the formation of chain-like MP clusters.Moreover,NPs were found to hinder the translation of MPs along the field direction both in uniform and gradient magnetic field.As the NP concentration increased,the translation of MPs became difficult.