Methods of strengthening ceramics and repair materials.pptx

in detail explanation of methods of strengthening ceramics

1. METHODS OF STRENGTHENING CERAMICS

2. MINIMIZING THE DEVELOPMENT OF STRESS RAISERS: Stress raisers are discontinuities in ceramic & metal-ceramic structures and other brittle material that cause a stress concentration in these areas. Abrupt changes in shape/ thickness in ceramic contour can act as stress raisers - Thus, the incisal line angles on anterior tooth prepared for ceramic crown should be rounded.

3. Creases/folds of platinum foil/gold foil substrate that become embedded in ceramic leave notches acting as stress raisers. PFM is considered more stronger than All Ceramic .... care must be taken to avoid large localized stresses (occlusal high points). Polishing and glazing reduces crack propagation.

4. Develop residual compressive stress within the surface of the material by thermal tempering: Most common method of strengthening. Rapidly cooling(quenching)the surface while it is hot & in the softened (molten) state. As molten core solidifies, it tends to shrink but outer skin remains rigid. The pull of the solidifying molten core, as it shrinks, creates residual tensile stresses in the core & residual compressive stresses within the outer surface.

5. Develop residual compressive stress by properly matching thermal expansion coefficients: Consider three layers of porcelain, the outer two layers of the same composition and coefficient of thermal contraction and the inner layer of a different composition with a higher coefficient of thermal contraction . When these layers are bonded together and bonded structure were allowed to cool to room temperature .The inner layer having a higher coefficient of thermal contraction would tend to contract more as it cooled . Thus, on cooling to room temperature the inner layer would produce axial and hoop compressive stresses in the adjacent outer layer which increases the fracture resistance and survival probability of the ceramic restoration.

6. Develop residual compressive stress by lon exchange method or chemical tempering: More sophisticated & effective method of developing residual stress. If a sodium-containing glass is placed in a bath of molten potassium nitrate, potassium ions in the bath, exchange places with some sodium ions in the surface of glass & remain in place after cooling. Since potassium ion is about 35% larger than sodium ion, the squeezing of potassium ion into the place of sodium creates very large compressive stress.

7. Dispersion Strengthening Reinforcing with dispersed phase of different material that is capable of hindering crack from propagating through the material . Dental ceramics containing primarily glass phase can be strengthened by increasing the crystal content Leucite Lithia disilicate Alumina Magnesia-alumina spinel Zirconia

8. Transformation toughening Zirconia (ZrO2) ceramic is a good example for this mechanism. The material is polymorph occurring in three forms: monoclinic (M) at room temperature tetragonal (T) ≥ 1170C cubic(C) ≥ 2370°C

9. Transformation from the monoclinic to the tetragonal phase is associated with a 5% volume decrease. Reversely, during cooling, the transformation from the tetragonal to the monoclinic phase is associated with a 3% volume expansion. The inhibition of these transformations can be achieved by adding stabilizing oxides ( CaO , MgO, Y203), which allow the existence of tetragonal-phase particles at room temperature.

10. When stress develops in the tetragonal structure and a crack in the area begins to propagate, the tetragonal grains transform to monoclinic grains. The associated volume expansion results in compressive stresses at the edge of the crack and extra energy is required for the crack to propagate further .

11. Minimising the number of porcelain firing cycles Several chemical reactions occur during porcelain firing temperature & increase in concentration of crystalline leucite in PFM can greatly affect the thermal contraction coefficient of porcelain. Changes in leucite content due to multiple firings can alter thermal coefficient of contraction of porcelain. If values exceed that of metal, mismatch occurs leading to failure in the bonding.

12. Minimising tensile stress through optimal design Using strong core materials with appropriate thickness. Conventional Feldspathic Porcelains should not be used as core in posterior areas, because occlusal forces can easily subject them to tensile stresses that exceed the tensile strength of core ceramic. Sharp line angles should be rounded off. Adequate amount of occlusal reduction. Adhesive cements are preferred.

13. PORCELAIN FAILURE MECHANICAL OR CHEMICAL FAILURES: 1. Metal failure: Incomplete degassing. Incorrect metal conditioner. Reused metal alloy 2-Porcelain-to-Metal Fractures: (Most common site for short-term failures to occur. Most common for base metals.) A.(adhesive failure modes) Contamination of metal surface (oxide-metal interface failure) Contamination of oxide surface (oxide-porcelain interface failure) Porcelain metal interface failure-oxide layer wasn’t formed B.(Cohesive failure modes) Oxide- oxide failure: the oxide layer is too thick, Thick oxide layers should be sandblasted prior to porcelainizing to minimize the oxide thickness.

14. PORCELAIN FAILURE A- porcelain fracture: (Most common site for long-term failures to occur. Design or fatigue problems.) Design or procedural errors: a. Too little bulk of metal b. Sharp angles in porcelain c.Improper margin design d. Inadequate framework design 2. Malocclusion or impact stresses 3. Thermal contraction incompatibility: a.Built -in stresses generate cracks at pores b. Thermal fatigue propagates cracks

15. B- Intra-Porcelain Failures: "Gray or Black" shades in porcelain color : a. Insufficient opaque porcelain b. Improper opaque firing* c.Porcelain oven contamination 2. Porcelain surface cracks: a. Improper cooling rate b. Thermal cycling c. Over-glazed or under-fired porcelain d.Improper porcelain selection 3. Porcelain surface roughness: a. Improper finishing and polishing agents b. Acid dissolution (topical fluorides )

16. PORCELAIN REPAIR SYSTEMS : Silane coupling agent + Acrylic (e.g., FUSION, George Taub Products, Inc.) B. Silane coupling agent + Composite ( e,g ., PULPDENT Porcelain Repair Kit) C. Silica particles applied through air borne particle abrasion+ silane coupling agent + Composite

17. STEPS TO BE FOLLOWED :