Two methods of fabricating large cross section structural ceramic parts of complex shape (e.g. turbine engine rotors) are by an injection molding process or a slip casting process. In injection molding, a ceramic powder such as silicon nitride with the appropriate additives to promote densification and to impart the desired material properties is mixed with an organic binder in such a way that the mixture attains the properties of a thermoplastic material. It is then shaped by injection molding techniques and equipment well known in the plastics industry. The organic binder is removed by a thermal process involving liquification, pyrolysis and distillation. The resulting porous ceramic green body is densified by hot isostatic pressing or sintering.
The non-destructive removal of the organic binder however, remains the yield limiting step in the process. For successful densification by hot isostatic pressing or cold isostatic pressing followed by sintering, the parts must emerge from the binder removal step in an externally crack free condition. Internal flaws must be minimized in order to maintain dimensional tolerances. These conditions become more difficult to attain as the parts become larger in cross section and complexity. Lower yields of acceptable parts result.
In slip casting, a ceramic powder such as silicon nitride with the appropriate additives to promote densification and to impart the desired material properties is mixed with a liquid vehicle, typically water, dispersants and organic binder in such a way that the mixture attains a suitable low viscosity for pouring or pumping into a porous mold. The porous mold adsorbs excess liquid vehicle leaving a solid component of ceramic powder and binder, saturated with liquid vehicle in the spaces between ceramic particles. The component is removed from the mold and dried to remove the residual liquid vehicle. The organic binder is removed by a thermal process involving liquification, pyrolysis and distillation. The resulting porous ceramic green body is densified by hot isostatic pressing or sintering.
The non-destructive removal of the residual liquid vehicle in a hick cross section slip cast component requires a slow controlled drying cycle. For successful densification by hot isostatic pressing or cold isostatic pressing followed by sintering, the parts must emerge from the binder removal step in an externally crack free condition. Internal flaws must be minimized in order to maintain dimensional tolerances. These conditions become more difficult to attain as the parts become larger in cross section and complexity. Lower yields of acceptable parts result.