Introduction:
Ceramics are refractory, polycrystalline compounds, usually inorganic, including silicates, metallic oxides, carbides and various refractory hydrides, sulfides, and selenides. Oxides such as Al2O3, MgO, SiO2, and ZrO2 contain metallic and nonmetallic elements and ionic salts, such as NaCl, CsCl, and ZnS. Exceptions to the preceding include covalently bonded ceramics such as diamond and carbonaceous structures such as graphite and pyrolized carbons.
Ceramics in the form of pottery have been used by humans for thousands of years. Until recently, their use was somewhat limited because of their inherent brittleness, susceptibility to notches or microcracks, low tensile strength, and low impact strength. However, within the last 100 years, innovative techniques for fabricating ceramics have led to their use as “high tech” materials. In recent years, humans have realized that ceramics and their composites can also be used to augment or replace various parts of the body, particularly bone. Thus, the ceramics used for the latter purposes are classified as bioceramics. Their relative inertness to the body fluids, high compressive strength, and aesthetically pleasing appearance led to the use of ceramics in dentistry as dental crowns. Some carbons have found use as implants especially for blood interfacing applications such as heart valves. Due to their high specific strength as fibers and their biocompatibility, ceramics are also being used as reinforcing components of composite implant materials and for tensile loading applications such as artificial tendons and ligaments.
Unlike metals and polymers, ceramics are difficult to shear plastically due to the (ionic) nature of the bonding and minimum number of slip systems. These characteristics make the ceramics nonductile and are responsible for almost zero creep at room temperature. Consequently, ceramics are very susceptible to notches or microcracks because instead of undergoing plastic deformation (or yield) they will fracture elastically on initiation of a crack. At the crack tip the stress could be many times higher than the stress in the material away from the tip, resulting in a stress concentration which weakens the material considerably. The latter makes it difficult to predict the tensile strength of the material (ceramic). This is also the reason ceramics have low tensile strength compared to compressive strength. If a ceramic is flawless, it is very strong even when subjected to tension. Flawless glass fibers have twice the tensile strengths of high strength steel (~7 GPa).
Ceramics are generally hard; in fact, the measurement of hardness is calibrated against ceramic materials. Diamond is the hardest, with a hardness index of 10 on Mohs scale, and talc (Mg3Si3O10COH) is the softest ceramic (Mohs hardness 1), while ceramics such as alumina (Al2O3; hardness 9), quartz (SiO2; hardness 8), and apatite (Ca5P3O12F; hardness 5) are in the middle range. Other characteristics of ceramic materials are (1) their high melting temperatures and (2) low conductivity of electricity and heat. These characteristics are due to the chemical bonding within ceramics.
Ceramics used in fabricating implants can be classified as nonabsorbable (relatively inert), bioactive or surface reactive (semi-inert), and biodegradable or resorbable (non-inert). Alumina
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 .
|