Commercial solid freeform fabrication (SFF) systems, which have been developed for fabrication of wax and polymer parts for form and fit and secondary applications, such as moulds for casting, etc., require further improvements for use in direct processing of structural ceramic and metal parts. Defects, both surface as well as internal, are undesirable in SFF processed ceramic and metal parts for structural and functional applications. Process improvements are needed before any SFF technique can successfully be commercialized for structural ceramic and metal processing. Describes process improvements made in new SFF techniques, called fused deposition of ceramics (FDC) and metals (FDMet), for fabrication of structural and functional ceramic and metal parts. They are based on an existing SFF technique, fused deposition modelling (FDM) and use commercial FDM systems. The current state of SFF technology and commercial FDM systems results in parts with several surface and internal defects which, if not eliminated, severely limit the structural properties of ceramic and metal parts thus produced. Describes systematically, in detail, the nature of these defects and their origins. Discusses several novel strategies for elimination of most of these defects. Shows how some of these strategies have successfully been implemented to result in ceramic parts with structural properties comparable to those obtained in conventionally processed ceramics.

A nonlinear rheological model combining elastic, viscous, and yielding phenomena is developed in order to describe the rheological behavior of materials which exhibit a yield stress. A key feature of the formulation is the incorporation of a recoverable strain; it has a maximum value equal to the critical strain at which the transition from an elastic solid-like response to a viscous shear thinning response occurs. An analysis is presented to enable determination of all the model parameters solely from dynamic measurements which are easily accessible experimentally. A rigorous correlation, analogous in form to the Cox–Merz rule, is shown to exist between the steady shear viscosity and the complex dynamic viscosity in terms of a newly defined ‘‘effective shear rate.’’ Experimental data obtained for a 70 vol % suspension of silicon particles in polyethylene indicate agreement with theoretical predictions for both the dynamic and steady shear behavior.

A novel process is discussed for producing a wide variety of ceramic powders with unique physical and chemical characteristics. Silicon, Si3N4, and SiC powders were produced from CO2 laser‐heated gas‐phase reactants; a detailed description of this laser‐driven process is presented. The physical, chemical, and crystalline nature of the resultant powders and the effect of process variables are discussed in Part II. In this process, reactant gases are rapidly heated by CO2 laser radiation and decompose, causing particles to nucleate and grow rapidly. Analytical models of fluid flow, heat transfer, heating rates, and powder‐formation mechanisms are discussed. The powders produced in this process are very fine (<0.1μm), spherical, nearly monodispersed in size, extremely pure, and loosely agglomerated.

Fused deposition of ceramics (FDC) is a solid freeform fabrication technique based on extrusion of highly loaded polymer systems. The process utilizes particle loaded thermoplastic binder feedstock in the form of a filament. The filament acts as both the piston driving the extrusion and also the feedstock being deposited. Filaments can fail during FDC via buckling, when the extrusion pressure needed is higher than the critical buckling load that the filament can support. Compressive elastic modulus determines the load carrying ability of the filament and the viscosity determines the resistance to extrusion (or extrusion pressure). A methodology for characterizing the compressive mechanical properties of FDC filament feedstocks has been developed. It was found that feedstock materials with a ratio (E/Z a ) greater than a critical value (3610 5 to 5610 5 s -1 ) do not buckle during FDC while those with a ratio less than this range buckle.

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We present processing (green and sintered), part shrinkage and warping, microstructural characterization, and mechanical properties of Si3N4 made by fused deposition of ceramics (FDC), using optical microscopy, scanning electron microscopy, and X‐ray diffraction. The mechanical properties (fracture strength, fracture toughness, and Weibull modulus) are also reported. Proper FDC build parameters resulted in dense, homogeneous, near‐net‐shape Si3N4, with microstructures and mechanical properties similar to conventionally processed material. Mechanical properties are shown to be isotropic, while there is some degree of microstructural texturing (preferred β‐Si3N4 grain orientation) in sintered components.

Si, Si3N4, and SiC powders which possess a unique set of characteristics were produced by a laser‐driven gas‐phase synthesis process. The powders have a fine particle size (<0.1 μm), are spherical, have a narrow range of particle sizes, are free of hard agglomerates, have a high degree of phase purity, and have a high absolute purity (<0.1% including oxygen). A detailed analysis of the physical, chemical, and crystalline characteristics of Si and Si3N4 is presented. A brief discussion of our initial work with SiC is also included. The dependence of particle characteristics on the various process parameters (laser power, cell pressure, gas composition) is discussed and related to a model of the powder‐synthesis process.