Karl BerrothFCT Ingenieurkeramik GmbH96528 Rauenstein, Germany

E-mail: k.berroth@fct-keramik.dewww.fct-keramik.de

Silicon Nitride Ceramics for StructuralComponents in Avionics and Space

Tab. 1 Properties of ceramics used for lightweight structures in avionics and spaceinstruments

Process Engineering

ceramics in combination with fabri-cation technologies, allowing alsorather large, lightweight and highlycomplex designed structures withhigh precision in design and smoothsurface roughness show potential, tooutperform the actually mainly usedsilicon carbide grades, and – more-

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Silicon nitride and siliconcarbide at FCTFCT Ingenieurkeramik GmbH is devel-oping and producing silicon nitrideand silicon carbide materials sinceabout 25 years. Different grades arein production, but also permanentR+D activities are in place in order, tooptimize specific properties andgenerate new grades with improvedperformance and increased compo-nent reliability. This is not only truefor the materials side, but also for ourprocessing procedures for the pro-duction of ceramic components.Further on, this paper is focused tosilicon nitride ceramics.Now a newly developed family of sil-icon nitride ceramics is available ineven large and complex shapedcomponents. With a CTE of down to<1,3 x 10-6/K, thermal conductivityof up to 85 W/m·K at 20 °C, 4-pointbending strength of up to 1100 MPaand the ability to get polished tooptical grade mirror surface withoutcoating. These materials have opened newfields for further application in avion-ics and space. Such newly developed

IntroductionNowadays, lightweight, stiff andstrong materials with a low CTE arehighly demanded for structural andoptical applications in avionics andspace. Due to the excellent combination ofproperties, dense sintered (SSN), gaspressure sintered (GPSN) hotpressed (HPSN) or hot isostaticpressed (HIPSN) silicon nitride andsilicon carbide ceramics and com-posites are candidate materials tofulfill design and optical engineersrequirements. Truss structures andspiders for telescopes, instrumentbase plates as well as housings fortelescopes and gyros made of siliconcarbide and silicon nitride have beenapproved and got into serial produc-tion.

State of the artIn this paper, state of the art, newlydeveloped grades as well as alreadyvalidated and successfully tested andused materials will be presented.Properties as well as behavior inambient and changing atmospheresand temperatures are explained. Butthese materials are not only suitedfor components in flying equipment,but also for the secure and contami-nation free processing of high purityproducts, for mechanical and chem-ical engineering as well as for metalforming, foundry industry and forequipment, used for production andtesting of electronic components likewafers. Processing routes, dimen-sions, shapes, tolerances, testingparameters and routines for inspec-tion as well as materials and compo-nent properties will be addressed. A lot of R+D work was done withthese non oxide ceramics during thepast three decades. They so gotindustrialized and are producedaround the world. Components likeballs for roller bearings, seal rings formechanical seals, linings of equip-ment for mechanical and chemicalengineering and electronics as wellas burner nozzles, heat exchangers,immersion heater sheaths advancedkiln furniture, wear parts and – moreand more upcoming – also armorplates are widely used in many fieldsof industrial technology.

Material

Density Young CTE TC 4PBS Tough

ρ[g/cm3]

E [GPa]

α[· 10-6/K]

λ[W/m·K]

σ[MPa]

k1c[MPa·m½]

SiO2 ULE 2,21 91 0,03 1 32 2

Silicon nitride ceramics

Si3N4 FSNI 3,25 310 1,4 25 750 6,5

Si3N4 HTC 3,32 310 1,5 85 750 7,5

Si3N4 HTCHP 3,34 320 1,5 65 1050 7,5

Si3N4 HiPUR 3,21 330 1,3 25 1020 6,5

Si3N4 TiN 3,45 350 1,6 25 950 7,5

Silicon carbideceramics

SSiC FSC 3,14 430 2,2 145 510 4

CESIC®HB 2,95 350 2,3 145 320 3,7

MF 2,65 249 2,1 121 149 4,6

SiSiC 90 3,1 380 2,5 140 290 3,5

SiSiC 80 3,03 360 2,4 150 290 3,5

SiC LPS 3,25 410 2,6 90 500 5

SiCf/Sic CVI 2,6 200 2,5 18 500 15

Various others

B4C 2,5 410 4,8 28 500 3,5

Al2O3 S Cryst 4 400 6 20 450 4

Al2O3 SiCw 3,75 390 5 40 500 8,5

Silicon Nitride to Silicon Nitride

Braze Area 78,5 mm2

Tensile Load [N]

Failure Stress[MPa]

Glue 1 3620 46

Glue 2 4211 54

Braze 1 6525 83

Braze 2 1177 15

Braze 3 5828 74

Braze 4 5360 68

Tab. 2 Strength of glued and activebrazed silicon nitride

materials which are used in avionicsand space structures.

Connecting proceduresBesides the materials and processingproperties, also the need of connect-ing ceramic components with eachother or with other materials likemetal, composites or plastics is ofmajor importance, because not allshapes and dimensions could berealized in monolithic ceramics.Whenever production facilities cometo a border or if the geometric struc-

ture requires to separate it into twopieces for being able to realize therequested geometry, a connectingprocedure of ceramic components isnecessary to have. FCT therefore alsohas investigated means of connect-ing• ceramic with ceramic components• ceramic with metal components• ceramic with plastic componentswith a focus on ceramics with ceram-ics. Different approaches are to bedetailed:• gluing• brazing• shrink fit• clamping • screwing.For lightweight structures, mostimportant is gluing and brazingceramic to ceramic components. Thisfacilitates to produce highly complexand large structures, using smallerindividual parts and by means ofbrazing, also repair work gets possi-ble for structures with defects likechips, cracks or wharpage. Tab. 2 shows results with two gluesand 5 different active brazes. Twocylindrical samples were glued/brazed together at two ground facesand then tested under tensile loaduntil fracture. For organic glues, theproblem of embrittlement at cryo-genic temperatures has to be solvedby specific glues. For active brazes,the temperature range and morelikely the activator elements have tobe adjusted in order to reach an acti-vation and binding at lowest possi-ble temperature.

Silicon nitride with highthermal conductivityIn order to overcome the lack of highthermal conductivity, which for stan-dard grades of silicon nitride is poor,compared to silicon carbide, a R+Dproject was started at FCT in coop-eration with customers to developgrades with increased thermal con-ductivity, keeping the high strengthand fracture toughness as well as thevery low CTE. As target, a value of atleast 70 W/m·K was set at a 4-pointbending strength of at least700 MPa. The production process-ing has to be feasible with therequest of the ability to make tubu-lar structures with more than 1 m inlength and also complex 3-D struc-tures for beam connector elements.State of the art now is shown inTab. 3. Several grades have beenachieved, densified by either uniaxi-al hot pressing or gas pressure sin-tering with thermal conductivities

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over with silicon nitride –, add a newmaterial with very promising proper-ties and application options andalternatives to both, metals, com-posites and ceramics in aviation andspace instruments.Components made of these materi-als are available on a commercialbase with diameters up to 500 mmand length up to 1300 mm. Thematerial thickness ranges from0,125 mm up to 70 mm. Mayorproperties of those silicon nitrideand silicon carbide grades are pre-sented in Tab. 1 in comparison with

Process Engineering Tab. 3 Thermal conductivity of various silicon nitride grades and SSiC

Fig. 1 Housing structure (l.) and final assembly of an avionic reconnaissance telescopecamera (r.)

Fig. 2 Spider base for small satellite made of Si3N4

from 45 – 85 W/m·K. The lower val-ues reached by hot pressing showvery high strength values up to1100 MPa. The gas pressure sinteredgrades reach up to 85 W/m·K at astrength level of 750 MPa and stillhigh fracture toughness. CTE is stillat a very low value, much lower thanthe ones for silicon carbide. Somegrades are already available as com-ponents, other are still only in labscale sample size.With the company´s grades, –already produced since many years –and the newly developed, optimizedand partially qualified for avionicsand space, a new material family of silicon nitride grades is avail-able on a commercial base for rea-sonably large, highly complex struc-tural parts. They offer significantlyincreased mechanical performancesin regard to strength and fracturetoughness compared to silicon car-bide and glass ceramic materials,which may also help to get muchhigher reliability in components.Another parameter is the CTE of sili-con nitride grades which is alwaysmuch lower than all the ones of sili-con carbide grades and all otherhigh strength and tough ceramics(Tab. 1).

Avionics and space experienceSince about years we started to pro-duce and supply ultra light weighthighly stiff structures with very lowCTE silicon nitride ceramics for air-borne reconnaissance telescopecamera systems (Fig. 1).

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Process Engineering

Fig. 3 Principle of truss structure Fig. 4 Beam connectors and beam ends

Fig. 5 Testing set up for silicon nitridebeams and connected beams for satellitetruss structure

Since 2006 FCT is in progress toqualify silicon nitride also for spacetelescopes (Fig. 2). Some facts aredescribed in cited literature. Qualifi-cation of scale 1 preflight compo-nents was reached in 2010/2011and since then FCT is qualified sup-plier for space structures. Since 2011 FCT is producing andsupplying flight model componentsfor the first space telescopes havinga silicon nitride truss structure(Fig. 3). In 2012 FCT has successful-ly completed this by delivery of allcomponents for this rather largestructure of an earth observationsatellite. The truss structure is madeof specific lightweight beams andbeam connectors (Fig. 4–5). Theintegration into the space instru-ment is ongoing. Also weight savingis the replacement of steel balls inball joints by silicon nitride (Fig. 6). Italso prevents corrosion. Mainly in high temperature applica-tions and in corrosive media, the useof bolts and nuts wit threads havehelped to facilitate highly effectiveand reliable installations and fixa-tions (Fig. 7). Here FCT replacessteel, titanium or other metals andpolymers. Also for high temperatureengineering we do quite a bit oftubes, beams and profiles with sili-con nitride and silicon carbide whichare highly stiff and lightweight –with low thermal mass –, are thermalshock resistant and can be used upto temperatures of 1300 °C for sili-con nitride and 2000 °C for siliconcarbide (Fig. 8).In some applications FCT replacesNi-based alloys like HASTELLOY e.g.

Fig. 6 Silicon nitride spring rings (l.) andball-and-socket joint (r.)

Fig. 7 Nuts and bolts with thread made of silicon nitride and silicon carbide

in high temperature applicationsrunning up to 1300 °C with verystrong thermal, chemical corrosionand creep requirements. Another application is in flying elec-tronics specifically for highlymechanically and thermally stressedsubstrates. Here, mainly disc andplate like shapes are required withdiameters up to 420 mm and thick-ness ranging from 0,125 up to2,0 mm. These discs and plates typ-ically have to be ground and lappedor even must be polished.With hot pressed grades, direct pol-ishing to optical grade surface finish

some of the up to date used materi-als. One mayor constraint is – up tonow – however the limited size ofcomponents, which should be typi-cally less than 1,5 m in length andless than 700 mm in diameter. Andadditionally, it was never validatedand used for space instruments.But finally, one grade of siliconnitride is now validated and qualifiedfor the truss structure of a satellitetelescope and so other componentsfor flight instruments can be pro-duced in the future. The truss com-ponents have passed the requiredproof tests and their integration intothe satellite is in progress.On the other side, avionic reconnais-sance cameras with housing struc-tures – made of a second grade ofsilicon nitride – are meanwhile suc-

cessfully flying on jet airplanes sinceabout at least 5 years now withoutfailure. Components of silicon nitride andsilicon carbide according customerdesign are available at FCT Inge-nieurkeramik GmbH. For furtherinformation, please contact us.

AcknowledgementThanks to ThalesAlenia Space,France, and CNES for financial sup-port in the development of “Siliconnitride with high thermal conductiv-ity”.

References[1] Berroth, K.; Devilliers, G.; Luichtel, G.:

Silicon nitride for lightweight stiffstructures for optical instruments, Pro-ceedings of SPIE conference onOPTICS + PHOTONICS conference,Paper number 7425–26

[2] Berroth, K.: Silicon Nitride and SiliconCarbide Ceramics for structural Com-ponents in Avionics and Space. Pro-ceedings of 12th European Confer-ence on spacecraft Structures, Materi-als & Environmental Testing, March2012, Nordwjik, Paper s3d2

[3] Devilliers, Ch., Lasic, T.; Boban, D.;Cornillon, L.; Tanzilla, D.; Autzaid, S.;Berroth, K.: Si3N4 ceramic application,development and qualification resultsfor truss structure of large space tele-scope. Proceedings of ICSO 2012,Internat. Conference on Space Optics2012

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Process Engineering

without any coating can beachieved. This is caused by the fullydensified, fine grained, homogenousand void free microstructure of thesilicon nitride. Silicon carbide how-ever always shows micro poresand/or carbon particles as imperfec-tions and therefore requires coatingswhen it is used for mirrors.

ConclusionsSilicon carbide ceramics in differentgrades are already widely used inavionics and space. Silicon nitride isnew for this application. Although itshows some properties like mechan-ical strength and fracture toughnessas well as a very low CTE – muchlower than for silicon carbide grades– it has potential, to outperform

Fig. 8 Tubes and profiles of SSiC and Si3N4