1 - Porcelain fused alloys Flashcards
Describe the properties of porcelain.
- good aesthetics
- micro cracks form at fitting surface
- prone to mechanical failure
- doesn’t withstand biting forces
Define compressive strength.
Stress to cause fracture
Define elastic modulus (rigidity).
Stress required to cause change in shape
Define brittleness/ductility.
- dimensional change experienced before fracture
- brittle = stretches 0.5% before fracture
- ductile = stretches 3.5% before fracture
Define hardness.
Resistance of surface to indentation or abrasion.
What are the mechanical properties of porcelain.
- brittle
- hard
- strong (high compressive strength, low tensile strength)
- rigid
What are the mechanical properties of alloys?
- hard
- strong
- ductile
- rigid
How are porcelain metal restorations formed?
- heated to high temperatures to form metal oxide
- alloy acts a support and limits strain that porcelain experiences
How does thermal expansion affect porcelain fused crowns?
- matching thermal expansion is vital
- means when heated and cooled the materials expand and contract at similar rates which prevents cracks
What are the different types of alloy used in porcelain fused alloys?
- high gold alloy
- low gold alloy
- silver palladium
- nickel chromium
- cobalt chromium
What properties must an alloy have to work well in a porcelain fused alloy?
- good wetting (good bond)
- similar thermal expansion to porcelain
- not discolour porcelain
- high rigidity
- high MP above fusion temperature with porcelain
What are the different suggestions of bonding for porcelain fused alloys?
- mechanical
- stressed skin
- chemical
Describe mechanical bonding in porcelain fused alloys.
Surface irregularities
Describe stressed skin bonding in porcelain fused alloys.
Slight difference in thermal contraction leads to compressive forces
Describe chemical bonding in porcelain fused alloys.
Electron sharing in oxides
