Dental Materials PDF

NATURE OF METAL AND ALLOY DR.WAYEL MOHAMMED HURAIB  Metal De nition A solid material which is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity (e.g. iron, gold, silver, and aluminium) fi ...

NATURE OF METAL AND ALLOY DR.WAYEL MOHAMMED HURAIB  Metal De nition A solid material which is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity (e.g. iron, gold, silver, and aluminium) fi  NOBLE METALS Noble metals have been used for inlays, crowns and FDPs because of their resistance to corrosion in the mouth. Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are the eight noble metals. However, in the oral cavity, silver can tarnish and therefore is not considered a noble metal.  BASE METALS These are non-noble metals. They are important components of dental casting alloys because of their in uence on physical properties, control of the amount and type of oxidation and their strengthening effect. Examples: Chromium, cobalt, nickel, iron, copper, manganese. fl  Metal Restorations Metal restorations and prostheses are an integral part of dentistry. Metals are among the strongest materials and provide strength and durability to any structure.  Metal Restorations There are two ways of constructing a metal restoration: I- Direct : have been used in modern dentistry since the introduction of direct lling gold and amalgam in the 19th century. II- Indirect : were introduced into the dental profession with the patenting of the centrifugal casting machine and the lost wax technique by William H. Taggart in 1907. fi  Alloy De nition De ned as a metal containing two or more elements, at least one of which is metal and all of which are mutually soluble in the molten state. fi fi  What is the meaning of Alloys ? Noble metal content is associated with greater corrosion.  CLASSIFICATION OF DENTAL CASTING ALLOYS ACCORDING TO USE A. Alloys for all metal and resin veneer restorations (e.g. inlays, posts, resin and composite veneered crowns and FDPs). B. Alloys for metal-ceramics restorations (e.g. PFM crowns and FDPs). C. Alloys for removable dentures (e.g. RPD frames and complete denture bases).  CLASSIFICATION OF DENTAL CASTING ALLOYS Proper Custom Impression Tray Extension CLASSIFICATION OF DENTAL CASTING ALLOYS BASED ON YIELD STRENGTH AND PERCENT ELONGATION Type I Soft Type II Medium Type III Hard Type IV Extra-hard  CLASSIFICATION OF DENTAL CASTING ALLOYS ACCORDING TO NOBILITY (ADA 1984) High noble metal alloys (HN) Contains > 40 wt% Au and > 60 wt% noble metals Noble metal alloys (N) Contains > 25 wt% of noble metals Base metal alloys Contains < 25 wt% of noble metals  CLASSIFICATION OF DENTAL CASTING ALLOYS CLASSIFICATION ACCORDING TO HARDNESS Small inlays, Class III and Class V cavities which are not subjected to great TYPE I SOFT stress. TYPE II MEDIUM Abutments, pontics, full crowns. Inlays, crowns and bridges, situations where there may be great stresses TYPE III HARD involved. They usually can be age hardened. Inlays subjected to very high stresses, partial denture frameworks and long TYPE IV EXTRA-HARD span bridges. They can be age hardened.  CLASSIFICATION OF DENTAL CASTING ALLOYS ACCORDING TO MAJOR ELEMENTS A. Gold alloys B. Silver alloys C. Palladium alloys D. Nickel alloys E. Cobalt alloys F. Titanium alloys  FUNCTIONS OF CONSTITUENTS The lost wax casting technique has been used since ancient times to convert wax patterns to cast metal.  GENERAL REQUIREMENTS OF CASTING ALLOYS 1. They must not tarnish and corrode in the mouth. 2. They must be suf ciently strong for the intended purpose. 3. They must be biocompatible (nontoxic and nonallergenic). 4. They must be easy to melt, cast, cut and grind (easy to fabricate). fi  GENERAL REQUIREMENTS OF CASTING ALLOYS 5. They must ow well and duplicate ne details during casting. 6. They must have minimal shrinkage on cooling after casting. 7. They must be easy to solder. fl fi  GOLD ALLOYS (FOR ALL-METAL RESTORATIONS) Pure gold is a soft and ductile metal and so is not used for casting dental restorations and appliances in its pure state. It is alloyed commonly with copper, silver, platinum, nickel and zinc. Alloying gold with these metals not only improves its physical and mechanical properties but also reduces its cost.  FUNCTIONS OF CONSTITUENTS Gold It provides tarnish and corrosion resistance and has a desirable appearance. It also provides ductility and malleability.  FUNCTIONS OF CONSTITUENTS Copper It is the principal hardener. It reduces the melting point and density of gold. It gives the alloy a reddish color. It also helps to age harden gold alloys.  Proper FUNCTIONS Custom Impression OF CONSTITUENTS Tray Extension Silver It whitens the alloy, thus helping to counteract the reddish color of copper. It increases strength and hardness slightly.  FUNCTIONS OF CONSTITUENTS Platinum It increases strength and corrosion resistance. It also increases melting point and has a whitening effect on the alloy. It helps reduce the grain size.  FUNCTIONS OF CONSTITUENTS Palladium It is similar to platinum in its effect. It hardens and whitens the alloy. It also raises the fusion temperature and provides tarnish resistance. It is less expensive than platinum.  PROPERTIES OF GOLD ALLOYS COLOR Traditionally, the gold alloys were gold colored. The color of modern gold alloys can vary from gold to white. It depends on the amount of whitening elements (silver, platinum, palladium, etc.) present in the alloy.  PROPERTIES OF GOLD ALLOYS MELTING RANGE Ranges between 920–960 °C. The melting range of an alloy is important. It indicates the type of investment required and the type of heating source needed to melt the alloy.  PROPERTIES OF GOLD ALLOYS YIELD STRENGTH The yield strength for : Type III — 207 MPa Type IV — 275 MPa  PROPERTIES OF GOLD ALLOYS HARDNESS The hardness indicates the ease with which these alloys can be cut, ground or polished. Gold alloys are generally more user friendly than the base metal alloys which are extremely hard. The hardness values Type III — 121 MPa Type IV — 149 MPa  PROPERTIES OF GOLD ALLOYS ELONGATION It indicates the ductility of the alloy. A reasonable amount is required especially if the alloy is to be deformed during clinical use, e.g. clasp adjustment for removable partial dentures, margin adjustment and burnishing of crowns and inlays. Type I alloys are easily furnished. Alloys with low elongation are very brittle. Type III—30–40% Type IV—30–35%.  PROPERTIES OF GOLD ALLOYS MODULUS OF ELASTICITY This indicates the stiffness/ exibility of the metal. Gold alloys are more exible than base metal alloys fl fl  PROPERTIES OF GOLD ALLOYS TARNISH AND CORROSION RESISTANCE Gold alloys are resistant to tarnish and corrosion under normal oral conditions. This is due to their high noble content.  PROPERTIES OF GOLD ALLOYS CASTING SHRINKAGE All alloys shrink when they change from liquid to solid. The casting shrinkage in gold alloys is less (1.25–1.65%) when compared to base metal alloys.  PROPERTIES OF GOLD ALLOYS CASTING SHRINKAGE The shrinkage occurs in three stages: 1. Thermal contraction of the liquid metal. 2. Contraction of the metal while changing from liquid to solid state. 3. Thermal contraction of solid metal as it cools to room temperature. Shrinkage affects the t of the restoration. fi  PROPERTIES OF GOLD ALLOYS BIOCOMPATIBILITY Gold alloys are relatively biocompatible.  PROPERTIES OF GOLD ALLOYS CASTING INVESTMENT Gypsum-bonded investments can be used for low fusing gold alloys.