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Engineered Interfaces in Fiber Reinforced Composites Part 9

Tham khảo tài liệu 'engineered interfaces in fiber reinforced composites part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 224 Engineered interjuces in fiber reinforced compositex 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 PRĐ-166 fiber is significantly rougher than the other two fibers which is attributed to its relatively large grain size 0.5 pm . The Nextel 610 fiber although polycrystalline is very smooth because of its extremely fine grain size. It contains 0.4 0.7 wt FcsOs and about 0.5 wt SiOs the hitter 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. Reaction barrier coatings on AljOj fibers Most 2-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 .