ABSTRACT
BACKGROUND: The brittleness and low flexural strength of ceramic material hinder, to a certain extent, its application in prosthodontic dentistry. Zirconia is enhanced in its flexural strength and toughness by the transformation toughening mechanism, which makes up for the brittleness of the traditional all-ceramic material.OBJECTIVE: To preliminarily explore a new machinable zirconia ceramic material and investigate sintering properties of dental machinable zirconia/La-monazite diphase ceramics with nano-zirconia/La-monazite diphase ceramics. DESIGN: By adjusting the composition and ratio of raw materials, and by adopting different preparation and sintering method, this study was intended to measure the related parameters and to explore the best preparation and sintering method. SETTING: Department of Prosthodontics, College of Stomatology Shanghai Ninth People's Hospital Affiliated to Shanghai Jiao Tong University; Lab of Advanced Inorganic Material Technology, School of Material Science and Engineering, Shanghai University. MATERIALS: There were 3 mol yttria-containing tetragonal zirconia polycrystals (3Y-TZP) (size≤50 nm, purity 99.99%, Yixing Xinxing Zirconia-products Co., Ltd.) and La-monazite (purity 99.99%, Baotou Rare-earth Phosphate Institution). METHODS: Experiments were performed at the Department of Prosthodontics, College of Stomatology Shanghai Ninth People's Hospital Affiliated to Shanghai Jiao Tong University from January 2004 to December 2006. The pilot experiments found that the zirconia with less than 15% of lanthanum phosphate was high in strength but poor in machinability while more than 20% of lanthanum phosphate was decreased significantly in its strength. Therefore, 15%, 18% and 20% (volume percentage) of la-monazite was added to 3Y-TZP. The green bodies of the three groups were compacted by cold isostatic pressing (200 MPa) and were sintered in air atmosphere at different temperatures: 1 560 ℃, 1 580 ℃ and 1 600 ℃ to make the ceramic samples. MAIN OUTCOME MEASURES: The volume density, porous rate (Archimedes method) and three-point bending strength (EZ-100 universal testing machine) were tested of all the ceramic samples.RESULTS: ①With the increase of sintering temperature, zirconia/La-monazite ceramics with 15%, 18% and 20% lanthanum phosphate was increased in its bulk and density. The density was the highest for 1 600 ℃ and the respective density of the three groups were 5.77 g/cm3, 5.42 g/cm3 and 5.39 g/cm3. The porous rate decreased with the increasing temperature and was the lowest at 1 600 ℃ (0.88%, 1.21%, 1.49% respectively). There was no significant difference in volume and density at different temperatures (P > 0.05). ②The flexure strength of diphase ceramic with 18% and 20% lanthanum phosphate increased with the temperature increasing to 1 580 ℃. At 1 580 ℃, the flexural strength reached the highest level, respectively (772.22±43.43) MPa, (216.03±25.20) MPa and (157.21±9.79) MPa. When the temperature reaches 1 600 ℃, the strength was decreased. CONCLUSION: Zirconia/La-monazite diphase ceramics can be prepared by adopting cold isostatic pressing (200 MPa) and sintering at 1 580 ℃.