CERAMICS ARE EXTREMELY RESISTANT MATERIALS, NON-DUCTILE AND WITH A HIGH MELTING TEMPERATURE. CONVENTIONAL METHODS OF MANUFACTURING BY MACHINING, PLASTIC DEFORMATION OR CASTING ARE INAPPROPRIATE OR IMPOSSIBLE FOR THESE MATERIALS. THE CERAMIC PROCESSES CONSIST IN SHAPING POWDER BEFORE CONSOLIDATION OF THE OBJECT AT A HIGH TEMPERATURE. SEVERAL DIFFERENT WAYS OF SHAPING EXIST AND OPERATE AT DIFFERENT STATES OF MATTER.
In processes using liquid methods, ceramic powders are dispersed in a solvent (water, alcohol, etc.) to result in a suspension (or slurry in the case of clay suspensions) of the desired properties. These suspensions usually have non-Newtonian behaviour, that is to say, their viscosity changes with the stresses exerted on the fluid, which can be used in the shaping processes such as casting or deposition techniques. A rheofluidifying suspension with a flow threshold has, for example, a low consistency during the stirring and pouring phases, but its viscosity will increase sharply when it ceases to be agitated, which prevents sedimentation during the storage phases and “streaks” after a deposit or coating. Processes that use liquid methods are mould casting, tape casting and dip coating processes.
Processes using the plastic route deform a paste to shape the object to be produced. Clay paste is naturally plastic, but this is not the case for technical ceramic powders, which need to be mixed with organic materials to obtain a rheological behaviour equated with abnormal fluid. Ceramic objects are then shaped by extrusion or injection.
Processes using the granular dry route are intended to agglomerate the fine powders in a controlled manner to impart a good flow for filling the moulds and sufficient plasticity to deform them under the action of pressing. One way to achieve this is, for example, to produce a suspension and spray it in droplets, which are dried by a hot air stream (atomization process). The granules obtained are spherical clusters of granules of the initial powder, and also contain binders and plasticizers previously added to the suspension.
The final step of the ceramic process is to consolidate the granular structure obtained by one of the methods described above through high temperature by a process known as “sintering“. At the beginning of sintering, the granules constituting the object “fuse” together to form bridges ensuring consolidation of the granular structure. The porosity of this structure is typically 40%. Continuing sintering leads to densification (porosity reduction), which is accompanied by a reduction in the volume of the part (dimensional shrinkage). Granule growth can also take place in the last step of the sintering process.
Mastery of the shaping and sintering processes is of great importance since they directly affect the microstructure of the part (size and shape of the pores, granules, presence of defects, etc.) and therefore, its final properties. The expertise of the ceramicist is applied to adapt the manufacturing process of the part to the needs associated with its use.
The CTTC has the skills in the science of ceramic processes and the equipment necessary to assist in the implementation of new products from the prototype to the industrialisation phase.
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