Processing, Characterization and Modeling of Non-Random Porous Ceramic Structures

Abstract

Processing of non-random porous ceramic structures via fused deposition process is discussed. structures are characterized experimentally and statistically based on their compressive strength. Finite element modeling is used to understand the effect of stress concentration leading to the strength degradation ofthese brittle elastic solids. Introduction Porous ceramic materials are of significant technological interest due to their applications in molten metal filters, light weight core for sandwich panels, radiant burners, catalyst supports, sensors and bone grafts [1-2]. The porosity may be needed in the structures to reduce the weight of the structure at the non-critical areas, to increase the activity of the ceramics by increasing surface area or to separate the wanted from the unwanted materials during filtering. But in all the cases, a better control of the pore geometry and improvements of the mechanical properties ofthe porous structures are important to improve the reliability ofthe structures. Various processing techniques have been utilized to fabricate porous ceramic materials. Replamineform process was utilized to fabricate porous bioceramic implants to duplicate the macroporous microstructures of corals that have interconnected pores [3]. Porous alumina ceramics have been fabricated using pore former or foaming agent that evolves gases during sintering at elevated temperatures [4]. Porous Hydroxyapatite (HAp) ceramic blocks were also fabricated using HAp slurry mixed with foaming agent followed by sintering at elevated temperature [5]. Shrout et al. and Rittenmyer et al. [6-7] reported fabrication of 3-3 piezocomposites using a mixture of volatilizable plastic spheres and PZT powder, in a process known as BURPS (BURned-out Plastic Spheres). Unfortunately, all of these processes form structures with randotnly arranged pores with a wide variety of sizes and have limited flexibility to control pore volumes and porosity distribution in the final structure. In this paper, we discuss about porous ceramics with non-random pore volumes, shapes and sizes, which have been processed using solid freeform fabrication (SFF) methods. SFF offers tremendous flexibility in varying the porosity parameters which controls the strength ofthese ceramic structures as well. Theoretical and experimental characterization of porous materials is not new and several theories have already been postulated to characterize the mechanical strength of polyct:ystalline porous ceramics. These theories to characterize the mechanical strength can be classified into three broad categories: (1) reduction in cross-section area approach, (2) stress concentration approach and (3) effective flaw size approach. Most of these studies in predicting the porositystrength relationship have been limited to the fitting capability of the equations towards the available experimental data and no attempt has been made to quantitatively access the effects of porosity parameters on the strength degradation ofthe porous ceramic structures.