Nanocomposites in which the constituents are mixed on a nanorneter scale can
provide important advantages in the Selective Laser Sintering (SLS) and Selective Laser
Reactive Sintering (SLRS) processes. The larger surface area and grain boundaries in the
nanocolnposites compared to that in the conventional microcomposites are expected to
enhance the solid state diffusion during laser irradiation as well as during any other
subsequent processes. Our strategy is to design and develop nanocomposites in which one
nanosize cOlnponent has a lower melting point than the other nanosize component, either of
which can serve as the matrix phase. The nanoscale dispersion of the low melting
component can aid the sintering process during SLS or SLRS. Nanocomposite powders of
AI203-COOx, Ab03-NiO, A1203-CO and A1203-Ni have been synthesized by sol-gel
processing and are evaluated by SLS.Mechanical Engineerin

Birmingham, B.R.

Chi, F.

Johnson, F.

Manthiram, A.

Marcus, H.L.

English

Texas ScholarWorks

DEVELOPMENT OF NANOCOMPOSITES FORSOLID FREEFORM FABRICATIONA. Manthirarn, F. Chi,L. F.Johnson, B. R.Birrningharn andH.L. MarcusCenter for Materials Science and Engineering, ETC 9.104The University of Texas at AustinAustin, TX 78712ABSTRACTNanocomposites in which the constituents are mixed on a nanorneter scale canprovide important advantages in the Selective Laser Sintering (SLS) and Selective LaserReactive Sintering (SLRS) processes. The larger surface area and grain boundaries in thenanocolnposites compared to that in the conventional microcomposites are expected toenhance the solid state diffusion during laser irradiation as well as during any othersubsequent processes. Our strategy is to design and develop nanocomposites in which onenanosize cOlnponent has a lower melting point than the other nanosize component, either ofwhich can serve as the matrix phase. The nanoscale dispersion of the low meltingcomponent can aid the sintering process during SLS or SLRS. Nanocomposite powders ofAI203-COOx, Ab03-NiO, A1203-CO and A1203-Ni have been synthesized by sol-gelprocessing and are evaluated by SLS.INTRODUCTIONNanostructured materials are characterized by ultrasmall building blocks whosesizes are of the order of 1-100 nanometers. These materials offer potentially useful andunusual combinations of mechanical, electrical, magnetic and optical properties that areotherwise unachievable in conventional materials [1]. The properties are controlled by acomplex interplay among the building blocks and the interfaces between them that aredetermined primarily by the synthesis and processing conditions. Nanostructured materialspromise to advance a range of technologies and there is intensive worldwide activity onthese materials both from scientific and technological points of view.While the terms "nanocrystalline" and "nanophase" materials refer generally tosingle phase solids of nanometer size, the term "nanocomposite" refers to multiphase solidsof nanometer size [2]. The solid phases in a nanocomposite could be of different nature.They could be metals, inorganic ceramics or polymers or combinations thereof but allmixed on a truly nanometer scale. The phases could be amorphous, semicrystalline orcrystalline.Nanocomposites exhibit ultraheterogeneity or nanoheterogeneity, which offerseveral processing advantages. Because the different phases are mixed on a nanoscale, thediffusion length for solid-solid reaction is much smaller than that in conventional materials.The larger surface and grain boundary energies associated with nanocomposites provide thedriving force for enhanced reactions. These factors reduce the reaction temperature as wellas time. Nanocomposites have been shown to sinter close to theoretical densities in severalsystems such as Ab03-Si02, Ab03-Ti02, Si02-MgO, etc. [3]. Nanocomposite gels storemuch higher metastable energy than homogeneous gels, and this extra energy enhances64densification. In addition, nanocomposites seem to undergo crystallization anddensification silnultaneously unlike homogeneous gels. Homogeneous gels, due to theiratomic scale mixing, often undergo crystallization before densification occurs which eitherleads to lower densities or necessitates higher temperatures for sintering. It is clear thatnanocomposites have the potential to offer lower processing temperatures and flaw-freeparts with higher densities.Our objective is to design and develop nanocomposites that can optimize SelectiveLaser Sintering (SLS) and selective Laser Reactive sintering (SLRS). Nanocomposites canbe synthesized by sol-gel processing or other "soft chemistry" techniques. Although bothsol-gel processing and solid freeform fabrication are well known, the idea of utilizing sol-gel derived nanocomposite powders that are specifically designed and synthesized tooptimize SLS or SLRS have not been exploited. Our strategy is to select ceramic-ceramicor ceralnic-metal nanocomposite systems in such a way that one nanosize component has alower melting temperature than the other nanophase component, either of which can serveas the Inatrix phase. The objective is to achieve sintering aid from the low melting phaseduring SFF processing. Because the low melting component is dispersed on a nanoscale inthe matrix phase, it is expected to aid the sintering process, during SFF or any othersecondary processing, in a much better way than the mechanically mixed microcompositepowders. By proper selection of composite systems and control of agglomeration duringsol-gel processing, the ability to achieve full density parts can be enhanced. Wepresent in this paper our preliminary results with A1203-CoOx, A1203-NiO, A1203-Co andA1203-Ni nanocomposites. CoO, NiO, Co and Ni have melting points of respectively,1795, 1984, 1495 and 1455 °C, which are lower than that of Al203 (2050 °C).EXPERIMENTALThe ceramic-ceramic nanocomposite powders of A1203-CoOx, and A1203-NiOwere synthesized by a sol-gel method [4]. The steps involved in the synthesis of Al203-coax are summarized in Fig. 1. To an aqueous solution of CO(N03)2. 6H20 maintainedat about 80°C, required amount of a solution of aluminum tri-sec-butoxide in iso-propylalcohol was added under constant stirring. Aluminum-tri-sec-butoxide hydrolyzes duringthis process to give large suspended particles of Al(OHh, which was then dissolved byadding nitric acid to obtain a homogeneous sol of A13+ and C02+. The sol was heatedgently on a hot plate to form a gel, which was then dried rapidly on the hot plate. The drygel was then heated in a muffle furnace to decompose the nitrate and hydroxide, whichresulted in a mixture of A1203-C0304. A similar procedure was adopted to obtain A1203-NiO also. The A1203-C0304, and A1203-NiO powders were also fired at differenttemperatures ranging from 300-1250 °c to evaluate the reaction process. The ceramic-ceramic nanocomposites so obtained were reduced in a stream of hydrogen at 400-1100 0Cto obtain the ceramic-lnetal nanocomposites Al203-Co, and Al203-Ni.The composite powders were characterized by x-ray diffraction, electronmicroscopy and density Ineasurements. Since the large surface area associated with thenanosize powders lead to an adsorption of water, C02 etc. on the surface, the powderswere baked at about 200°C for about 24 h before scanning with the laser beam. Singlelayer tests were performed with various power levels and scan speeds. The specimensobtained were characterized by x-ray diffraction and microscopy.65tearious temperatureIC (N03h. 6H20 in watertirring at 80 OC0Mixed under sAl(OHh precipitate+Cobalt nitrate solutionPeptization withHN03 at 80 °cAl3+ + C02+ solDry on a hot-pIaI Dry gel ICalcination at v~ Al20 3 + C0304 powder mixture II Al(OC4H9h in isopropyl alcohol IFig.1 Synthesis scheme for Al203-C0304 nanocomposite powderRESULTS AND DISCUSSIONPowder synthesis and characterization:The phase identification by x-ray of a nominal composition containing an atom ratioof Al:Co = 2: 1 is summarized in Table 1. It can be seen that C0304 is formed during theinitial stages of drying on the hot plate. Although the exact temperature of hot-plate wasnot measured, it was presumed that C0304 formation can occur at temperatures as low as2000C because firing of pure Co(N03h.6H20 at 200°C for 20 h gives C0304. Up toabout 4500C, mostly the reflections of C0304 are discernible in the x-ray pattern.Aluminum oxide or AlOOH present are very small in particle size and could not be detectedby x-ray diffraction. In the region 580-900 °C, aluminum oxide was found to exist asy-A1203 having a cation-deficient spinel structure. The presence of y-A1203 is, forexample, seen in the x-ray pattern (Fig. 2) of a powder mixture fired first at 580°C in airand then at 700 0C in hydrogen atmosphere. A small reflection corresponding to y-A1203is seen around 28 = 67 ° among strong reflections of Co metal. Above 1000 °C, anyaluminum oxide present is in the form of a-Al203 having the corundum structure.66Table I Phase identification by x-ray diffraction of the A1203-Co304 nanocomposites afterheating at various temperatures for a constant time of 20 hTemperature(OC)Dried on a hot-plate30045058065070080090010001250Phase identificationC0304C0304C0304C0304 + y-Ah0 3C0304 + y-Ah0 3C0304 + y-Ah0 3C0304 + y-A1203 + CoAh04C0304 + y-Al203 + CoAl20 4CoAl204 + a-Al203CoAl20 4ColorBlackBlackBlackBlackBlackBlackDark blueDark blueDark blueBluer------------------Co-----,•Co•AI203•Co•78 74 70 66 62 58 54 50 46 42 382-Theta in degreeFig. 2 X-ray diffraction pattern of 75 mole % Al203 - 25 mole % C0304 (Al : Co ratio=2:1) nanocomposite after firing first at 580 OC in air and then at 700 °C in Hz for 20 hThe oxides of cobalt and aluminum can react to give the spinel oxide CoA1204,which is blue in color. The change in color from black to blue around 800 °C (Table 1)seems to suggest that the formation of CoA1204 occurs around this temperature.However, the formation of CoAl204 spinel can start much below 800 °C and we cannotrule out its presence below 800 °C from x-ray data since both C0304 and CoAl204 havespinel structure with close lattice parameters. The broad nature of the reflections at lowertemperatures and their overlap make it extremely difficult to determine the exact temperature67at which CoAh04 formation begins. Nevertheless, we could clearly see the formation ofthe spinel NiAh04 in the analogous system AIZ03-NiO at temperatures as low as 320 °C(Fig. 3). Formatiqn ofspinel phases around 300 °C has also been shownin several othersystems such as NiMnz04 [5]. Furthermore, formation ofCoAlzQ4hasbeensuggestedtooccur around 500 QC frorn an observation of a decline in the catalytic activity of C0304supported on "/-AlzQ3 [6]. The catalytic activity in this system is due to. C0304 and thedegradation of catalytic activity corresponds to a conversion ofC0304 to CoA1204. So, itis likely that the formation of the CoAlz04 spinel occurs at temperatures below 500 OC.A. !'I iAI204• NiO1000 C.........:::;;...-<70 62 54 46 38 302·Theta in degreesFig. 3 X-ray diffraction pattern of 50 mole % Al203 - 50 mole % NiO (Al : Ni ratio= 2: 1) nanocomposite after firing at various temperatures for about 20 h6850 nmFig. 4 TEM micrograph of 75 mole % A1203 - 25 mol % C0304 (Co: Al ratio = 1:2)nanocomposite after firing at 580°C in air for 20 hThe TEM micrograph of the A1203-C0304 nanocomposite powder after heating at580°C in air for 20 h is shown in Fig. 4. The large particles are about 50 nm in size andwere presumed to be C0304 since they were shown to be rich in Co by EDS analysis. Thesmall particlesarelessthan 10nm and were shown to be rich in Al by EDS analysis. Theseobservations are consistelltwith the x-ray results. The TEM image of thenanocompositepowder after heating at 700 DC for 20 h is shownin Fig. 5. The particles have grownsignificantly toabout150nmduring this process. However, the rise in firing temperaturedid not have any significant influence on the size and distribution of the agglomerates. AnSEM micrograph of the powder heated at 700 DC is shown in Fig. 6, which indicates anagglomerate size of about 100 Jlm.Selective laser sintering:Preliminary SLS experiments with the A1203-C0304 nanocomposites indicatedblowing up ofthe.powder under.the laser beam. A.low powder density of about 20 %together with theadsorbedmoisture<seem to cause this problem. The powder was thenball-milled for about 30 min and subjected to SLSunderanin situ baking around 200 DC.This Inodificationalloweduse of high enough laser power without the powder beingblown. Duringthisprocess,the color ofthe powder changedfrom black to blue indicatingthe reaction ofA1203andC0304underthe laserbealntofonn the spinel oxide CoA1204.A top view after SLSof.a 83.2 •• mole% .. Al203-16.8mol}%Co304 •• nanocomposite isshown in Fig. 7. •.. The corresponding SEM micrograph of the cross section is shown inFig. 8. Althoughthe>powdersinterslocally during<theSLS process to give relativelyhigher density, large cracks were found to occur. The low density of the initial powderwith large porosity was suspected to cause this problem. Similar results were alsoobtained with Al203-NiO.6950 nmFig. 5 TEM micrograph of 75 mole % Ah03 - 25 mole % C0304 (Co: Al ratio =1:2) nanocomposite after firing at 700°C in air for 20 hFig. 6 SEM micrograph of75 mole % Al203 mole % C0304(CO : Al ratio =1:2) nanocomposite after firing at 700°C in air for 20 h707 Photograph of the top view of 83.2 mole % Ab03 - 16.8 mole % C0304nanocomposite after firing at 580°C in air for 20 h and subjecting to SLS.10m Top surface of the layer~Fig. 8 SEM micrograph of 83.21TIole % - 16.8 mole % C0304nanocomposite after firing at 580°C in air for 20 h and subjecting to SLS.71In order to circumvent the above difficulties, strategies to the density of theinitial powders without sacrificing much of the nanoscale mixing were pursued. One wayto achieve higher densities in the powder is to annealthe.nanocornpositesatrelativelyhigher telnperatures (.... 1000 0 C). However, such an annealing around 1000 °c of theA1203-C0304 nanocomposite will result in the formation of the spinel oxide CoAb04,which will in essence consume the low melting component C0304 and jeopardize ourapproach to SLS. Therefore, to demonstrate the feasibility of our objective, we decided topursue the ceramic-metal nanocomposite Ab03-CO, which will not undergo any chemicalreaction during the preannealing of the powder at higher temperatures. Accordingly,A1203-CO nanocomposites were first obtained by reducing the Ab03-C0304 oxidenanocomposite in hydrogen around 1000 oC. The powder was then annealed in N2 atm at1350 °c for 8 h to increase the density, and reduced again in hydrogen around 1000 °c toconvert back any cobalt oxide that might have formed by reaction with the residual oxygenduring N2 annealing. The resulting Al203-Co powder was found to have a density ofabout 42 % and sinter under the laser beam. The single layer test coupons appear to havebetter structural integrity with much reduced macrocrack formation. An SEM photographof the powder after SLS is shown in Fig. 9.9 SEM micrograph of 50 mole % Al203 - 50 mole % Co nanocomposite afterfiring at 1350 °C in N2 for 8 h, 1000 0C in for 8 h and subjecting to72CONCLUSIONSNanocolnposite powders of Ah03-COOx, Ah03-NiO, Ah03-CO and Ah03-Nihave been successfully synthesized by sol-gel processing. The powders have beeninvestigated by SLS. Low densities (about 20 %) of the initial powders not only lead to aformation of macrocracks during SLS, but also prevent use of high enough laser powerthat can cause sintering. Increasing the initial powder densities to about 40 % seems toavoid this difficulty significantly and give structurally sound specimens. Further increasein the densities of the initial powders promises to enhance the success of our strategy, andsuch experiments are cunently being pursued.ACKNOWLEDGEMENTFinancial support by the DARPA-ONR grant No. N00014-92-J-1394 and ONRgrant No. N00014-92-J-1514 is gratefully acknowledged.REFERENCES1. R. P. Andres et aI, 1. Mater. Res. 4, 704 (1989).2. S. Komarneni, 1. Mater. Chern. 2, 1219 (1992).3. A. M. Kazakos, S. Komarneni and R. Roy, J. Mater. Res. 5, 1095 (1990).4. E. Breval, Z. Deng, S. Chiou and C. G. Pantano, J. Mater. Sci. 27, 1464 (1992).5. X. X. Tang, A. Manthiram and J. B. Goodenough, J. Less-Common Metals 156, 357(1989).6. E. Garbowski et aI, Appl. Catalysis, 64, 209 (1990).73

Development of Nanocomposites for Solid Freeform Fabrication

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Development of Nanocomposites for Solid Freeform Fabrication

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