Two-Step Sintering of Partially Stabilized Zirconia for Applications in Ceramic Crowns

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 (T 1 ) of 1300, 1375 and 1400 • C for 5 min, followed by a decrease to the second step temperature (T 2 ) and holding at T 2 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.Materials 2020, 13, 1857 2 of 20 non-cubic unit cell [10][11][12][13][14][15][16]. To prepare ceramics with high translucency, it is necessary to sinter to high density (>99.9% theoretical density), avoid the presence of secondary phases (particularly at the grain boundaries) and, in the case of an optically anisotropic material such as Y-TZP, to keep the grain size small with respect to the wavelength of visible light [12,13,[15][16][17]. Porosity is considered to be the main cause of scattering [10,11,13], but as pore size is related to grain size [11,18], a reduction in grain size is still expected to be helpful in reducing scattering. The translucency of Y-TZP ceramics can also be improved by lowering the amount of alumina additive (added to increase resistance to low temperature degradation [19][20][21]) and by preparing tetragonal/cubic zirconia composite materials [17,[22][23][24][25]. However, reducing alumina content and increasing the amount of cubic zirconia phase will reduce the low temperature degradation resistance and toughness of zirconia [21,24,26]. Hence, optimizing sintering parameters in order to obtain samples with high density and fine grain size is a topic of interest.The fabrication of Y-TZP ceramics is usually carried out using solid-state sintering at a high temperature (1350-1550 • C) [26][27][28]. The high sintering temperature promotes not only densification but also grain growth [29][30][31] and the formation of the cubic phase of zirconia [26,27]. Since the properties of polycrystalline ceramics are controlled by the microstructure, it is important to control the grain growth while maintaining high density. However, highly dense ceramics with micr...

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