Zirconia Ceramics Research Articles

BN nanosheets reinforced zirconia composites: An in-depth microstructural and mechanical study

This paper deals with the effect of hydroxylated boron nitride nanosheets (BNNS) incorporation on the microstructural and mechanical features of zirconia ceramics. Few-layered BNNS were synthesized via a simple hydroxide-assisted planetary ball milling exfoliation technique. 3 mol% yttria tetragonal zirconia polycrystal (3Y-TZP) with 2.5 vol% BNNS powders were prepared by an environmentally friendly process in water, and spark-plasma sintered at three temperatures to explore the in-situ reduction of the functionalized BNNS. An exhaustive study by (S)TEM techniques was performed to elucidate the influence of the sintering temperature on the matrix and the 3Y-TZP/BNNS interfaces, revealing that BNNS were homogeneously distributed throughout the matrix with an abrupt transition at 3Y-TZP/BNNS interfaces. BNNS effectively hindered slow crack growth, thus increasing the composite’s crack growth resistance by about 30 %. A 1 MPa·m1/2 rising R-curve was also induced by crack bridging.

The Manufacturing Process and Corrosion Properties of Magnesium Matrix Composites: A Review

Magnesium-based biomaterials are a new generation of biodegradable metals that are playing a major role in the development of future implant materials. Magnesium-based materials can corrode too rapidly during physiological conditions and lose their usefulness before bones heal. This review aims to provide a comprehensive overview of the selection of suitable matrix and reinforcing materials as well as suitable process techniques to achieve better corrosion resistance targeted for dental implant applications. It also discusses the advantages and disadvantages of testing methods and corrosion media commonly used for biomedical applications. Specific search strategies are carried out in electronic databases: Springer, Pubmed, and Sciencedirect as well as google without limiting years. Papers are selected after reviewing their title, abstract, and full text. Then study the objectives, criteria, methods, and results. Based on the available literature, we can conclude that for magnesium-based composite matrices it is preferable to use commercially available Mg AZ91D alloys to investigate their corrosion behavior in vitro and in vivo environments. While for the composite reinforcement, bioceramic materials can be used, including Hydroxyapatite (HA) added with Zirconia ceramics (ZrO2) or Titanium metal oxide (TiO2). Furthermore, the process of making ceramic composites uses powder metallurgy (PM) and squeeze casting methods. Then, corrosive testing can use the commonly used electrochemical methods, namely Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) with corrosive SBF media.

A review on surface texturing of zirconia ceramics for dental applications

A review on surface texturing of zirconia ceramics for dental applications

Effect of forming strategy on surface morphology and properties of zirconia ceramics formed by laser powder bed fusion

This paper investigates the effects of different forming processes on the surface morphology and dimensional accuracy of zirconia ceramics, and selects the optimal forming process for mechanical property analysis. A laboratory-built ceramic laser 3D printer was used to carry out the forming experiments on the specimens. Under the condition that only the forming process of the experimental target is changed, the effects of powder layering process, scanning spacing and scanning strategy on the surface morphology and properties of zirconia ceramics are investigated. The use of two powder spreading and scanning forming processes in the sample forming process effectively improved the forming defects of zirconia ceramics caused by powder splattering, and the zirconia ceramics had a better forming effect. Under this condition, the dimensional accuracy of the specimens increases and then decreases with the decrease of the scanning spacing, where the highest dimensional accuracy is obtained when the scanning spacing is 0.158 mm; among different scanning strategies, the best surface morphology is obtained by forming specimens with the island scanning strategy. In addition, the mechanical properties of the ceramics formed from zirconia showed that the Young's modulus and Vickers hardness of the samples increased by approximately 53.6 % and 15.7 %, respectively, compared to the traditional forming process.

Generation mechanism and temporal–spatial evolution of electron excitation induced by an ultrashort pulse laser in zirconia ceramic

Femtosecond lasers have been applied in the machining of zirconia (ZrO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{ZrO}}_2}$$\\end{document}) ceramics because of their ultrashort pulse duration and high peak power. However, the high-precision micromachining of zirconia remains challenging owing to an unclear understanding of the ultrafast laser–material interaction mechanisms. In this study, the transient processing phenomenon induced by a femtosecond laser pulse, namely electron excitation (filament), occurring on a picosecond to nanosecond timescale, was directly observed and quantitatively investigated inside ZrO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{ZrO}}_2}$$\\end{document}. A pump-probe imaging method comprising a focusing probe beam integrated with a high-speed camera was used. The evolution process, type variation, and related interaction mechanisms of the filament were revealed under different parameters, including laser pulse energy, sample position, and pulse duration. Finally, the generation mechanism of multiple filaments was clarified and successfully elucidated. This study reveals the ultrafast laser–material interaction mechanisms and the high-precision processing in the laser drilling of zirconia ceramics.

Bonding of Composite Cements Containing 10-MDP to Zirconia Ceramics Without Dedicated Ceramic Primer.

To measure zirconia-to-zirconia microtensile bond strength (µTBS) using composite cements with and without primer. Two Initial Zirconia UHT (GC) sticks (1.8x1.8x5.0 mm) were bonded using four cements with and without their respective manufacturer's primer/adhesive (G-CEM ONE [GOne] and G-Multi Primer, GC; Panavia V5 [Pv5]), and Panavia SA Cement Universal [PSAu], and Clearfil Ceramic Plus, Kuraray Noritake; RelyX Universal (RXu) and Scotchbond Universal Plus [SBUp], 3M Oral Care). Specimens were trimmed to an hour-glass shaped specimen whose isthmus is circular in cross-section. After 1-week water storage, the specimens were either tested immediately (1-week μTBS) or first subjected to 50,000 thermocycles (50kTC-aged μTBS). The fracture mode was categorized as either adhesive interfacial failure, cohesive failure in composite cement, or mixed failure, followed by SEM fracture analysis of selected specimens. Data were analyzed using linear mixed-effects statistics (α = 0.05; variables: composite cement, primer/adhesive application, aging). The statistical analysis revealed no significant differences with aging (p = 0.3662). No significant difference in µTBS with/without primer and aging was recorded for GOne and PSAu. A significantly higher µTBS was recorded for Pv5 and RXu when applied with their respective primer/adhesive. Comparing the four composite cements when they were applied in the manner that resulted in their best performance, a significant difference in 50kTC-aged μTBS was found for PSAu compared to Pv5 and RXu. A significant decrease in µTBS upon 50kTC aging was only recorded for RXu in combination with SBUp. Adequate bonding to zirconia requires the functional monomer 10-MDP either contained in the composite cement, in which case a separate 10-MDP primer is no longer needed, or in the separately applied primer/adhesive.

A deep-neural network potential to study transformation-induced plasticity in zirconia

Zirconia (ZrO2) ceramics uniquely exhibit transformation-induced plasticity, allowing plastic deformation prior to failure, setting them apart from most other ceramics. However, our understanding of ZrO2 plasticity is hindered by the challenge of simulating stress-induced atomic-scale phase transformations, owing to lack of an efficient interatomic potential accurately representing polymorphism and phase changes in ZrO2. In this work, we introduce a novel deep neural network interatomic potential, DP-ZrO2, constructed using a concurrent-learning approach. DP-ZrO2 reproduces properties of various ZrO2 phases, matching their phase diagrams as well as transformation pathways with accuracy comparable to ab initio density functional theory. Leveraging DP-ZrO2, we conducted molecular dynamics simulations of temperature-induced interphase boundary migration and nanocompression. These simulations demonstrate the potential’s efficiency and applicability in studying deformation microstructures involving phase transformations in ZrO2. Our approach opens the door to large-scale simulations under complex loading conditions, which will shed light on the conditions favouring ZrO2 transformation-induced plasticity.

Densification, microstructure, and nanomechanical evaluation of pulsed electric sintered zirconia-silicon nitride reinforced Ti-6Al-4 V alloy

In this study, the influence of silicon nitride (Si3N4) and zirconia (ZrO2) ceramics was examined on the titanium alloy using the pulsed electric current sintering technique to investigate the microstructural behavior, densification, and nanomechanical properties of these composites. Si3N4 and ZrO2 were dispersed in Ti-6Al-4 V at a functional pressure of 50 MPa, a sintering temperature of 1200 °C, a heating rate of 100 °C/min, and a holding time of 10 min. The ternary composite samples were prepared viz, Cs1 (Ti6Al4V-3 vol.% Si3N4-15 vol.% ZrO2), Cs2 (Ti6Al4V-3 vol.% Si3N4-10 vol.% ZrO2), and Cs3 (Ti6Al4V-3 vol.% Si3N4-5 vol.% ZrO2). The bulk morphology of the composites was studied using scanning electron microscopy (SEM) equipped with EDS, and the phase contents were identified with an X-ray diffractometer (XRD). The relative density results for the tri-composites showed that the Cs1 sample recorded the highest at 99.94%, producing a fully dense sintered composite. However, there was a drop in the relative density of composites Cs2 and Cs3, recording 97.73% and 97.11%, respectively, comparable to the unreinforced Ti-6Al-4 V alloy with 98.65%. The nanoindentation examination conducted for the trio-composites showed linear mechanical responses/improvement, with Vickers hardness, from 589.31 to 865.70 MPa; nano hardness, from 6.466 to 9.441 GPa, and elastic modulus, from 113.52 to 185.95 GPa.

Effect of sintering temperature on the microstructures and mechanical properties of ZrO2 ceramics fabricated by additive manufacturing

The optimization of the sintering process for ceramic stereolithography-based additive manufacturing is attracting high interest important due to its correlation with the final properties of the fabricated parts. In this work, a photosensitive zirconia suspension with low viscosity and high solid loading (48 vol%) was prepared using surface-modified zirconia ceramic powders, resulting in an improvement in long-term stability. The correlations between the sintering temperature (1400–1520 °C) and mechanical properties were investigated. The results showed that and the as-sintered zirconia parts exhibited a relatively high density of 99.5 ± 0.1 % and three-point flexural strength of 821 ± 65 MPa as well as excellent fracture toughness (4.37 ± 0.22 MPa m1/2) sintering at 1480 °C, which was attributed to the highly homogeneous microstructure without obvious pores or crack. The obtained insights regarding the correlation between sintering temperature, relative density, and flexural strength can provide practical guidance for the future applications of zirconia ceramics.

Micromilling of high-aspect-ratio ceramic thin walls with customized micro PCD end mills

With the increasing application of ceramic structural parts, how to realize the ductile cutting of ceramics and high-quality machining of ceramic structural parts have become an important research direction. In this paper, the micromilling of fully sintered zirconia ceramics is studied using customized PCD end mills with three different rake angles. The critical thickness for microcutting of ceramics during the micromilling process is theoretically analyzed and experimentally identified. Then, the ductile cutting of ceramic materials is to be realized by controlling the key cutting parameter (feed per tooth fz) considering the critical cutting thickness. Under the condition of ductile cutting, the high-aspect-ratio vortex curved thin walls of fully sintered zirconia ceramics are successfully fabricated. Furthermore, the tool wear of the customized PCD end mill is analyzed. The tool geometric parameters that are beneficial to ensure both ductile cutting and high tool life are identified.

STABILITY OF BIOFUNCTIONALIZED HIGH-PERFORMANCE OXIDE CERAMICS AGAINST RADIATION STERILIZATION

While high-performance ceramics like alumina and zirconia exhibit excellent wear resistance, they provide poor osseointegration capacity. As osseointegration is crucial for non-cemented joint prostheses, new techniques have been successfully developed for biofunctionalizing high-performance ceramic surfaces. Stable cell adhesion can be achieved by covalently bound specific peptides. In this study we investigate the effect of sterilization processes on organo-chemically functionalized surfaces.To enhance the performance of alumina-toughened zirconia ceramics (ATZ), a 3-aminopropyldiisopropylethoxysilane (APDS) monolayer was applied and coupled with cyclo-RGD peptides (cRGD) by using bifunctional crosslinker bis(sulfosuccinimidyl)suberat (BS³). The samples were sterilized using e-beam or gamma-sterilization at 25 kGy, either before or after biofunctionalization with cRGD. Functionalization stability was investigated by contact angle measurements. The functionality of cRGD after sterilization was demonstrated using proliferation tests and cytotoxicity assays. Immunofluorescence staining (pFAK, Actin, DAPI) was conducted to evaluate the adhesion potential between the samples and human mesenchymal stem cells (hMSCs).Functionalized samples before and after sterilization showed no significant difference regarding their contact angles. A proliferation test demonstrated that the cells on functionalized samples proliferate significantly more than on untreated samples before and after sterilization. hMSCs showed a significant higher proliferation on gamma sterilized samples compared to all other groups after 14 days. It was confirmed that the samples did not exhibit cytotoxic behavior before or after sterilization. Fluorescence microscopy demonstrated that both, cells on sterilized and on non-sterilized samples, expressed high levels of pFAK-Y397.The investigated functionalization enables improved adhesion and proliferation of hMSCs and is stable against the investigated sterilization processes. This is of importance as the option of having a sterile product enables the start of the translation of this biofunctional coating towards preclinical and subsequently first-in-man applications.Acknowledgments: We acknowledge the financial support of the Federal Ministry of Education and Research, BMBF (13GW0452A-C).

Zirconia Ceramics.

Zirconia ceramics have become popular among other dental ceramics thanks to their biological, mechanical, optical, and aesthetic properties. CAD/CAM (computer-aided design/ computer-aided manufacturing) technology improvement has played a vital role in the increased popularity of zirconia ceramics; easy computer manipulation significantly expanded the possibility of using different types of restorations. Zirconia ceramics have a broad spectrum of indications in prosthetic dentistry, from simple restorations to complex structures supported by dental implants. A good orientation in the classification, features, and manipulation of zirconia ceramics is the main key to success.

Adsorption of oral antibacterial agents on zirconia surfaces with different crystal systems.

Zirconia ceramics are widely used as dental prosthetics owing to their high biocompatibility, excellent mechanical strength, and aesthetic properties similar to color tones of natural teeth. However, there exists a growing demand for the facile attachment of antibacterial properties in long-term dental restoration. Thus, in this study, we evaluated the adsorption ability of cetylpyridinium chloride (CPC) and benzalkonium chloride (BKC)-quaternary amines widely used as antibacterial substances in commercial toothpaste and other oral care products-onto zirconia surfaces with tetragonal and monoclinic crystal structures. Although tetragonal zirconia has been widely used in dental prosthetic materials such as crowns etc., monoclinic zirconia has also been used under oral conditions because of long-term implantation. When antibacterial molecule loading on zirconia powders under simulated oral conditions, it was revealed that monoclinic zirconia adsorbed approximately five times more CPC and BKC per unit area compared with that of tetragonal zirconia. Moreover, in tetragonal zirconia, the adsorption amounts of both CPC and BKC increased slightly with growing Y2O3 content as a stabilizer. This phenomenon was attributable to the formation of complexes between rare earth elements (REE) such as Y2O3 in zirconia and quaternary amines such as CPC and BKC. In this study, the antibacterial molecular adsorption ability of dental zirconia was observed, and new advantages of zirconia in dental applications were discovered.

Study of the Effect of Nanostructured Grains on the Radiation Resistance of Zirconium Dioxide Ceramics During Gas Swelling under High-dose Irradiation with Helium Ions

Study of the Effect of Nanostructured Grains on the Radiation Resistance of Zirconium Dioxide Ceramics During Gas Swelling under High-dose Irradiation with Helium Ions

ELECTRON-BEAM SYNTHESIS OF ZIRCONIA CERAMIC COATINGS IN THE FOREVACUUM PRESSURE RANGE

We report the results of our experimental investigations on the synthesis of ceramic protective coatings based on zirconium oxide stabilized with yttrium oxide (ZrO<sub>2</sub>-Y<sub>2</sub>O<sub>3</sub>) by electron-beam evaporation of a solid target in the forevacuum pressure range (5-50 Pa). The mass-to-charge composition of the beam plasma was measured in the course of target evaporation. The obtained thick coatings (over 100 μm) have a columnar structure. The article includes analysis of the factors that affect changes in morphology, hardness, and tribological properties of the obtained coatings.

A comparative analysis of mechanical and optical behavioral patterns of translucent zirconia ceramics.

This research aimed to assess and compare the translucency and mechanical properties of partially stabilized zirconia in contrast to lithium disilicate, particularly within the context of translucent zirconia. The experimental design entails examining fifty samples, with ten drawn from each of the five distinct categories of ceramic materials, as part of this in vitro study. Translucency is measured using the Konica Minolta CM-3600D spectrophotometer, assessing Delta E through Lab values against white and black backgrounds. Flexural strength is analyzed via a 3-point bend test on a universal testing machine, with a controlled crosshead speed set at 1 mm/min. The study included the five categories of ceramic materials, each consisting of ten samples: High-strength zirconia (Katana HT), translucent zirconia (e.max Zircad MT and Cercon ht ML), and lithium disilicate (Press MT and LT). The Konica Minolta CM-3600D spectrophotometer is utilized to measure the translucency parameter. This involves determining the color difference (Delta E) by comparing the L*a*b values against both white and black backgrounds. The flexural strength (FS) of zirconia and lithium disilicate materials was analyzed through a 3-point bend test, aiming to compare their respective strengths. The testing procedure was carried out on a universal testing machine with a controlled crosshead speed set at 1 mm/min. The FS was calculated using the formula σ = FL/πR3 for circular disks, where σ represents the FS, F is the fracture load, L is the span length in millimeters, and R is the radius of the disk. The Student's t-test was employed for statistical analysis. The mean translucency parameter for e.max Press MT (6.33 ± 1.05) was significantly greater than all the specimens investigated. The Cercon ht ML exhibited a slightly higher translucency (2.18 ± 0.52) compared to e.max Zircad MT (1.49 ± 0.69), with a statistically significant difference (P = 0.022). Conversely, the FS of e.max Zircad MT (26.97 ± 2.06) was significantly greater (P < 0.001) than that of Cercon ht ML (23.25 ± 2.36). Notably, the Katana HT material demonstrated the highest load strength (32.92 ± 3.10), a statistically significant difference compared to its counterparts (P < 0.001). Among the materials tested, lithium disilicate ceramics exhibited the highest translucency, with its MT variant demonstrating the lowest strength. Katana HT displayed significantly greater biaxial FS compared to translucent zirconia, surpassing even lithium disilicate. Translucent zirconia proved to be notably more translucent than high-strength zirconia. Within the category of translucent zirconia, e.max Zircad MT exhibited substantially higher FS than Cercon.

The impact of CO2 laser on the bond strength of translucent zirconia and traditional zirconia with the resin cement: In vitro study

Background/Aim: The aim of this study is to compare the impact of CO 2 laser on traditional zirconia and translucent zirconia in terms of the bonding strength to resin cement. Material and Methods: In this in-vitro study 20 zirconia disks (10 mm diameter and 2 mm thickness) were assigned to two groups (n = 10). Group 1: 10 discs of traditional zirconia, Group 2: 10 discs of translucent zirconia, CO 2 lasers were used for pretreatment of zirconia surface, respectively. Dentinal disks were cemented on zirconia disk using dual-curing resin cement. Shear bond strength tests were performed at a crosshead speed of 1 mm/min after 24 h distilled water storage. Data was analyzed by T Student's test. Results: The means and standard deviations of shear bond strength values in Group 1, and Group 2 were 1.15, 0.38, 1.99 , 0.65 MPa respectively. Data showed that application of CO 2 laser resulted in a significant higher shear bond strength of resin cement to translucent zirconia ceramics (p &lt; 0.05). Conclusions: Application of CO 2 laser treatment (3W) increases the bond strength to the resin cement of translucent zirconia compared to traditional zirconia.

Marginal chipping factor in machinable zirconia and lithium disilicate ceramic veneer restorations

Background:The marginal edges of the laminate veneer ceramic restorations are very critical areas that play a very determinant role in the success of this type of esthetic restorations.Aim:To evaluate the marginal chipping factor of laminate veneer restorations prepared from both zirconia lithium disilicate materials. Materials and methods:Two typodont teeth representing the upper central incisors were used to prepare laminate veneers with an overlapping incisal design. These veneers were fabricated in two thicknesses (0.5 mm and 0.3 mm) using three types of multilayer zirconia materials with varying yttria percentages (3Y, 5Y, and 3Y/5Y) as well as lithium disilicate computer-aided-design (CAD) blocks. The veneers were divided into four groups based on the yttria percentage and further subdivided into two subgroups for the thickness. Top-view images of the veneer margins were captured and imported into the Image Pro Plus software. The average periphery of each veneer margin was 30 mm. The lengths of the chipped margins were measured under a stereomicroscope, and the CF was calculated.Results: One-way ANOVAsuggested significant differences among the ceramics. e.max CAD ceramic veneers exhibited the highest mean CF values (4.72, 5.9), whereas 3Y/5Y zirconia veneers demonstrated the lowest mean CF values (0.74, 1.54) for 0.5 mm and 0.3 mm thicknesses, respectively. Independent t-tests indicated no significant difference in the CF between 0.5 mm and 0.3 mm thicknesses for each ceramic material.Conclusion:Different ceramic materials exhibited varying levels of marginal chipping, with zirconia ceramics demonstrating lower CF than lithium disilicate ceramics. Reducing the veneer thickness from 0.5 mm to 0.3 mm did not significantly affect the CF.

Mechanism underlying ultraviolet-irradiation-induced discoloration of dental zirconia ceramics stabilized with 3 and 5 mol% yttria

The present study aimed to analyze the mechanism underlying the color alteration of dental zirconia ceramics stabilized with 3 and 5 mol% yttria (3YZ and 5YZ, respectively) caused by 254-nm ultraviolet (UV) irradiation. Fully sintered specimens of 3YZ (Zpex, Tosoh) and 5YZ (Zpex Smile, Tosoh) were irradiated with 254-nm UV at 4 mW/cm2 (up to 60 min). Color alteration (ΔE) was analyzed using the CIE L*a*b* colorimetric system. The UV-induced excitation of oxygen vacancies, which capture two electrons in zirconia (i.e., F center) was analyzed using photoluminescence (PL) and electron spin resonance (ESR) spectroscopy. Furthermore, the spontaneous change in ΔE after cessation of UV irradiation was evaluated. UV irradiation increased ΔE to 13.4 in 3YZ and 10.3 in 5YZ within the initial 10 min of irradiation, and then, ΔE reached a plateau. A PL peak was observed at 435 nm, which decreased with UV irradiation. In addition, UV irradiation generated an ESR signal attributed to the F+ centers formed via one-electron detachment from F centers. The signal intensities of both 3YZ and 5YZ increased with irradiation time; however, the signal intensity was higher in 3YZ than that in 5YZ. After cessation of UV irradiation, ΔE gradually decreased to approximately 2, which took a few weeks. The discoloration of 3YZ and 5YZ caused by UV irradiation was due to the excitation of F centers. Although the specimens spontaneously reverted to their original color over time, clinical procedures involving UV irradiation, such as photo-functionalization, should be performed with caution to avoid damaging the esthetics of dental zirconia restorations.

The metameric effect of monolithic zirconias with varying yttrium ratios.

To evaluate the metameric disparities among monolithic zirconia materials with differing yttrium compositions across various lighting conditions. Thirty-six square-shaped zirconia samples measuring 10 × 10 × 0.5 mm were prepared from monolithic zirconia materials with three different yttrium contents. A 0.2 mm thick layer of polymerized dual-polymerizable self-adhesive resin cement was created using a silicone mold with the same dimensions as the prepared zirconia specimens. To evaluate metamerism, color measurements were conducted using a spectrophotometer device on a neutral gray background in a color measurement cabinet that offers four different illumination environments. All samples underwent aging by subjecting them to 10000 thermal cycles using a thermal cycle tester. Following thermal aging, color measurements were taken once more, and the data were recorded using the CIE L*, a*, b* color system. Two-way ANOVA and Post-hoc Bonferroni tests were employed to analyze the data. It was observed that there was no statistical difference among the color measurements made in different illumination environments of the monolithic zirconia ceramics used to evaluate metamerism (P > .05). This observation remained consistent both before and after thermal aging. After thermal aging, the color of monolithic zirconia materials exhibited a tendency towards red and yellow hues, accompanied by a decrease in brightness levels. It can be stated that different illumination conditions did not affect the metamerism of monolithic zirconia materials, but there was a color change in monolithic zirconia materials after a thermal aging period equivalent to one year.