SINGLE TOOTH CAST RESTORATIONS: Fixed Prosthodontics Page 1 Lecture: "Dental Gold Casting Alloys" INTRODUCTION AND REVIEW: A. General Requirements for Casting Alloys: 0. Fabrication requirements vs final properties: 1. Physical Properties: a. Reasonably low melting point (Flow) b. Moderately high density (Castability) Platinum = 21.45 gms/cm 3 Palladium = 12.02 Cobalt = 8.90 Silver = 10.50 Nickel = 8.90 Iron = 7.87 Gold = 18.88 Chromium = 7.18 Copper = 8.96 c. Low coefficient of thermal expansion (α) Tooth = 9-11 ppm/ °C Gold Alloys = 18 Amalgam = 25 PFM Alloys = 14 Composite = 25-45 2. Chemical Properties: a. Chemical corrosion (tarnish) resistance b. Electrochemical corrosion resistance c. Solubility (solderability) 3. Mechanical Properties: a. High E (stiffness) b. Moderately high YS and hardness (resistance to plastic deformation) c. High percent elongation (burnishability) d. Hardenable by heat treatment (retention of polish) 4. Biological Properties: a. Biocompatible: no toxic soluble phases b. Non-reactive in the oral environment B. Review of Corrosion of Metals: 1. Types: Chemical corrosion, Electrochemical corrosion 2. Requirements for Electrochemical Corrosion = Anode,Cathode, Circuit, Electrolyte 3. Electrochemical Corrosion Types: a. Based on electrode variations: (1) Galvanic corrosion (macro-galvanic) (2) Local galvanic corrosion (micro-galvanic, structure-selective corrosion) b. Based on electrolye variations: (3) Concentration cell corrosion (crevice corrosion) c. Based on stress variations: (4) Stress corrosion
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SINGLE TOOTH CAST RESTORATIONS: Fixed Prosthodontics Page 1 Lecture: "Dental Gold Casting Alloys" INTRODUCTION AND REVIEW: A. General Requirements for Casting Alloys: 0. Fabrication requirements vs final properties: 1. Physical Properties: a. Reasonably low melting point (Flow) b. Moderately high density (Castability)
2. Chemical Properties: a. Chemical corrosion (tarnish) resistance b. Electrochemical corrosion resistance c. Solubility (solderability) 3. Mechanical Properties: a. High E (stiffness) b. Moderately high YS and hardness (resistance to plastic deformation) c. High percent elongation (burnishability) d. Hardenable by heat treatment (retention of polish) 4. Biological Properties: a. Biocompatible: no toxic soluble phases b. Non-reactive in the oral environment B. Review of Corrosion of Metals: 1. Types: Chemical corrosion, Electrochemical corrosion 2. Requirements for Electrochemical Corrosion = Anode,Cathode, Circuit, Electrolyte 3. Electrochemical Corrosion Types: a. Based on electrode variations: (1) Galvanic corrosion (macro-galvanic) (2) Local galvanic corrosion (micro-galvanic, structure-selective corrosion) b. Based on electrolye variations: (3) Concentration cell corrosion (crevice corrosion) c. Based on stress variations: (4) Stress corrosion
CLASSIFICATION, DEFINITIONS, AND TERMINOLOGY: A. Classification for Dental Casting Alloys: 1. Full Gold Crown and Bridge Alloys (based on precious metal content): a. ADA Classification System (see phase diagrams on next page): (1) Type I ≥ 83% Au+ (Non-Heat Hardenable) -- inlay (2) Type II ≥ 78% Au+ ( " " " ) -- inlay, onlay, crown (3) Type III ≥ 78% Au+ (Heat Hardenable) -- onlay, crown (4) Type IV ≥ 75% Au+ ( " " ) -- crown, bridge b. Effects of Alloying Components: (1) Gold (Au) → (2) Copper (Cu) → (3) Silver (Ag) → (4) Palladium (Pd) → (5) Platinum (Pt) → (6) Zinc (Zn) → 2. Other Crown and Bridge Casting Alloys: a. ADA Classification System: (1) High gold alloys (IMMUNE): (2) Low gold alloys(SEMI-IMMUNE): (3) Gold-substitute alloys (PASSIVE): (a) Ag-Pd (b) Pd-X (4) Base metal alloys (PASSIVE): (a) Ni-Cr (with and without Be) (b) Co-Cr (c) Fe-Cr (5) Titanium alloys (PASSIVE): (6) Other alloys (Al Bronzes, Brasses) (ACTIVE): b. Effects of Alloying Components in Other Alloys: (1) Cr, Ti → (2) Ni, Co, Fe → B. Terminology:
1. Precious Metal = containing metals of high economic value such as gold, platinum, palladium, silver, (rhodium), (iridium), (rhuthenium), and (osmiun).
2. Noble Metal = a precious metal that is resistant to tarnish. This excludes silver by
this definition.
3. Low Gold Alloys = Alloys containing less that 75% gold (less than 50 atomic percent 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
A. Distortion: 1. Margins: Probability highest in thinner portions of pattern. a. Wax Deformation: During improper removal or handling of pattern. b. Investment Expansion/Contraction c. Premature Quenching: Wait until button loses red color. B. Surface Irregularities: 1. Fine Surface Roughness: Due to inherent particle size of investment material. 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. 2. Surface Defects: a. Nodules: caused by air bubbles trapped on the pattern during investing. (Use surfactant; paint pattern; vacuum invest; vibrate investment) b. Ridges or Veins: caused by poor wetting causing water films on pattern. (Use surfactant; vacuum invest; vibrate investment carefully). 3. Gross Surface Defects:
a. Fins: caused by cracked investment (from overheating) being penetrated by the casting alloy.
C. Incomplete Castings: 1. 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. 2. Incomplete External Shape: a. Insufficient casting pressure. b. Excessive back pressure from investment. c. Suck back into sprue.
MULTIPLE CHOICE STUDY QUESTIONS: 1. What is the melting temperature for gold alloys for casting inlays? a. 740 °C b. 800 °C c. 820 °C d. 888 °C e. 930 °C 2. The advantage of high density for casting alloys is: a. Generation of high centrifugal casting pressures. b. Reduced specific heat for rapid melting. c. Better flow in the mold space on the investment. d. Less turbulence in the sprue channel. e. More resistance to penetration into the investment material. 3. The coefficient of thermal expansion and contraction for most cast metal alloys is
in the range of: a. 5 ppm/ °C b. 10 ppm/ °C c. 14 ppm/ °C d. 18 ppm/ °C e. 25 ppm/ °C 4. Which one of the following causes tarnishing of gold alloys intraorally? a. Sulphide ions b. Chloride ions c. Mercury d. Fluoride ions e. Phosphoric acid 5. The burnishability of margins requires which ONE of the following mechanical
properties? a. High modulus b. High yield strength c. High hardness d. High percent elongation e. High toughness 6. For optimal electrochemical corrosion resistance, how many phases should be
present in the cast structure? a. 1 phase b. 2 phases c. 1 high gold phase and any other phase d. As many as possible e. It does not matter 7. Which ONE of the following types of corrosion does NOT affect gold casting
alloys? a. Galvanic corrosion b. Local galvanic corrosion c. Concentration cell corrosion (crevice corrosion) d. Stress corrosion
8. What is the basis of the ADA classification system for gold casting alloys? a. Noble metal content b. Gold content c. Copper content d. Platinum content e. Precious metal content 9. Which one of the following elements has a major effect on the melting temperature
of a gold casting alloy? a. Au b. Pt c. Fe d. Cu e. Zn 10. Which one of the following elements has the greatest positive effect on the
corrosion resistance of a gold casting alloy? a. Au b. Pt c. Cu d. Zn e. In 11. Which one of the following elements is added to gold casting alloys as an oxygen
scavenger during the alloying process? a. Au b. Pt c. Fe d. Cu e. Zn 12. Which one of the following alloying elements in non-gold containing casting alloys
does not contribute to oxide production during casting? a. Cr b. Ti c. Co d. Ni e. Pt 13. Which one of the following elements is not included under the definition for noble
metals? a. Gold b. Silver c. Platinum d. Palladium e. Rhodium 14. Which one of the following is not a base metal system? a. Fe-Cr b. Ni-Cr c. Ti d. Pd-Ag e. Co-Cr
15. Which one of the following categories of casting alloys is based on optimal IMMUNE corrosion behavior of the materials?
a. High-gold casting alloys b. Low-gold casting alloys c. Gold-substitute alloys d. Base metal alloys e. Titanium alloys 16. What is the HARDENING MECHANISM that is available for Type III and IV gold
casting alloys? a. Work hardening b. Spinoidal decomposition c. Disorder-order hardening d. Precipitation hardening e. Solution hardening 17. Type IV gold casting alloys with two phases are harder, but also run the risk of
