Aqueous slip casting of complex-shaped ceramic components

Abstract

Results of theoretical and experimental studies of the process of aqueous slip casting of complex-shaped ceramic components using plaster molds are given. A technology for manufacture of complex-shaped homogeneous high-quality preforms from quartz ceramics such as aerial fairing shells is proposed.The casting of aqueous slip into plaster molds is a currently employed technique for shaping thin-walled ceramic components, for example, aerial fairing shells [1,2]. The buildup of a preform on the surface of a complex-profile mold involves two major processes:-capillary suction of the liquid phase of the slip by the plaster mold and movement of slip particles towards the mold and their deposition on the mold surface;-deposition of slip particles on the mold surface under the action of gravity.One of the two processes becomes prevailing depending on the density, viscosity and grain composition of the slip. A judicious trade-off between these parameters is a major concern in developing a technology for shaping preforms. With the former process predominant, a more dense and uniform packing of particles is achieved, and the preform shape exhibits higher strength characteristics.Apart from the capillary suction and gravity forces, a slip particle is subjected to the buoyancy force of liquid phase (static lift) and to the viscous force of the medium. The particle's velocity vector is controlled by the direction and intensity of all the forces involved. With allowance for Stokes' law the equation of motion of a particle towards the curved surface of a mold takes the formwhere m is the mass of the particle; V p is the velocity of motion of the particle towards the preform surface; V f is the speed of water filtration; r is the radius of the particle; h is the viscosity of the medium (the slip); q is the horizontal slope of the tangent to the curved surface of the preform; P g is the gravity force, P g = mg; P a is the buoyancy force, P a = m 0 g; r is the particle density; r 0 is the slip density; m 0 is the slip mass; m = r(4pr 3 /3); m 0 = r 0 (4pr 3 /3).Equation (1) describes the transient motion of a particle from the state of rest to a fully established quasi-stationary motion when the inertial force on the left side of the equation can be neglected. According to [3], one has the solution V V P P r V g r g a p f f = + -+ -6 2 9 2 0 p h q = r r h q cos ( ) cos . (2) Allowing for 2 8 2 0 gr V g ( ) r r h -=(V g is the gravitational deposition velocity of a slip particle), expression (2) can be written asThe particle velocity V p is a major factor controlling the preform buildup rate; by analogy with Eq. (3), the preform buildup rate for a curved surface can be written aswhere h f is the portion of the wall thickness that was built up by capillary filtration of water in the plaster mold; h g is the

Gelcasting of alumina based ceramic cores containing yttria for single crystal and directional solidification blades

“…8,9 The aqueous slurry prepared by gelcasting can fill up mould cavities of complex parts under no pressure, and the complexity of shape attained by gelcasting is limited only by the ability of mould design. 10 A rapid prototyping method is developed by combining selective laser sintering and gelcasting technique for the fabrication of complex shaped Al 2 O 3 ceramic parts. 11 Based on the investigation of the effects of suspension composition on the drying, debinding and pressureless sintering processes, a stable Al 2 O 3 /NiO binary oxide aqueous suspension with optimal monomer, dispersant and solid composition is defined, and then complex shaped Al 2 O 3 /Ni nanocomposites without degraded mechanical properties are successfully fabricated by a gelcasting process.…”