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the metal-enamel system. A wide spectrum of industrial and domestic applications
currently make use of these coatings (e.g., the treatment of components for
household use, the protection of interior walls of reactors for chemical processes,
the protection of mechanical components of rotary heat exchangers, and aircraft
turbojets). Enameled steel components are appreciated for their esthetic properties
as well as for their chemical characteristics. In particular, enameling of metal
substrates is used to provide protection against chemical corrosion for applications
in hostile environments such as in reactors for chemical processes. In fact, from a
functional point of view, vitreous enamel coatings have an excellent resistance to
chemical corrosion processes [
1
-
3
]. Steel enameling is also used to obtain low
roughness surfaces, having no fouling, and/or antimicrobial properties. Vitreous
enamel coatings surfaces are also characterized by high values of hardness (up to
800 HV) giving to coated components a good resistance to tribological phenomena
such as abrasive wear [
4
]. Compared to other coatings for metals, such as ther-
mally sprayed ceramics, vitreous enamel coatings are characterized by a chemical
and not only a physical adhesion to the substrate achieved by a graded interface
that is developed during the coating firing process. Enameled steel components are
also subjected to internal stresses. From a macroscopic point of view, these
internal stresses contribute to prevent the instability of enameled steel plate when
subjected to impact event [
5
]. But, the presence of residual stresses also affects the
coating failure due to cracks onset and propagation, the spallation of the coating,
the shape changes of the coated components and in general can influence the
performance of the entire coated parts. Even if enameling is nowadays an indus-
trial practice, some of the mechanical performances of enameled composites have
not been studied and others are not completely understood. Based on this fact, the
present work is intended to add an original contribution about one of the main
physical and microstructural aspects that characterize the enamel coating: the
residual stress and its relationship with the local characteristics of the enamel
matrix.
Residual stresses are introduced in the enameled substrate during the manu-
facturing process. In fact, during the cooling to room temperature thermal mis-
match stress develops due to the difference between the coefficients of thermal
expansion (CTE) of the vitreous enamel material and the metal substrate. Different
methods have been used to measure the residual stresses in coatings: mathematical
modeling [
6
,
7
], mechanical methods [
8
], and material removal techniques [
9
].
Each technique has certain advantages and limitations; their applicability is
determined by such factors as shape, dimensions, materials of the coating and the
substrate, knowledge of the constituents' properties and processing conditions. The
experimental methods can be either destructive or non destructive, but all use a
global approach to the evaluation of the residual stress. The computational models
may be better tools to identify potential problems and decide what changes in the
coating process are necessary to control the residual stresses. However, most
models of residual stresses assume that coatings are homogeneous in structure and
therefore cannot predict stress concentrations. But, for a more exhaustive
mechanical characterization, the study of the local distribution of the residual
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