The Growth of Ceramics in Aerospace and Defence
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Transcript of The Growth of Ceramics in Aerospace and Defence
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THE GROWTH OF CERAMICS IN
AEROSPACE AND DEFENCE
John Cotton
This work by Lucideon is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike
4.0 International License.
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INTRODUCTION
Over the last 20 years the demands of the
Aerospace and Defence sectors on materials
have consistently focussed on low density
(leading to lightweight components), high
specific strength and/or stiffness (maximising the
performance of the lightweight materials), and
high hardness (for wear resistance and ballistic
protection). Reducing the weight of aerospace
components has obvious benefits in terms of
increasing the effectiveness of the fuel burned,
either in increasing the range or allowing greater
payload to be carried for the same amount of
fuel. In defence applications, a weight reduction
of personal protection (armour/helmet etc)
reduces the load on the individual soldier,
allowing him to carry more munitions making him
more effective, and increasing his agility and
manoverability. Similarly, military vehicles benefit
from reduced weight, making them more easily
transported (airlifted) into the theatre of
operations.
However, these weight reductions must not be
achieved at the expense of performance - hence
the sector’s drive for new, lightweight, high
performance materials.
In addition to light weighting, there are also
specific application areas where a material’s
extreme properties allow it to outperform the
competition to yield benefits in terms of fuel
burn, emissions, payload and survivability.
On the face of it, ceramic materials, characterised
as they are by low toughness and by brittle, often
catastrophic failure, should have little attraction
for the aerospace and defence sectors, which
demand the ultimate in performance and
reliability. They are however generally low density
materials offering weight benefits over
competing metallic materials. Moreover, ceramics
have high specific strength and stiffness and
there are several applications in these sectors
where the ‘ceramic option’ has become the norm
rather than the exception. Examples include:
Personnel and vehicle armour where the material
often competes with economically more
attractive alternatives but where its improved
performance wins out; ultra high temperature
ceramics for applications where no alternative
materials exist; and thermal barrier coatings for
applications where the temperature of operation
exceeds the capability of metallic competitors.
HIGH IMPACT CERAMICS
The history of armour development has been one
of continual change with the armour system
development being driven by changes in the level
and complexity of the threat and enabled by the
availability of improved materials and technology.
The demise of plate armour with the advent of
gunpowder and development of the musket is
but one example. Today the threats which
armour systems face are severe. A modern high
velocity anti tank round will strike its target with
the same kinetic energy as two Range Rovers
travelling at 75 mph – except in this case the
impact is concentrated into an area of a few sq
cm.
The realisation that a relatively weak and brittle
ceramic material could provide ballistic
protection spread after WWII though there are
several ancient examples where minerals and
rocks have been used for body armour. Early
research and development work in UK, Europe,
and particularly in USA, culminated in the first
commercial ceramic armour system made by
Coors Ceramics and used with considerable
success in Vietnam. The system however was
heavy with front and back plates weighing ~7kg.
In addition, the plates were bulky and restricted
the soldier’s movement and agility.
Overcoming these drawbacks of weight and bulk,
coupled with the proliferation of higher energy
armour-piercing munitions, has been the driving
force for development of ceramic armour
systems ever since.
HOW CERAMIC ARMOUR WORKS
Ceramic armour systems are a multilayer
structure, each layer of which contributes to the
overall performance of the armour. Typical
ceramic armour for personnel may be
constructed as per Fig. 1.
1. The front or ‘strike’ face of the ceramic is
protected from day to day damage by a soft
but resilient, usually fabric, coating which is
also designed to prevent fragments of
shattered projectile from being ejected from
the armour system during impact.
2. The ceramic tile which forms the heart of the
system and acts to:
- blunt the projectile thereby spreading the
area of impact and reducing the localised
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stress - to do this the ceramic needs to be
harder than the material used in the
projectile.
- deflect and rotate the projectile to
reduce its effective length and further
spread the impact area – this is often
achieved by designing the shape of the tile
surface.
- fracture the projectile such that
the trajectory of the individual fragments
are more easily deflected and the kinetic
energy of the individual projectile particles
is reduced - to do this the ceramic must
survive the impact longer than the
projectile, i.e. there is a delay before the
ceramic fractures during which time the
projectile is damaged – this short period is
often called the dwell time.
- ultimately the ceramic tile fractures in a
controlled manner to develop a 'pool' of
ceramic debris which then acts to abrade
the projectiles as they pass into the tile.
Front
anti-spall
layer
Ceramic
plate Interlayer
Rear
retaining
layer
Adhesive joints
Fabric
encapsulation
Figure 1. Schematic representation of the
internal structure of a ceramic armour
component
3. An interlayer which controls the acoustic
impedance of the interface with the ceramic
tile thereby inhibiting or delaying the reflected
shock wave into the ceramic. This has the
effect of improving the performance of the
ceramic by increasing the dwell time (the
period of time before ceramic fracture
occurs). A number of layers may be used to
achieve this effect. The interlayer region may
also be relatively ductile spreading the load
area onto the next layer.
4. An energy absorbing retaining layer
which is intended to capture the remaining
parts of the projectile and prevent
penetration. Usually this layer is composed of
a strong fibre reinforced composite which can
absorb the remaining projectile energy by
translating it into damage in the perpendicular
plane.
Figure 2. Ballistic impact on ceramic armour plate
Each layer is adhesively bonded to the next.
The whole multilayer structure may also be
encased in a fabric envelope to protect the
system from damage in handling and to help
cushion the wearer in use.
To work effectively each of these individual parts
must perform and in doing so influence and
support the operation of its neighbour.
Over the years several ceramic materials have
been assessed for armour systems with the most
promising, alumina, boron carbide, silicon carbide,
going on to be used in commercial systems.
Throughout this development period the drivers
for the technology have been stopping power,
multi-hit capability, weight, and effective volume.
Stopping power has developed as the kinetic
energy and hardness of armour piercing
projectiles have increased; multi-hit capability has
constrained armour design and dimensions,
whilst weight and effective volume have driven
the search into low density, high hardness
ceramic materials.
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Currently the drive is to reduce the burden on the
fighting soldier by reducing the all up weight of
the armour system including the hard armour
plates, the soft armour protection in the flak
jacket and the helmet. Issues such as temperature
management, adding functionality to the armour
and armour design to improve body coverage
without restricting movement and agility are
important – but the main priority remains
reduction of the system weight.
The armour includes a new form of ceramic plate
that can withstand more bullet strikes than
current plates. It also includes bicep, leg and rib
protectors.
Weight and bulk are also major considerations for
vehicle armour.
Fighting vehicles need to be airlifted into the
theatre of operations so must fit into and be
capable of being lifted by airborne transport. On
the ground the vehicles are required to perform
at higher speeds and with greater manoverability
without compromise in protection and fighting
capability. In addition they must not increase the
burden on fuel supply as this represents a
strategical aspect in any operations.
As well as ballistic threats, vehicles are also the
target for thermal (e.g. shaped charge) weapons
and, increasingly, to blast from mines and IEDs.
In general, different armours and defensive
strategies are needed to cope with the different
types of threat. Outboard ceramic armour can be
effective against ‘long rod’ penetrators such as
APFSDS rounds whereas pre-detonation systems
and electric armour are designed to defeat hand
held shaped charge weapons.
Maintenance and in theatre repair of damaged
armour is essential if fighting vehicles are to
remain in front line service. Modular armour
systems which can be replaced in the field
together with in-situ repair technologies which
have been developed from rapid manufacturing
processes are intended to improve the field life of
fighting vehicles and hence minimise replacement
and logistics costs.
Application Demands Solutions Trends
Personnel protection
Ballistic performance
Multihit capability
Reduced system weight
Temperature control
Flexibility
Reduced bulk
Increased functionality
Ceramic (alumina, silicon carbide, boron carbide, CMC) hard plate backed by high strength woven composite
Lower density materials
Soft armour systems
Incorporation of HUMS and comms systems
Lighter weight systems
Improved ballistic performance allowing reduced thickness
Improved ergonomics and fit of flak jacket (better weight distribution)
SMART materials
Direct write and printed electronics
Vehicle protection
Lightweight armour systems
Protection against multiple threats – ballistic (long rod), thermal (shaped charge)
Multiple armour systems
Vehicle and armour design
Armour placement to meet likely threat
Reactive armour
New high performance alloys
Retrofit systems
In field repair of damaged armour
Modular designs
New armour concepts (e.g. electric armour)
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ULTRA HIGH TEMPERATURE CERAMICS
Ceramic materials have traditionally been known
for their refractoriness and high melting points
and it is this ability to withstand the extremes of
temperature which make them uniquely capable
of fulfilling some of the most demanding
requirements of the Aerospace and Defence
sectors. Carbides and borides of zirconium,
hafnium, and tantalum have some of the highest
melting points available and hence are candidates
for airframe and engine components for missiles
and high performance jets where surface
temperatures in excess of 2000°C may be
encountered. Typical applications for UHTCs are
rocket nozzles, nosecones and leading edges of
wings and stabilisers on hypersonic missiles,
reverse thrust petals and thrust diverters all
demanding extreme high temperature resistance,
chemical stability and abrasion resistance, ideally
in a lightweight component. Designs of high
velocity missiles are moving towards sharp,
leading edge components to minimise drag and
improve performance – high velocities however
equate to high leading edge temperatures and it
is here that UHTCs are expected to perform.
Hence the UHTC should be low density in itself,
or efficient enough to be effective as a thin
coating or stand alone component.
Melting points of UHTCs:
Material Melting Point (°C) Density (g/cm3)
B4C 2445 2.52
SiC 2730 3.2
TiB2 2970 4.52
NbB2 3050 6.97
Zr B2 3200 6.08
HfB2 3250 10.5
NbC 3500 7.6
ZrC 3530 12.2
HfC 3890 6.8
Many of these demanding applications are one shot and short lived and hence long term survival of the component is secondary to its short term performance – in these cases the oxidation resistance of the candidate carbides and borides, which is often the Achilles heel of these materials, ceases to be an issue. The same refractory behaviour which make UHTCs attractive also makes the materials difficult to manufacture with shape limited, very high temperature processing being required to densify them. Typically, this may involve hot pressing or hot isostatic pressing, reaction sintering, plasma or vapour deposition techniques.
The very high temperature environments in which
UHTCs are expected to perform also present a
significant problem in terms of their testing and
evaluation. Conventional furnaces are unable to
achieve the operation temperatures of these
materials so novel bespoke high temperature
testing solutions are required. These may include
induction furnaces, arc furnaces or focussed
energy sources, e.g. lasers. High temperature
bespoke testing solutions such as these require
careful design and expert selection of
construction and insulation materials. In many
cases, control of furnace atmosphere is also
required.
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Application Demands Solutions Trends
Airframe
- Nosecones
- Sharp leading edge components
Thrust diverters and reverse thrust petals
Engine components
- Rocket nozzles
Lightweight
Thermal stability
High melting point
High thermal conductivity
Oxidation resistance
- May be only short lived
Complex shapes
UHTCs (borides and carbides of Tantalum, Zirconium, Titanium
And Hafnium)
Composites of the above
Oxidation resistant (glassy) coatings
Multifunctional materials sustaining thermal and mechanical loads
Composite structures
Improved fracture toughness, thermal conductivity, oxidation resistance
Coatings
THERMAL BARRIERS TO INCREASE OPERATING TEMPERATURES FOR TURBINES
The efficiency of gas turbine engines is dictated
by the maximum temperature of operation,
however this is constrained by the properties of
the materials used in the highly stressed
components of the combustion and expansion
zone of the turbine - these start to lose strength
as the temperature approaches the melting point
of the alloys used. Insulating the components
from the hot gas stream using ceramic thermal
barrier coatings or TBCs has made it possible to
increase the operating temperature of turbines
and thereby win gains in performance, CO2
emissions reduction or fuel burn. The use of TBCs
can allow turbine temperature increases of up to
150°C without increasing metal temperatures and
this in turn equates to efficiency gains of about
10%.
Typically thermal barrier coatings used on turbine
components are composed of materials such as
yttria stabilised zirconia (YSZ) usually applied to
the component surface using coating techniques
such as plasma spraying either in air or vacuum,
flame spraying using a high velocity oxy-flame
(HVOF) system, or physical vapour deposition
(PVD). The coatings used are thin (<1mm), have a
thermal conductivity in the range of 1W/mK and,
by virtue of the deposition method, are
composed of complex structures. TBCs work
through a combination of low intrinsic thermal
conductivity coupled with radiation reflectance
effects. This is achieved with ceramic TBCs by
control of porosity and pore structure coupled
with manipulation of the microstructure to
achieve a functionally gradient coating.
Figure 3. Turbine blade with thermal barrier
coating on the aerofoil (NASA/courtesy of
nasaimages.org)
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Figure 4. Typical structure of yttria stabilised
zirconia TBC (Golosnoy et al J. Ther. Spray
Techn., Feb. 2009)
One of the major issues associated with the use
of ceramic TBCs on metal components is the
thermal expansion mismatch between the
coating and the metal surface which creates
interfacial stresses during thermal cycling and if
not controlled can rapidly lead to de-lamination
and spalling of the coating. The expansion
mismatch is overcome by the use of an interlayer
or bond-coat which mitigates these stresses
through its ductility or by acting as a functionally
gradient layer. The use of bond coats however
can have consequences for the long-term
performance of the TBC.
Application Demands Solutions Trends
TBCs
On turbine blades, stators, blade tip clearance rings, and combustion chambers.
On exhaust manifold and turbocharger components for high performance automotive engines
Thermal insulation
Resistance to thermal cycling and thermal shock
Corrosion resistance
Long life (>16000 hours at operating temperature)
Application to complex shaped components
Even higher operating temperatures
Thermally sprayed and PVD coatings of YSZ
Dopants to modify coating structure
Corrosion resistant bond coats suited to fuels from different sources
Laser machining of air bleed holes for film cooling
Non-line of sight application methods
Nanomaterials
Functionally gradient structures
Novel TBC structures arising from new application methods
Oxidation and corrosion of the bond coat during
operation causes the growth of scale underneath
the TBC which if it progresses causes the layer to
fall off exposing the metal surface to the
increased gas temperatures ultimately leading to
component failure. This problem is enhanced by
the fact that the TBC is porous (up to ~15%) and
contains microcracks.
The structure of TBCs can also change with use.
Sintering and grain growth can occur in service,
and this changes both the mechanical and
thermal properties of the coating – usually
resulting in reduced performance. Corrosion of
coatings and reaction with constituents of the
fuel may also occur. In these instances the
forensic capabilities of the scanning electron
microscope coupled with more sophisticated
surface analysis techniques such as XPS (X-ray
Photoelectron Spectroscopy), or SIMS
(Secondary Ion Mass Spectroscopy) are
invaluable in identifying the cause and indicating
the solution to the corrosion problem.
CONCLUSION
The Aerospace, and particularly the Defence
sectors, have driven and continue to drive the
development of advanced materials, demanding
increased performance to meet increasingly
difficult applications. Ceramic materials have a
vital role to play in meeting these demands either
as part of sophisticated multi-material systems or
where their unique capabilities make them the
only candidates for the job.
by Lucideon
ABOUT LUCIDEON
Lucideon is a leading international provider of
materials development, testing and assurance.
Through its offices and laboratories in the UK, US
and the Far East, Lucideon provides materials
and assurance expertise to clients in a wide range
of sectors, including healthcare, construction,
ceramics and power engineering.
The company aims to improve the competitive
advantage and profitability of its clients by
providing them with the expertise, accurate
results and objective, innovative thinking that
they need to optimise their materials, products,
processes, systems and businesses.
ABOUT THE AUTHOR
JOHN COTTON - BUSINESS DEVELOPMENT MANAGER, AEROSPACE & DEFENCE
John is a Chartered Engineer who holds a Degree
in Applied Physics and is a Fellow of the Institute
of Mining Minerals and Materials (IOM3). John
serves on the Ceramic Science Committee of
IOM3 and is a member of Peer Review College for
the Engineering and Physical Sciences Research
Council (EPSRC). John also acts as a Technology
Expert for Materials KTN.
With over forty years of experience in advanced
materials – specialising in refractories and
technical ceramics at Lucideon, John is an expert
in all aspects of materials R&D and problem-
solving. From identifying and solving production
issues to advising on application design and
performance, John has worked with
manufacturers, systems integrators and end-
users to make a real difference to their
businesses.
John has contributed to several materials
textbooks, composed a large number of papers
and is a frequent presenter at conferences
worldwide.
ADVANCED MATERIALS
Throughout his term at Lucideon John has
worked with a range of advanced materials
including both monolithic and composites for
applications such as fuel cells, lightweight
materials for airframe and sporting goods, as well
as sensors, actuators, and high temperature and
wear resistant components.
AEROSPACE AND DEFENCE
John's experience in aerospace and defence
materials incorporates ceramic armour,
lightweight and high temperature composites
and coatings for thermal and corrosion
management.
CERAMICS
John has been involved in a range of ceramic
projects including the development of sinterable
silicon nitride ceramics, evaluation of ceramic
materials as electrochemical gas sensors, design
and manufacture of ceramics for engine
components, and design of dies and
development of extrusion technology for the
production of thin ceramic and metal powder
tapes.