Zirconia
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Questions and Answers

What is the atomic number of Zirconium?

  • 42
  • 41
  • 39
  • 40 (correct)

    • In which year was Zirconium first discovered?

  • 1790
  • 1785
  • 1789 (correct)
  • 1795

    • What is the name of the mineral that is a free oxide of Zirconium?

  • Urania
  • Zircon
  • Silicate oxide
  • Baddeleyite (correct)

    • Why can't Zircon and Baddeleyite be used as primary materials in dentistry?

    <p>Because of impurities of various metal elements</p> Signup and view all the answers

    What is the purpose of the complex and time-consuming processes involved in producing pure Zirconia powders?

    <p>To separate the impurities of various metal elements</p> Signup and view all the answers

    What is the main advantage of using green state/pre-sintered Zirconia over fully sintered Zirconia?

    <p>It is less expensive</p> Signup and view all the answers

    What is the main disadvantage of milling fully sintered Zirconia?

    <p>It causes wear of diamond burs</p> Signup and view all the answers

    What happens to the frameworks of Zirconia during the sintering process?

    <p>They shrink by 20-25%</p> Signup and view all the answers

    What is the temperature range in which the tetragonal phase of zirconia is stable?

    <p>1170 – 2370 ◦C</p> Signup and view all the answers

    What is the characteristic of the Martensitic phase transformation?

    <p>It involves coordinated shifts in lattice position</p> Signup and view all the answers

    What is the purpose of adding stabilizing oxides to pure Zirconia?

    <p>To retain the tetragonal structure at room temperature</p> Signup and view all the answers

    What is the result of the tetragonal to monoclinic transformation?

    <p>A shear strain and a large volume increase</p> Signup and view all the answers

    What is the benefit of the Stress induced transformation toughening process?

    <p>It counteracts the external tensile stresses</p> Signup and view all the answers

    What is the characteristic of the Cubic phase of zirconia?

    <p>It is stable above 2370 ◦C to melting temperature 2680 ◦C</p> Signup and view all the answers

    What is the type of zirconia commonly used in dentistry?

    <p>Yttria tetragonal zirconia polycrystal (3Y-TZP)</p> Signup and view all the answers

    What is the range of flexural strength of Yttria tetragonal zirconia polycrystal (3Y-TZP)?

    <p>900-1200 MPa</p> Signup and view all the answers

    What is the main advantage of sintering in CAD/CAM zirconia restorations?

    <p>Easy milling without wear of the milling tools</p> Signup and view all the answers

    Which of the following is a disadvantage of the CAD/CAM sintering method?

    <p>Difference in the sintering shrinkage of the framework &amp; the enlargement values</p> Signup and view all the answers

    What is the main advantage of hard machining of zirconia?

    <p>Time saving as there is no need for sintering</p> Signup and view all the answers

    What is the main disadvantage of hard machining of zirconia?

    <p>The milling system has to be very tough &amp; wear resistant</p> Signup and view all the answers

    What is an example of a CAD/CAM system that uses the sintering method?

    <p>Cercon (Dentsply International)</p> Signup and view all the answers

    What is the term for the phenomenon where zirconia is exposed to various conditions in the oral cavity?

    <p>Low thermal degradation</p> Signup and view all the answers

    What is a common problem associated with zirconia restorations?

    <p>All of the above</p> Signup and view all the answers

    What is the term for the breakdown of zirconia due to exposure to saliva, temperature changes, acidification, and cyclic loading?

    <p>Low thermal degradation</p> Signup and view all the answers

    Which of the following reasons contributed to the limited success of Magnesia partially stabilized zirconia (Mg-PSZ)?

    <p>Large grain size and low stability</p> Signup and view all the answers

    What is the primary benefit of adding CeO2 to Y-TZP in Ceria-stabilized tetragonal zirconia polycrystalline (Ce-TZP)?

    <p>Enhance mechanical properties and hydrothermal ageing resistance</p> Signup and view all the answers

    What is the minimum percentage of Al2O3 required in Zirconia-toughened alumina (ZTA)?

    <p>50% by weight</p> Signup and view all the answers

    What is the primary advantage of using Alumina-toughened zirconia (ATZ) over Y-TZP?

    <p>Improve strength and fracture toughness</p> Signup and view all the answers

    What is the composition of In-Ceram Zirconia, a type of alumina and zirconia material?

    <p>Al2O3 (62%), ZrO2 (20%), Li2O3 (12%), SiO2 (4.5%), CaO (0.8%), and other oxides (0.7%)</p> Signup and view all the answers

    What is the maximum flexural strength of In-Ceram Zirconia?

    <p>650 Mpa</p> Signup and view all the answers

    Which of the following methods is NOT used for the fabrication of Zirconia dental restorations?

    <p>Injection molding</p> Signup and view all the answers

    What is the method used to manufacture the blanks for soft machining of Zirconia?

    <p>Cold isostatic pressing</p> Signup and view all the answers

    What is the result of water vapor attacking the Zr-O bond?

    <p>Stress accumulation due to movement of -OH</p> Signup and view all the answers

    What is the effect of the presence of water on the zirconia surface?

    <p>Microcracks and esthetic degradation occur</p> Signup and view all the answers

    What determines the grain size of zirconia?

    <p>Sintering conditions and temperature</p> Signup and view all the answers

    What is the effect of lowering the stabilizer content in the grains?

    <p>Promotes the t→m transformation</p> Signup and view all the answers

    What is the requirement for improving stability in zirconia?

    <p>Y2O3 mol% 3.5 &lt; x &lt; 8µm</p> Signup and view all the answers

    What is the result of the LTD process?

    <p>Reduction in flexural strength and risk of spontaneous catastrophic failure</p> Signup and view all the answers

    What is affected by the LTD process?

    <p>Monoclinic phase content, flexural strength, and surface roughness</p> Signup and view all the answers

    What is the purpose of applying primer in zirconia?

    <p>To remove excess water after 20 sec</p> Signup and view all the answers

    Study Notes

    Zirconium and Zirconia

    • Zirconium (Zr) is a metal with atomic number 40, first discovered in 1789 by Martin Klaproth.
    • Zirconium does not occur in nature in a pure state, but can be found in conjunction with silicate oxide (Zircon) or as a free oxide (Baddeleyite).
    • These minerals cannot be used as primary materials in dentistry due to impurities and radioactive materials.

    Fabrication of Zirconia

    • Zirconia frameworks can be milled in an enlarged form to compensate for sintering shrinkage (20-25%).
    • Green state/pre-sintered zirconia is more porous, less mechanical, and less dense, whereas fully sintered zirconia is more mechanical, dense, and less porous.
    • Green state/pre-sintered zirconia is less time-consuming and less expensive, but may cause surface defects due to heat generation.

    Crystalline Phases of Zirconia

    • Monoclinic phase: stable at room temperature up to 1170°C, has lower mechanical properties.
    • Tetragonal phase: stable at 1170-2370°C, has the highest strength and fracture toughness.
    • Cubic phase: stable above 2370°C to melting temperature 2680°C, has moderate mechanical properties.
    • The lattice transformation between these phases is called Martensitic, which is reversible and temperature-dependent.

    Phase Transformation and Stress Induced Transformation Toughening

    • The addition of stabilizing oxides (CaO, MgO, Y2O3) to pure Zirconia allows the retention of the tetragonal structure at room temperature.
    • The T to M phase transformation can occur in a more beneficial way, known as Stress Induced Transformation Toughening (SIT), which increases fracture toughness.

    Classification of Zirconia Ceramics

    • Yttria Tetragonal Zirconia Polycrystal (3Y-TZP): the most common type of zirconia available in dentistry for fabrication of crowns and FPDs.
    • Magnesia Partially Stabilized Zirconia (Mg-PSZ): not successful due to presence of porosity, large grain size, and low stability.
    • Ceria-Stabilized Tetragonal Zirconia Polycrystalline (Ce-TZP): has improved mechanical properties, hydrothermal aging resistance, and fracture toughness.
    • Zirconia-Toughened Alumina (ZTA) and Alumina-Toughened Zirconia (ATZ): developed to reduce grain size, increase critical stress, and improve strength.

    Machining of Zirconia

    • CAD/CAM zirconia dental restorations can be fabricated by either soft machining of pre-sintered blanks or hard machining of fully sintered blanks.
    • Soft machining: advantages include easy milling without wear of the milling tools, but disadvantages include time-consuming and sintering shrinkage differences.
    • Hard machining: advantages include time-saving and superior fit, but disadvantages include high hardness and wear of the milling tools.

    Problems with Zirconia

    • Low thermal degradation: a phenomenon where zirconia is exposed to various conditions in the oral cavity, decreasing its mechanical strength.
    • Esthetic problem: water vapor attacks the Zr-O bond, breaking it and leading to a stress accumulation, causing microcracks and esthetic degradation.
    • Bonding to zirconia: a problem that affects the monoclinic phase content, flexural strength, and surface roughness.
    • Chipping of veneering porcelain: another problem associated with zirconia.

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