Zirconia-based Ceramics Research Articles

Sintering in a graphite powder bed of alumina-toughened zirconia/carbon nanotube composites: a novel way to delay hydrothermal degradation

Zirconia-based ceramics have been introduced in biomedical applications, for example, in hip implants. Certain zirconia composites are prone to spontaneously transform from the tetragonal phase to the monoclinic phase during long-term storage in the presence of moisture at low temperatures. This phenomenon is time-dependent and can be accelerated by water or water vapour. Herein, we report strong experimental evidence of a delayed t→m ZrO2 phase transformation in alumina-toughened zirconia (ATZ) and ATZ/multi-walled carbon nanotube (MWCNT) composites when pressureless sintered in air in a graphite powder bed. The m-ZrO2 phase in ATZ and ATZ/MWCNT composites sintered in a graphite powder bed after hydrothermal ageing for 20h at 134°C decreased by 81% and 87%, respectively, compared to an ATZ sample sintered in an alumina powder bed. The enhanced hydrothermal stability could be attributed to the formation of a thin continuous alumina protective layer covering the surface of the ceramic composites.

Electrophoretic Deposition of Zirconia Multilayered Constructs

Zirconia-based ceramics have gained considerable interest for several applications (e.g. solid electrolytes in fuel cells and in oxygen sensors, thermal barrier coatings and biomaterials for dental and orthopaedic applications) due to their high mechanical strength, improved fracture toughness and easy affordability. We constructed both a manual and an automated electrophoretic deposition set-ups, which allowed us to obtain multilayers of alternating tetragonal and cubic phases by layering nanometric sized powders of yttria doped zirconia (Y-TZP) with different mol percentages of yttria (3 % and 8 %). The rationale behind the design of these multilayer constructs was to optimize the properties of the final ceramic by combining the high mechanical toughness of the tetragonal phase of zirconia together with the high ionic conductivity of its cubic phase. These multilayered constructs were proved to have good mechanical integrity and a clearly defined interface between the cubic zirconia grains layer and the tetragonal zirconia grains layer.

Contemporary Ceramics and Cements

Contemporary Ceramics and Cements

Depth-of-cut errors in ELID surface grinding of zirconia-based ceramics

Based on the analysis of a surface grinding system and the material removal mechanism, a mathematic model has been proposed to predict the accumulated error between the total set depth-of-cut (DoC) and the total actual depth-of-cut (ADoC) in multi-pass surface grinding of zirconia-based ceramic materials. Design of Experiments (DoE) approach has been implemented to carry out experiments. The influence of the set DoC of each grinding pass, the total set DoC, and the grinding wheel velocity on the accumulated errors in ADoC has been investigated in detail in surface grinding of zirconia-based ceramics both with electrolysis in-process dressing (ELID) and without ELID. It has been observed that the accumulated DoC errors increase faster in the first few passes and gradually reach a saturation after a certain total DoCs (about 8–10 grinding passes) and a higher step DoC leads to a faster (fewer passes) saturation of the accumulated DoC errors. Compared to grinding without ELID, it has been found that ELID-grinding is characterized with better process stability and ELID offers positive effects on material removal rate (MRR), especially in case of removing large volume of material with the same grinding parameters.

Genotoxicity test of self-renovated ceramics in primary human peripheral lymphocytes.

Zirconia-based ceramics is widely used in dentistry. Different compositions of ceramics have different features. Our self-renovated ceramics become more machinable without scarifying its dental restoration properties after adjusting ratio of lanthanum phosphate (LaPO4)/yttrium oxide (Y2O3). In order to evaluate its safety, here, we tested its genotoxicity in primary human peripheral lymphocytes. The human lymphocytes cultured on three groups of different ratios of LaPO4/Y2O3 diphase ceramics for 6 days showed little effect of growth inhibition and similar effect of growth trend to the negative control. Furthermore, single-cell gel electrophoresis (comet assay) indicated that there was no significant difference of the value of tail moment between the tested ceramics and negative control, the IPS Empress II (P > 0.05). Our findings implicate that our self-renovated ceramics do not induce DNA damages in human peripheral lymphocytes and support their future clinic application.

Type of Failure of Zirconia-Based Ceramics in Dental Laboratory in Misurata, Libya

Zirconia (ZrO2) is used to fix restorations as a core material because of its mechanical properties, aesthetics, and compatibility. This study aims to analyze the failure types in ZrO2-based restoration fabricated in a dental laboratory in Misurata, Libya. Data were collected from laboratory records for a 30-day period with follow-up for five months. About 6% of the total restorations had defects with different percentages, 46% of which were fractures, 29% of which had weak bonding between ZrO2frameworks and veneers, 18% of which had cracks, and 7% of which had shade defects. Although ZrO2is a suitable material for dental restorations, defects may occur and lead to the failure of dental restorations. A thorough study is necessary to analyze the cause of failurein zirconia-based restorationsand to improve the properties to produce a versatile dental restorative material.

Research Progress on the Transformation, Fracture Mechanism and Properties of Structural Ceramics at Cryogenic Temperatures

Currently,the rapid development of cryogenic techniques makes them required for a wide range of applications such as aeronautics astronautics,superconducting fields,nuclear fusion and so on.Structural ceramics are promising cryogenic structural materials which are prior to some kinds of metals and polymer materials in some specific applications.The research history and progress of several classical types of structural ceramics at cryogenic temperatures are reviewed.The contents of the present paper include the transformation mechanism and properties of zirconia-based ceramics,alumina ceramics and basic mechanical properties and fracture mechanisms of Si3N4and SiC ceramics at cryogenic temperatures.

Studying tribological characteristics of alumina- and zirconia-based ceramics

Tribotests of ceramic specimens of various compositions (Al2O3-1% TiO2, ZrO2-5.3% Y2O3, Al2O3-15% (ZrO2 + 5.3% Y2O3)) were carried out under dry friction conditions. It has been shown that all of the specimens have a high wear resistance, while the Al2O3-15% (ZrO2 + 5.3% Y2O3) specimens have the lowest wear rate and the highest microhardness. This is due to the fine-grained structure of these specimens and their higher density compared to that of the other ceramic compositions.

Influence of Sintering Temperature on the Translucency of Sintered Zirconia by Cold Isostatic Pressing

Zirconia-based ceramics exhibit excellent mechanical properties and biocompatibility in dental applications. However, the production of translucent zirconia that offers resemblance to real teeth remains a challenge. This study aims to fabricate zirconia compacts by cold isostatic pressing (CIP) and investigate the influence of sintering temperature on translucency, microstructure, hardness, and density of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). Zirconia stabilized with 3 mol% yttria (3Y-TZP) was pressed by uniaxial pressing and later by CIP to produce green bodies in pellet form. Subsequently, the green bodies were sintered at different temperatures (1100 °C to 1300 °C). The specimens were then investigated in terms of translucency, density, and hardness. X-ray diffraction was also performed and the microstructure of the specimens was observed under a scanning electron microscope (SEM). Density and light transmittance tests results showed that zirconia sintered at 1200 °C exhibits the highest density (5.957 g/cm3) and light transmittance intensity. Vickers hardness test showed that higher sintering temperatures result in higher hardness of the sintered zirconia. SEM micrographs illustrate the effect of microstructural changes on the translucency of zirconia. A temperature of 1200 °C is found to be the recommended sintering temperature at which zirconia exhibiting optimum translucency and mechanical properties is produced. CIP is found to be a suitable consolidation method to produce high-density translucent zirconia.

Effect of sandblasting on surface roughness of zirconia-based ceramics and shear bond strength of veneering porcelain.

This study aims to investigate the effect of sandblasting on the surface roughness of zirconia and the shear bond strength of the veneering porcelain. Pre-sintered zirconia plates were prepared and divided into four groups. Group A were not treated at all; group B were first sandblasted under 0.2 MPa pressure and then densely sintered; group C and D were sintered first, and then sandblasted under 0.2 MPa and 0.4 MPa pressures respectively. Surface roughness was measured and 3D roughness was reconstructed for the specimens, which were also analyzed with X-ray diffractometry. Finally after veneering porcelain sintering, shear bond tests were conducted. Sandblasting zirconia before sintering significantly increased surface roughness and the shear bond strength between zirconia and veneering porcelain (p<0.05). Sandblasting zirconia before sintering is a useful method to increase surface roughness and could successfully improve the bonding strength of veneering porcelain.

Effect of four surface treatment methods on the shear bond strength of resin cement to zirconia ceramics- a comparative in vitro study.

Improving the retention of zirconia-based ceramics is desirable in order to avoid the failure of crowns and fixed partial dentures .This can be achieved by creating micromechanical retention using surface treatments. Therefore, it becomes necessary to constantly compare and re-evaluate the influence of different surface treatment methods on the bond strength . To evaluate the effect of four different surface treatments on shear bond strength between zirconia surface and resin cements. Observational study. Twenty five zirconia plate samples were prepared based on ISO standards and were divided into five groups and each group was subjected to following five different surface treatments : no treatment, sandblasting with 110 μm alumina, sandblasting with 250 μm alumina, acid etching with 9.6% hydrofluoric acid and laser radiation on the surface. All the samples were surface disinfected and were embedded in blocks of autopolymerising resin to check shear bond strength on the universal testing machine. Statistical analysis used-data was analyzed using one-way ANOVA and a Post Hoc Bonferroni test. Analysis of the data showed that the highest shear bond strength values were obtained with laser treatment (18.120 ± 0.8159 Mpa). The lowest values were obtained with control group (9.166 ± 0.569 Mpa). Laser treatment increased the shear bond strength values significantly (p<0.05). Surface treatments increased the bond strength between zirconia and resin cement and carbon dioxide laser could be an effective surface treatment for increasing bond strength.

Low Temperature Degradation of Y-TZP Ceramic for Dental Applications

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.

Deformation behavior of zirconia-based porous ceramics

It has been studied a porous ceramics obtained from nanocrystalline ZrO2 powders. Samples were obtained by pressing and sintering of compacts, the porosity of ceramic samples was from 20 to 80%. The average grain size in the studied ceramics increased from 0.4 to 4 microns with increasing sintering temperature and duration of isothermal holding from 1400 to 1600 °C and 1 to 5 hours, respectively. It was shown that the "stress – strain" diagrams on the initial stage of deformation has a nonlinear behavior with high parabolic factor of strain-stress curves. It has been shown that fracture of the materials was observed from the elastic area and has a rod-like and cellular-like parts in its structure.

The isothermal conductivity improvement in zirconia-based ceramics under 24 GHz microwave heating

Abstract Under 24-GHz millimetre-wave irradiation heating ionic conductivity of zirconia base ceramics was up to 20 times higher than that of a conventionally-heated sample at the same temperature of 400 °C. The degree of enhancement could be altered by changing the stabilising atom from Y to Yb. Enhancement of ionic conduction was prominent in the setup condition of larger self-heating ratio and larger MMW absorbing materials. The isothermal improvement of ionic conductivity under MMW irradiation would be ascribed to the non-thermal effect.

A Review of Surface Treatment Methods to Improve the Adhesive Cementation of Zirconia-Based Ceramics

In spite of high mechanical strength, zirconia-based ceramics (ZrO2) has poor bond strength after conventional bond cementation procedures, requiring different surface treatment methods (STMs). This review gathered information about the STM for adhesive cementation (AC) to ZrO2in the PubMed database, consideringin vitrostudies pertaining to AC for acid-resistant ceramics (ZrO2) limited to peer-reviewed papers published in English between 1965 and 2013 in dental journals. Different STMs have been proposed for ZrO2: air-abrasion (laboratory or chairside) with silica- (Si-) coated aluminum particles, the use of materials containing phosphate monomers, primer or silane application, laser irradiation, Si vapor phase deposition, and selective infiltration etching. In conclusion, STMs improve bond strength of resin luting cement to ZrO2mainly when tested in short time. STMs must be correlated to the type of ZrO2and the resin cement.

Maintaining the mechanical strength of La-, Y-co-substituted zirconia porous ceramics through the superplastically foaming method

The superplastically foaming method was adopted to make closed-pore inclusive zirconia-based ceramics. Lanthanum oxide was added to monoclinic or tetragonal yttria-stabilised zirconia to reduce the thermal conductivity of the matrix. Sintering and superplastic deformation led to a solid solution and transformation to the cubic phase. The resulting superplastically foamed porous ceramics having a porosity of 45% had only 40% of the thermal conductivity of the fully densified ceramics having the same composition. This value was comparable to that of conventionally fabricated porous ceramics with the same composition and porosity. The superplastically foamed ceramics had 60%, while conventionally fabricated ceramics had only 20%, of the mechanical strength of the fully dense ceramics.

Mesoscopic Monte Carlo simulations of microstructure and conductivity of ZrO2–glass composites

Zirconia-based refractories are ceramics that serve as furnaces for the production of high-quality speciality glasses. These materials are obtained by cooling a melt composed of different oxides and oxide precursors. The microstructure of these refractories consists of monoclinic zirconia embedded in a three-dimensional interconnected amorphous phase. Using mesoscopic Monte Carlo simulations, the present paper addresses the effect of the amount of amorphous phase and temperature on the microstructure of the material. The simulated microstructures resemble those obtained using tomography experiments. A theoretical model describing the electrical behavior of the material as a function of temperature and composition is also reported. In this model, the conductivity of the ZrO2 particles is assumed to be constant, but the overall conductivity of the sample depends on its tortuosity τ. Comparison with experimental data suggests that the present model provides a realistic picture of the electrical behavior of zirconia-based ceramics and leads to quantitative predictions.

Addition of a pontic to all-ceramic Turkom-Cera fixed partial denture restorations

High-strength all-ceramic materials are commonly used in dentistry. When complications occur in an all-ceramic restoration, the restoration is usually replaced. This article describes the time-saving ability and cost-effectiveness of this novel technique for the addition of a pontic in two complicated clinical cases. Turkom-Cera™ [Turkom-Ceramic (M) Sdn. Bhd.] with aluminum oxide (99.98%) is an all-ceramic system that offers the option of addition of a new pontic to the sintered framework. The new pontic was cut off from an alumina blank [Turkom-Ceramic (M) Sdn. Bhd.], moistened, and attached to the framework using alumina gel [Turkom-Ceramic (M) Sdn. Bhd.]. The framework was veneered with veneering porcelain (Vita VM 7; VITA Zahnfabrik). The two cases presented here involving the addition of a pontic to sintered framework were followed up for at least 1 year. No complication was detected or reported by the patients. Alumina- and zirconia-based ceramics are particularly suitable for for all-ceramic restorations in high-stress bearing areas. However, replacement of a failed all-ceramic restoration is not the most practical solution, considering both cost and tooth-related factors. This attractive feature of the Turkom-Cera allows the repair of a fractured ceramic coping or the addition of a new pontic to restorations.

Experimental study and simulation of plastic deformation of zirconia-based ceramics in a pulsed electric current apparatus

Plastic deformation by compression of cylindrically shaped zirconia (ZrO2)-based ceramics in a pulsed electrical current apparatus was studied using a combined experimental and theoretical approach. Both fully dense electrically insulating 3Y–ZrO2 and electrically conductive 3Y–ZrO2–TiCN 60/40 (vol.%) ceramics were subjected to a compressive load at temperatures above 1200°C. Deformed non-conductive 3Y–ZrO2 samples were concave shaped, whereas the composite samples exhibited a different behaviour depending on the electrical current path within the set-up. A convex shape was obtained when the current was freely flowing through them, while they started to become concave shaped when the samples were separated from the graphite pressing punches by relatively low conductive silicon carbide disks. The secondary titanium carbonitride (TiCN) phase in the composite materials exhibited a grain boundary pinning effect, which limited coarsening of their microstructure. The influence of current flow on the shape of the deformed ceramic samples was interpreted in terms of the temperature distribution generated during hot deformation. Finite-element simulations, coupling thermal, electrical and mechanical fields, were used to explain the deformation behaviour of the different samples. A subsequently coupled thermal–electrical and mechanical analysis procedure was developed for this aim. Special attention was paid to the materials and interactions properties used during modelling. The modelling results are in good agreement with the experimental data, so that the developed finite-element approach and code can be used for the analysis of near net shaping of ceramic parts assisted by an electrical field.

Electrophoretic Deposition of Zirconia Nanoparticles

Zirconia-based ceramics have gained considerable interest for several applications (e.g. solid electrolytes in fuel cells and in oxygen sensors, thermal barrier coatings and biomaterials for dental and orthopedic applications) due to their high mechanical strength, improved fracture toughness and easy affordability. Generally, one of the main disadvantages of ceramic materials is their brittleness, i.e. low fracture toughness. Electrophoretic deposition (EPD) has recently gained interest in the ceramics processing due to its wide versatility, allowing the fabrication of laminated and graded ceramics, fiberreinforced ceramic matrix composites, coatings, and nanostructures. Moreover, EPD has been pointed out as an effective technique to obtain complicated 3D shape structures. This makes EPD a promising technique to obtain ceramic constructs for several applications like solid oxide fuel cells, thermal barrier coatings and biomaterials. In our research group, we have developed both a manual and automated EPD set-up to obtain tetragonal and cubic zirconia multilayered ceramic constructs. Multi layers of alternating tetragonal and cubic phases with a clearly defined interface were successfully obtained by an EPD layering process with submicron sized powders of Y-TZP with different mol percentages of yttrium oxide (3 % and 8 %). Finally, we investigated how to obtain 3D shape structures by EPD and we applied this technique to the dental materials field in order to obtain dental crowns.