Dental cast base metal alloys (2)


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Applied Dental Material
Second Year

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Dental cast base metal alloys (2)

  1. 1. ‫بسما اهللا الرحنمنا الرحمي ِ‬ ‫ِ ِنمْ ِ ِ َّ ِنمْ ِ َّ ِ م‬ ‫}وحاحللَدْ عقدة مل ن لسنان ي‬ ‫ِناَ َدْقُ قُ َدْ ِناَ م ً ِّ ِّ ِناَ يِ‬ ‫}يفقهوحا قول ي }82‬ ‫ِناَ َدْ ِناَ قُ ِناَ َدْيِ‬ ‫}72‬ ‫ا الﻌﻅﻳما ا صدقا اهلل‬
  2. 2. Dental Casting Alloys II Cast ‫ا‬Base ‫ا‬Metal ‫ا‬Alloy
  3. 3. Alternatives to Gold Alloys      Due ‫ا‬to ‫ا‬continuous ‫ا‬increased ‫ا‬price ‫ا‬of ‫ا‬gold, ‫ا‬ alternative ‫ا‬dental ‫ا‬casting ‫ا‬alloys ‫ا‬has ‫ا‬been ‫ا‬ introduced ‫ا‬for ‫ا‬Types ‫ا‬III ‫ا &ا‬IV ‫ا‬gold ‫ا‬alloys ‫ا‬ such ‫ا‬as: Economy ‫ا‬gold ‫ا‬alloys Silver ‫ا‬palladium ‫ا‬alloys Palladium ‫ا‬silver ‫ا‬alloys Base ‫ا‬metal ‫ا‬alloys
  4. 4. Definition: Cast base metal alloys which do not contain noble metals. They are substitutes for gold alloys type III & IV.
  5. 5. Uses in Dentistry 1. Removable partial denture framework. 2. Full denture bases. 3. Crown and bridge. 4. Dental implants.
  6. 6. Base Metal Alloys Used in Dentistry    Cobalt ‫ا‬chromium ‫ا‬alloy Nickel ‫ا‬chromium ‫ا‬alloy Titanium ‫ا‬and ‫ا‬titanium ‫ا‬alloys
  7. 7. Cobalt Chromium Alloys Composition: Major elements 90% by weight Minor elements 10% by weight 1. Molybdenum: 3- ‫ا %6ا‬by ‫ا‬weight. 1. Cobalt: 35-65% ‫ا‬by ‫ا‬weight. 2. Silicon and Manganese. 2. Chromium: 28- ‫ا %03ا‬by ‫ا‬weight. 3. Nickel: 0-30% ‫(ا‬It ‫ا‬is ‫ا‬used ‫ا‬interchangeably ‫ .3 ا‬Carbon: 0. ‫ا %2ا‬by ‫ا‬weight. with ‫ا‬Cobalt). It is a substitute for Type IV gold alloy
  8. 8. Nickel Chromium Alloys Composition: Minor elements 10% by weight Major elements 90% by weight 1. Nickel: 70-80% 2. Chromium: 12-20%. 1. 2. 3. 4. Molybdenum: 3- 6% Aluminum: 2-6% Silicon and Manganese. Beryllium: 0.5%. It is a substitute for Type III gold alloy
  9. 9. Role of each element: Major elements: 1- Cobalt 2- Nickel Strength Hardness Modulus of elasticity Strength Hardness Modulus of elasticity Ductility Nickel is allergic [female>male] Therefore nickel free base metal alloys has been introduced
  10. 10. 3- Chromium: It increases tarnish &corrosion resistance by formation of passive layer M [oxide layer with specific properties] M a A j o r Passive layer: a- Thin b- Uniform c- Non porous d- Adherent e-Transparent
  11. 11. Minor Elements: • To increase strength, hardness & decrease ductility: -Molybdenum as grain refiner -Carbon 0.2% as discontinuous precipitate in the grain boundaries -Aluminum reacts with nickel forming intermetallic compound which precipitates inside the solid solution alloy This causes precipitation hardening • To improve cast-ability: -Silicone & Manganese *Increase fluidity of molten alloy *Act as deoxidizer -Beryllium *Decrease the melting temperature Beryllium vapor is carcinogenic and may lead to fibrosis of the lungs. Therefore many alloys are available now free of beryllium.
  12. 12. Carbon content is very critical •If it is more than 0.2% continuous carbide precipitation at the grain boundaries Strength Hardness The alloy can not be used in dentistry Ductility
  13. 13. Avoid carbon pick up by avoid using: carbon containing investment carbon crucible for melting improper adjusted flame ordinary casting atmosphere Therefore we should use: carbon free investment ceramic crucible for melting proper adjusted flame (acetylene & oxygen) vacuum casting atmosphere
  14. 14. Pure Titanium and Titanium Alloys: Composition: 1-Pure titanium: There are 4 grades of pure titanium according to: N.B .Titanium has the property of allotropy Its atoms are arranged in different crystalline forms by changing the surrounding temperatures Oxygen 0.18- 0.4% Iron 0.2- 0.5% 885°C α Form Stabilizer is aluminum β form Stabilizers are copper, palladium and vanadium aluminum Therefore titanium alloys have been introduced to stabilize either α , ß or both to get different properties
  15. 15. CobaltNickelTitanium chromium chromium Biocompatibility Nickel allergic Gold type III IV nobility Beryllium vapor is carcinogenic Excellent due to the presence of Passive layer Excellent due to presence of noble metals Gold Resistance to tarnish & corrosion Re-passivate in nanosecond if is scratched 70% 65% Pt 1% 2% Pd 3% 3%
  16. 16. Cobaltchromium Physical properties Nickelchromium Titanium & Titanium alloys Color Lustrous silvery white ,if properly finished & polished Melting temperature Higher than gold .˙.- Need complicated & expensive technique for melting - Highly reactive to atmosphere . ˙.Vacuum casting is essential Sag resistance during -soldering -ceramometallic restoration Higher sag resistance than gold, since they have higher melting temperature Density They are lighter than gold .˙.- Difficult in casting, they need higher casting pressure. - Comfortable in upper restorations
  17. 17. Mechanical Properties Cobaltchromium Nickelchromium Modulus of elasticity Double that of gold [stiffer] Yield strength Comparable to gold Ultimate strength Comparable to gold Ductility [%elongation] Less ductile than gold Hardness .˙. used in thin section .˙.Less burnish-able than gold Harder than gold .˙.Difficult in finishing &polishing But retain this surface for longer time Titanium & Titanium alloys
  18. 18. Mechanical properties of pure Titanium are similar to gold alloy type III and IV. While those of Titanium alloy are similar to Cobalt Chromium and Nickel chromium alloys.
  19. 19. Casting Investment Melting Cobaltchromium Nickelchromium Carbon-free phosphate or silicate investment with vents Titanium With very stable oxides Oxygen Acetylene- Electric Furnace Quartz crucible Casting machines Cooling Centrifugal under vacuum Special casting machine using centrifugal + air pressure under vacuum Bench cooling
  20. 20. CobaltCasting chromium Finishing & polishing Nickelchromium Titanium & Titanium alloys No pickling because it attack the passive Difficult but retain their polished surface Special technique for finishing & polishing Sandblasting + Electrolytic Polishing Recasting Cannot be recast due to difficulties during casting [technique sensitive]
  21. 21. Cobaltchromium Microstructure Heat treatment Joining Nickelchromium Titanium &Titanium alloys Large grain The alloys can not be heat-treated because the mechanical properties are neither improved or controlled by heat treatment. By soldering using: Hard low fusing solder Fluoride containing flux [capable of removing the passive layer at the site of soldering] Anti-flux
  22. 22. Difficulties in casting titanium   Very high melting temperature when compared to other base metal alloys .˙. It needs - special melting technique [electric] - special investment [with very stable oxides] - vacuum Very low density when compared to other base metal alloys .˙. It needs high casting pressure to fill the mold
  23. 23. Shaping of Titanium [Alternative to casting]   Milling the restoration from titanium piece Spark erosion of titanium piece using electric discharge to prepare the restoration
  24. 24. Important Questions  1. 2. 3. 4. 5. 6. 7. Give reason (s) for: Dental casting alloys should be of solid solution Ductility is essential for dental casting alloys Stiffness is required in dental casting alloy Increasing platinum & palladium % in noble ceramometallic alloys Tin & Indium should be incorporated in noble ceramometallic alloys Construction of dental appliances from Type IV should be made in thick section