ceramics; mahalaxmi

Question 1

Define ceramics

Answer 1

Dental ceramics are nonmetallic, inorganic
materials primarily containing compounds of
one or more metallic or nonmetallic oxides,
borides, carbides, and nitrides, as well as complex mixtures of these materials.

Question 2

Give the classification of dental ceramics

Answer 2

Based upon:

1. Composition: Feldspathic porcelain, leucite
   reinforced porcelain, aluminous porcelain, alumina, glass infiltrated alumina, glass infiltrated spinel, glass infiltrated zirconia, and glass ceramic.
2. Fusion temperature: High (1315°C–1370°C), medium (1090°C–1260°C), low (870°C–1065°C), and ultra-low (<850°C) fusing.

Question 3

Describe all-ceramic materials in detail

Answer 3

All-ceramic materials have a greater amount of crystalline phase (35–99 vol%). This increased
crystallinity is responsible for improved mechanical properties owing to their contribution as crystalline reinforcement and stress-induced transformation. On the other hand, increased crystallinity also increases the opacity of the material, which is not a desirable property for a tooth-colored restorative material.
Hence, these materials are used as a core material over which a more translucent veneer porcelain is layered to create the final morphology and shade of the restoration. The increased crystal content combined with the much smaller crystal size provides improved flexural strength.

Question 4

Give reasons for low tensile strength of ceramics

Answer 4

Griffith’s microcracks
The covalent bonds in porcelain should actually produce much greater strength. But it is well known that porcelain breaks easily. This low tensile strength is attributed to the presence of many microcracks (called Griffith’s microcracks, named after their discoverer) which with their large radius at the tip causes large
tensile stresses at their tips leading to crack
propagation.

Question 5

Discuss the methods to strengthen ceramics.

Answer 5

Strengthening of ceramics can be accomplished by either of the two methods.

1. Development of residual compressive stresses within the surface of the material
2. Interruption of crack propagation through the material.

DEVELOPMENT OF RESIDUAL COMPRESSIVE STRESSES WITHIN THE SURFACE OF THE MATERIAL

1.1 Development of residual compressive
stresses.
1.2 Reduced number of firing cycles.
1.3 Optimal design of prostheses.
1.4 Ion exchange
1.5 Thermal tempering

INTERRUPTION OF CRACK PROPAGATION THROUGH THE MATERIAL

2. 1 Dispersion strengthening.
3. 2 Transformation toughening