Ceramics 1 Flashcards
describe the different types of porosity
smooth-surfaced porosity: DECREASES strength in DIRECT PROPORTION to the amount present (10% porosity = 10% decrease)
- rough-surfaced porosity:can decrease strength due to stress conc. effects–> 10% porosity decrease in strength
why should porcelain not be used in stress-bearing areas? how do we compensate for this?
is brittle and has relatively low tensile properties; to compensate for this, it must be supporting eg by metal coping;
- a metal cast where the strength of the resto is derived from; however this leads to esthetic problems (metallic shade, less translucent
- OR reinformed porcelain core e.g. aluminous porcelain for jacket crown; is TWICE as resistant to fractures compared to unsupported; but light transmission is limited due to the presence of the aluminous crystals
- however this is not good for post restos as it does not provide enough strengths
how can we close griffith cracks?
- glazing
- place the resto under compression–they close when compressed; for the crack to propagate the external force must FIRST OVERCOME the compressive strength; will result in an increase in ceramic strength
what are the methods of surface compression to close griffith’s cracks?
- ion exchange strengthening– larger ions, such as K+, are substituted for smaller ions such as Na by immersing cermaic in sol’n containing K; then heat to allow the K to diffuse in and the Na to diffuse out –> cause ceramic on surface to be compression
- thermal tempering - rapidly cooling the surface and SLOWLY cooling the core; surface will be placed under compression; can also use a layer of porcelain on surface with slightly lower CTE–>will expand less than core
- transformation toughening – manipulation of microstructure of zirconia; changes the way atoms are stacked at different temps (polymorphic transformation); zirconia, at room temp, has a monoclinic structure; above 1150 it converts to its tetragonal form; accompanied by shrinking during heating and equivalent expansion during cooling (SHRINK during HEATING and EXPAND during COOLING)
describe alumina sintered ceramics (properties)
-alumina dispersed in glassy ceramics
- alumina and matrix are of similar CTE
- bonds well to matrix
- flexural strength high)
describe leucite sintered ceramics
- contains KAlSi2O6; has up to 45% tetragonal leucite ;
has increased compressive and flexural strength
has increased THERMAL contraction
describe the process of making HEAT PRESSED ceramics
ceramic is injected uner waxes mold and sintered; then the ingots are slowly heated to 1150 then pressure is applied continuously–feeds more material into the mold space to COMPENSATE FOR SINTERING SHRINKING
properties: stronger than feldspathic glass; also has an excellent fit
system advantages: –avoid large voids, produce high density ceramics with small crystals
disadv: equipment cost and LOW STRENGTH of leucite type
describe the process of creating slip-casting
- condense aq porcelain on refractory die
- the porosity in the refractory die helps condensation by absorbing the water from the slip by CAPILLARY ACTION
- -fired at high temp on refractory die
- the fired porous core is then glass-infiltrated
- molten glass is drawn into the pores by capillary action at high temp
adv: reduced porosity, fewer defects from processing, and igher toughness than conventional feldspathic porcelain
what does CAD CAM stand for?
Computer
Assisted
Design
Computer
Assisted
Manufacturing
CAD/CIM where I = integrated
typical properties of CAD CAM ceramics?
low ductility
fracture toughness is TWICE of feldspathic porcelain–>less prone to fracture
describe the effects of topical fluorides on ceramics
acidulated phosphate fluoride contains a WEAK SOL’N of HF acid that etch ceramic; repeated applications will result in:
- loss of surface luster
- increased plaque retention
- discoloration from staining
porcelains are INSOLUBLE and INERT in the oral environment, but are readily etched by HF
ye
what is the pH of acidulated phosphate fluoride?
ph 3
what are the major advantages of dental ceramics?
- biocompatible
- chemical inert
- wear resistant
- esthetic
what are the major disadvantages of ceramics?
- difficult to fabricate
- expensive (related to fabrication)
- abrasive to natural teeth
- high clinical failure rates
