As noted in the previous section, we distinguish chemically similar materials as ?ceramics? and ?glasses? by the presence or absence, respectively, of crystallinity . A sophisticated form of crystalline ceramics are the ?glass-ceramics,? which are first produced like ordinary glassware and are then transformed into crystalline ceramics by a careful heat treatment. The advantage of the initial, glass stage is that the product can be formed into a complex shape economically and precisely by conventional glass-forming technology. The advantage of the subsequent crystallization is that the final microstructure is fine grained with little or no residual porosity. Such a microstructure tends to provide have good resistance to mechanical shock due to the elimination of stress concentrating pores. It should be noted that the crystallization process is not always 100% complete, but that the residual glass phase effectively fills the grain boundary volume, helping to create the pore-free structure .optimal mechanical performance in a ceramic. Glass-ceramic products typically Conventional glass ceramics are based on composition systems such as Li2OAl2O3- SiO2, which produce crystalline phases with exceptionally low thermal expansion coefficients and subsequent resistance to thermal shock. An additional, critical component of the composition of conventional glass ceramics is the addition of a few mol% of a nucleating agent such as TiO2 which facilitates the characteristic, fine-grained crystalline microstructure. Glass-ceramics for biomedical applications are more typically based on
compositions similar to the Bioglass system. Conveniently, P2O5 serves as a nucleating agent in the same way as TiO2. Low-alkali (0 to 5 wt%) silica glass ceramics , known as Ceravital, have been successfully used for more than ay decade as implants in middle-ear surgery to replace bone damaged by chronic infection (Hench 1993). In Japan, a two-phase silica-phosphate glass-ceramic has been developed. Known as A/W glass-ceramic, it consists of an appetite phase, Ca10(PO4)6-(OH1F2), a wollastonite phase, and a residual glassy matrix. A/W glass-ceramic has been used successfully in hundreds of patients for replacing part of the pelvic bone and in vertebral surgery. An easy-to-machine silica-phosphate glass-ceramic has been developed in Germany, which contains phlogopite (a type of mica) and apatite crystals. Finally, it should be noted that compositional ranges in which bioglasses and Bioglass-ceramics bond effectively with bone and other tissues is generally limited. For example, small additions of certain oxide components such as A12O3 and TiO2 can inhibit bone bonding in these systems.
5.8
CERAMIC-MATRIX COMPOSITES
As noted in , zirconia ceramics are increasingly popular alternatives to alumina ceramics because of their relatively high fracture toughness values. Ceramic-matrix composites (CMC’s) are proving to have even higher values of fracture toughness, comparable to that in some common structural metal alloys. (See Table 5.1) In CMC’s,
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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