SAMPE Journal - September/October 2020 - 22

FEATURE / OPTIMUM NUTPL ATE PERFORMANCE
alloy, the aluminum WCA was found to be 68±7° after being solvent wiped and 48±6° when abraded.
When plasma treated, the WCA dropped to 25±4°.
The same trend is observed in the aluminum 2024
data. Here the water contact angle is 79±6° after
solvent wiping which can be reduced to 51±4° with
abrasion or further reduced to 28±4° when using
the APC. While not as drastic as the nutplate treatment, this reduction shows a significant increase
in the aluminum surface energy following plasma
cleaning and activation.
Typically, poor bond performance is expected
from hydrophobic surfaces especially those with
large water contact angles like the ones exhibited by
the solvent wiped nutplates. Hydrophilic surfaces,
on the other hand, are indicative of higher surface
energies which typically lead to increased bond
performance. The wetting angle of the nutplate
and the aluminum substrate surface after sanding
is drastically lower than that obtained beforehand.
However, only plasma exposure transformed the
extremely hydrophobic surface after solvent wiping (WCA >120°) to an extremely hydrophilic surface with WCA <5°. Similar trends are observed for
the aluminum substrates though the differences in
water contact angle are less extreme. The extremely low water contact angle achieved by the NPC indicates that the plasma has successfully removed
any contaminants present on the nutplate bonding
surface. In addition, the surface energy of the nutplate has been maximized by the plasma cleaning
process. This ensures that the nutplate is optimally
prepared for installation.

Material

Aluminum 7075

Aluminum 6061

Aluminum 2024

Surface Finish

Bare Metal

Anodized

Bare Metal

Surface Prep

WCA (°)

Solvent Wipe

71±7°

Abrasion

45±4°

Plasma

10±1°

Solvent Wipe

55±4°

Abrasion

42±1°

Plasma

25±1°

Solvent Wipe

68±7°

Abrasion

48±6°

Plasma

25±4°

Solvent Wipe

79±6°

Abrasion

51±4°

Plasma

23±4°

Table 2. Water contact angle of multiple aluminum alloys after surface
preparation.
Figure 6. Maximum load for nutplates bonded to aluminum for varying surface preparation methods.

Push-Off Testing
Push-off testing was performed using the dome
style CB6010 CR3 nutplates. The maximum load
before failure for the bonded nutplates was tested
and is summarized in Figure 6. For push-off testing, aluminum 6061 substrates were prepared using a consistent surface preparation by grit blasting
all of the substrate materials. This was done in order to limit the number of variables and ensure that
any difference in performance was the result of the
nutplate surface preparation. The nutplates with
an IPA wipe disbonded at a load of 345±23 lbs. This
increased to 441±34 lbs following abrasion. When
using only a 5 second cleaning with the NPC, the
maximum load for the nutplates after plasma was
845±74 lbs.
Figure 7 shows images of the nutplate and
substrate coupon after push-off testing. Nutplates
bonded using the IPA wipe or abrasion exhibited
a high degree of interfacial failure mode between
Figure 7. Bond failure interfaces after IPA wipe (left), abrasion
(center) and plasma (right).

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SAMPE Journal - September/October 2020

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