Finite element simulations of ceramic powder compaction and sintering in the making of a micro heat exchanger

One major problem in processing ceramics using conventional powder-based compaction and sintering technique is the non-uniform relative density distribution and the residual stresses induced during these processes. The non uniformities may cause high differential sintering rate, introducing cracks and shape distortion in the sintered product. The capability in predicting the relative density distribution and residual stresses will therefore enhance our ability to design and fabricate ceramic components with internal cavities. In this thesis, finite element models for compaction process are developed based on theory of plasticity using the modified Drucker-Prager/cap model in soil mechanics and a recent hyperbolic cap model. A finite element model for sintering process based on viscoplasticity constitutive law is also presented. The models allow simulations to be performed before experiments to select feasible processes. The results from finite element simulation are consistent with findings of other researchers and in agreement with our experimental observations in making various features in our meso-scale heat-exchanger. The compaction models indicate that severe local shear yielding during unloading may have caused micro-cracks since the sites that yielded in shear during top punch removal are coincident with the sites where micro-cracks originate. The sintering model implies that sintered products are more uniform in their final relative density and residual stress distribution regardless of their geometry and their base states as inherited from the two different compaction models. In addition, simulations reveal that local effects such as differences in relative density, residual stresses and geometry do not seem to significantly affect the final grain size and sintering time needed to densify a ceramic compact. An in-depth study of shear yielding and its relationship with compact cracking is needed to improve the current compaction model. To fine-tune the sintering model, sinter forging needs to be conducted to obtain experimental material parameters.