Selective Laser Sintering of Zirconia
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67 Mat. Res. Soc. Symp. Proc. Vol. 625 ©2000 Materials Research Society
EXPERIMENT Material Preparation Granulated yttria-stabilized zirconia powder from Tosoh Ceramic Corporation was selected as the base material for the casting mold. The Tosoh powder consists of zirconia crystallites, 24 nm average size, granulated into 50 m size spheres with a water-soluble binder. To minimize post-process shrinkage and prevent granulate dissolution during infiltration, the granulated particles were pre-sintered for one hour at 500'C and two hours at 1250'C to pyrolyze the binder and densify each particle. The sintering schedule was determined from a sintering map, shown as Figure 1 [7]. The sacrificial SLS binder, an 80:20 molar blend of methylmethacrylate and butylmethacrylate, respectively, was produced by methods discussed elsewhere [8]. This particular copolymer binder was chosen because it completely unzips during firing, leaving no contaminating residue. The copolymer binder was spray dried into fine particles using an Anhydro Laboratory 1 Model Spray Drier. The density of the binder was measured using a Quantachrome Ultrapycnometer 1000.
R 1 e 1 0.9 a t 0.8 i
Volume Diffusion (II)
20 hr
Volume Diffusion (I) 0.2 hr
v 0.7 e
0.6
2hr
ZIRCONIA Part. Dia. = 24 nm Ext.P = 0 MPa
D 0.5 e n 0.4
0.3 t y 0.2 800
I 1000
1200
Boundary Diffusion (I) 1400
1600
1800
Temperature, 'C
Figure 1: Zirconia Sintering Map Material ProcessinQ and Characterization
Material Processing and Characterization
The pre-sintered zirconia powder and 30vol% copolymer were mixed in a rolling mill for 24 hours. An SLS Model 125 Workstation was used to build a series of bend test specimens. A layer thickness of 0.005 inches, scan spacing of 0.003 inches and CO 2 laser power of 5 watts 68
were used. Laser scan speed was 16 in/s, and laser spot size was approximately 0.02 inches (500im). To prevent part curling, the powder bed was pre-heated to 80BC [9]. Five green bend specimens were produced and tested per ASTM Cl 161-94 using a four point bend apparatus. Density was measured using the Archimedes principle with ethanol. Roughness (Ra) was measured using a Surfanalyzer 5000. The remaining green bend specimens were measured and weighed. The specimens were infiltrated with a colloidal solution of hydrous zirconia (Nyacol ZrlOO/20). Wetting of the green parts was improved by adding a surfactant to the colloid. After a number of infiltration and curing cycles, the green shapes were pyrolyzed at 50OBC to bum off the copolymer binder, ramped to 150OB3C and soaked for 10 hours to promote zirconia crystallization and grain growth. After the firing cycle, each specimen was re-weighed and re-measured. Samples were set aside for density measurements, flexural testing, microscopy and roughness measurements. The infiltration and firing sequence was continued for two more cycles. Material characterization was done after each firing step. X-ray diffraction was used to determine the resulting zirconia phases in the final mold material. To measure the min
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