Abstract
Partially-stabilized zirconia is used in ceramic crowns due to its excellent mechanical properties and bio-inertness but does not match the natural color and translucency of tooth enamel. To reduce scattering of light and improve translucency, the grain size of zirconia ceramics should be less than the wavelength of visible light (0.4–0.7 μm), and porosity should be eliminated. The aim of the present work was to study the effect of two-step sintering of a commercial powder (Zpex Smile, Tosoh Corp., Tokyo, Japan) on the grain size and translucency of zirconia for use in ceramic crowns. Samples were sintered at a first step temperature (T1) of 1300, 1375 and 1400 °C for 5 min, followed by a decrease to the second step temperature (T2) and holding at T2 for 5–20 h. Samples were also conventionally sintered at 1450 °C for 2 h for comparison. Two-step sintered samples with an almost equal density, smaller grain size and narrower grain size distribution compared to conventionally sintered samples could be sintered. However, the translucency of two-step sintered samples had lower values compared to conventionally sintered samples. This is due to the slightly higher porosity in the two-step sintered samples. Density and translucency of both conventionally and two-step sintered samples could be increased further by using a ball milled powder.
Highlights
Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics are used as ceramic crowns due to their advantageous properties, such as biocompatibility, esthetics, strength, durability and ease of customization [1,2]
The two-step sintering technique relies on the difference in kinetics between grain boundary diffusion and grain boundary migration to suppress grain growth in the final stage of sintering [43]
Conventional and two-step sintering experiments were conducted on a commercial Y-TZP powder in order to prepare sintered samples with high density and small grain size
Summary
Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics are used as ceramic crowns due to their advantageous properties, such as biocompatibility, esthetics, strength, durability and ease of customization [1,2]. The optical properties of zirconia do not match those of tooth enamel, limiting their application. The esthetic properties of Y-TZP ceramics can be improved using veneering with porcelain, but this can cause a reduction in mechanical properties, due to fracture of the porcelain veneer [2,5,6,7]. This has led to research to improve the translucency of Y-TZP ceramics to allow their use as monolithic restorations without the need for porcelain veneer [8,9]. Light is scattered by porosity, by secondary phases and at grain boundaries if the material is optically anisotropic, i.e., has a Materials 2020, 13, 1857; doi:10.3390/ma13081857 www.mdpi.com/journal/materials
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