Advanced Ceramics

Virtually Bulletproof

December 1, 2010
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Refined zirconia is being used to produce the strongest and most reliable all-ceramic dental material to date.

Partially stabilized zirconia's physical properties make it an ideal material for dental restorations.


Zirconia is a popular dental material for the creation of dental restorations such as copings for crowns, frameworks for bridges, and custom implant abutments for implant cases. Recently, this advanced material has been refined to produce the strongest and most reliable all-ceramic to date: BruxZir(r) Solid Zirconia, a full-contour zirconia restoration with no porcelain overlay.*

A failing amalgam restoration.

Properties

Also known as zirconium oxide or zirconia, zirconium dioxide (ZrO2) is commercially available in two basic forms: naturally, as the mineral baddeleyite; and synthetically, as derived from zircon sand (ZrSiO4). Zirconia powder is synthesized from zircon sand (ZrO2*SiO2) using a solid-state reaction process. Several oxides are added to zirconia to stabilize the tetragonal and/or cubic phases: magnesium oxide (MgO), yttrium oxide (Y2O3), calcium oxide (CaO) and cerium (III) oxide (Ce2O3), and others.

BruxZir is manufactured from yttria-stabilized zirconia (YSZ) powder, which exhibits superior mechanical properties such as high strength and flexibility. YSZ surpasses the strength limitations of traditional fine ceramics and has the potential for use in a variety of applications, including telecommunications, renewable energy, and dental restorations.

Partially stabilized zirconia's physical properties make it an ideal material for dental restorations. Typical zirconia materials exhibit flexural strength of more than 1200 MPa. However, because of post-powder processing, BruxZir Solid Zirconia dental restorations are able to exceed that threshold with flexural strengths of up to 1465 MPa.

The fracture toughness for partially stabilized zirconia is high because of the phase transformation toughening that occurs in the material. The toughening mechanism comes into play when a crack is encountered. The cubic grains constrain the tetragonal precipitates that want to expand and release associated energy. When these grains are faced with a propagating crack tip, the tetragonal phase is released and allowed to change back to the more stable monoclinic phase. This results in the associated volumetric expansion, effectively closing the advancing crack through a kind of self-healing event. This also means that the material features high impact resistance.

Zirconia exhibits excellent resistance to thermal shock. With its relatively low thermal expansion numbers, the material remains very stable in the mouth.

Finally, zirconia has a natural opaque white hue. Glidewell Laboratories has recorded advancements that allow zirconia to be changed into a more desirable translucent natural ivory shade, which is more lifelike than typical snow-white zirconia.

A failing amalgam restoration (from above) is replaced with a BruxZir Solid Zirconia crown.

Refining Process

The lab's scientists start with high-purity powders and create better chemistry by refining particulates via size reduction and blending. The laboratory then creates a green pre-form with very high pre-bisque firing density by using unique consolidation processes that enable the smallest particulates to be as close as possible before the machining starts. In doing so, the lab also reduces the elongation factor, which means a more accurate crown dimension. After machining, the part is sintered to full density.

The resulting material features a small grain size, which improves flexural strength and fracture toughness. As a crack moves through a material's grain boundaries, it is deflected by the material's grains. If a material has many grains to deflect and take energy out of the force of the crack, it becomes inherently stronger. A focus on smaller particulates has also created improved translucency.

Material Toughness

Brittle materials may exhibit significant tensile strength by supporting a static load, but toughness indicates how much energy a material can absorb before mechanical failure. Fracture toughness is a property that describes the ability of a material with inherent microstructural flaws to resist fracture via crack growth and propagation.

Various methods have been devised to modify the yield strength, ductility, and fracture toughness of both crystalline and amorphous materials. Fracture toughness is a quantitative way of expressing a brittle material's resistance to fracture when a crack is present. This is one of the most important properties of any brittle material for virtually all design applications. If a material has a high value of fracture toughness, it will probably undergo ductile fracture.

Transformation toughening was a breakthrough in achieving high-strength ceramic materials with a high value for fracture toughness. For the first time, a ceramic material was available with an internal mechanism for actually inhibiting crack propagation. A crack in a traditional ceramic travels all the way through the ceramic with little inhibition, resulting in immediate and brittle fracture and catastrophic failure. The partially stabilized zirconia exhibits a fracture toughness that is three to six times higher than normal zirconia and most other ceramics. Partially stabilized zirconia is so tough that it can be struck with a hammer or even fabricated into a hammer for driving nails.

These innovations led to the development of BruxZir Solid Zirconia, which is indicated for bruxers and grinders as an esthetic posterior alternative to metal occlusal PFMs or cast-metal restorations. Designed and milled using CAD/CAM technology, BruxZir is sintered for 6.5 hours at 1530øC. The final BruxZir crown or bridge emerges nearly chip-proof and is diamond polished and glazed to a smooth surface.

Another beneficial physical characteristic of BruxZir is its wear properties-diamond polishing the BruxZir crown provides long-term life for opposing enamel surfaces. This wear compatibility has been validated in enamel wear "in-vitro" studies, and clinical studies are currently under way as well.

For more information, call (800) 854-7256 or visit www.bruxzir.com.

Modified and reprinted with permission from Glidewell Laboratories' Inclusive magazine, (c)2010. BruxZir is a registered trademark of Glidewell Laboratories.


*Developed by Glidewell Laboratories

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