Biomedical Engineering Reference
In-Depth Information
them prone to ion diffusion from the substrate toward the coating
surface. This feature can severely hamper the nucleation of the calcium
phosphate-rich layers and thus the formation of crystalline HCA. In
fact, only a small percentage of multi-valent cations is sufficient to
completely hinder the bioactivity of a glass and thus its bone bonding
ability [1]. Multi-layer coatings and careful optimization of both the
glass composition and the firing schedule should be good strategies to
achieve this goal.
8.3 GLAZING
A thin, glossy, and glass-like coating formed on the surface of a ceramic
substrate is a ''glaze.'' The glass, synthesized by a melting and quenching
route, are usually applied to the substrate by dipping, spraying, pouring,
painting, and so on (as already described for enameling). An optimized
thermal treatment must be carefully developed at temperatures slightly
above the liquid flow temperature of the glass, obtaining layers with a
thickness of about 30-150
m. After the firing process, the coated sam-
ples should be annealed, in order to obtain tension-free glass coatings.
More precisely, in order to have an amorphous layer, after the heat
treatment, the coating can be simply annealed; otherwise, in order to
obtain a glass-ceramic coating, the coated materials must be thermally
treated with a nucleation and growth process, based on the characteristic
temperatures of the pure glass (Chapter 7).
In the biomedical field, glazing can be successfully used to coat zirconia
and alumina by bioactive glasses and glass-ceramics [7]. An intermediate
layer often forms between the coating and the substrate - for example,
between a glass coating and a zirconia substrate there is a ''composite''
layer made of glassy phase and zirconia particles. During the thermal
treatment above its melting point, the glass diffuses within the zirconia
substrate, so the zirconia granules are surrounded by a glassy matrix,
leading to the formation of a ''composite'' layer, which assures a
continuity of thermal and mechanical properties from the zirconia
substrate to the glass coating.
In order to study the adherence of the coatings, the relative crack
resistance can be qualitatively evaluated using indentation techniques.
Indentation techniques work on the basis that an indenter (diamond
tip) pressed into a harder material will travel less far into the material
than it would into a softer material. Because of the complex loading
μ
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