Thin coatings
Ceramic coatings are obtained in different ways (chemical vapor deposit (CVD),
plasma spraying, sol-gel, lazer assisted deposition, etc.). For materials used in
orthopedics, they are made on the surface of metallic parts of the prosthesis which
are in direct contact with the bone, so as to enable integration with latter. The
ceramic coating should particularly be chosen according to its biological properties,
Bioceramics 515
the nature of the substrate to be covered, the required nature of the bonding between
the substrate and the deposition of the physical, chemical and mechanical properties.
The example of the hydroxyapatite coatings by plasma spraying on a hip
prosthesis in titanium or titanium alloy illustrates all the complexities of processing
of this type of bioceramic. HAP powder is introduced in the inside in a carrier gas on
the jet of a plasma torch before being sprayed on the substratum prepared
mechanically for this operation (sand blasting). This technique poses a number of
problems, essentially due to the partial and superficial decomposition of HAP
particles at a very high temperature. Consequently, the deposit obtained is composed
of different phases heterogenously distributed in the deposit. These different foreign
phases (CaO, TCP, TTCP, oxyapatite) will, depending on their quantity, modify the
chemical, mechanical and biological behavior of the biomaterial [RAN 96]. For
example, the rehydration of CaO as Ca(OH)2 induces a large increase of the
molecular volume, thus causing the formation of cracks in the deposit, and even its
detachment from the surface. On the other hand, the presence of phases which are
more soluble (TCP, TTCP) than the HAP may help nucleation of a layer of neo-
formed apatite in vivo and the biological activity of biomaterials.
Taking into account the inherent disadvantages of high working temperatures
required for plasma spraying of HAP, other techniques of covering metallic
substrate at lower temperatures have been developed at the laboratory level: the sol-
gel method [MIY 95] and electrodeposition [DUC 90]. However, most of these
techniques require some heating to obtain a cohesive layer and its adherence to the
substrate. Other techniques, using supersaturated solutions to promote precipitation
of apatites closer to biological apatites, have been suggested. These help to obtain
coatings on surfaces which were up to now hardly suitable for covering by
conventional techniques of depositing [GRO 96]. The electrolysis of a
supersaturated metastable solution also yields a coating when the metal to be
covered is placed as a cathode [ROY 93]. However, the main disadvantage of these
techniques is the slow speed of formation of the deposit and the poor quality of
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 .
|