Dental Ceramics Properties

Atomic Composition of Ceramics

• Ceramics are composed of inorganic nonmetal glass phase with some crystalline fillers like Silica (Si), Zirconia (Zr), and Alumina (Al)
• The glassy phase affects translucency, but makes ceramics weaker and more prone to crack propagation
• The crystalline phase improves mechanical properties, but alters esthetics

Properties of Ceramics

• Biocompatibility
• Excellent esthetics
• High surface hardness
• Thermal insulators with a coefficient of thermal expansion close to natural tooth structure
• Mechanical and optical properties vary according to microstructure
• Brittle with good resistance to compressive stresses, but poor under tensile and shear stresses, failing by crack propagation

Classification of Dental Ceramics

• Classified by microstructure, processing method, fusing temperature, translucency, and fracture resistance
• Free-hand layering, casting, pressing, milling, and CAD-CAM are different processing methods

Development of Ceramics

• Two approaches: developing high-strength ceramics and minimizing material-related restoration failure
• High-strength glass ceramics (lithium disilicate, zirconia-reinforced lithium silicate) and partially stabilized zirconia

Thermal Contraction and Expansion

• Coefficient of thermal expansion and contraction should be considered when selecting veneering porcelain
• Mismatch between coping and veneering porcelain can leave the porcelain in residual compression, providing additional strength

Chemical Reactions and Thermal Contraction

• Chemical reactions occur during porcelain firing temperatures, affecting crystalline leucite concentration and thermal contraction coefficient
• Leucite is a high-expansion crystal phase that affects thermal contraction coefficient

Crack Propagation and Toughening

• Dispersion strengthening and transformation toughening are methods to interrupt crack propagation
• Dispersion strengthening involves adding particles to the ceramic, while transformation toughening uses yttria-stabilized zirconia

Dental Porcelain

• Consists of a glassy alumino-silicate glass matrix with crystalline phases (feldspar, quartz, alumina) embedded
• Can be classified into feldspathic porcelain, lithium disilicate-based, and zirconia-reinforced lithium silicate ceramics

Feldspathic Porcelain

• Glassy matrix with crystalline phases
• Indications: monolithic restorations, single full-coverage crowns, inlays, onlays, and veneers
• Processing methods: powder and liquid, CAD-CAM, and heat pressing

Lithium Disilicate-Based Ceramics

• High translucency and flexural strength (360 MPa)
• Indications: thin veneers, inlays, onlays, partial crowns, and minimal invasive crowns
• Methods of fabrication: pressable forms and CAD-CAM

Zirconia Lithium Silicate (ZLS) Ceramics

• Dual microstructure with lithium meta-silicate and zirconium oxide in solution
• Fine-grained, homogeneous structure with high load characteristics and easy milling and polishing
• Examples: Vita suprinity, Celtra Duo, and Celtra Press

Fluoroapatite-Based Ceramics

• Veneering material only, on zirconia or glass ceramics
• Methods of fabrication: free-hand layering and pressing

Polycrystalline Zirconia

• Most commonly used, with 95-98% zirconia crystals
• Advantages: high flexural strength (1200-1400 MPa) and transformation toughening

Types of Zirconia

• 3Y-Tetragonal Zirconia Polycrystals (TZP): first generation, used as a core only, with high strength and fracture toughness
• 2nd Generation: same microstructure as 1st gen, but with changes in grain size, sintering temperature, porosity content, and alumina content
• 3rd Generation: 4 mole% Y-PSZ, with 25% cubic phase and high translucency
• 4th Generation: 5 mole% Y-PSZ, with high translucency and flexural strength (500-800 MPa)