Water-based Tape Casting of Ceramics and Fabrication of Ceramic Laminates

Doktorsavhandling, 1999

The objective of the work presented in this dissertation has been twofold: to develop a water-based system for tape casting and to build laminar structures from the tapes. A set of requirements on the binder was specified and latexes were identified as suitable for water-based tape casting, mainly owing to their low viscosity and high polymeric content. A systematic approach was used to evaluate different formulations throughout the processing chain to find possibilities and limitations. Rheological measurements were used to characterise stability and flow behaviour of the suspensions and to study interactions between constituents in the suspension. The effect of the dispersant concentration was studied. It was found that, below optimum dosage, i.e. monolayer coverage, bridging flocculation can occur and the dispersant can adsorb onto latex particles. Tape casting experiments were done to investigate possible casting rates, resulting thickness and the thickness at which spontaneous cracking occurs. The cast tapes were then evaluated with regard to their quality, green density and homogeneity. It has been shown in this work that very high solids loading (> 55 vol%) can be reached in alumina systems thanks to efficient dispersants and latex binders with a high polymeric content. This in combination with an efficient drying system of the tape caster enabled high casting rates to be reached. The lowest viscous formulations also gave tapes thicker than 0.5 mm without cracking. Significant differences between binders were observed in this work. For example, in a comparison between anionic and nonionic latexes, the nonionic latex gave a more homogeneous packing of the green tape and higher final density. In the anionically stabilised latexes, phase separation between surfactant and polymer occurred, creating pore channels in the tape. The work on laminar structures focused mainly on ceramic laminates with crack deflecting ability. These types of structures have been shown to be more damage tolerant and to have superior thermal shock resistance. Examples of laminates of this type are SiC/graphite and dense/porous laminates of alumina or SiC using fugitive particles. A technique for fabricating the latter type of laminates was developed in this work. The composite is a laminate of alternating porous and dense layers of the same material. To be able to co-sinter these types of layers, it is necessary that the added fugitive particles do not affect the stability of the suspension, i.e. the packing of the ceramic matrix in the layers should be the same. Hereby the dense and porous layers sinter with the same total shrinkage, and the fugitive particles (mainly starch particles) leave voids too large to sinter themselves and will shrink only by the same amount as the surrounding matrix, leaving porosity. The advantage of such a composite is that no chemical reaction or thermal mismatch is present between the layers. During casting, when the suspension passes under the casting blade, the shearing can cause particles and polymer to be oriented in the direction of the flow. This can be used deliberately to enhance mechanical, electrical or thermal properties. However, it can also cause sintering anisotropy, making close dimensional tolerances difficult to control. The conditions that give rise to shrinkage anisotropy were studied. Water-based systems were shown to be on a par with or better than organic solvent-based systems. A desirable particle orientation benefits from a high volume fraction, elongated or plate-like particles and a high shear rate. This was exploited in making a laminate structure with alternating porous and dense layers from a coarse low sinterability powder and a fine plate-like powder for the respective layers.