Studio del comportamento meccanico di smalti porcellanati per substrati metallici
Creators: name: Rossetti, Luigi <1978>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: ING-IND/14 Progettazione meccanica e costruzione di macchine

description: Composite porcelain enamels are inorganic coatings for metallic components 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 raw materials and elements and in
particular it is based on boron-silicate glass added with metal oxides(1) of titanium, zinc, tin,
zirconia, alumina, ecc. These additions are often used to improve and enhance some important
performances such as corrosion(2) and wear resistance, mechanical strength, fracture
toughness and also aesthetic functions. The coating process, called enamelling, depends on
the nature of the surface, but also on the kind of the used porcelain enamel. For metal sheets
coatings two industrial processes are actually used: one based on a wet porcelain enamel and
another based on a dry-silicone porcelain enamel. During the firing process, that is performed
at about 870°C in the case of a steel substrate, the enamel raw material melts and interacts
with the metal substrate so enabling the formation of a continuous varying structure. The
interface domain between the substrate and the external layer is made of a complex material
system where the ceramic vitreous and the metal constituents are mixed. In particular four
main regions can be identified, (i) the pure metal region, (ii) the region where the metal
constituents are dominant compared with the ceramic vitreous components, (iii) the region
where the ceramic vitreous constituents are dominant compared with the metal ones, and the
fourth region (iv) composed by the pure ceramic vitreous material. It has also to be noticed
the presence of metallic dendrites that hinder the substrate and the external layer passing
through the interphase region. Each region of the final composite structure plays a specific
role: the metal substrate has mainly the structural function, the interphase region and the
embedded dendrites guarantee the adhesion of the external vitreous layer to the substrate and
the external vitreous layer is characterized by an high tribological, corrosion and thermal
shock resistance. Such material, due to its internal composition, functionalization and
architecture can be considered as a functionally graded composite material. The knowledge of
the mechanical, tribological and chemical behavior of such composites is not well established
and the research is still in progress. In particular the mechanical performances data about the
composite coating are not jet established. In the present work the Residual Stresses, the
Young modulus and the First Crack Failure of the composite porcelain enamel coating are
studied. Due to the differences of the porcelain composite enamel and steel thermal properties
the enamelled steel sheets have residual stresses: compressive residual stress acts on the
coating and tensile residual stress acts on the steel sheet. The residual stresses estimation has
been performed by measuring the curvature of rectangular one-side coated specimens. The
Young modulus and the First Crack Failure (FCF) of the coating have been estimated by four
point bending tests (3-7) monitored by means of the Acoustic Emission (AE) technique(5,6). In
particular the AE information has been used to identify, during the bending tests, the
displacement domain over which no coating failure occurs (Free Failure Zone, FFZ). In the
FFZ domain, the Young modulus has been estimated according to ASTM D6272-02. The
FCF has been calculated as the ratio between the displacement at the first crack of the coating
and the coating thickness on the cracked side. The mechanical performances of the tested
coated specimens have also been related and discussed to respective microstructure and
surface characteristics by double entry charts.