Methods Of Strengthening Dental Ceramics

Methods Of Strengthening Dental Ceramics

Write in detail methods of strengthening ceramic.
Or
Write methods of strengthening dental ceramics
Answer:

Methods used for strengthening of dental ceramic are based on the following mechanism:

Development of residual compressive stress:

• Ion exchange or chemical tempering
• Thermal tempering
• Thermal compatibility.

Interruption of crack propagation.

• Dispersion of crystalline phase
• Transformation toughening.

Development of Residual Compressive Stresses within the Surface of Metal:

In this method, residual compressive stresses are introduced within the surface of glass and ceramic objects. Strengthening is gained by virtue of the fact that developing of tensile stresses before any net tensile stress develops; must first negate these residual stresses.

Chemical Tempering or Ion Exchange:

• It is one of the most effective methods of introducing residual compressive stresses into the ceramic.
• In this process, the larger K⁺ ions exchange the smaller Na⁺ ions.
• When dental porcelain possessing sufficient soda (Na₂O) is immersed in a KNO₃ salt bath at 400°C for 4 hours, K⁺ ions will replace or exchange some of the Na⁺ ions located close to the surface layers.
• The K⁺ ions are 35% larger than the Na⁺ ions, creating larger residual compressive stresses in the surface of the glass subjected to this treatment.
• This surface compression results in increased strength of porcelain.

Thermal Tempering:

• On rapid cooling, the surface of the object from the molten state can introduce residual compressive stresses.
• The rapid cooling produces skin of glass surrounding a soft (molten) core.
• During solidification, the molten core tends to shrink, but the outer skin remains rigid.
• This shrinkage in the molten core creates residual tensile stress in the core and residual
  compressive stresses within the outer surface.

Thermal Compatibility: This method applies to porcelain fused metals:

• The metal and porcelain should be created with a slight mismatch in their thermal contraction coefficient.
• Typically porcelain coefficient of thermal expansion coefficient between 13.0 – 14.0 × 10 ^-6 /°C and metals have between 13.5 – 14.5 × 10 ^-6/°C.
• The difference of 0.5 × 10-6/°C in the thermal expansion between metals and porcelain causes the metal to contract slightly more than does the ceramic during cooling after firing the porcelain.
• This condition puts the ceramic under slight residual compression which makes it less sensitive to tensile stresses. ‘These are known as thermally compatible system.

Interruption of Crack Propagation through the Material:

A dispersed phase which is capable of obstructing the crack propagation through the material is reinforced into the glasses or ceramics to strengthen them. Two different types of dispersions are used to interrupt crack propagation.

Dispersion of Crystalline Phase:

• Al₂O₃ is added to glasses as a dispersed phase to strengthen them.
• Al₂O₃ is a tough crystalline material which can prevent crack propagation through them and strengthen the glass.
• The technique has found application in dentistry in the development of aluminous porcelain for porcelain jacket crowns.

Transformation Toughening:

• A crystalline material such as Partially Stabilized Zirconia is incorporated into glasses or ceramics.
• Partially stabilized zirconia is capable of undergoing change in crystal structure when placed under stress and can improve the strength.
• As the refractive index of partially stabilized zirconia is much higher than that of surrounding glass matrix results in scattering of light.
• As it passes through the bulk of porcelain, this scattering produces an opacifying effect that may not be esthetic in most restorations.