Engineered Interfaces in Fiber Reinforced Composites Part 2 pptx

14 Engineered interjaces in fiber reinforced composites reinforcements like glass, silica, and alumina, but are less effective with alkaline surfaces like magnesium, asbestos, and calcium carbonate (Plueddemann, 1974). 2.2.4. Chemical bonding Chemical bonding is the oldest and best known of all bonding theories. Physical adsorption mechanisms discussed in Section 2.2.2 depend on van der Waal forces or the acid-based interaction, while chemical bonding mechanism is based on the primary bond at the interface. A chemical reaction at the interface is of particular interest in the study of polymer matrix composites because it offers a major explanation for the use of silane coupling agents on glass fibers embedded in thermoset and amorphous thermoplastic matrices. Surface oxidative treatments of carbon fibers have been known for many years to promote chemical bonding with many different polymer resins. Recent work (Buxton and Baillie, 1995) has shown that the adhesion is a two-part process: the first part is the removal of a weak layer of a graphitic-like structure from the fiber surface particularly at low levels of treatment; and the second part is chemical bonding at the acidic sites. However, much further work is still needed to verify this hypothesis. In this mechanism of adhesion, a bond is formed between a chemical group on the fiber surface and another compatible chemical group in the matrix, the formation of which results from usual thermally activated chemical reactions. For example, a silane group in an aqueous solution of a silane coupling agent reacts with a hydroxyl group of the glass fiber surface, while a group like vinyl on the other end will react with the epoxide group in the matrix. The chemical compositions of the bulk fiber and of the surface for several widely used fiber systems are given in Table 2.2. It is interesting to note that except for glass fibers, the chemical composition of the surface does not resemble that of the bulk fiber, and oxygen is common to all fiber surfaces. Further details regarding the types of surface treatments commonly applied to a variety of organic and inorganic fibers and their effects on the properties of the interfaces and bulk composites are given in Chapter 5. 2.2.5. Reaction bonding Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. Chapter 2. Characterization of interfaces Table 2.2 Elemental composition of fibersa 15 Fiber Bulk Surface analysis Probable functional group E-glass Si, 0, AI, Ca, Mg, B, S, 0, AI -Si-OH, -SiOSi Carbon C, 0, N, H, metal C, 0, H JZOOH, C-OH, C=O F, Fe, Na impurities (inner core), borate B (outer core) C (outer core), 0, N Boron (B/W core) W2B~, WB4 Bz03 as methyl B-OH, B-0-B Silicon carbide Si, W (inner core), Si, C Si-0-Si, Si-OH (SiC,/W core) "After Scolar (1974) Special cases of reaction bonding include the exchange reaction bond and the oxide bond. The exchange reaction bond occurs when a second element in the constituents begins to exchange lattice sites with the elements in the reaction product in thermodynamic equilibrium (Rudy, 1969). A good example of an exchange reaction is one that takes place between a titanium-aluminum alloy with boron fibers. The boride compound is initially formed at the interface region in an early stage of the process composed of both elements. This is followed by an exchange reaction between the titanium in the matrix and the aluminum in the boride. The exchange reaction causes the composition of the matrix adjacent to the compound to suffer a loss of titanium, which is now embedded in the compound. This eventually slows down the overall reaction rate. The oxide bond occurs between the oxide films present in the matching surfaces of fiber and matrix. The reaction bond makes a major contribution to the final bond strength of the interface for some MMCs, depending on the fiber-matrix combination (which determines the diffusivity of elements from one constituent to another) and the processing conditions (particularly temperature and exposure time). A general scheme for the classification of interfaces in MMCs can be made based on the chemical reaction occurring between fiber and matrix according to Metcalfe (1974). Table 2.3 gives examples of each type. In class I, the fiber and matrix are mutually non-reactive and insoluble with each other; in class 11, the fiber and matrix are mutually non-reactive but soluble in each other; and in class 111, the fiber and matrix react to form compound(s) at the interface. There are no clear-cut definitions between the different classes, but the grouping provides a systematic division to evaluate their characteristics. For pseudoclass 1 composites that include B-AI, stainless steel-A1 and Sic-A1 systems, hardly any interaction occurs in solid state diffusion bonding, but a reaction does occur when the A1 matrix is melted for liquid infiltration. In general, in most CMCs, chemical reaction hardly occurs between fiber (or whisker) and matrix. However, an extremely thin amorphous film can be formed, 16 Engineered interfaces in jiber reinforced composites Table 2.3 Classification of fiber-metal matrix composite systemsa Class I Class I1 Class I11 w-cu W-Cu(Cr) eutectics W-Cu(Ti) A1203-CU W-Nb C-AI ( > 700 "C) A1203-Ag C-Ni AI2Q3-Ti BN coated B W-Ni &Ti B-Mg Sic-Ti B-AI Si02-AI Stainless steel-A1 Sic-AI aAfter Metcalfe (1974) originating from the oxide present on the fiber surface, due to the limited fiber- matrix reaction, e.g., between alumina whisker and zirconia matrix (Becher and Tiegs, 1987), or resulting from the decomposition of the metastable Sic fibers in Sic matrix (Naslain, 1993). The reaction compound thereby formed normally has a low fracture energy and is soft compared to the fiber or matrix. It acts as a compliant layer for the relaxation of residual thermal stresses and promotes longitudinal splitting along the fiber length. 2.2.6. Mechanical bonding Mechanical bonds involve solely mechanical interlocking at the fiber surface. Mechanical anchoring promoted by surface oxidation treatments, which produce a large number of pits, corrugations and large surface area of the carbon fiber, is known to be a significant mechanism of bonding in carbon fiber-polymer matrix composites (see Chapter 5). The strength of this type of interface is unlikely to be very high in transverse tension unless there are a large number of re-entrant angles on the fiber surface, but the strength in longitudinal shear may be significant depending on the degree of roughness. In addition to the simple geometrical aspects of mechanical bonding, there are many different types of internal stresses present in composite materials that arise from shrinkage of the matrix material and the differential thermal expansion between fiber and matrix upon cooling from the processing temperature. Among these stresses, the residual clamping stress acting normal to the fiber direction renders a synergistic benefit on top of the mechanical anchoring discussed above. These mechanisms provide major bonding at the interface of many CMCs and play a decisive role in controlling their fracture resistance and R-curve behavior. Further details of these residual stresses are discussed in Chapter 7. Chapter 2. Characterization of interfaces 17 2.3. Physico-chemical characterization of interfaces 2.3.1. Introduction Composite interfaces exist in a variety of forms of differing materials. A convenient way to characterize composite interfaces embedded within the bulk material is to analyze the surfaces of the composite constituents before they are combined together, or the surfaces created by fracture. Surface layers represent only a small portion of the total volume of bulk material. The structure and composition of the local surface often differ from the bulk material, yet they can provide critical information in predicting the overall properties and performance. The basic unknown parameters in physico-chemical surface analysis are the chemical composition, depth, purity and the distribution of specific constituents and their atomic/microscopic structures, which constitute the interfaces. Many factors such as process variables, contaminants, surface treatments and exposure to environmental conditions must be considered in the analysis. When a solid surface is irradiated with a beam of photons, electrons or ions, species are generated in various combinations. An analytical method for surface characterization consists of using a particular type of probe beam and detecting a particular type of generated species. In spectroscopy, the intensity or efficiency of the phenomenon of species generation is studied as a function of the energy of the species generated at a constant probe beam energy, or vice versa. Most spectro- scopic techniques are capable of analyzing surface composition, and some also allow an estimation of the chemical state of the atoms. However, it may be difficult to isolate the contributions of each surface layer of the material being probed to these properties. Since most surface analysis techniques probe only the top dozen atomic layers, it is important not to contaminate this region. For this reason and particularly to reduce gas adsorption, a vacuum always has to be used in conjunction with these techniques. The emergence of ultrahigh vacuum systems of less than loT6 Pa (or 7.5 x Torr), due to rapid technological advances in recent years, has accelerated the development of sophisticated techniques utilizing electrons, atoms and ions. Amongst the currently available characterization techniques, the most useful ones for composite interfaces are: infrared (IR) and Fourier transform infrared (FTIR) spectroscopy, laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), ion scattering spectroscopy (ISS), solid state nuclear magnetic resonance (NMR) spectroscopy, wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS) and the measurement of the contact angle. A selected list of these techniques is presented in Table 2.4 along with their atomic processes and the information they provide. Each technique has its own complexity, definite applications and limitations. Often the information sought cannot be provided by a single technique. This has resulted in the design of equipment that utilizes two or more techniques and obtains different sets of data from the same surface of the sample (e.g. ISSjSIMS two-in-one and XPS/AES/SIMS three-in-one equipment). Adamson (1982), Lee (1989), Castle and Watts (1988) and Ishida (1994) 18 Engineered interfaces in fiber reinforced composites have presented excellent reviews of most of these techniques, with Ishida (1994) being particulalry informative for characterization of composite materials. In addition to surface analytical techniques, microscopy, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning tunneling microscopy (STM) and atomic force microscopy (AFM), also provide invaluable information regarding the surface morphology, physico-chemical inter- action at the fiber-matrix interface region, surface depth profile and concentration of elements. It is beyond the scope of this book to present details of all these microscopic techniques. 2.3.2. Infrared and Fourier transform infrared spectroscopy IR spectroscopy, one of the few surface analytical techniques not requiring a vacuum, provides a large amount of molecular information. The absorption versus frequency characteristics are obtained when a beam of IR radiation is transmitted through a specimen. IR is absorbed when a dipole vibrates naturally at the same frequency as the absorber, and the pattern of vibration is unique for a given molecule. Therefore, the components or groups of atoms that are absorbed into the IR at specific frequencies can be determined, allowing identification of the molecular structure. The FTIR technique uses a moving mirror in an interferometer to produce an optical transformation of the IR signal as shown in Fig. 2.6. During this operation, the source radiation is split into two: one half is reflected into the fixed mirror and the other half transmitted to the moving mirror. If the mirrors are placed equidistant from the beam splitter, their beams will be in phase and reinforce each other. In contrast, the beams that are out of phase interfere destructively. An interferogram is produced from the equations involving the wavelength of the radiation, and a Fourier analysis is conducted to determine the relation between the intensity and frequency. FTIR can be used to analyze gases, liquids and solids with minimal preparation and little time. This technique has been extensively applied to the study Fixed mirror - Movable mirror- Unmodulated incident ,, \e Source Splitter 1 Detector Fig. 2.6. Schematic diagram of an interferometry used in the FTIR spectroscopy. After Lee (1989). Chapter 2. Characterization of interfaces 19 Table 2.4 Techniques for studying s Technique ,urface structures and composition" ~~~ Atomic process and type of information Microscopy Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) Scanning tunneling microscopy (STM) Atomic forcc microscopy (AFM) An analytical SEM consists of electron optics, comprehensive signal detection facilities, and a high-vacuum environment. When the primary electron beam is targctcd at the specimen, a portion of the electrons is backscattered from the upper surface of the specimen. The electrons in the specimen can also be excited and emitted from the upper surface which are called secondary electrons. Both backscatterd and secondary electrons carry the morphological information from the specimen surface. The microscope collects these electrons and transmits the signals to a cathode ray tube where the signals are scanned synchronously. providing morphological information on the specimen surface. Environmental SEMs are a special type of SEM that work under controlled environmental conditions and require no conductive coating on the specimen with the pressure in the sample chamber only 1 or 2 orders magnitude lower than the atmosphere. TEM is composed of comprehensive electron optics, a projection system, and a high-vacuum environment. When a portion of high voltage primary electrons is transmitted through an ultrathin sample, they can be unscattered and scattered to carry the microstructural information of the specimen. The microscopes collect the electrons with a comprehensive detection system and project the microstructural images onto a fluorescent screen. The ultimate voltage for a TEM can generally be from IO to 1000 keV, depending on the requirement of resolving power and specimcn thickness. The STM, like other scanning probe microscopes, relies on the scanning of a sharp tip over a sample surface. When the tip and sample are very close so that the electron clouds of tip and sample atoms overlap, a tunneling current can be established through voltage differences applied between the two electrodes. When a raster scan is made, the relative height coordinate z as a function of the raster coordinate x and y reflects the surface topography of the sample. The STM is limited to conducting materials as it is based on the flow of electrons. In AFM, a sharp tip integrated with a soft spring (cantilever) deflects as a result of the local interaction forces present between the apex of the tip and the sample. The deflection of this cantilever can be monitored at its rear by a distance sensor. The forces existing between tip and sample, when they are close, can be van der Waals, electrostatic or magnetic force. Atomic-scale friction, elasticity and surface forces can also be measured. AFM can be employed for both conductive and non-conductive specimens, without having to apply a high vacuum, presenting a major advantage over STM. 20 Engineered interfaces in Jiber reinforced composites Table 2.4 (Contd.) Technique Atomic process and type of information Spectroscopy Auger electron spectroscopy (AES) X-ray photoelectron spectroscopy (XPS) Secondary ion mass spectroscopy (SIMS) Ion scattering spectroscopy (ISS) Infrared (IR) and Fourier transform infrared (FTIR) spectroscopy Raman spectroscopy (RS) The sample surface is bombarded with an incident high energy electron beam, and the action of this beam produces electron changes in the target atoms; the net result is the ejection of Auger electrons, which are the characteristics of the element. Because of the small depth and small spot size of analysis, this process is most often used for chemical analysis of microscopic surface features. When a sample maintained in a high vacuum is irradiated with soft X-rays, photoionization occurs, and the kinetic energy of the ejected photoelectrons is measured. Output data and information related to the number of electrons that are detected as a function of energy are generated. Interaction of the soft X-ray photon with sample surface results in ionization from the core and valence electron energy levels of the surface elements. The sample surface is bombarded with a beam of around 1 keV ions of some gas such as argon and neon. The action of the beam sputters atoms from the surface in the form of secondary ions, which are detected and analyzed to produce a characterization of the elemental nature of the surface. The depth of the analysis is usually less than a nanometer, making this process the most suitable for analyzing extremely thin films. In ISS, like in SIMS, gas ions such as helium or neon are bombarded on the sample surface at a fixed angle of incident. The ISS spectrum normally consists of a single peak of backscattered inelastic ion intensity at an energy loss that is characteristic of the mass of surface atom. From the pattern of scattered ion yield versus the primary ion energy, information about elements present on the sample surface can be obtained at ppm level. The absorption versus frequency characteristics are obtained when a beam of IR radiation is transmitted through a specimen. The absorption or emission of radiation is related to changes in the energy states of the material interacting with the radiation. In the IR region (between 800 nm and 250 pm in wavelength), absorption causes changes in rotational or vibrational energy states. The components or groups of atoms that absorb in the IR at specific frequencies are determined, providing information about the molecular structure. The FTIR technique employs a moving mirror to produce an optical transformation of the IR signal, with the beam intensity after the interferometer becoming sinusoidal. FTIR has been extensively used for the study of adsorption on polymer surfaces, chemical modification and irradiation of polymers on the fibersurfaces. The collision between a photon of energy and a molecule results in two different types of light scattering: the first is Rnyleigh scattcring and the second is Raman scattering. The Raman effect is an inelastic collision where the photon gains energy from or loses energy to the molecule that corresponds to the vibrational energy of the molecule. Surface-enhanced Raman spectroscopy has been successfully used to obtain information about adsorption of polymers onto metal surfaces, polymer-polymer interaction and interdiffusion, surface segregation, stress transfer at the fiber-matrix interface, and surface structure of materials. Chapter 2. Characterization of interfaces 21 Table 2.4 (Contd.) Technique Atomic process and type of information ~ Nuclear magnetic resonance (NMR) spectroscopy In NMR technique, a sample is placed in a magnetic field which forccs thc nuclei into alignment. When the sample is bombarded with radiowaves, they are absorbed by the nuclei. The nuclei topple out of alignment with the magnetic field. By measuring the specific radiofrequencies that are emitted by the nuclei and the rate at which the rcalignment occurs, the spectroscope can obtain the information on molecular structure. "After Adamson (1982), Lee (1989) and Ishida (1994) of adsorption on surfaces of polymers (Lee, 1991) and of chemical modification and irradiation of polymers on the fiber surfaces, including silane treated glass fibers (Ishida and Koenig, 1980; Garton and Daly, 1985; Grap et al., 1985; Miller and Ishida, 1986; Liao, 1989; DeLong et al., 1990). Fig. 2.7 shows typical IR spectra of glass fiber-epoxy matrix composites with and without an amino silane coating on the fiber. 2.3.3. Laser Raman spectroscopy Laser Raman spectroscopy uses a light scattering process where a specimen is irradiated monochromatically with a laser. The visible light that has passed into the specimen causes the photons of the same wavelength to be scattered elastically, while I11111111 Wave number (cm-'1 2000 1600 1200 800 Fig. 2.7. Spectra of a glass fiber-epoxy matrix composite (a) before and (b) after hydrolysis. After Liao (1989). 22 Engineered interfaces in jiber reinforced composites it causes the light of slightly longer or shorter wavelengths to be scattered inelastically. The inelastic proportion of the photons imparts energy to the molecules, which are collected for analysis. An interesting feature of the Raman spectroscopy is that certain functional groups or elements scatter incident radiation at characteristic frequency shifts. The vibrational frequency of the group or element is the amount of shift from the exciting radiation. Functional groups with high polarizability on vibration can be best analyzed with Raman spectroscopy. Raman and IR spectroscopies are complementary to each other because of their different selection rules. Raman scattering occurs when the electric field of light induces a dipole moment by changing the polarizability of the molecules. In Raman spectroscopy the intensity of a band is linearly related to the concentration of the species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to exist for charge with molecular vibration. The concentration of the absorbing species is proportional to the logarithm of the ratio of the incident and transmitted intensities in the latter technique. As the laser beam can be focused to a small diameter, the Raman technique can be used to analyze materials as small as one micron in diameter. This technique has been often used with high performance fibers for composite applications in recent years. This technique is proven to be a powerful tool to probe the deformation behavior of high molecular polymer fibers (e.g. aramid and polyphenylene benzobisthiazole (PBT) fibers) at the molecular level (Robinson et al., 1986; Day et al., 1987). This work stems from the principle established earlier by Tuinstra and Koenig (1970) that the peak frequencies of the Raman- active bands of certain fibers are sensitive to the level of applied stress or strain. The rate of frequency shift is found to be proportional to the fiber modulus, which is a direct reflection of the high degree of stress experienced by the longitudinally oriented polymer chains in the stiff fibers. In the case of carbon fibers, two bands are obtained: a strong band at about 1580 cm-' and a weak band at about 1360 cm-', which correspond to the Ezs and AI, modes of graphite (Tuinstra and Koenig, 1970). The intensity of the Raman- active band, AI^ mode, increases with decreasing crystalline size (Robinson et al., 1987), indicating that the strain-induced shifts are due to the deformation of crystallites close to the surfaces of the fibers. The ratio of the intensities of the two modes, Z(Alg)/Z(Ezg), has been used to give an indirect measure of the crystalline size in carbon fibers (Tuinstra and Koenig, 1970). Table 2.5 gives these ratios and the corresponding average crystal diameter, La, in the graphite plane, as determined by X-ray techniques. Typical examples of strain dependence of the Raman frequencies is shown in Fig. 2.8 for two different carbon fibers, and the corresponding plots of the shifted Raman frequency are plotted as a function of the applied strain in Fig. 2.9. Enabled by the high resolution of spectra, which is enhanced by the use of spatial filter assembly having a small (200 pm) pin hole, the principle of the strain-induced band shift in Raman spectra has been further extended to the measurement of residual thermal shrinkage stresses in model composites (Young et al., 1989; Filiou et al., 1992). The strain mapping technique within the fibers is employed to study the Chapter 2. Characterization of interfaces 23 Table 2.5 Intensity ratio of Raman bands I(AI,)/I(E2J and the corresponding apparent crystal diameter, La, for various carbon fibers" Thornel 10 Union Carbide Thornel 25 Thornel 50 Thornel 75 Thornel 40 Morganite I Morganite I1 H.M.G. 50 Hitco Fortafil 5-Y Great Lakes 0.85 0.40 0.29 0.25 0.30 0.22 0.83 0.56 0.25 50 120 155 170 150 200 50 80 180 I I I 1525 1545 1565 1585 1605 1625 Raman Frequency (crn-') 1525 1545 1565 1585 1605 1625 Raman Frequency (ern-') Fig. 2.8. Laser Raman spectra obtained (a) for a polyacrylonitrile (PAN)-based HMS4 carbon fiber, and (b) for a pitch-based P75S carbon fiber. After Robinson et al. (1987). [...]... schematically in Fig 2. 21 Neglecting the effect of gravity, the droplet shape can be defined by the following expression: J5 = 2 b F ( 4 , K ) + n E ( 4 , 4 1 I (2. 17) where the parameters are: e L=-, (2. 18) XI (2. 19) (2. 20) Chapter 2 Characterization of interfaces K = dl - (!y = sin-ld- n2 - 1 37 (2. 21) (2. 22) n2 - a2 F ( 4 ,K ) and E ( $ , K) are elliptical integrals of the first and second kind, respectively... 40 Engineered interfaces in fiber reinforced composites Kim, J.K and Mai, Y.W (1993) Interfaces in composites In Structure and Properties of Fiber Composites, Materials Science and Technology, Series Vol 13 (T.W Chou ed.) Ch 6, VCH Publication, Weinheim, Germany, pp 23 9 -28 9 Kim, J.K., Mai, Y.W and Kennedy, B.J (19 92) Surface analysis of carbon fibers modified with PVAL coating and the composite interfaces. .. solution resulting in an absorption of energy, which is detected as an NMR Spectrometers are also available for high resolution solid state NMR Nuclei in 32O analyzer I -\ = t I Electron energy anatyzer fl I -I * : Fig 2. 16 A combination of SIMS and ISS After Lee (1989)  32 Engineered interfaces in fiber reinforced composites bc HSCH CH CH SIIOCH~)~ c ‘b2a2 (a) 3 20 0 150 100 50 k 0 PPM Fig 2. 17 NMR Spectra... heat-treated Kevlar 49 fiber- poxy resin composite At applied strains of (a) 0.60% (b) 1.90% and (c) 2. 5% After Galiotis (1993a,b) 26 Engineered interfaces in jiber reinforced composites Table 2. 6 XPS analysis, elemental composition of carbon fibers" Carbon fibers T300 C(%) O(%) N(%) S(%) Si(%) Na(%) Unsized Sized 81.5 12. 7 20 .0 5.3 0.8 - - - - 0.8 - 79 .2 "After Cazeneuve et al (1990) In XPS, only large... than in AES 28 Engineered interfaces in fiber reinforced composites Auger Elec‘&on XPS Electron SIMS and ISS Ion Excitation Fig 2. 12 A comparison of XPS,AES, SIMS and ISS reactions After Lee (1989) In AES, an energetic beam of electrons strikes the atoms of the sample in a vacuum and electrons with binding energies less than the incident beam energy may be ejected from the inner atomic level, creating... various polymer resins determined from the contact angle Fiber' 'I Fig 2. 21 A liquid droplet attachcd to a monofilament Gilbert et al (1990) Engineered interfaces in fiber reinforced composites 38 Table 2. 7 Interlaminar shear strength (ILSS),AES atomic percent, contact angle, 0, and surface energy, ys data for untreated and electrochemically oxidized pitch-based carbon fiber" Carbon fiber ILSSb (MPa)... - 25 $ Y E O E - 1 c 2 + v) -25 v) & 0 L 2 -50 0 0 20 0 400 600 800 1000 -75 Axial distance (pm) (4 2 50 g c - E + 25 0 ' v) L -25 e LL 2 5 2 -50 a i 0 0 20 0 400 600 800 1000 -75 Axial distance (pm) (b) 100 2 50 0 + v) 1 -50 -100 Applied strain =2. 5% 0 (4 0 20 0 400 600 800 1000 -150 Axial distance (pm) Fig 2. 10 Fiber strain and interfacial shear stress (IFSS) profiles along the fiber length for a heat-treated... (Kim et al., 1994): one involves the testing of single fiber (or multiple fibers in some cases) microcomposites in which individual fibers are embedded in specially constructed matrix blocks of various shapes and sizes; and the other uses bulk laminate composites to measure the interlaminar/intralaminar properties Much of the discussion presented in this chapter follows that given in our recent publications.. .Engineered interfaces i jiber reinforced composites n 24 uo 7 E, - ‘5 - r a mu c E u’c 0) 3 3 a 9 Fig 2. 9 Variation of the position of the 1580 cm-’ peak with fiber strain (a) for a polyacrylonitrile (PAN)-based HMS4 carbon fiber, and (b) for Thornel 50 carbon fiber After Robinson et al (1987) stress transfer mechanisms across the fiber- matrix interface in the fiber fragmentation... roughening will result in a smaller 8, on the chemically equivalent but rough surface This will increase the apparent surface tension of the solid surface, ysv In contrast, however, if for a smooth surface 0 is greater than 90°, roughening the surface will increase Or still further, leading to a decrease in ysv 36 Engineered interfaces i fiber reinforced composites n 2. 3.11.3 Contact angle on a cylindrical . - Fig. 2. 16. A combination of SIMS and ISS. After Lee (1989). 32 Engineered interfaces in fiber reinforced composites bc HSCH CH CH SIIOCH~)~ c ‘b2a2 (a) 3.k 20 0 150 100. magnitude lower than in AES. 28 Engineered interfaces in fiber reinforced composites SIMS and ISS Ion Excitation XPS Auger Electron Elec ‘&on Fig. 2. 12. A comparison of. having to apply a high vacuum, presenting a major advantage over STM. 20 Engineered interfaces in Jiber reinforced composites Table 2. 4 (Contd.) Technique Atomic process and type of information