1.c. ceramics - materials, joining and applications

7/23/2019 1.c. Ceramics - Materials, Joining and Applications

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Ceramics - materials, joining and

applications

Ceramics are an incredibly diverse family of materials whose members span traditionalceramics (such as pottery and refractories) to the modern day engineering ceramics (such

as alumina and silicon nitride) found in electronic devices, aerospace components andcutting tools.

Whilst the most extravagant claims of the 19!s in favour of advanced ceramic materials

(such as the all ceramic engine) have largely proved inaccurate, it is true to say thatceramics have established themselves as "ey engineering materials.

When used in con#unction with other materials, usually metals, they provide added

functionality to components thereby improving application performance, once the

appropriate #oint design and technology have been identified.

Ceramic materials

Ceramics exhibit very strong ionic and$or covalent bonding (stronger than the metallic

bond) and this confers the properties commonly associated with ceramics% high hardness,

high compressive strength, low thermal and electrical conductivity and chemicalinertness.

&his strong bonding also accounts for the less attractive properties of ceramics, such as

low ductility and low tensile strength. &he wider range of properties, however, is not

widely appreciated. 'or example, whilst ceramics are perceived as electrical and thermalinsulators, ceramic oxides (initially based on *aCu+) are the basis for high

temperature superconductivity. iamond, beryllia and silicon carbide have a higher

thermal conductivity than aluminium or copper.

Control of the microstructure can overcome inherent stiffness to allow the production ofceramic springs, and ceramic composites have been produced with a fracture toughness

about half that of steel.

&he main compositional classes of engineering ceramics are the oxides, nitrides and

carbides. &he &able gives the general properties of the most used ceramics.

Table 1 Properties of ceramics

Ceramic Melting

point

(°C)

Density

(g/cm)

!trengt"

(MPa)

Coefficient of

t"ermal

e#pansion

T"ermal

cond$cti%ity

(&/m')

lastic

Mod$l$s

(*Pa)



(# 1+-/°C)

*e+ -/! /.1 -0 2.0 -1! 0!!

3l-+/ -!! 0.! 0 .! 0! /!

4r+- -2!! . 12 1!. 19 10!

3l5 19!! /./ 001 0.0 1! /-!

6i/ 50 19!! /.- -1! /.! 12 12

*0C -/! -. /! 0./ - 0!

6iC -2!! /.- 10! 0./ ! -1!

WC -/22 1. !! .- 2!!

iamond /!!! /. 1!! !. -!!! !!

#ides

3luminium oxide (3l-+/) and 7irconia (4r+-) are the most commonly used engineeringgrade oxide ceramics, with alumina being the most used ceramic by far in terms of both

tonnage and value.

.itrides

6ilicon nitride (6i/ 50), and aluminium nitride (3l5) are the main advanced engineeringceramics in this category. &here is a wide range of grades and types of these materials,

particularly of silicon nitride with each grade having specific properties

Carbides

6ilicon carbide (6iC) is widely used for its high thermal conductivity, corrosion

resistance and hardness, although as an engineering ceramic its toughness is lower thanthat of some silicon nitride grades. *oron carbide (*0C) is the third hardest industrial

material (after diamond and cubic boron nitride) and is used for components needing very

high wear performance.

Ceramic-based composites

Ceramics are used as the reinforcement of composite systems such as 8: (glass

reinforced plastics) and metal matrix composites such as alumina reinforced aluminium(3l$3l-+/). 3dvanced ceramic materials are also used as the matrix materials in

composites. Currently the most widely available materials are based on 6iC and carbon.



oining

&here are many possible techni;ues for #oining ceramics to themselves and to dissimilar

materials. &hese technologies range from mechanical fixturing to direct bonding. Fig.1 gives an overview of these methods.

0ig'1' n o%er%ie2 of

processes for

joining

ceramics

&he selection of one of these techni;ues to manufacture a particular component will

depend on a number of factors including%

• desired component function eg strength, electrical insulation or wear resistance

• materials to be #oined

• operational temperature

• applied stress• re;uired level of #oint hermeticity

• component design

• cost

Whilst all these considerations must be ta"en into account, generally the two important

factors are the similarity of the materials to be #oined and the re;uired temperature

capability. Fig. 2 gives the temperature capability of a number of #oining media.



0ig'3' Temperat$re

capability of

a n$mber of

joining media

When #oining ceramics to metals it is necessary to create an interface between the

materials. <n general the interface must accommodate the following%

• the difference in coefficient of thermal expansion (C&=)

• bond type ie ionic$covalent for ceramics ranging to the metallic bond

• crystallographic lattice mismatch between the ceramic and metal

pplications

Compared to metals and plastics, ceramics are hard, noncombustible and inert. &hus theycan be used in high temperature, corrosive and tribological applications. &hese

applications rely on combinations of properties that are uni;ue to industrial ceramics andwhich include%

• retention of properties at high temperature

• low coefficient of friction (particularly at high loads and low levels of lubrication)

• low coefficient of expansion

• corrosion resistance

• thermal insulation

• electrical insulation

• low density

=ngineering ceramics are used to fabricate components for applications in manyindustrial sectors, including ceramic substrates for electronic devices ( Fig. 3),

turbocharger rotors ( Fig. 4), and tappet heads for use in automotive engines. +ther

examples of where advanced ceramics are used include oilfree bearings in food processing e;uipment, aerospace turbine blades, nuclear fuel rods, lightweight armour,

cutting tools, abrasives, thermal barriers and furnace$"iln furniture.



0ig'' Ceramic s$bstrates for electronic

de%ices

0ig'4' Ceramic t$rboc"arger rotor

assembly made from silicon

nitride

Courtesy of NGK/NTK Spark

Plug Co

!$mmary

When selecting a material for use in a specific component the applicability and suitability

of the candidate materials need to be considered in detail. When a ceramic material is

being selected the fitnessforpurpose criteria that should be applied include%

• operational environment atmosphere, temperature, applied stress, fatigue,

exposure time• predictable excursions beyond the usual, including mechanical impact or rapid

heating$cooling

• design ceramic materials are relatively intolerant of abrupt changes in cross

section such as notches, holes and corners

• #oining the role of the #oint, its operational conditions and performance

re;uirements and the #oining techni;ues suitable for manufacture

• cost as with all materials selection and component design ;uestions, the cost and

availability of the raw materials and all necessary fabrication techni;ues must be

considered in the light of their suitability to provide a component with the

re;uired performance profile at a viable cost

'uture development is li"ely to come from improved processing and fabricationtechni;ues that will lower component costs or improve behaviour, an increasing demand

for higher performance materials necessitating the use of more ceramics. Whilst it is

difficult to predict new materials, improvements in existing ones can be readily foreseen.&he most significant area of development is li"ely to be in the ceramic matrix

composites.



Whilst existing composites based on 6iC will improve as porosity levels are reduced by

improved processing techni;ues, the development of high temperature oxidebased

composites is li"ely to provide a competitor material system with wider applicability inthe near future. <n the future we can expect to see a still greater contribution to industrial

growth and technological development from these materials.

1.c. ceramics - materials, joining and applications

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