Manufacturing of ceramic parts was achieved by selective laser treatment of a
preceramic polymer (polysiloxane) loaded with ceramic filler powder (alumina). Thin layers
of polymer/filler powder mixture were sequentially cured with a CO2-laser (λ=10.6 µm)
thereby generating the geometrical shape of the part. Subsequently, the cured thermoset part
was annealed in nitrogen atmosphere at 600 to 1000 °C to convert the compact into a Si-OC/Al2O3 micro-composite material. Dimensional changes upon pyrolysis (∆l/l0 ≈ 3 %) can be
controlled by adjusting the polymer-to-filler ratio and the heat treatment conditions. The new
process is called Selective Laser Curing (SLC).Financial support of Fonds der Chemischen Industrie and Deutsche
Forschungsgemeinschaft is gratefully acknowledged.Mechanical Engineerin

Buhler, P.

Greil, P.

Niebling, F.

Otto, A.

Sindelar, R.

English

Texas ScholarWorks

  Solid freeform fabrication of ceramic parts from filler loaded preceramic polymers  R. Sindelar*, P. Buhler*, F. Niebling**, A. Otto**, P. Greil*  * University of Erlangen-Nuremberg, Department of Materials Science ** University of Erlangen-Nuremberg, Department of Manufacturing Technology Erlangen, Germany  Abstract Manufacturing of ceramic parts was achieved by selective laser treatment of a preceramic polymer (polysiloxane) loaded with ceramic filler powder (alumina). Thin layers of polymer/filler powder mixture were sequentially cured with a CO2-laser (λ=10.6 µm) thereby generating the geometrical shape of the part. Subsequently, the cured thermoset part was annealed in nitrogen atmosphere at 600 to 1000 °C to convert the compact into a Si-O-C/Al2O3 micro-composite material. Dimensional changes upon pyrolysis (∆l/l0 ≈ 3 %) can be controlled by adjusting the polymer-to-filler ratio and the heat treatment conditions. The new process is called Selective Laser Curing (SLC).  1. Introduction  Various solid freeform fabrication techniques for manufacturing of ceramic parts have been developed, including Selective Laser Sintering, Stereolithography, Laminated Object Manufacturing, Fused Deposition Modelling and a variety of printing techniques [1-10]. All of these techniques use a polymeric binder phase to stabilise the shape of a ceramic powder part. Prior to high temperature sintering, however, the binder has to be removed and a high porosity of typically 30-50 % gives rise for pronounced dimensional changes. The selective laser sintering process offers the possibility to compact the powder directly by local heating [11]. Alternatively a porous template can be produced which may be infiltrated subsequently with porcelain slurry [12] or silicon melt [13]. The selective laser sintering techniques require a high temperature in the laser spot to trigger local sintering processes. Due to high temperature gradients residual stresses can cause local thermal shock failure and interlayer crack formation. In the selective laser curing process, however, significantly lower temperatures are applied (< 300 °C) and thermal gradient induced damaging is expected to be significantly reduced and dimensional accuracy will be improved.  Preceramic polymers suffer from a pronounced shrinkage up to 30 % (linear) upon pyrolysis due to the difference between the density of the polymer precursor (ρpolymer 1.0-1.3 g/cm³) and the ceramic residue (ρceramic > 2-3 g/cm³). The shrinkage associated with the polymer to ceramic conversion can be reduced down to 0 % when the polymer is loaded with active or inert fillers [14]. Active fillers react with the decomposition products of the polymer to form a rigid network of carbide or oxide product phases which results in a pronounced reduction of shrinkage.  The aim of the present work was to study the novel selective laser curing process for shaping of a polysiloxane loaded with 50 vol.-% alumina powder as an inert filler. 133  Polysiloxanes offer a high ceramic yield (> 80 wt.-%), are stable in ambient environment and show a high transmission for laser light of 85 % (relative) to achieve volumetric heating. The alumina filler was applied to reduce shrinkage during subsequent thermal processing. During laser curing the filler offers control over the processing window by means of its transmittance for laser radiation, heat capacity and heat conductivity.  2. Experimental Procedure 2.1 Materials  A Poly(methylsilsesquioxane) (PMS) [(CH3)0.96(OH)0,036(OC2H5)0.004SiO1,5]n with n ≈ 9000 (Festharz MK, Wacker Chemie Burghausen, FRG) was mixed with 50 vol.-% alumina powder (Gilox 63, Alcoa, FRG) with a mean particle size of 20 µm. The PMS is a white powder with a medium particle size of 8.5 µm and a melting point of 50 °C. The elemental composition is 42 wt.-% Si, 36 wt.-% O, 18 wt.-% C and 4 wt.-% H. Si-OH and Si-OC2H5 functional groups in the partially crosslinked molecular structures, Fig. 1, give rise for a condensation curing reaction when the polymer is heated to 150 - 250 °C. H2O and C2H5OH are the main condensation products to be released upon curing. 1 wt.-% of Aluminium-acetyl-acetonat (C15H21AlO6) was added as a catalyst to accelerate the curing reaction.  R = CH3, reactive side-chains: OH and OEt = OC2H5  Fig 1: Supposed molecular structure of polymethylsilsesquioxane [15].  The powder was prepared by dry mixing polymer, catalyst and filler in a ball mill equipped with polyethylene lining for 16 h with approximately 60 rpm. The polymer and the filler were dried prior to the mixing step, alumina at 130 °C and PMS at 40 °C under reduced pressure of 10³ Pa. The weight loss of the polymer after drying was 0.07 wt.-%. For FTIR- and wetting measurements the plain polymer powder (without filler and catalyst) was used.  2.2 Laser curing and pyrolysis  The curing experiments were carried out with a 200 W cw-CO2-laser (wavelength 10.6 µm) in a standard Selective Laser Sintering machine (M 250, EOS Munich, FRG). The focused gaussian beam diameter was 400 µm and the scanning speed of the laser was adjustable from 0 m/s up to 3 m/s. Parts composed of up to 40 single layers were manufactured with a single layer thickness of 100 µm. Table 1 summarises the processing variables. 134   Process variable Investigated range of values Scan velocity [vscan] 80 – 200 mm/s Laser power [Elaser beam] 6.9 W Line energy =scanbeamlaserline vEE  34 – 86 Ws/m Hatching distance [hs] 200 µm Layer thickness [pa] 100 µm  Tab 1: Range of process variables varied in the selective laser curing experiments.  After the selective laser curing the parts were subsequently pyrolysed under flowing nitrogen atmosphere with a flow rate of 0.2 l/min in an electrically heated tube furnace. The specimens were placed in a steel insert and the maximum pyrolysing temperature was 1000 °C.  2.3 Characterisation  The structural rearrangement reactions associated with the thermal processing were analysed by infrared spectroscopy (FT-IR) (Impact 420, Nicolet). Wetting of the alumina filler by the molten polymer was examined by measurement of the wetting angle according to the sessile drop method in a microscope (Leitz, Wetzlar, Germany) equipped with a heater. Dense alumina plates served as the substrates and the temperature was varied in the range from 70 to 110 °C in air. The linear dimensional change during heating of the cured part was measured with a differential dilatometer (Netsch - Dilatometer 1600, Netsch Gerätebau, Selb, FRG) using a Quartz-glass rod as a reference.  The degree of curing was determined by a thermo-barometric analysis technique [16, 17]. For this purpose the gaseous curing products (ethanol and water) leaving the polymer during the curing reaction were collected in a closed reaction tube which was evacuated prior to the specimen heating. The degree of curing, αcur, was normalised to the maximum pressure, Pcurmax, where the gaseous reaction products, C2H5OH and H2O, are supposed to have completely left the polymer.  maxcurcurcur PP=α            (1) Accordingly, the degree of pyrolysis, αpyr, was correlated with the amount of gaseous decomposition species released upon pyrolysis heating.  maxpyrpyrpyr PP=α           (2)  Fig. 2 shows αcur and αpyr of the PMS as a function of temperature. 135   020406080100100 150 200 250 300 350 400 450Temperature [°C]Degree of Curing  αcur  [%]  a) 020406080100400 500 600 700 800 900 1000Temperature [°C]Degree of Pyrolysis  αpyr  [%]  b)  Fig. 2: Degree of curing a) and of pyrolysis b) as derived from thermo-barometric gas analysis for poly(methylsilsesquioxane).  3. Results 3.1 Laser heating  The extension of the heat affected zone was estimated by exposure of one trace on the powder bed with the CO2-laser (6.9 W, 125 mm/s scan-velocity), Fig 3. The width of the trace was measured to be 500 µm and the depth 160 µm.  136    Fig. 3: Laser cured trace and microstructure of the polymer wetted filler material.  Most important for the distribution of the laser energy in the powder bed is a volumetric laser energy absorption and heating [18-22]. FT-IR was used to get results for the coupling of laser radiation with a wave length of 10.6 µm to Poly(methylsilsesquioxane). The relative transmission of Festharz MK is 85 % and the transmission of alumina is given in literature [23] and is better than 97 %. Due to high transmission of the polymer the energy can couple in the surface of the filler powder. Depending on the size, shape and concentration of the filler in the polymer matrix the local heating conditions can be varied in a wide range. Fig. 4 shows the results of a FE simulation of the SLC process with different coupling depths. A comparison of simulation and the laser cured traces is described in [23].  01000200030004000500060007000-10 40 90 140 190Time [ms]Temperature [°C]  Depth of coupling:   5  µm   Depth of coupling:  10    µm  Depth of coupling: 100    µm  Fig. 4: Peak temperatures in the polysiloxane/alumina mixture as calculated by FE simulation for three different coupling depths.  3.2 Wetting and curing  When the laser interacts with the polymer/powder mixture melting of the PMS-polymer starts at 50 °C. With increasing temperature a decrease of the wetting angle of the polymer melt on alumina from > 40° at 70 °C to > 10° at 130 °C was observed, Fig. 5. 200 µm 100 µm 137  0102030405060 80 100 120Temperature [°C]Contact Angle [°] Fig. 5: Wetting angle of Poly(methylsilsesquioxane) on dense alumina in air.  Further heating above 120 °C triggers the curing reactions: Si OH OH Si+ Si O Si + OH2Si C2H5O OH Si Si O Si ++ C2H5OH Water and alcohol may leave the thin layers (< 100 µm) without extended formation of porosity. With increasing temperature and time the degree of curing increases reaching a maximum of approximately 0.42 at a line energy of 86 Ws/m, Fig. 6. The laser power was kept constant and the different line energies were realized by different scanning velocities. A degree of curing of at least 35 % was found to be necessary to maintain the geometrical shape of a laser cured part. 01020304050600 20 40 60 80 100Line Energy [Ws/m]Degree of Curing [%]80 mm/s100 mm/s125 mm/s150 mm/s200 mm/s Fig. 6: Degree of curing vs. line energy at different scanning velocities at 6.9 W laser power.  138  A downscaled turbine wheel was manufactured with a line energy of 46 Ws/, a hatching distance of 200 µm and a layer thickness of 100 µm, Fig. 7. After laser curing the part was pyrolysed at 700 °C. A heating and cooling rate of 300°C/h and a dwell time of 4 h were applied.    Fig. 7: Demonstration part, a downscaled turbine wheel fabricated by selective laser curing and subsequent pyrolysis of poly(methylsilsesquioxane)/alumina precursor powder.  4. Conclusion  Selective Laser Curing is a novel process for solid freeform fabrication of ceramic parts from laser light curable preceramic polymers. Poly(methylsilsesquioxane) loaded with Al2O3 filler was converted to a Si-O-C/Al2O3 micro composite ceramic. A low dimensional change (< 3 %) and a wide range of possible polymer and filler compositions make this novel process particularly interesting for ceramic based rapid prototype manufacturing.  5. Acknowledgements  Financial support of Fonds der Chemischen Industrie and Deutsche Forschungsgemeinschaft is gratefully acknowledged.  6. Literature  1 Halloran, J. W.: Freeform fabrication of ceramics. British Ceramic Transactions, 1999, Vol. 98, No. 6, 299-303 2 Cawley, J. D.: Solid freeform fabrication of ceramics. Current Opinion in Solid State and Materials Science, 4 (1999), 483-489 3 Lenk, R.: Rapid Prototyping of Ceramic Components. Advanced Engineering Materials 139  2000, 2, No. 1-2, 40-47 4 Karapatis, N. P.; van Griethuysen, J.-P. S.; Glardon, R.: Direct rapid tooling: a review of current research. Rapid Prototyping Journal, Vol. 4, No. 2, 1998, 77-89 5 Subramanian, K.; Vail, N.; Barlow, J.; Marcus, H.: Selective laser sintering of alumina with polymer binders. Rapid Prototyping Journal, Vol. 1, No. 2, 1995, 24-35 6 Brady, A.; Halloran, J. W.: Solid freeform fabrication of Ceramics via Stereolithogaphy. Naval research reviews, 1998, 39-43 7 Doreau, F.; Chaput, C.; Chartier, T.: Stereolithography for Manufacturing Ceramic Parts. Advanced Engineering Materials 2000, 2, No. 8, 493-496 8 Brady, G. A.; Halloran, J. W.: Stereolithography of ceramic suspensions. Rapid Prototyping Journal, Vol. 3, No. 2, 1997, 61-65 9 Hinczewski, C.; Corbel, S.; Charier, T.: Stereolithography for the fabrication of ceramic three-dimensional parts. Rapid Prototyping Journal, Vol. 4, No. 3, 1998, 104-111 10 Vail, N.; Balasubramanian, B.; Barlow, J. W.; Marcus, H. L.: A thermal model of polymer degradation during selective laser sintering of polymer coated ceramic powders. Rapid Prototyping Journal, Vol. 2, No. 3, 1996, 24-40 11 Company brochure: Phenix Systems, rue Georges Besse, 63100 Clermont-Ferrand, France, 2001 12 Petzold, J.: New Freeform Porcelain Modelling. cfi/Ber. DKG 78 (2001) No. 3, S. D10-D13 13 Löschau, W.; Lenk, R.: Rapid Prototyping of Complex Shaped Parts of Si/SiC-Ceramics by Laser Sintering. Tagungsband EUROMAT 99, München, 27. - 30. September 1999, WILEY-VCH Verlag GmbH, Weinheim 1999, S. 145-149 14 Greil, P.: Active Filler controlled Pyrolysis of Preceramic Polymers. J. American Ceramic Society. 78 (1995), 835-848 15 Takahashi, T.; Kaschta, J.; Münstedt, H.: Melt Rheology and Structure of Silicone Resins. Rheol. Acta 40 (2001) 5, 490-498 16 Wild, M. J.; Buhler, P.: On the phase composition of polymethylsiloxane derived ceramics. J. Mater. Sci. 33 (1998), 5441-5444 17 Schmidt, H.; Buhler, P.; Greil, P.: Pyrolytic conversion of Poly(methylsiloxane) to silicon (oxy)carbide ceramics. Fourth Euro-Ceramics, Vol.1, Riccione, Italy, Oct. 1995, edited by C. Galassi (Gruppo Editoriale Faenza Editrice S.p.A., 1994), 229-236 18 Niebling, F.; Otto, A.: FE-simulation of the Selective Laser Sintering Process of Metallic Powders. Geiger, M.; Otto, A.: Laser Assisted Net Shape Engineering 3, Meisenbach Verlag, Bamberg, 2001, S. 371-382 19 Sun, M; Beaman, J.: A Three Dimensional Model for Selective Laser Sintering, Proc. of Solid Freeform Fabrication, The University of Texas at Austin, Austin, Texas (1991), 102-109 20 Wang, X.C.; Kruth, J.P.: A Simulation Model For Direct Selective Laser Sintering of Metal Powders. 1st Int. Seminar on Progress in Innovative Manufacturing Engineering, Geneva, Italy, June 20-22, 2001, S. 57-71 21 Niebling, F.; Otto, A.: FEM-gestützte Prozessbetrachtung des Direkten 140  Metallaserstrahlsinterns. Vollertsen, F.; Kleiner, M.: Idee - Vision - Innovation. Meisenbach Verlag, Bamberg, 2001, S. 419-426 22 Tolochko, N. K.; Laoui, T.; Khlopkov Y. V.; Mozzharov, S. E.; Titov, V. I.; Ignatiev, M. B.: Absorptance of powder materials suitable for laser sintering. Rapid Prototyping Journal, Vol. 6, No. 3, 2000, 155-160 23 Nyquist, R. A.; Kagel, R.O.: Infrared Spectra of Inorganic Compounds. Academic Press Inc., New York 1971 24 Sindelar, R.; Niebling, F.; Buhler, P.; Greil, P.; Geiger, M.: Selective Laser Curing of Preceramic Polymers. Geiger, M.; Otto, A.: Laser Assisted Net Shape Engineering 3, Meisenbach Verlag, Bamberg, 2001, 447-458  141

Solid Freeform Fabrication of Ceramic Parts from Filler Loaded Preceramic Polymers

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Solid Freeform Fabrication of Ceramic Parts from Filler Loaded Preceramic Polymers

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