Papers by Keyword: Low Temperature Degradation

Abstract: Low-temperature-degradation-free (LTD-free) zirconia-toughened alumina (ZTA) has been developed as an alternative to alumina and zirconia. It has been reported that ZTA has more beneficial mechanical characteristics than alumina, because of the stress-induced transformation and the spontaneous transformation of the zirconia phase that occurs for some ZTA in a hydrothermal environment. To achieve high-reliability artificial-joint prostheses, it is necessary to improve both the mechanical characteristics and phase stability of the joint material simultaneously. Thus, in this study, we evaluate the wear characteristics and corrosion resistance of LTD-free ZTA using a hip wear simulator, so as to confirm the long-term stability and reliability of this material in clinical use. The ZTA with advantageous mechanical characteristics used in this study has extremely high wear and corrosion resistance. Therefore, this ZTA could be quite useful as a bearing material in artificial joints for longer-term clinical use. It is also confirmed that the newly developed LTD-free ZTA is extremely stable under sliding conditions. Thus, it can be expected that the initial superior surface characteristics will be maintained through long-term clinical use.

Abstract: The effect of manganese oxide (MnO₂) and aluminum oxide (Al₂O₃) addition of up to 1 wt% in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), sintered in air at 1450oC, was examined. The low temperature degradation resistance was observed by immersing the sample in Ringer solution at 37oC over a period of 8 weeks, whereby the weight loss and the SEM image of sample was analyzed to generate the degradation pattern of the sample. Additions of 0.6wt% Al₂O₃/ 0.4wt% MnO₂ were found to be beneficial in retarding the ageing of the ceramics sample. Therefore the sample was deemed beneficial in the use of orthopedic applications.

Abstract: The demand for tetragonal zirconia as a dental restorative material has been increasing because of its excellent mechanical properties and resemblance to natural tooth color, as well as its excellent biological compatibility. Cerium oxide (CeO₂) has been added to yttria-stabilized zirconia (Y-TZP), and studies have demonstrated that the stability of the tetragonal phase can be significantly improved. Y-TZP with 5wt% CeO₂ as a second stabilizer was developed via colloidal process, followed by a suitable sintering process. According to the literature, the sintering process is the most crucial stage in ceramic processing to obtain the most homogeneous structure with high density and hardness. This study aims to investigate the effect of sintering temperature on the mechanical properties of nanostructured ceria–zirconia fabricated via colloidal processing and slip casting process with cold isostatic pressing (CIP). Twenty-five pellet specimens were prepared from ceria–zirconia with 20 nm particle size. CeO₂ nanopowder was mixed with Y-TZP nanopowder via colloidal processing. The consolidation of the powder was done via slip casting followed by CIP. The samples were divided into five different sintering temperatures with. Results from FESEM, density and hardness analyses demonstrated statistically significant increase in density and hardness as the sintering temperature increased. The hardness increased from 4.65 GPa to 14.14 GPa, and the density increased from 4.70 to 5.97 (g/cm3) as the sintering temperature increased without changing the holding time. Sintering Ce-Y-TZP at 1600 °C produced samples with homogenous structures, high hardness (14.14 GPa), and full densification with 98% of the theoretical density.

Abstract: Low temperature degradation free Zirconia toughened alumina (ZTA) has been developed. It is reported that ZTA has higher mechanical strength compared to alumina due to the stress induced transformation and spontaneously transformation of zirconia phase on some ZTA have been occurred. For achieving the higher reliability of artificial joint prosthesis alternative to alumina and other ceramic materials, it is necessary to improve and validate the both mechanical characteristics and phase stability at the same time. We evaluated that microstructure, mechanical characteristics and phase stability of newly developed ZTA (BIOCERAM^® AZUL). It was confirmed that four-point bending strength and weibull modulus were extreamly high, and ZTA has higher reliability. There were no significant changes and deterioration in four-point bending strength, crystal structure and wear property with and without accelerated aging test. Newly developed ZTA not only with high mechanical characteristics but also with phase stability could be quite useful as bearing materials in artificial joints for longer clinical use.

Abstract: The use of tetragonal zirconia as a dental restorative material has recently increased because of its unique mechanical and optical properties, as well as high biological compatibility with the oral cavity environment. However, the mechanical properties of zirconia can be severely degraded, which leads to the failure of dental restorations. This review focuses on the low-temperature degradation of dental zirconia and its effects on the properties of zirconia and on the oral environment. The purpose is to show the importance of this negative phenomenon and suggest guidelines for minimizing the aging of zirconia that is used as a dental restoration material.

Abstract: An in vitro study was conducted to study the effect of hydrothermal aging on the phase stability, microstructure and mechanical properties of dental 3Y-TZP ceramics. It’s found that dental 3Y-TZP showed a time-dependent tetragonal to monoclinic phase transformation during aging. This transformation resulted in an increase of the mechanical properties at the initial stage of aging, probably due to the generated superficial compressive stress. However, once the stress became large enough to cause severe grain fall off, micro-crackings and surface flaws, the mechanical properties of dental 3Y-TZP started to drop drastically. Dental 3Y-TZPs doped with Al₂O₃ and sintered at low temperatures showed improved aging resistance.

Abstract: Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic has been recently introduced into prosthetic dentistry for the fabrication of crowns and fixed partial dentures (FPDs). The mechanical properties of Y-TZP are the highest ever reported for the all-ceramic materials. This is favorable for the fabrication of multi-unit posterior bridges and the substantial reduction in core thickness. However, Y-TZP ceramic is susceptible to low temperature degradation (LTD), which is detrimental to the long-time survival and aesthetics of zirconia restorations in vivo. This review summarizes the characterization, mechanisms, and influencing factors of the LTD in dental Y-TZP ceramic. In addition, the recent trend of exploring high aging resistant zirconia-based dental ceramics is discussed.

Abstract: Effect of surface charges induced by polarization treatment on Low Temperature Degradation (LTD) in 3mol%Y-doped ZrO₂ was studied. Samples were polarized by applying voltage (7kV/cm) at 200°C for various time (1~30min). LTD acceleration test was conducted using polarized samples. LTD was inhibited on negatively charged surface in all the polarized samples, which is independent on polarization time. We carried out thermally stimulated depolarization current (TSDC) analysis for investigate polarization mechanism. Two polarization elements were confirmed: orientation polarization and space charge polarization. A comparison of the result of LTD acceleration test and TSDC analysis indicates that orientation polarization is considered superior element in inhibition of LTD.

Abstract: This work follows a study on hydrothermal aged 3Y-TZP bioceramics, which showed that the surface X-ray diffraction data from moisture exposed samples give distorted results, reflecting a simple linear growth of a partially transformed layer from the surface into the interior. There is no indication for a leveling off or retardation of this growth at elevated temperatures (134°C) and here we present evidence that this is probably true at body temperatures as well. However, the rate constants at body temperature for the studied material are low and indicate a long lifetime. It should be noted that this statement is specific and other materials with minor changes to chemistry or microstructure may behave much better or much worse under those conditions. Furthermore slow crack growth and crack interactions are not yet investigated and may necessitate a minimum of low-temperature degradation susceptibility to ensure reliable long-time use.

Abstract: The purpose of this study was to investigate the influence of in vitro low-temperature degradation (LTD) treatments on the structural stability and mechanical properties of two commercial Y-TZP ceramics candidate for dental application. TZ-3YS and TZ-3YS-E powder were chosen because of there minor differences of chemical composition. The two powders were compacted at 200MPa using cold isostatic pressure, and densely sintered at 1500°C and 1450°C for 2hr respectively. Two methods of in vitro LTD treatments were performed, the first is to autoclave specimens in steam at 134°C, 2bar, for 1 to 5hr; the second is to immerse specimens into 4% acetic acid at 80°C for 168 hr. XRD was used to identify the crystal phases, flexural strength were tested according to ISO 6872 standard, surface microstructure was evaluated by Atomic Force Microscopy (AFM). Results show that the relative content of monoclinic phase was increasing with the prolonged aging time, the TZ-3YS was more sensitive to LTD compared with TZ-3YS-E. LTD tests did not necessary reduce the flexural strength of Y-TZP ceramics, the nucleation and growth of monoclinic phase were detected by AFM, and the surface microstructure induced by LTD was not identical between the two Y-TZP ceramics. AFM was a sensitive method to evaluate the transformation of Y-TZP ceramics. To choose appropriate Y-TZP powder for dental application, it is mandatory to examine the microstructure before and after LTD.