Strengthening porous metal skeletons by metal deposition from a nanoparticle dispersion

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references.The accuracy of solid freeform fabrication processes such as three-dimensional printing (3DP) and selective laser sintering (SLS) must be improved for them to achieve wide application in direct production of metal parts. This work seeks to reduce sintering and deformation of porous metal skeletons during liquid-metal infiltration by reinforcing the skeletons with metal deposits. This can be accomplished by depositing a metal from a suspension of nanometer-scale iron particles. The nanoparticle deposits from the suspension concentrate in regions of high stress when the solvent is removed by drying. The particles are sintered to create a dense structure that reinforces the porous skeleton-reducing deformation and creep. Generically, this work studies a process for metal deposition from a liquid carrier with unique transport characteristics compared to traditional metal deposition processes such as plating, chemical vapor deposition, evaporation, and sputtering. This process of depositing metal from nanoparticle suspensions is studied using a commercial product of iron nanoparticles. The processed iron particle suspension contains significant carbon from the organic dispersants used to stabilize the suspension. Gas adsorption, X- ray diffraction, and SEM imaging were used to show that the carbon aids reduction of any iron oxide on heating and strongly influences the densification characteristics. The iron nanoparticles are applied to porous steel skeletons produced by sintering stainless steel powder. These are then heated to typical steel infiltration temperatures of 1284 C. The nanoparticle deposits are shown to reduce creep deflections at infiltration temperatures by up to 95% and reduce shrinkage by up to 60%.(cont.) The best results are obtained by repeating the process of applying the nanoparticles, drying the solvent, and sintering them to 700 C up to four times. The performance in magnetic materials can also enhanced by applying a magnetic field along the magnetic particles. This magnetic field concentrates the nanoparticle deposits into the contact points between the skeleton particles where they provide optimal benefit.by Nathan Brad Crane.Ph.D