Slip casting involves the dewatering of a colloidal suspension of mineral particles in a porous mould, leaving a coating that can be sintered after drying. It has been in use for clay-based ceramics since the 18th century. Slip casting of non-clay materials is a relatively new technique as it was previously thought that clay was, necessary to produce a slip. Schwerin, in 1910, was the first to successfully slip cast a non-clay material. Since then, slip casting has been applied to oxide and non-oxide ceramics as well as metals and cermets.
Slip Casting of Non-Plastic Materials
The slip casting of non-plastic materials is difficult owing to the nature of non-clay colloids. The size of non-clay particles must be reduced to that of the colloidal range (~ 1 µm) resulting in roughly spherical particles with typically negatively charged surfaces. Such slips do not exhibit the characteristics exhibited by clay-based slips: inherently high specific surface area SSA; edge face flocculation; dense particle packing of preferred orientation. With non-clay slips, a binder is also required in order to enhance the low dry strength of slip cast articles. A slip that will be stable for a period of time sufficient to produce homogeneous castings must be well deflocculated and characterised by a high solids loading ~ 20-30 vol % (typically ~ 60wt %) and thixotropic rheology.
Slip Casting Techniques
There are two common methods for producing shapes by slip casting. These are drain slip casting and solid slip casting. In drain slip casting, the slip is left in the mould until the desired wall thickness is obtained, at which point the excess slip is drained from the mould. Solid slip casting involves keeping the slip in the mould and replenishing it during the entire process until a solid cast is achieved. Thixotropic casting is a specialised form of solid slip casting, involving highly thixotropic slips of very high solids loadings, a technique that has been used in the refractories industry. The higher the solids loading, the more pronounced is the thixotropy, to the limiting point beyond which gelation cannot be reversed by vibration.
Thixotropic casting can be defined as vibratory solid slip casting of highly concentrated and highly thixotropic non-plastic slips in porous moulds. Rapid setting results from the fact that the solids loading is near the limit for gelation and so no topping up of the mould is needed and only a negligible amount of dewatering need occur. Vibratory forming methods are well documented in the literature for refractory cements and cement pastes. However, thixotropic casting differs from vibratory forming of refractory cements in that setting in the former is solely dependent on gelation induced by dewatering of the suspension while, in the latter, it is dependent on the bonding of the cement.
Thixotropic Casting Slips
As for casting slips in general, a thixotropic casting slip should be optimally deflocculated. Further, in the case of thixotropic casting slips, the solids content must be maximised such that the suspension is only just fluid when subjected to vibration. Solids loadings up to 78 vol% can be achieved through gap-grading, optimal deflocculation, and the use of moderately viscous suspending liquids, although experience by the authors with this technique has revealed that solids loadings of 40-50 vol% (typically -70-80 wt%) are more common in practice. Thixotropic casting is suited only to slips with low colloidal content. Thus, clay-based slips or non-clay slips with high SSA are not appropriate for thixotropic casting. This is because it is difficult to overcome the strong thixotropy of such slips by means of vibration, whereas the "false-thixotropy" of deflocculated coarse non-plastic slips is easily broken down. Furthermore, the high SSA of colloidal clay-based slips or non-clay slips limits the maximum solids loading attainable.
Flexibility of The Thixotropic Casting Process
Thixotropic casting, like slip casting in general, is well suited to the production of complex shapes. It is also a cost effective and simple process requiring little capital outlay. In addition, thixotropic casting, unlike conventional slip casting, provides the added advantages that come with having a very high solids loading: i.e. rapid setting due to natural gelation and high dimensional tolerances due to the negligible drying shrinkage which results from the minimal water content. A further advantage of thixotropic casting is its potential for the elimination of differential sedimentation.
Thixotropic Casting and Its Suitability for Processing Ceramic Matrix Composites
The utility of thixotropic casting is in eliminating differential sedimentation and such a wet-forming process is ideal for the preparation of CMCs. If mixture homogeneity is retained during the forming stage, the resulting CMC will be characterised by microstructural uniformity and therefore will attain optimal strength and toughness. The elimination of differential sedimentation can be achieved through the use of a casting slip that is fluid only during the vibratory mixing and pouring stage. The near instant gelation rate will ensure that homogeneity will be retained during the setting stage in the casting mould.