Mechanical size effects in thin copper foils. An experimental study
Kein Vorschaubild vorhanden
Autor:innen
Autor:in (Körperschaft)
Publikationsdatum
2004
Typ der Arbeit
Dissertation
Studiengang
Institute for Mechanical Systems
Typ
11 - Studentische Arbeit
Herausgeber:innen
Herausgeber:in (Körperschaft)
Betreuer:in
Übergeordnetes Werk
Themenheft
DOI der Originalpublikation
Link
Reihe / Serie
Reihennummer
Jahrgang / Band
Ausgabe / Nummer
Seiten / Dauer
Patentnummer
Verlag / Herausgebende Institution
ETH Zürich
Verlagsort / Veranstaltungsort
Zürich
Auflage
Version
Programmiersprache
Abtretungsempfänger:in
Praxispartner:in/Auftraggeber:in
Zusammenfassung
The goal of this work is the investigation of the effect of several size parameters
on the mechanical behavior of thin copper foils in tensile testing, in particular the
question is whether a smaller sample has a different mechanical behavior than a
larger one. Attention is paid to the most relevant size parameter, the thickness,
and the influence of the microstructure of the foils, a factor which has not been
accounted for systematically in literature up to now.
Copper foils with 10, 20 and 34 μm thickness are tensile tested in-house, thicker
foils (50, 100 and 250 μm) by a project partner (Laboratory of Materials for Mecha-
tronics and Electrical Engineering, University of Applied Sciences Augsburg, Prof.
Villain). For the tensile tests, a new setup was built which extends a previous setup
developed by [Mazza, 1997] and allows for an automatic testing of the samples at a
controlled strain rate. The standard strain rate applied is ˙≤ = 10−4 1/s. Samples
with a dogbone shape are tested, their geometry is scaled according to the thick-
ness. The samples are produced by wet etching of rolled and electrodeposited copper
foils (standard sample type, “as-received samples”). Some of the samples are heat
treated after etching (“heat treated samples”).
As the microstructure of a crystalline solid has a significant influence on its
mechanical behavior it is characterized in detail. Different techniques such as met-
allography, X-ray diffraction and electron backscatter diffraction are applied for
this task. The rolled samples (10 and 20 μm thick) have a strong cube texture with
elongated grains with an oblate cross-section (typical length 100 μm, small diameter
5 μm, long diameter 30 μm). The electrodeposited samples have a columnar grain
structure with a weak fibre texture. Heat treatment changes the microstructure of
the rolled foils considerably. The grains are equi-axed with an average diameter of
15 μm. Thus, the 10 and 20 μm heat treated foils have only 1-2 grains per thick-
ness. Rolling texture components with 〈111〉 parallel to the rolling direction form
the preferred orientations, some grains are still in cube orientation.
The most important result of the tensile tests is that the thickness of the foils
has an influence on the mechanical behavior in the size regime studied. When
the thickness is reduced from 250 to 10 μm the fracture strain decreases for the
as-received foils from approximately 20% to 0.2% and for the samples with heat
treatment from 35% to 15%. The tensile strength increases with smaller thickness
for the as-received samples if the surface roughness is taken into account for the stress
calculations (the surface roughness of the thinner foils is a considerable fraction of the
total thickness). The 10 μm as-received foils have the highest tensile strength which
is 400 MPa. The heat treated samples do not show a pronounced size dependence
of the tensile strength.
xi
To explain the effects observed, in particular the size dependence of the fracture
strain and the tensile strength as well as the low fracture strain of the 10 and 20 μm
as-received foils (in the order of 0.2%), the surfaces of the fractured samples and the
microstructure of the samples are analyzed in detail.
The analysis of the fracture surfaces shows for all samples a failure by necking
in thickness direction. Meaning that samples which show macroscopically a low
fracture strain, i.e. a behavior which is typical for brittle materials, display micro-
scopically large plastic deformations, i.e. a ductile behavior. This discrepancy can be
explained by a strongly localized deformation: a sample fails as soon as the stresses
in a cross-section reach a critical value; there is hardly any redistribution of strain,
which is typical for ductile material behavior. This is also reflected by an analysis of
the microstructure after the tensile test. The as-received samples do not show large
microstructural changes with respect to the unloaded state except at the location of
breakage. In comparison to that, the heat treated foils show a moderate elongation
of the grains and a strong increase in surface roughness after tensile testing. This
increase can be explained by the formation of slip bands at the surface and by the
rotation of grains out of their original plane. The rotation of grains is facilitated by
the low number of grains per thickness in the heat treated samples, as grains, which
are in contact with the surface, can deform more easily.
The general trend that thinner samples have a smaller fracture strain is believed
to be caused by a combination of various mechanisms. Firstly, local reductions in
cross-section by an imperfect sample geometry and by statistically random, plastic
deformations are more critical for thinner samples. Secondly, surface grains can
deform more easily and hence the number of grains per thickness has an impact on
the mechanical behavior. Thirdly, in thinner samples there are less grains which
could result in a smaller number of activated gliding systems. Fourthly, dislocations
cannot build up large plastic deformations in small grains.
The influence of other parameters such as width and length of a sample, strain
rate and orientation with respect to the rolling direction were studied as well. In
comparison with the thickness, they only have a small influence on the mechanical
behavior of the foils tested.
It has to be stressed that the size dependence found in this work was measured
in a tensile test, i.e. a test where no considerable strain gradients occur. Experi-
mental verification of size effects in loading situations, where no strain gradients are
present, is scarce (e.g. [Weiss et al., 2002] and [Espinosa et al., 2004]). This work
also shows that, for the explanation of the effects observed, a thorough examination
of the microstructure of the samples tested is mandatory. As the influence of many
parameters has to be taken into account in detail, the experimental study of size
effects turns out to be a complicated topic.
Besides the experimental details, this work shows the characterization results
for the microstructure of the copper foils before and after tensile testing as well as
the tensile test results for various parameters. The influence of many factors on
the mechanical behavior of thin foils is discussed thoroughly and the tensile test
behavior is explained by means of a simple geometrical model.
Schlagwörter
Mechanical properties of alloys and metals (metallurgy), Tensile tests (materials testing), Meral plates + metal sheets + metal films (metal products), Zugversuche (Materialprüfung), Mechanische Eigenschaften von Legierungen und Metallen (Metallurgie), Metallfilme + Metallfolien + Metallfeinbleche (Metallprodukte)
Fachgebiet (DDC)
620 - Ingenieurwissenschaften und Maschinenbau
Veranstaltung
Startdatum der Ausstellung
Enddatum der Ausstellung
Startdatum der Konferenz
Enddatum der Konferenz
Datum der letzten Prüfung
ISBN
ISSN
Sprache
Englisch
Während FHNW Zugehörigkeit erstellt
Nein
Zukunftsfelder FHNW
Publikationsstatus
Veröffentlicht
Begutachtung
Fachlektorat/Editorial Review
Open Access-Status
Lizenz
Zitation
SIMONS, Gerd, 2004. Mechanical size effects in thin copper foils. An experimental study. Zürich: ETH Zürich. Verfügbar unter: https://irf.fhnw.ch/handle/11654/49558