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Gold alloys-ppt


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Gold alloys-ppt

  1. 1. Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License. Copyright 2008, Dr. Stephen Bayne. The following information is intended to inform and educate and is not a tool for self-diagnosis or a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional. You should speak to your physician or make an appointment to be seen if you have questions or concerns about this information or your medical condition. You assume all responsibility for use and potential liability associated with any use of the material. Material contains copyrighted content, used in accordance with U.S. law. Copyright holders of content included in this material should contact with any questions, corrections, or clarifications regarding the use of content. The Regents of the University of Michigan do not license the use of third party content posted to this site unless such a license is specifically granted in connection with particular content objects. Users of content are responsible for their compliance with applicable law. See for more information. Mention of specific products in this recording solely represents the opinion of the speaker and does not represent an endorsement by the University of Michigan. Viewer discretion advised: Material may contain medical images that may be disturbing to some viewers.
  2. 2. GOLD CASTING ALLOYS Stephen C. Bayne University of Michigan School of Dentistry Ann Arbor, Michigan
  3. 3. INDIRECT RESTORATIONS Overview of Errors * • • • • • • • Impressions Models/Casts Waxing Investing Casting Finishing/Polishing Cementing ERRORS: 0.1 to 0.2% * ----------+1.5 to 1.7% ** -1.5 to 1.7% -----** ------ Calculation of ideal permissible error: 2 x 25 µm / 10,000 µm = 0.5% *Image source: Steve Bayne, University of Michigan, 2008 **Image source: Undetermined ** ** ** **
  4. 4. GENERAL REQUIREMENTS 0. Fabrication requirements vs final properties: 1. Physical Properties: a. Reasonably low MP (flow) b. Moderately high density (castability) → Platinum Gold Palladium Silver Copper c. = 12.45 gms/cm3 = 18.88 = 12.02 = 10.50 = 8.96 Cobalt Nickel Iron Chromium = 8.90 = 8.90 = 7.87 = 7.17 Low coefficient of thermal expansion (α )→ Tooth PFM alloys Gold alloys = 9-11 ppm/° C = 14 = 18 Amalgam = 25 Composite = 35-45 2. Chemical Properties: a. Chemical corrosion (tarnish) resistance b. Electrochemical corrosion resistance c. Solubility (solderability)
  5. 5. GENERAL REQUIREMENTS (continued) STRESS 3. Mechanical Properties: a. High E (stiffness) b. Moderately high YS and H (resistance to plastic deformation) c. Hardenable by heat treatment (retention of polish) x x Yield Strength (or Hardness) Modulus (Stiffness, Slope of Line) STRAIN 4. Biological Properties: a. Biocompatible: no toxic soluble phases b. Non-reactive in the oral environment Graph source: Steve Bayne, University of Michigan, 2008
  6. 6. REVIEW OF CORROSION 1. Types: Chemical Corrosion, Electrochemical Corrosion 2. Requirements for Electrochemical Corrosion: Anode, Cathode, Circuit, Electrolyte Images source: Steve Bayne, University of Michigan, 2008 3. Electrochemical Corrosion Categories: a. Galvanic Corrosion (macro-galvanic) b. Local Galvanic Corrosion (structure-selective corrosion) c. Concentration Cell Corrosion (crevice corrosion) d. Stress Corrosion
  7. 7. CLASSIFICATION OF ALLOYS 1. Full Gold Crown and Bridge Alloys (based on precious metals) a. ADA Classification System (see phase diagrams) (1) Type I ≥ 83% Au+ (Non-heat hardenable) -- inlay (2) Type II ≥ 78% Au+ (Non-heat hardenable) -- inlay, onlay, … (3) Type III ≥ 78% Au+ (Heat hardenable) -- onlay, crown (4) Type IV ≥ 75% Au+ (Heat hardenable) -- crown, bridge b. Effects of Alloys Components: (1) Gold (Au) → Corrosion resistance (2) Copper (Cu) → Hardness (3) Silver (Ag) → Counteract orange color of copper (4) Palladium (Pd) → Increase MP and hardness (5) Platinum (Pt) → Increase MP (6) Zinc (Zn) → Prevent oxidation during melting (O2 getter)
  8. 8. TERMINOLOGY 1. Precious Metal = containing metals of high economic value such as gold, platinum, palladium, silver, (rhodium), (iridium), (rhuthenium), and (osmium). 2. Noble Metal = a precious metal that is resistant to tarnish. This excludes “silver” by definition. 3. Low Gold Alloys = Alloys containing <75% gold (less than 50 a/o gold) which means that gold atoms represent less than every other atom. 4. Gold-substitute Alloys = precious metal alloys not containing gold. 5. Base-Metal Alloys = alloys not containing precious metals to impart their corrosion resistance.
  9. 9. Low Gold Alloys High Gold Alloys WEIGHT PERCENT GOLD 10 20 30 40 1100 1000 50 60 70 75 80 90 95 100 LIQUIDUS SOLIDUS 900 800 TEMPERATURE ( ° C ) 85 DENTAL ALLOYS 700 Random Solid Solution 600 500 400 300 Ordered Solid Solution 200 100 0 CU 10 20 30 40 Graph source: Steve Bayne, University of Michigan, 2008 50 60 70 ATOMIC PERCENT GOLD 80 90 100 AU
  10. 10. AU AU-AG-CU Tenary Phase Diagram 5 10 15 20 α 25 Type IVI 30 T Type I Type II Type III 35 40 45 50 AG α+β Graph source: Steve Bayne, University of Michigan, 2008 CU
  11. 11. COMMERCIAL EXAMPLES Graph source: Steve Bayne, University of Michigan, 2008
  12. 12. PROCESSING CYCLES CAST GOLD ALLOYS 890°C Trt alloy inve s tmen t Tm TIME Alloy LCTE = 16-18 ppm/°C Graph source: Steve Bayne, University of Michigan, 2008
  13. 13. CASTING PROBLEMS for Gold Alloys A. Distortion: 1. Margins: Probability highest in thinner portions of pattern. a. Wax Deformation: improper removal or handling of pattern. b. Premature Quenching: wait until button loses red color. c. Investment Expansion/Contraction: a b c Image source: Steve Bayne, University of Michigan, 2008
  14. 14. CASTING PROBLEMS for Gold Alloys B. Surface Irregularities: 1. 2. 3. Fine Surface Roughness: Inherent particle size of investment a. High W/P ratio increases surface roughness. b. Low W/P ratio decreases investment adaptation or flow. c. Prolonged burnout encourages investment decomposition. d. Overheating alloy encourages investment decomposition. e. Overheating alloy encourages reaction with investment. Surface Defects: a. Nodules: air bubbles trapped on the pattern during investing. (Use surfactant; paint pattern; vacuum invest; vibrate) b. Ridges or Veins: poor wetting causing water films on pattern. (Use surfactant; vacuum invest; vibrate investment carefully). Gross Surface Defects: a. Fins: cracked investment (from overheating) ALLOY Surface roughness INVESTMENT Nodules Ridges or veins Fins Image source: Steve Bayne, University of Michigan, 2008
  15. 15. CASTING PROBLEMS for Gold Alloys C. Incomplete Castings: 1. 2. Internal Porosity: due to improper solidification. a. Improper Spruing: Diameter too small or too long. b. Low Temperature: Investment or metal too cold. c. Included Gases: Contaminated gold or oxidized old gold. d. Occluded Gases: Improper burnout of pattern. Incomplete External Shape: a. Insufficient casting pressure. b. Excessive back pressure from investment. c. Suck back into sprue. Internal porosity Gas bubble Incomplete margin Image source: Steve Bayne, University of Michigan, 2008