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energies
Article
A Novel Framework to Study the Role of Ground and Fumed
Silica Fillers in Suppressing DC Erosion of Silicone Rubber
Outdoor Insulation
Alhaytham Y. Alqudsi 1,*, Refat A. Ghunem 1,2 and Eric David 1
Citation: Alqudsi, A.Y.; Ghunem,
R.A.; David, E. A Novel Framework
to Study the Role of Ground and
Fumed Silica Fillers in Suppressing
DC Erosion of Silicone Rubber
Outdoor Insulation. Energies 2021,14,
3449. https://doi.org/10.3390/
en14123449
Academic Editor: Pawel Rozga
Received: 28 April 2021
Accepted: 7 June 2021
Published: 10 June 2021
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1École de Technologie Supérieure, Montréal, QC H3C 1K3, Canada; refat.ghunem@nrc-cnrc.gc.ca (R.A.G.);
eric.david@etsmtl.ca (E.D.)
2Metrology Research Center, National Research Council Canada, Ottawa, ON K1A 0R6, Canada
*Correspondence: Alhaytham-yousef-j.alqudsi.1@ens.etsmtl.ca
Abstract:
This paper investigates the effect of ground and fumed silica fillers on suppressing DC
erosion in silicone rubber. Fumed silica and ground silica fillers are incorporated in silicone rubber at
different loading levels and comparatively analyzed in this study. Outcomes of the +DC inclined
plane tracking erosion test indicate a better erosion performance for the fumed silica filled composite
despite having a lower thermal conductivity compared to the ground silica composite. Results of
the simultaneous thermogravimetric and thermal differential analyses are correlated with inclined
plane tracking erosion test outcomes suggesting that fumed silica suppresses depolymerization and
promotes radical based crosslinking in silicone rubber. This finding is evident as higher residue
is obtained with the fumed silica filler despite being filled at a significantly lower loading level
compared to ground silica. The surface residue morphology obtained, and the roughness determined
for the tested samples of the composites in the dry-arc resistance test indicate the formation of a
coherent residue with the fumed silica filled composite. Such coherent residue could act as a barrier
to shield the unaffected material underneath the damaged surface during dry-band arcing, thereby
preventing progressive erosion. The outcomes of this study suggest a significant role for fumed
silica promoting more interactions with silicone rubber to suppress DC erosion compared to ground
silica fillers.
Keywords: HVDC outdoor insulators; silicone rubber; fumed silica; ground silica; dry-band arcing;
erosion performance
1. Introduction
With the rising awareness of the impacts of fossil fuel-based electricity generation on
climate change, solutions for integrating renewable energy sources into the existing electric
grid infrastructure have been investigated. Utilizing a high voltage direct current (HVDC)
transmission system would facilitate such integration by enabling an efficient transmission
of electric power over long distances from remote renewable energy sources such as hydro,
wind and solar farms to load centers [
1
,
2
]. Accordingly, HVDC outdoor insulators should
be designed to ensure the reliability of the power transmission system. Silicone rubber’s
(SiR) characteristic hydrophobicity makes it highly desirable for use as a housing material
in polymeric outdoor insulators. SiR, however, is susceptible to erosion caused by dry-band
arcs sustained under heavily polluted conditions. Incorporating silica fillers in composite
formulations of SiR was considered for enhancing the thermal conductivity and, in turn,
the erosion performance of SiR.
Meyer et al., in [
3
], highlighted the correlation between the thermal conductivity and
the erosion resistance of their silica filled SiR composites. It was concluded that the increase
in silica filler loading from 10 to 50 wt% (percent by weight) caused a significant increase
in the thermal conductivity, which resulted in lower eroded masses in the inclined plane-
tracking and erosion test (IPT). El-Hag et al., in [
4
], illustrated that adding fumed silica
Energies 2021,14, 3449. https://doi.org/10.3390/en14123449 https://www.mdpi.com/journal/energies
Energies 2021,14, 3449 2 of 15
by 10 wt% to SiR would result in a comparable erosion performance with 50 wt% micro
silica filled SiR. This observation was attributed to the role of fumed silica in favorably
bonding with the silicone rubber matrix. Nazir et al., in [
5
], reported an improvement in
the IPT erosion performance of their hybrid SiR composites containing nano silica and
aluminum nitride fillers with an increase in the nano silica loading level. An increase in
the composite thermal conductivity was observed with an increased loading of nano silica
fillers. Ramirez et al., in [
6
], explained that the high specific surface area of the fumed
silica filler facilitates a better interaction with SiR as a result of the increased concentration
of the silanol groups interacting with the siloxane chains of the polymer. Ansorge et al.,
in [
7
], highlighted the effect of an additional factor influencing the erosion performance
of silica filled SiR that is related to the material curing temperature. It was reported in [
7
]
that the erosion performance of micro silica filled room temperature vulcanized (RTV) SiR
showed higher erosion depths under the IPT compared to high consistency silicone rubber.
This outcome was attributed to the improved filler-polymer bonding at high temperature
curing. Similar findings were reported in [
3
], highlighting the effect of high temperature
curing on the erosion performance of silica filled SiR composites.
Several research studies conclude that the erosion performance of SiR worsens under
DC voltage, particularly +DC, compared to AC [
8
,
9
]. The dry-band arcing exhibited by
the insulators under DC voltage is of higher relative severity compared to that of AC in
terms of arc discharge duration and leakage current magnitude [
9
]. Ghunem et al., in [
10
],
illustrated the effect of increasing silica filler loading and, subsequently, the composite
thermal conductivity on delaying the inception of stable eroding DC dry-band arcs in SiR.
Comparable magnitudes of the third detail wavelet component of leakage currents were
obtained between the SiR composites filled with silica and alumina trihydrate (ATH) at
30 wt%, despite the additional effect of the water of hydration of ATH in suppressing
erosion [
10
]. These findings suggest the presence of erosion suppression mechanisms asso-
ciated with silica’s interaction with the silicone matrix. Kone et al., in [
11
], demonstrated
the effect of the silica filler size and loading level on the integrity of the silica residue
produced under the IPT. The coherency and porosity of such residue could shield the
SiR material against progressive erosion under DC dry-band arcing [
11
]. In an earlier
study [
12
], fumed silica was found to have a significant effect in suppressing DC erosion
as compared to nano ATH and sub-micron boron nitride (BN) in SiR composites, despite
the comparable thermal conductivities reported for all of the composites. Accordingly,
the literature suggests that the role of silica fillers in suppressing the DC erosion in SiR is
more than simply improving the composite thermal conductivity.
The literature indicates that the use of silica fillers in SiR composites, with their
different particle sizes and loading levels, would improve the erosion performance of the
composites as a result of an increase in the thermal conductivity of the composite. Merely
using the +DC IPT as a means to rank the erosion performance of such composites without
considering any additional analytical tools would overshadow the true role of the silica
size in suppressing the DC erosion of SiR. Moreover, this would limit the role of the silica
filler in merely improving the composite thermal conductivity. This paper introduces a
framework to thoroughly investigate the role of fumed silica and ground silica fillers on
suppressing the DC erosion of SiR. The study would enable using a number of analytical
tools with outcomes that could be correlated with the IPT outcomes. This study, in turn,
could ultimately support the developments in SiR outdoor insulators for their reliable use
in the HVDC electric grid.
2. Materials and Methods
Fumed silica and ground silica, whose properties are shown in Table 1, are used
as the fillers for this study. Based on the literature, fumed silica was selected due to its
high specific surface area facilitating a favorable interaction with silicone at small weight
fractions in the composite. Ground silica, on the other hand, can be filled at much higher
loading levels to replace a significant portion of the SiR material and, subsequently, reduce
Energies 2021,14, 3449 3 of 15
the cost of the composite. A two-part RTV SiR is used in the study, where Part A is the main
potting compound and Part B is the crosslinking agent. Part A and Part B are maintained
at a weight ratio of 10:1, respectively. Weighed portions of the filler are added to Part A
and mixed using a ROSS high shear mixer until all of the filler is added to the mixture.
Part B is then added and mixed for one minute to be later poured into IPT specimen molds
and degassed under a vacuum. The mixture is cured at room temperature for a 24 h time
period, followed by thermal treatment at 85
◦
C for 3 h. An unfilled SiR was also prepared
for selected tests in this study.
Table 1. Filler properties and prepared composites.
Filler Type Supplier Filler Code Particle Size
(µm)
Specific Surface
Area (m2/g)
Specific
Gravity
Composite
Formulation
Fumed silica Sigma Aldrich FS07 7×10−3390 2.3 SiR + 5 wt% FS07
Ground silica US Silica GS10 10.5 1NA 22.65 SiR + 30 wt% GS10
1Median particle size. 2Not applicable for micro-sized fillers.
The +DC IPT is used in this study as part of the electrical analysis to assess the
erosion performance of the prepared composites. The test setup is set as per the IEC 60587
standard [
13
] and modified for +DC testing as per the recommendations in [
9
]. The test
voltage was set at +3.5 kV for a 6 h run time with a contaminant flow rate of 0.3 mL/min and
a contaminant conductivity value of 2.5 mS/cm. A digital Mitutoyo 571-200 micrometer
with an accuracy of 0.1 mm was used to measure the erosion depth of 10 specimen samples
for each composite.
Leakage currents for the tested specimens during the IPT are acquired using a National
Instruments NI USB-6356 data acquisition device at a sampling rate of 7 kHz. A sample
window is then applied to capture the first 468 samples from each second. The root-mean-
square value (RMS) of the leakage current is computed and stored as a single value for
the 468 acquired samples at every second. Computing the RMS value for every second
of the test run would suffice for representing the change in the leakage current values
during the test and would provide a practical approach for acquiring the data with smaller
storage requirements instead of saving the entire current waveform of 468 samples per
second. Following the analytical approach presented in an earlier study [
12
], a statistical
boxplot method is used to observe the distribution of the RMS leakage current values
acquired during the IPT in 20 min time intervals. This statistical analysis allows one to
observe the evolution of the dry-band arc from the intermittent state to the stable severe
state, which is reflected in the changing distribution of the RMS leakage current values
between consecutive time intervals in the boxplot. An increase in the dry-band arcing
stability and severity is reflected in a reduction in the non-conducting periods of the leakage
current. This corresponds to a transition from the initial intermittent state of the dry-band
arc, which is characterized by a large number of nonconducting periods and frequent
RMS leakage current values below 1 mA. Figure 1shows the +DC IPT test setup used in
the study [10].
The dry-arc resistance test is used in this study as a method to produce a controlled
and quick heat ablation through the sample thickness rather than progressive erosion on
the surface, as is the case with the IPT. The test will enable the fast production of tested
composites whose surface residue could be analyzed for drawing preliminary conclusions
regarding the role of the filler on defining the surface residue characteristics that could
be observed using microscopy. The test setup is set as per the ASTM D 495 [
14
] standard,
which utilizes tungsten electrodes to generate low current arcs under high voltages on the
tested composites. The arc is generated in current steps that define the current magnitude
and the duty cycles of the rms AC voltage applied, as per the schedule set in [
14
] and
with each current step lasting for one minute. For this study, only the first 4 current steps
of the 7 steps in [
14
] are used in the dry-arc resistance test; i.e. total test run of 4 min.
Detailed description of the on and off duration of the current in these 4 current steps are
Energies 2021,14, 3449 4 of 15
described in details in [
14
]. The first 3 current steps of the test (denoted here as cycles 1
to 3) are analogous to an intermittent state of the dry-band arc. The degree of intermittency
decreases as test goes from the first current step (cycle 1) to the third (cycle 3). The fourth
current step (cycle 4), on the other hand, is analogous to the stable eroding state of the
dry-band arc without intermittency. All the current steps used have a constant current
magnitude of 10mA. Modifying the existing test setup to generate a DC dry-band arc could
impair the proper functionality of the power electronics of the setup controlling the duty
cycles. Figure 2shows the dry-arc resistance test setup.
Figure 1.
+DC inclined plane-tracking and erosion test (IPT) setup with the leakage current acquisi-
tion system.
Figure 2. Dry-arc resistance test with images of the test during the 4 cycles of operation.
Simultaneous thermogravimetric–differential thermal analysis (TGA–DTA) was per-
formed on the prepared composites under nitrogen (N
2
) and air atmospheres to understand
the thermal decomposition characteristics of the composites. The heating rate was set at
25
◦
C/min for a temperature span from 80 to 800
◦
C. Thermal conductivity measure-
ments for the prepared composites were acquired using a thermal conductivity analyzer
instrument as per the ASTM D7984 standards [
15
], which enables the acquisition of mea-
surements in short test times without the use of a vacuum chamber. These measurements
are necessary to understand the relationship between the composite thermal conductivity
and the erosion performance of the composites.
Surface residue on post tested specimens of the +DC IPT, dry-arc resistance test and
TGA are analyzed using two various methods. The surface morphology of the samples
is observed using scanning electron microscopy (SEM) on 20 nm gold sputter coated
surfaces and a laser confocal microscope. Surface roughness analysis on the samples is
performed using a Keyence VR-5000 optical microscope. Quantitative representation of
the surface roughness is performed by means of the average roughness parameter, R
a
,
whose computation details can be found in [
16
]. All of the aforementioned tools serve to
Energies 2021,14, 3449 5 of 15
observe the effect of the filler on the residue characteristics of the composite in terms of
coherency and roughness.
3. Results and Discussion
3.1. Erosion Performance
Figure 3shows the +DC IPT outcomes of the study. The results preliminarily indicate a
better erosion performance of the FS07 filled composite compared to that of GS10.
Figure 3a
shows a higher average erosion depth obtained for the GS10 filled samples compared to
the FS07 filled samples. Figure 3b further illustrates the inferior erosion performance of the
GS10 filled composite through images of the post tested specimen, indicating larger eroded
areas with the composite compared to those filled with FS07. It is important to note that the
better erosion performance of the FS07 filled SiR does not necessarily conclude a superior
erosion performance for the composite as compared to the GS10 filled SiR. The outcomes
simply show that the FS07 filled SiR had a better or comparable erosion performance to
the GS10 filled SiR, despite being filled at one sixth of the filler loading level of the GS10
filled SiR. This highlights a significant role for fumed silica in suppressing erosion and
potentially facilitates its use as a co-filler with ground silica in practical formulations of SiR
composites of high filler loadings that could be used in industry.
Figure 3.
(
a
) +DC IPT erosion depth outcomes for the tested composites. (
b
) Images of the post tested +DC IPT compos-
ite specimens.
Table 2shows the measured thermal conductivity of the prepared composites. The ther-
mal conductivity values acquired were consistent with those found in [
3
,
4
]. The increase
in the weight fraction of the GS10 and FS07 fillers in the composite leads to a significant
increase in the composite thermal conductivity [
3
]. It is important to note that despite
having twice the thermal conductivity of the FS07 filled SiR, the GS10 filled SiR showed
inferior erosion performance, which suggests that that the thermal conductivity is not the
main governing factor in suppressing the erosion of SiR under DC voltage. In an earlier
study [
12
], it was found that the favorable interaction of fumed silica with the SiR matrix
was more decisive in determining the erosion performance of SiR under the +DC IPT than
the improvement of the composite thermal conductivity using BN fillers. The difference in
DC and AC erosion in silica filled SiR was thoroughly investigated and discussed in studies
such as [
11
] and is not the subject of this work. Rather, this study presents a practical
framework for highlighting the prominent role of the fumed silica-silicone interactions on
suppressing the DC erosion of SiR. According to Hshieh in [
17
], the silica-ash layer formed
during the combustion of silicones produces a barrier effect that shields the silicone mate-
rial against the influx of heat, preventing further combustion of the material. Accordingly,
the presented framework aims to highlight the role of the silica filler size and its interaction
with SiR in promoting the formation of a coherent residue with a barrier shielding effect
that enhances the erosion performance of SiR, as shown in the outcomes of Figure 3.
Energies 2021,14, 3449 6 of 15
Table 2.
Thermal conductivity measurements (k) of the composites based on the 15 acquired mea-
surements of each composite with a precision of ±1%.
Composite Minimum k
(W/m·K)
Maximum k
(W/m·K)
Average k
(W/m·K)
SiR + 5 wt% FS07 0.169 0.205 0.188
SiR + 30 wt% GS10 0.400 0.430 0.409
A statistical boxplot representation of the RMS leakage current of the composites
during the +DC IPT is illustrated. Figure 4shows the RMS leakage current waveform
obtained for one of the GS10 filled SiR composites and its corresponding boxplot analysis.
The boxplot shows the leakage current distribution for the first 3 h of the test, which was
comprised of 12 20-min time intervals. Each bar shown in the boxplot represents the value
distribution of the RMS leakage current values acquired during that time interval of the test.
For example, the bar in the third time interval represents the RMS leakage current values
acquired from minute 60 to minute 80 of the IPT. The bar width in the boxplot represents
the distribution of the leakage current values during any given time interval. The top and
bottom of the bar and the circled marker represent the 75th and 25th percentile and the
median value of the RMS leakage current during that time interval, respectively. In Figure 4,
the DC dry-band arc is shown to develop through two distinct stages in terms of arc stability
and severity. The reduction in the bar width is an indication of the changing nature of the
dry-band arc, from intermittent to stable with less nonconducting periods. As illustrated
in [
10
,
12
], the initial stage of the dry-band arc is intermittent with inconsiderable erosion
noted on the composite surface. The subsequent stage, however, is stable with reduced
nonarcing periods leading to severe erosion of the composite. The inception of the stable
dry-band arc stage was suggested to be dependent on the rate of formation of surface
residue promoted with thermo-oxidation at temperatures just below 200 ◦C by Si-C bond
scission, as illustrated in the Andrinov mechanism [
18
]. This residue would reduce the rate
of evaporation of the liquid contaminant in the IPT, leading to the development of a stable
dry-band arc [10].
Figure 4.
(
a
) Root-mean-square (RMS) leakage current for a GS10 filled silicone rubber (SiR) sample
during the +DC IPT and (
b
) corresponding statistical boxplot representation for the first 12 20-min
time intervals, first 240 min, of the test.
Figure 5shows the statistical boxplots for a number of tested composites. The results
indicate a faster inception of a stable eroding dry-band arc for the GS10 filled SiR compared
to that of FS07 by 40–60 min (about 20% of the total testing duration). Similar outcomes were
found in [
12
] for fumed silica filled composites against BN filled composites, which were
attributed to the favorable interaction of fumed silica with SiR and a possible delay in the
formation of the early surface residue by thermo-oxidation of the silicone volatiles.
Energies 2021,14, 3449 7 of 15
Figure 5.
Statistical boxplot outcomes for selected samples of the +DC IPT tested composites during
the first 12 20-min time intervals of the test.
3.2. Thermogravimetric–Differential Thermal Analysis
Figure 6shows the TGA–DTA outcomes of the study. The TGA plot shown in Figure 6a
is conducted for the prepared silica composites and the unfilled SiR. All of the composites
and the unfilled SiR begin depolymerization at 400
◦
C, which, according to Camino et al.
in [
19
], represents the scission of the Si-O bonds in SiR to produce cyclic oligomer volatiles.
The rapid depolymerization of the unfilled SiR eventually leaves a low remnant residue
of about 14.5 wt%. The TGA plot illustrated in [
4
] showed similar low remnant residues,
while other studies [
20
,
21
] have shown the complete depolymerization of an unfilled SiR
at the end of a TGA test. This variation could be attributed to a number of issues, such as
the difference in suppliers and material preparation methods. For this study, 14.5 wt%
was considered as the additional residue, possibly fused or crosslinked residue, produced
for an unfilled SiR under TGA in an N
2
atmosphere. Beyond 400
◦
C, the FS07 composite
decomposes at a slightly higher rate than that of GS10, which is still considered comparable
despite having a much lower filler loading.
Figure 6b shows the DTGA plot for both of the composites under an N
2
atmosphere.
The DTGA plot suggests the presence of multiple decomposition peaks, with the second
one starting at temperatures higher than 500
◦
C. Camino et al., in [
19
], reported a radical-
based crosslinking mechanism involving the homolytic scission of Si-CH
3
bonds in SiR,
which competes with depolymerization during the second decomposition stage at elevated
temperatures. At the onset of the 500
◦
C temperature, the decomposition rate of the FS07
filled composite becomes lower than that of the GS10 filled composite, as observed in the
shaded region of the DTGA plot in Figure 6b. This observation may suggest the influence of
FS07 on suppressing depolymerization and promoting radical based crosslinking, despite
being filled in SiR at one sixth of the filler loading level of the GS10 composite. In other
words, the DTGA peaks appearing more distinctively with FS07 as compared to the GS10
filled SiR suggest interactions between fumed silica and the SiR matrix to promote radical-
based crosslinking to a greater extent as compared to the interactions between ground
silica and SiR. The rate of SiR depolymerization was found to be subject to the mobility
and flexibility of the SiR siloxane chains, as indicated by Delebecq et al. in [
22
] and
Hamadani et al. in [
23
]. It was reported in [
6
] that the high silanol group concentration on
the fumed silica’s surface favorably interacts with the siloxane chains of SiR. Accordingly,
this interaction could suppress the depolymerization and volatilization of SiR, as explained
in [
23
]. To further support this conclusion, the DTA of both composites was conducted
in an air atmosphere. The DTA plot shown in Figure 6c indicates the exothermic peaks
obtained for both composites, which represent the combustion of the volatile SiR oligomers
Energies 2021,14, 3449 8 of 15
produced in depolymerization. Clearly, the suppressed depolymerization of the FS07 filled
SiR leads to a lower exothermic peak compared to that of GS10.
Figure 6.
(
a
) Thermogravimetric analysis (TGA) for the prepared composites and the unfilled SiR in
an N
2
atmosphere. (
b
) Corresponding differential thermogravimetric analysis (DTGA) plot for the
silica filled composites. (c) DTA for the prepared composites in an air atmosphere.
The final wt% of TGA remnant residues (R
TGA
) obtained for both composites was
found to be comparable, with the GS10 filled SiR having a slightly higher remnant residue
by a difference of only 1.8%, despite being loaded at six times the filler loading of the FS07
filled SiR. This further indicates the role of FS07 in suppressing SiR depolymerization and
promoting radical-based crosslinking. To clarify this quantitively, based on the computation
illustrated in [
4
,
11
], the final assumed residue R
asm
if the composite polymer and filler
components independently decompose without interaction is calculated as follows:
Rasm = (14.5% ×WSiR)+Wfiller (1)
where W
SiR
and W
filler
are the weight fractions of the SiR and the filler in the composite,
respectively. As mentioned earlier, the 14.5% represents the undecomposed portion of
SiR that was found in the TGA plot of Figure 6a. The additional residue R
add
obtained,
which accounts for the role of the filler interaction with the SiR polymer, is calculated
as follows:
Radd = RTGA −Rasm (2)
Table 3shows the calculated additional residue for each composite. Clearly, the ad-
ditional residue obtained for the FS07 filled SiR, 52.6%, is much higher than that of GS10,
33%, by a factor of 1.6. This difference in the additional residue differentiates between
the effect of each filler and its interaction with SiR on suppressing depolymerization and
promoting crosslinking. The higher additional residue for the FS07 filled SiR could indicate
Energies 2021,14, 3449 9 of 15
a better suppression of depolymerization and a higher degree of crosslinking exhibited by
the composite during TGA.
Table 3. Calculation of the additional residue Radd of the composites.
Composite WSiR (%) Wfiller (%) RTGA (%) Rasm (%) Radd (%)
SiR + 5 wt% FS07 95 5 71.4 18.8 52.6
SiR + 30 wt% GS10 70 30 73.2 40.2 33
Figure 7shows an SEM image of the obtained TGA residues for both of the compos-
ites. The GS10 filled SiR TGA residue surface was observed to be of a coarser nature in
comparison to the FS07 filled SiR TGA residue. This could in part be a result of the higher
particle size of GS10, as shown in the image, or a result of the lower crosslinking and
higher volatilization leading to a more porous residue compared to that of the FS07 filled
SiR. The FS07 filled SiR, on the other hand, appears to promote a coherent residue with
radical-based crosslinking. The weakness of the GS10 filled SiR residue can be certainly
observed in terms of the propagating surface fractures shown in Figure 7, which are not
present in the FS07 filled SiR residue, indicating coherency in the residue characteristics of
the latter. The coherency of the residue was proposed to have a barrier shielding effect on
the SiR material against the progressive erosion of silica filled SiR composites under DC
voltage [
11
]. These observations shown in the TGA plots and residues could be correlated
with the erosion performance of the composites illustrated earlier, indicating the role of
the FS07 filler and its interaction with SiR in promoting a more coherent residue, which,
in turn, suppresses the progressive erosion of SiR and enhances the erosion performance
of SiR.
Figure 7.
(
a
) Scanning electron microscopy (SEM) images for the TGA residue for the SiR + 30 wt% GS10
and (b) TGA residue for the SiR + 5 wt% FS07 under an N2atmosphere.
3.3. Residue Morphology Using the Dry-Arc Resistance Test
The formation of high additional residue with SiR composites could improve the
erosion performance of SiR composites as a result of the formation of a coherent residue that
shields the composite against an influx of heat from dry-band arcing. To better investigate
this possible correlation, the dry-arc resistance test is utilized as a fast and controllable
test for preparing eroded samples of the composites whose residues can be observed.
Figure 8shows the microscopic images obtained for the eroded pits of the post-tested
silica composites of the dry-arc resistance test. Figure 8a,b clearly shows that the residue
obtained from the FS07 filled SiR is more coherent, with less cracks and surface splitting
compared to the GS10 filled SiR residue, which is seemingly rougher with porous surfaces.
This observation is also confirmed by the SEM images shown in Figure 8c,d. Through
SEM, Nazir et al., in [
24
], reported similar observations with corona-aged SiR composites
showing less cracks with nano silica filled SiR compared to micro silica. The integrity of
Energies 2021,14, 3449 10 of 15
the residue could be attributed to the role of the radical-based crosslinking promoted by
FS07 interacting with SiR, leading to a more stable residue with coherency characteristics
similar to that shown in the TGA residue of Figure 7. These observations could explain the
better erosion performance obtained for the FS07 filled SiR, as shown in Figure 3.
Figure 8.
Microscopic images of dry-arc resistance post-tested samples at a magnification of 50 for
(
a
) SiR + 10 wt% GS10 and (
b
) SiR + 5 wt% FS07 composites, and SEM imaging at a magnification of
15 k for (c) the SiR + 30 wt% GS10 and (d) SiR + 5 wt% FS07 composites.
To further validate the applicability of using the dry-arc test for observing the residue
morphology of eroded composites, SEM was used to observe the eroded residue of the post
IPT tested composites for comparison against those obtained under the dry-arc resistance
test. Figure 9shows the images obtained using SEM for the IPT tested composites. As can be
seen in Figure 9, the surface morphology of both of the composites under the IPT are similar
to their counterparts in the dry-arc resistance test in terms of roughness and coherency.
This similarity further justifies the use of the dry-arc test as part of this mechanistic study.
Though the experimental conditions involved in both of the tests are completely different,
such as the absence of a wet contaminant in the dry-arc resistance test, the interest of this
study is to observe the heat ablation effect of the arc on the composites and analyze the
eroded residue characteristics accordingly. Creating this joule heating effect using either
test does not necessarily dictate that similar testing methods or experimental conditions
are to be followed. With this understanding, the dry-arc resistance test is advantageous
in terms of the higher degree of controllability obtained with stimulating fast SiR erosion
as a result of sustaining the arc at one fixed location above the sample during the test.
The difference between the testing conditions of the tests has no effect on changing the
residue characteristics of the composites, as can be seen in Figure 9, which further justifies
the use of the dry-arc resistance test.
Energies 2021,14, 3449 11 of 15
Figure 9.
SEM imaging at a magnification of 15 k for (
a
) the SiR + 30 wt% GS10 and (
b
) SiR + 5 wt%
FS07 composites tested using the +DC IPT.
Though the formation of coherent residue could enhance the erosion performance of
the silica filled SiR composites, Delebecq et al., in [
21
], did explain that the carbon content of
the residue could increase with increased SiR crosslinking. This increase in carbon content,
however, would not significantly impact the composite during the IPT to cause a tracking
failure, as explained in [
11
]. According to Kumagai et al., in [
25
], the analysis of the residue
formed as a result of dry-band arcing in RTV SiR was found to contain 1 wt% of elemental
carbon, which was considered insignificant for tracking. A simple demonstration of this
would be achieved by testing the composites using the dry-arc resistance test for 10 s during
the 1st current step of the test in cycle 1. Figure 10 shows the surface residue obtained for
both composites after 10 s of the test in cycle 1 with equal electrode spacing. The FS07 filled
composite shows a tendency to form a slightly higher burnt residue, possibly containing
carbon, during the test compared to the GS10 filled composite. Still, however, the difference
is insignificant, which is in line with [11,25].
Figure 10.
Surface residue of (
a
) the SiR + 30 wt% GS10 and (
b
) SiR + 5 wt%FS07 composites tested
using the dry-arc resistance test for the first 10 s of cycle 1.
3.4. Surface Roughness of Eroded Composites
Analyzing the surface roughness of eroded silica filled SiR composites could further
elaborate on the role of silica fillers in the DC erosion performance of SiR composites.
The two elements that are associated with the roughness analysis are waviness and av-
erage roughness. Waviness describes the texture of the overall surface profile along a
defined displacement axis, while average roughness describes the short-wavelength (high
frequency) variations superimposed on the waviness along the same displacement axis [
16
].
Figure 11 shows the 3D topography and corresponding waviness profiles for the silica
filled samples eroded using a dry-arc test. Clearly, the waviness of the FS07 filled SiR
composite indicates a smoother surface with lower variations in the peak heights and valley
depths within different segments of the profile. It is important to highlight that the erosion
depth of the fumed silica filled SiR composite was found to be of higher value compared
to that of the ground silica filled composite in the dry-arc resistance test, as shown in
Figure 11c,d.
This contrasts the +DC IPT outcomes of this study, which have been high-
Energies 2021,14, 3449 12 of 15
lighted in
Figure 3a.
This implies that the use of the dry-arc resistance test in its standard
testing conditions to assess the erosion performance of the composites in terms of erosion
depth would not suffice. Rather, further modification of the testing conditions of the
dry-arc resistance test are required to produce erosion depth ranking outcomes similar to
those of the IPT for these specific composites. As mentioned earlier, the dry-arc resistance
test is only used for the purpose of producing controlled arcing for post-testing residue
analysis and not as means to rank the erosion performance of the composites in terms of
the erosion depths. Using the dry-arc test in its standard form as a means to compare the
erosion performance of the composites could work if both of the composites had equivalent
filler loadings. An example of this would be in using the dry-arc resistance setup to test the
erosion performance of a 30 wt% GS10 filled SiR against a 5 wt% FS07 + a 25 wt% GS10
filled SiR or a 10 wt% FS07 + a 20 wt% GS10 filled SiR composites.
Figure 11.
Three-dimensional topography of the (
a
) the SiR + 30 wt% GS10 and (
b
) SiR + 5 wt%
FS07 composites tested using the dry-arc resistance test. Corresponding waviness profiles for
(c) SiR + 30 wt% GS10 and (d) SiR + 5 wt% FS07 composites.
Figure 12 shows the statistical boxplot for the average roughness value distribution,
R
a
, of the sampled areas shown in the plot. A total of 10 profile lines were taken for each
sampled area. Accordingly, each boxplot shows the median, 25th and 75th percentile and
the minimum and maximum values for R
a
within the 10 profiles. The wider variation and
higher median value of R
a
for the GS10 filled composite indicates a rough profile within
small segments of the composites, which could indicate a higher degree of surface porosity
compared to the FS07 filled SiR composite.
Based on Figures 11 and 12, the GS10 filled composite surface residue is of higher
roughness with the overall surface waviness and within the smaller segments of the surface.
Surface roughness could significantly impact the erosion performance of the composite
due to a number of reasons. Under the salt-fog test, Deng et al., in [
26
], explained that
rough SiR surfaces with large filler particle sizes tend to cause a higher impairment of the
hydrophobicity retention properties of SiR. In their study, leakage currents developed at
higher magnitudes with SiR composites having small filler particle sizes and less surface
roughness. On the other hand, Kozako et al., in [
27
], illustrated that the addition of
nano silica to their SiR composites did not have much of an influence on changing the
Energies 2021,14, 3449 13 of 15
hydrophobic properties of SiR, despite a slight increase in the surface roughness with
respect to the unfilled SiR. Moreover, it is possible that the rough surface texture of the
residue at early stages of the IPT could interrupt the smooth flow of liquid contaminant
during the test, leading to more localized dry-band arcs being formed on the insulator
surface as a result of liquid contaminant being trapped within small, eroded pits.
Figure 12.
Statistical boxplot representation of 10 values of R
a
for the sampled areas shown for each
composite. The center bar represents the median value, the top and bottom box edges represent
the 25th and 75th percentile values, respectively, while the top and bottom markers represent
the minimum and maximum values of Ra, respectively.
4. Conclusions
The presented paper illustrated the role of fumed silica and ground silica fillers in
suppressing the DC erosion of SiR through a novel framework. The erosion performance
outcomes suggest that fumed silica and its interaction with SiR were effective in promoting
the formation of a coherent shielding residue, which resulted in suppressing the DC erosion
of SiR. This was found despite the higher composite thermal conductivity of the ground sil-
ica filled composite, which further supports the influence of the filler’s interface interactions
over enhancements in the thermal conductivity on suppressing DC erosion. Simultane-
ous TGA–DTA analysis shows the significant influence of fumed silica in suppressing
depolymerization and promoting radical-based crosslinking at high temperatures in SiR as
a result of its favorable interaction with the siloxane chains of the polymer tethering their
flexibility and mobility during depolymerization. This results in the formation of a much
higher additional residue with the fumed silica filled composite, despite being filled at one
sixth of the loading level of the ground silica filled composite. The formation of a higher
additional residue could result in a higher carbon content, which still would not be enough
to promote a tracking failure during the IPT. The microscopy conducted on the eroded
composites from the dry-arc resistance test shows coherency and low surface fracture in
the residue of the fumed silica filled composite. This could also explain the better erosion
performance of the composite as a result of the shielding effect of the coherent residue
preventing progressive erosion. Moreover, the surface morphology outcomes of the dry-arc
tested composites are consistent with those of the IPT, which validates the use of the dry-arc
test as part of this framework. The surface roughness outcomes show a rougher surface
waviness and higher values of R
a
for the ground silica filled composite, which could further
indicate the weakness and porosity of the residue, leading to an inferior performance under
the IPT. The overall conclusion of the study suggests a significant role for the silica filler
size in suppressing the erosion of SiR under DC voltage as a result of its influence on the
eroded residue characteristics.
Energies 2021,14, 3449 14 of 15
Author Contributions:
Conceptualization, A.Y.A., R.A.G. and E.D.; methodology, A.Y.A., R.A.G.
and E.D.; software, A.Y.A.; validation, R.A.G. and E.D.; formal analysis, A.Y.A., R.A.G. and E.D.;
investigation, A.Y.A.; resources, R.A.G. and E.D.; data curation, A.Y.A.; writing—original draft
preparation, A.Y.A.; writing—review and editing, A.Y.A., R.A.G. and E.D.; visualization, A.Y.A.;
supervision, R.A.G. and E.D.; project administration, R.A.G. and E.D.; funding acquisition, R.A.G.
All authors have read and agreed to the published version of the manuscript.
Funding:
The authors of the paper would like to thank the Natural Sciences and Engineering Research
of Canada (NSERC) and the National Research Council Canada (NRC) for their financial support.
Acknowledgments:
The authors would like to thank Souheil-Antoine Tahan, Simon Laflamme,
Mohammad Saadati and Joel Grignon for providing the permission, technical help and advice
needed to complete the microscopic part of this study.
Conflicts of Interest: The authors declare no conflict of interest.
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