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الكلية كلية طب الاسنان
القسم ترميم ومعالجة الاسنان
المرحلة 3
أستاذ المادة احمد غانم مهدي الهلال
4/27/2011 3:14:39 PM
Dental porcelain
The term ceramic is defined as any product made essentially from a nonmetallic material by firing at a high temperature to achieve desirable properties. The term porcelain refers to a family of ceramic materials composed essentially of kaolin, quartz, and feldspar, also fired at high temperature. Dental ceramics for ceramic-metal restorations belong to this family and are commonly referred to as dental porcelains.
The laboratory portion of a ceramic restoration is usually made in a commercial dental laboratory by a skilled technician working with specialized equipment to the shape and shade specifications provided by the dentist. Skilled technicians and artisans are also employed by the manufacturers of artificial denture teeth to produce the many forms, types, and shades necessary in this application of porcelain. Dental ceramics were first used in dentistry in the late 1700s.
Porcelain jacket crowns were developed in the early 1900s. They consisted of feldspathic or aluminous porcelain baked on a thin platinum foil and can be considered the ancestors of all-ceramic crowns. Because their low strength, however, porcelain jacket crowns were limited to anterior teeth. In the 1960s, the poor match in thermal expansion (and contraction) between framework alloys and veneering ceramics, which often led to failures and fractures upon cooling, stimulated the development of leucite-containing feldspathic porcelains. The problem was solved by mixing controlled amounts of high-expansion leucite with feldspar glass at the manufacturing stage. This allowed the adjustment of the coefficient of thermal expansion of feldspathic porcelains to very narrow specifications. This invention led to considerable improvement in the reliability of ceramic-metals and allowed ceramic materials to be bonded to a metal framework. During cooling, the thermal contraction of the metal framework is slightly higher than that of the veneering ceramic, thus placing the internal surface of the ceramic in compression. Because ceramics are stronger in compression than in tension, this property is used to advantage to provide increased resistance to shattering.
Classification of dental ceramics:
Dental porcelains are classified into four groups:
1. High fusing porcelain (1300-1370 ?C).
2. Medium fusing (1100-1250 ?C).
3. Low fusing (850-1100 ?C).
4. Ultra low fusing < 850?C.
The high and medium fusing porcelain are used for denture teeth, while the low and ultra low fusing is used for crown and bridge construction.
Composition:
1. Feldspar.
2. Quartz.
3. Kaolin.
4. Pigments and flux.
1. Feldspar:
It’s the main component of the porcelain about 80% by weight. It’s a mixture of potassium alumino silicate (K2O.AL2O3.6SIO2) and sodium alumino silicate (Na2O.Al2O3.6SiO2). The feldspar fused when it melts forming translucent glass matrix.
2. Quartz:
It forms about 15%, it acts as strengthening agent and present as fine crystalline dispersion throughout the glossy phase that is produced by the melting of the feldspar.
3. Kaolin:
It’s about 4%, it’s a hydrated aluminum silicate (Al2O.2SiO2.2H2O) act as a binder and increasing the ability to mold the unfired porcelain and aid in forming a workable mass of porcelain during molding, it’s opaque in color.
4. Pigments and flux:
It represents less than 1%; some coloring pigments (metallic oxide) are added in small quantities to provide wide variety of colors, e.g. iron oxide (brown shade), titanium oxide (yellow shade), and cobalt oxide (blue shade).
Fluxes (low fusing glass) added to reduce the temperature that is required to set the porcelain powder particles at low temperature enough, so that the alloy to which it is fired, do not melt or get deform.
APPLICATIONS:
Ceramics have three major applications in dentistry:
(1) ceramics for metal crowns and fixed partial dentures.
(2) all-ceramic crowns, inlays, onlays, and veneers, when esthetics is a priority,
(3) ceramic denture teeth.
Bonding between ceramic to metal.:
1. Mechanical bond:
By mechanical interlocking of porcelain with the roughness of metal coping (sandblasting the metal surface with aluminum oxide particles).
2. Vander Wall’s bonding:
It’s the attraction between the atoms or molecules, it is adhesion related to the extent to which the metal is melted by the porcelain, the better wetting, the stronger the Vander Wall’s adhesion (bond).
3. Compression (physical bond):
The coefficient of thermal expansion for the metal should be slightly higher than that of porcelain, so that the porcelain will draw toward the metal when the restoration cools after firing.
4. Chemical bonding:
Metal and porcelain reacts chemically in an oxidized atmosphere at approximately 1000?C to bond together (indicated by the formation of an oxide layer on the metal). The fused porcelain absorbs ions from the metal to produce a chemical bond between them.
*** 20% of the bond is due to the first three factors, and 80% of the bond is due to chemical bond.
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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