JEC COMPOSITES MAGAZINE - Issue #131 - Novembre/December 2019 - 66

TECHNOLOGY assembly
found on the surface after the process in combination with the SLE® material. Taking a closer look at the results of the XPS analysis, it can be seen
that, in most cases, the surface shows less nitrogen after removing the
SLE® sheet - with a slight increase in the amount of carbon (likely in the
same proportion as the pure adhesive tape material composition, which
stays on the surface and lowers the amount of measurable nitrogen due
to a higher (thicker) carbon coverage). Supporting this finding is the fact
that most surfaces show an increase in nitrogen after wiping (cleaning)
with IPA. Some of the results concerning the nitrogen content do not
show a trend nor are highly significant, probably because the removal
of the adhesive tape is not areal but insular/island-like - resulting in a
non-homogeneous surface, maybe a scattered surface with slight residues and probably clean surface areas as well.
Overview and future steps
Considering the results of contact angle, IR spectroscopy and XPS
measurements on the surfaces, the SLE® material probably has no direct
influence on the CFRP material in terms of significant surface changes
(chemical composition) next to a mechanical influence zone (if covered
with SLE® during the process). No destructive influence or incorporation of foreign materials visible or measurable with the above-mentioned
analytic systems can be found on the evaluated areas. Furthermore, only
a slight amount of residues, probably originating from the adhesive tape,
is likely to be found on the surface in the areas mechanically affected by
the drilling or trimming process. The area covered by the SLE® material
without mechanical influence is likely to show only a slight additional
amount of (foreign) material detected by IR spectroscopy and also a little surface change detected by contact angle measurements - and the
cleaning by wiping with IPA induces some change in the contact angle
values determined without SLE® processing. The XPS results support
this finding and also show that the possible residues are probably not
very thick as nitrogen is still slightly detectable on the CFRP surface - as
the nitrogen source is the CFRP's resin and cannot be found in significant amounts in the adhesive tape composition itself. So, the thickness
is very likely to be not areal or insular or under 10 nm (locally), which is
the depth of analysis of the XPS measurement itself. The IPA cleaning
probably reduces the residues originating from the adhesive tape on the
CFRP surface - so a further evaluation of different cleaning methods is
needed to provide a procedure with almost no residues and a pure and
clean surface. Such evaluations of adhesive tapes and cleaning processes are in the focus of the current development work, as well as adhesive
bonding tests to evaluate the long-term stability of the surface.
Nevertheless, processes such as atmospheric pressure plasma or UV laser pre-treatment should be able to handle the residues and prepare the
CFRP resin surface appropriately for adhesive bonding processes.
Lifetime evaluation in drilling use cases
The use of MGC's SLE® offers substantial advantages for titanium and
CFRP machining in terms of machining quality and lasting optimization
of running tool costs. The SLE® sheet is applied as a solid lubricant on the
surfaces to be processed and can be removed without residue after processing. Dry lubrication occurs as a result of the SLE® machining and has
a trailing lubrication effect, whereby no continuous application over the
entire component is required. This represents a significant advantage, es-

Fig. 4: Schematic representation of the clamping,
sample plate and processing segments

Fig. 5: Application of SLE® on the good side of
the sample plate with drilling process from the back

pecially in the case of curved components and complex freeforms.
Applying an SLE® sheet is an additional step in the production process,
but it can be parallelized to the machining process and is significantly
compensated by the reduced rework and optimized tool life. To demonstrate the influence of SLE® on the service life of tools, wear and machining quality were investigated by drilling tests with coated and uncoated
drills. Based on the investigations, the difference between the processes
with and without SLE® can be highlighted. The following configurations
were compared in the evaluation series:
-- 1,000 holes with non-coated drill bit with SLE®
-- 1,000 holes with coated drill bit with SLE®
-- 1,000 holes with non-coated drill bit without SLE®
-- 1,000 holes with coated drill bit without SLE®
Two coated and two uncoated drilling tools from a renowned German
supplier were used for the tests. 300 x 300 x 3 mm test plates mounted on a special holder in the machine room were used. The plates were
quasi-isotropic layers of non-crimp carbon fibre fabrics produced by
infusion. The sample components were provided with 100 holes each
at four intervals. After each interval, a visual inspection was performed
and the plate was rotated by 90 degrees back on the device. This way,
the last holes of the respective intervals were compared in the centre of
the sample plate and a direct comparison could be made. Figure 4 shows
the schematic structure of the experimental plate and the four intervals.
The drilling trials were performed on a 3-axis CNC machine, with the

jec composites magazine / N°131 November - December 2019

JEC COMPOSITES MAGAZINE - Issue #131 - Novembre/December 2019

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #131 - Novembre/December 2019