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Thermal Spray Deposition of Metals on Polymer Substrates
by
Bobby Anand
A thesis submitted in conformity with the requirements for the degree of Master of Applied Sciences
Department of Mechanical & Industrial Engineering University of Toronto
The roughness averages presented in the figure above, demonstrate the consistency of
the generated mechanical treatment methodology.
3.3.2 SEM-Images of Surface Topography
Analysis of the substrate surfaces before and after mechanical treatment was done to
determine the existence of any polymer ‘damage’. As the roughness values for both PTFE
and HDPE were within ± 0.05 µm for both the smooth and rough surfaces, examination
of the microstructure was performed to understand the topography of each specimen.
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Figure 3.2: x200 magnification SEM-Image of PTFE/Teflon®, Smooth (A) and Rough (B)
Figure 3.3: x200 magnification SEM-Image of HDPE, Smooth (A) and Rough (B)
Figure 3.2 (A), shows a smooth PTFE surface provided by the manufacturer at Ra values
of roughly 0.20 µm. The surface prior to mechanical treatment is clear of liquid or solid
contaminants. Any water remaining on the surface dried at room temperature, and
contaminants such as grease and dust were cleaned using a 99% isopropyl wash. As the
specimen was produced through a skiving process small lines can be seen that are
directly correlated to the flatness of the extrusion blade in the manufacturing process.
These small lines do not interfere with the overall surface roughness but may play a small
role in overall adhesion. In contrast, for Figure 3.2 (B) the roughened surface averages
at an Ra value of approximately 1.60 µm. The small white particles seen in the SEM-
image, of Figure 3.2 (b) are embedded aluminum oxide abrasive media, that reduce
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overall adhesion between the polymer and metal coating. Valleys and peaks have
appeared on the roughened sample which will aid in mechanical interlocking and lead to
higher adhesion strengths.
Similar results as the PTFE sample can be witnessed for the HDPE in Figure 3.3 above.
The smooth HDPE substrate is seen in Figure 3.3 (A) where the skiving patterns similar
to that in Figure 3.2 (A) can be observed. The magnitude of the skive marks are slightly
higher than that of the smooth PTFE surface, and will be considered when examining
adhesion strength with the metallic coatings. However, as the overall roughness average
evaluated along the spray direction, parallel to the skive imprints, remains within the
smooth criteria the sample is considered suitable for metallization. Only a few dust
particles are observed on the substrate, along with a few indentations from handing the
specimen, that will be cleaned using the same procedure as described for smooth PTFE
samples. Conversely, the rough sample HDPE sample in Figure 3.3 (B) exhibits a similar
average roughness as the rough PTFE sample in Figure 3.2 (B) however, there is
significantly more residue remaining after the mechanical treatment. Additional
consideration must be taken when chemically cleaning and preparing the samples for
metallization.
3.4 Thermal Wire-Arc Coatings
3.4.1 Coating Deposition and Limitations
Thick zinc coatings of approximately 260 µm were obtained on all PTFE and HDPE
samples, regardless of the specimen’s surface Ra value. Aluminum coatings were
obtained on the smooth and rough PTFE substrates, but the coatings delaminated from
HDPE substrates within the first two passes of the wire-arc spray. The coating deposition
rate for these tests will be measured by the increase in coating thickness per pass of the
wire-arc spray, as the samples are significantly smaller than the spray area of 1 total pass
(Sample Area = 25.81 cm2). The deposition rates for the smooth virgin specimens and
their mechanically treated counterparts vary slightly, as the roughened surfaces will
capture a higher volume of metal on the first pass. The coating thickness per pass of the
spray are listed below.
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Table 3.3: Coating Deposition Per Pass on Rough & Smooth Polymer Surfaces
Substrate Coating Coating Deposition (µm per pass)
PTFE Al 44.0 ± 1.00
PTFE Zn 37.0 ± 1.00
HDPE Al 0
HDPE Zn 38.0 ± 0.50
For consistency within the adhesion pull tests, all tests were performed once the overall
coating thickness had reached 260 ± 10 µm. Therefore, 6 passes of aluminum spray and
7 passes of the zinc spray were enough to reach the desired coating thickness. If any
alterations to the coating was performed, it will be illustrated in subsequent section of this
study, along with the coating deposition per pass.
Figure 3.4 and Figure 3.4 are SEM images of the coating surfaces of aluminum and zinc
respectively. For consistency, only coatings on roughened PTFE will be examined. The
calculated porosity for the following sections is dependent on the polishing procedure
after cold mounting the coated samples. The number of detectable pores will increase as
the polishing process is further refined. All porosity measurements and calculations can
be found in Appendix A.
Figure 3.4: (Left) SEM Cross-sectional View of ~260 µm thick aluminum coating onto roughened PTFE,
(Right) ImageJ processed visualization of pores (white) on the top surface of the coating (black).
In Figure 3.4, the thick aluminum coating completely covers the entire PTFE substrate,
and exhibits interesting splat behaviors. According to Hale et al. [45] , in the study of in-
flight particle measurements for aluminum particles, the average size of the particles
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projected from a thermal wire-arc would be 33 to 53 µm, and splats with diameters ranging
from 100 to 400 µm can be observed on the outer most layer. Porosities of the coating
must also be evaluated, according to Oerlikon Metco, the supplier of the wire-arc and
coating material, the typical porosity of aluminum coatings should be 1 - 2 vol.% [43].
However, the porosity of the coating was by examining an imaging processing software,
ImageJ. SEM images were taken of the coating cross-section and it was determined that
the porosity based was approximately 10 ± 2.5 %. This porosity result is expected to
remain consistent throughout the report, as the spraying parameters will remain
consistent in all tests.
Figure 3.5: (Left) SEM Cross-sectional View of ~260 µm thick zinc coating onto roughened PTFE, (Right)
ImageJ processed visualization of pores (white) on the top surface of the coating (black).
The zinc coating on roughened PTFE is shown in Figure 3.5. According to Johnston et
al. [46], air-borne zinc droplets at the appropriate arc current and air pressure can have
particle diameters varying between 28 – 37 µm. However, in the SEM of the zinc topcoat,
it can be noted that splats with diameters of 50-250 µm are produced. The existence of
these splats is predicted to be caused by similar effects experienced in the aluminum
coating. The main cause of increase droplet size would be splashing due caused by low
substrate temperatures and molten droplet accumulation on the substrate surface. The
porosity is predicted to be higher than that of the aluminum coating by Oerlikon Metco,
approximately 4 vol.% [42]. Using ImageJ, the porosity determined by analyzing the
cross-sectional areas was roughly 4.8 ± 1.1%. However, this porosity percentage will vary
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based on the degree of polishing done to the cold-mounted sample. Refining the polishing
process will increase the number of visible pores in the cross-sectional view, therefore
the calculated porosities are estimates of the overall coating.
3.4.2 SEM-Images of Coating Interface
In this section, samples from tests 1 – 8 illustrated in Table 3.1 will be cut, and polished
to examine their interfacial features. The SEM described in Section 2.5.2 will be used to
understand any significant differences or patterns that may contribute to enhancing or
reducing adhesion capabilities between the metal coating and polymer substrate. SEM
images of the metal-to-polymer interface will be presented below, followed by an
explanation of any notable features or properties that should be highlight for analysis.
First, comparisons between zinc coatings and aluminum coatings will be made on a
roughened PTFE surface, as coatings were not achievable with aluminum onto HDPE.
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Figure 3.6: x250 Magnification Overview (OV1-2) SEM images of ~260 µm Thick Coating Aluminum onto PTFE (20), Along with x1.0k Magnified Images (A-D) of Specified Interlocking Mechanisms.
The SEM image in Figure 3.6 (OV1 & OV2) captures interesting features in the interface
between the metal and polymer. The image does reveal evidence of micro-scaled
delamination that was not visible prior to cold mounting. The existence of small pores or
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cracks may come from the coating process as wire-arc deposition typically results in
highly porous coatings.
Nevertheless, it can be observed that a large amount of interlocking has occurred
between the metal and polymer substrate. Figure 3.6 (A to D) closely examines
significantly deeper interlocking that occurs throughout the length of the boundary. The
existence of these deep interlocking areas will greatly increase the overall adhesion
strength between the coating and substrate. Additionally, from images (B) and (C) in
Figure 3.6, the deep interlocking suggests that the metal coating may be flowing deeper
into the polymer substrate, as the overall temperature of the system increases. If the
sample was polished further, image (B) would expose the complete metal link witnessed
in the image, this is evident as for image (C) the interlocking completely encapsulates the
polymer substrate. Images (A) and (D) represent typical cases of mechanical interlocking
that is witness on metal-to-metal surfaces after thermal coating, however image (D) does
support the suggestion of polymer flow, as witnessed in images (B) and (C). Finally, the
entire length of the boundary shows signs of minor and major interlocking occurring
between the metal and polymer that will increase the adhesion strength of the overall
thick coating.
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Figure 3.7: x250 Magnification Overview (OV1-2) SEM images of ~260 µm Thick Zinc Coatings onto PTFE (20),
Along with x1.0k Magnified Images (A-D) of Specified Interlocking Mechanisms.
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Similar to the interlocking mechanisms found between the aluminum coated rough PTFE
in Figure 3.6, interlocking also occurs with the zinc coated roughened PTFE samples,
thought to a lesser extent. In Figure 3.7, four areas were examined for the interlocking
phenomena and overall boundary adhesion condition. Immediately, it should be noted
similar patterns of micro-delamination can be observed on the coating-to-substrate
interface. These patterns would most likely arise due to similar reasons, previously
discussed for the aluminum coated samples, delamination due to the coating process and
polymerization during cold mounting. However, and more importantly, the microscopic
imaging reveals that the amount of interlocking occurring at the interface between metal
and polymer is significantly lower than with the aluminum coating. This observation may
result in a lower adhesion strength with the zinc coated polymers, which may provide
evidence that temperature plays a significant role in the adhesion process between metal
coatings and polymer substrates. In Figure 3.7, images (A) and (B) show deep
interlocking zones that were similar to the zones found on the aluminum coating in Figure
3.6. However, the interlocking found on the zinc coated images (A) and (B) suggest that
the particles adhered to the surface at significantly colder temperatures than that of
aluminum which represents the properties as mechanical interlocking. Chen et al., [44]
produced zinc coated ABS coupons using an electric wire-arc and presented a similar
interlocking phenomenon. However, in their results adhesion was further promoted by an
increased surface roughness in the polymer substrate which resulted from the 3D printing
manufacturing process. The overall interlocking achieved by Chen et el, was similar to
that of images (C) and (D) in Figure 3.6 where micro level adhesion occurs between the
metal and polymer substrate, but without the deeper hooks found with the aluminum
coating. Thus, according to Table 3.2, the temperature of the impacting zinc particles may
play a significant role in the existence of these deep hooks.
3.5 Adhesion Pull-Test Results and Analysis
3.5.1 Polymer Sample Data
Adhesion pull-tests were performed on samples from tests 1-8, that achieved
approximately 260 µm thick coatings, and that did not show delamination. The data of all
adhesion tests performed can be found in the figure below.
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Figure 3.8: Adhesion Strength Pull-Test Data on Tests 1-8.
Figure 3.8 presents the adhesion strength data for all the tests with the blue bars
representing zinc coated samples and the orange representing aluminum coated
samples. The error bars in the figure represents the standard deviation from the average
determined from the pull tests performed on 5 samples. Therefore, a total of 40 samples
were tested, of which only 30 achieved a metal coating. Additionally, all substrates are
labelled on the x-axis starting with smooth PTFE and ending with Rough PE. On the
smooth PTFE samples the wire-arc spray was able to deposit thick aluminum and zinc
coatings.
The zinc coating onto smooth PTFE with an Ra value of approximately 0.20 µm had an
adhesion strength of 0.43 MPa. In comparison, the aluminum coating on the same
substrate had almost double the adhesion strength, approximately 0.76 MPa. The
aluminum coating has significantly higher adhesion strength than the zinc on the same
substrate, which is most likely induced by the higher degree of mechanical interlocking.
Additionally, this trend is exhibited on the mechanically treated PTFE samples with an Ra
value of about 1.60 µm. The zinc coated rough PTFE exhibited an adhesion strength of
about 0.85 MPa, and the aluminum coated sample produced roughly 1.37 MPa. The
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aluminum coating generated almost double the adhesion strength than the zinc
specimen, and the mechanical treatment of the substrates increased overall adhesion in
both the aluminum and zinc coatings. The adhesion strength of the zinc coating increased
by 98% (factor of 2.0) when introducing mechanical treatment, and the aluminum coating
by roughly 80% on PTFE samples. Thus, introducing a roughened surface increased the
overall hooking phenomena evaluated in Section 3.4, which resulted in a significant
increase in the overall adhesion strength of the system regardless of the metallic coating.
For the HDPE samples, the coatings demonstrated similar results with a contrast to the
aluminum tests. All smooth HDPE samples had an Ra value of approximately 0.20 µm
and their rough counterparts, an Ra of roughly 1.60 µm. Now, thick zinc coatings were
achieved on both smooth and roughened HDPE, however aluminum was unable to
adhere regardless of the surface preparation. The zinc samples produced adhesion
strengths of roughly 0.49 MPa and 0.72 MPa, on the smooth and roughened samples
respectively. The increase in adhesion strength contributed by the mechanical treatment
is present in the HDPE samples, at approximately 47 % increase from the smooth sample
to the roughened sample. Nevertheless, mechanical treatment of both polymers resulted
in higher adhesion strength.
Table 3.4: Adhesion Test Results of Aluminum and Zinc Coated, PTFE and HDPE ( "0" = delaminated)
Substrate PTFE/Teflon® HDPE
Smooth Rough Smooth Rough
Zinc 0.43 0.85 0.49 0.72
Aluminum 0.76 1.37 0 0
3.5.2 Sample Delamination
The HDPE substrates experienced delamination between the first and second pass of the
electric wire-arc. The SEM image below in Figure 3.9 illustrates aluminum delaminating
on a roughened HDPE, and an SEM image of the substrate after removing all delaminated
aluminum pieces.
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Figure 3.9: SEM-image of aluminum delamination on right, and on left a photo of the delamination specimen.
It should be noted that in Figure 3.9, droplets have deposited onto the specimen roughly
where there were peaks and valleys generated by the mechanical treatment. However,
one very important feature should be recognized between the delaminated sample and
the mechanically treated one presented in Figure 3.3. Specifically, in the smooth HDPE
surface the presence of grooves generated through the skiving process used by the
manufacturer vanish upon mechanical treatment. However, these grooves are faintly
observed after delamination of aluminum, which suggests that the sample becomes
smoother after thermally spraying the aluminum. As aluminum is at a significantly higher
temperature than zinc, especially due to the rapid oxidation of the air-born aluminum
particles, the HDPE may be melting during deposition causing delamination of the
coating.
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3.5.3 Results and Discussions
All presented in section 3.1 were evaluated using SEM imaging of the interface between
the metallic coating and polymer substrate. From these eight tests, the following results
were achieved.
• A thick aluminum coating on smooth PTFE was obtained
• Aluminum was not able to adhere to HDPE
• Aluminum coatings have roughly double the adhesion strength of zinc,
regardless of the substrate roughness.
• Mechanical treatment of the polymer substrates enhanced the adhesion
strengths of all tests
• Mechanical treatment of HDPE did not aid in achieving an aluminum coating
The results of the microscopic imaging and adhesion tests had suggested that the
temperature of deposition and substrate may play significant roles in promoting or
reducing adhesion capabilities. As discussed in section 3.4.2, a significantly higher
degree of hooking may have been influenced by metal deposition close to the glass-
transition temperatures of the polymer substrates where there was enhanced mechanical
interlocking induced by flow of the polymer substrate. Temperature measurements and
examination at elevated substrate temperatures must be conducted to determine if there
are any thermal influences on metal-to-polymer adhesion.
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Chapter 4 Coating onto Heated Substrates
Temperature Analysis of Wire-Arc Deposition onto Polymers
4.1 Introduction
Results have shown that a thick coating of aluminum can be deposited on PTFE, in
contrast to HDPE where, regardless of surface preparation, aluminum will not adhere.
However, zinc coatings are obtained on both polymers and no delamination is seen on
the smooth HDPE and PTFE substrates. The polymers have similar thermal properties;
however, PTFE has a glass-transition temperature of 115-125 °C and HDPE has a
maximum operation temperature of roughly 80 °C. Additionally, the HDPE surface
experiences melting around 125 – 138 °C, suggesting that the PTFE begins to experience
plastic behavior while HDPE begins to melt. Results in Chapter 3 have suggested that
aluminum coated PTFE experiences a high degree of hooking, which may be a result of
deposition near the glass transition of the material. Thus, this section will examine the
influence of manipulating substrate temperature to determine the associated effects on
metal-to-polymer adhesion.
First, temperature measurements will be collected with 4 K-type thermocouples capable
of a 0.2 second response time connected to a DAQ as described in Section 2.6.3. As
mechanically treated and smooth polymer substrates will experience the same
temperature profile, only temperature measurements on the roughened samples will be
taken. Once the temperature data is acquired during deposition of zinc and aluminum
coatings onto both substrates, observation of the substrate topography will be conducted
with a 2-mode thermal spray gun and furnace, at temperatures simulating spray
conditions. Then, adhesion pull-tests will be conducted on all samples listed in Table 3.1
at elevated substrate temperatures. Specifically, rough and smooth PTFE samples will
be heated slightly below the glass transition temperature of 115 °C, and HDPE will be
heated to between 40-50 °C, half of the maximum operation temperature instructed by
the manufacturer. Cross-sectional SEM images of the interface between the heated
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substrate and metallic coatings will be compared to the room temperature SEM images
presented in Section 3.4.2.
Figure 4.1: A typical temperature profile for Electric Wire-Arc Spraying using a Guided Arm
Figure 4.1 above, represents a zoomed in temperature profile of one complete pass of
the electric-wire arc torch over a substrate initially at room temperature that is sprayed
with aluminum. All figures examined later will have compressed profiles, because of the
increased number of passes shown. The temperature profile has been divided into 4
sections, starting off with section (A) pre-spraying conditions. In (A), the substrate
temperature prior to thermal spraying will be illustrated to define the heated or room
temperature samples. Then, section (B) presents a sudden increase in temperature
induced by introducing the air jet and electric arc within 6 inches of the sample surface.
The temperature of the substrate reaches a maximum during deposition in section (C),
and temperature fluctuations are observed that represent alternate heating and cooling
caused by progressive passes of the guiding arm for the wire-arc spray gun. The 3 peaks
will be consistent throughout the temperature profiles as the number of passes the guiding
arm requires to obtain 1 full coating on the polymer samples. Finally, Section (D)
illustrates the cooling phase between spraying, additionally, examination of any signs of
delamination is performed at this time. The time for cooling between samples vary based
on the heated and room temperature substrates and will be defined for each test.
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4.2 In-situ Temperature Measurements during Thermal Spraying
The temperature data presented below is for mechanically treated substrates initially at
room temperature. Upon examination, it was determined that differences between smooth
and rough samples were negligible. However, the temperature profiles of the smooth
specimens will only be examined if there are any significant changes to the
measurements. Additionally, both smooth and mechanically treated temperature
measurements and figures can be found in the appendices.
4.2.1 Thermal Spraying: Aluminum onto PTFE and PE
Figure 4.2 shows the temperature variation of the roughened PTFE/Teflon® surface
during ten passes of the wire arc spray torch depositing aluminum.
Figure 4.2: Temperature data of a 10 Pass Electric-Wire Arc Deposition of Aluminum onto Roughened PTFE
Substrate at Room Temperature
First, the substrate starts at an atmospheric temperature of approximately 25 °C, then
during the first pass, as aluminum is deposited onto the polymer surface temperatures
reach a maximum of approximately 86 °C. A constant cooling period of 2 minutes is
maintained between coatings to permit complete solidification of the most recently
deposited layer. The minimum temperature after the cooling period of the first pass was
roughly 33 °C. However, with each proceeding pass of the spray gun there exists residual
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thermal energy prior to the next coating. This residual thermal energy stabilizes within the
testing parameters between the 3rd and 4th coating. The maximum achievable
temperature during the testing period was recorded as approximately 88 °C, with a
minimum resting temperature after cooling of roughly 39 °C. All recorded temperatures
have exhibited behavior within the maximum of 88 °C, similarly the minimum temperature
after cooling maintained at roughly 39 °C on the 10th and final pass for the test. Finally,
similar results were obtained for the PE samples, however a complete aluminum coating
was not achieved.
4.2.2 Thermal Spraying: Zinc onto PTFE and HDPE
Figure 4.3 illustrates a ten-pass coating process of zinc deposited on a roughened
PTFE/Teflon® surface. Deposition on the HDPE samples will not be examined, as the
thermal properties are like that of PTFE, besides HDPE having double the specific heat
of PTFE. However, the deposition of zinc onto HDPE can be found in Appendix B.
Nevertheless, some discrepancies in the temperature measurements will be described
further in the explanation.
Figure 4.3: Temperature data of a 10 Pass Electric-Wire Arc Deposition of Zinc onto Roughened PTFE
Substrate at Room Temperature
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Similar to the previous specimen in Figure 4.2, the test begins at an atmospheric
temperature of approximately 24 °C, then as the first coating of zinc is deposited onto the
polymer the temperature reaches roughly 36 °C. As the maximum temperature of the
substrate while depositing zinc is lower than the maximum temperature during aluminum.
Additionally, as zinc has a lower thermal conductivity than aluminum and the peak
temperatures during deposition are relatively low, the cooling period was adjusted to
roughly 30 seconds between passes. The minimum temperature achieved after the first
pass was roughly 27 °C, almost 3 degrees higher than ambient temperature. Patterns like
that of the aluminum deposition is observed while spraying zinc onto the polymer
substrates. The minimum temperature after the cooling phase, and maximum
temperature upon deposition of the metal gradually increases with proceeding passes.
The highest temperature, obtained at the 8th pass was roughly 44 °C, and the maximum
temperature after cooling obtained at the 10th and last pass was approximately 34 °C.
Thus, the polymer substrates are experiencing significantly different thermal cycling when
depositing zinc or aluminum. Although the temperatures of the substrates after cooling
only vary by 4-5 degrees, the maximum temperature experienced during deposition of
aluminum is almost double that when depositing zinc. Returning to results of chapter 3,
zinc has little trouble adhering to both polymer substrates, however, regardless of surface
preparation aluminum was not able to adhere to the HDPE. Thus, using the temperature
profiles examined above, the polymer substrates will be heated with two different methods
that mimic the thermal spray conditions.
4.3 Polymer Substrate Topography: Furnace Heated
In this section, examination of the polymer surface topography using SEM was conducted
for two heating tests. These two tests will attempt to characterize any surface differences
or changes in surface morphology using surface roughness testing.
4.3.1 Furnace Heated: Surfaces of PTFE/Teflon®
First, to determine the effects of surface heating during aluminum deposition, samples of
PTFE and HDPE were mechanically treated and placed within a furnace (see Figure 2.12)
at a resting temperature of 90 °C. Then, the samples were placed horizontally inside the
furnace for 30 minutes. Placing the samples horizontally prevented any gravitational
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effects that may displace some of the polymer substrate while heating. After 30 minutes,
the substrates were taken out and immediately tested to determine their average Ra
values.
All SEM-images below were captured at relatively the same location, additionally all
figures will be organized under the same testing specimen. Specifically, if a figure
examines the virgin surface of a polymer, mechanical treatment of the polymer will be
conducted, and examination will be performed at the same location to maintain accuracy.
Each proceeding image will be the next step in the samples testing method (i.e. a PTFE
sample will be purchased as virgin smooth, then mechanically treated, and finally heated).
Figure 4.4: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin PTFE, and Smooth
Furnace Heated PTFE
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The first observation from Figure 4.4 is the skiving pattern obtained during processing of
the material sheet. These patterns do not impact the roughness of the surface but aid in
observation of topography changes on the substrate after furnace heating. Nevertheless,
when comparing the virgin sample to the furnace heated counterpart no differences can
be observed. The average roughness values of the virgin samples were roughly 0.22 µm,
and after heating for 30 minutes at 90 °C, the resulting Ra value was 0.20 µm, both with
a 0.02 µm deviation. Thus, the temperature alone had negligible effects on the surface
roughness under thermal spray conditions.
In the next test, the virgin smooth PTFE sample illustrated in Figure 4.4, has a noticeably
higher roughness average. The roughness of the previous smooth samples was
approximately 0.22 ± 0.02 µm, but for the smooth samples for this specific test the Ra
values were roughly 0.68 ± 0.03 µm. This increase in virgin roughness is an uncontrollable
variable resulting from discrepancies in the manufacturing process. The discrepancies
have created a deep wave-like pattern on the virgin polymer substrate that has
contributed to the overall surface roughness. However, as these samples will be
mechanically treated, the roughness of the virgin polymer will not affect the analysis of
the test.
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Figure 4.5: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin PTFE, Sandblasted
Rough PTFE, and Furnace Heated Rough PTFE
51
The relatively smooth PTFE samples are taken and mechanically treated with aluminum
abrasive, which result in samples with RA values of 1.58 ± 0.06 µm. Small white particles
can be observed on the ‘sand-blasted’ SEM images, which are residual sand and dust
from the surface treatment. These samples are then placed in furnace and allowed to
heat for 30 minutes at 90 °C, resulting in the ‘Heated + SB’ SEM images. The PTFE
roughened substrate experienced negligible changes to the surface roughness and
observed topography. Specifically, the roughened samples were scratch tested and
determined to have RA values of approximately 1.54 ± 0.06 µm.
4.3.2 Furnace Heated: Surfaces of HDPE
The HDPE substrate will be examined in this sub-section. Furthermore, all results
determined for both polymers will be summarized at the end of Section 4.3.3.
Figure 4.6: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin HDPE, and Smooth Furnace Heated HDPE
52
The tests performed on the roughened and smooth PTFE samples of the last section,
were repeated for the HDPE samples under the same conditions. This surface
examination will attempt to reveal temperature influences on the HDPE polymer, in
contrast to the negligible effects on the PTFE.
Accordingly, in Figure 4.6, SEM imaging of the surface topography of the smooth virgin
HDPE and smooth furnace heated HDPE are presented. All images in the figure above
will focus in the same relative area, specifically an observable ripple in the substrate
surface. First, it was observed that the non-heated virgin HDPE clearly defines the
irregularities in the rippling area. Additionally, this distortion in the polymer runs parallel
to the skiving patterns which are visually distinguishable when examining the surface at
400x magnification. Measuring the roughness parallel to the surface irregularities, the Ra
value was determined to be approximately 0.16 ± 0.01 µm.
Then, the smooth virgin samples were heated under the same conditions previous
described (30 mins at 90 °C) for the PTFE samples. The first noticeable difference in the
SEM images between the non-heated and heated surfaces is that the ripple and skiving
patterns are less visible under similar SEM parameters. The main stem of the ripple,
which is parallel to the skiving patterns, remains detectable, however, many of the smaller
features illustrated in the non-heated images become undetectable. Despite these
observations, the average roughness value for the heated substrates were roughly 0.15
± 0.01 µm. Therefore, although the analysis of the SEM imaging provided some evidence
that HDPE may be subject to softening at elevated temperatures, the roughness tests
deem the results inconclusive. The investigation of the mechanically treated substrates
may provide enough evidence on this claim.
53
Figure 4.7: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin HDPE, Sandblasted
Rough HDPE, and Furnace Heated Rough HDPE
54
A new smooth virgin HDPE sample was selected to examine the surface topography at
elevated temperatures. First, in Figure 4.7, surface images of the smooth virgin sample
were captured to highlight skiving patterns and any discrepancies that may be used to
distinguish between the surface preparation methods. Additionally, it should be noted that
the prevalent feature of the virgin samples; the skiving pattern, has proven present in all
tested polymer specimens. Diagnosis of the virgin surface roughness provided an Ra
value of roughly 0.24 ± 0.01 µm, parallel to the skiving pattern. Although slightly higher
than the roughness of the previous sample, the difference is negligible after mechanical
treatment.
The sand-blasted samples contain small ripples and debris, generated by the aluminum
oxide abrasive, that will be used to interpret any changes in surface topography. All
skiving marks, indicating smooth regions of the polymer, prior to mechanical treatment
have vanished and been replaced with the course and rugged exterior, expected of
sandblasting. Furthermore, the roughness average of the mechanically treated HDPE
substrates was 1.59 ± 0.06 µm, virtually identical to the sand-blasted PTFE samples.
Finally, after heating the samples for the specified time and temperature, the heated SEM
images were produced. However, similar conclusions to the smooth heated HDPE are
examined in the topography. Very little can be concluded besides small signs of softening
at the rippling areas, but there is a significant change in the overall roughness. The
evaluated Ra value for the rough heated HDPE was approximately 1.31 ± 0.05 µm, almost
0.28 µm lower than the non-heated rough samples. This decrease in roughness is roughly
18% lower than the roughened surface but is the highest change in topography examined
out of all tested samples. The influence of elevated temperatures does influence
roughened PTFE surfaces, but is this effect is difficult to detect in an isolated environment.
55
4.3.3 Furnace Heated: Results and Discussion
The figure below summarizes the roughness data acquired in the tests performed in
Sections 4.3.1 and 4.3.2. Each test was performed with 3 different samples, which were
all characterized at the virgin, sand blasted, and heated conditions. The roughness data
for each test can be found in Appendix C.
Figure 4.8: Evaluated Ra Values of Various Polymer at virgin, sand blasted, and heated conditions. Heating
Method: Furnace. (0 – Smooth, 20 – Rough)
In Figure 4.8, roughness data is separated in order of the tests performed starting with
the rough samples (#20) of a specified polymer, then the smooth samples (#0).
Additionally, the small error bars on top of the colored bars indicated the standard
deviation of 9 measurements per sample in a set of 3 per test. Furthermore, the black
bars represent the roughness of the virgin polymer purchased from McMaster-Carr [18,
19], the gold bars represent sand blasted samples, and the red represents the roughness
of heated samples. Now, as previously stated, the roughness of the PTFE 20 sample
prior to mechanically treating was significantly higher due to the existence of waves
created by errors in the manufacturing process. However, after mechanically treating the
surface via sandblasting, the overall roughness of the PTFE 20 was identical to the sand
blasted HDPE 20. The conclusions reached from the furnace heating test were:
56
1. No noticeable changes to surface topography and roughness were
observed for the PTFE samples, regardless of surface preparation.
2. Smooth Heated HDPE displayed small noticeable physical changes in the
surface topography that had no effect on overall surface roughness.
3. Heating a Roughened HDPE specimen in an isolated environment resulted
in approximately 18% reduction of surface roughness.
The examination of the influence of temperature on the smooth and roughened polymer
surfaces provided evidence of small surface changes that governs the need for further
investigation. As these tests were performed in an isolated environment, where only
temperature was the governing variable for surface alterations, the influences of the air-
jet and metal droplet impingement were omitted. An alternate test that evaluates the same
parameters examined above was constructed using a MasterCraft™ 3-Position Heat
Gun. This new test will mimic similar temperature conditions like the electric wire-arc,
while providing a low air-jet impingement on the substrate surface.
4.4 Polymer Substrate Topography: Heat Gun
4.4.1 Heat Gun: Temperature Measurements and Air Jet Impingement
Furnace heating the polymer substrates provided information on the effects of
temperature on polymers, now, this section will focus on using a MasterCraft™ 3-Position
heat gun to determine the effects of substrate heating via a heated air jet. The heat gun
has built in fans that will be used to mimic the wire-arc’s air jet, however at a lower air
velocity.
Now, a brief explanation of the typical temperature profile that will be experienced by the
polymer substrates via the heat gun will be provided below. This specific test was
performed on roughened HDPE; however, all tests have experienced similar temperature
profiles with negligible variations. Temperature data along with roughness data for the
preceding section can be found in the Appendix C.
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Figure 4.9: Standard Temperature Profile for All Heat Gun Tests performed on Smooth PTFE Substrate
In Figure 4.9, the temperature profile of 5 heating intervals are examined starting at an
ambient temperature of approximately 23 °C. These heating intervals will mimic the
heating and cooling periods experienced by the electric wire-arc during deposition.
Generally, the heat gun will attempt to heat the substrate for roughly 14 seconds, this is
the approximate time it takes to reach the first peak of roughly 84 °C. The cooling phase
will be maintained at roughly 2 minutes in between each heating interval, this will allow
the temperatures to reach below glass transition for PTFE and below melting for HDPE.
In this specific test, the only discrepancy from the remaining temperature profiles, is the
reduced cooling phase in the first heating interval. However, this can be ignored, as the
profile is for smooth PTFE which has proven to show negligible affects below its glass
transition temperature evaluated during furnace testing. Nevertheless, the temperature of
the substrate immediately after the first cooling phase is roughly 55 °C, and the
temperature increases by 3 degrees to 58 °C at the final cooling phase. Additionally, from
the first heating phase to the last there is an increase of roughly 10 degrees resulting in
roughly 94 °C. These heating results will vary depending on the polymer; however, the
overall patterns will remain the same. Furthermore, these temperature profiles replicate
the nature of the temperature profile presented in Figure 4.2 and Figure 4.3, the aluminum
and zinc sprayed roughened PTFE, respectively.
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4.4.2 Heat Gun: Surfaces of PTFE/Teflon®
As previous stated in Section 4.4.1, the heating tests performed for the furnace heated
samples will be conducted using the described heat gun. The following tests will begin
with the PTFE samples, then HDPE, and will conclude with a summarization of results of
both heating methods. Finally, overall test conclusions will be made on the effects of
isolated heating, and heating aided with an air jet onto the specified polymer substrates.
Figure 4.10: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin PTFE, and Gun
Heated Smooth PTFE
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In the figure above, a smooth unheated virgin PTFE specimen has been examined for
skiving marks that will aid in the observation of topographical changes after heating. The
Ra value of the surface was approximately 0.14 ± 0.01 µm, therefore smooth with very
faint features that can be seen in the 400x magnified images. Then, after performing the
heat test at 5 intervals of 80-90 °C, the previously faint skiving marks became more
defined. However, heating the substrate successfully defined deep markings, but the
remainder of the surface discrepancies remained the same in comparison to the unheated
virgin sample. This observation is further supported, as the average roughness of the
heated area was roughly 0.19 ± 0.02 µm. Thus, regardless of the heating aided with an
air-jet, the PTFE smooth substrate experiences negligible effects on the surface
topography. Additionally, the results of both heating tests; furnace heating, and the heat
gun, provided enough evidence that the smooth PTFE surface does not experience
significant change to the surface topography.
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Figure 4.11: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin PTFE, Sandblasted Rough PTFE, and Gun Heated Rough PTFE
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SEM images of smooth ‘virgin’ PTFE substrates were taken, then the samples were
mechanically treated until an average roughness of 1.55 ± 0.07 µm was achieved. As
expected, rippling occurred throughout the surface where valleys and peaks can be seen
in the ‘sand-blasted’ SEM images. Finally, after heating the substrate, the final SEM
images were captured, and the roughness analyzed. No conclusions could be made after
observation of the ‘heated + sb’ SEM images, this is consistent with the other furnace and
heat gun tested PTFE samples. The determined Ra value for the heated surface was
roughly 1.45 ± 0.11 µm, this test had the largest deviation of all heated PTFE samples.
Additionally, the overall roughness decreased by 0.10 µm, however, with a slight increase
in the surface deviation of 0.04 µm. The furnace heated samples experienced a drop of
only 0.04 µm from the unheated and heated samples, however with the introduction of an
air-jet increased that difference by 0.06 µm. This suggests that the substrate is beginning
to exhibit slight plastic behavior the closer the overall temperature gets to the materials
glass transition temperature.
4.4.3 Heat Gun: Surfaces of HDPE
Two HDPE samples were examined before and after the heat gun experiment under the
SEM. In the furnace test previously discussed, the existence of softening patterns was
observed correlating to the substrate temperature rise. Now, with the introduction of the
air-jet and thermal cycling, which more accurately represents the electric wire-arc, the
emphasis will be on examining any enhancement to this softening phenomenon.
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Figure 4.12: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin HDPE, and Gun
Heated Smooth HDPE
First, the examination of an extremely smooth HDPE virgin sample was conducted and
in Figure 4.12 above, virtually no skiving marks can be identified. However, instead of the
typical skiving marks that are present in almost all virgin smooth polymer samples, there
ripples that can be seen at 200x and 400x magnification. Additionally, the surface
roughness after SEM examination of the surface was identified as approximately 0.15 ±
0.02 µm. Notice, that this Ra value is almost identical to the value determined for the
smooth PTFE sample in Figure 4.10, where virtually no surface features were observed.
Now, after subjecting the smooth virgin polymer to the heat gun procedure, the gun
heated smooth HDPE SEM- images were acquired. Immediately, distinct differences are
examined on the substrate surface where previously no prominent feature could be
examined. Clearly defined skiving marks oriented parallel to one another are visible
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throughout the substrate surface. These emerging features clearly indicate softening of
the heating zone, where the temperature creates a viscous surface that the air-jet
modifies. However, the average roughness of the substrate after heating was
approximately 0.21 ± 0.03 µm, slightly higher than the unheated counterpart. The
appearance of these skiving features on the sample upon heating may contribute to this
minor change in surface roughness. Nevertheless, the observations of the furnace heated
HDPE suggested evidence that softening, and material flow was occurring on the
substrate surface. Now, with the addition of focused jet impingement, distinct differences
are recognized with developing skiving features. This emerging pattern suggests possible
material flow aided by the air-jet, along with supporting the softening concept described
previously. Finally, examination of the effect of the heat gun on the roughened HDPE
samples will resolve any concerns surrounding the discrepancies in these predictions.
64
Figure 4.13: 200x and 400x Magnification SEM Images of the Surfaces of Smooth Virgin HDPE, Sandblasted
Rough HDPE, and Gun Heated Rough HDPE
65
Finally, the last heat gun experiment will start by examining the smooth virgin PTFE
surface prior to any heating. In the figure above, in the virgin row, the presence of many
deep and faint skiving marks can be identified throughout the substrate surface. Following
the trends previously described, the presences of these markings will slightly increase the
overall surface roughness but will not attribute to any influence on the analysis of the gun
heated rough HDPE sample. Nevertheless, the virgin smooth surface of the HDPE
sample was approximately 0.16 ± 0.02 µm. Then, the substrate was sand blasted until a
uniform surface roughness of 1.59 ± 0.09 µm was achieved. In the ‘sand blasted’ row of
the figure, the common rippling pattern is observed on the surface that represent peaks
and valleys generated through the mechanical surface treatment. These observable
surface features will be the primary focus, that will provide indication for softening and top
surface material flow. The heat gun experiment on the smooth virgin HDPE suggested
that the skiving patterns created by the manufacturer, that represent a ‘slippery’ surface
will appear after the heating experiment. Therefore, attention to any presence of skiving
patterns will be taken after completing the 5-interval heat test at 90 °C. Finally, after
heating the substrate, SEM imaging of the substrate surface were taken and presented
in the ‘Heated’ row of the figure. Ripples are observed throughout the substrate surface,
as expected from the roughened sample, however as anticipated, skiving marks have
begun to emerge across the heating zone of the sample. In the figure above, you can see
in the ‘Heated’ row, skiving marks like that of the smooth virgin sample prior to any
mechanical treatment or heating. Additionally, the presence of these features suggest
that the surface roughness has drastically reduced, which can be verified with the
examined Ra value of approximately 0.34 ± 0.08 µm. Thus, upon heating the sample in
a procedure mimicking an electric wire-arc spray test, the roughened HDPE surface
experiences approximately 79% reduction in overall surface roughness. Additionally, this
provides evidence that softening of the substrate surface does exist for rough HDPE
surfaces when spraying aluminum via electric wire arc.
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4.4.4 Heat Gun: Results and Discussion
The following figure is collection of the roughness data of the furnace heating and heat
gun tests examined throughout Chapter 4. Additionally, as the virgin and sand blasted
samples differ depending on the test, all presented values for those sets derive solely
from the heat gun procedure. Finally, the roughness data associated with the figure can
be found in the Appendix C.
Figure 4.14: Evaluated Ra values of specified polymers (0 – smooth, 20 – rough) at the virgin, sand blasted,
furnace heated, and post-heat gun states.
In Figure 4.14, like Figure 4.8, the polymers are organized starting with the roughened
PTFE (#20) substrate, followed with its smooth virgin counterpart (#0). Then, the HDPE
samples were illustrated in similar fashion, starting with roughened and ending with
smooth. Additionally, the black bars represent the virgin samples of the heat gun test,
prior to any heating or mechanical treatment. The orange bars represent the average
roughness of the surface after mechanically treating the substrates, these bars are only
present for samples indicated with the grit size of 20. Finally, the two heating bars are
represented as red for furnace heating examined in Section 4.3.3, and the blue bar
representing the heat gun results in the most recent analysis. Any error bars in the
experiment are associated with the standard roughness deviation evaluated for a sample
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set of 3 polymers per test. Summarizing points made in the furnace heating section, along
with the ones created for the heat gun during its analysis, the conclusions are:
1. Negligible differences in the surface topography and roughness were
observed for all PTFE samples, regardless of surface preparation and
heating method.
2. Smooth HDPE experiences no change in surface roughness or topography
when furnace heated, however small increases in Ra are examined after
Heat gun testing, along with emerging skiving features.
3. Roughened HDPE experiences an 18% reduction in surface roughness
with furnace heating, and an almost 79% decrease when introducing an
air-jet
These concluding points help emphasis the existence of surface modification on HDPE
during thermal spraying. The HDPE substrate experiences softening during deposition,
which results in higher possibilities of coating delamination. This occurs because the
temperature of depositing aluminum droplets onto the HDPE softens the outer layer,
preventing any mechanical interlocking at the interface. The possibility of manipulating
this substrate temperature will be evaluated, as heating the samples close to their
respective softening/glass-transition temperatures may affect adhesion at the metal-to-
polymer interface.
4.5 Heated Substrate Characterization
Now that the temperature effects on the topography of the polymer have been
established, this section will focus on the manipulating the temperature of the substrates
during the spraying procedure. Specifically, the average temperature of the polymers will
be raised to specific temperature zones. These zones are, the glass-transition
temperature of PTFE, and a high operation temperature where softening occurs for
HDPE. In Chapter 3, it was discovered that interfacial hooking aided in higher adhesion
between aluminum and PTFE. Examination of the effects of manipulating substrate
temperature on adhesion strength and interlocking properties will be the primary focus
that builds on the temperature and air-jet influences constructed in the previous section.
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Additionally, the differences between coating metal onto room temperature and heated
substrates will be established with examination of the interface through SEM-imaging.
4.5.1 Aluminum onto Heated PTFE and HDPE
In this section, only the heated rough PTFE temperature profile will be examined, as all
HDPE samples at elevated temperatures delaminated prior to establishing an aluminum
coating. Additionally, it should be assumed that all PTFE samples experience a relatively
similar profile, regardless of surface roughness, while spraying aluminum. Furthermore,
the number of achievable coatings has increased from the room temperature tests
performed in Chapter 3, which arises due to a slight change in the surface preparation.
Specifically, iso-propyl alcohol was left on the substrate longer which allowed it to
evaporate naturally resulting in a higher coating thickness. In contrast, the room
temperature samples were sprayed with pressurized air to remove any excess iso-propyl
on the surface. However, examination of the adhesion strength of the first bonding layer
will be conducted at the point of delamination mimicking the conditions of the room
temperature tests. Finally, the variable transformer used to power the heater cartridges
were adjusted to 70 Volts, in order to establish a substrate temperature of 100 °C, only
about 15°C below the glass transition temperature of PTFE (it ranges from 115°C to
125°C, see Table 2.1).
Figure 4.15: Temperature verse time graph of aluminum depositing onto a roughened PTFE substrate heated to roughly 100 °C, via heat cartridges and a variable transformer
69
In Figure 4.15 above, the rough substrate (Ra ~ 1.58 µm) was heated and stabilized at
an elevated temperature of roughly 96 °C. This temperature was achieved by setting up
the Powerstat® variable transformer [35] to regulate the voltage at 70 – 75 Volts,
increasing the substrate temperature gradually until the desired temperature was
achieved. Then, the electric wire-arc procedure for aluminum was conducted to achieve
a thick metal coating prior to the delamination conditions.
First, a sudden drop in temperature from 96 °C to roughly 90 °C was observed, and as
previously stated, this is caused by the air-jet hitting the substrate surface prior to the
molten aluminum droplets. However, as PTFE has a thermal conductivity of 0.25 W/m·K,
much lower than that of the thermocouple 1 mm bead, this sudden 6-degree temperature
drop does not signify a significant drop in the substrate temperature. Thus, ignoring the
sudden dip in temperature at the start of the first pass, the temperature of the substrate
rapidly increases to roughly 127 °C, slightly above the glass transition temperature of
PTFE (see Table 2.1). Then, after a 2-minute cooling period, the temperature rapidly
drops to approximately 98 °C, closer to the desire substrate temperature. However, the
maximum temperature after the second pass drastically decreases to roughly 115 °C,
which better represents the substrate temperature as we observe it gradually increase to
126 °C at the final pass. Therefore, the first pass did not appropriately indicate the
substrate temperature, but the second pass will be considered a more accurate
measurement. Nevertheless, we observe patterns that are comparable to the aluminum
deposition onto room temperature PTFE in Figure 4.2 and Figure 4.3. Specifically, that
the temperature of the substrate after the cooling phase gradually increases from roughly
96 °C to 104 °C at the last pass. This occurs due to the low thermal conductivity of the
substrate, as the sample is heated and cooled, progressively higher residual thermal
energy is maintained within the polymer. Additionally, we see the same increase in the
peak temperature after each pass of the wire-arc like depositing zinc onto HDPE at room
temperature. Finally, as the substrate’s base temperature remained within the specified
condition, and an approximately 425 ± 10 µm aluminum coating was achieved.
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4.5.2 Zinc onto Heated PTFE and HDPE
A zinc coating of 10 passes was achieved on both PTFE and HDPE substrates,
regardless of surface roughness, and just prior to any signs of delamination. The following
are the temperature measurements of the heated substrates, and the effects of molten
zinc on the surface temperature.
Figure 4.16: Temperature verse time graph of zinc depositing onto a roughened PTFE substrate heated to
roughly 106 °C, via heat cartridges and a variable transformer
In the figure above, zinc was deposited roughened PTFE surface (Ra ~ 1.59 µm) initially
at 106 °C which immediately experienced a temperature drop of roughly 16 °C. The
substrate experiences a decrease in overall substrate temperature because of impinging
air. Once the spray moves away the substrate heats up again and after approximately 1
minute the temperature of the substrate increases from 89 °C to roughly 103 °C.
Furthermore, the temperature of the substrate progressively drops until a minimum
temperature of roughly 84 °C is recorded as the substrate temperature. Similar patterns
are examined for the minimum spray peak, as the first pass resulted in an 89 °C substrate,
the final pass decreased the temperature to exactly 80 °C. The most important
observation is that, as we increase the number of passes the overall temperature
difference form the heated substrate to the valleys during spraying are gradually
71
decreasing. The 16-degree difference after the first pass, gradually decreased to an
almost 4 °C difference between the spraying condition and after the ‘heating’ phase.
In conclusion, the high temperature PTFE substrate experiences a cooling effect when
zinc is being deposited onto it. Additionally, this phenomenon did not prevent the zinc
from adhering to the surface, which achieved a thick coating. The average coating
thickness of the zinc onto the roughened and smooth PTFE substrates is approximately
370 ± 10 µm.
Figure 4.17: Temperature verse time graph of zinc depositing onto a roughened HDPE substrate heated to
roughly 55 °C, via heat cartridges and a variable transformer
In contrast to the zinc deposition on the heated PTFE substrate in Figure 4.16, the
roughened HDPE substrate does not experience cooling. Rather, the substrate
experiences a stabilized heating pattern with a reduction in cooling rate. First, the HDPE
substrate’s temperature is increased by the heating plate with a variable transformer set
to 55 volts. The plate raises the temperature of the samples to roughly 55 °C, a
temperature where softening of the polymer occurs, and the spray test begins with the
first coating only increases the temperature of the substrate by half a degree to 55.5 °C.
This pattern is like the zinc deposition onto heated PTFE, as the first pass does not
accurate depict the substrate temperature, however in the second pass a sudden
increase is observed to almost 57 °C. The temperature of the substrate peaks at the 5th
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pass at roughly 60 °C during spraying, however after the 1-minute cooling phase, the
temperature drops to 50.6 °C. This decrease in substrate temperature is consistent
throughout all heated substrates, as the number of passes increase, the substrate
temperature after cooling decreases which indicates a reduction in the magnitude of
thermal energy contained within the system. However, as the passes increase beyond
the 5th, the temperature of the substrate slightly decreases during zinc deposition.
In conclusion, the zinc deposited onto the HDPE substrate surface increases the overall
substrate temperature by roughly 5 °C at the 5th pass. Then, the substrate experiences a
loss of residual thermal energy resulting in a minor decrease of 2.5 °C from the maximum
peak achieved at the final coating. However, a thick coating was achieved on the smooth
and roughened HDPE surface of approximately 380 ± 10 µm.
4.5.3 SEM-Imaging of Coating-Substrate Interface
Specimens from all 8 tests described in Table 3.1 were performed with the heated
polymer substrates for 10 passes of the electric wire-arc with both metals. The average
calculated thickness of the aluminum and zinc coatings, as previously described, are…
• 425 ± 10 µm for… PTFE (Smooth and Roughened)
• 370 ± 10 µm for… PTFE (Smooth and Roughened)
• 380 ± 10 µm for… HDPE (Smooth and Roughened)
The SEM-imaging of aluminum onto heated rough PTFE and zinc onto heated rough
PTFE will be examined thoroughly, as these samples experienced the most significant
difference in adhesion strength in the testing performed in Chapter 3. However, a brief
examination of all successful tests will be provided after evaluating the roughened PTFE
samples.
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Figure 4.18: x250 Magnification Overview (OV1-2) SEM images of Aluminum onto Heated PTFE (20), Along
with x1.5k Magnified Images (A,C, and D) and x800 Magnification for (B) of Specified Interlocking
Mechanisms.
74
The interface between aluminum and the rough heated PTFE was examined in Figure
4.18 (OV1 and OV2) at x250 magnification. The SEM images presented were separated
into two sections, the image to the left represents the typical pattern observed throughout
the cross-section. The image on the right was selected, as it illustrates an uncommon
region in the interface where the intensity of ‘hooking’ is low, but still present.
Starting with the image to the left, it is apparent that the substrate-to-metal interface
experiences a high degree of mechanical interlocking. There are several locations in this
selected zone, that experiences a very deep hooking phenomenon. This deep hooking
was present in the room temperature samples, but at a lower degree as observed in
Section 3.4.2. However, this degree at which mechanical interlocking occurs on a room
temperature versus a heated substrate will be evaluated when examining the adhesion
strength associated with the heated sample. Now, the cross-sectional image to the right
inspects a low mechanical interlocking area that is relatively uncommon throughout the
examined sample. Although, this sample differs from the rigid curling patterns examined
in the previous image, deep interlocking can be observed throughout the length of the
interface. A deeper examination of these interlocking zones is illustrated in Figure 4.18,
images (A) to (D).
Starting with Figure 4.18 (A), the polymer and metal overlap and become tangled similar
to interlocking that was examined previously in Figure 4.6 (C). This ring-like interlocking
mechanism occurs instantaneously during deposition, as the molten metal droplets
redistribute the softened polymer substrate. Then, as a single branch of the molten metal
plunges deep within the specimen and intersects with another branch of a different or the
sample droplet, they combine to make the ring-like interlock. This pattern can also be
observed on image (B), where almost 5 branches are penetrating deep within the
polymer. Starting from the left of image (B), the first two branches have begun to create
a ring-like structure, and the two branches following them appear to have solidified just
prior to intersecting. Finally, a large aluminum fragment appears to be separated from the
coating itself, however if further polishing was performed on the cold-mounted specimen,
a connection would be found between the large fragment and the coating. Thus, these
zones indicate areas which the adhesion strength of the coating a significantly higher than
that of smoother samples. Analysis of images (C) and (D) do not provide any evidence of
75
‘deep’ hooking, or ring-like structures, however aluminum branches can still be observed
throughout the interface. These interlocking locations will aid overall adhesion strength,
but not to the extent observed in images (A) and (B).
Figure 4.19: x250 Magnification Overview (OV1-2) SEM images of Zinc onto Heated PTFE (20), Along with x1.5k Magnified Images (A-D) of Specified Interlocking Mechanisms.