Critical effects of thermal processing conditions on grain size and microstructure of dental Y-TZP during layering and glazing

Abstract

The grain size of 3Y-TZP is critical because it must remain below a certain size in order to remain tetragonal during aging in the oral cavity. However, fabrication protocols regularly do not accommodate this critical size factor. Nevertheless, grain size measurements generally are not considered or undertaken. Further, when they are, they are thermally etched, which alters the material by grain growth. The didactic purpose of the present work is to provide a strategy for the application of suitable experimental methods for the examination of grain growth in 3Y-TZP, without the risk of grain growth during thermal etching. Examination of the effects of the different applicable thermal conditions is done in order to demonstrate the extents of grain growth in dental restorations following sintering, layering, and glazing. 3Y-TZP nanoscale powders were prepared by coprecipitation using aqueous solutions of zirconyl nitrate and yttrium nitrate, precipitated by ammonium hydroxide, and calcined at 800 °C. Uniaxially and cold isostatically pressed disks were bisque-fired at 800 °C for 2 h, after which they were planed and polished (0.1 μm). The 27 samples were sintered at 1400 ºC, 1500 ºC, or 1600 °C for 1, 2, or 4 h. They then were post-treated by annealing at 750 °C for 1 min under mechanical vacuum and then by post-annealing at 750 °C for 1 min in air. As the pre-polish was retained following sintering, SEM images could be used to determine the grain sizes by standard grain size measurement methods without having to impose thermal etching. These data then were implemented in a grain growth kinetics assessment. The pre-polishing procedure and SEM imaging were successful in enabling accurate grain size measurements without sample alteration. The grain growth as a function of temperature and time exhibited trends approximately logarithmic and linear, respectively. The diffusion mechanism was suggested to be of boundary control by a high-solubility solute in a doped system. Significantly, only sintering at 1400 °C for 1–2 h retained the grain size below the critical limit of ~ 300 nm for resistance to aging in the oral cavity. Also, significantly, ~ 7% grain growth, with a range of 2–14%, occurred during the modest thermal conditions used for both annealing and post-annealing. Sintering, annealing, and post-annealing reduce the number of grain boundaries and hence reduce light scattering, resulting in increasing translucency. Further, the use of the lower sintering temperature of 1400 °C still can yield a very high bulk density (≥ 99.62%), but with a relative increase in light scattering and reduced translucency, making this more suitable for masking dark stump shades. More critically, this sintering temperature for only 1–2 h is necessary in order to avoid risk of the tetragonal → monoclinic phase transformation during the aging process. Further, these conditions are sufficient to allow a limited number of layering and glazing procedures.