Ceramics strengthening techniques


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Ceramics strengthening techniques

  1. 1. Ceramics strengthening techniques By Radwa El-Dessouky
  2. 2. Griffith flaws They are minute submicroscopic surface defects (scratches and cracks) present on the glass surface and act as stress concentration centers when subjected to tensile stresses
  3. 3. •Possible causes of ceramic surface micro-cracks: •CTE Mismatches between veneer and core porcelains. •Heat generation during grinding and adjustment . •the destructive, repetitive masticatory force that occurs in the oral cavity.
  4. 4. Compressive forces In case of tensile forces, the forces tend to open crack sites, resulting in crack propagation Compressive forces, however, tend to approximate the edges of surface cracks Tensile forces
  5. 5. •These imperfections may result from: Incomplete fusion of particles during sintering due to improper Firing time or temperature If one crystal is out of line or twisted as compared to its neighbor, the bonds between them may be stretched or distorted causing weakness of ceramic structure.
  6. 6. Ions of the same charge (positive or negative) may cause electrostatic repulsion leading to stresses in this region and finally cracks may occur.
  7. 7. The sizes and shapes of porcelain particles can be an important factor;
  8. 8. Thermal stresses which may develop during cooling can create internal flaws, causing the fused porcelain particles to separate at their interface porosity .
  9. 9. •Fatigue refers to the degradation of strength over time. • Clinically, ceramic crowns must function in the presence of moisture, externally from saliva and internally from a cementing agent . • Two types of loading conditions can lead to fatigue: Cyclic (repetitive) loading & static loading. In the oral environment, there is a combination of both conditions.
  10. 10. •The possible mechanism of ceramic’s fatigue: I. a chemical reaction between water molecules and glass surfaces . II. The absorbed moisture lowers the energy required for crack propagation .The pre-existing flaws grow to critical dimensions. III. Since stress concentration increases with length, crack propagation continues until the load is removed or fracture occurs.
  11. 11. Reduction of tensile stresses •Metal •Tooth •Advanced ceramic core Dispersion strengthening Bonding to
  12. 12. PFM System Fusing the porcelain to an oxide coated metal provides rigid support against propagation of ceramic surface cracks when exposing to tensile stresses •The currently used metal-ceramic systems: i. Nobel metal alloy systems: high gold, low gold and gold free Ii`Base metal alloy systems: Ni-Cr, Ti
  13. 13. Waxing-up the framework Spruing
  14. 14. Supported framework fabrication for high-gold and gold-reduced alloys Framework fabrication for palladium-based and base metal alloys Checking the ceramic space with silicon key
  15. 15. sandblasting After oxidation firing cycle
  16. 16. Application of opaquer layer (2 layers) with a brush and then, firing After ceramic veneering, firing and finishing
  17. 17. Success of Metal-ceramic systems depends on a strong bond between the metal and fused ceramic which can be; •Mechanical Interlocking: the roughened alloy surface produced by sandblasting provides irregularities into which porcelain can flow. •Van der Waal forces or "wetting bonds” Depends on surface tension of porcelain in the liquid state (its contact angle and wettability)
  18. 18. A contact angle greater than 90 degrees indicates a lack of wetting and, consequently, lack of adhesion .
  19. 19. • External surface compression : thermal expansion coefficient [CTE] of the veneered porcelain must be slightly less than that of the metal alloy. During cooling, the porcelain is held in a state of compression as shrinkage of the metal occurs. Chemical bonding: through the oxide layer at porcelain-metal interface a chemical bonding between the porcelain to metal occurs.
  20. 20. •For precious metal ceramic bond: Bonded tin foil: platinum foils are electroplated with a layer of tin oxide to which aluminous ceramic is attached •For Base metal ceramic bond: Base metals can form oxides on their surface through an oxidative firing
  21. 21. 1.Inadequate esthetic, this can be due to: 1. loss of translucency found in natural teeth. In addition, the underlying metal color often penetrates the porcelain making it appear grayer than the surrounding teeth .
  22. 22. 2. Opaque porcelains are used to mask the metal coping; however, they are highly reflective causing a less than natural appearance . In an attempt to avoid this reflection, metal-ceramic restorations are often over contoured. 2. Galvanism . 3.allergic reactions in the gingiva.
  23. 23. An all-ceramic restoration in which crown is bonded to the underlying dentin and Dentin bonded any available enamel ceramics using a composite resin–based luting material This method reinforces the ceramic structure with no need to internal strengthening mechanisms.
  24. 24. The bond is mediated through; 1.Mechanical interlocking; through sandblasting and etching [a hills and valleys microscopic appearance]. When the resin flows over the etched surface, it flows into surface imperfections, and when the resin hardens, the imperfections act as undercuts firmly bonding the resin in place. 2.Chemical bond; through application of silane coupling agent which bonds through the Si molecule to the silica in the porcelain and to the acrylic bonding agents in restoration.
  25. 25. Technique The internal surface of the sandblasted crown is etched with hydrofluoric acid After etching ,The crown is opaque white. Silane is applied to the crown
  26. 26. A gingival retraction cord is placed. The adjacent teeth are protected with Teflon tape. The tooth is etched with 37% phosphoric acid.
  27. 27. resin cement is applied gingival retraction cord is removed Final result
  28. 28. LUMINEERS A veneering system which can be fabricated so thin that tooth reduction is not usually necessary. Its structure and manufacturing resemble that of EMPRESS I.
  29. 29. •How do Lumineers differ from traditional porcelain veneers? •They are thinner. The typical Lumineers will measure on the order of 0.2 to 0.3 mm thick while the traditional veneer’s thickness ranged from 0.4 to 0.8mm. •This decreased thickness means that a dentist can bond an ultra-thin Lumineer directly to an unprepared tooth, without creating an end result that is grossly over contoured. •Lumineers ® can be placed using a "no drilling / no shots [anesthesia]" protocol.
  30. 30. •Disadvantages: •Poor esthetic as the little amount of opaquer is unable to mask the tooth shade. • over contoured or look too toothy: if nodrilling technique is applied.
  31. 31. •Indications: •The patient demands a no-drilling placement process: ex; people with dental phobias. •The patient demands a totally reversible procedure: using a no-drilling technique offers the possibility that they can be removed if the patient is unhappy with the appearance they have created . •(In-between visits to avoid the problems associated with appearance, roughness, or thermal sensitivity.)
  32. 32. high strength ceramics •Pressable ceramics (Empress I & IPS e.max) •Infiltrated ceramics (InCeram Alumina, InCeram spinnel & InCeram Zirconia) (pure a •Machinable ceramics luminous core &pure zirconia core)
  33. 33. When a tough, crystalline material such as alumina (Al2O3) is added to a glass, the glass is toughened and strengthened, because the crack cannot pass through the alumina particles as easily as it can pass through the glass matrix.
  34. 34. •The magnitude of increased strength depends on:  the crystal type; toughness and their geometrical shape.  crystal size; small crystals are better.  the inter-particle spacing; close approximation is better relative CTE to the glass matrix as a close match between the CTE of crystalline material and the surrounding glass matrix increases
  35. 35. quartz Aluminous reinforcement Lithium disilicate leucite Mica
  36. 36. Residual compressive stresses Ion exchange Thermal tempering glazing
  37. 37. I. Thermal tempering: • rapidly cooling the glass surface while the center is hot and in the molten state produces a skin of rigid glass surrounding the molten core. •As the molten core solidifies it tends to shrink. The pull of the solidifying molten core, as it shrinks, creates residual compressive stresses within the outer surface,
  38. 38. Limitations; Simple shapes are required such that uniform stresses distributions can occur, Dental restorations, however, are characterized by complex shapes, sharp angles and varying thickness.
  39. 39. 2. Glazing: •By coating the core ceramic with a thin layer of a veneering ceramic having a slightly lower (CTE). •This mismatch allows the core material to contract slightly more upon cooling; leaving the veneering ceramic in residual compression
  40. 40. 3) Ion Exchange or chemical tempering: •This process involves the exchange of larger K+ ions for the smaller Na + ions (a common constituent of a variety of glasses). •By placing the glass in a bath of molten potassium nitrate, K+ ions in the bath exchange places with some of Na + of the glass particles. The K+ is larger than the Na + . crowding of the K+ ions in place previously occupied by the smaller Na + ion creates residual compressive stresses in the surfaces of the glass .
  41. 41. Limitation the depth of the compression zone is less than 100 μm, so that this effect would be easily worn out after long– term exposure to certain inorganic acids.
  42. 42. •Proper patient selection [ex; bruxism or deep bite are contraindicated]
  43. 43. •Using strong core materials with appropriate thickness; since these stresses are distributed on the inner surface (core material is in tension). Occlusal force Compressive stresses Tensile stresses cracking
  44. 44. •Adequate amount of occlusal reduction as too little interocclusal space during tooth preparation can be a potential cause of fracture under occlusal loading
  45. 45. •The marginal designs generally accepted during ceramic crowns preparation are; •deep chamfer • flat shoulder • shoulder with rounded internal angles. •Acute angled preparations [beveled or featheredge]finish lines are to be avoided
  46. 46. •Non-uniform finish line causes the porcelain at the cervical region to vary in thickness with a potential for premature fracture during fabrication procedures, in the process of seating or after cementation. •All transitions and line angles are to be rounded to avoid stress concentrations.
  47. 47. •Accurate registration of occlusion, avoiding the premature contacts which may act as stress bearing zones on the ceramics. •Adequate cement gap (internal relief) to avoid tensile stresses exerted from excess luting cement on ceramic crown
  48. 48. •Adhesive cementation is preferred because conventional cements are strong in compression and weak in tension. In case of a FPD, The connectors are the weakest point and the most stress bearing area
  49. 49. •To reinforce the connector area: 1. Increasing the connectors height to at least 4mm. However, in the posterior region subjected to higher loads, the 2. The minimal recommended connector height connector cross section may be limited by area is 12–16 mm2 the short clinical although this may interfere molar crowns. with biological and esthetic considerations.
  50. 50. Continuous change in dimensions Sudden change Increasing the Radius of curvature at the connectors area in the gingival embrasure to 0.45 mm increases the fracture resistance as it allows the crack to propagate smoothly from the gingival embrasure toward the pontic smoothly .
  51. 51. Recommendations •Good condensation technique •Programmed firing schedule •High pressure compaction •Vacuum firing. . •Glazing. • Gradual cooling is important to avoid stresses development and cracking
  52. 52. 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 and cubic(C) ≥ 2370°C. Pure zirconia at room temperature Pure zirconia at 1170 C
  53. 53. 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.
  54. 54. The inhibition of these transformations can be achieved by adding stabilizing oxides (CaO, MgO, Y2O3), which allow the existence of tetragonal-phase particles at room temperature.
  55. 55. 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.