PH I LI PS TECHNICAL REVIEWVOLUME 35, 1975, NO.5

Boron filament: a light, stiff and strong materialA. C. van Maaren, O. Schob and W.Westerveld

Philips produce tungsten wire in large quantities for their incandescent lamps. Tungstenwire is also ideally suited as a core for boron filament, which has attracted increasingattention in recent years as a basis for strong, stifl and light structural materials. It istherefore not surprising that Philips have also given serious thought to the production ofbaron filament. Research carried out at the Central Laboratories of the Lighting Divisionhas created knowledge that makes it possible to produce boron filament of very highquality. Boronfilament is approximately as strong as the strongest steel wire, twice asstijf and three times as light. By embedding it with the appropriate orientation in anepoxy resin, a structural material is obtained which combines high strength and stiffnesswith low weight. The boron wire made by Philips has been used in an experimentalsection for the cargo-hold door of a future space shuttle.

Introduetion

Houses, cranes, gears and innumerable other largeand small structures have to be not just strong but alsostiff. The suspension bridge built over the Menai Straitin Wales between 1819 and 1826 is strong but was orig-inally not stiff enough: it swayed so much in gales thatthe horses pulling the mail-coach could not keep theirfeet. It was later stiffened and IS still in use, carryingmodern traffic [11.

Strong and stiff structures require strong and stiffmaterials such as steel. In the last twenty or thirty yearsengineers have been concerned with discovering ordevising materials that are also light. The importanceof this last property becomes obvious when we consider

A. C. van Moaren and Dr O. Schob are with the Philips LightingDivision; W. Westerveld was also with the Lighting Division untilhis retirement in 1974.

that in many bridges, cranes and vehicles the main loadcarried is the actual weight of the structure itself; theyare in fact much heavier than their useful load. Theneed for light and strong materials is even more vitalin aircraft and spacecraft.

It is now possible to make materials that are just asstrong and stiff as steel but considerably lighter. Theseare the modern reinforced materials sometimes referredto as fibre or filament composites. They consist oforiented strong and stiff filaments of one material in amatrix of another tougher and more flexible material.The situation is thus the reverse of that in traditionalreinforced materials such as reinforced concrete and

[1] J. E. Gordon, The new science of strong materials, PenguinBooks, 1968. This paperback gives a very clear generalaccount of materials science.

126 A. C. VAN MAAREN et al. Philips tech. Rev. 35, No. 5

glass-fibre-reinforced polyester, where the bars andfibres are in fact tougher than the matrix.Only a few types of fibre or filament are suitable for

modern composites, and boron filament is one of these.In its single-crystal form boron is, after carbon (dia-mond), the element with the highest strength and stiff-ness. Like diamond, however, it is also brittle and can-not be drawn into filaments. Boron filament is there-fore a composite (a 'clad wire'), formed by depositingboron on a wire core. Because of the high temperaturenecessary for this process, the choice of core material isvery limited; as a rule, tungsten is used.Philips are interested in boron filament, partly be-

cause the company itself produces large quantities oftungsten wire and partly because of the importancethat Philips also attach to materials that are strong,stiff and light. Some of the components in productionmachinery, for example, are subjected to large forcesand accelerations. To acquire experience in this fieldwe undertook an investigation into the making ofboron filament with a tungsten core. The knowledgegained has enabled us to produce boron filament ofpreviously unequalled quality quickly and reproduciblyon a laboratory 'scale.This investigation forms the chief topic of the present

article. It is preceded by an introduetion dealing withthe concepts of strength and stiffness, the effect offaults in the materialon strength, and strong filamentsand filament composites. The article ends with a de-scription of how boron filament is made into a struc-tural material and some examples of its application.

Strength and stiffness

Strength and stiffness are easily defined in the caseof a test bar in tension. If the bar is subjected to asteadily increasing tensile load, strain first occurs, fol-lowed eventually by fracture (fig. 1). To begin with,the deformation is elastic: the bar returns to its originalstate when the load is removed. The strain e, i.e. therelative extension M/I of the rod, and the tensile stressa are proportional to each other (Hooke's law):

a=Ee.

The proportionality constant E is the 'stiffness' or'Young's modulus' of the material. Beyond a certainextension the deformation becomes plastic: the bar nolonger returns to its original state when the load isremoved'. The tensile strength ofthe material (at) is thetensile force at which the bar finally breaks, dividedby the original cross-sectional area.A material can also be subjected to a shear stress,

and a stiffness constant and a strength can likewise bedefined for that situation.

The important point about the new materials, just asfor steel, is that they can be loaded heavily in tension.Materials that can accept compressive stress - such asstone or concrete, which are strong in compression butweak in tension - have been freely available sinceancient times. We shall therefore confine the rest of ourdiscussion to tensile-load situations.

Strength of materials in theory and practice

The strength found in a perfect single-crystal barsubjected to a homogeneously distributed tensile stressought to depend entirely on the strength of the chemicalbond between the atoms. A measure of this bondstrength is the surface tension y (surface energy per unitarea). It can be shown that the 'theoretical tensilestrength' ath is given approximately by:

atb = VEy/a, (2)

where a is the distance between two adjacent crystalplanes, at right angles to the direction of tension, whichwill be separated when the bar breaks.

The surface tension - a familiar concept and aneasily measured quantity for liquids - is a quantitythat can also be estimated reasonably well for solidsubstances [21.

In the simple theory from which (2) was derived yis proportional to E, so that ath is proportional toE!Va. Materials that are both stiff and light (E large, asmall) should therefore also be strong - an interestingconclusion in view of the need for strong and stiff mat-

(1)

t

-8

Fig. 1. The tensile stress a as a function of the strain s (the'stress-strain curve') of a test bar with a tensile load (schematic).The ratio ale in the linear initial part of the curve is the stiffnessor Young's modulus E ofthe material. The tensile force at whichthe bar breaks, divided by the original cross-sectional area, isknown as the tensile strength (at).

Philips tech. Rev. 35, No. 5 BORON FILAMENT 127

erials that are also light. There are a number of elementsin the second and third periods of the periodic table,or cornpounds of these elements, e.g. B, C, B4C, SiC,Si3N4, BN, AlN and A1203, which are light and verystiff. The strength of the chemical bond in these sub-stances is also apparent from their high melting points.

Equation (2) can be derived from some very simple argu-merits [3] based on principles put forward by A. A. Griffith inabout 1920. If a bar is stretched, we add to it a strain energy of[oae per unit volume. Since we are only concerned with an order-of-magnitude calculation here, we shall assume for simplicitythat Hooke's law continues to apply until the bar breaks. Wet hen have: Jadé = + Ba = ± a2 /E. The strain energy between twocrystal planes is thus +a2a/E per unit area. As soon as this energystarts to exceed the surface energy 2y of two fracture planes thebar will break. From this we obtain equation (2) but with a factorof 2 before the root sign. Closer examination shows that thecorrect factor in front of the root sign also depends on the typeof bond in the material [4J.

In spite of the theoretical predictions, the light andstiff materials B, C, B4C, etc., in their normal forms arepractically incapable of withstanding tensile loads. Thestrengths of other materials measured in practice alsoremain far below the theoretical values given by equa-tion (2). The theoretical strength CTth for steel is about30000 Njrnm''. The strength of ordinary commercialsteels is only 500 Njrnm>, and that of very strong steels(piano wires) 3000 Njrnrn''.

In about 1920 A. A. Griffith [5] showed from anexperiment on thin glass fibres that the theoreticalstrength is not simply a fictitious quantity. For thetheoretical strength of glass he obtained a value ofabout 15000 N/mm2. That of ordinary glass is about200 Njmm>. Thin, newly drawn glass fibres, however,are far stronger, and Griffith found that as his fibresgot thinner they also got stronger. Extrapolation tozero thickness gave a value of 12000 Njmrn", which isnot so very far below the theoretical value.

The weakness of brittle materials such as glass, boronand sapphire in their usual form must be ascribed tothe notch effect from scratches on the surface, crackson the inside or, more generally, any irregularity in thematerial. If a bar in tension contains a notch (jig. 2),the stress is not homogeneously distributed but is muchgreater at the edge of the notch than the average stress(stress concentration). Once the theoretical strength isreached at this edge, the material breaks there, thestress concentrates at the new edge, the materialbreaks there again, and so the process continues.Griffith calculated that a brittle bar with internaldisc-shaped cracks at right angles to the direction oftension has a strength (Je given by:

where c is the radius of the largest disc [6]. If c is one

atomic spacing, CTe is equal to the theoretical strengthatll. Cracks of only 100 atomic spacings in Griffith'smodel will decrease the strength by a factor of 10.

In glass the sensitive place is its surface: ifthe surfaceis carefully protected from damage, glass can be

Fig.2. 'Lines of force' in a bar with a notch (schematic). Thedirection and density of these lines indicate the direction and sizeof the main stress. The bar has a tensile load in the longitudinal(vertical) direction. There is a concentration of the lines of forceat the edge of the notch.

stronger than the strongest steel. Extreme strengths arefound in whiskers. These are thin, needle-like singlecrystals (e.g. of Alz03, SiC or graphite) in which,because of the reduced volume, the probability ofirregularities is extremely small.

In metals an entirely different mechanism is respon-sible for failure to attain the theoretical strengths. Sincethe metal bond is relatively omnidirectional in nature,atomic surfaces can slide relatively easily in relation toeach other through the movement of dislocations.Under load, metals therefore often exhibit markedplastic deformation (flow) before they break: they areto a greater or lesser degree ductile. Metal whiskers arealso relatively strong owing to the absence of disloca-tions. In boron, graphite and the other light, stiff, non-metallic materials mentioned, dislocations cannot move

(3)

[2] See for example B. F. Orrnont, Dokl. Akad. Nauk SSSR 106,687,1956, and Zh. n'eorg. Khimii 3, 128J, J958.

[3] See the book by J. E. Gordon rn, pp. 68 ff.[4] A. Kelly, Strong solids, Clarendon Press, Oxford 1966, p. 11.[5] A. A. Griffith, Phil. Trans. Ray. Soc. A 221, 163, 1920.f6] See A. Kelly [4], p. 45.

128 A. C. VAN MAAREN et al. Philips tech. Rev. 35, No. 5

because ofthe covalent and hence markedly directionalnature of the atom bonds.

However, while ductility is the weak feature of ametal, it is also its great advantage, since the crackingprocess is halted as soon as it occurs because plasticdeformation evens out the stress concentrations at theedge of the crack before local fracture occurs.

Plastic deformation (flow) and brittle fracture (pro-pagation of cracks) are theoretically possible in anymaterial. Which of the two processes occurs in aparticular case will largely depend on the temperatureand the nature of the deformation.

of the types now under investigation. To illustrate theproperties of boron filament we have compared itsstrength at and stiffness E with those of some otherstrong filaments and fibres in Table 1. The specificstrengths at!e and specific Young's moduli El e are alsoshown (e is the density). These are appropriate figuresof merit for assessing the quality of strong, light andstiff materials.

Steel is a good, if not the best, representative of metalfilaments. It is strong and stiff but heavy. Tungsten andmolybdenum are just as strong, considerably stiffer butalso much heavier. Of the metal filaments, beryllium [7]

Fig.3. Fracture surface of a boron-epoxy composite. The fracture clearly started at the fila-ments. It can also be seen that the adhesion between the filaments and the matrix is very good.

Strong filaments and filament composites

The glass fibres and whiskers referred to abovefurnish striking examples of the fact that materials at-tain their greatest strength in the form ofthin filaments.In a nutshell, this is because it is only in this form thatthe perfect state can be very closely approached. Thismore or less perfect state may be monocrystalline(whiskers), polycrystalline (graphite filament) oramorphous (glass, boron filament).

The production of very strong materials must there-fore be based on filaments and boron filament is one

Table I. Tensile strength at, Young's modulus E and density e ofsome filaments and wires, together with their specific strengthatle and specific stiffness Elo .

at E o atle E/e103 105 103 105 107

N/mm2 N/mm2 kg/m3 Nrn/kg Nm/kg

Steel 4.2 2.1 7.8 5.4 2.7Beryllium 1.3 2.4 1.8 7.0 13

Quartz glass 6.6 0.73 2.2 30 3.3

Graphite 2.5 2.1 2.0 13 11Boron 3.0 3.8 2.6 12 15

SiC whiskers 21 4.9 3.2 66 15

Philips tech. Rev. 35, No. 5 BORON FILAMENT

also merits some attention because it is so light; un-fortunately it is toxic and very expensive. Glass fibresare strong and light but not stiff. The quartz-glassfibre included in the Table is the strongest of the glassfibres.

Of the filaments that are strong, stiff and light,graphite filament and boron filament are the only onesnow being tested on a wide scale for various aeronaut-ical and aerospatial purposes. Like boron, graphitecannot be drawn into filament. Graphite filaments areobtained by heating filaments of organic material in aninert atmosphere under a tet;tsile load and thus car-bonizing them [81.

Whiskers are by far the strongest form of any mat-erial. However, unlike steel or boron filament, theycannot be made in lengths exceeding several centi-metres; this means that special processing is required,which will not be discussed here.

Construction with individual filaments is difficult.Structural elements - strips, sheets, struts, tubes -can be produced by embedding the filaments in anothermaterial, the carrier or matrix. In the resultant 'com-posite' the matrix protects the filaments or fibres fromdamage and also ensures that forces are distributedevenly over the filaments. The maximum advantagecan be derived from the filaments if they are keptparaIIel and as close as possible to each other over theentire length of the structural element by a stronglyadhesive matrix. The resultant elements are strong andstiff in one direction and can be combined to formunits that are strong and stiff in a number of directions.Epoxy resin is a suitable matrix for boron filament,largely because the adhesion is so good (fig. 3).The similarity between modern composites and the

older ones such as reinforced concrete or glass-fibre-reinforced polyester is only superficial. The now gen-eraIly accepted designation of 'reinforced materials'for the modern composites is not in fact a very appro-priate one, since these are not reaIlymaterials that arereinforced by filaments - like reinforced concrete -but very strong filaments embedded in a matrix.Moreover, the wires or bars in older materials oftenact as 'crack-stoppers': cracks in the matrix stop shortat the filaments. The original rather brittle materialsthus acquire a certain toughness. In modern com-posites, on the other hand, the matrix is usuaIly tough-er than the filaments which are not intended to stopcracks. This difference can clearly be seen if we attemptto break glass-fibre-reinforced polyester and a boron-filament/epoxy composite. Since glass has a relativelylarge strain and boron filament relatively little strain,the fracturing process starts from the matrix in thefirst material and from the filaments in the second(see fig. 3).

Fig. 4. Schematic representationof the production of boronfilament. Tungsten wire (W) pas-ses through a reduction chamberand then through two boron-deposition chambers. The wire iselectrically heated; the mercurytraps (M) also provide contactwith thewire. Thewire is cleanedin the' reduction chamber andboron is deposited on it in thesubsequent two chambers. B thefinished boron filament.

129

The manufacture of boron filament

Boron filament is made by heating a tungsten wireelectricaIly in a gas mixture containing a boron com-pound. The gas reacts at the wire and deposits boronon it. In our investigations we used wire temperaturesranging from 950°C to 1320 oe and a gas mixture ofH2 and BCl3. The reaction at the wire produces bothsolid boron and gaseous HCI. Only about 10% of theexpensive BCI3 is converted to boron in this reaction.Efficient recovery of the unused BCl3 lowers the costof boron filament considerably.

Our .best results were obtained with a continuousproduction process in which the tungsten wire firstpasses through a reduction chamber and then througha number ofboron-deposition chambers (figs:4 and 5).The 'reduction chamber is included for cleaning the

[7] N. P. Pinto and J. P. Denny, Metal Progress 91, No. 6, 107,June 1967.

[8] M. M. Tang and R. Bacon, Carbon 2, 211 and 221, 1964.W. Johnson and W. Watt, Nature 215, 384, 1967, and NewScientist 41, 398, 19'69. . . .

130 A. C. VAN MAAREN et al. Philips tech. Rev. 35, No. 5

Fig.5. A battery of boron-deposition reactors in which boron filament was produced continuously for several months.

Philips tech. Rev. 35, No. 5 BORON FILAMENT 131

a

b

Fig. 6. a) Longitudinal section through a boron filament in whichthe boron is amorphous. b) As (a), but with crystalline boron.Strong filaments are invariably amorphous. Filaments ofcrystalline boron are weak. The sections were photographed witha polarizing microscope. which makes the structure in (b) visible.

surface of the tungsten wire. The chambers are sealedto the gas mixtures by mercury traps through whichthe tungsten wire passes. These traps also act as theelectrical connections for the heating current. Thecurrent, and hence the ternperature ofthe wire, can thusbe adjusted separately for each chamber. The diam-eter of the incoming tungsten wire is usually 12.5 (.Lm

and that of the outgoing boron filament 100 f-lm.There are three vital factors involved in this process:

the variety of the boron modifications, the man ner inwhich the boron coating begins to form and grows, andthe formation of tungsten borides in the core. We shallnow refer to these factors in explaining how to obtainthe best boron filament.

Boron modifications

The surface of a strong boron filament has a 'corn-on-the-cob' structure (see the photograph on the titlepage). Examination of the boron wire by X-ray dif-fraction shows that the boron is practically amorphous,with no diffraction lines. This modification is meta-stable, tending to change to a crystalline rnodificationwith poor mechanical properties (jig. 6). Below 300°Cthe rate of change is completely negligible, so thatthere is no danger of recrystallization if the finished

T

] 4m/min_V

Fig.7. T-v diagram (T temperature, v wire speed) with linesshowing where filaments with a diameter of 25, 30, ... 60 fLllloccur in an experimental boron-deposition chamber of length25 cm. The lines were obtained by interpolation between diam-eters found at four wire-feed rates and six temperatures.Filaments formed at temperatures in the grey region above thedashed line exhibit a surface containing crystalline regions (fig. 8)and are always relatively very weak.

filament is not used above that temperature. However,at a température of 1000 °C the filament would recrys-tallize within a minute.

Our investigations revealed that in the production ofboron filament there is a critical ternperature for thewire. Below this ternperature only the desired amor-phous form occurs, while above it the unwanted crys-talline modification also occurs. With the wire station-ary the critical ternperature is 1000 oe. In a system inwhich the wire moves the critical température is higherand increases with the speed of the wire. Fig. 7 showsthe combinations of wire ternperature and processingrate that were necessary to produce filament of a par-ticular diameter in one of our experimental boron-deposition chambers. Filaments formed in the regionabove the dashed line are relatively weak and includepatches of an abnormal structure (jig. 8) where thematerial has recrystallized. The explanation for thisconnection between critical température and wirespeed is evident: the boron is obviously depositedin the amorphous condition and the faster it is re-moved from the chamber, the higher the permissibletemperature.

Fig. 8. The surface of a filament formed at a ternperature abovethe critical ternperature (the grey region in fig. 7). The boron inthe patches of different structure is crystalline (p-rhombohedral).

132 A. C. VAN MAAREN er al. Philips tech. Rev. 35, No. 5

In our tests with various boron-deposition chamberswe found no conditions in which strong amorphousboron could be obtained at a wire temperature exceed-ing 1320 oe.

The fact that the critical temperature increases withincreasing wire speed is of great importance in practice,because the strongest wires are obtained at the highesttemperature (below the critical temperature, of course),and the temperature may clearly be high if the wirespeed is high enough. Running the wire faster will alsospeed up the production and hence reduce costs.

Nucleation and growth of nodules

The creation of a corn-on-the-cob structure like thatshown in the title page, with large nodules, probablytakes place in the following way. When a particularpart of the wire enters the boron-deposition chamber,boron nuclei form at its surface. These grow in alldirections until they cover the entire surface of thewire, and then they all grow in a radial direction untilthat part of the wire leaves the chamber. Every noduleis derived from a single nucleus.

Impurities in the surface of the wire or in the gasmixture lead to irregular formation of nuclei andhence to 'activated' growth of nod u les (fig. 9).Appreciably larger smooth nodules then form at thesurface (fig. JO). Filaments in which this developmentis observed are always relatively weak (at less than1500 Njrnm"). Flaws of this kind can be avoided bycarefully cleaning the wire and the gas mixture before-hand.

In general, a higher formation temperature producesa stronger filament. The filament in fig. J la was madeat 1230 oe and has a tensile strength of 4000 Nmirn".The one in fig. 11b was made at 970 oe and its tensilestrength is 1500 Nzmrn". The weaker of these twofilaments does not have the large perfect nodules ofthose seen on the title page, but a finer structure. Theexplanation is simple. It can be seen in fig. 7 that forthe same processing time thicker filaments are formedat higher temperatures, i.e. the growth rate increaseswith temperature. If the number of nuclei formed persecond is approximately constant, the growth ofnodules at a low ternperature will not keep pace withthe formation of nuclei and growth will therefore keepstarting again from new nuclei. If the gas mixture con-tains too much BCl3, filaments of the type illustratedin fig. lIb are also formed at high temperatures; therate at which nuclei form is evidently faster in thatcase.

The filament temperature must therefore be as highas possible provided it does not exceed the criticaltemperature; in other words, it should be just below thecritical temperature. If for example a boron filament of

a b

Fig.9. Longitudinal sections through filaments with activatedgrowth of nodules, a) starting from the tungsten substrate wire,b) from an imperfection in the boron layer.

diameter 100 fLm is to be made from a tungsten wire ofdiameter 12.5 fLm, it is impossible to achieve this, evenapproximately, for all parts of the growing filament ifthe process is limited to a single boron-depositionchamber. This is because boron is a conductor at thehigh temperatures necessary here, so that the currentdensity and hence the temperature in the thick filamentat the lower end of the chamber would be much lowerthan in the thin filament at the top. It would thereforebe impossible te deposit good-quality filament at thetop and bottom of the chamber simultaneously. Thisis why the process is split up between two boron-deposition chambers (see fig.4); the layer typicallygrows in diameter from 12.5 to 40 fLm in the first andfrom 40 to 100 iJ_min the second.

Formation of tungsten borides in the core

The strength of a filament in which these variousdefects (figs. 8, 9, 10 and lib) have been successfullyavoided is determined by the core. During the pro-duction of the filament a transition area of tungstenborides is formed between the tungsten core andthe boron layer, and in a broken boron filament it

Fig. 10. Surface of a boron filament in which activated growth(fig. 9) has occurred.

Philips tech. Rev. 35, No. 5 BORON FILAMENT 133

a

bFig. 11. a) Surface ofa boron filament made at 1230 -c. b) As (a),but at 970°C. The tensile strengths of these filaments were 4000and 1500 N/mm2 respectively.

is often possible to distinguish radial cracks that haveobviously been initiated in this region (fig. 12).The negative influence of the core has been clearlydemonstrated by tests in which extremely high tensilestrengths (7000-11000 Njrnm'') have been attained byetching the core away 191.

By etching layer after layer from filaments and carry-ing out X-ray-diffraction analyses after every etching,we have found that the transition zone contains theentire range of tungsten borides -W2B, WB, W2B5

and WB4. As would be expected, the W2B occurs closeto the tungsten core, the W B4 close to the boron layer.

The boride W2B behaves rather differently from theothers. If 2W is made to react with B to form W2B, theatomic volume increases by no less than 50 % noi Thuswhen W2B forms, the core expands and easily causescracks in the boron coating. When the other boridesare formed, the atomic volume decreases by about 20 %or less. Here the contraction of the core tends to pullthe boron coating more tightly together, until it sep-arates: this improves the strength of the filament. Theabsence of W2B in the cores of very strong filamentsconfirms this.

As long as there is some tungsten remaining in thecore the whole series of borides will continue to beformed during the production of the filament. Conse-quently, to eliminate the W2B, the formation ofborides must be allowed to continue until first thetungsten and then the W2B have completely disap-peared. This means that the process temperature mustbe made as high as possible and the processing time aslong as possible within the range permitted by the con-ditions discussed above.

In principle, thecelirnination ofthe W2B can be mademore effective by starting with a very small quantity of

tungsten, i.e. with a very thin wire. We made boronfilaments starting from tungsten wires of various diam-eters and tested the resultant tensile strengths. Thethickness of the finished boron filament was chosen ineach case to give a density of 2.6 g/cm". The results areshown in Table If. The filaments are indeed strongerwith thinner substrate wire. Unfortunately, 6-f1.111diam-eter tungsten wire is so much more expensive thanthe usual l2.5-fLm wire that the increased strength canonly be obtained at appreciably higher cost.

Table 11. Tensile strength <1t of boron fibre as a function of thediameter of the tungsten substrate wire (0,,). The diameter ofthe boron filament (0n) is selected in each case to make thedensity of the finished filament equal to 2.60 x 103 kg/m3.

0" (iLm) I 0B (fLm) I <1t (103 N/mm2)

6 50 4.758.9 70 4.0912.5 100 3.7518 150 2.90

Fig. 12. Cross-section of the core of a boron filament. Starting atthe centre, we have first tungsten, then various tungsten boridesand finally boron. Radial cracks spread from the core, throughthe transition region consisting of borides, into the boron.

We also approached the boride problem in quite adifferent way - by depositing a layer of SiC on thetungsten wire ahead of the boron, thus forming a dif-fusion barrier between the tungsten and the boron. Wedid indeed manage to eliminate boride formation com-pletely, but we were unable to deposit a layer of good-quality boron on the SiC.

[9 ·R. Kochendörfer and H. Jahn, Kunststotfe 59, 859, 1969.[10] R. P. I. Adler and M. L. Hammond, Appl. Phys. Letters 14,

354, 1969.

134 A. C. VAN MAAREN el al.

The strength of boron filament

The tensile strengths that we find in the filaments wehave produced can be divided into three ranges clearlyassociated with the factors discussed above. The veryweak filaments containing crystalline boron (fig. 8) willnot be considered here.

We find tensile strengths of less than about1500 N/mm2 mainly in filaments in which activated,irregular growth has occurred (fig. 9). This appears aslarge smooth nodules at the surface (fig. 10), and canbe attributed to irregularities such as thickness varia-tions in the substrate wire or contamination on thesubstrate or in the gas.

We find tensile strengths between about 1500 and3200 Njrnrn'' mainly in filaments of the type illustratedin fig. llb, in which, for some reason or other, nodulegrowth has lagged behind the formation of the nuclei,so that a perfect corn-on-the-cob structure is not ob-tained.

Finally, in filaments that are3200 Njrnrn'' the strength is limited

stronger thanprincipally by

crack formation starting from the core as a resultof the formation of tungsten borides. In the strong-est filaments there is no W2B in the core, as we sawearlier.

To summarize, we have successfully produced strongboron filament of homogeneous quality, on a labor-atory scale, in a continuous process in reactors withtwo boron-deposition chambers. Flaws of the kindsillustrated in figs. 8, 9, 10 and lIb were avoided in the

Philips tech. Rev. 35, No. 5

Fig. 13. Making in strips onefilament thick ('pre-pregs') aboron-epoxy composite.

process by starting from tungsten wire with a thorough-ly clean and smooth surface, and bya correct choice ofthe ternperature, speed of the wire and composition ofthe reaction gas. The strength of the filament thenobtained is at least some 3200 N/mm2. We can alsomake filaments of appreciably greater strength (about5000 Njrnrn'') reproducibly, but only at much greatercost because of the much more expensive 6-[J.ITI tung-sten wi re that then has to be used.

Incorporating boron filament in composites

As stated earlier, epoxy resin is a suitable matrix forboron filament. A composite can be made from thesetwo component materials in two stages. The first con-sists in making strips one filament thick, known as 'pre-pregs'. These are formed by passing the filamentthrough a resin solution and winding it round a drumwith a spacing of about 20 urn between turns (fig. 13).When the desired number of turns have been formedon the drum, a thin piece of woven glass-fibre is ap-plied to provide the necessary lateral adhesion andmake the pre-pregs into strips that can be handledeasily. The resin is then cured by gently heating thedrum. Finally, the pre-preg is cut through at rightangles to the filaments and removed from the drum.

The next stage is to combine pre-pregs to formlaminates. They are stacked in the desired fashion andthe resin fully cured under pressure. If the pre-pregsare stacked with the filaments all in the same direction,

Philips tech. Rev. 35, No. 5 BORON FILAMENT 135

Fig. 14. Laminates of boron-epoxy composite and somestructural elements reinforcedwith the composite.

a laminate is obtained that is extremely strong andrigid in one direction. The strength and stiffness canbe 'spread' over a range of directions by stacking thesuccessive pre-pregs with their filaments at an angle,usually a right angle. Since the pre-pregs are veryflexible, laminates of a great variety of shapes can bemade from them (fig. 14).

If it is desired to make objects with circular sym-metry such as pressure cylinders, rods or shafts fromcomposite materials, the resin-impregnated filament issometimes wound directly in a number of successivelayers on a jig of the desired shape (filament winding).

Examples of application

We will now briefly describe a few applications ofour boron filament.

Interest in modern filament composites is almostentirely confined to the aerospace industry, because oftheir high cost. Most of our filament has also beenmade for that application. The international space-research programme includes the construction of aspace laboratory or 'sky lab'. Its link with Earth willbe maintained bya space shuttle. This vehicle willlandlike an aeroplane on return to Earth and will thereforebe able to make the two-way trip many times. Eachtime it re-enters the atmosphere it will be subjected tovery large forces. Under a contract with ELDO (theEuropean Space Vehicle Launcher DevelopmentOrganization), Fokker-VFW have been engaged in a

study for the construction of the cargo door for thespace shuttle. The door is to be about 20 m long and5 m wide. An experimental model of a representativepiece of the door, measuring about 55 by 75 cm, hasbeen built and tested. It consisted of boron-filamentcomposite coated with titanium on both sides. In viewof our experience in this field Fokker delegated thedevelopment and production of the required compositeto Philips. We have been able to meet the specificationin full. Because of the frictional heat developed onre-entry the material has to be able to withstand a tem-perature of 350 oe for a totalof 100 hours. For thisreason the material chosen for the matrix was not theusual epoxy resin but polyimide resin, which is resistantto higher temperatures.

An entirely different type of application is in thewire-tension meters used in winding coils of copperwire. In automatic winding machines the wire feed hasto be adjusted to make the tension in the wire almostconstant. This is usually not difficult for cylindricalcoils but for other shapes of coil - e.g. square ones -the wire speed varies so greatly that the feed is difficultto control. Wire-tension meters were made by Philipsfor determining just how the wire tension does in factvary. In these meters the wire travels over the end ofa leaf spring, whose deflection is converted into anelectrical signal by strain gauges. To enable wire-tension changes to be transmitted almost instanta-neously the spring has to be extremely stiff and light.In effect this means that its resonant frequency must

136 BORON FILAMENT Philips tech. Rev. 35, No. 5

be much higher than the frequencies of the variationsin the wire-tension, which are often of the order of1000Hz. The resonant frequency is inversely propor-tional to the square root of the specific stiffness. Bymaking the spring of boron-filament composite theresonant frequency was raised to 6000 Hz. This caseis characteristic of the many problems in whichmechanical changes have to be transmitted veryquickly. Such problems are enconntered in computerperipherals.A final example is that of a long thin support arm

that had to be stiffened. In the Philips Industrial Equip-

Summary. Boron is a light and very stiff element and shouldtheoretically also be very strong. Like other brittle substances,however, it will not take an appreciable tensile load in its normalform. This weakness is due to the notch effect of scratches, cracksor other imperfections. It most nearly approaches the perfectcondition in the form of thin filaments. The strength of filamentsis employed in modern composite materials, by embedding thefilaments in a matrix such as epoxy resin. Of the known strong,stiff and light filaments - whose properties make them speciallyattractive for aeronautical and space applications - only boronand graphite filament have been widely tested.In the investigation reported here on boron filament an

electrically heated tungsten wire is fed continuously through anumber of reaction chambers containing a gas mixture thatreacts with the hot wire, depositing boron on it. In strong fila-ments the boron is practically amorphous and its surface shows

ment Division the field of a cyclotron magnet had tobe measured accurately with a Hall element in thecourse of some experiments. Because the area to becovered was large and there was limited vertical clear-ance, the Hall element had to be supported by a longthin non-magnetic arm with a minimum of deflection.These conditions were satisfied by employing a squarealuminium tube 4 m long and with a cross-section ofonly 25X 25 mm, stiffened with boron-epoxy laminates.Before stiffening, the arm deflected by about 35 mmunder its own weight; the stiffening reduced this toabout 3 mm.

a regular pattern of nodules, the 'corn-on-the-cob' structure. Ifthe temperature is too high, crystalline boron is formed. Impuri-ties on the tungsten substrate or in the reaction gas lead toactivated growth and hence to large nodules at the surface. Ifthe temperature is too low or the gas composition incorrect,nuclei form too quickly for the growth and the nodules at thesurface exhibit a substructure. In the absence of these defects thestrength is determined by the core, which generally also containstungsten borides in addition to the tungsten. The borides includeW2B, which has a relatively large atomic volume and conse-quently has an adverse effect on the strength. Tungsten and W2Bcan be avoided in the finished filament by starting from a verythin tungsten wire.

Some of the boron filament has been used in the constructionof an experimental test-piece for the cargo door of a spaceshuttle.