The Strength and Hydrothermal Stability of Y-TZP Ceramics for Dental Applications

Effects of dental grinding, sandblasting, fatigue, and aging on the biaxial flexural strength of yttria partially stabilized tetragonal zirconia ceramics were evaluated. Grinding lowers the mean strength and reliability under static and cyclic loading. In contrast, sandblasting provides a powerful tool for surface strengthening; it also hinders the propagation of the diffusion-controlled transformation during exposure to aqueous environments. By introducing moderate porosity, a reasonable compromise between the mechanical strength and the elastic modulus has been achieved. Such biscuit-sintered specimens also exhibited full hydrothermal stability. Novel experimental dental root posts were designed that exhibit certain advantages over the zirconia posts already in clinical use.     

髋关节整体替代陶瓷材料

许多材料在医学领域应用广泛,例如,整体替换硬组织或软组织的元件(如骨盆、骨头、关节、植牙等)、修补、诊断或矫正仪器(如起搏器、心脏阀等)。这些材料不仅要有好的力学性能,还要保持长期稳定,不能与人体相排斥。由于陶瓷材料在生理环境中具有强度高、生物相容性强和稳定性好的优点,人们研究用陶瓷材料替换骨骼。从20 世纪70 年代起,欧洲人用陶瓷组件置换整个髋关节。这些组件主要由氧化铝和氧化锆单体制成。然而,在有水环境中,氧化锆会发生低温降解。目前人们的研究重点在于提高陶瓷组件的强度和耐磨性,同时缩小其尺寸并延长其使用寿命。研究中使用的材料是氧化锆增韧的氧化铝复合陶瓷和其它氧化铝复合陶瓷,不再是单体陶瓷。另外,还可以使用氧化铝和氧化锆功能梯度复合材料。该梯度材料可以利用电泳沉积法(EPD)制得,其表面为纯氧化铝,中心部分为均匀的氧化铝、氧化锆复合材料,中间过渡部分是呈连续梯度渐变的氧化铝、氧化锆复合材料,烧成后会产生剩余热应力。设计这样的梯度结构是为了使复合材料具有最大表面压应力和最小内部张应力,与纯氧化铝组件相比,提高了强度和耐磨性。     

Impact of sandblasting on the mechanical properties and aging resistance of alumina and zirconia based ceramics

Bioinert zirconia and alumina ceramic devices are widely used, both in orthopaedics and in dentistry. In order to improve their bonding with bone tissues or dental resin cements, their surfaces are often roughened at different scales. In this work, we have investigated the effects of the same sandblasting treatment on alumina, zirconia and a zirconia-toughened alumina, focusing on their mechanical performance and the interplay between surface defects and residual stresses. Additionally, we explored the impact of the treatment on the hydrothermal aging of the two zirconia-containing materials. Residual stresses generated during sandblasting were always predominant over surface defects but their effect varied with the material: while they had a weakening effect on alumina, they reinforced both zirconia-containing materials. Finally, we found that the monoclinic grains at the surface of sandblasted zirconia recrystallized into tetragonal nanograins after annealing and this led to an increased resistance to aging.     

Ultra-fine Yttria-Stabilized Zirconia for dental applications: A step forward in the quest towards strong,translucent and aging resistant dental restorations

To decrease the light scattering caused by birefringence of the tetragonal phase of dental Yttria-Stabilized Zirconia (YSZ), two main strategies are followed: 1) increasing Y₂O₃ content to have a larger amount of non-birefringent cubic phase or 2) decreasing grain size below 100 nm to reduce their scattering coefficient. Both strategies might affect mechanical properties and aging resistance. This study shows that increasing the stabilizer content enhances both translucency and aging resistance, at the expense of mechanical properties. Nanometric-sized YSZ show, instead, very interesting compromise. Nanometric-sized zirconia stabilized with 3 mol.% of Y₂O₃ possesses high strength and toughness (above 1600 MPa and 3.3 MPa m^1/2), aging resistance and translucency, thanks to its fully-stabilized tetragonal nano-grains. Nanometric-sized zirconia stabilized with 1.5 mol.% of Y₂O₃ has the best mechanical performances (strength above 1500 MPa and toughness of 4.8 MPa m^1/2), still showing aging resistance and intermediate opacity. These results highlight the interest of moving towards Yttria-containing transformable tetragonal nano-ceramics for dental applications.     

Enhancement of mechanical properties of ceria-calcia stabilized zirconia by alumina reinforcement

Zirconia ceramics stabilized using 10 mol% CeO₂ and 1 mol% CaO were studied with the addition of small amounts of α-alumina. The elaboration process of five different compositions was done by wet mixing of powders using 0, 2.5, 5, 10, and 15 wt% alumina, followed by pressing and sintering. The 2.5 wt% alumina addition reduced the grain size, which led to an increase in hardness and the 10 wt% alumina samples showed the maximum mechanical strength (around 1000 GPa), measured by the ball on three balls bending test. The fraction of monoclinic phase around Vickers indentations is reduced by the presence of alumina, but the transformability and resistance to cracking by Vickers indentations are still much higher than for 3Y-TZP. The hydrothermal degradation resistance was also improved by the addition of alumina, with only a very small increase of monoclinic phase of about 1 % in volume after aging for 30 h in standard autoclaved conditions. The enhancement in mechanical properties and LTD resistance leads the path to explore the use of these materials in biomedical applications.     

Phase Stability and Mechanical Properties of Zirconia and Zirconia Composites

Monolithic zirconia materials (3Y‐TZP, 10Ce‐TZP, and 12Ce‐TZP) and their composites with 30 vol% alumina were produced. Low‐temperature aging degradation (LTAD) and mechanical properties of materials were investigated. For assessment of phase stability in the materials, aging experiments were performed in water at 90°C for 32, 64, and 128 days. The aging phenomenon was characterized and monitored using X‐Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Four‐point bending was used to determine the flexural strength of materials before and after aging treatment in water at 90°C for 2, 4, and 6 months. The aging experiments resulted in different phase transformation rates for the materials studied. The 12Ce‐TZP containing materials showed the highest resistance to low‐temperature aging and 3Y‐TZP containing materials showed the highest bending strength. When compared, no change in flexural strength was observed between the materials not exposed to aging and the aged materials.     

Analysis of zirconia mechanical properties after application of a protocol to simulate clinical aging

In order to test the effects of hydrothermal and clinically related aging on zirconia, monolithic disc-shaped samples were milled, sintered and polished from two high translucency zirconia , 3 mol % (HT) and 5 mol % (XT). Samples were divided into three groups: non-aged: control (CT); hydrothermal aging (HA - autoclave aging for 12.5 h at 134 °C, 2 bar); in vitro clinically-related aging (CRA - chewing simulation for 1.2 million cycles followed by 50,000 thermocycles and acidic exposure in HCl, pH 1.2, for 15 h). Mechanical properties (flexural strength, fatigue, hardness and elastic modulus) were analyzed and compared using the analysis of variance (at a level of 5% significance). In vitro clinically-related aging significantly decreased fatigue strength of XT zirconia with no effects on HT zirconia. Surface hardness and elastic modulus were not affected (p = 0.591 and 0.392 respectively). Hydrothermal aging increased fatigue strength for both materials and decreased the surface hardness and modulus of HT zirconia (p ≤ 0.001). Hydrothermal aging and in vitro clinically-related aging have different effects on the mechanical properties of zirconia , when used to simulate five years of clinical service.     

Processing and properties of tape-cast alumina/zirconia laminates composites

In the present work, laminar ceramic structures formed by layers of alumina and partially stabilized zirconia were fabricated by water-based tape casting. Rheological, physical and mechanical properties of slurries and laminates were evaluated. The laminates consisted of stacked alumina and zirconia green tapes produced by thermopressing. Pyrolysis was carried out at 450 °C and sintering at 1500 °C. The alumina/zirconia laminates were studied for a better understanding of the formation behavior and crack propagation at the laminate interface. The flexural strength values of laminates depend on the stress state on their surface. The laminates with the highest amount of zirconia layers presented low strength values (6.7 MPa), while the laminates with more alumina layers had a higher strength level (57.7 MPa). This is because these laminates have alumina layers on the surface which are in a state of residual compressive stress.     

Functional gradient coating of alumina on net shaped zirconia implant: Improved strength,aging resistance,and role of residual stress

Aging of zirconia in vivo has been widely discussed as a potential cause of implant degradation over time. Hydrothermal degradation is sensitive to composition, process conditions, and microstructure leading to an emphasis on secondary phases, and grain boundary engineering for aging resistance. However, surface coatings, resultant residual stress, and associated physical constraint for phase stabilization are insufficiently explored. Herein a novel ceramic dough processing facilitated the formation of a functional gradient alumina coating (20–50 µm) below the critical thickness, on net-shaped green zirconia dental implant while preserving the fine machined threads. Residual stress (~ ?0.8 GPa) after sintering improved the characteristic strength by ~ 45% with a simultaneous contribution to profound phase preservation after in vitro aging. Thus, the compositional gradient coating on green zirconia components using alumina-based slurries is a facile surface modification technique to inhibit moisture-induced aging.     

氧化铝、氮化硅和碳化硅的疲劳特性与寿命预测

分析了几种典型的结构陶瓷在长期载荷下的失效特性和差别。认为长期失效的本质是强度衰减,建立了一个疲劳失效的强度衰减模型和提出了寿命预测方法。分别研究了氧化铝、氮化硅和碳化硅几种常用工程陶瓷在常温和高温下的疲劳特性和差异。采用三点弯曲的受力方式测试了不同载荷水平下的断裂时间。结果表明：碳化硅的疲劳门榄值超过强度的80％,而且受温度影响最小;氧化铝的静疲劳受微小裂纹扩展控制;氮化硅的高温疲劳主要是蠕变机制导致强度衰减,疲劳门槛值不超过强度50％。由实验研究了氮化硅的高温静疲劳、动疲劳和循环疲劳三者在相同温度和相同应力峰值变化下的寿命关系,结果与计算一致。     

Nacre-like alumina composites reinforced by zirconia particles

Nacre-like alumina is a class of bio-inspired ceramic composite manufactured by field-assisted sintering of green bodies made primarily of alumina platelets with an anisotropic microstructure. Here we investigate the addition of zirconia particles to enhance the mechanical properties of the composite. The resulting structure is a nacre-like anisotropic structure which features deflection and reinforcement during crack propagation. Monoclinic zirconia has no impact on the mechanical properties of the composite while tetragonal zirconia improves its fracture resistance properties. Both types of zirconia seem to slow down grain growth during sintering. The addition of zirconia stabilised in the tetragonal phase is thus a good option to obtain a composite with a fine microstructure and higher mechanical properties than a standard nacre-like alumina, with a flexural strength of 626 ± 39 MPa and a crack initiation toughness of 6.1 ± 0.6 MPa.m^0.5.     

Al2O3增强ZrO2陶瓷的制备及性能研究

本文采用热分解法制备Al2O3微粉、化学共沉淀法制备(Y,Ce)—ZrO2超细粉,通过适当工艺制备出ZrO2／Al2O3复合陶瓷。经研究发现,添加Al2O3,可抑制ZrO2晶粒的长大,提高基体的强度和韧性。当Al2O3含量达到30%(质量分数)时,复合陶瓷的抗弯强度为986MPa,断裂韧性为13.7MPa*m1／2。材料性能的提高可归结为Al2O3颗粒的弥散增韧和ZrO2陶瓷的相变增韧叠加作用的结果。     

振动搅拌对掺铁尾矿砂水泥稳定碎石混合料的影响研究

为综合分析铁尾矿砂水泥稳定碎石混合料路用性能的影响因素,本文对不同铁尾矿砂掺量(矿料质量的0%、5%、10%、15%、20%、25%)及不同成型方式(传统连续搅拌、振动搅拌)的水泥稳定碎石混合料的物理力学性能、耐久性及微观结构进行试验研究。研究结果表明,铁尾矿砂的加入能够提高水泥稳定碎石混合料的无侧限抗压强度、间接拉伸强度、水稳定性、抗冻性。当铁尾矿砂用量为10%时,水泥稳定碎石混合料的强度、水稳定性、抗冻性达到最大值;当铁尾矿砂掺量为5%时,水泥稳定碎石混合料的疲劳寿命最长。振动搅拌制备试样较连续搅拌有更高的强度、水稳定性、抗冻性及耐疲劳性能;振动搅拌制备的铁尾矿水泥稳定碎石混合料内部水泥水化程度更高,水化产物分布更均匀,能够降低混合料因应力集中而破坏的风险。     

Highly translucent and strong ZrO2-SiO2 nanocrystalline glass ceramic prepared by sol-gel method and spark plasma sintering with fine 3D microstructure for dental restoration

Balance of better mechanical strength and good translucency for dental restorative materials is always a challenge. A translucent glass ceramic/ceramic with improved mechanical properties or a strong glass ceramic/ceramic with good translucency would therefore be interesting for dental application. Nanocrystalline glass ceramics (NCGC) attract a lot attention because of their superior optical and mechanical properties. This study aims to obtain ZrO₂-SiO₂ nanocrystalline glass-ceramic that possesses high mechanical strength as well as excellent translucency by controlling the content, size, and connection of nanocrystalline ZrO₂ in a ZrO₂-SiO₂ glass-ceramic material. Toward this end, well-homogenized nano-powders with three different compositions, 45%ZrO₂-55%SiO₂ (molar ratio, 45Zr), 55%ZrO₂-45%SiO₂ (55Zr), and 65%ZrO₂-35%SiO₂ (65Zr)_, were synthesized, followed by a fast sintering process. Highly-translucent nanocrystalline glass ceramics composed of tetragonal ZrO₂ were obtained. Samples with high zirconia content showed that the structure of the skeleton was predominately built by nano-sized ellipsoidal ZrO₂ particles bonded by grain boundaries, with amorphous SiO₂ filling the voids between the ZrO₂ particles. The achieved flexural strength measured by piston-on-three-ball test was as high as 1014 MPa. To our knowledge, this is one of the highest flexural strength values of glass ceramics ever reported, which is higher than transparent zirconia and alumina ceramics. The 3D structure of nanocrystalline zirconia in silica matrix did enhance the flexural strength of the NCGC. The results of this study suggest that the new ZrO₂-SiO₂ NCGC has great potential of using as dental restoration.     

Properties of alumina 10 vol% zirconia composites—The role of zirconia starting powders

Zirconia toughened alumina (ZTA) materials are applied for cutting tools, wear parts and in biomedical applications. Due to the constraint of the rigid alumina matrix, ZTA materials with up to 10 vol% zirconia addition (AZ10) do not require addition of stabilizer oxides. AZ10 materials based on submicron sized alumina and four different submicron to nanoscale zirconia powders were manufactured by hot pressing at temperatures between 1475?1600 °C. Results show that the powder choice has a strong influence on mechanical properties, evolution of microstructure and phase composition. Best results with strength up to 850 MPa, fracture toughness values of 8.5 MPa√m and invulnerability to overfiring were obtained with zirconia powders showing the coarsest yet most homogeneous primary particle size and a low degree of agglomeration. Ultrafine but hard agglomerated powders lead to materials with extremely inhomogeneous microstructure and inferior properties.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

氮化硅陶瓷牙科修复材料研究进展

氮化硅具有比氧化铝、氧化锆等牙科陶瓷修复材料更好的力学性能、化学稳定性和生物相容性,已经用于生物骨科修复.近年来陆续有学者将氮化硅陶瓷引入到牙科修复领域,文章在介绍氮化硅陶瓷性能特点的基础上,综述了氮化硅陶瓷牙科种植体、氮化硅桩核冠及氮化硅饰面瓷的研究进展,并展望了氮化硅在牙科修复领域的未来研究方向.     

Alumina toughened zirconia from yttria coated powders

Alumina toughened zirconia (ATZ) ceramics are attractive materials for biomedical implants and other engineering applications requiring high strength and abrasion resistance at ambient temperature. As the toughness of conventional ATZ composites is moderate it was tried to improve the mechanical properties by starting from a very tough and transformable 2.5Y-TZP powder derived from powder coating. 2.5Y-TZP and ATZ materials with 20–40 vol% of alumina were produced and tested with respect to mechanical properties, microstructure and phase composition. ATZ materials with a bending strength of up to 1900 MPa and a fracture toughness of 4.8–5.5 MPa √m were obtained. Transformability of the TZP distinctly declines with rising alumina content. Calculation of cooling and transformation stresses of the materials qualitatively confirms the measured toughness values. Alumina addition initially induces tensile residual cooling stress in the zirconia matrix which facilitates phase transformation. Formation of compressive stresses during transformation limits the transformability and toughness.     

THE EFFECT OF THE REPLACEMENT OF FREE SILICA BY ALUMINA AND ZIRCONIA IN ELECTRICAL PORCELAIN1

Effect on electrical porcelain of the replacement of free silica by alumina and zirconia.—Since it had been suggested that free silica may be detrimental to the mechanical and the dielectric strengths of electrical porcelain, a series of sixteen batches was prepared in which various proportions of the free silica were replaced, weight for weight, by alumina, zirconia or combinations of both, and bars and disks, after burning at cone 8 l/2 to 9 or at cone 12, were tested for shrinkage, transverse strength, impact strength, heat resistance, absorption of moisture and dye penetration. The results show that resistance to sudden temperature changes can be greatly improved by substituting zirconia,—that the danger of overburning can be greatly decreased by substituting alumina and that the mechanical strengths can be increased by substituting either or both; but in the case of dielectric strength, no improvement was obtained by substituting for the free silica.     

Surface strength of balls made of five structural ceramic materials evaluated with the Notched Ball Test (NBT)

In high performance hybrid bearings the balls are conventionally made of silicon nitride ceramics. There are some disadvantages such as costs or the higher stiffness of silicon nitride compared to steel. Therefore, alternative materials are under investigation. The surface strength is one of the most important criteria for the qualification of the spherical components in the application. It has to be evaluated for each new material (or new surface finish).The recently standardised Notched Ball Test (NBT) enables one to determine the surface strength (tensile strength) of balls, which is strongly influenced by the surface finish and volume flaw populations. In this paper the NBT strength of five candidate materials for rolling elements (silicon carbide, silicon nitride, alumina, zirconia and zirconia toughened alumina) is investigated. Fractography is performed to evaluate the direct correlation between the defects found at the surface and the measured strength.