Ceramics 2 Flashcards
what are the uses of felspar/leucite ceramics used for?
- ceramic metal restos (mostly feldspar-based ceramics), all ceramic restos, partial dentures, and denture teeth
describe the PFM bonding mechanisms
mechanical bonding– produced by sandblasting the casting alloy before bonding to porcelain
-chemical: mostly occurs because of the mixing of the OXIDE layer on the alloy surface and the oxide of the porcelain–> initial oxide is partially eliminated by sandblaster, but when temp rises the oxide reforms; it encourages GOOD WETTING of the porcelain; at the high temp, the two oxides diffuse into one another and create a mixed oxide zone
- in gold alloys, Fe promotes iron oxide film
in other alloys, In, Sn, or Zn is added
however, if the alloy is overheated, the elements that create the oxide film are depleted –> no oxide
if an overly thick layer is created, it weakens the mixed oxide layer –> should be sandpblasted to minimize oxide film
too great a mismatch of CTE between P and M creates DEBONDING and CRAZING
- when cooling, they must SRHINK at the SAME RATE so the stressed that may compromise the bond do not form; however, the metal canhave a slightly larger thermal coeff–>porcelain will be placed under compression (griffith cracks?)
ye
ceramo-metal alloys are used for crowns and bridges
ye
what are the majoy advantages of metal ceramics?
- more conservative tooth prep (metal is only a 0.5 margin, porcelain is 1.25mm)
- stronger, tougher
- margins
- long-span bridge
- more successful in posterior restos
what are the major disadvantages of metal ceramics?
inferior esthetics–opaque
properties of lithium disilicate? (glass ceramic)
- zero porosity
- high strength
- high toughness
- translucency
- pigmentation
- opalescence
- low or negative thermal expansion
- high temp stability
- machinability
- high chemical durability
- biocompatibility
describe how glass ceramics are formed?
heat pressed
- ceramic is injected UNDER PRESSURE into the mold space then sintered
what advantages does /lithium disilicate glass ceramic have over feldspathic?
stronger than feldspathic, plus an excellent fit
what are the advantages and disadv of heat pressing lithium disilicate?
avoid large voids
produce high density ceramics with small crystals
disadv: equipment costs
describe the process and logic behind creating glass-infiltrated alumina?
- ceramic powder is built up on POROUS
- produces porous low density core or coping
- glass poweder is placed in core, thn fired–>melts and flows into pores to create HIGH DENSITY GLASS CERAMIC CORE
what is inceram spinell use for?
anterior crown, veneers, inlays and onlays; is the most TRANSLUCENT
what is inceram alumina use for?
anterior crowns, bridges, and single cfowns in the post region, high translucency
what is inceram xirconia used for?
post crowns and bridges
- moderate translucency
spinell, alumina,and zirocnia is order of increase translucency
spinell > alumina > zirconia
spinell, alumina, zirconia in order of increasing flexural strength
zirconia > alumina > spinell
what are the methods used to improve porcelain low tensile and transverse strength
- substrate reinforced
- crack arresting
- surface compression/crack closing
- griffith crack elimination
how does substrate reinforocing improve low tensile and transverse strength in porcelain?
core is VENEERED with CONVENTIONAL porcelain
how does crack arresting inmprove the low tensile and transverse strength in porcelain?
increase crystalline phase –> improves resistance to crack propagation
what is ceramming?
process of ppting crysalline phase in glassy matrix increase strength
what does toughening of ceramic depend on?
- crystal type
- size
- volume
- interparticle spacing
- CTE
why are small and uniformly dispersed particles favourable in porcelains?
if they are large and non-uniformly dispersed, they leave large large regions of unreinforced glass;
as well, the particles may not have the same CTE as the glass, so if they are large and clumped together, they may create tensile stress and could fracture the glass
how does surface compression/crack closing work?
place SURFACE of glass resto under compression-closes griffith’s cracks; for crack propagation the external force must first overcome the compressive stress;
what is griffith crack elimination?
ceramic fused and bonds to metal, eliminates cracks
what are the best types of porcelain to use in stress-bearing situations?
glass-infiltrated core porcelain
what are the best porcelains for esthetics?
feldspathic porcelain
bond strength always decreases with time
ye
what metals are added to promote oxides forming for PFMs?
- in gold alloys, Fe promotes iron oxide film
in other alloys, In, Sn, or Zn is added
