molten alumina. The low viscosity of molten alumina and its high melting temperature (-2070°C) preclude the melt spinning process so that slurry and sol-gel spinning processes have been developed to avoid the melting step. A particular advantage of the sol gel spinning process is the ability to control the fiber diameter in the range of 1-7 pm. Scanning electron microphotographs and surface roughness profiles of three alumina fibers, PRD-166, Nextel 610 and Saphikon fibers, are shown in Fig. 5.38. It is noted that the surface of the PRD-166 fiber is significantly rougher than the other two fibers, which is attributed to its relatively large grain size (4.5 pm). The Nextel 610 fiber, although polycrystalline, is very smooth because of its extremely fine grain size. It contains 0.4-0.7 wt% Fe202 and about 0.5 wt% SO2, the latter is to reduce the final grain size. The roughness of the fiber and the relative magnitude of the thermal expansion coefficient between fiber and matrix are the predominant factors determining the fracture behavior of the composite involving interface debonding and subsequent fiber pull-out. Representative properties of some alumina fibers are given in Table 5.16. 5.5.6.2. RMC t iori hcrrric.r COLI t iiigs on A 1203 fibers Most a-alumina fibers are not readily wetted by most metals, due to their low surface energy, particularly if the fibers are in the form of short whiskers. The wettability of these fibers and whiskers can be improved by a CVD process of a thin metallic coating, such as Ni (Sutton, 1966) or Ni alloys containing active metals like Ti (Noone et al., 1969) for a molten silver matrix. A duplex Ti-Ni coating further promotes the wetting and improves significantly the bonding, as revealed by the improvement in composite tensile strength. The fracture mode changes from interfacial failure to matrix shear failure with the coated fibers. The Ti-Ni coatings are also found to be effective for other matrices like A1 and Ni-Cr alloy (Nicholas, 1968). Fig. 5.38. Scanning electron microphotographs of (a) PRD-166, (b) Nextel 610 and (c) Saphilkon A1203 fibers, showing different surface roughness profiles. After Chawla (199% Fig. 9.25. p. 330. Reproduced by permission of Chapman & Hall. q P Table 5.16 Mechanical properties of major oxide fibers” Properties Diameter (pm) Density (g/cm3) Tensile strength (MPa) Young’s modulus (GPa) Specific strength (IO6 cm) Specific modulus (1 O6 cm) Fiber FP PRD-166 Saffil RF Saffil HA Safimax Fiberfrax Nextel 312 Nextel 440 20 20 1-5 1-5 3.0 1-7 I1 11 3.9 4.2 3.3 3.4 3.3 2.73 2.1 3.1 > 1400 2070 2000 I500 2000 1000 1720 1720 380 380 300 > 300 300 105 152 220 > 3.7 5.0 6.2 4.5 6.2 3.8 6.5 5.7 > 970 920 930 > 900 930 390 570 720 ~ ~ ~_____ “After Birchall (1986). Fiber FP: r-Al203 yarn (Du Pont). PRD-166: AI2O3-ZrO2 yarn (Du Pont). Saffil RF: 5% Si02/AI20, staple (ICI). Saffil HA: 5% Si02/Al2O3 staple (ICI). Safimax: 4% Si02/AI2O3 semi-continuous, standard density (ICI). Fiberfrax: 50% SiO2/Al2O3 staple (Carborundum). Nextel 312: 24% sio2/14% B203/A1203 (3M). Nextel 440: 28% Si02/2% B2O3/AI2O1 (3M). c & 5 226 Engineered interfaces in fiber reinforced composites Another good example of interfacial modification can be found in alumina fiber- glass matrix composites that are essentially an oxide-oxide system. A series of intermediate compounds has been identified by Aksay and Pask (1975). The reaction product gives rise to a strong chemical bonding at the interface region and thus a brittle fracture behavior of the composite (Michasle and Hellman, 1988; Maheshw- ari et al., 1989). Tin dioxide, Sn02, is known to have no mutual solubility with aluminum up to 1600°C (Barczak and Insley, 1962), and has a low solubility in silica (Manfred0 and McNally, 1984). This knowledge has been applied by Chawla et al., (1993) to PRD-166 and Saphikon-single crystal alumina fibers. The Sn02 coating prevents chemical reactions that otherwise occur with the glass matrix. The bonding Fig. 5.39. Scanning electron microphotographs of fracture surfaces of (a) uncoated and (b) Sn02 coated PRD-166 A1203 fiber reinforced glass matrix composites. After Chawla (1993). Fig. 9.26 and Fig. 9.27, p. 333. Reproduced by permission of Chapman & Hall. Chapter 5. Surfuce treatments qf,fibers and effects on composite properties 227 400 200 ID a 0- E 3 2 - m -200- a c v1 X at the fiber-Sn02 interface is purely mechanical, whereas that between SnO2 and glass is a combination of chemical and mechanical bonds. Fig. 5.39 shows a characteristic planar brittle fracture and pull-out fibers in uncoated and Sn02 coated PRD- 166 fiber-glass matrix composites, respectively. The major toughening mechanisms in the coated fiber composite are mainly crack bridging and crack deflection (Chawla, 1993). The beneficial effects of Sn02 coating on A1203 fiber has also been demonstrated in flexure and compression tests (Siadati et al., 1991). A rnicromechanics analysis of the residual thermal stresses present in glass matrix composites with and without Sn02 coating has been studied by Chawla (1993), and a summary is given in Fig. 5.40. Both the radial and axial stresses in the fiber are greater for the coated fibers than the uncoated fibers, whereas these stresses remain almost constant in the matrix. From the composite toughness viewpoint, the presence of the high tensile radial stress at the fiber-coating and coating-matrix interfaces is deemed particularly desirable. It is also interesting to note that there is a large axial stress discontinuity at the interface region when the coating layer is present. (bl - PRD-166 Fiber Matrix Coating - 1 I I I I Radial distance (p) 5 Radial distance @rn) Fig. 5.40. Distributions of thermal residual stresses in the (a) radial and (b) axial directions of SnOz coated PRD-166 Al2O3 fiber reinforced glass matrix composites: (. .) uncoated fiber; (-) coated fiber. After Chawla (1993), Fig. 9.29, p. 335. Reproduced by permission of Chapman & Hall. 228 Engineered interfaces in jber reinjorced composites The improvement in fracture toughness of a Nextel 480 mullite (3Al2O3-2SiO~) fiber in a glass matrix has also been achieved by incorporating a BN coating on the fiber surface (Vaidya et al. 1992). The uncoated fiber composite shows a brittle and planar fracture, while those containing BN coated fibers exhibit extensive fiber pull- out, in a similar manner shown for SnOz coated PRD-166 fibers (Fig. 5.39(b)). However, when a very thin, say about 0.3 pm, coating is applied, no BN layer is observed after the process, because the thin coating becomes easily oxidized, followed by vaporization of the oxidation product. Otherwise, the BN coating tends to decompose during the hot pressing of the matrix material. This indicates that the choice of coating thickness is an important factor which controls the effectiveness of the coating material. Ha and Chawla (1993) and Ha et al. (1993) used a similar BN coating successfully to obtain tough mullite fiber-mullite matrix composites. A duplex SiC/BN coating is also recommended for use to reduce the interface bond strength. A diffusion barrier coating has also been successfully applied to aluminide-based intermetallic matrix composites (Misra, 1994). 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Sci. Leu. 13. 305- 309. Li. P.X., Ma. Z.Y. and Liu, G.B. (1989). In Interfhces in Metal Matrix Compo.vite.y. (R.Y. Liu ed.). Thc 3M Society. pp. 307-316. Li, Q, (1990). In Proc. ICCI-III, ControNed Inrerjuce Structures (H. Ishida ed.), Elsevier Sci. Pub., New pp. 351-356. York. Li, Z.F. and Netravali, A.N. (1992). Surface modification of UHSPE fibers through allylamine plasma deposition. 11. effect on fiber and fiber/epoxy interface. J. Appl. Polym. Sci. 44, 319 332. [...]... Pittsburgh, PA pp 2 59- 264 Singh, R.N and Gaddipati, A.R ( 199 1) A uniaxially reinforced zirconia-silicon carbide composite J Muter Sci 26, 95 7 Singh, R.N ( 199 3) Interfacial properties and high temperature mechanical behavior of fiber reinforced ceramic fiber reinforced ceramic composites Mater Sci Eng A 166, 185- 198 236 Engineered interfaces in fiber reinforced composites Strife, J., Prewo, K.M ( 198 2) Silicon... Kelly ( 197 3), Marston et al ( 197 4), Atkins ( 197 5) and Harris ( 198 0), and these are reviewed recently by Kim and Mai ( 199 1a, b, 199 3) Reviews on failure mechanisms are also available for MMCs (Ochiai, 198 9; Taya and Arsenault, 198 9; Clyne and Withers, 199 3), CMCs (Davidge, 198 9; Warren and Sarin, 198 9; Evans, 198 9; Ruhle and Evans, 198 9; Chawla, 199 3), and cementitious fiber composites (Mai, 198 5; Cotterell... continuous alumina fiber reinforced glass matrix composites J Am Ceram Soc 71, 725-731 Morin, D ( 197 6), Boron carbide-coated boron filament as reinforcement in aluminium alloy matrices J Less Common Metals 47, 207-213 Misra, A.K ( 199 4) Modification of the fiber/ matrix interface in aluminide-based intermetallic matrix composites Composites Sci Technol 50, 3748 Morgan, R.J and Allred, R.E ( 199 3) Aramid fiber. .. pull-out As the external loading continues and the crack propagates, the broken fibers are pulled out from the matrix (Fig 6.1(e)), resulting in a continuation of the post- 244 Engineered interfaces in fiber reinforced composites debonding frictional work The pull-out energy (Cottrell, 196 4; Kelly, 197 0) is the work done against sliding friction in extracting the broken fiber Based on the work done...234 Engineered interfaces in fiber reinfbrced composites Li, Z.F., Netravali, A.N and Sachse, W ( 199 2) Ammonia plasma treatment of ultra-high strength polyethylene fibers for improved adhesion to epoxy resin J Mater Sci 27, 4625-4632 Liao, Y.T ( 198 9) A study of glass fiber- epoxy composite interface Polym Composites 10, 424-428 Lowden, R.A ( 199 1) In Advanced Composite Materials Ceramic Trans., 19, American... 242 Engineered interfaces in jiber reinforced composites 6.1.2 Fiber- matrix interface debonding in mode 11 shear For a composite containing fibers whose maximum strain is greater than that of matrix (i.e q > ern), the crack propagating in the matrix is halted by the stiff fiber if the current level of stress is not high enough as shown in Fig 6.l(b) Alternatively, the crack may pass around the fiber. .. T ( 199 3) Influence of silane coupling agents on interlaminar fracture in glass fiber fabric reinforced unsaturated polyester laminates J Mater Sci 28, 1725-1723 Takayanagi, M., Kajiyama, T., Katayose, T ( 198 2) J Appl Polym Sci 21, 390 S 391 7 Tissington, B., Pollard, G and Ward, I.M ( 199 1) A study of the influence of fiberlresin adhesion on the mechanical behavior of ultrahigh modulus polyethylene fiber. .. fiber composites J Muter Sei 26, 82 -92 Vaidya, R.U., Fernando, J., Chawkd, K.K and Ferber, M.K ( 199 2) Effect of fiber coating on the mechanical properties of a Nextel-480 fiber reinforced glass matrix composites Mater Sei Eng A151, 161-1 69 Vaughhan, D.J ( 197 8) The use of coupling agents to enhance the performance of aramid reinforced composites Polym Eng Sci 18, 167-1 69 Verpoest, I and Springer, G.S ( 198 8)... ( 197 2) Interfacial characterization of silicon carbide coated boron reinforced aluminum matrix composites J Mater Sci 7, 91 9 -92 8 Prouhet, S., Camus, G., Labrugere, C., Gette, A and Martin, E ( 199 4) Mechanical characterization of S i c fiber/ SiC (CVD) matrix composites with a BN-interphase J Am Ceram Soc 77, 6 49- 6 59 Riess, G., Bourdeux, M., Brie, M., Jouquet, G ( 197 4) Carbon Fibers - Their Place in Modern... Novak, R.C ( 196 9) Fracture in graphite filament reinforced epoxy In Composite Materink: Testing and Design ASTM STP 460 ASTM, Philadelphia, PA, pp 54&5 49 Chaptcr 5 Surface treatments qf’,fibers and effects on composite properties 235 Outwater, J.O ( 195 6) In Proc 11th Annual Tech Cont Reinforced Plastics Div SPI Sec 9- B Plueddemann, E.P ( 197 2) Cationic organofunctional silane coupling agents In Proc 27th . Arsenault, 198 9; Clyne and Withers, 199 3), CMCs (Davidge, 198 9; Warren and Sarin, 198 9; Evans, 198 9; Ruhle and Evans, 198 9; Chawla, 199 3), and cementitious fiber composites (Mai, 198 5; Cotterell. Mai ( 199 1a). 242 Engineered interfaces in jiber reinforced composites 6.1.2. Fiber- matrix interface debonding in mode 11 shear For a composite containing fibers whose maximum strain is. Eng. A 166, 185- 198 . Singh, R.N. ( 199 3). Interfacial properties and high temperature mechanical behavior of fiber reinforced 236 Engineered interfaces in fiber reinforced composites Strife,