Sintering Behavior of Gehlenite, Part II. Microstructure and Mechanical Properties Dechang Jia and Waltraud M. Kriven* , ** ,w Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 A self-toughened gehlenite (2CaO . Al 2 O 3 . SiO 2 or ‘‘C 2 AS’’) ceramic with randomly distributed platelet grains was prepared by the organic steric entrapment (PVA) route. The gehlenite ce- ramic had a density of 2.698–2.875 g/cm 3 , corresponding to a relative density of 90%–96%. The platelet gehlenite grains had an average thickness of 3.670.8 lm and a width of 12.973.7 lm, respectively, with an average aspect ratio of 3.6. The three-point bending strength, fracture toughness, and Young’s modulus at- tained were 142.1712.1 MPa, 2.3270.12 MPa . m 1/2 , and 10876.8 GPa, respectively. Fractography as well as Vickers indentation crack propagation profiles showed that crack deflec- tion, crack blunting, and pinning effects due to the randomly distributed platelet grains were considered to be responsible for the good mechanical properties of the gehlenite ceramic. I. Introduction T HE organic steric entrapment (PVA and PEG) method was invented in our laboratory 1 and used to fabricate a variety of one-, two-, and three-component oxides including alumina, cristabolite, mullite, YAG, xenotime, calcium mono- and tri-al- uminates, as well as mono-, di-, and tri-calcium silicates, cor- dierite, leucite, zircon, the components of Portland cement, and various other titanates and phosphates. 2–15 The technique is highly recommended because it has higher powder yield than do other chemical synthesis methods and it can even be extended to chemicals that decompose in water. 2 In a recent paper by us, the PVA method was used to prepare macro-/mesoporous gehlenite (2CaO Á Al 2 O 3 Á SiO 2 or C 2 AS) ceramic with a very good pore self-forming ability, but our original target was to prepare a dense gehlenite ceramic to find a ceramic matrix candidate more compatible with dicalcium silicate, which could exhibit a phase transformation toughening effect. 16,17 Thus, in this paper, opti- mized sintering processing was used to obtain a relatively dense gehlenite ceramic and its microstructure and mechanical prop- erties were investigated; its toughening mechanisms were also discussed on the basis of observations on indentation crack propagation profiles together with fractographs. II. Experimental Procedure (1) Materials The powder was chemically designed according to the stoichio- metric proportions for gehlenite (2CaO Á Al 2 O 3 Á SiO 2 or C 2 AS). The processing flowchart for the gehlenite ceramic is shown in Fig. 1. Calcium nitrate tetrahydrate (Ca(NO 3 ) 2 Á 4H 2 O) (reagent grade, Aldrich Chemical Co., Milwaukee, WI) and aluminum nitrate nonahydrate (Al(NO 3 ) 3 Á 9H 2 O) (reagent grade, Aldrich Chemical Co.) were used, respectively, to obtain calcium and aluminum oxides. A Ludox SK colloidal SiO 2 (25 wt% SiO 2 sol, DuPont, Wilmington, DE) was used to supply the SiO 2 . These cation sources were dissolved in stoichiometric ratios in deionized water. To improve the solubility of the Ludox SK, the pH value of the solution was adjusted by addition of nitric acid. Once the cat- ion sources were completely dissolved, the 5 wt% PVA solution (polyvinyl alcohol, 205S, Celanese Chemicals Inc., Dallas, TX) was added. Water was evaporated by continuous magnetic stir- ring and later by hand stirring with glass rods during heating on a hot plate, until the decomposition of the organic/inorganic pre- cursors had fully taken place. The products of the decomposed organic/inorganic precursors were ground and calcined at a heat- ing rate of 51C/min, in an air atmosphere in a box furnace at 8001C for 1 h, and white powders were obtained. To obtain finer particles, the calcined powder derived from the PVA technique was subjected to wet attrition milling at 50 rpm for 1 h in a jar, 85 mm in diameter  105 mm in height. For one batch of powders (30 g), 700 g of ZrO 2 balls Ca(NO 3 ) 2 •4H 2 O De-ionized water Al(NO 3 ) 3 •9H 2 O PVA solution Ludox Precursor solution preparation by PVA route: Water evaporation Powder calcination Powder wet attrition milling Solvent evaporation Sintering Cold press + CIP Samples Powder sieving 125 °C, in an oven Magnetic stirring Heated on a hot plate; magnetic stirring and hand stirring 800 °C/1h, in a furance 50 rpm/1h, with a ball to charge weight ratio of ~23 30-40# 10 MPa + 300 MPa 1500 °C/2 h Fig. 1. Processing flowchart of the gehlenite (2CaO Á Al 2 O 3 Á SiO 2 or ‘‘C 2 AS’’) ceramic made by the PVA method. A. Bandyopadhyay—contributing editor *Member, American Ceramic Society. **Fellow, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: kriven@uiuc.edu. On leave from the Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin, China. Manuscript No. 22387. Received October 23, 2006; approved April 27, 2007. J ournal J. Am. Ceram. Soc., 90 [9] 2766–2770 (2007) DOI: 10.1111/j.1551-2916.2007.01835.x r 2007 The American Ceramic Society 2766