Schwickerath adhesion tests of porcelain veneer and stereolithographic additive-manufactured zirconia

The adhesion between zirconia core and porcelain is an essential factor that affects the long-term stability of zirconia-based restorations. This paper was aimed to evaluate the adhesion between commercially available porcelain veneering materials and digital light processing (DLP)-manufactured zirconia. The adhesion characteristics were compared with those of veneer and zirconia core fabricated by conventional subtractive manufacturing. The DLP-manufactured zirconia substrates were veneered by one experienced dental technician with the following porcelains: IPS e.max Ceram (DE), VM9 (DV), and VintageZR (DZ). The subtractive-manufactured substrate was veneered by VintageZR (SZ) only. The results showed that the zirconia fabricated by DLP showed adequate bonding when veneered by commercial dental porcelains. Among the DLP-manufactured zirconia substrates with different brands of dental porcelains, DZ showed a significantly higher bond strength and strain energy release rate than the other two groups (p < 0.05). No significant statistical difference in bond strength or strain energy release rate was observed between substrates veneered by the same porcelain (p > 0.05). On the basis of the results, the adhesion between DLP-manufactured zirconia and the veneer ceramic can be affected by the brand of the dental porcelain. The zirconia fabricating method did not affect bonding of the zirconia/porcelain structure.     

Dental ceramic prostheses by stereolithography-based additive manufacturing: potentials and challenges

ABSTRACT

The current method for making dental ceramic prostheses in a subtractive manner causes a severe waste of materials while requires excessive manual works that bring in the uncertainty for control of quality. The rapid development and commercialisation of additive manufacturing (AM) has aroused interest and wonders both in material and dental communities about their potentials and challenges in fabricating of ceramic prostheses in a materials-saving manner. In this work, AM approach was applied to fabricate the dental bridges and implants made of zirconia. The achieved geometries and dimensional accuracy are used to demonstrate the potential of this technique in fabricating of dental ceramic prostheses, whereas the observed macro and micro defects formed during the treatment process is used to reveal the challenges facing in order to adapt this technology into real dental practice. Suggestions are provided for future development of the technology, particularly on minimising the processing defects.     

Gelcast zirconia ceramics for dental applications combining high strength and high translucency

Tetragonal zirconia polycrystals (TZP) represent a favorite material for monolithic ceramic dental restorations. However, all approaches employed so far to improve the translucency of dental zirconia ceramics are accompanied by a significant decline in strength. In this investigation, we developed dental 3Y-TZP ceramics that can provide excellent strength combined with enhanced translucency. The machinable tetragonal zirconia discs and blocks were prepared from fine mesostructured zirconia particles stabilized with 3 mol% of yttria using the gelcasting method. Zirconia ceramics with an average biaxial strength of 1184 MPa and translucency of 41.1% for a 1 mm thick sample were obtained. Due to its unique microstructure, this tetragonal ceramic provided a favorable combination of high translucency comparable to the high-translucent, tetragonal/cubic 4Y-TZP and very high strength achievable only in the pure tetragonal 3Y-TZP. The applicability and resistance to low-temperature degradation of the new dental ceramics was demonstrated.     

Digital light processing additive manufacturing of in situ mullite-zirconia composites

Digital light processing (DLP) can produce small series ceramic parts with complex geometries and tiny structures without the high cost of molds usually associated with traditional ceramic processing. However, the availability of feedstock of different ceramics for the technique is still limited. Mullite-zirconia composites are refractory materials with diverse applications, nevertheless, their 3D printing has never been reported. In this work, alumina and zircon were used as raw materials for additive manufacturing by DLP followed by in situ mullite and zirconia formation. Thus, coarse zircon powder was milled to submicrometric size, alumina-zircon photosensitive slurries were prepared and characterized, parts were manufactured in a commercial DLP 3D printer, debound, and sintered at different temperatures. The printed parts sintered at 1600 °C completed the reaction sintering and reached a flexural strength of 84 ± 13 MPa. The process proved capable of producing detailed parts that would be unfeasible by other manufacturing methods.     

Effects of Y-TZP blank manufacturing control and addition of TiO2 nanotubes on structural reliability of dental materials

Titanium dioxide (TiO₂) nanotubes have been applied to enhance the mechanical and biological properties of dental materials. Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) have been increasingly used in dentistry as a substructure for crowns and fixed partial prostheses. Aside from its optimal clinical results, Y-TZP is prone to failures due to microstructure-related defects introduced in the manufacturing process that may lower its structural and clinical reliability. The purpose of this study was to evaluate the role of the manufacturing process of blanks as well as their original composition modification by addition of TiO₂ nanotubes (0%, 1%, 2% and 5% in volume) while controlling all manufacturing steps. Materials were subjected to a biaxial flexural strength test, a fractographic qualitative analysis by scanning electron microscopy (SEM), a microstructure evaluation in field emission-SEM and X-ray diffraction. Values of flexural strength were subjected to ANOVA, Tukey (α = 0.05) and Weibull statistics. Grain size values were subjected to Kruskal-Wallis and Dunn tests (α = 0.05). Highlights of the results include that for experimental Y-TZP added 2% vol TiO₂ nanotube ceramics presented flexural strength values at 577 MPa and Weibull modulus (m) at 8.1. The addition of TiO₂ nanotubes in different blends influenced experimental Y-TZP properties, leading to lower flexural strength, although they presented higher m than the commercial Y-TZP. Nanotubes also led to bigger grain sizes, more pores and a slight increase in the monoclinic phase, influencing the microstructure of Y-TZP. Y-TZP blank manufacturing control as well as addition of TiO₂ nanotubes led to higher m values and, hence, greater structural reliability.     

Three-dimensional complex construct fabrication of illite by digital light processing-based additive manufacturing technology

Illite is a group of clay minerals that are expected to be widely used in catalyst fabrication, radioactive element adsorption, and so forth, due to its excellent adsorption properties. However, the shape control limitation of the illite product should be overcome to maximize its utilization and properties. We herein propose additive manufacturing (AM) as one of the best solutions to solve this structural drawback. Digital light processing (DLP) technology with the film-type of the material supplying system was adapted instead of the general vat-type DLP system to increase illite printability. The photo-curability and printability of illite-contained photocurable suspension were optimized. The color effect due to different ferric oxide content in yellow- and white-illite which influence the photopolymerization process also adjusted thoroughly. White illite showed better photo-curability and could be increased solid loading than yellow illite. The defect-free illite products with three-dimensional complex structures, which cannot be produced by typical ceramic processes, were obtained by DLP technology for both yellow- and white-illite after sintering at 1100°C. The overcoming of shape control limitation of illites by ceramic AM proved in this study has excellent potential for expanding illite utilities in various applications.     

Improving wettability,antibacterial and tribological behaviors of zirconia ceramics through surface texturing

3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramics are promising restorative materials being extensively used for fabricating dental prosthodontics. However, peri-implant inflammation and the severe abrasive wear on occlusive natural teeth are two critical problems in the clinical application of zirconia dentures. The paper aims to improve the antibacterial and tribological performance of 3Y-TZP ceramics through laser surface texturing. Three types of surface textures, including micro-honeycombs, micro-composite grids, and micro-grooves, were fabricated onto the zirconia specimens. The effects of different microtextures on the surface behaviors, including wettability, bacteria adhesion, and wear behavior, of 3Y-TZP ceramics were rigorously studied. The results indicate that the introduction of microtextures can change the solid-liquid contact of the zirconia surface, thus affecting its wettability. Wettability is a decisive factor that determinines the antibacterial behavior of textured zirconia ceramics. A hydrophobic surface is more conducive to inhibiting the adhesion, extension, reproduction of bacteria and thus achieves a superior antibacterial performance. The examined microtextures yield the improvement of wear resistance for the zirconia ceramics, but their performances depend on the texture density and the structural strength. The results obtained can provide technical guidance for the design and application of microtextures in the restorative dental fields.     

Additive manufacturing of electrofused mullite slurry by digital light processing

Mullite, one of the main refractory materials, has several applications that may demand tiny structures with complex geometries, and digital light processing (DLP) can produce such parts with outstanding dimensional precision and surface quality. In this work, electrofused mullite powder was used as a raw material for additive manufacturing by DLP. Photosensitive mullite suspensions were developed and their rheological behavior, stability, and thermal decomposition were investigated. Mullite parts were printed from suspensions with different ceramic loadings, debound, and sintered at different temperatures (from 1500 to 1650 °C). Density and strength increased with an increase in both solid loading and sintering temperature. Printed parts from slurry with 50 vol% of solid loading sintered at 1650 °C reached a relative density of 97.7 ± 0.3 % and flexural strength of 95.2 ± 5.0 MPa.     

Extrusion-based additive manufacturing of functionally graded ceramics

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.     

Characterization of zirconia specimens fabricated by ceramic on-demand extrusion

The Ceramic On-Demand Extrusion (CODE) process is a novel additive manufacturing method for fabricating dense (~99% of theoretical density) ceramic components from aqueous, high solids loading pastes (>50?vol%). In this study, 3?mol% Y₂O₃ stabilized zirconia (3YSZ) specimens were fabricated using the CODE process. The specimens were then dried in a humidity-controlled environmental chamber and afterwards sintered under atmospheric conditions. Mechanical properties of the sintered specimens were examined using ASTM standard test techniques, including density, Young’s modulus, flexural strength, Weibull modulus, fracture toughness, and Vickers hardness. The microstructure was analyzed and grain size measured using scanning electron microscopy. The results were compared with those from Direct Inkjet Printing, Selective Laser Sintering, Lithography-based Ceramic Manufacturing (LCM), and other extrusion-based processes, and indicated that zirconia specimens produced by CODE exhibit superior mechanical properties among the additive manufacturing processes. Several sample components were produced to demonstrate CODE’s capability for fabricating geometrically complex ceramic components. The surface roughness of these components was also examined.     

Fabrication of ZrO2 and ATZ materials via UV-LCM-DLP additive manufacturing technology

The development of photosensitive slurries for additive manufacturing is attracting high interest due to their correlation with the final properties of the fabricated parts. Lithography-based ceramics manufacturing (LCM), i.e. digital light processing (DLP), is one of the techniques that is receiving high attention. Within the different ceramic materials, ZrO₂ and ATZ are the most studied due to their potential applications including in the dental field. In this work, the fabrication of ZrO₂ and ATZ materials via LCM-DLP is shown. A comparison between one- and two-step procedures to develop UV-curable ZrO₂ and Al₂O₃-ZrO₂ slurries was performed. The printed parts with a relative density of ∼99% for both ZrO₂ and ATZ were subjected to 4-point bending measurements, obtaining flexural strength values around 800 MPa for both ZrO₂ and ATZ materials prepared by using the one-step slurry.     

Effect of Fe2O3 doping on color and mechanical properties of dental 3Y-TZP ceramics fabricated by stereolithography-based additive manufacturing

In this study, iron(III) oxide (Fe₂O₃)-doped zirconia (3Y-TZP) ceramics with desirable mechanical and color properties for dental restorations were fabricated by stereolithography-based additive manufacturing. Six zirconia ceramic paste specimens with high solid loading (58 vol%) and reasonably low viscosity were prepared according to doped content of Fe₂O₃ (0–0.14 wt%). Zirconia ceramics were fabricated using commercial stereolithography three-dimensional printer and sintered at 1500 °C for 4 h to obtain final dense parts with a relative density of above 99%. Effects of Fe₂O₃ doping on microstructure, mechanical properties, and color of 3Y-TZP ceramics were investigated. Results indicate that Fe₂O₃ exhibited little effect on the shrinkage and density of colored ceramics compared to uncolored ceramics. Average grain size of 3Y-TZP ceramics sintered at 1500 °C increased with increasing content of Fe₂O₃. X-ray diffraction analysis showed that tetragonal phase was dominant phase structure of white and colored 3Y-TZP ceramics, and monoclinic phase increased with increasing Fe₂O₃ content. Compared to uncolored specimens, Fe₂O₃ exhibited negative effects on three-point flexural strength (mean > 879.70 MPa), Vickers hardness (mean > 12.14 GPa), and indentation fracture toughness (mean > 4.23 MPa m^1/2) of the colored specimens. With the increase in the content of Fe₂O₃ from 0 to 0.14 wt%, L* (black–white index) value decreased from 83.39 to 79.54, a* (green–red index) value increased from −2.28 to −0.74, and b* (blue–yellow index) value increased from 1.15 to 17.94. Chromaticity (L*, a*, b*) fell within the range of natural tooth color, indicating that it is suitable for dental application because of its color compatibility with natural teeth. In addition, the transmittance slightly decreased with increasing Fe₂O₃ content. Thus, Fe₂O₃-doped 3Y-TZP ceramics can be used as potential candidates for aesthetic dental restoration materials.     

Fabrication of highly bioactive zirconia ceramics via grain-boundary activation for dental implants

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics with outstanding mechanical properties and aesthetic origins are expected to be used in dental implant applications. However, tetragonal zirconia ceramics are not bioactive, which affect the osseointegration and reliability as dental implant materials. Herein, in this study, Y-TZP ceramics were modified by grain-boundary activation via coating a bioactive glass (BG) sol with different content on the crystal surfaces of zirconia powder and followed by being gelled, dried, granulated, low-temperature treated, molded and sintered at 1450°C for 3 h in air. The effects of BG content on the morphology, phase compositions, mechanical properties, in vitro mineralization ability and cell biological properties of the bioactivity modified Y-TZP ceramics were evaluated. The BG additive did not affect the tetragonal–monoclinic phase transformation of ZrO₂. However, the addition of BG decreased the flexural strength of the modified Y-TZP ceramics compared to that of Y-TZP. The in vitro mineralization results showed that a homogeneous apatite layer was produced on the surface of the Y-TZP ceramics when they were immersed in the simulated body fluid for 21 days. The cell response results indicated that the bioactive surface modification of Y-TZP ceramics could promote cell adhesion, propagation and osteogenic differentiation performance. Thus, our research results suggest that the highly bioactive Y-TZP ceramics could be a potential candidate for dental implant material.     

Additive manufacturing of fiber reinforced ceramic matrix composites: Advances,challenges, and prospects

Fiber reinforced ceramic matrix composites (FRCMCs) have been used in various engineering fields. Additive manufacturing (AM) technologies provide new methods for fabricating FRCMCs and their structures. This review systematically reviews the additive manufacturing technologies of FRCMCs. In this review, the progress for additive manufacturing of FRCMCs were summarized firstly. The key scientific and technological challenges, and prospects were also discussed. This review aims to motivate the future research of the additive manufacturing of FRCMCs.     

玻璃渗透氧化锆全陶瓷牙科材料的制备和表征

开发了一种适合渗透多孔四方相氧化锆的玻璃,并通过玻璃渗透工艺制备了氧化锆-玻璃全陶瓷牙科材料. 研究表明,该渗透玻璃在渗透温度下(1100~1200℃)具有合适的粘度、良好的渗透性和化学相容性,且热膨胀系数与氧化锆匹配;熔融态玻璃通过毛细管作用力填充预烧后的多孔四方相氧化锆坯体的孔隙,形成氧化锆和玻璃相互交融的致密的三维网络结构,渗透过程中没有发生氧化锆从四方相到单斜相的转变. 该氧化锆-玻璃复合材料的弯曲强度和断裂韧性分别为400 MPa和5.5 MPa×m1/2,较氧化玻璃复合材料分别提高了32%和41%.     

Zirconia-multiwall carbon nanotubes dense nano-composites with an unusual balance between crack and ageing resistance

Yttria stabilized zirconia (Y-TZP) ceramics are used in a wide variety of applications, such as orthopaedic and dental implants. Y-TZP offers indeed a unique combination of biocompatibility and mechanical properties (high crack resistance for a ceramic). However, the major drawback of Y-TZP is their lack of stability: zirconia is prone to ageing, especially under humid atmosphere. Increasing the ageing resistance of Y-TZP led so far to a decrease of toughness and crack resistance. Here we show that the addition of a small volume fraction of multiwall carbon nanotubes (MWCNT) in a polycrystalline nano-structured Y-TZP sintered under specific conditions (Spark Plasma Sintering) leads to a material exhibiting a balance between ageing and crack resistance never reached before.     

Acid etching and silica coating effects on Y-TZP topography and ceramic/resin cement bond strength

Zirconia-based (Y-TZP) dental prostheses' retention loss remains one of the most frequently reported difficulties in dental practice. Selective Infiltration Etching (SIE) treatment has been proposed to alter the final Y-TZP topography improving its bond strength to resin cement. SIE consists of glass film deposition on the Y-TZP surface before glass removal with 10% hydrofluoric acid etching. This study's objective was to investigate the effects of SIE protocol steps on Y-TZP topography and its consequence on its bond strength to resin cement. The SIE protocol was divided into two main steps: silica coating (first step) and glass dissolution by hydrofluoric acid (second step), and the null hypothesis was that the SIE treatment has the same bond quality independently of the HF exposition time. Atomic Force Microscopy (AFM), Scanning Electron and Transmission Electron microscopy were used to characterize the zirconia surfaces. The bond strength was evaluated by the interfacial fracture toughness test, and data were analyzed by ANOVA and Tukey's Test (5%). As the main result, it was shown that, when exposed during sufficient time, the 10% HF can etch zirconia at room temperature, which is responsible for topographic changes, significantly improving the bond quality. Additionally, no glass components or dihedral angles modifications were observed at Y-TZP grain boundaries after silica coating. With the study limitations, it was possible to infer that the surface-modifying agent in SIE protocol is the HF etching, and the null hypothesis was rejected.     

Influence of resin composition on rheology and polymerization kinetics of alumina ceramic suspensions for digital light processing (DLP) additive manufacturing

Alumina ceramics with optimized microstructures and mechanical properties were obtained by the attractive digital lighting processing (DLP) additive manufacturing methodology in the present study. A acrylate-based resin system was designed for the alumina powders with a mean particle size of 0.5 μm. The influence of oligomer on the viscosity and polymerization kinetics of the ceramic suspensions has been elaborately discussed by rheology, curing depth and photo-DSC characterizations. The results indicated that the introduction of oligomer has improved the cross-linking density of resins and decreased the critical dose of energy for resin polymerization, which contributed to a tougher ceramic-resin slice with higher dimensional accuracy. Densifying processes including debinding and high temperature sintering of the ceramic parts were conducted according to the TG-DTA characterizations, alumina ceramics with uniform microstructures and eliminated delamination or intralaminar cracks were finally obtained. The flexural strength was 471 MPa for the ceramics obtained from the resin composition containing 20 wt% oligomer, Weibull modulus for the ceramics were determined to be 17.31 by evaluating thirty all sides polished ceramics, indicating the highly uniform property of the ceramics fabricated by DLP additive manufacturing.     

Y-TZP,Ce-TZP and as-synthesized Ce-TZP/Al2O3 materials in the development of high loading rate digital light processing formulations

Y-TZP, Ce-TZP and Ce-TZP/Al₂O₃ materials are widely investigated in dentistry. Digital Light Processing (DLP) is considered as a breakthrough technology for the dental field to fine print Y-TZP green parts. High loading photocurable formulations (>45 vol%) with Y-TZP, Ce-TZP commercial powders and Ce-TZP/30 vol% Al₂O₃ as-synthesized powder suitable to DLP printing were achieved in this study. A low specific surface area (5–13 m²/g) of particles without any pores and 1 wt% to 2 wt% of steric dispersant are required to obtain high loading formulations. The as-synthesized composites provide these properties by increasing the calcination temperature from 800 °C to 1200 °C. The as-prepared ceramic formulations based on the same photocurable resin exhibit a curing behavior suitable to DLP process for Y-TZP formulations (thickness > 50 μm in few seconds with a high conversion rate) in comparison with ceria ceramic. The ceria is a strongly UV absorbing material and a specific formulation is developed to obtain 80% of conversion and a cured thickness of 75 μm in 0.5 s.     

Accurate printing of a zirconia molar crown bridge using three-part auxiliary supports and ceramic mask projection stereolithography

Zirconia ceramic is a widely used material for dental restoration. Stabilized zirconia all-ceramic teeth have excellent mechanical properties, biocompatibility, and aesthetic properties. At present, the CAD/CAM technique for zirconia all-ceramic dental prosthesis leads to low material efficiency and high tool wear. Although restorations fabricated using additive manufacturing are gaining attention, it is still very challenging to obtain accurate shapes and proper mechanical properties in zirconia restorations. In this investigation, a type of three-part auxiliary support was adopted and added to the occlusal surface to fabricate a typical molar crown bridge. A ceramic solid content of 40 vol% acrylic-based slurry was prepared, and a molar crown bridge was fabricated using mask projection stereolithography. The experimental results showed that the average dimensional error of the printed green body was ±150 μm. The density of the sintered ceramic parts was 6.026 g/cm³, and the three-point bending strength was 541 ± 160 MPa, which is higher than that of human dentin (160 MPa), but lower than that of CAD/CAM zirconia (900–1200 MPa). Although the proposed process still needs to be optimized to improve the mechanical properties and reliability of the crown bridge, the mask projection process combined with the adopted three-part auxiliary supports are capable of individualized manufacturing of complex zirconia crown bridges.