Aqueous milling and near net shape forming of SiAlON ceramics

Abstract

A relatively new technique known as “Gelcasting” has the potential for producing near net shaped ceramics in large or small quantities without the limitations of conventional processing techniques such as difficult binder removal, formation of density gradients in the green state or limited shape complexity. The gelcasting technique involves dispersing a ceramic powder in a solution containing an organic monomer. Green ceramic articles are then formed by pouring the dispersion into a mould and polymerising the monomer to form a gel that retains the shape of the mould. Drying and sintering follow to produce a dense near net shape ceramic component. A major disadvantage to the gelcasting technique has been the need to use toxic acrylamide monomer as a gelling agent.

Two aspects of the gelcasting process were examined in this study with a view to adapting it to the production of SiAlON components: (i) dispersion of multi component oxide and nonoxide ceramic powder mixes in aqueous media; and (ii) replacement of the organic monomer with a non-toxic polysaccharide (gellan gum) while attempting to retain controllable gelling properties.

Milling aqueous based slurries has several advantages over their organic counterparts. Use of aqueous slurries reduces material costs, waste disposal and potential fire hazards. Aqueous milling also takes advantage of the enhanced effects of electrostatic forces which can aid in obtaining higher solids loadings.

The SiAlON in this study was produced from the ceramic powder components Si3N4, AlN, Y2O3, and Al2O3. These components are usually milled in isopropylalcohol (IPA) to prevent hydrolysis of the AlN component. Hydrolysis of AlN in an aqueous media is a self sustaining process producing hydroxyl ions which raise the pH which in turn increases the rate of hydrolysis. The rate of hydrolysis has been reported to be significantly reduced by maintaining a pH below 7. AlN can be purchased with a hydrophobic protective coating. However, the milling process necessary to homogenise the powder components will remove this coating.

Commercial SiAlON ceramics utilise synthesised 21R polytypoid as a replacement for AlN. This Al and N rich SiAlON phase, Al7O3N5, is resistant to hydrolysis during aqueous milling.

Betz 1190 dispersant was used in conjunction with low pH to reduce the viscosity and raise the solids content of the slurries. The dispersant is a polymer with positively charged head groups distributed along it’s length. These head groups are attracted to the particles primarily by hydrogen/chemical bonding promoting dispersion in two ways; (i) the individual particles have a net positive charge, due primarily to the dispersant charge, this promotes long range repulsion between particles and (ii) the polymeric portion of the adsorbed dispersant acts as a physical (steric) barrier around the ceramic particles preventing short range particle-particle interaction. The low pH of the solution enhances the positive surface charge on the ceramic particle surfaces.

Gellan gum has been developed as a substitute for agar in microbiological media. It has rheological properties superior to the those of agar and exhibits a gelling mechanism which could replace acrylamide monomer. Gellan gum can be prepared over a wide range of pH values, including acidic conditions. This provides versatility when selecting the method of dispersion. Gelled solutions can be formed with as little as 0.05 wt % in water. In addition it has low viscosity when heated above 75°C. It has several advantages over agar such as lower viscosity for a given solution concentration and the ability to form irreversible gels by the addition of divalent cations.