Polycrystalline Dental Ceramics

By Mohamed Mahmoud Abdul-Monem

Dental Biomaterials DepartmentFaculty of Dentistry

Alexandria UniversityEgypt

What are polycrystalline cermaics?

• Solid-sintered, monophase ceramics are materials that are formed by directly sintering crystals together without any intervening matrix to from a dense, air-free, glass-free, polycrystalline structure.

• There are several different processing techniques that allow the fabrication of either solid-sintered aluminous-oxide or zirconia-oxide frameworks.

Aluminous oxide

• The first fully dense polycrystalline material for dental applications was Procera® AllCeram alumina (Nobel Biocare) with a strength of approximately 600 MPa.

• The alumina powder is pressed and milled on a die and sintered at about 1600°C, leading to a dense coping but with approximately 20% shrinkage

• Alumina core ceramic is indicated for anterior and posterior crowns.

• Alumina cannot be acid etched to produce micromechanical retention silica-coated alumina particles thus sandblasting the surface with is required to ensure sufficient resin bonding.

Zirconia

• Zirconia has unique physical characteristics that make it twice as strong and tough as alumina-based ceramics.

• Zirconia occurs as a natural mineral called baddeleyite. This mineral contains 80–90% zirconium oxide. The major impurities are usually TiO2, SiO2 and Fe2O3.

• This oxide exists in three different crystal structures: monoclinic at room temperature, tetragonal at ~1200°C and cubic at 2370°C.

• Zirconium oxide is transformed from one crystalline state to another during firing.

• At the firing temperature, zirconia is tetragonal and at room temperature, it is monoclinic, with a unit cell of monoclinic occupying about 4.4% more volume than when tetragonal.

Zirconia phase transformation

• ZrO2 adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures.

• The volume expansion caused by the cubic to tetragonal to

monoclinic transformation induces large stresses, and these stresses cause ZrO2 to crack upon cooling from high temperatures.

• When the zirconia is blended with some other oxides, the tetragonal and/or cubic phases are stabilized.

• Effective stabilizers include magnesium oxide (MgO), yttrium oxide (Y2O3, yttria), calcium oxide (CaO), and cerium(III) oxide (Ce2O3).

• Zirconia is often more useful in its phase 'stabilized' state. Upon heating, zirconia undergoes disruptive phase changes.

• By adding small percentages of yttria, these phase changes are eliminated, and the resulting material has superior thermal, mechanical, and electrical properties.

• In some cases, the tetragonal phase can be metastable. If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion.

• This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia.

Zirconia transformation toughening

• Zirconia may be in the form of blocks that are milled to create the frameworks (CAD/CAM).

• Mostly, they are fabricated from a porous block, milled oversized by about 25%, and sintered to full density in a 4 - 6 hours cycle.

• Alternatively, fully dense blocks are milled. However, this approach requires approximately 2 hours of milling time per unit whereas milling of the porous block necessitates only 30 to 45 minutes for a three-unit bridge.

Properties of zirconia

• Low thermal conductivity (20% that of alumina)• Chemically inert ant corrosion resistant• Flexural strength 900 Mpa• Fracture toughness 8-10 MPa · m1/2

• High fracture resistance• Wear of opposing dentition(Monolithic Zirconia)• Difficulty in adjusting occlusion

Fracture toughness of zirconia

• Fracture toughness of zirconia tends to increase with increasing grain size (0.9µm-1.4 µm).

• There is a decrease in strength caused by very large grain size(1.8 µm) caused by premature phase transformation leading to microcracking

Hydrothermal degradation of zirconia

• Hydrothermal degradation of zirconia occurs between 200-400 ˚C .

• Longer exposure times at oral temperature may also degrade zirconia leading to increased surface roughness,fragmanted grains and microcracks .

Zirconia toughened alumina ZTA

• 70-90% alumina• 10-20% zironia• Toughened by a stress-induced transformation

mechanism of zirconia leading to compressive stresses within alumina.

• The strength of alumina is doubled and toughness is increase 2-4 times .

Methods of strengthening Dental ceramics

Strengthening Methods

Strengthening brittle materials

Introduction of residual

compressive stresses

Ion strenghtening Thermal compatibility

Interruption of crack propagation

Incorporation of crystalline phase

Heat treatment ceramming

Methods of designing to

Minimize tensile stress through

designAvoid stress raisers

1.Strengthening of brittle materialsa.Introduction of residual compressive stresses

Ion strengthening• Replacing smaller ions

by relatively larger ones • As a result crack growth

from surface flaws is more difficult

Thermal compatibilty• In PFM ,metals and

porcelain are designed with a slight mismatch in COTE (metal slightly higher)

• Metal contracts more on cooling

• This leaves porcelain in residual compression

1.Strengthening of brittle materialsb.Interruption of crack propagation

Incorporation of crystalline phase

• Tough crystaline material as alumina or leucite is added to galss in a particulate form,the glass is toughened and strengthened (Dispersion tougheneing)

Heat treatment ceramming• A glass cerammic material is

fabricated in vitreous or noncrystalline state and then converted to a crystalline state by heat treatment to induce partial devitrifaction

• Crystalline particles,needles or plates formed during ceramming serve to interrupt cracks

2.Design of dental restorations to :

a.Minimize tensile stress• In case of PFM ,metal

copings act as the foundation.

• The strong yet more ductile metal prevents the interior of porcelain portion of the crown from being subjected to tensile stresses.

b.Avoid stress raisers• Sharp line angles in

preparation should be removed.

• Sharp line angles in coping surface should be avoided

• Sudden changes in porcelain thickness should be avoided.

Thank you