![]() Currently, full-contoured (anatomical-shaped) monolithic zirconia dental restorations are offered, which could abbreviate or extinguish the dental laboratory work on zirconia-based restorations. Most dental zirconia systems indicate structural dyeing (coloring) to enhance the esthetic. Traditionally, zirconia is dull white in color and its opacity can mask the underneath structure. Most importantly, the CAD-CAM technique has the ability to produce zirconia restorations with sufficient precision for dental use. Both CAD-CAM processes have three main steps: acquisition of digital data, computer processing and designing, and fabrication of the zirconia structure. In the other method, the zirconia structure is milled from a pre-sintered block, reaching its final mechanical properties after sintered, which produces structural shrinkage that can be partly compensated at the designing stage, and the fit of the zirconia restoration will be warranted. The disadvantages are the great wear of the grinding tools (burs) and the population of flaws produced during the machining that may lower the mechanical reliability of the structure. One method mills the fully sintered block of zirconia with no distortion (shrinkage) to the final structure. Zirconia structures used for dental purposes are fabricated using CAD-CAM (computer-aided design and computer-aided manufacturing) technology in two possible ways. In this case, the 4% volume increase becomes beneficial, essentially squeezing the crack to close and increasing toughness, known as transformation toughening. It turned out that the highly localized stress ahead of a propagating crack is sufficient to trigger zirconia grains to transform in the vicinity of the crack tip. Metastable means that trapped energy still exists within the material to drive it back to the monoclinic phase. Consequently, zirconia-based ceramics used for biomedical purposes typically exist as a metastable tetragonal partially stabilized zirconia (PSZ) at room temperature. ![]() This effect has been attributed to a surface energy difference. However, another way of stabilizing the tetragonal phase at room temperature is to decrease the crystal size (the critical average grain size is <0.3 μm). The tetragonal phase is stabilized at lower dopant concentrations than the cubic phase. It is also important to consider that the stabilization of the tetragonal and cubic structures requires different amounts of dopants (stabilizers). So, as the monoclinic phase does not form under normal cooling conditions, the cubic and tetragonal phases are retained, and crack formation, due to phase transformation, is avoided. Ceria (CeO 2), yttria (Y 2O 3), alumina (Al 2O 3), magnesia (MgO) and calcia (CaO) have been used as stabilizing oxides. This could result in the formation of ceramic cracks if no stabilizing oxides were used. During this zirconia phase transformation, the unit cell of monoclinic configuration occupies about 4% more volume than the tetragonal configuration, which is a relatively large volume change. The zirconia tetragonal-to-monoclinic phase transformation is known to be a martensitic transformation. On cooling, the transformation from the tetragonal to the monoclinic phase starts at 1052 ☌, peaks at 1048 ☌, and finishes at 1020 ☌, exhibiting a hysteresis behavior. Upon heating, the monoclinic phase of zirconia starts transforming to the tetragonal phase at 1187 ☌, peaks at 1197 ☌, and finishes at 1206 ☌. Yttria is added to stabilize the crystal structure transformation during firing at an elevated temperature and improve the physical properties of zirconia. ĭental zirconia is, most often, a modified yttria (Y 2O 3) tetragonal zirconia polycrystal (Y-TZP). Incidentally, Zr and Ti are two metals commonly used in implant dentistry, mostly because they do not inhibit the bone forming cells (osteoblasts), which are essential for osseointegration. Zirconium (Zr) is a very strong metal with similar chemical and physical properties to titanium (Ti). ![]() Zirconia (zirconium dioxide, ZrO 2), also named as “ceramic steel”, has optimum properties for dental use: superior toughness, strength, and fatigue resistance, in addition to excellent wear properties and biocompatibility. Currently, zirconia-based ceramics are the most studied, challenging researches for different reasons. The most popular dental ceramic systems are silica-, leucite-, lithium disilicate-, alumina-, and zirconia-based materials.
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