v1 Jan 2014

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.

by Lucideon 2

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.