Alloys for Cast Metal Restorations

• Crown and bridge alloys- Porcelain fused-to-metal alloys

• Porcelain surface- Metal alloy substructure - Porcelain fused metal crown

• Porcelain has good aesthetics - But microcracks tend to form at fitting surface due to large biting forces - Makes it prone to mechanical failure- Alloys withstand large stresses readily as have good mechanical properties

• Stress to cause fracture

• Stress/strain ratio- Stress required to cause change in shape

• Dimensional change experienced before fracture

• Resistance of surface to indentation or abrasion

• Strength (compressive/tensile)- Brittleness/ductility- Elastic modulus (rigidity)- Hardness is not

• Initial gradient = Elastic modulus (steeper gradient means more rigid)- Red dots at end of curve = fracture stress- If small gap between fracture stress and proportional limit = brittle- If large gap between FS and PL = ductile

Brittle = CeramicsDuctile = Alloys

• Hard so surface withstands abrasion/indentation well- Quite rigid so large stress required to cause strain- Strong so high compressive strength- Low tensile strength so tendency to form surface defects leads to fracture at low stress- Quite brittle so low fracture toughness- Not ductile- Maximum strain is approx 0.1% before fracturing

• Alloys much stronger- Much harder and more rigid- More ductile- Withstand greater degrees of permanent strain when subjected to large stresses like biting

• Metal oxide bonded to both the porcelain and the alloy

• Helps eliminate defects/cracks on porcelain surface

• Alloy supports and limits strain that porcelain experiences- More rigid so change shape very little and return to original dimensions- Helps it not reach level for brittle failure

• Both porcelain and alloy should have similar thermal expansion coefficients - Due to needing to be fired in furnace then cooling - If didn’t have similar then defects would occur- So they expand at same rate when heated and contract at same rate when cooled

• High gold alloy- Low gold alloy- Silver palladium (AgPd)- Nickel chromium (NiCr)- Cobalt chromium

• AKA good wetting- Means goo surface contact- Porcelain forms bond with metallic oxides on surface- To ensure the restoration does not fall apart in patients mouth and swallow

• Porcelain is 14ppm per degree C- To avoid setting up stresses during fusing of porcelain to alloy- So defects and micro-cracks don’t form on cooling stage

• Needs to avoid discolouration of porcelain as porcelain has good aesthetics- Ag in AgPd can produce green discolouration- Copper not used in high gold alloy for this reason

• High bond strength- Good hardness- High elastic modulus (rigid) to support porcelain and prevent fracture

• Must be higher than fusion temp of porcelain- Or creep may occur

• Gradual increase in strain (permanent) experienced under prolonged application of stress (

• Gold (Au) = 80%- Platinum/Palladium (Pt/Pd) = 14% - Silver (Ag) = 1%- Small amount Indium, Tin- No Copper (green hue)