Metal ceramics /certified fixed orthodontic courses by Indian dental academy


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Metal ceramics /certified fixed orthodontic courses by Indian dental academy

  1. 1. Metal ceramic INDIANDENTALACADEMY Leader in continuing dental education 02/27/14 1
  2. 2. MECHANISM OF PORCELAIN –METAL ATTACHMENT Four theories have been proposed to explain the processes that lead to porcelain-to-metal bonding: 1. Van der waals forces. 2. Mechanical retention. 3. Compression bonding. 4. Direct chemical bonding. 02/27/14 2
  3. 3. VAN DER WAALS FORCES. 1. The attraction between charged atoms that are in intimate contact yet do not actually exchange electrons is derived from van der waals forces. 2. These secondary forces are generated more by a physical attraction between charged particles than by an actual sharing or exchange of electrons in primary(chemical) bonding. 3. Van der waals forces are generally weak, because nearly all the positive and negative charges present in these atoms are satisfied in 02/27/14 3
  4. 4. 4. It is also believed that bonding entails some measure of true adhesion based on the extent to which the metal substructure is wetted by the softened dental porcelain. 5. The better the wetting of the metal surface, greater the vanderwaal’s forces. 6. Furthermore, porcelain’s adhesion to metal can be diminished or enhanced by alterations in the surface characters(texture) of the porcelain-bearing surface on the substructure. 02/27/14 4
  5. 5. 7. A rough, contaminated metal surface will inhibit wetting and reduce the vanderwaals bond strength. On the other hand, a slightly textured surface, created by finishing with uncontaminated aluminum oxide abrasives and followed by air abrasion(blasting) with 50 microns aluminium oxide, reportedly will promote wetting by the liquid porcelain. 8. Improved wetting is then accompanied by an increase in adhesion through vanderwaals forces. 02/27/14 5
  6. 6. MECHANICAL RETENTION: 1. The porcelain-bearing area of a metal casting contains many microscopic irregularities into which opaque porcelain may flow when fired. 2. Air abrading the metal with aluminum oxide is believed to enhance mechanical retention further by eliminating surface irregularities ( stress concentrations) while increasing the overall surface area available for bonding. 02/27/14 6
  7. 7. 3. Despite it’s presence, mechanical retention’s contribution to bonding may be relatively limited. 4. Dental porcelain does not require a roughened area to bond to metal. In fact porcelain will fuse to a well polished surface, but some surface roughness is effective in increasing bonding forces. 02/27/14 7
  8. 8. COMPRESSION BONDING Dental porcelain is strongest under compression and weakest under tension. Hence , if the coefficient of thermal expansion of the metal substructure is greater than that of the porcelain placed over it, the porcelain should be placed under compression on cooling. 1. When cooling a restoration with a fullporcelain veneer, the metal contracts faster than the porcelain but is resisted by the porcelain’s lower coefficient of thermal expansion. 02/27/14 8
  9. 9. 2. This difference in contraction rates creates tensile forces on the metal and corresponding compressive forces on the porcelain. Without the wraparound effect created in a full porcelain restoration, there is less likelihood this compression bonding will develop fully. 02/27/14 9
  10. 10. THERMAL EXPANSION Generally substances increase in the length and volume when they are heated. This phenomenon is called as thermal expansion. The specific rate of change in length of a particular substance per unit change in temperature is called coefficient of linear expansion. The rate of change in volume is called coefficient of cubical expansion. These may generally be called coefficient of thermal expansion or simply thermal expansion. 02/27/14 10
  11. 11. RELATION BETWEEN METAL AND PORCELAIN When porcelain is fused to metal, three possible relations can exist in thermal expansion: 1. Thermal expansion (or contraction) is greater in porcelain than in metal. 2. Thermal expansion (or contraction) is equal between metal and porcelain. 3. Thermal expansion (or contraction) is greater in metal than in porcelain. 02/27/14 11
  12. 12. THERMAL EXPANSION IS GREATER IN PORCELAIN THAN IN METAL. Greater thermal expansion in porcelain means that during the time after porcelain has lost thermoplastic fluidity in the course of cooling, but after melting of porcelain at high temperature, porcelain is apt to contract to be smaller and shorter than metal until it reaches room temperature. Therefore, assuming that they are separated, there will be a difference in length between them. 02/27/14 12
  13. 13. Hence porcelain becomes shorter after cooling although they had the same length before heating. In the ceramo-metallic system, porcelain side is subjected to tensile stress while the metal side is subjected to compressive stress as they are fused together. As a result, the porcelain, which is very weak against tensile stress, will crack immediately. 02/27/14 13
  14. 14. THERMAL EXPANSION IS EQUAL BETWEEN METAL AND PORCELAIN As metal and porcelain expand or contract at the same rate, there will be no difference in dimensions between them at all. As a result, porcelain receives no stress from metal and thus cracking does not occur in the stable porcelain unless undue external force is applied. It is very difficult, however, to obtain the identical curves for coefficient of thermal expansion between porcelain and metal, and under ordinary conditions there is a discrepancy to some extent. 02/27/14 14
  15. 15. THERMAL EXPANSION IS GREATER IN METAL THAN IN PORCELAIN. In general, this thermal expansion relationship exists between metal and porcelain in the dental metal-ceramic system. The objective of such a relationship is to obtain the most stable assembly after firing. Fractures do not usually occur since porcelain has very high compression strength, although the porcelain side is subjected to compressive stress as the metal contracts more than porcelain during cooling to ambient temperature after firing. 02/27/14 15
  16. 16. However, this does not mean that cracking will never occur. If there is a significant difference in thermal expansion between metal and porcelain, a shearing force acts on their interface, and if stress is sufficiently great, cracking, or fracture may occur. 02/27/14 16
  17. 17. CHEMICAL BONDING The single most significant mechanism of porcelain-metal attachment is a chemical bond between dental porcelain and the oxides on the surface of the metal substructure. There are those who believe that two mechanisms might exist within the chemical (or molecular) bonding theory. According to one hypothesis,the oxide layer is permanently bonded to the metal substructure on one side while the dental porcelain remains on the other. 02/27/14 17
  18. 18. The oxide layer itself is sandwiched in between the metal substructure and the opaque porcelain. This sandwich theory is undesirable in that a thick oxide layer might exist that would weaken the attachment of metal to porcelain. The second, and more likely, theory suggests that the surface oxides dissolve, or are dissolved by the opaque porcelain layer. The porcelain is then brought into atomic contact with the metal surface for enhanced wetting and direct chemical bonding so metal and porcelain share electrons. 02/27/14 18
  19. 19. From a chemical standpoint, both covalent and ionic bonds are thought to form but only a monomolecular( single) layer of oxides is believed to be required for chemical bonding to occur. 02/27/14 19
  20. 20. Thank you For more details please visit 02/27/14 20