Test One: Dental Ceramics Flashcards
How can brittle materials be strengthened?
1- surface smoothing/removing surface flaws, like glazing porcelain, which is only temporary
2- residual compressive stresses
3- interruption of crack propogation
How does stress concentration determine the behavior of brittle materials?
Under tension, brittle materials are weak because the pulling forces propagate the stress through the surface cracks. Under compression these materials are strong bc the cracks are being held closed
How do residual compressive stresses strengthen brittle materials and what are ways to induce this?
The material is altered so that there are compressive forces on the surface with tension on the inside. Methods: ion exchange, thermal tempering, and CTE mismatch
Ion exchange method
Glass with sodium is placed in a potassium nitrate bath. The sodium ions are replaced by the larger potassium ions which creates compressive forces on the surface
Thermal tempering method
Outer surface of glass is cooled by jets of air, then as the inner core cools it pulls on the outer layer and compresses it–used in windshields and diving masks
CTE (coefficient of thermal expansion) mismatch method
Inner layer of high thermal expansion glass sandwiched between lower expansion glass then allowed to cool. Inner layer contracts more and puts compressive forces on the outer layers–used in pfm crowns
How can crack propagation become interrupted?
Brittle materials are reinforced with a dispersed crystalline phase that will interrupt crack propagation–ie: rip-stop nylon
How can dental restorations be designed to accommodate for ceramic weakness?
1- minimizing tensile stress, by having metal comings in pfms to reduce tension on the porcelain
2-avoiding stress concentration by rounding line angles of the prep and maintaining relatively uniform porcelain thickness
How do glass modifiers affect silica glass?
Glass modifiers are metal ions that weaken the bond between silica tetrahedra which will lower the melting temp for easier handling. Too many modifiers can reduce chemical durability
Why is a self glaze porcelain preferred over an applied glaze?
With lower firing porcelains chemical durability is reduced. Applied glazes contain more modifiers which reduces resistance to leaching by oral fluids
How is feldspar used in porcelains?
When mixed with various metal oxides and fired to high temperatures, it can form a glass which is able to flow slightly at porcelain firing temperatures and allow the porcelain powder particles to coalesce and sinter together.
How are porcelain denture teeth retained in acrylic bases?
Retention of porcelain teeth to the denture base is provided by mechanical interlocking. The anterior teeth have projecting metal pins which become surrounded with the denture base resin during processing, whereas the posterior teeth are equipped with diatoric spaces into which the denture base resin may flow.
How does oxide formation help form a bond between porcelain and metal?
Gold alloys for porcelain bonding contain some base metals. The base metals form a surface oxide which forms a chemical bond with the porcelain in addition to mechanical retention.
Two reasons for condensing porcelain
lower firing shrinkage and less porosity in the fired porcelain
Why is porcelain vacuum fired?
It minimizes porosity to increase translucency of the porcelain and maintain strength. This is performed by changing the pressure in the furnace to shrink the air bubbles within the porcelain
