Functionalization of Surface Layers on Ceramics
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Ceramics are often prepared with surface layers of different composition from the bulk, to impart a specific functionality to the surface or to act as a protective layer for the bulk material. Here I describe a general process by which functional surface layers with a nanometer-scale compositional gradient can be readily formed during the production of bulk ceramic components. The first concept regarding this process was established by Ishikawa et al. in 1998. The basis of this approach is to incorporate selected low-molecular-mass additives into either the precursor polymer from which the ceramic forms, or the binder polymer used to prepare bulk components from ceramic powders. Thermal treatment of the resulting bodies leads to controlled phase separation (“bleed out”) of the additives, analogous to the normally undesirable outward loss of low-molecular-mass components from some plastics; subsequent calcination stabilizes the compositionally changed surface region, generating a functional surface layer. This approach is applicable to a wide range of materials and morphologies, and should find use in catalysts, composites, and environmental barrier coatings.