12 Questions
Which of the following is a method of strengthening brittle materials like dental ceramics?
Minimizing stress concentrations
What is the recommended maximum connector height for posterior fixed partial dentures?
Greater than 4 mm makes the anatomic form too bulky and unaesthetic
Which type of porcelain should not be used as the core of ceramic crowns?
Feldspathic porcelains
What is the purpose of broadening the radius of curvature of the gingival embrasure portion of the interproximal connector?
To minimize stress concentrations
What is the primary purpose of designing ceramic prostheses to avoid exposure of the ceramic to high tensile stresses?
To increase the strength and durability of the restoration
What is the primary purpose of adjusting the occlusion on a porcelain surface?
Contact points rather than contact area will act as stress raisers in the porcelain surface as well as within the internal surface of the crown
Which of the following techniques involves the replacement of smaller ions with larger ions to create residual compressive stresses?
Ionic exchange strengthening (ion stuffing)
What is the primary purpose of introducing residual compressive stresses on the surface of a dental ceramic restoration?
To increase the strength and fracture resistance
In thermal tempering, how are residual compressive stresses introduced on the surface of the ceramic restoration?
By rapid cooling of the surface from the molten state
Which technique involves the addition of a dispersed phase of a different material to hinder crack propagation in dental ceramics?
Dispersion strengthening
What is the recommended difference in the coefficient of thermal expansion between the core and veneer materials to induce residual compression in the ceramic surface?
0.5 × 10^-6/°C
Which of the following strengthening techniques involves a change in the crystal structure of the material during crack propagation?
Transformation toughening
Study Notes
Methods of Overcoming Deficiencies of Ceramics
- Two categories: methods of strengthening brittle materials and methods of designing components
Methods of Designing Components
- Minimize stress concentrations by minimizing the effect of stress raisers
- Stress raisers include: • Discontinuities in ceramic structure • Abrupt changes in shape or thickness in the ceramic contour • Sharp line angles in the preparation • Unadjusted occlusion on a porcelain surface • Contact points rather than contact area on the porcelain surface and internal surface of the crown
- Optimal design of ceramic prostheses should: • Avoid exposure of ceramic to high tensile stresses • Have no sharp angles or marked changes in thickness • Not use conventional feldspathic porcelains as the core of ceramic crowns • Have appropriate connector height (greater than 4 mm can be bulky and unaesthetic) • Broaden the radius of curvature of the gingival embrasure portion of the interproximal connector
Strengthening Brittle Materials
- Induced surface compression methods: • Surface Glaze & Glass infiltration • Ionic exchange strengthening (ion stuffing) • Thermal tempering • Coefficient of thermal expansion & contraction
- Surface Glaze & Glass infiltration: • Applies a translucent, low-fusing porcelain to the surface • Fills surface defects • Can potentially fill in cracks and place the surface under compression
- Ionic exchange strengthening (ion stuffing): • Replaces smaller Na+ ions with larger K+ ions • Creates residual compressive stresses (700 MPa) in the surface of the glass • Neutralizes tensile stresses developed during service
- Thermal tempering: • Involves rapid cooling of the restorations' surface from the molten state • Introduces residual compressive stresses • Produces a skin of rigid glass surrounding the soft core • Creates residual compressive stresses within the outer surface
- Thermal compatibility: • Core and veneering porcelain should be selected with slight mismatch in thermal contraction coefficient • Difference of 0.5 × 10–6/°C in thermal expansion between core and veneer causes development of residual compression in the ceramic surface
Interruption of Crack Propagation
- Dispersion strengthening: • Reinforces ceramics with a dispersed phase of a different material • Hinders crack propagation through the material • Dental ceramics containing primarily a glass phase can be strengthened by increasing the crystalline phase
- Transformation toughening: (not mentioned in detail)
Explore methods used to overcome deficiencies in dental ceramics through strengthening techniques and component design. Learn how to minimize stress concentrations and prevent stress raisers in ceramic structures.
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