Using Layered Manufacturing to Create Textured Microstructures in Si 3 N 4 Ceramics
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Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
FDC process, the ceramic loaded thermoplastic binder encounters a variety of shear stresses and shear stress gradients during part building. For this reason it was thought that texture could be intentionally induced in FDC parts through a similar seeding process. If this is the case, it would be possible to make custom designed microstructures on a scale never before achieved (on the order of the size of the nozzle used, 250 to 750 am). The aim of this study is to investigate the possibility of introducing microstructural texture into FDC parts, thereby enhancing properties in specific directions. 2.0 EXPERIMENTAL PROCEDURE The process of Si 3N4 part fabrication for FDC has many sequential operations. The feedstock material, i.e. filaments, are fabricated first. Filaments are then used to fabricate green parts by FDC. These constitute two extrusion steps that the powder/binder mixture undergo on route to a finished part, which should aid in the alignment of the beta silicon nitride seed particles. The green parts are subjected to thermal binder removal and gas pressure sintering operations. Filaments were fabricated using Honeywell's AS800 powder, beta silicon nitride seed powder made at Rutgers University, and RU9 binder (Honeywell). Many process improvements have considerably increased the homogeneity and powder dispersion in the filament. The details of filament fabrication can be found elsewhere [5]. 2.1 FDC PART MANUFACTURING, BINDER BURNOUT AND SINTERING Green FDC part manufacturing was performed using a modified Stratasys FDM® 1650. The standard build conditions were: a 406 pim diameter nozzle, road width of 508 Am, slice thickness of 254 Am, liquefier temperature of 170 'C, and a build envelope temperature of 42 'C. A negative offset between adjacent roads was used for part building, but the flow rate was knowingly decreased to a value of 90% which lead to final parts that were not fully dense. This was done to minimize road distortion caused by the smearing of neighboring roads which commonly happens during dense part building. For the purposes of this study, it was not necessary to produce fully dense parts, although future work will include such parts. Optimization of FDC parameters to obtain fully dense green parts has been established earlier. Details of process optimization can be found elsewhere [6]. Post processing techniques of parts after FDC, i.e., binder removal and sintering were developed at Rutgers University and Honeywell Technology Center respectively. Further details of the binder removal cycle development can be obtained from reference [7]. It should be noted that binder removal cycles are similar to those of injection molded parts, and sintering cycles are essentially identical to those of FDC parts made by conventional ceramic processes. 2.2 MATERIALS CHARACTERIZAION As mentioned above, 10 vol.% of the total ceramic powder used to make the FDC filament feedstock was P-Si3N4 seed particles. The addition of these part
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