Slip Casting of ZrB₂–SiC Composite Aqueous Suspensions

Abstract

Zirconium diboride (ZrB 2 ) is a material of particular interest because of the excellent and unique combination of high melting point, high electrical and thermal conductivity, and chemical inertness against molten metals or nonbasic slags. [1] These properties make it an attractive candidate for ultra-high temperature applications where corrosionwear-oxidation resistance is demanded. [2] Sintering is conducted at temperatures higher than 1700 8C depending on the sintering techniques (pressure assisted or pressureless) and/or sintering aids used. [3][4][5][6][7][8] ZrB 2 sintered bodies are usually manufactured with relatively simple geometrical shapes because their brittleness and high hardness make difficult the mechanical machining. Moreover, the requested diamond machining is usually expensive. The electrical conductivity of ZrB 2 would permit the production of complex shaped parts by electrical discharge machining (EDM); however, such a method induces the oxidation of the external surfaces with the consequent formation of superficial flaws. The production of near-net shape components by slip casting could be an advantageous technique to overcome these drawbacks.Aqueous dispersions of ZrB 2 with different dispersants were the subject of previous studies, in particular for the production of 3D components by freeze-form extrusion. [9,10] Recently, high solid loading (45 vol%) slurries of ZrB 2 -SiC composite for slip casting were prepared by using polyethylenimine (PEI) as dispersant and nearly fully dense cones were obtained. [11] The present work deals with the production of near-net shapes from ZrB 2 -based compositions by using slip casting as forming technique. Because it is well-known that the presence of silicon carbide (SiC) in the composition ensures improved strength, toughness, and oxidation resistance with respect the pure ZrB 2 , a ZrB 2 -based composite containing 20 vol% of SiC as secondary phase has been chosen and Si 3 N 4 was used as sintering aid. [2,[12][13][14][15] The effect of two commercial ammonium polyacrylates (Duramax D3005 and Dolapix PC33) on the electrostatic stabilization of the casting slips was investigated in order to select the most proper formulations. In particular, zeta potential measurements in water both on the single powders and on the pre-mixed composite powder were performed. As final components, crucibles of different thickness were produced as green bodies, sintered at 2150 8C and then characterized to highlight the effect of the dispersants on the final microstructures.

Ice templating of ZrB2–SiC systems

“…As observed in previous works [22,23], Dolapix is less effective than Duramax in stabilizing the composite suspensions, but the effectiveness of the two poly-electrolytes results exchanged for the SiC powder. Moreover, Dolapix PC33 is the most effective in optimizing the electro-steric repulsions between particles, because of the higher molecular weight.…”

“…According to previous studies [22,23], Duramax D3005 promotes a better stabilization of monophasic ZrB 2 and ZrB 2 -SiC slurries compared to Dolapix PC33. Conversely Dolapix is more effective in stabilizing SiC powder.…”

Continuous C fibre composites with a porous ZrB2 Matrix

“…The powders were homogenized through ball milling for 24 h in ethanol using Si 3 N 4 balls, dried by rotary evaporator and sieved. With those powder mixtures, aqueous slurries were prepared according to previous studies [12]. The composites were fabricated infiltrating the 1-D fabrics with the two types of slurries by hand lay-up and subsequently stacking them in a 0-90°configuration as illustrated in Fig.…”

From random chopped to oriented continuous SiC fibers–ZrB2 composites

“…For the matrix of the CMCs, two compositions were prepared (see Table 1): -ZrB 2 À 20vol% ZrSi 2 ðCMC-ZÞ -ZrB 2 À 4vol% Si 3 N 4 ; ðCMC-SÞ The powders were homogenized through ball milling for 24 h in ethanol using Si 3 N 4 balls, dried by rotary evaporator and sieved. With those powders mixtures, aqueous slurries were prepared using Dolapix PC33 (Zschimmer & Schwarz, Lahnstein-Rhein, Germany) as dispersant and Versicol E11 (Allied Colloids, Bradford, UK) as binder, according to previous studies [19,20]. The slurries were prepared using a solid loading of 22 wt%.…”