METHODS OF STRENGTHENING DENTAL CERAMICS - Dr MEENU MERRY C PAUL

METHODS OF STRENGTHENING DENTAL CERAMICS - Dr MEENU MERRY C PAUL

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  METHODS OF STRENGTHENING DENTAL CERAMICS DrMEENU MERRY C PAUL Professor-Prosthodontics Malabar Dental College
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  METHODS OF STRENGTHENING CERAMICS Select stronger & tougher ceramics  Develop residual compressive stress within the surface of the material by thermal tempering  Develop residual compressive stress by properly matching thermal expansion coefficients  Develop residual compressive stress by Ion Exchange  Dispersion Strengthening
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  6. Transformation Toughening 7.Reduce the tensile stress in the ceramic by appropriate selection of stiffer supporting materials 8. Minimize the number of porcelain firing cycles 9. Minimize tensile stress through Optimal design ……Design the ceramic FPD prosthesis to minimize the magnitude of tensile stresses &stress concentrations during function 10. Adhesively bond ceramic crowns to tooth structure
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  Minimizing the developmentof stress raisers Stress raisers are discontinuities in ceramic & metal-ceramic structures and other brittle materials that cause a stress concentration in these areas Abrupt changes in shape/ thickness in ceramic contour can act as stress raisers Thus, the incisal line angles on anterior tooth prepared for ceramic crown should be rounded
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  Minimizing the developmentof stress raisers Creases/ folds of platinum foil/gold foil substrate that become embedded in ceramic leave notches acting as stress raisers PFM is considered more stronger than All Ceramic ….care must be taken to avoid large localized stresses (occlusal high points) Polishing & glazing reduces crack propagation
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  DEVELOP RESIDUAL COMPRESSIVESTRESSES WITHIN THE SURFACE OF THE MATERIAL BY THERMAL TEMPERING …most common method of strengthening Rapidly cooling(quenching)the surface while it is hot & in the softened (molten) state As molten core solidifies,it tends to shrink, but outer skin remains rigid The pull of the solidifying molten core, as it shrinks, creates residual tensile stresses in the core & residual compressive stresses within the outer surface
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  DEVELOP RESIDUAL COMPRESSIVESTRESS BY PROPERLY MATCHING THERMAL EXPANSION COEFFICIENTS
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  DEVELOP RESIDUAL COMPRESSIVESTRESSES BY ION EXCHANGE More sophisticated & effective method of developing residual stress…..Also called CHEMICAL TEMPERING If a sodium –containing glass is placed in a bath of molten potassium nitrate, potassium ions in the bath, exchange places with some sodium ions in the surface of glass & remain in place after cooling. Since potassium ion is about 35% larger than sodium ion, the squeezing of potassium ion into the place of sodium creates very large compressive stress
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  DISPERSION STRENGTHENING Reinforce theglass with a dispersed phase of a different material that is capable of hindering a crack from propagating through the material …..Crack blocking ability of crystalline particles Dental ceramics containing primarily a glass phase can be strengthened by increasing the crystal content of LEUCITE ,LITHIA DISILICATE , ALUMINA, MAGNESIA-ALUMINA SPINELL, ZIRCONIA etc
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  TRANSFORMATION TOUGHENING Ceramics containingZIRCONIA crystals(CERCON, LAVA) undergo transformation toughening of ZrO2 from a tetragonal crystal phase to a monoclinic phase at the tips of cracks that are in the region of tensile stress On heating,Zirconia…..tetragonal form On cooling to room temp ….back to monoclinic form With addition of YTTRIUM OXIDE ,zirconia is maintained in tetragonal form at room temp YTTRIA (yttrium oxide) stabilized zirconia ceramic is called CERAMIC STEEL
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  TRANSFORMATION TOUGHENING Compressive forcesaround the crack caused by transformation & expansion of Zirconia crystals around the crack
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  MINIMIZE THE NUMBEROF PORCELAIN FIRING CYCLES Several chemical reactions occur during porcelain firing temp & increase in concentration of crystalline leucite in PFM… It can greatly affect the thermal contraction co efficient of porcelain Changes in leucite content due to multiple firings can alter thermal coefficient of contraction of porcelain If values exceed that of metal, mismatch….
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  MINIMIZE TENSILE STRESSTHROUGH OPTIMAL DESIGN Tougher & stronger ceramics …… sustains high tensile stresses Conventional Feldspathic Porcelains should not be used as core in posterior areas, because occlusal forces can easily subject them to tensile stresses that exceed the tensile strength of core ceramic Sharp line angles should be rounded off Lateral excursive & protrusive movements should be checked
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  References Phillip’s Science ofdental materials Craig’s Restorative dental materials