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A novel technique to characterise and evaluate the interfacial fracture toughness of coatings
thesis
posted on 2017-01-31, 04:35 authored by Elambasseril, JoeCoatings are often used as a covering to enhance the quality and protect the material
to which they are applied to. The interface between the coating and substrate is the
weakest part of the bimaterial system. Through accurate prediction of the interfacial
fracture toughness, coatings can be produced that are more reliable as unanticipated
failures are less likely to occur. Currently the tests that are carried out in this field
have limitations and are only able to test certain coating–material combinations or
are only suitable for certain loading conditions.
A simple method using circumferentially notched tensile (CNT) specimen was
developed for quantitatively evaluating the adhesion of coatings on substrates. The
method is based on common tensile test of materials and is competent for estimating
the mixed mode fracture toughness over a range of phase angles. In the first stage the
0, 15, 30, 45 and 60 degree notch angled mild steel cylindrical substrates with
electroplated nickel were studied in detail using finite element models. In the second
stage the nickel coated steel CNT specimens were tensile tested with a well defined
precrack at the interface. In-situ acoustic signals acquisitions and scanning electron
microscope were carried out to obtain the crack initiation (critical load) and
propagation. From the critical load, critical interface energy release rate as a function
of mode mixity was evaluated using J- integrals. The results showed an increase in
the interfacial fracture toughness values with the increase in phase angle. In the third
stage, the studies were also conducted to evaluate the fracture properties of TiN and
TiAlN coatings using CNT specimens. The combined results of computational and
experimental analysis showed that any defect or stress concentration at the interface
can significantly weaken the adhesion of coating. It has been concluded that this new
proposed method is an easier testing method to evaluate the adhesive properties of
both ductile and brittle coatings on metallic substrates compared to traditional
methods.
History
Campus location
AustraliaPrincipal supervisor
Raafat IbrahimYear of Award
2011Department, School or Centre
Mechanical and Aerospace EngineeringCourse
Doctor of PhilosophyDegree Type
DOCTORATEFaculty
Faculty of EngineeringUsage metrics
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