Solid Freeform Fabrication of Functionally Graded Materials Abstract:
Functionally Graded Materials (FGMs) have a wide range of potential commercial applications which consist of components requiring wear-resistant surfaces such as engine components, rocket nozzle and so forth . Solid freeform fabrication (SFF), especially Freeze-form Extrusion Fabrication (FEF) technology, an economical method feasible for grading among many compatible plastic, ceramic and metal materials combinations. In this paper, I describe how FEF method build Al2O3/ZrO2 graded parts and it used to test the graded material compositions by energy dispersive spectroscopy (EDS) after drying, binder burnout and sintering these parts. While ZrC/W graded test bars were fabricated and used to test mechanical properties and microstructures. Keywords: Functionally graded materials, Solid freeform fabrication, Extrusion 1 Introduction
Fabrication of functionally graded materials (FGMs) are materials that are produced with multiple materials in graded fashion taking advantage of complementary material properties. In addition, ceramic materials are often used in high-temperature applications for their superior heat resistance. However, pure ceramic materials are hard to use in high-stress environments and are often difficult to use in building complex geometries by using traditional processes due to its poor fracture toughness . Thus, the metal-ceramic or ceramic-ceramic material is desirable. To prepare FGMs for objects with complex shape and three-dimensional (3D) compositional gradients, solid freeform fabrication (SFF) methods would be required. Traditional manufacturing methods of FGMs objects include slip casting, tape casting, thermal spraying, vapor deposition; While shortcomings of these techniques include the extensive use of toxic organic solvents, long fabrication time, low repeatability, irregularly shaped pores, and thin structures [2,3]. Most importantly, these methods have limit controls over complex shapes. Solid free-form fabrication (SFF) is a group of additive manufacturing technologies that create 3D objects layer by layer, which can deliver different materials to the building areas and have the ability to build components with FMGs. The most developed SFF techniques consist of Extrusion Freeform Fabrication (EFF), Fused Deposition Modeling (FDM) and Robocasting and the EFF was the first technique to produce FGMs from ceramic oxides graded to stainless steel or from different types of ceramics (oxides and non-oxides) [3,4,5]. The Freeze-form Extrusion Fabrication (FEF), one of the currently available EFF technologies, was well suited to directly generate complex-shaped FGMs objects with extrusion and deposition of aqueous colloidal pastes. In this process, slurry can achieve higher than 50 vol% solids loading with 2–4 vol% organic binder. Also, FEF method is able to build relatively larger parts because it can make a green body in an environment below the freezing point of water, which means the paste would solidify after the deposition of each layer during the fabrication process . 2 Experimental Procedure
To demonstrate the feasibility of FEF for producing FGMs, some graded parts were created by using Al2O3 and ZrO2 pastes and ZrC and ZrC/W pastes. ZrO2 powders (TZ-3Y, Tosoh Corporation, Tokyo, Japan), Al2O3 powders (Almatis Aluminum Oxide A16SG Grade), ZrC powders (0.35μm,Attrition-milled) and W powders (0.6~1μm, Sigma Aldrich, St. Louis, MO) were used. The solids loading of the Al2O3 paste was 45 vol.% and 1 vol.% Methocel (Methocel*F4M, Dow Chemical Company) was used as a binder added into the paste to achieve consistent extrusion behaviors and to assist in forming a stronger green body after drying and Darvan C (R.T. Vanderbilt Company, Inc.) was used as a dispersant and deionized water was utilized . Powder, Darvan C and water were mixed in a Nalgene bottle and then ball milled for...
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