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 = Ceramics Ductile = 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 (<EL) • Occurs when material temp is more than half its melting point

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