Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
SELF-HEALING CONCEPT TO IMPROVE THE
MECHANICAL PERFORMANCE OF VITREOUS
ENAMEL COATED STEEL
Andrea Zucchelli*, Giuseppe Palombarini*, Fabrizio Tarterini*, Luca Pignatti‡, Raffaele
Poletti‡, Alberto Pirazzoli‡
* University of Bologna, V. Risorgimento 2, 40136, Bologna, Italy
‡ SMALTIFLEX S.p.A., R&D, via dell'Industria, 115, 41038, S. Felice sul Panaro (MO), Italy
Tel: +39 051 209 3454; Mobile: +39 339 34 66 937
Fax: +39 051 209 3412
e-mail: a.zucchelli@unibo.it
Nanoparticles of alumina were introduced in a blue enamel widely used to protect heat exchanger components,
and the enamel was used to coat sheets of a low carbon steel. The composite structures were investigated in
order to evaluate both the enamel-to-steel adhesion and the capability of the nanoparticles to allow a selfmending effect against deep brittle crack propagation phenomena within the enamel. The mechanical behaviour
and crack sensitivity of steel samples coated using enamels both with and without additions of nanoparticles of
alumina were submitted to impact load and bending tests and then analysed by means of different techniques to
comparatively evaluate extent and morphology of the mechanical damages. It is shown that the addition of
nanoparticles of γ-Al2O3 allows the enamel coating to display better mechanical performance in terms of
increased post-elastic behaviour under bending condition, as well as of improved adherence and reduced spalling
tendency under impact load conditions. The morphological differences between nanomodified and standard
specimens at the steel-enamel interface are pointed out and discussed with reference to their effects on adhesion.
Keywords: Enamel coatings, metal oxide nanoparticles, crack self-healing capability, enamel
toughness, microstructural characterization, acoustic emission
1
Introduction
The demand of surface engineered materials for components to be submitted to severe inservice conditions is growing, in particular for applications where increased mechanical
performances and resistance to corrosion and wear are required. In this regard, a very
promising field of research and development is represented by self-healing materials and
coatings, i.e. materials allowing in-service or maintenance damage to be repaired by the
action of already contained components, without the necessity of external actions or
restoration.
Different strategies are being proposed to achieve self-repairing effects in mechanically
damaged metals, ceramics, polymers, cements and, even more, composite materials.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
Healing agents can be introduced in a polymeric matrix within microcapsules and released
where cracks are able to open some capsule: the interaction between the agent and particles of
a catalyst also introduced in the matrix can give rise to local polymerisation effects suitable to
bond the crack surfaces and stem the crack propagation [1]. A similar approach has been
followed by introducing hollow glass fibres in a polymer-matrix composite: an agent stored
within the fibres outflows where fibres are eventually broken by local high stresses, allowing
both damage detection and in situ restoring effects to be achieved [2, 3]. A method for
enhancing the toughness of ceramic materials is based on the introduction in the matrix of
particles of a compound such as zirconium oxide and zirconium compound which, through its
transformation from tetragonal to monoclinic phase and the compressive stress states induced
by the consequent volume increase, can counteract the tensile stresses at the tip of a crack
mending the propagation of brittle fracture [4].
An important field of application is represented by protective coatings, which play a
determining role in the many cases where the chemical or mechanical performance of a
component is controlled by the surface properties of the selected material. In this regard,
considerable attention is being addressed to coatings constituted by nanosized components for
their unique chemical, physical and mechanical properties. Among different classes of
coatings, a remarkable interest is concerning vitreous enamels used to protect components to
be realised using a low carbon steels. Enamels for metallic components are inorganic coatings
constituted by a ceramic-vitreous matrix containing randomly dispersed specific additives.
The matrix is made by a mixture of raw materials and in particular by a boron-silicate glass
added with oxides of elements such as titanium, zinc, tin, aluminium, etc. The aim of these
additions is to improve important properties such as mechanical strength, fracture toughness,
resistance to corrosion, wear and fatigue, as well as the aesthetic appearance of the
component. The enamelling process varies depending on the nature of both substrate and
selected porcelain enamel. Two industrial processes are commonly used to coat low carbon
steel: one based on a wet porcelain enamel, the second based on a dry-silicone porcelain
enamel.
Two main aspects are to be considered for an effective use of enamel coatings: the functional
performances of the coating itself (mechanical behaviour and chemical inertness) and its
adhesion to the substrate. In particular, a premature failure of coatings because of poor
adhesion at the interface can be the cause of remarkable damages and economic losses [5].
Significant enhancement can be obtained in both mechanical resistance and adhesion of
enamels to steel by introducing nanoparticles of metal oxides in the enamel frits. The addition
of nanoparticles of alumina was found a positive way to improve the adhesion of two different
vitreous enamels to a substrate of low carbon steel. The result was ascribed to the effects that
the nanoparticles are able to exert on amount and size of dendrites forming at the enamel-steel
interface during the firing stage of the application process [6].
In the present work, the addition of nanoparticles of alumina to a vitreous enamel was
considered as a promising way (i) to improve enamel-steel adhesion, and (ii) to introduce a
compound allowing the enamel to self-mend crack propagation phenomena (the self-healing
action). To this purpose, a blue enamel was selected as the base material for the addition of
nanoparticles because it widely application and strategic importance in heat exchanger
applications. In particular in air-heaters and air-pre-heaters used on units firing sulfur bearing
fuels the heating elements and nearby structures are subjected to medium-cold corrosion. In
this specific cases a successfully adopted passive corrosion control method is based on the
enamelling of the heating elements.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
The main enamel required performance for such applications are the high corrosion resistance
[5,7], the good adhesion of the coating to the substrate, the good mechanical (strength and
toughness) and wear resistance. Nevertheless mechanical performance and adhesion of
enamel coatings, nowadays available for such applications, can be improved to obtain in
higher in service reliability.
The expected reinforcing effects, induced by the nanoalumina adding, have been investigated
by means of mechanical tests as well as by means of comparative analyses of the extent and
morphology of damages observed in tested samples prepared both with and without additional
nanoparticles.
2
Experimental details
Sheets of 0.9 mm thick of a very low carbon steel (Table 1) were coated with two blue
enamels prepared by Smaltiflex S.p.A., Italy, following an internal standard procedure. A
standard enamel was prepared by mixing two different types of frits (Table 2) with clays. A
nanomodified enamel, in turn, was prepared adding 0.25 wt. % of nanoparticles of γ-alumina
to the standard enamel. The added particles were less than 100 nm in size. Both mixtures were
milled in ball mills for 4 hours in order to obtain wet blends containing particles of controlled
size (~50 ± 6 μm). The steel sheet were pre-treated according to the following procedure: (1)
degreasing at 60°C with alkaline degreasing bath, (2) acid attack in a 5 vol.% solution of
sulphuric acid at 60°C, (3) room temperature water bath, (4) nickel deposition by immersion
in a 1.2 % of NiSO4 bath, (5) room temperature water bath, (6) immersion in a 0.3%
neutralizer solution at 60°C, (7) drying up at 110° . Both enamels were applied to pre-treated
steel by wet spraying. The wet coated specimens were dried at ~40°C and then fired in a
radiant tubular furnace for ~6 minutes at 850°C. The thickness of vitreous coating was
measured on each enamelled side of all specimens according to the procedure schematised in
Fig. 1.
Table 1: steel sheets chemical composition (%)
Element C
Mn
Si
P
S
Al
wt.%
Cu
0.003 0.310 0.030 0.025 0.040 0.025 0.032
Table 2: Composition of enamel frits in wt.% of oxides (as given by the producers)
Oxides SiO2 B2O3 Na2O K2O CaO Li2O MnO CoO NiO TiO2 ZrO2
Frit 1
49
7
3
3
7
9
3
3
2
8
10
Frit 2
55
7
3
2
7
8
2
2
3
5
11
3
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
A - Specimen geometry
for impact tests
100 mm
Thikness
measurement
ii
100 mm
250 mm
20 mm
B - Specimen geometry for four bending tests
Thikness measurement
positions
Figure 1: Geometry of specimens submitted to (A) impact and (B) bending tests. The crosses indicate the
positions selected for measurements of enamel thickness
As-enamelled samples were characterised by means of X-ray diffraction (XRD) and stylus
surface profilometry (radius pick-up 5 μm). The XRD patterns were recorded using a
computer-controlled goniometer and CoKα radiation, with 0.02° 2θ steps and 1 s counting
time.
Enamelled sheets were submitted to two different mechanical tests: four point bending test
and impact test. Different specimen geometries were adopted for each type of test: square
sheets (side of 100 mm) (Fig. 1-A), rectangular sheets (20x250 mm in size) for bending tests
(Fig. 1-B). All tested specimens were enamelled on both surfaces.
The four point bending tests were carried out according to ASTM D6272-02 using a well
known apparatus (Fig. 2-B) [9]: thrust cylinders with an outer span of 65 mm were joined to
the stationery yoke while counterpressure cylinders with an inner span of 25 mm and loading
cell were joined to the moving yoke. Tests have been done under displacement control setting
up the head test machine speed at 0.12 mm/sec.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
A - Drop weight apparatus scheme
Andrea Zucchelli et al.
B - Four point bending apparatus scheme
Punch
mobile head
Punch
guide
Specimen
load cell
Acosutic
emission
piezometric
transducer
H
Displacement
direction
Inner span
Specimen
Outer span
Specimen
support
cylindrical
support
Displacement
transducer
Figure 2: Schematic representation of the apparatus used for (A) impact tests, and (B) four point bending tests
Further information on failure phenomena occurring in the enamel coating during the bending
tests were collected by means the acoustic emission technique (AE): the onset and growing of
cracks cause the release of part of the strain energy stored inside the material in the form of
acoustic waves. In the present work the AE has been monitored by a Physical Acoustic
Corporation (PAC) PCI-DSP4 device equipped with a PAC R15 transducer setting up the
amplitude threshold at 40 dB. The scheme reported in Figure 2-B shows the apparatus for
bending tests equipped with the AE piezoelectric sensor.
The impact tests were performed using a device where a 2 kg weighing punch with an
hemispherical head (20 mm in diameter) falls onto the specimen starting from 1 m in height
and running into a cylindrical guide (Fig. 2-A) [8]. Damage and cracking effects arisen in
specimens submitted to impact tests were investigated by means of optical and scanning
electron microscopies (OM; SEM). Localised chemical analyses were carried out on fracture
surfaces by means of an electron microprobe device equipping the SEM, using the energy
dispersion spectroscopic technique (EDS).
3
Results and discussion
The enameling process of the steel sheets allowed external surfaces to be obtained with
roughness values (Ra, center line average) that, when measured adopting a stroke of 30 mm,
lie in the range 0.5-0.6 μm for both standard and nanomodified specimens. The XRD patterns
recorded for the same surfaces show the presence of a crystalline fraction in both types of
enamel, whose diffraction peaks correspond for both enamels to those attributable to silicon
oxide (Fig. 3). No indication was found on the presence of alumina in the modified enamel
due to the low concentration of added particles.
5
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
600
Andrea Zucchelli et al.
•
500
400
300
200
•
0
10
20
•
• •
100
30
40
50
•
•
60
•
70
•
80
90
Figure 3: XRD pattern measured on the external surface of a steel sample coated with the nanomodified enamel
Preliminary mechanical test has been performed by means of four point bending test,
monitored by means of the AE, in order to identify the influence of the nanomodification of
the coating to the flexural stiffness and the crack formation and propagation in a quasi static
loading condition.
The stress-displacement diagrams obtained by bending tests display three typical stages in the
mechanical response of specimens coated with both types of enamel: (i) linear elastic stage,
(ii) non-linear stage before plastic crisis, and (iii) non-linear stage after plastic crisis.
Therefore, two important transition points characterise the mechanical behaviour of the
enamelled specimens under bending loads: the yielding point (1), i.e. a transition from
linearity to non linearity before plastic crisis, and the plastic crisis point (2), i.e. a transition
from the non linear pre-plastic crisis stage to the post-plastic crisis stage. The diagram
reported in Fig. 4 shows that the specimen coated with the nanomodified enamel is stiffer than
the specimen coated with the standard enamel. In particular the estimated slopes of the linear
part of the diagrams are respectively: for the standard enamel coated sheets the slope is (0.159
± 0.003) kN/mm and for the nanomodified enamel coated sheets the slope is (0.168 ± 0.003)
kN/mm. So from the standard to the nanomodified enamel coated sheets it is possible to
estimate a bending stiffness increasing of ∼5%.
0.4
0.3
Stress NAN 1A
Stress (kN/mm2)
0.3
Stress STD 1A
0.2
Nano modif.
coat (2)
Standard coat
(2)
0.2
Nano modif.
coat (1)
Standard coat
(1)
0.1
0.1
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Displacement (mm)
Figure 4: Stress-displacement diagrams for steel specimens coated with standard and nanomodified enamels.
Four point related to deviation from linearity (1) and at the maximum stress before plastic crisis (2) are pointed
out. The standard deviation is reported for both stresses and displacements
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
Moreover, it results that the two previously outlined transition points occurred at values of
stress and displacement significantly higher for the nanomodified enamel-steel specimen. In
particular, as shown by data reported in Table 3, both transitions occurred for the
nanomodified specimen at higher values of stress and displacement: by ~9% and ~4%,
respectively, for transition point (1), and by ~7% and ~6%, respectively, for transition point
(2). Also to be considered is the area under stress-displacement curves, whose value can be
related to toughness. In this regard, the value of this area is higher for the nanomodified
specimen, by ~13% at the first transition point and ~16% at the second transition point (Table
3). All these results support the experimental evidence that the addition of alumina
nanoparticles has promote the coating stiffening and toughening.
Table 3: Mean values (M.V.) and standard deviations (St.D.) of stress and displacement at the deviation from
linearity (point 1 in Fig.4) and at the maximum stress before plastic crisis (point 2 in Fig.4)
Stress
(kN/mm2)
Characteristic
values at the
deviation from
linearity
Characteristic
values at
maximum stress
before plastic
crisis
First AE event
with appreciable
energy
AE event with the
highest energy
value
Displacement
(mm)
Area under
stressdisplacement
curves
(kN/mm)
M.V.
St.D.
M.V.
St.D.
M.V.
St.D.
Standard
0.225
coating (1)
0.007
1.340
0.027
0.160
0.004
Nano modif.
0.245
coating (1)
0.008
1.390
0.042
0.180
0.006
Standard
0.266
coating (2)
0.008
1.780
0.036
0.271
0.007
Nano modif.
0.286
coating (2)
0.009
1.880
0.047
0.314
0.009
Standard
0.188
coating (1)
0.006
1.109
0.022
0.111
0.002
Nano modif.
0.246
coating (1)
0.008
1.387
0.042
0.181
0.006
Standard
0.234
coating (2)
0.007
1.513
0.030
0.168
0.004
Nano modif.
0.278
coating (2)
0.008
2.049
0.051
0.361
0.010
To better understand the failure progression of the enamel coatings during bending tests the
AE was recorded and analysed following a parametric approach. AE events are the
consequence of failures inside the material which generate waves differing in terms of
duration, amplitude, number of counts and energy depending on the type of failure. The
parametric analysis were developed considering the above mentioned four parameters and
diagrams such as those shown in Fig. 5 and 6 were obtained. In particular, Fig. 5-A1 and 5-B1
show that the AE events generated by specimens coated with the nanomodified enamel are
characterised by amplitude and duration lower then those observed while bending standard
specimens.
7
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
This indicates that the two different enamel-steel samples underwent considerably differing
types of failure [10]: in fact, AE waves with low values of amplitude, duration and energy are
mainly related to ductile or tough fracture, while waves with high values of amplitude,
duration and energy are mainly related to brittle fracture.
The differences between diagrams of cumulative counts vs. energy, recorded for the two
enamel-steel specimens and reported in Fig. 6, support the hypothesis on the occurrence of
two different types of failure. Worth noting, the profile of accumulated AE event energy
given by the nanomodified specimen well below that one given by the standard specimen, an
important difference supporting the idea that the fracture in the nanomodified specimens is
tougher.
A - Standard enamel coating
0.4
1.2E+04
2.5E-06
A1
A2
0.3
2
8.0E+03
6.0E+03
4.0E+03
0.3
1.5E-06
0.2
Stress STD 1A
Eac (J) STD 1A
0.2
1.0E-06
0.1
2.0E+03
AE Event Energy (J)
2.0E-06
Stress (kN/mm )
Duration (μsec)
1.0E+04
5.0E-07
0.1
0.0E+00
40
50
60
70
80
90
0.0
100
0.0E+00
0.0
Amplitude (dB)
0.5
1.0
1.5
2.0
2.5
Displacement (mm)
3.0
3.5
4.0
B - Nanomodified enamel coating
0.4
1.2E+04
B1
0.3
1.0E+04
2
8.0E+03
6.0E+03
4.0E+03
0.3
1.2E-07
0.2
Stress NAN 1A
Eac (J) NAN 1A
0.2
8.0E-08
0.1
AE Event Energy (J)
1.6E-07
Stress (kN/mm )
Duration (μsec)
2.0E-07
B2
4.0E-08
2.0E+03
0.1
0.0E+00
40
50
60
70
80
90
0.0
100
0.0E+00
0.0
Amplitude (dB)
0.5
1.0
1.5
2.0
2.5
Displacement (mm)
3.0
3.5
4.0
Figure 5: Diagram of AE event parameters and stress. AE event duration versus amplitude diagram for
specimens respectively coated with the standard (A1) and the nanomodified (B1) enamel. Stress and AE event
energy versus displacement for specimens respectively coated with the standard (A2) and the nanomodified (B2)
enamel
8
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
A - AE Event Cumulative Counts
AE Cumulative Event Count
2.0E+05
1.6E+05
1.2E+05
Count CUM STD 1A
Count CUM NAN 1A
8.0E+04
4.0E+04
0.0E+00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Displacement (mm)
B - AE Event Cumulative Energy
AE Cumulative Event Energy (J)
2.5E-05
2.0E-05
1.5E-05
Eac CUM (J) STD 1A
Eac CUM (J) NAN 1A
1.0E-05
5.0E-06
0.0E+00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Displacement (mm)
Figure 6: Diagrams of (A) AE cumulative counts per event, and (B) AE cumulative energy vs. displacement
under bending conditions
An additional evidence on the different mechanical behaviour displayed by the two types of
enamel coating is concerning the AE event energy distribution in the displacement domain,
illustrated in Fig. 5-A2 and 5-B2. In fact, the AE activity takes place in the linear elastic stage
for both types of specimens but, in the case of nanomodified specimens, the energy release
begins at lower values of displacement values (Table 3), a result that can be related to two
different stress intensity levels arising during the cracking failure process. In particular, the
low displacement value observed for the AE given by standard specimens seems to indicate a
behaviour more brittle than that of nanomodified specimens. A further point to be considered
in regard to the AE energy distribution is that the AE event with the highest energy content
takes place in stress-displacement domains which are different for the two types of enamels
(Table 3): in fact, the AE energy peak takes place in the non-linear stage before the plastic
crisis in the case of standard specimens, but in the non-linear stage after the plastic crisis in
the case of nanomodified specimens. Considering that AE events characterised by very high
energy contents can be associated to deep cracking effects in the coating, it can argued that:
(i) the steel specimens coated with the nanomodified enamel are less sensitive to failure than
those coated with the standard enamel and that (ii) they are able to better follow the steel
strain also in the plastic domain.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
The effects of impact load tests were investigated with the aid of observations carried out both
at the stereo and at the SEM microscopes. In particular, the after-impact fracture propagation
and distribution was evaluated by means of observations carried out on a same sample at
different times after the impact event. In Fig. 7 images taken at the stereo microscope at 0 hr,
2 hr and 5 hr after the impact are shown for both a standard (upper line) and a nanomodified
specimen (lower line). Considerable differences in failure progression and peeling effects can
be noted by comparative observations on the two coatings. Just after the impact, the
differences between the damaged areas are little different, in both cases with limited zones
where the coating was been crashed out.
Figure 7: Images at the stereo microscope for impacted specimens, taken at 0 hr, 2 hr and 5 hr after the impact,
for a standard specimen (upper line) and a nanomodified specimen (lower line). The considerable differences in
the progression of failure and peeling effects between the two coatings should be noted
In the nanomodified specimen, however, peeling effects are apparently absent and, most
important, the damaged area does not undergo significant modifications during the subsequent
hours. In contrast, peeling effects of increasing extent can be observed on the standard
coating, as revealed by the increase in the area displaying a brilliant white colour. By the
images reported in Fig. 7 it can be evaluate that, in 5 hr after the impact, the peeled area
increased on the standard enamel by ~50%, vs. an increase of ~3% on the nanomodified
enamel. This result is clearly indicative of the benefits that can be obtained by the addition of
nanoparticles to the base enamel. Moreover, the more detailed images reported in Fig. 8,
taken in heavily deformed zones of both specimens, show even better morphological
differences between the peeling effects allowing to argue that the nanomodified coating
displayed a significantly higher adherence to the base steel.
The observations at higher magnifications performed on the same specimens by a SEM
microscope give further information on the different morphologies displayed by the two
enamels in the impacted zones and, in addition, enlighten fracture surfaces showing a tougher
morphology in the case of the nanomodified enamel (Fig. 8-A vs. Fig. 8-B).
10
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
Figure 8: Images at the stereo microscope of impacted areas (concave side), taken ~5 hr after the impact, for (A)
a standard, and (B) a nanomodified specimen. Details of loading effects are concerning deeply deformed zones
(1) and (2), respectively. The higher adherence of the nanomodified enamel should be noted
SEM observations carried out at or near the enamel/steel interface show that Fe/rich dendrites
grew in both specimens (Fig. 9) with the following remarkable difference pointed out by
localized EDS analyses performed on the same zones and also reported in the Figure: a layer
of enamel still covered the metal substrate coated with the nanomodified enamel while, as
indicated by the high intensity peaks of Fe, zones of practically bare metal were present on the
standard specimen.
Figure 9: SEM images of impacted specimens (concave side) coated with the standard enamel (A, C and E) and
the nanomodified enamel (B, D and F). A, B: central parts of the impacted areas; C, D: fracture morphology of
the same parts; E, F: details on the differences in fracture extent and morphology between the standard and
nanomodified enamels, respectively
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
Spectrum 1
Counts (a.u.)
Standard coating
Counts (a.u.)
Spectrum 2
Energy (keV)
Spectrum 1
Counts (a.u.)
Nanomodified coating
Counts (a.u.)
Spectrum 2
Energy (keV)
Figure 9: SEM images of fracture surfaces observed on impacted specimens (concave side) coated with the
standard and nanomodified enamels. A distribution of Fe/rich dendrites can be seen in both samples, but the EDS
spectra reported aside indicate that a layer of enamel still cover the metal substrate only in the case of the
nanomodified specimen
4
Conclusions
Comparative bending and impact load tests carried out on sheets of a low carbon steel coated
with a standard enamel and the same enamel modified by the addition of nanoparticles of γalumina, and post-test observations on the damage extent and morphology carried out with
optical and electron microscope techniques, allow the following conclusions to be drawn:
- The added nanoparticles proved to be able to significantly improve the mechanical
resistance of the enamel to both bending and impact loads, as well as its adherence to the
metal substrate. In particular in bending tests it was observed a great difference in terms of
AE energy release between the standard and the nanomodified enamel coating. This fact can
be related to a different cracks formation and propagation in the nanomodified enamel
coating and a self-mending effect against brittle crack propagation phenomena can be
assumed;
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Andrea Zucchelli et al.
- The time crack propagation observed in enamels on impacted specimens is much lower for
the nanomodified enamel, as a reasonable toughening effect exerted by the nanoparticles.
- On heavily deformed samples, the enamel coating undergo peeling effect which, however,
are much more limited for nanomodified specimens where, contrary to standard specimens,
a residual film of enamel remains adherent to the base metal.
ACKNOWLEDGEMENTS
The authors wish to thank Ing. Giampaolo Campana for fruitful discussions and Drs. Silvia Tiberi Vipraio and
Mr. Tommaso Lanzellotto for their assistance to the experimental activities.
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