MC part 2

Ceramic Types

• Conventional dental porcelain comprises a vitreous ceramic primarily formed from silica (SiO2) and potash or soda feldspar; essential for creating aesthetic dental prostheses.
• Ternary-phase diagrams (K2O-Al2O3-SiO2) illustrate composition ranges of feldspathic porcelain utilized in MC prostheses and denture teeth.
• Feldspathic porcelain components include relatively pure and colorless feldspars requiring pigments for achieving natural tooth hues and aesthetic matching to surrounding teeth.
• Incorporation of opacifiers and glass modifiers is crucial to manage fusion temperatures, sintering temperatures, thermal contraction coefficients, and solubility rates of the ceramic materials.
• The initial mixing of ingredients followed by firing results in frit, which is ground into fine powder for further applications.

Feldspathic Porcelains

• Contain a mixture of oxides: SiO2 (52-65 wt%), Al2O3 (11-20 wt%), K2O (10-15 wt%), Na2O (4-15 wt%), and various additives enhancing performance.
• Termed 'porcelain' due to the combination of glass matrix with crystalline phases, which differs from traditional porcelain derived from kaolinite.
• Specific concentrations of soda, potash, and leucite are adjusted in MC porcelains to optimize sintering temperature and thermal expansion compatibility with metal copings.
• Opaque porcelains contain high levels of metallic oxide opacifiers, minimizing the thickness of the opaque layer and concealing underlying metals.

Importance of Leucite

• Leucite formation occurs during feldspar melting at temperatures between 1150 °C and 1530 °C, leading to incongruent melting, which affects porcelain's thermal expansion behavior.
• Leucite, with a thermal expansion coefficient of 20-25 × 10–6/K, is critical for controlling thermal contraction in feldspathic porcelains.
• Distinguished from glass-ceramics, feldspathic porcelains lack controlled nucleation and crystal growth processes.

Types of Feldspathic Porcelains

• Four types are recognized: ultralow- and low-fusing ceramics, low-fusing specialty ceramics, ceramic stains, and ceramic glazes (both autoglaze and add-on glaze).
• The abrasiveness of ceramics is influenced by the size and type of crystal fillers, impacting the wear of opposing enamel when not properly managed.
• Non-essential repeated firings risk devitrification and alterations in thermal expansion properties, increasing the chances of ceramic chipping or cracking.

Ultralow-Fusing Ceramics

• Duceram LFC, introduced in 1992, marked a significant development in ultralow-fusing ceramics for metal-ceramic prostheses featuring:
  • Hydrothermal glass incorporating water to disrupt glass networks, thus reducing the glass transition and firing temperatures.
  • Claimed self-healing properties via a hydrothermal layer formation along the ceramic surface.
  • Small crystal particle size (400-500 nm) enhances opalescence, reflecting varying light hues.
• Contemporary ultralow-fusing ceramics (sintering < 850 °C) utilize mild glass phases for enamel protection and reduced abrasiveness.
• Reduced concentrations of Al2O3 along with higher CaO, K2O, Li2O, and Na2O content enable smoother and less abrasive surfaces compared to conventional porcelains.
• Lower thermal expansion coefficients and lower sintering temperatures allow compatibility with lower fusion temperature alloys like Type 2 and 4 gold alloys.