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stress is an important task. In particular, the identification of regions of high stress
level in the coating structure is of major concern. In fact, the configuration of the
local structure of the material, such as the presence of bubbles or the existence of a
transition zone between the two constituents of the material, can affect the local
values of the stresses, thus promoting or delaying the onset of the fracture of the
material in service. This can be accomplished by directly evaluating the residual
stress on the real microstructure of the material, obtained from digital micrographs
processed to this task. Digital image processing, widely used in many fields of
sciences and engineering, has been applied in material sciences for quantitative
description of complex material microstructures [
10
,
11
] and, in particular, for
stress investigation in thermal spray coatings [
12
,
13
].
In this chapter, a procedure based on the analysis of digital images of enamel
coatings has been applied to the measurement of thermal stresses. The approach
considered uses the digital images filtered and converted into a graphical format to
perform finite element simulations on the real local structure of the material. In
this study, coatings manufactured with different technologies were considered. A
preliminary characterization of the microstructures was performed by quantitative
measurements on digital images. Then, the thermal stresses were evaluated taking
into account F.E.M. discretization of the coating structure of growing complexity.
The results of the local approach were finally related to the characteristics of the
microstructure.
9.2 Enamel Coating and Enameling Technology
Vitreous enamels are inorganic materials that are used as coatings for metallic
components. These materials are based on a special ceramic-vitreous matrix in
which specific additives are randomly dispersed. The ceramic-vitreous matrix is
made by a mixture of various row materials and elements and in particular it is based
on boron-silicate glass added with metal oxides of titanium, zinc, tin, zirconia,
alumina, etc. These additions are often used to enhance some important perfor-
mances such as corrosion and wear resistance, mechanical strength, fracture
toughness, and also esthetic functions. The compound obtained by mixing these
rough materials is then fired at about 1,300 C and quickly cooled into cold water, as
shown in the scheme of Fig.
9.1
. At the end of this process, the ceramic-vitreous
matrix is obtained and it is commonly called frit.
In order to obtain the vitreous enamel raw materials, specific additives have to
be mixed and milled with the frits. These additives are necessary to increase the
adherence of the enamel to the metal substrate and also to give special properties
and functions to the final coat. The mixing-milling process can be realized by a
wet or a dry process. In the wet process, the vitreous enamel raw material consists
on a slip in which the small grain of the frits and of the additives are homoge-
neously dispersed in water. In the case of the dry milling process, the addition of
silicone oil enables the production of fine powder made by frits and additives
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