In view of the fact that the covalent carbon-carbon bond is a very strong one, we should expect linear chain polymers such as polyethylene to be potentially very strong and stiff. What one needs for realizing this potential is full extension of molecular chains. The orientation of the polymer chains with respect to the fiber axis and the manner in which they fit together (i.e., order of crystallinity) are controlled by their chemical nature and the processing route. During the 1970s and 1980s considerable effort has gone towards realizing this potential in the simple linear polymer and impressive results have been obtained on a laboratory scale. Allied Corporation announced in the mid 1980s an extended chain ultrahigh molecular weight trade name with impressive properties.
In the mid 1970s reports of producing strong and stiff polyethylene fibers started to appear. Most of this work involved drawing of melt crystallized polyethylene to very high draw ratios. Tensile drawing, die drawing, or hydrostatic extrusion were used to obtain the high plastic strains required for obtaining a high modulus. Later developments have involved altogether different processing routes, two ways of achieving molecular orientation: a) without high molecular extension and b) with high molecular extension. resulted in moduli as high as 200 GPa. In all these methods, molecular orientation is achieved together with chain extension.
The chains are quite extended in this structure. A shish kebab structure consists of a continuous array of fibrous crystals, the shish kebabs, in which the molecular chains are highly extended. The third method of crystallization, and perhaps technologically the most important, leads to gels. Gels are nothing but swollen networks in which crystalline regions form the junctions. Essentially, an appropriate polymer solution is converted into gel which can be processed by a variety of methods to give the fiber. High molecular weight of the polymer and high concentration of teh solution for a given molecular weight promote gel-forming crystallization. The alignment and extension of chains is obtained by the drawing of gel fiber. One problem with this gel route is the rather low spinning rates of 1.5 m mind . At higher rates, the properties obtained are not very good. Allied Corporation launched in the mid 1980s an UHMW-PE fiber, called Spectra 900, obtained by the gel processing route. Spectra 900 fiber is very light with a density of 0.97 g cm-3. Its strength and modulus are slightly lower than those of aramid fibers but on a per unit weight basis, Spectra 900 has values about 30-40% higher than those of Kevlar. It should be pointed out that both those fibers as is true of most organic fibers, must be limited to low temperature applications. Spectra 900, for example, melts at 150°C. This solution spinning approach to producing high modulus and highstrength fibers has been successfully applied in producing the aramid fibers. We describe these in the next section.
Back to Top PageAramid Fiber is a generic name for a class of synthetic organic fibers called aromatic polyamide fibers. The U.S. Federal Trade Commission gives a good definition of an aramid fiber as "a manufactured fiber in which the fiber forming substance is a long chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatc rings" Researchers at the Monsanto and Du Pont companies were independently able to produce high modulus aromatic fibers. Only Du Pont, however has produced them commercially under the trade name Kevlar since 1971.
Nylon is a generic name for any long chain polyamide. Aramid fibers like Nomex or Kevlar, however, are ring compounds based on the structure of benzene as opposed to linear compounds used to make nylon. The basic chemical structure of aramid fibers consists of oriented para-substituted aromatic units, which makes them rigid rodlike polymers. The rigid rodlike structure results in a high glass transition temperature and poor soulbility, which makes fabrication of these polymers, by conventional drawing techniques, difficult. Instead, they are melt spun from liquid crystalline polymer solutions as described below.
Fabrication: Although the specific details of the manufacturing of aramid fibers remain proprietary secrets, it is believed that the processing route involves solutionpolycondensation of diamines and diacid halides at low temperatures. The most important point is that the starting spinnable solutions that give high strength and high modulus fibers have liquid crystalline order. Various states of polymer in soultion depends on the type of polymer chain. Two-dimensionl, liner, flexible chain ploymenr in soultion are called random coils. If the polymer chain can be made of rigid units, that is, rodlike, they can be represented like a random array of rods. Any associated solvent may contribute to the rigidity and to the volume occupied by each polymer molecule. With increasing concentration of rodlike molecules, one can dissolve more polymer by forming regions of partial order, that is, regions in which the chains form a parallel array. This partially ordered state is called a liquid crystalline state. When the rodlike chains become approximately arranged parallel to their long axes but their centers remain unorganized or randomly distributed, we have what is called a nematic liquid crystal. It is this kind of order that is found in the extended chain polyamides.
Liquid crystal solutions, because of the presence of the ordered domains, are optically anisotropic, that is birefringent. The parallel array of polymer chains in the liquid crystalline state become even more ordered when these solutions are subjected to shear. It is this inherent property of liquid crystal soultions which is exploited in the manufacture of aramid fibers. The charactersistic fibrillar structure of aramid fibers is due to the alignment of polymer crystallites along the fiber axis.
Organic Fibers Researchers at Du Pont discovered a spinning solvent for poly p-benzamide (PBA) and were able to dry spin quite strong fibers from tetramethylurea-LiCI solutions. This was the real breakthrough. The modulus of these as spun organic fibers was greater than that of glass fibers. p-Oriented rigid diamines and dibasic acids give polyamides that yield, under appropriate conditions of solvent, concenration, and polymer molecular weight, the desired nematic liquid crystal structure. One would like to have, for any solution spinning process a high molecular weight to obtain improved mechanical properties, a low viscosity to ease processing conditions, and a high polymer concentration to achieve a high yield. For para aramid, poly p-phenyleneterephthalamide(PPD-T), trade name Kevlar, the nematic liquid crystalline state is obtained in 100% sulfuric acid ata polymer concentration of about 20%. The polymer solution is often referred to as the dope. The various spinning processes available are classified as dry, wet and dry jet-wet spinning process. For aramid fibers, the dry jet wet spinning method is employed. It is believed that solution-polycondensation of diamines and diacid halides at low temperatures (near 00C) gives the aramid forming polyamides. Low temperatures inhibit by product generation and promote linear polyamide formation. The resulting polymer is pulverized, washed, and dried. This is mixed with a strong acid (e.g., concentrated suphuric acid) and extruded through spinnerets at 100 0C through about 1-cm air layer isto cold water (0-4 0 C). The fiber precipitates in the air gap and the acid is removed in the coagulation bath. The spinneret capillary and air gap cause rotation and alignment of the domains resulting in highly crystalline and oriented as-spun fibers.
Back to Top PageCarbon fibers are built by long carbon-carbon molecular chains yielding very stiff fibers. The trends have driven development of carbon fibers in two direction; high-strength (HS) fibers with very high tensile strength and a fairly high strain to failure (1-1.5%) and high modulus (HM) five with very high stiffness. Especially, the latter has found their use in advanced aerospace applications where the use of light weigh materials with high stiffness is essential. Carbon fibers have a low coefficient of thermal expansion, good friction properties, good X-ray penetration and is non-magnetic. The main drawback is the high cost and all carbon composites are relatively brittle.The polyacrylonitrile fibers are stabilized in air (a few hours at 250 0 C) to prevent melting during subsequent higher temperature treatment. The fibers obtained after this treatment are heated slowly in an inert atmosphere to 1000-1500 0 C. Slow heating allows the high degree of order present in the fiber to be maintained. The rate of temperature increase should be low so as not to destroy the molecular order present in fibers. in air (a few hours at 25O 0 C) to prevent melting during the subsequent higher-temperature treatment.
The initial stretching treatment of PAN improves the axial alignment of the polymer molecules. During the oxidation treatment the fibers are maintained under tension to keep the alignment of PAN while it transforms into rigid ladder polymer. In the absence of this tensile stress in this step, there will occur a relaxation and the ladder polymer structure will become disoriented. After the stabilzation treatment, the resulting ladder type structure has high glass transition temperature so that there is no need to stretch the fiber during the next stage, namely carbonization. There still are present considerable quantities of nitrogen and hydrogen. These are eliminated as gaseous waste products during carbonization, that is heating to 1000-1500 degree C. The carbon atoms remaining after this treatment are in the form of a network of extended hexagonal ribbons. Although these strips tend to align parallel to the fiber axis, the degree of order of one ribbon with respect to another is relatively low. This can be improved by further heat treatment at still higher temperatures (upto 3000 0 C). This is called the graphitization treatment. The mechanical properties of the resultant carbon fiber may vary over a large range depending mainly on the temperature of the final heat treatment. Hot stretching above 2000 0 C results in plastic deformation of fibers leading to an improvement in properties.
Cellulose is a natural polymer and is frequently found in a fibrous form. in fact, cotton fiber, which is cellulosic, was one ofthe first ones to be carbonized. It has the desirable property ofdecomposing before melting. It is inappropriate, however, for high-modulus carbon fiber manufacture because it has a rather low degree of orientation along the fiber axis, although it is highly crystalline. It is also not available as a tow of continuous filaments and is quite expensive. These difficulties have been overcome in the case of-rayon fiber, which is made from wood pulp, a cheap source. The cellulose is extracted-from wood pulp and continuous filament tows are produced by wet spinning. Rayon is a thermosetting polymer. T he process used for the conversion of rayon into carbon fiber involves the-same three stages: stabilization in a reactive atmosphere (air or oxygen, <400 0 C), carbonization (< 1500 0 C), and graphitization (> 2500 0 C). Various reactions occur during the first stage, causing extensive decomposition and evolution of H2 0, CO, C02 , and tar. The stabilization is carried out in a reactive atmosphere to inhibit tar formation and improve yield. Chain fragmentation or depolymerization occurs in this stage. Because of this depolymerization, stabilizing under tension, as done in the case of PAN precursor, does not work in this case. The carbonization treatment involves heating to about 1000 0 C in nitrogen. Graphitization is carried out at 2800 0 C but under stress. This orienting stress at high temperature results in plastic deformation via multiple slip system operation and diffusion. Figure 2.18 shows the process schematically. The carbon fiber yield from rayon is between 15 and 30% by weight compared to a yield of about 50% in the case of PAN precursors.
There are various sources of pitch but the three commonly used sources are polyvinyl chloride (PVC),, petroleum asphalt, and coal tar. Pitch-based carbon fibers have become attractive because of the cheap raw material and high yield of carbon fibers.
The same sequence of oxidation, carbonization, and graphitization is required for making carbon fibers out of pitch precursors. Orientation in this case is obtained by spinning. An isotropic but aromatic pitch is subjected to melt spinning at very high strain rates and quenched to give a highly oriented fiber. This thermoplastic fiber is then oxidised to form crosslnked structure that makes the fiber nonmelting. This is followed by carbonization and graphitization.
Commercial pitches are mixtures of various organic compounds with an average molecular weight between 400 and 600. Prolonged heating above 350 degree C results in the formation of a highly oriented, optically anisotropic liquid crystalline phase(mesophase). When observed under polarized light, anisotropic mesophase dispersed in an isotropic pitch appears as microspheres floating in pitch. The liquid crystalline mesophase pitch can be melt spun into a precursor for carbon fiber. The melt spinning process involves shear and elongation in the fiber axis direction and thus a high degree of preferred orientation is achieved. This orientation can be further developed during conversion to carbon fiber. The pitch molecules (aromatic of low molecular weight) are stripped of hydrogen and the aromatic molecules coalesce to form larger bidimensional molecules. Very high value of Young's modulus can be obtained. It should be appreciated that one must have the pitch in a state amenable to spinning in order to produce the precursor fiber. This precursor fiber is made infusible to allow carbonization to occur without melting. Thus, the pitches obtained from petroleum asphalt and coal tar need pretreatments. This pretreatment can be avoided in the case of PVC by means of a carefully controlled thermal degradation of PVC. The molecular weight controls the viscosity of the melt polymer and the melting range. Thus, it also controls the temperature and the spinning speed. Because the pithces are polydispersoid systems, thir molecular weights can be adjusted by solvent extraction or distillation.
The thermal treatments for all precursor fibers serve to remove non carbon elements in the form of gases. For this, the precursor fibers are stabilized (they become black) to ensure that they decompose before melting. Carbon fibers obtained after carbonzation contain many growin in defects because the thermal energy sipplied at these low temperatures is not enough to break already formed carbon-carbon bonds. That is why these carbon fibers are very stable upto 2500-3000 0 C when they change to graphite. The decomposition of the precursor fiber invariably results in a weight loss and a decrease in fiber diameter. The weight loss can be considerable - from 40 to 90% depending on the precursor and treatment. The external morphology of the fiber, however, is generally maintained. Thus, precursor fibers with transverse sections in the form of kidney bean, dog bone, or circle maintain this form after conversion to carbon fiber.
At the microscopic level, carbon fibers possess a rather heterogeneous microstructure. Many workers have attempted to characterize the structure of carbon fibers and there are in the literature a number of models. There exists a better understanding of the structure of PAN-based carbon fibers. Essentially a carbon fiber consists of many graphitic lamellar ribbons oriented roughly parallel to the fiber axis with a complex interlinking of layer planes both longitudinal and lateral.
The density of the carbon fiber varies with the precursor and the thermal treatment given. It varies in the range of 1.6-2.2 g cm -3.Note that the density of the precursor being generally between 1. 14 and 1.19 cm -3. As mentioned above, the degree of order, and consequently the modulus in the fiber axis direction, increases with increasing graphitization temperature. Even among PAN carbon fibers we can have a series of carbon fibers: for example, high tensile strength but medium Young's modulus (HT) fiber (200-300 GPa); high Young's modulus(HM) fiber (400 GPa); extra- or superhigh tensile strength (SHT) and superhigh modulus type (SHM) carbon fibers. The mesophase pitch based carbon fibers show rather high modulus but low strength levels (2 GPa). Not unexpectedly, the HT type carbon fibers show a much higher strain to .failure value than the HM type. The former are more widely used. The mesophase pitch based carbon fibers are used for reinforcement, while the isotropic-based fibers are more frequently used as insulation and fillers. For high-temperature applications involving carbon fibers, it is important to take into account the variation of inherent oxidation resistance of carbon fibers with modulus.
The carbon fibers produced from various precursor materials are fairly,,good electrical conductors, Although this had led to some work toward a potential use of carbon fibers as current carriers for electrical power transmission, it has also caused extreme concern in many quarters. The reason for this concern is that if the extremely fine carbon fibers accidentally become airborne (suring manufacutre or service) they can settle on electrical equipment and cause short circuiting.
Anisotropic as the cabron fibers are, they have two principal coefficients of thermal expansion, namely transverse or perpendicular to the fiber axis and parallel to the fiber axis. Carbon fibers have found variety of application in the aerospace and sporting goods industries. Cargo bay doors and booster rocket casing in the US shuttle are made of carbon fiber reinforced epoxy composites. Modern commercial aircrafts also use carbon fiber reinforced composites. Among other areas of application of carbon fibers, one can cite variuos machinery items such as turbine, compressor, and windmill blaes and flywheels; in the field of medicine the applications include both equipment as well as implant materials (e.g., ligament replacement in knees and hip joint replacement).
Back to Top PageBoron is an inherently-brittle material. It is commercially made by chemical vapor deposition of boron on a substrate, that is, boron fiber as produced is itself a composite fiber. In view of the fact that rather high temperatures are required for this deposition process, the choice of substrate material that goes to form the core of the finished boron fiber is limited. Generally, a fine tungsten wire is used for this purpose. A carbon substrate can also been used. The first boron fibers were obtained by Weintraub by means of reduction of a boron halide with hydrogen on a hot wire substrate.
The real impulse in boron fiber fabrication, however, came only in 1959 when Talley used the process of halide reduction to obtain amorphous boron fibers of high strength. Since then, the interest in the use of strong but light boron fibers as a possible structural component in aerospace and other structures has been continuous, although it must be admitted that this interest has periodically waxed and waned in the face of rather stiff competition from other so-called advanced fibers, in particular, carbon fibers.
Boron fibers are obtained by chemical vapor deposition (CVD) on a substrate. There are two processes:
Thermal Decomposition of a Boron Hydride This method involves low temperatures, and, thus, carbon coated glass fibers can be used as a substrate. The boron fibers produced by this method, however, are weak because of a lack of adherence between the boron and the core. These fibers are much less dense owing to the trapped gases.
Reduction of boron Halide : Hydrogen gas is used to reduce boron trihalide:
In this process of halide reduction, the temperatures involved are very high, and, thus, one needs a refractory material, for example, a high melting point metal such as tungsten, as a substrate. It turns out that such metals are also very heavy. This process, however, has won over the thermal reduction process despite the disadvantage of a rather high-density substrate (the density of tungsten is 19.3 g cm -3) mainly because this process gives boron fibers of a very high and uniform quality. There are many firms producing boron fibers commercially using this process.
In the process of BCI3, reduction, a very fine tungsten wire (10-12 micron diameter) is pulled into a reaction chamber at one end through a mercury seal and out at the other end through another mercury seal. The mercury seats act as electrical contacts for resistance heating of the substrate wire when gases (BCl3, + H2,) pass through the reaction chamber where they react on the incandescent wire substrate. The reactor can be a one- or multistage, vertical or horizontal, reactor. BCl3 , is an expensive chemical and only about 10% of it is converted into boron in this reaction. Thus, an efficient recovery of the unused BCl3, can result in a considerable lowering of the boron filament cost.
There is a critical temperature for obtaining a boron Fiber with optimum properties and structure. The desirable amorphous form of boron occurs below this critical temperature while above this temperature there occur also crystalline forms of boron that are undesirable from a mechanical properties viewpoint. With the substrate wire stationary in the reactor, this critical temperature is about 1000'C. In a system where the wire is moving, this critical temperature is higher and it increases with the speed of the wire. Fibers formed in the region above the dashed line are relatively weak because they contain undesirable forms of boron as a result of recrystallization. The explanation for this relationship between critical temperature and wire speed is that boron is deposited in an amorphous state and the more rapidly the wire is drawn out from the reactor, the higher the allowed temperature is. Of course, higher wire drawing speed also results in an increase in production rate and lower costs. Boron deposition on a carbon monofilament (-35 micron diameter) substrate involves precoating the carbon substrate by a layer of pyrolytic graphite. This coating accommodates the growth strains that result during, boron deposition.
The structure and morphology of boron fibers depend on the conditions of deposition: temperature, composition of gases, gas dynamics, and so on. While theoretically the mechanical properties are limited only by the strength of the atomic bond, in practice, there always are present structural defects and morphological irregularities that lower the mechanical properties. Temperature gradients and trace concentrations of impurity elements inevitably cause process irregularities. Even greater irregularities are caused by fluctuations in electric power, instability in gas flow, or any other operator-induced variables.
Depending on the conditions of deposition, the elemental boron has been observed in various crystalline polymorphs. The form produced by crystallization from the melt or chemical vapor deposition above 1300 degreeC is beta-rhombohedral. At temperatures lower than this, if crystalline boron is produced, the most commonly observed structure is alpha-rhombohedral. Boron fibers produced by the CVD method described above have microcrystalline structure that is generally called "amorphous". This designation is based on the characteristic X-ray diffraction pattern produced by the filament in the Debye-Scherrer method, that is, large and diffuse halos with d spacings of 0.44, 0.25, 0.17, 1.4, 1.1, and 0.091 nm, typical of amorphous material. Electron diffraction studies, however, lead one to conclude that this "amorphous" boron is really a microcrystalline phase with grain diameters of the order of 2 nm
Based on X-ray and electron diffraction studies. One can conclude that amorphous born is really monocrystalline beta-rhombohedral. In practice, the rpesence of microcrystalline phases (crystals or groups of crystals observable in the electron microscope) constitutes an imperfection in the fiber that should be avoided. Larger and more serious imperfections generally result from surpassing the critical temperature of deposition or the presence of impurities in the gases. When boron fiber is made by deposition on a tungsten substrate, as is generally the case, then depending on the temperature conditions during deposition, the core may consist of, in addition to tungsten, a series of compounds, such as W2 , WB, W 2 B5, and WB4. The various tungsten boride phases are formed by diffusion of boron into tungsten. Generally, the Fiber core consists only of WB 4, and W2, B5. On prolonged heating, the core may completely be converted into WB4. As boron diffuses into the tungsten substrate to form borides, the core expands from its original 12.5 micron (original tungsten wire diameter) to 17.5 micron. The SiC coating is a barrier coating used to prevent any adverse reaction between B and the matrix such as Al at high temperatures. The SiC barrier layer is vapor deposited onto boron using a mixture of hydrogen and methyldichlorosilane.
The boron fiber surface shows a "corn-cob" structure consisting of nodules separated by boundaries. The nodule size varies during the course of fabrication. In a very general way, the nodules start as individual nuclei on the substrate and then grow outward in a conical form until a filament diameter of 80-90 micron is reached, above which the nodules seem to decrease in size. Occasionally, new cones may nucleate in the material, but they always originate at an interface with a foreign particle or inclusion.
Boron fibers have inherent residual stresses that have their origin in the process of chemical vapor deposition. Growth stresses in the nodules of boron, stresses induced by the difusion of boron into the tungsten core, and stresses generated by the difference in the coefficient of expansion of deposited boron and tungsten boride core, all contribute to the residual stresses and thus can have a considerable influence on the fiber mechanical properties. The compressive stresses on the fiber surface are due to the quenching action involved in pulling the fiber out from the chamber. Morphologically, the most conspicuous aspect of these internal stresses would appear to be the frequently observed radial crack, from within the core to just inside the outer surface, in the transverse section of these fibers.
It is well known that brittle materials show a distribution of strengths rather than a single value. Imperfections in these materials lead to stress concentrations much higher than the applied stress levels. Because the brittle material is not capable of deforming plastically in response to these stress concentrations, fracture ensues at one or more such sites. Boron fiber is indeed a very brittle material and cracks originate at preexisting defects located at the boron-core interface or at the surface. As mentioned in the beginning, boron fiber in itself is a composite fiber. It is a consequence of the discontinuity between the properties of boron and tungsten borides. This discontinuity cannot be eliminated totally but can be minimized by a proper core formation and proper bonding between the core and the boron deposit.
Due to the composite nature of the boron fiber, complex internal stresses and defects such as voids and structural discontinuities result from the presence of a core and the deposition process. Thus, one would not expect boron fiber strength to equal the intrinsic strength of boron. The average tensile strength of boron fiber is 3-4 GPa, while its Young's modulus is between 380 and 400 GPa.
An idea of the intrinsic strength of boron is obtained in a flexure test. It would be expected that in flexure, assuming the core and interface to be near the neutral axis, critical tensile stresses would not develop at the core or interface. Flexure tests on boron fibers lightly etched to remove any surface defects gave a strength of 14 GPa. Without etching the strength was half this value.
There has been some effort at NASA Lewis Research Center to improve the tensile strength and toughness (or fracture energy) of boron fibers by making them larger in diameter. Commercially produced 142-gm diameter boron fiber shows tensile strengths less than 3.8 GPa. The tensile strength and fracture energy values of the as-received and some limited-production run larger-diameter fibers showed improvement after chemical polishing. Fibers showing strengths above 4 GPa had their fracture controlled by a tungsten-boride core, while fibers with strengths of 4 GPa were controlled by fiber surface flaws. The high-temperature treatment, improved the flber properties by putting a permanent axial contraction strain in the sheath.
Boron has a density of 2.34 g cm-3 (about 15% less than that of aluminum). Boron fiber with the tungsten core has a density of 2.6 g cm-3 for a fiber of 100 micron diameter. Its melting point is 2040 0 C and it has a thermal expansion coefficient of 8.3 x 10-6 oC-1 up to 315 0 C.
Boron Fiber composites are in use in a number of U.S. military aircraft, notably the F-14 and F-15, and in the U.S. Space Shuttle. Increasingly, boron fibers are being used for stiffening golf shafts, tennis rackets, and bicycle frames. One big obstacle to the widespread use of boron Fiber is its high cost compared to that of other fibers. A major portion of this high price is the cost of the tungsten substrate.
