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Additive manufacturing of zirconia parts by indirect selective laser sintering
Available online at www.sciencedirect.com
Journal of the European Ceramic Society 34 (2014) 81–89
Additive manufacturing of zirconia parts by indirect selective laser sintering Khuram Shahzad a , Jan Deckers b , Zhongying Zhang a , Jean-Pierre Kruth b , Jef Vleugels a,∗ a
Department of Metallurgy and Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium b Division PMA, Department of Mechanical Engineering, KU Leuven, Celestijnenlaan 300B, B-3001 Heverlee, Belgium Received 30 April 2013; received in revised form 22 July 2013; accepted 25 July 2013 Available online 30 August 2013
Abstract Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO2 –PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO2 suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 ◦ C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y2 O3 stabilized ZrO2 parts to 92 and 85%, respectively. © 2013 Elsevier Ltd. All rights reserved. Keywords: Additive manufacturing; Indirect selective laser sintering; Zirconia; Polymer/ceramic microspheres
1. Introduction Selective laser sintering (SLS) is an additive manufacturing (AM) technique used to produce three dimensional parts in a layer by layer way starting from a computer aided design (CAD) model. The CAD model is divided into slices which are subsequently consolidated by the AM technology into complex components. A schematic of the SLS process is shown in Fig. 1. The part building process consists of two main steps, i.e., powder layer deposition and laser sintering. The powder layers are deposited by a conventional roller or scraper system. The sintering step refers to the irradiation of the deposited powder layer by selective laser beam scanning which locally sinters the powder according to the predefined part slice geometry. The part building platform is lowered and the process is repeated until the part is completed. Near-net-shape fabrication without using dies or moulds, short production times and the ability to produce parts with high geometrical complexity are key benefits of SLS.1
∗ Corresponding author at: Department of Metallurgy and Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, Bus 2450, B-3001 Heverlee, Belgium. Tel.: +32 16 321244; fax: +32 16 321992. E-mail address: [email protected] (J. Vleugels).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.07.023
Selective laser sintering (SLS) of ceramic components can be categorized as direct or indirect. Indirect SLS involves melting of a sacrificial organic binder phase to produce green parts. The green parts are subsequently debinded and furnace sintered to produce ceramic parts. Direct SLS does not involve a sacrificial binder phase and the ceramic parts are produced by direct sintering or melting. Direct SLS of ceramics can be further divided into powder based and slurry based direct SLS. In powder based direct SLS, the packing density of the powder layers is low, resulting in a modest sintered density and cracks due to thermal stresses in the parts.2 Hagedorn et al. however developed a direct selective laser melting (SLM) process to produce fully dense Al2 O3 –ZrO2 parts with eutectic composition.3 Powder bed preheating by means of a diffused CO2 laser beam up to 1700 ◦ C was required to avoid thermal cracks. Although powder bed preheating from the top could effectively eliminate thermal cracks, the maximum part height was limited to 3 mm.3 Slurry based direct SLS on the other hand has the advantage of starting from more homogeneous and highly packed powder layers. Slurry layers are commonly deposited by doctor blading4,5 or spray deposition6 and subsequently dried. The production of porcelain parts with a sintered density of 86% by direct slurry based SLS has been reported.4 The parts however
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slurry infiltration and isostatic pressing were applied to increase the green density of SLS Al2 O3 –PP parts.13 Near fully dense (98%) Al2 O3 parts were realized by slurry based indirect SLS using fully hydrolyzed polyvinyl alcohol as fugitive binder.14 The part building rate however was modest (∼0.89 mm3 /s) because of the additional drying step.15 This work reports on the feasibility to fabricate complex shape zirconia parts by powder based indirect SLS. Zirconia–polypropylene microsphere composite starting powder was prepared by thermally induced phase separation (TIPS). The influence of the SLS process parameters on the green parts was assessed and pressure slurry infiltration (PI) and warmisostatic pressing (WIP) were used to improve both the green composite and sintered ceramic density. Fig. 1. Schematic of the SLS set-up.
had a low strength because of microstructural inhomogeneities and thermal cracks.4 Klocke et al. spray deposited 3 mol% yttriastabilized ZrO2 layers from a slurry.6 The effect of the laser scan speed on the density of the parts was investigated by varying the scan speed between 50 and 150 mm/s, at a fixed laser power of 13 W, scan spacing of 0.01 mm and layer thickness of 0.01 mm. The density of the sintered parts was reported to decrease with increasing laser energy density, with a highest density of ∼76% at a scan spacing of 50 mm/s. Because of the high cooling rate, cracks were formed in the parts.6 Powder based indirect SLS allowed to produce crackfree polymer–ceramic composite parts using conventional SLS equipment.7–10 Since polymer parts produced from semicrystalline polymers had a higher density than that of amorphous polymers, semi-crystalline polymers are preferred over amorphous polymers to be used as binder phase.11,12 However, there is a disadvantage associated with semi-crystalline polymers as these polymers show a 4–5% volume shrinkage upon solidification what can cause component distortion. To avoid distortion, SLS of semi-crystalline polymer based material is conducted by heating the complete powder bed and growing part into the SLS window, i.e., the temperature window between the onset of polymer melting during heating and crystallization during cooling. Only the additional heat needed to locally melt the polymer is provided by laser beam scanning. After the completion of the part, the powder bed containing the component is slowly cooled to room temperature. In general, polymers like polyamide and polypropylene with a wide SLS window >15 ◦ C are preferred as it is easier to control the powder bed preheating temperature.11,12 Spherical micrometer sized powders are generally preferred to produce uniform powder layers by roller deposition. For composite powders, a high homogeneity is essential to produce strong green parts.7,10 Spherical Al2 O3 –polyamide (PA) composite microspheres produced by thermally induced phase separation (TIPS) were reported to have a good flowability allowing depositing homogeneous powder layers suitable for SLS. The sintered density of the parts produced from this powder however was only ∼50%.9 Post processing techniques like
2. Experimental details 2.1. Starting powder preparation Co-precipitated 3 mol% Y2 O3 –ZrO2 (grade TZ-3Y, Tosoh, Japan) powder with a d50 ∼ 30 nm was used as structural material and isotactic polypropylene (PP) with an average molecular weight (Mw ) of 12,000 (Mw /Mn = 2.4, Sigma–Aldrich) was used as fugitive binder. Xylene (p-xylene, reagent grade, Sigma–Aldrich) was used as solvent during thermally induced phase separation. Ethanol (Chem-lab NV, Belgium) was used for washing. The different steps involved in the PP–ZrO2 composite powder synthesis are schematically presented in Fig. 2. The TZ-3Y powder was first deagglomerated in xylene on a multidirectional mixer (Type Turbula) using TZ-3Y ZrO2 milling balls in a polyethylene container for 24 h at 79 rpm. After removing the milling balls, the deagglomerated suspension was diluted
Fig. 2. Schematic of the polymer/ceramic composite powder synthesis by TIPS.
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into a 800 ml suspension in a 2 l glass flask. The suspension was continuously mechanically stirred under a nitrogen gas flow and heated to 133 ◦ C to obtain a 9 wt% PP xylene solution based particle suspension. After complete PP dissolution, the suspension was naturally cooled to room temperature to induce precipitation. The precipitates were allowed to settle, allowing decantation of the xylene. The powders were washed multiple times with ethanol to remove residual xylene and dried in air at 65 ◦ C. Composite powder batches with 30 and 40 vol.% ZrO2 were produced for selective laser sintering. The size of the agglomerates was measured by laser diffraction (Mastersizer Plus, Malvern, UK), whereas the morphology was studied by scanning electron microscopy (SEM, XL30-FEG, FEI, The Netherlands). Differential scanning calorimetry (DSC, Model-2920, TA instruments, USA) was used to determine the thermal properties of the composite powders. 2.2. Selective laser sintering (SLS) and post-SLS processing The PP/ZrO2 composite powder was loaded into the powder feed container of a Sinterstation 2000 (DTM Corporation/3D Systems), equipped with a 100 W CO2 laser (f100, Synrad, USA) with a wavelength of 10.6 m and a laser beam diameter of 400 m. Test files were prepared in the computer to set up the geometries and SLS parameters, including powder bed preheating temperature, powder layer thickness and laser parameters (laser power, scan spacing and scan speed). The laser scanning process was conducted in nitrogen atmosphere. After part fabrication, the powder bed was allowed to cool down before collecting the parts. The non-sintered composite powder was sieved and reused. In order to increase the density of the SLS parts, pressure slurry infiltration (PI) and warm isostatic pressing (WIP) were applied. For slurry infiltration, an aqueous 30 vol.% TZ-3Y powder suspension was prepared by multidirectional mixing (type Turbula) for 24 h and electrostatic stabilization by nitric acid addition of a pH of 5. For pressure infiltration, the ZrO2 suspension and SLS parts were inserted into a stainless steel vessel and manually pressurized for 5 min at 3, 16 or 32 MPa (see Fig. 3). The infiltrated parts were dried in an oven at 60 ◦ C for 2 h. The weight of the parts before and after infiltration was recorded. For WIPing, the same device was used in combination with a heating jacket, as indicated in Fig. 3. Both SLSed and SLSed/PI components were WIPed. The SLS parts were vacuum packed in nitrile rubber bags (TNT® Blue disposable gloves, Ansel limited, Malaysia) and isostatically pressed at 64 MPa and 135 ◦ C for 5 min. After post-SLS processing, the parts were heated at 0.1 ◦ C/min in air to 600 ◦ C for debinding (Carbolite, Sheffield, UK) and sintered in air at 1450 ◦ C for 2 h (Nabertherm, D-2804, Germany). The density of the sintered ceramics was measured by the Archimedes method in ethanol. To calculate the amount of shrinkage of the SLS parts during post-processing, the dimensions of cubic parts were measured by a digital vernier calliper in the X–Y direction and the Z-direction, i.e., the building direction. The sintered samples were cross-sectioned and polished
Fig. 3. Experimental set-up used for pressure infiltration and warm isostatic pressing.
for microstructural examination (SEM, XL30-FEG, FEI, The Netherlands). Polished samples were thermally etched in air at 1350 ◦ C for 20 min using a heating rate of 20 ◦ C/min. 3. Results and discussion 3.1. ZrO2 –PP composite characteristics During temperature induced phase separation (TIPS), a polymer is dissolved in a solvent by heating, followed by phase separation induced during cooling of the solution. The morphology of the precipitated polymer depends on the polymer concentration. As schematically presented in Fig. 4, when a homogeneous polymer solution of critical composition (the point at which the binodal and spinodal curves meet) is cooled from a temperature above the binodal line, a spontaneous liquid–liquid phase separation into a polymer rich and polymer lean phase will take place when entering the stable region B. Further cooling is followed by crystallization and the formation of a bi-continuous membrane morphology. In the case of a polymer concentration above or below the critical point, the liquid–liquid phase separation is not spontaneous and will only occur when the concentration fluctuation is large enough to overcome the potential barrier when entering the metastable region A upon cooling. The liquid–liquid phase
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Fig. 6. Particle size distribution of PP–ZrO2 powder produced by TIPS.
Fig. 4. Polymer-solvent phase diagram.
separation in this region may follow a nucleation and growth mechanism. In a solution with a polymer concentration above the critical point and below the monotectic point (point m in Fig. 4), the polymer rich phase tends to form the matrix and rejects the polymer lean phase, resulting in cellular membrane structures when solidified.16 When the polymer concentration is below the critical concentration, as in this experimental work, polymer rich phase droplets are formed in a polymer lean matrix phase. The addition of Al2 O3 powder during TIPS of a PP–Xylene solution allowed processing PP–Al2 O3 composite microspheres.12 Using the same approach, PP–40 vol.% ZrO2 (d50 ∼ 49 m) and PP–30 vol.% ZrO2 (d50 ∼ 27 m) were produced by TIPS from a 9 wt% PP–xylene solution. The obtained composite particles are spherical (see Fig. 5) with a wide size distribution in the 5–150 m range (see Fig. 6). The SLS window of the composite powders was measured by DSC to be ∼20 ◦ C (Fig. 7), which implies the powder bed can be preheated in a temperature region that is wide enough to avoid distortion of the parts due to polymer crystallization during the SLS process. The
Fig. 5. SEM micrograph of PP–30 vol.% ZrO2 microsphere powder produced by TIPS.
Fig. 7. DSC curve of PP–40 vol.% ZrO2 composite powder. TOM and TOC are the onset temperatures of melting during heating and crystallization during cooling, respectively.
powder showed a good flowability and uniform powder layers could be roller deposited, as shown in Fig. 8. 3.2. Fabrication of green parts For the indirect SLS process of polymer/ceramic composite powder, the powder bed was preheated to 145 ◦ C, just below the melting point of the polymer binder phase in the SLS temperature window (see Fig. 7). The polymer in the laser scanned selected region was locally molten by the extra laser irradiation energy. The laser energy density was controlled by varying the SLS parameters, i.e., the laser power, layer thickness, laser beam scan speed and laser beam scan spacing. In this work, the powder
Fig. 8. Roller deposited composite powder bed containing some SLS parts.
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Fig. 9. Investigated energy densities as a function of the experimental SLS parameters (a) and corresponding PP–30 vol.% ZrO2 parts (b).
layer thickness was fixed at 0.13 mm. The laser energy density can be calculated as: e=
P svl
(1)
With, e, the laser energy density (J/mm3 ), P, the laser power (Watts), s, the scan spacing (mm), v, the scan speed (mm/s) and, l, the layer thickness (mm). In order to find the best parameter combination to obtain strong enough green parts for the post processing, a parametrical study was conducted. The laser scanning parameters were varied between 3 and 7 W of laser power, 0.1–0.2 mm of scan spacing and 500–1250 mm/s of scan speed, to build parts with a dimension of 15 mm × 15 mm × 10 mm. The investigated energy densities, calculated according to Eq. (1), are presented as a function of the corresponding SLS parameter combinations in Fig. 9a. In these tests, the laser energy density was varied from 0.09 to 1.07 J/mm3 . At high laser energy density, fumes were generated during laser scanning indicating polymer degradation. At the lowest laser energy density, the parts delaminated because the polymer phase was not molten. This implied the laser energy density had to be carefully adjusted in this range, i.e., just enough to melt the polymer phase avoiding polymer degradation. The parametrical study was performed using both PP–40 vol.% ZrO2 and PP–30 vol.% ZrO2 starting powders. The corresponding PP–40 vol.% ZrO2 SLS parts however were too
fragile to perform any post processing. The SLS PP–30 vol.% ZrO2 parts and their corresponding SLS parameters are shown in Fig. 9. Most of the SLSed green parts were strong enough for the post processing, although the parts scanned by a too high laser energy density showed some PP degradation. The density of the green parts was measured geometrically and found to slightly decrease with increased laser energy density. The parts sintered with a laser energy density below 0.2 J/mm3 reached 36% of the theoretical composite density. At higher laser energy density, the density decreased because of PP degradation and an adhering porous volume of microspheres. The quality of the parts was also judged by visual inspection. The parts with sharp edges and smooth surfaces were considered to be the best. Based on the information gained from this analysis, 4 SLS parameter settings were selected, as summarized in Table 1, to assess the possibility to produce parts of more complex geometry. As presented in Fig. 10, the first 3 shapes could be produced with all selected parameter sets. However, only the two parameter sets with the highest laser energy density allowed manufacturing the most complicated component. This also implied that a higher laser energy density can result in stronger green parts, providing PP degradation can be avoided. Because of the higher laser energy density, the parameter set (a) described in Table 1 was selected to prepare parts of 15 mm × 15 mm × 10 mm and some challenging geometries to investigate the post-SLS processing operations.
Table 1 Selective laser sintering parameters, laser energy density and green density. Parameter set
Laser power (W)
Scan speed (mm/s)
Scan spacing (mm)
Laser energy density (J/mm3 )
Green density (% TDa )
(a) (b) (c) (d)
3 3 3 5
1250 1250 1250 1250
0.10 0.15 0.20 0.20
0.185 0.123 0.092 0.154
36 37 36 36
a
Theoretical density of the PP–zirconia composite.
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Fig. 10. Complex parts produced according to the SLS parameter settings a–d, specified in Table 1.
3.3. Post-SLS processing The effect of pressure infiltration (PI) and warm isostatic pressing (WIPing) on the green and sintered density of the SLSed parts is summarized in Tables 2 and 3. The parts produced using the optimum SLS parameter (parameter set (a) of Table 1) had a density of 36% of the theoretical density of the composite material. Debinding and sintering of these parts resulted in highly porous ceramic parts with a sintered density of only 32%. As shown in Table 4, a linear shrinkage of ∼30% in the X/Y-direction and slightly higher shrinkage of 31% in the building direction (Z-direction) was measured after sintering.
The green SLS parts were infiltrated with a 30 vol.% ZrO2 aqueous suspension. Three infiltration pressures 3, 16 and 32 MPa were applied since no spontaneous infiltration was observed due to the hydrophobic nature of PP. Applying a 3 MPa infiltration pressure increased the weight of the SLS part 1.39 times, resulting in an increased green density from 36 to 43%. A further increase in weight (1.69 times) and green density (up to 45%) was observed when infiltrating at 16 MPa. When infiltrating at 32 MPa however, all parts fractured. The increased green density was also reflected in a higher sintered ceramic density of 49 and 54% when infiltrated at 3 and 16 MPa, respectively. Upon sintering, a ∼30% linear shrinkage in the X/Y-direction and 31% in the Z-direction was measured for the PI SLS parts (see Table 4), similar as for the SLS parts without PI. WIPing was more effective compared to PI, allowing increasing the SLS part density up to 90% and sintered density up to 92%. When WIPing a pressure infiltrated part, the green density was increased from 45 to 83%, resulting in a sintered ZrO2 density of 85%. The highest density was obtained when WIPing the SLS parts before debinding and sintering. Fracture surfaces of the as-SLS and WIPed SLS composites are shown in Fig. 11. The microstructure of the SLS material is not homogeneous and consists of joined starting powder microspheres. The inter-agglomerate voids however remained due to a limited or total lack of plastic flow during SLS (see Fig. 11a). A more homogeneous microstructure was observed after WIPing (see Fig. 11b and c), clearly revealing an increased density due to plastic deformation. The microstructures of a selection of sintered ZrO2 parts are compared in Fig. 12. The sintered SLS part only had a density of 32% and the layered structure can still be observed in the sintered material, as shown in Fig. 12a. The porosity originated from the intergranular voids in the deposited microsphere layers, since the microspheres themselves were fully densified during
Fig. 11. Scanning electron micrographs of the green samples produced by SLS (a) and SLS + WIP (b and c).
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Table 2 Effect of infiltration pressure on the green and sintered density of SLS parts. Infiltration pressure (MPa)
Infiltration ratio (mass ratio after/before infiltration)
PI density (g/cm3 ) (%TDa )
Sintered density (g/cm3 ) (%TDb )
3 16
1.39 1.69
1.23 (43) 1.46 (45)
2.96 (49) 3.25 (54)
a b
Theoretical density of the PP–zirconia composite. Theoretical density of zirconia.
Fig. 12. SEM micrographs of cross-sectioned sintered samples produced by SLS (a and b), SLS + PI (c), SLS + WIP (e and f) and SLS + PI + WIP (d, g, and h).
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Table 3 Effect of PI (16 MPa) and WIP (64 MPa/135 ◦ C) on the part density. SLS density (g/cm3 ) (% TD*)
PI density (g/cm3 ) (% TDa )
WIP density (g/cm3 ) (% TDa )
Sintered density (g/cm3 ) (% TDb )
0.88 (36)
– 1.46 (45) – 1.46 (45)/
– – 2.21 (90) 2.66 (83)
1.98 (32) 3.25 (54) 5.57 (92) 5.17 (85)
a b
Theoretical density of the PP–zirconia composite. Theoretical density of zirconia.
Table 4 Dimension and linear shrinkage of the green SLS part before and after post-processing. Material
X and Y-dimension (linear shrinkage %) (mm)
Green SLSed SLSed/debinded/sintered SLSed/PI/debinded/sintered SLSed/WIPed/debinded/sintered SLSed/PI/WIPed/debinded/sintered
15.34 10.70 10.79 7.94 9.59
± ± ± ± ±
0.11 0.11 (30%) 0.10 (30%) 0.08 (48%) 0.34 (38%)
sintering (see Fig. 12b). The sintered microstructure of the pressure infiltrated ZrO2 has a higher density and homogeneity, as shown in Fig. 12c. The large intergranular pores were eliminated with a residual porosity of ∼5 m in the sintered WIPed part, shown in Fig. 12e and f. However, a large crack was observed in the centre of the part. The high amount of shrinkage during WIPing could be the reason for this crack formation as the sintered parts showed a linear shrinkage of ∼48% in the X/Ydirection and ∼52% in the building direction (see Table 4). The sintered microstructure of the combined PI and WIPed part (see Fig. 12g and h) is similar as for the WIPed part (Fig. 12e and f), but without large crack formation. The linear shrinkage of the SLS part after PI, WIP and sintering was about ∼38% in the X/Y-direction and 42% in the Z-direction (see Table 4). Residual porosity however was also present in the PI/WIPed ZrO2 . The microstructure in the dense areas in-between the large pores is submicrometer grain sized (see Fig. 12d), as characteristic for a Y-TZP ceramic, indicating that the selected sintering conditions were appropriate and the inhomogeneity present in the green SLS microstructure is responsible for the presence of the larger pores in the final microstructure. Although the sintered density of the SLSed/WIPed ZrO2 is higher than that of SLSed/infiltration/WIPed ceramic, the latter combined post-SLS processing techniques are preferred because of the presence of the large central crack in the former. The density of the ZrO2 parts achieved by this indirect SLS process is lower than the fully dense parts of Al2 O3 –41.5 wt% ZrO2 made by SLM.3 Whereas the SLM process only allows producing parts of 3 mm in the building direction,3 the indirect SLS process has no size limitations. Moreover, the surface roughness of the parts (Ra ∼ 5 m, Rz ∼ 51 m) produced by indirect SLS is lower than the parts produced by SLM with an Rz ∼ 150 m.3 The surface roughness is also lower than that of the alumina parts (Ra ∼ 14 m, Rz ∼ 114 m) produced by indirect SLS.13 The smaller particle size of the ZrO2 starting powder could be the reason for the improved surface morphology.
Z-dimension (linear shrinkage %) (mm) 10.51 7.25 7.25 5.03 6.08
± ± ± ± ±
0.09 0.05 (31%) 0.06 (31%) 0.04 (52%) 0.18 (42%)
Fig. 13. Complex parts produced by the SLS green (a) PI + sintered, (b) WIPed + sintered and (c) routes.
Some complicated shape green and sintered parts obtained using different post-SLS processing techniques are presented in Fig. 13, illustrating the shaping capability of the technology. 4. Conclusions Homogeneous polypropylene (PP)–ZrO2 composite starting powder with spherical morphology and suitable thermal properties was produced by thermally induced phase separation. A CO2 laser power of 3 W, scan velocity of 1250 mm/s and scan
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spacing of 0.1 mm at a powder layer thickness of 130 m were found to be the optimum SLS parameters combining sufficient strength of the SLS parts to allow damage-free manipulation and avoiding PP degradation. Based on the density and the ability to produce complex shaped parts, the optimum SLS parameters (P = 3 W, v = 1250 mm/s and l = 0.1 mm) were selected to produce parts for post-SLS processing. Parts produced from PP–40 vol.% ZrO2 powder were very fragile, independent of the applied SLS parameters, and even showed delamination. Increasing the polymer concentration to 70 vol.% allowed manufacturing stronger parts. The sintered density of the SLS parts was only 32% but could be increased to 54% by pressure infiltration (PI) at 16 MPa with a 30 vol.% ZrO2 suspension. Warm isostatic pressing of the composite SLS parts allowed increasing the green density to 90% and the sintered ceramic density to 92% TD. However, cracks were observed in the cross-section of the sintered WIPed components. Combining PI and WIPing allowed producing crack-free complicated Y-TZP parts with a density of 85%. Acknowledgements This work was financially supported by the Flemish Institute for the Promotion of Scientific Technological Research in Industry (IWT) under project SBO-DiRaMaP and the Research Fund of KU Leuven under project GOA/08/007. References 1. Gibson I, Rosen DW, Stucker B. Additive manufacturing technologies: rapid prototyping to direct digital manufacturing. New York, Heidelberg, Dordrecht, London: Springer; 2010. 2. Bertrand Ph Bayle F, Combe C, Goeuriot P, Smurov I. Ceramic component manufacturing by selective laser sintering. Appl Surf Sci 2007;254:989–92.
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3. Hagedorn Y-C, Balachandran N, Meiners W, Wissenbach K, Poprawe R. SLM of net-shaped high strength ceramics: new opportunities for producing dental restorations. In: Proc. solid freeform fabric. symp.. 2011. p. 536–46. 4. Tian X, Li D, Heinrich JG. Rapid prototyping of porcelain products by layerwise slurry deposition (LSD) and direct laser sintering. Rapid Prototyping J 2012;18:362–73. 5. Yen HC, Tang HH. Study on direct fabrication of ceramic shell mold with slurry-based ceramic laser fusion and ceramic laser sintering. Int J Adv Manuf Technol 2012;60:1009–15. 6. Klocke F, Derichs C, Ader C, Demmer A. Investigations on laser sintering of ceramic slurries. Prod Eng Res Dev 2007;1:279–84. 7. Subramanian K, Vail N, Barlow J, Marcus H. Selective laser sintering of alumina with polymer binders. Rapid Prototyping J 1995;1:24–35. 8. Gill TJ, Hon KKB. Experimental investigations into the selective laser sintering of silicon carbide polyamide composites. Proc Inst Mech Eng Part B: J Eng Manuf 2004;218:1249–56. 9. Deckers J, Shahzad K, Vleugels J, Kruth JP. Cold/quasi isostatic pressing assisted indirect selective laser sintering of alumina components. Rapid Prototyping J 2012;5(18):409–19. 10. Shahzad K, Deckers J, Boury S, Neirinck B, Vleugels J, Kruth JP. Preparation and indirect selective laser sintering of alumina/PA microspheres. Ceram Int 2012;38:1241–7. [11].Dickens JED, Lee BL, Taylor GA, Magistro AJ, Ng H, McAlea KP, et al., Sinterable semi-crystalline powder and near-fully dense article formed therewith. US Patent RE39354E. 12. Drummer D, Rietzel D, Kühnlein F. Development of a characterization approach for the sintering behaviour of new thermoplastics for selective laser sintering. Phys Procedia 2010;5:533–42. 13. Shahzad K, Deckers J, Kruth JP, Vleugels J. Additive manufacturing of alumina ceramics by indirect selective laser sintering and post processing. J Mater Process Technol 2013;213:1484–94. 14. Tang HH, Chiu ML, Yen HC. Slurry-based selective laser sintering of polymer-coated ceramic powders to fabricate high strength alumina parts. J Eur Ceram Soc 2011;31:1383–8. 15. Tang HH, Yen HC. Ceramic parts fabricated by ceramic laser fusion. Mater Trans 2004;45:2744–51. 16. Van de Witte P, Dijkstra PJ, Van den Berg JWA, Review Feijen J. Phase separation processes in polymer solutions in relation to membrane formation. J Membr Sci 1996;117:1–31.
Journal of the European Ceramic Society 34 (2014) 81–89
Additive manufacturing of zirconia parts by indirect selective laser sintering Khuram Shahzad a , Jan Deckers b , Zhongying Zhang a , Jean-Pierre Kruth b , Jef Vleugels a,∗ a
Department of Metallurgy and Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium b Division PMA, Department of Mechanical Engineering, KU Leuven, Celestijnenlaan 300B, B-3001 Heverlee, Belgium Received 30 April 2013; received in revised form 22 July 2013; accepted 25 July 2013 Available online 30 August 2013
Abstract Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO2 –PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO2 suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 ◦ C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y2 O3 stabilized ZrO2 parts to 92 and 85%, respectively. © 2013 Elsevier Ltd. All rights reserved. Keywords: Additive manufacturing; Indirect selective laser sintering; Zirconia; Polymer/ceramic microspheres
1. Introduction Selective laser sintering (SLS) is an additive manufacturing (AM) technique used to produce three dimensional parts in a layer by layer way starting from a computer aided design (CAD) model. The CAD model is divided into slices which are subsequently consolidated by the AM technology into complex components. A schematic of the SLS process is shown in Fig. 1. The part building process consists of two main steps, i.e., powder layer deposition and laser sintering. The powder layers are deposited by a conventional roller or scraper system. The sintering step refers to the irradiation of the deposited powder layer by selective laser beam scanning which locally sinters the powder according to the predefined part slice geometry. The part building platform is lowered and the process is repeated until the part is completed. Near-net-shape fabrication without using dies or moulds, short production times and the ability to produce parts with high geometrical complexity are key benefits of SLS.1
∗ Corresponding author at: Department of Metallurgy and Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, Bus 2450, B-3001 Heverlee, Belgium. Tel.: +32 16 321244; fax: +32 16 321992. E-mail address: [email protected] (J. Vleugels).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.07.023
Selective laser sintering (SLS) of ceramic components can be categorized as direct or indirect. Indirect SLS involves melting of a sacrificial organic binder phase to produce green parts. The green parts are subsequently debinded and furnace sintered to produce ceramic parts. Direct SLS does not involve a sacrificial binder phase and the ceramic parts are produced by direct sintering or melting. Direct SLS of ceramics can be further divided into powder based and slurry based direct SLS. In powder based direct SLS, the packing density of the powder layers is low, resulting in a modest sintered density and cracks due to thermal stresses in the parts.2 Hagedorn et al. however developed a direct selective laser melting (SLM) process to produce fully dense Al2 O3 –ZrO2 parts with eutectic composition.3 Powder bed preheating by means of a diffused CO2 laser beam up to 1700 ◦ C was required to avoid thermal cracks. Although powder bed preheating from the top could effectively eliminate thermal cracks, the maximum part height was limited to 3 mm.3 Slurry based direct SLS on the other hand has the advantage of starting from more homogeneous and highly packed powder layers. Slurry layers are commonly deposited by doctor blading4,5 or spray deposition6 and subsequently dried. The production of porcelain parts with a sintered density of 86% by direct slurry based SLS has been reported.4 The parts however
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slurry infiltration and isostatic pressing were applied to increase the green density of SLS Al2 O3 –PP parts.13 Near fully dense (98%) Al2 O3 parts were realized by slurry based indirect SLS using fully hydrolyzed polyvinyl alcohol as fugitive binder.14 The part building rate however was modest (∼0.89 mm3 /s) because of the additional drying step.15 This work reports on the feasibility to fabricate complex shape zirconia parts by powder based indirect SLS. Zirconia–polypropylene microsphere composite starting powder was prepared by thermally induced phase separation (TIPS). The influence of the SLS process parameters on the green parts was assessed and pressure slurry infiltration (PI) and warmisostatic pressing (WIP) were used to improve both the green composite and sintered ceramic density. Fig. 1. Schematic of the SLS set-up.
had a low strength because of microstructural inhomogeneities and thermal cracks.4 Klocke et al. spray deposited 3 mol% yttriastabilized ZrO2 layers from a slurry.6 The effect of the laser scan speed on the density of the parts was investigated by varying the scan speed between 50 and 150 mm/s, at a fixed laser power of 13 W, scan spacing of 0.01 mm and layer thickness of 0.01 mm. The density of the sintered parts was reported to decrease with increasing laser energy density, with a highest density of ∼76% at a scan spacing of 50 mm/s. Because of the high cooling rate, cracks were formed in the parts.6 Powder based indirect SLS allowed to produce crackfree polymer–ceramic composite parts using conventional SLS equipment.7–10 Since polymer parts produced from semicrystalline polymers had a higher density than that of amorphous polymers, semi-crystalline polymers are preferred over amorphous polymers to be used as binder phase.11,12 However, there is a disadvantage associated with semi-crystalline polymers as these polymers show a 4–5% volume shrinkage upon solidification what can cause component distortion. To avoid distortion, SLS of semi-crystalline polymer based material is conducted by heating the complete powder bed and growing part into the SLS window, i.e., the temperature window between the onset of polymer melting during heating and crystallization during cooling. Only the additional heat needed to locally melt the polymer is provided by laser beam scanning. After the completion of the part, the powder bed containing the component is slowly cooled to room temperature. In general, polymers like polyamide and polypropylene with a wide SLS window >15 ◦ C are preferred as it is easier to control the powder bed preheating temperature.11,12 Spherical micrometer sized powders are generally preferred to produce uniform powder layers by roller deposition. For composite powders, a high homogeneity is essential to produce strong green parts.7,10 Spherical Al2 O3 –polyamide (PA) composite microspheres produced by thermally induced phase separation (TIPS) were reported to have a good flowability allowing depositing homogeneous powder layers suitable for SLS. The sintered density of the parts produced from this powder however was only ∼50%.9 Post processing techniques like
2. Experimental details 2.1. Starting powder preparation Co-precipitated 3 mol% Y2 O3 –ZrO2 (grade TZ-3Y, Tosoh, Japan) powder with a d50 ∼ 30 nm was used as structural material and isotactic polypropylene (PP) with an average molecular weight (Mw ) of 12,000 (Mw /Mn = 2.4, Sigma–Aldrich) was used as fugitive binder. Xylene (p-xylene, reagent grade, Sigma–Aldrich) was used as solvent during thermally induced phase separation. Ethanol (Chem-lab NV, Belgium) was used for washing. The different steps involved in the PP–ZrO2 composite powder synthesis are schematically presented in Fig. 2. The TZ-3Y powder was first deagglomerated in xylene on a multidirectional mixer (Type Turbula) using TZ-3Y ZrO2 milling balls in a polyethylene container for 24 h at 79 rpm. After removing the milling balls, the deagglomerated suspension was diluted
Fig. 2. Schematic of the polymer/ceramic composite powder synthesis by TIPS.
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into a 800 ml suspension in a 2 l glass flask. The suspension was continuously mechanically stirred under a nitrogen gas flow and heated to 133 ◦ C to obtain a 9 wt% PP xylene solution based particle suspension. After complete PP dissolution, the suspension was naturally cooled to room temperature to induce precipitation. The precipitates were allowed to settle, allowing decantation of the xylene. The powders were washed multiple times with ethanol to remove residual xylene and dried in air at 65 ◦ C. Composite powder batches with 30 and 40 vol.% ZrO2 were produced for selective laser sintering. The size of the agglomerates was measured by laser diffraction (Mastersizer Plus, Malvern, UK), whereas the morphology was studied by scanning electron microscopy (SEM, XL30-FEG, FEI, The Netherlands). Differential scanning calorimetry (DSC, Model-2920, TA instruments, USA) was used to determine the thermal properties of the composite powders. 2.2. Selective laser sintering (SLS) and post-SLS processing The PP/ZrO2 composite powder was loaded into the powder feed container of a Sinterstation 2000 (DTM Corporation/3D Systems), equipped with a 100 W CO2 laser (f100, Synrad, USA) with a wavelength of 10.6 m and a laser beam diameter of 400 m. Test files were prepared in the computer to set up the geometries and SLS parameters, including powder bed preheating temperature, powder layer thickness and laser parameters (laser power, scan spacing and scan speed). The laser scanning process was conducted in nitrogen atmosphere. After part fabrication, the powder bed was allowed to cool down before collecting the parts. The non-sintered composite powder was sieved and reused. In order to increase the density of the SLS parts, pressure slurry infiltration (PI) and warm isostatic pressing (WIP) were applied. For slurry infiltration, an aqueous 30 vol.% TZ-3Y powder suspension was prepared by multidirectional mixing (type Turbula) for 24 h and electrostatic stabilization by nitric acid addition of a pH of 5. For pressure infiltration, the ZrO2 suspension and SLS parts were inserted into a stainless steel vessel and manually pressurized for 5 min at 3, 16 or 32 MPa (see Fig. 3). The infiltrated parts were dried in an oven at 60 ◦ C for 2 h. The weight of the parts before and after infiltration was recorded. For WIPing, the same device was used in combination with a heating jacket, as indicated in Fig. 3. Both SLSed and SLSed/PI components were WIPed. The SLS parts were vacuum packed in nitrile rubber bags (TNT® Blue disposable gloves, Ansel limited, Malaysia) and isostatically pressed at 64 MPa and 135 ◦ C for 5 min. After post-SLS processing, the parts were heated at 0.1 ◦ C/min in air to 600 ◦ C for debinding (Carbolite, Sheffield, UK) and sintered in air at 1450 ◦ C for 2 h (Nabertherm, D-2804, Germany). The density of the sintered ceramics was measured by the Archimedes method in ethanol. To calculate the amount of shrinkage of the SLS parts during post-processing, the dimensions of cubic parts were measured by a digital vernier calliper in the X–Y direction and the Z-direction, i.e., the building direction. The sintered samples were cross-sectioned and polished
Fig. 3. Experimental set-up used for pressure infiltration and warm isostatic pressing.
for microstructural examination (SEM, XL30-FEG, FEI, The Netherlands). Polished samples were thermally etched in air at 1350 ◦ C for 20 min using a heating rate of 20 ◦ C/min. 3. Results and discussion 3.1. ZrO2 –PP composite characteristics During temperature induced phase separation (TIPS), a polymer is dissolved in a solvent by heating, followed by phase separation induced during cooling of the solution. The morphology of the precipitated polymer depends on the polymer concentration. As schematically presented in Fig. 4, when a homogeneous polymer solution of critical composition (the point at which the binodal and spinodal curves meet) is cooled from a temperature above the binodal line, a spontaneous liquid–liquid phase separation into a polymer rich and polymer lean phase will take place when entering the stable region B. Further cooling is followed by crystallization and the formation of a bi-continuous membrane morphology. In the case of a polymer concentration above or below the critical point, the liquid–liquid phase separation is not spontaneous and will only occur when the concentration fluctuation is large enough to overcome the potential barrier when entering the metastable region A upon cooling. The liquid–liquid phase
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Fig. 6. Particle size distribution of PP–ZrO2 powder produced by TIPS.
Fig. 4. Polymer-solvent phase diagram.
separation in this region may follow a nucleation and growth mechanism. In a solution with a polymer concentration above the critical point and below the monotectic point (point m in Fig. 4), the polymer rich phase tends to form the matrix and rejects the polymer lean phase, resulting in cellular membrane structures when solidified.16 When the polymer concentration is below the critical concentration, as in this experimental work, polymer rich phase droplets are formed in a polymer lean matrix phase. The addition of Al2 O3 powder during TIPS of a PP–Xylene solution allowed processing PP–Al2 O3 composite microspheres.12 Using the same approach, PP–40 vol.% ZrO2 (d50 ∼ 49 m) and PP–30 vol.% ZrO2 (d50 ∼ 27 m) were produced by TIPS from a 9 wt% PP–xylene solution. The obtained composite particles are spherical (see Fig. 5) with a wide size distribution in the 5–150 m range (see Fig. 6). The SLS window of the composite powders was measured by DSC to be ∼20 ◦ C (Fig. 7), which implies the powder bed can be preheated in a temperature region that is wide enough to avoid distortion of the parts due to polymer crystallization during the SLS process. The
Fig. 5. SEM micrograph of PP–30 vol.% ZrO2 microsphere powder produced by TIPS.
Fig. 7. DSC curve of PP–40 vol.% ZrO2 composite powder. TOM and TOC are the onset temperatures of melting during heating and crystallization during cooling, respectively.
powder showed a good flowability and uniform powder layers could be roller deposited, as shown in Fig. 8. 3.2. Fabrication of green parts For the indirect SLS process of polymer/ceramic composite powder, the powder bed was preheated to 145 ◦ C, just below the melting point of the polymer binder phase in the SLS temperature window (see Fig. 7). The polymer in the laser scanned selected region was locally molten by the extra laser irradiation energy. The laser energy density was controlled by varying the SLS parameters, i.e., the laser power, layer thickness, laser beam scan speed and laser beam scan spacing. In this work, the powder
Fig. 8. Roller deposited composite powder bed containing some SLS parts.
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Fig. 9. Investigated energy densities as a function of the experimental SLS parameters (a) and corresponding PP–30 vol.% ZrO2 parts (b).
layer thickness was fixed at 0.13 mm. The laser energy density can be calculated as: e=
P svl
(1)
With, e, the laser energy density (J/mm3 ), P, the laser power (Watts), s, the scan spacing (mm), v, the scan speed (mm/s) and, l, the layer thickness (mm). In order to find the best parameter combination to obtain strong enough green parts for the post processing, a parametrical study was conducted. The laser scanning parameters were varied between 3 and 7 W of laser power, 0.1–0.2 mm of scan spacing and 500–1250 mm/s of scan speed, to build parts with a dimension of 15 mm × 15 mm × 10 mm. The investigated energy densities, calculated according to Eq. (1), are presented as a function of the corresponding SLS parameter combinations in Fig. 9a. In these tests, the laser energy density was varied from 0.09 to 1.07 J/mm3 . At high laser energy density, fumes were generated during laser scanning indicating polymer degradation. At the lowest laser energy density, the parts delaminated because the polymer phase was not molten. This implied the laser energy density had to be carefully adjusted in this range, i.e., just enough to melt the polymer phase avoiding polymer degradation. The parametrical study was performed using both PP–40 vol.% ZrO2 and PP–30 vol.% ZrO2 starting powders. The corresponding PP–40 vol.% ZrO2 SLS parts however were too
fragile to perform any post processing. The SLS PP–30 vol.% ZrO2 parts and their corresponding SLS parameters are shown in Fig. 9. Most of the SLSed green parts were strong enough for the post processing, although the parts scanned by a too high laser energy density showed some PP degradation. The density of the green parts was measured geometrically and found to slightly decrease with increased laser energy density. The parts sintered with a laser energy density below 0.2 J/mm3 reached 36% of the theoretical composite density. At higher laser energy density, the density decreased because of PP degradation and an adhering porous volume of microspheres. The quality of the parts was also judged by visual inspection. The parts with sharp edges and smooth surfaces were considered to be the best. Based on the information gained from this analysis, 4 SLS parameter settings were selected, as summarized in Table 1, to assess the possibility to produce parts of more complex geometry. As presented in Fig. 10, the first 3 shapes could be produced with all selected parameter sets. However, only the two parameter sets with the highest laser energy density allowed manufacturing the most complicated component. This also implied that a higher laser energy density can result in stronger green parts, providing PP degradation can be avoided. Because of the higher laser energy density, the parameter set (a) described in Table 1 was selected to prepare parts of 15 mm × 15 mm × 10 mm and some challenging geometries to investigate the post-SLS processing operations.
Table 1 Selective laser sintering parameters, laser energy density and green density. Parameter set
Laser power (W)
Scan speed (mm/s)
Scan spacing (mm)
Laser energy density (J/mm3 )
Green density (% TDa )
(a) (b) (c) (d)
3 3 3 5
1250 1250 1250 1250
0.10 0.15 0.20 0.20
0.185 0.123 0.092 0.154
36 37 36 36
a
Theoretical density of the PP–zirconia composite.
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Fig. 10. Complex parts produced according to the SLS parameter settings a–d, specified in Table 1.
3.3. Post-SLS processing The effect of pressure infiltration (PI) and warm isostatic pressing (WIPing) on the green and sintered density of the SLSed parts is summarized in Tables 2 and 3. The parts produced using the optimum SLS parameter (parameter set (a) of Table 1) had a density of 36% of the theoretical density of the composite material. Debinding and sintering of these parts resulted in highly porous ceramic parts with a sintered density of only 32%. As shown in Table 4, a linear shrinkage of ∼30% in the X/Y-direction and slightly higher shrinkage of 31% in the building direction (Z-direction) was measured after sintering.
The green SLS parts were infiltrated with a 30 vol.% ZrO2 aqueous suspension. Three infiltration pressures 3, 16 and 32 MPa were applied since no spontaneous infiltration was observed due to the hydrophobic nature of PP. Applying a 3 MPa infiltration pressure increased the weight of the SLS part 1.39 times, resulting in an increased green density from 36 to 43%. A further increase in weight (1.69 times) and green density (up to 45%) was observed when infiltrating at 16 MPa. When infiltrating at 32 MPa however, all parts fractured. The increased green density was also reflected in a higher sintered ceramic density of 49 and 54% when infiltrated at 3 and 16 MPa, respectively. Upon sintering, a ∼30% linear shrinkage in the X/Y-direction and 31% in the Z-direction was measured for the PI SLS parts (see Table 4), similar as for the SLS parts without PI. WIPing was more effective compared to PI, allowing increasing the SLS part density up to 90% and sintered density up to 92%. When WIPing a pressure infiltrated part, the green density was increased from 45 to 83%, resulting in a sintered ZrO2 density of 85%. The highest density was obtained when WIPing the SLS parts before debinding and sintering. Fracture surfaces of the as-SLS and WIPed SLS composites are shown in Fig. 11. The microstructure of the SLS material is not homogeneous and consists of joined starting powder microspheres. The inter-agglomerate voids however remained due to a limited or total lack of plastic flow during SLS (see Fig. 11a). A more homogeneous microstructure was observed after WIPing (see Fig. 11b and c), clearly revealing an increased density due to plastic deformation. The microstructures of a selection of sintered ZrO2 parts are compared in Fig. 12. The sintered SLS part only had a density of 32% and the layered structure can still be observed in the sintered material, as shown in Fig. 12a. The porosity originated from the intergranular voids in the deposited microsphere layers, since the microspheres themselves were fully densified during
Fig. 11. Scanning electron micrographs of the green samples produced by SLS (a) and SLS + WIP (b and c).
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Table 2 Effect of infiltration pressure on the green and sintered density of SLS parts. Infiltration pressure (MPa)
Infiltration ratio (mass ratio after/before infiltration)
PI density (g/cm3 ) (%TDa )
Sintered density (g/cm3 ) (%TDb )
3 16
1.39 1.69
1.23 (43) 1.46 (45)
2.96 (49) 3.25 (54)
a b
Theoretical density of the PP–zirconia composite. Theoretical density of zirconia.
Fig. 12. SEM micrographs of cross-sectioned sintered samples produced by SLS (a and b), SLS + PI (c), SLS + WIP (e and f) and SLS + PI + WIP (d, g, and h).
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Table 3 Effect of PI (16 MPa) and WIP (64 MPa/135 ◦ C) on the part density. SLS density (g/cm3 ) (% TD*)
PI density (g/cm3 ) (% TDa )
WIP density (g/cm3 ) (% TDa )
Sintered density (g/cm3 ) (% TDb )
0.88 (36)
– 1.46 (45) – 1.46 (45)/
– – 2.21 (90) 2.66 (83)
1.98 (32) 3.25 (54) 5.57 (92) 5.17 (85)
a b
Theoretical density of the PP–zirconia composite. Theoretical density of zirconia.
Table 4 Dimension and linear shrinkage of the green SLS part before and after post-processing. Material
X and Y-dimension (linear shrinkage %) (mm)
Green SLSed SLSed/debinded/sintered SLSed/PI/debinded/sintered SLSed/WIPed/debinded/sintered SLSed/PI/WIPed/debinded/sintered
15.34 10.70 10.79 7.94 9.59
± ± ± ± ±
0.11 0.11 (30%) 0.10 (30%) 0.08 (48%) 0.34 (38%)
sintering (see Fig. 12b). The sintered microstructure of the pressure infiltrated ZrO2 has a higher density and homogeneity, as shown in Fig. 12c. The large intergranular pores were eliminated with a residual porosity of ∼5 m in the sintered WIPed part, shown in Fig. 12e and f. However, a large crack was observed in the centre of the part. The high amount of shrinkage during WIPing could be the reason for this crack formation as the sintered parts showed a linear shrinkage of ∼48% in the X/Ydirection and ∼52% in the building direction (see Table 4). The sintered microstructure of the combined PI and WIPed part (see Fig. 12g and h) is similar as for the WIPed part (Fig. 12e and f), but without large crack formation. The linear shrinkage of the SLS part after PI, WIP and sintering was about ∼38% in the X/Y-direction and 42% in the Z-direction (see Table 4). Residual porosity however was also present in the PI/WIPed ZrO2 . The microstructure in the dense areas in-between the large pores is submicrometer grain sized (see Fig. 12d), as characteristic for a Y-TZP ceramic, indicating that the selected sintering conditions were appropriate and the inhomogeneity present in the green SLS microstructure is responsible for the presence of the larger pores in the final microstructure. Although the sintered density of the SLSed/WIPed ZrO2 is higher than that of SLSed/infiltration/WIPed ceramic, the latter combined post-SLS processing techniques are preferred because of the presence of the large central crack in the former. The density of the ZrO2 parts achieved by this indirect SLS process is lower than the fully dense parts of Al2 O3 –41.5 wt% ZrO2 made by SLM.3 Whereas the SLM process only allows producing parts of 3 mm in the building direction,3 the indirect SLS process has no size limitations. Moreover, the surface roughness of the parts (Ra ∼ 5 m, Rz ∼ 51 m) produced by indirect SLS is lower than the parts produced by SLM with an Rz ∼ 150 m.3 The surface roughness is also lower than that of the alumina parts (Ra ∼ 14 m, Rz ∼ 114 m) produced by indirect SLS.13 The smaller particle size of the ZrO2 starting powder could be the reason for the improved surface morphology.
Z-dimension (linear shrinkage %) (mm) 10.51 7.25 7.25 5.03 6.08
± ± ± ± ±
0.09 0.05 (31%) 0.06 (31%) 0.04 (52%) 0.18 (42%)
Fig. 13. Complex parts produced by the SLS green (a) PI + sintered, (b) WIPed + sintered and (c) routes.
Some complicated shape green and sintered parts obtained using different post-SLS processing techniques are presented in Fig. 13, illustrating the shaping capability of the technology. 4. Conclusions Homogeneous polypropylene (PP)–ZrO2 composite starting powder with spherical morphology and suitable thermal properties was produced by thermally induced phase separation. A CO2 laser power of 3 W, scan velocity of 1250 mm/s and scan
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spacing of 0.1 mm at a powder layer thickness of 130 m were found to be the optimum SLS parameters combining sufficient strength of the SLS parts to allow damage-free manipulation and avoiding PP degradation. Based on the density and the ability to produce complex shaped parts, the optimum SLS parameters (P = 3 W, v = 1250 mm/s and l = 0.1 mm) were selected to produce parts for post-SLS processing. Parts produced from PP–40 vol.% ZrO2 powder were very fragile, independent of the applied SLS parameters, and even showed delamination. Increasing the polymer concentration to 70 vol.% allowed manufacturing stronger parts. The sintered density of the SLS parts was only 32% but could be increased to 54% by pressure infiltration (PI) at 16 MPa with a 30 vol.% ZrO2 suspension. Warm isostatic pressing of the composite SLS parts allowed increasing the green density to 90% and the sintered ceramic density to 92% TD. However, cracks were observed in the cross-section of the sintered WIPed components. Combining PI and WIPing allowed producing crack-free complicated Y-TZP parts with a density of 85%. Acknowledgements This work was financially supported by the Flemish Institute for the Promotion of Scientific Technological Research in Industry (IWT) under project SBO-DiRaMaP and the Research Fund of KU Leuven under project GOA/08/007. References 1. Gibson I, Rosen DW, Stucker B. Additive manufacturing technologies: rapid prototyping to direct digital manufacturing. New York, Heidelberg, Dordrecht, London: Springer; 2010. 2. Bertrand Ph Bayle F, Combe C, Goeuriot P, Smurov I. Ceramic component manufacturing by selective laser sintering. Appl Surf Sci 2007;254:989–92.
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