Conference Review

Composite Materials -ICCM V_________ K.K. Chawla

University ofIllinois at Urbana-Champaign

Fig~re 1. Effect of strain rate on fracture appearance of GFRP specimens. (a) Quasi­~tatl~ test (X2.2). (b) Intermediate rate test (x2.4). Impact test: (c) pull-out of fiber layerIn WIP region (x2.0), (d) unbroken gauge section viewed by transmitted light showingmatrix cracks (x2.8), and (e) unbroken gauge section viewed by reflected light showinqdebonding around cracks (x2.6).

15307000

1100086000

Conventional CompositeMaterials Materials

Table l. Benefits of Use of CompositeMaterials in Helicopters

Total partsFasteners

b) Intermediate rate test [ x 2.4r

which areof necessity tied to designchanges. It would be desirable, forexample, to translate the benefits ofcomposites technology in helicoptermanufacture (see Table I) to the au­tomobile industry resulting in bet­ter performance and operatingefficiently at equal or lower cost .

a) Quasi-static test ( x 2.2)

composites work for you; and (c) useof computer aided design (CAD) andcomputer aided manufacturing(CAM). As an example of a success­ful conceptual change in design, Dr.Forney cited Grumman's X-29, for­ward swept wing airplane which hasreduced drag, requires a smaller en­gine and is thus more fuel efficient.The required stiffness in this planeis obtained by the use of advancedcomposite materials. Yet anotheraerospace example is the Starshipaircraft of Beech which supports anadjustable forward wing and is madeof carbon/epoxy composites. Dr.Forney then cited the need for con­ceptual changes in manufacturing

INTRODUCTION

The series International Confer­ence on Composite Materials (lCCM)was started in 1975. The 5th confer­ence was held in San Diego, Califor­nia, July 30-August 1, 1985. In1975, Dr. lain Le May, the editor ofJournal of Engineering Materialsand Technology, invited me to re­port on the 1975 ICCM for hisjournal. Since then, I have unwit­tingly become a sort of unofficialrapporteur of the various ICCM's.l ,2,3My objective is to continue this ef­fort by reporting on the develop­ments in this very active field basedon the papers presented at the ICCMV and informal discussions withcolleagues.

The ICCM V General Chairmanwas Dr. Bill Harrigan who provideda sneak preview of the conferencein the June 1985 issue of theJournal of Metals by summarizingthe contributions to one metal ma­trix composites session." The confer­ence's opening address , called theScala Lecture after the originator ofthe ICeM series, Pete Scala, wasdelivered by Dr. R.C. Forney, Execu­tive Vice-President, DuPont Com­pany. His talk was entitled "Ad­vanced Composites: The Technologi­cal Linchpin in the StructuralRevolution." We are entering a com­posite age, according to Dr. Forney.He attributed this to improvementsin materials that form composites(e.g. much stiffer and stronger fi­bers are available now plus therehas been a great improvement inthe toughness of resin matrices) andadvances in fabrication technology.Dr. Forney was less enthusiasticabout the progress made so far inconceptual change and a systems ap­proach with regard to the compositematerials. These would involveworking together of various conven­tional discipline teams, new indus­trial partnerships, and institutionalinteractions. He stressed the impor­tance of viewing the composites as"material systems substitutes" rath­er than simple replacement of con­ventional materials. The conceptualchange demanded of the engineersinvolves. (a) starting with a functionand designing a material and notvice-versa; (b) making anisotropy of

JOURNAL OF METALS· December 1985 25

Figure 2. Fracture surface of Nb-wire rein­forced Mo(Si, Geb

Finally, Dr. Forney mentionedsome important economic trendswherein composites can expect a sig­nificant growth potential. Forexample, according to Dr. Forney,there is a general trend of shiftingfrom large, mass production unitsto small, flexible units. Thus, wherein the past one had available a lim­ited selection, the future would becharacterized by variety and divers­ity. Short life items would give wayto long life goods, and energy con­suming items would tend toward en­ergy conserving items. These trends,in turn, would and are leading tointernational linkages (e.g , in ad­vanced fibers) and growth forcomposites. Last but not least, thereare second order benefits that shouldfollow from these trends. For exam­ple, if the usage of composite materi­als leads to a lighter vehicle, itshould follow that the cost of highwaybuilding and maintenance would godown. Use of composite materialsshould ease the demand on ores andminerals, including petroleum. Thisshould go hand in hand with lessenergy consumption and less envi­ronmental pollution. Dr. Forney esti­mated that the composites marketwill reach the mark of $23 billionin 1990.

The rest of the conference was inthe form of 26 parallel sessions. Itwould be convenient to describe thehighlights under the following head­ings corresponding to the matrixtype employed:• Polymer Matrix Composites (PMC)

(Figure 1)• Ceramic Matrix Composites (CMC)

(Figure 2)• Metal Matrix Composites (MMC)

(Figure 3)I shall try to describe the salientdevelopment features and the prob­lem areas in each of the abovefields .

POLYMER MATRIX COMPOSITES(PMC's)

Carbon and Kevlar fiber reinforcedpolymer matrix composites, besidesthe conventional glass fiber rein­forced composites, continue to be onthe forefront. Aerospace continues tobe the primary user industry ofPMC's and carbon fiber reinforcedpolymer matrix continues to be themajor type of composites in use . S-2glass fiber, stronger than convention­al E glass fiber and cheaper thanthe very high strength S-glass fiber,is finding applications in pressurevessels, aerospace components, andtactical and strategic weapons. Themain action in the PMC's, however,has been in developing tougher poly­mer matrix materials.

Commonly, thermosetting resins(epoxies and polyesters) are used aspolymer matrices. These are typical­ly characterized by a low toughnessand low. strain to failure, difficultprocessing, and small shelf-life. Alsobeing thermorigid, it is not possi­ble to recycle them. All these prob­lems are minimized with thermoplas­tic resins. Actually, the use of ther­moplastic matrices in composites isnot new . They have been used, almost

exclusively so far, in short fiber re­inforced composites using injectionmolding techniques. What is new isthe development of some new ther­moplastic resins for use with contin­uous fibers. Superior toughness ofcarbon fiber reinforced polyethere­therketone (PEEK) and K-Polymerhas been shown to be due to thesuperior toughness of these matrices.Additionally, these thermoplasticmatrices offer short molding cycle,infinite prepreg shelf life, capabilityof fusion bonding, improved repair­ability and recycling (scrap reuseetc.) . The disadvantages includehigher processing temperatures andsensitivity to solvents. PEEK, an ICIproduct, is an aromatic thermoplas­tic with a mix of crystalline andamorphous components. Typicalstrain to fracture is 25-30% forPEEK vs . 3-4% for epoxy. Amongother efforts at improving the poly­mer matrix toughness, one may citeK-polymers (DuPont) which are afamily of thermoplastic polyimidesand polyphenylene sulfide (PPS)which is an aromatic sulfide, semi­crystalline thermoplastic with ahigher end use temperature thanepoxies. Phillips Petroleum intro­duced continuous carbon fiber rein­forced PPS prepreg in 1984.

Dispersing an elastomeric or rub­bery phase in a resin (thermosettingor thermoplastic) can also lead toan improved toughness. Some newthermosetting bismaleimides (BMl's)with higher glass transition temper­ature (Tg) than the conventionalBMI's have been reported. The enduse temperature is high, in therange of 150-200°C. But, they re­main brittle. Some increase intoughness has been obtained by ad­dition of toughening agents.

Among the range of other aspectsof PMC's covered in the papers

Figure 3. Illustrates the microstructure of the fracture surface of the longitudinal specimen: (a) before heat treatment, x 3000; (b) after heattreatment at 540°C for 80 hr., x3000; and (c) at 580°C for 20 hr., x 3000.

a

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b c

JOURNAL OF METALS· December 1985

presented at ICCM V, one may cite:acoustic emission monitoring of dam­age accumulation during monotonictensile loading, delamination in car­bon fiber reinforced epoxy laminates,mechanically fastened joints, longterm degradation and life time pre­diction of aromatic polyamide fibers,development work involving applica­tions of carbon/epoxy composite ma­terials for primary structural compo­nents in commercial satellites, prob­lems regarding metallization (for sat­isfactory electrical performance) ofwaveguides made of carbon fiberPMC's, use of industrial robots inthe processing of PMC's, and, lastbut not least, environmentally in­duced damage in PMC's. It turnsout that fluctuating relative humidi­ty is a more serious problem than asteady state saturation. Environmen­tal damage also depends on the pasthistory of exposure to cyclingenvironment.

METAL MATRIX COMPOSITES(MMC'S)

In the field of MMC's, in additionto boron and carbon fibers, there isnow a wide variety of SiC and Al203reinforcements. ICI, under the tradename of "saffil", produces low cost($30/kg) short staple alumina fibers.The fiber is o-alumina with about 4%Si. The latter acts as a crystalgrowth inhibitor. This fiber has beensuccessfully incorporated into Al ma­trix by a variety of routes such aspowder metallurgy, stir casting,squeeze casting, etc. The great at­traction of any metal matrix compos­ite is, of course, the high tempera­ture capability. In this respect, saffilfibers reinforced Al composites arequite promising since they are ableto retain strength at high tempera­tures. Their room temperatureproperties, however, are not muchdifferent from those of the matrix.Then, there is continuous a-A1203 fi­ber of DuPont (trade name FP fiber).DuPont has made, for some timenow, composites of FP fibers in Aland Mg matrices by liquid metal in­filtration in vacuum and underpressure. It turns out that in thiscase 2-3% Li must be added to Alin order to obtain wetting. Mg andits alloys wet FP without anyproblem. With saffil fibers, usingsqueeze casting technique for exam­ple, one does not need wettingagents. Pressures of 20-30 MPa areemployed in the industrial practiceof squeeze casting. There is a vastunexplored area of detailed charac­terization of the interface in mostany MMC. Currently, there aboundsa lot of confusion with regard towettability, reaction bonding etc. In-

ternal stresses due to the thermalexpansion mismatch between thecomponents, effect of the processingroute, matrix alloy chemistry andheat treatments, interfacial reactionat high temperatures are some ofthe critical areas where some morefundamental research is required.One basic point that must be bornein mind is that in these composites,matrix should not. be regarded as asecondary component. The finalinsitu structure of matrix will be afunction of a number of variablesjust mentioned, and consequently,there is plenty of room for control­ling the composite properties. Inother words, matrix .is not just abinding medium to hold the fiberstogether. This realization is comingin the PMC's as well where, asmentioned above, some tougher ma­trices are being tried in place of con­ventional epoxies and polyesterswhich only served to hold the fiberstogether.

SiC is available in form of contin­uous fibers, short fibers, whiskers,and particles. There are two typesof continuous SiC fibers. First,Nicalon (Nippon Carbon Co.) fibers,in the form of multifilament yarn,are produced by the Yajima processof controlled pyrolysis of spun organ­ic precursors much like in the car­bon fiber manufacture. These arevery fine 00-20 j.Lm diameter) withas-produced strength of - 2 GPa anda Young's modulus or 200 GPa.Pure ~-SiC has a modulus of over400 GPa. The reason for thisdescripancy is that Nicalon fibersare not pure ~-SiC but have someSi02 and free carbon mixed withSiC. Second, AVCO monofilament fi­bers are produced by CVD. Theseare rather thick fibers (diameter 142urn) but are a purer variety of SiCand thus the properties in generalare superior than those of Nicalonfibers. A detailed comparison ofthese two types of SiC fibers hasrecently been made by DeCarlo.5

SiC in the form of short fibers,whiskers, and particles is being usedas a reinforcement of mainly AI· andits alloys. Again, a wide variety oftechniques involving powder metal­lurgy, casting, diffusion bonding,etc., is being employed to fabricatethese composites. The main advan­tage of particulate composites is thatconventional metalworking equip­ment can be used for secondaryworking. Since these irregular parti­cles have an aspect ratio of -3, thedirectionality of properties in thewrought piece is not excessive. Thiscan be of advantage since isotropyis usually desirable. Most signifi­cantly, particulate SiC is very cheap.

The great bottleneck in the MMC'sseems to be their generally low frac­ture toughness. It would appear thatthe crack blunting ability of the duc­tile matrix is not being exploited toadvantage. It is expected that a con­siderable research effort will beexpended in this area.

Commercial applications of MMC'sare coming up, albeit slowly. AI-Sialloy matrix containing Al203 (+Si02) short fibers are being used inpistons for automotive engines byToyota Motor Co. Connecting rodsand some sporting goods are otherexamples. These are other limiteddefense related applications. Mars­den" has recently reviewed some ofthe commercial potentials of MMC's.

CERAMIC MATRIX COMPOSITES(CMC's)

Ceramic matrix composites are arelatively new entrant in the com­posite field. Contrary to MMC'swhere the matrix failure strain ismore than that of fiber, in CMC'sthe matrix strain is generally lessthan that of fiber. Thus, the compos­ite strength is controlled by the fail­ure strain of the matrix. Making rel­atively ductile fibers bridge themicrocracks generated in brittle ce­ramic matrix results in reducedcrack openings, requiring higherstrain levels for crack propagationsto occur. Consequently, we have anincreased fracture strain in thecomposite. Also, a weak fiber/matrixinterface will make the microcracksgenerated in the brittle matrix todeviate and not propagate throughthe fibers. An example of this mech­anism is the CMC:Nb wire rein­forced molybdenum disilicide (MoSi2).The MoSi2 matrix has germaniumadded to improve the oxide surfacelayer and as a sintering aid. Nbwire forms a Si02-Se02 coating dur­ing sintering which is resistant tooxidation even when microcracksform in the matrix. This Ge contain­ing interfacial layer causes a radialcrack deviation, preventing themicrocracks in the brittle matrixfrom entering into the Nb wires. Thefracture is thereby delayed until theNb wire fracture strain is attained.

Other examples of CMC's includecarbon fiber and SiC fiber reinforcedSiC matrix obtained by chemical va­por impregation and reaction bondingof liquid silicon impregnant. Theresulting mechanical properties arepromising. In particular, C fiber re­inforced SiC showed fracture tough­ness (KIC) values of up to 10 MPavmcomparedwith 1.5-3.5 MPa vm forsintered and polycrystalline reactionbonded SiC. SiC (Nicalon type) rein­forced lithium alumimosilicate ma-

JOURNAL OF METALS • December 1985 27

trix and glass-ceramic matrix areother promising CMC systems.

Many of the problems mentionedfor MMC's regarding characteriza­tion and optimization of the inter­face region, effect of matrix chemis­try and processing routes etc., areequally valid for CMC's.

CLOSURE

Some 600 participants from 20countries made this San Diego con­ference truly an international eventand the biggest so far in the ICCMseries. The great difference in tech­nical content of this meeting fromthe earlier ones was, in my view,the rather large number of paperson metal matrix composites and asizable number on ceramic matrixcomposites. A rough breakdown ofpapers would be 55%, 35%, and 10%for PMC's, MMC's, and CMC's,respectively. It was in 1980, at theICCM III in Paris, that an authorityin polymer based composites in-

formed me, very solemnly, that met­al matrix composites were no goodand that they did not have anyfuture. Well, since 1980, we havewitnessed a tremendous surge of ac­tivity in the MMC field. That alarge proportion of work presentedat ICCM V was in the MMC area ispersonally very heartening to me.The ICCM had also a very largecommercial exhibit. The rather largenumber of companies displayingtheir techniques, capabilities, andwares in the area of MMC also testi­fied to the importance of metal ma­trix composites. Nevertheless, therestill remains the problem of makingwidespread use of composites of alltypes in non-aerospace areas areality. That challenge still remains.Low cost reinforcements are essen­tial for this, and the fact that thecost of carbon fibers has come downconsiderably over the years isencouraging in this respect.

ACKNOWLEDGEMENTS

I am grateful to the Department ofMetallurgy and Mining Engineering,University of Illinois at Urbana­Champaign for support. In particular,I am indebted to Professor C. A. Wertand Professor J.M. Rigsbee for theirmany kindnesses.

References

1. KK Chawla, "A Report on the InternationalConference on Composite Materials 1975," Trans.AMSE..J. Eng. Mater. and Technology, 97 (1975)pp. 371-381.2. K.K Chawla,· "Composite Materials-1980,"Mater. Sci. and Eng., 48 (1981) pp. 137-141.3. KK Chawla, "Composite Materials-Some Re­cent Developments," J. Metals, 35 (1983) pp. 82-86.4. W. Harrigan, "Conference Preview: Fifth Inter­national Conference on Composite Materials," J.Metals, 37 (1985) pp. 57-58.5. J.A. DiCarlo,. "Fibers for Structurally ReliableMetal and Ceramic Composites," J. Metals, 37(1985) pp. 44-46. 6. K Marsden, "CommercialPotentials for Composites," J. Metals, 37 (1985)pp. 59-62.7. W.C. Harrigan, J. Strife, and A.K Dhingra,(eds.), Fifth International Conference on CompositeMaterials: ICCM V, The Metallurgical Society,Warrendale, PA 15086.

If you want more information on this subject,please circle reader service card number 40.

NEWS&UPDATE~~~~~~~~~~~~(Continued from page 12)

Westinghouse Hanford, ORNL and Brush Wellman Win IR 100 AwardsWestinghouse Hanford of Rich­

mond, Washington, Brush WellmanInc. of Cleveland, Ohio, and OakRidge National Laboratory (ORNL)of Oak Ridge, Tennessee, are amongthe recent recipients of Research &Development Magazine's prestigious

.IR 100 Award. This award honorsthe 100 most significant technicaladvances of the year. WestinghouseHanford won for their improvementson a system to convert uraniumslurry into usable oxide powder,while Brush Wellman won for itsnew beryllium copper alloy, BrushAlloy 174. ORNL, on the other hand,received five of the awards for itsmultipurpose R&D.

Hanford's "advanced fluidized bedcalciner" is based on a design origi­nally developed by General ElectricCompany for use with nuclear fuelscrap. The improvements enable theadvanced calciner to be installed ina remotely operated glovebox enclo­sure and be fully automated.

While the original design and re­cent improvements were geared torecover and reuse uranium-pluto­nium oxide scrap, the developers an­ticipate wider use in the treatmentof toxic metals and other hazardouswastes. Says Dr. Robert J. Cash,manager of Westinghouse Hanford'steam, "I don't see why our fluidizedbed couldn't be used for the destruc­tion of dioxins or PCB's, maybe ina two-stage process where you'd get

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more time at high temperature."The advanced fluidized bed calciner

provides time, temperature and tur­bulence to change a liquid slurrysuch as ammonium diurante into auranium dioxide powder. The liquidis sprayed into a hot chamber whichincludes thousands of tiny metalbeads. These constantly movingbeads provide an efficient heat trans­fer media for the immediate trans­formation of the slurry droplets intodry particles of powder, which arecarried out by hot nitrogen gas andrecovered in a collector. Able to pro­duce a temperature of up to 700°Cwithout any plugging, the develop­ment team anticipates that the cal­ciners could attain temperaturesaround 1,000°C in transforming haz­ardous toxic materials into safe pow­ders or gases.

Brush Wellman's Alloy 174 wasspecifically designed to allow for thedenser packaging, miniaturizationand higher operating temperaturesrequired to today's electrical andelectronic applications. Provided asmill hardened strip, the alloy is animproved substitute for the phosphorbronzes and other copper base spe­cialty alloys. It also offers a highyield strength, improved electricaland thermal conductivity, superiorresistance to stress relaxation at ele­vated temperatures, and betterformability than competitive alloys.

With this unique combination of

properties, Alloy 174 is consideredthe preferred spring material in ap­plications where the traditional cop­per base materials are not adequate.These applications could be exploitedby the following industries: comput­ers, business electronics, telecommun­ications, automotive, energy and con­sumer product areas.

ORNL, winner of 45 of the IR 100Awards over the last nine years, re­cei ved five in 1985. They werehonored for:• A leach-resistant material for the

safe immobilization and disposal ofhigh-level radioactive nuclearwaste.

• A high performance surge arrestorto protect electrical equipment fromlightning, switching operations, ornuclear electromagnetic pulse.

• A highly sensitive display systemcapable of detecting complex mole­cules that control life processes upto 1000 times faster than withx-ray film.

• An instrument that permits simul­taneous measurement oftwo-dimen ­sional strain in solid materialspeciments at high temperatures.

• An improvement in reliability, sta­bility, and detection capability fora helium ionization detector usedin gas chromatography.The IR 100 Awards are selected

from among thousands of scientificand engineering achievements.

(Continued on page 44)

JOURNAL OF METALS • December 1985