polycrystalline dental ceramics
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Polycrystalline Dental CeramicsBy Mohamed Mahmoud Abdul-MonemDental Biomaterials DepartmentFaculty of DentistryAlexandria 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 oxideThe 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 1600C, 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.
ZirconiaZirconia 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 8090% zirconium oxide. The major impurities are usually TiO2, SiO2and Fe2O3.
This oxide exists in three different crystal structures: monoclinic at room temperature, tetragonal at ~1200C and cubic at 2370C.
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 zirconiaLow thermal conductivity (20% that of alumina)Chemically inert ant corrosion resistantFlexural strength 900 MpaFracture toughness 8-10 MPa m1/2High fracture resistanceWear of opposing dentition(Monolithic Zirconia)Difficulty in adjusting occlusion
Fracture toughness of zirconiaFracture toughness of zirconia tends to increase with increasing grain size (0.9m-1.4 m).There is a decrease in strength caused by very large grain size(1.8 m) caused by premature phase transformation leading to microcracking
Hydrothermal degradation of zirconiaHydrothermal 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 ZTA70-90% alumina10-20% zironiaToughened 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
1.Strengthening of brittle materialsa.Introduction of residual compressive stressesIon strengtheningReplacing smaller ions by relatively larger ones As a result crack growth from surface flaws is more difficultThermal compatibiltyIn 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 propagationIncorporation of crystalline phaseTough crystaline material as alumina or leucite is added to galss in a particulate form,the glass is toughened and strengthened (Dispersion tougheneing)Heat treatment cerammingA 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 raisersSharp line angles in preparation should be removed.Sharp line angles in coping surface should be avoidedSudden changes in porcelain thickness should be avoided.