Zirconia

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Questions and Answers

What is the atomic number of Zirconium?

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?

Because of impurities of various metal elements Signup and view all the answers

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

To separate the impurities of various metal elements Signup and view all the answers

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

It is less expensive Signup and view all the answers

What is the main disadvantage of milling fully sintered Zirconia?

It causes wear of diamond burs Signup and view all the answers

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

They shrink by 20-25% Signup and view all the answers

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

1170 – 2370 ◦C Signup and view all the answers

What is the characteristic of the Martensitic phase transformation?

It involves coordinated shifts in lattice position Signup and view all the answers

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

To retain the tetragonal structure at room temperature Signup and view all the answers

What is the result of the tetragonal to monoclinic transformation?

A shear strain and a large volume increase Signup and view all the answers

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

It counteracts the external tensile stresses Signup and view all the answers

What is the characteristic of the Cubic phase of zirconia?

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

What is the type of zirconia commonly used in dentistry?

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

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

900-1200 MPa Signup and view all the answers

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

Easy milling without wear of the milling tools Signup and view all the answers

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

Difference in the sintering shrinkage of the framework & the enlargement values Signup and view all the answers

What is the main advantage of hard machining of zirconia?

Time saving as there is no need for sintering Signup and view all the answers

What is the main disadvantage of hard machining of zirconia?

The milling system has to be very tough & wear resistant Signup and view all the answers

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

Cercon (Dentsply International) Signup and view all the answers

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

Low thermal degradation Signup and view all the answers

What is a common problem associated with zirconia restorations?

All of the above 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?

Low thermal degradation Signup and view all the answers

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

Large grain size and low stability 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)?

Enhance mechanical properties and hydrothermal ageing resistance Signup and view all the answers

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

50% by weight Signup and view all the answers

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

Improve strength and fracture toughness Signup and view all the answers

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

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

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

650 Mpa Signup and view all the answers

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

Injection molding Signup and view all the answers

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

Cold isostatic pressing Signup and view all the answers

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

Stress accumulation due to movement of -OH Signup and view all the answers

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

Microcracks and esthetic degradation occur Signup and view all the answers

What determines the grain size of zirconia?

Sintering conditions and temperature Signup and view all the answers

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

Promotes the t→m transformation Signup and view all the answers

What is the requirement for improving stability in zirconia?

Y2O3 mol% 3.5 < x < 8µm Signup and view all the answers

What is the result of the LTD process?

Reduction in flexural strength and risk of spontaneous catastrophic failure Signup and view all the answers

What is affected by the LTD process?

Monoclinic phase content, flexural strength, and surface roughness Signup and view all the answers

What is the purpose of applying primer in zirconia?

To remove excess water after 20 sec 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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