Aerospace & Defense Technology - December 2021 - 31

Tech Briefs
(a)
(b)
Front view and (b) elevation of cold sprayed samples set in holder prior to salt fog chamber testing.
high degree of controllability of thickness,
including nanometric dimensions.
It is understood that PVD and CVD have
lower application rates when compared
to other surface modification technologies
and generally do not provide coatings
as thick as some other treatments,
like thermal spraying a large area with
coatings over 100 µm. While both coating
methods can be easily performed in a
lab environment with the right equipment,
they are not suitable for performing
in-situ on a ship. Fully enclosing an
installed part is usually not realistic and
accessing tight spaces make either PVD or
CVD less than an ideal choice for coating
or repairing a component.
Plasma spray coatings use molten
metal particles to impact a substrate,
which then cool and solidify rapidly.
High temperatures result in microstructure
phase changes and subsequent
residual stresses once the coatings cool
down. These " quenching stresses " can
result in tens of MPa residual stresses for
ceramic reinforcements that can result
in extensive splat cracking. High residual
stresses may affect the margin to
yield stress that some other coating
methods may not inherently produce
because they do not reach the high temperatures
resulting from plasma spray.
In addition, plasma spray processes require
temperatures above 2000K for the
particles in-flight. On a ship where
there is limited electrical power, this
can unduly burden the turbine generators
from the high-energy draw.
Cold spray is a low energy method
that could be employed on ships to repair
failed or degraded components and
minimize the number of material deficiencies
onboard the ship. When lead
times for replacement parts are excessive
(>12 months), a cold sprayed coating
of the same or similar material
could be applied to reduce downtime
and repair costs. Additionally, cold
spray can also be used to provide protective
coatings to prevent material deterioration
from corrosion or mechanical
wear. To support the use of this
technology aboard ships, further research
must be conducted to determine
the mechanical properties and corrosion
resistance of various applicationcompatible
cold spray coatings.
Cold spray can be performed under a
range of pressures, with high pressure
normally resulting in a coating with
better overall adhesion and a harder
coating. Additionally, multiple reinforcements
may have a synergistic effect,
enhancing the properties of the
base coating more than any one reinforcement
alone. Starting with a highpressure
process would be a proof of
concept for dual particle reinforcement,
and future testing could closer simulate
ship limitations and capabilities as a
feasible possibility to repairing and
manufacturing components.
This work was done by Travis Norrell for
the Naval Postgraduate School. For more
information, download the Technical
Support Package (free white paper) at
www.aerodefensetech.com/tsp under
the Manufacturing & Materials category.
NPS-0021
Novel Whisker Mitigating Composite Conformal Coat
Assessment
European Union RoHS compliant commercial lead-free electronics used in DoD electronics have an
increased failure risk due to tin whisker short circuits unless properly mitigated by design.
Defense Strategic Environmental Research and Development Program, Alexandria, Virginia
A
n electronic assembly is created by
integrating thousands of parts from
multiple suppliers utilizing a host of circuit
card manufacturing processes. With
the lead elimination from electronics resulting
from the European Union
Reduction of Hazardous Substances
(RoHS) legislation, many of the heritage
aerospace and defense commercial off
the shelf (COTS) solder materials with
tin-lead have become obsolete. Most
notably there has been an increasing
cost and schedule pressure to use commercially
available pure tin part finishes
and lead-free solders in aerospace and
defense electronic systems.
Aerospace & Defense Technology, December 2021 www.aerodefensetech.com
Intro
Cov
ToC
+
-
A
µ
Since these materials have an increased
tendency to grow tin whiskers
that can cause short circuit failures and
decrease reliability, original equipment
manufacturers are relying on multiple
mitigation methods to manage the tin
whisker risk. One of the main mitigations
is the use of conformal coating to
31
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http://www.aerodefensetech.com/tsp http://www.aerodefensetech.com http://info.hotims.com/79420-922

Aerospace & Defense Technology - December 2021

Table of Contents for the Digital Edition of Aerospace & Defense Technology - December 2021

Aerospace & Defense Technology - December 2021 - Intro
Aerospace & Defense Technology - December 2021 - Sponsor
Aerospace & Defense Technology - December 2021 - Cov1
Aerospace & Defense Technology - December 2021 - Cov2
Aerospace & Defense Technology - December 2021 - 1
Aerospace & Defense Technology - December 2021 - 2
Aerospace & Defense Technology - December 2021 - 3
Aerospace & Defense Technology - December 2021 - 4
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Aerospace & Defense Technology - December 2021 - Cov3
Aerospace & Defense Technology - December 2021 - Cov4
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