Effect of airborne-particle abrasion of yttria-containing zirconia dental ceramics on mechanical properties before and after regeneration firing

The zirconia-resin adhesion, crucial in fixed dental prostheses, may be mechanically enhanced by airborne-particle abrasion (APA) induced surface roughening. APA may substantially increase the strength of 3Y-TZP, which is associated with (sub)surface compressive residual stresses through t-m phase transformation. In this work, the effects of APA and regeneration firing (RF) (1000 °C, 15 min) of various zirconia dental ceramics containing 3–5 mol.% of yttria were investigated. The phase composition, (sub)surface microstructural changes, and biaxial flexural strengths coupled with Weibull statistics and fractography were analysed and compared. The results show a significant increase in strength for 3Y and 4Y specimens after APA, ascribed to the t-m transformation toughening. However, APA substantially decreased the strength of 5Y variant. After RF, the ferroelastic domain switching phenomenon was presumably the persisting mechanism to withstand the propagation of APA-induced cracks in 3Y and 4Y zirconias, not being inferior to strength values of the as-sintered counterparts.     

Evolution of microstructure and residual stresses in gradually ground/polished 3Y-TZP

A comprehensive study of progressively ground/polished 3Y-TZP was performed with the aim of better understanding the mechanisms driving the microstructural modifications observed after such procedures, and identifying the processing parameters leading to optimal microstructures (i.e. ageing-protective and damage-free). Gradually ground/polished surfaces were produced, yielding four different topographies of increasing roughness (grades 1–4) and two different textures (unidirectionally, U, and multidirectionally, M). Phase transformation, microstructure and residual stresses were investigated by means of advanced characterization techniques. It was found that low-roughness mildly ground/polished specimens (i.e. 2-M/U) presented a nanometric layer with the ageing-related protective features generally associated with coarsely ground specimens. A lower limit for grain refinement in terms of surface abrasion was also found, in which partial recrystallization took place (i.e. 1-M/U). A mathematical relation was established between average surface roughness (S_a), monoclinic volume fraction (V_m) and surface compressive residual stresses, demonstrating that if the processing parameters are controlled, both V_m and residual stresses can be predicted by the measurement of S_a.     

Iron oxide colouring of highly-translucent 3Y-TZP ceramics for dental restorations

The influence of 0.01–2 mol% Fe₂O₃ powder addition on the microstructure, mechanical and optical properties, and hydrothermal stability of highly-translucent 3Y-TZP ceramics is assessed and compared with commercially available co-precipitated 0.18 mol% Fe₂O₃ doped ZrO₂ powder-based ceramics. Only those ceramics with up to 0.1 mol% Fe₂O₃ resulted in a proper shade for dental zirconia ceramics, with a typical composition of 87 vol% t-ZrO₂ and 13 vol% c-ZrO₂. The amount of cubic phase increased at higher Fe₂O₃ content. The hardness (∼13 GPa) and fracture toughness (∼3.6 MPa m^1/2) of the 0.01 mol% - 0.1 mol% Fe₂O₃ doped 3Y-TZP was comparable, whereas the hardness decreased above 0.5 mol% Fe₂O₃ and the fracture toughness decreased above 2 mol% Fe₂O₃. The hydrothermal ageing resistance slightly increased for Fe₂O₃ concentrations up to 1 mol%, whereas the translucency slightly decreased with increasing Fe₂O₃ content.     

Effect of grinding parameters on surface integrity and flexural strength of 3Y-TZP ceramic

Grinding parameter effects on grinding forces, the surface phase transformation of tetragonal to monoclinic (t-m), surface residual stress, surface roughness, and flexural strength of 3 mol% Yttrium stabilized Tetragonal Zirconia Polycrystal (3Y-TZP) were investigated. The results have shown that the grinding force, compressive residual stress, and surface roughness increase with the increase in depth of cut and the feed speed. On the other hand, they decreased as the wheel speed increased. Additionally, grinding improved the 3Y-TZP surface t-m phase transformation. The m-phase content increased with the increase in the depth of cut and decreased as the wheel speed increased. Finally, the flexural strength of ground 3Y-TZP diminished with the increase in the cut depth. The fracture and smear were observed on the dry ground surface since it has a higher surface roughness compared to that of wet grinding.     

The mechanical behavior of the material-tissue and material-material interface in dental reconstructions

In dental and craniofacial sciences, frequently the goal is to replace lost or damaged natural tissue with synthetic materials. For ideal function, these replacement materials must strongly bond to the existing tissue, but they also must form a hermetic seal that eliminates the passage of microorganisms and fluids that would lead to further tissue destruction or weakening of the interface or the individual materials, compromising the final outcome. Therefore, the study of interfaces is crucial, and the manner in which they can be tested to predict the likelihood of success is of great interest to the field.Because a variety of materials and material combinations are used for the repair or replacement of oral and craniofacial tissues, numerous types of material interfaces exist. A complete discussion of this important topic requires an examination of all of them. In this review article, the three different types of interfaces are treated separately. First, the interface between the tooth tissue and restorative material is explored, specifically by considering resin-based materials such as dental adhesives and composite, and the manner in which they interact with dentin and enamel. Second, the interaction between these same resin-based materials and other structures, such as oxide ceramic dental crowns, are explored, because these tooth replacement materials are typically fixed to the remaining tooth structure through the use of resin-based adhesives and cements, or repaired intraorally with similar materials. Finally, the interface between different synthetic materials, such as metals and ceramics, with dental porcelain used as an esthetic veneering material is addressed.     

Evaluation of the effect of low-temperature degradation on the translucency and mechanical properties of ultra-transparent 5Y-TZP ceramics

This study aimed to evaluate the effects of low-temperature degradation (LTD) on the translucency, flexural strength, and surface hardness of ultra-transparent 5Y-TZP ceramics. Three commercial zirconia materials were investigated, including 5Y-TZP and two 3Y-TZP samples. The LTD process was mimicked by hydrothermal aging at 134 °C for 20 h. The translucency parameters (TPs) corresponding to different thicknesses were analyzed. The surface volume fraction of the monoclinic phase was determined by X-ray diffraction (XRD). The three-point flexural strength was measured, and the surface hardness was measured by the indentation method. The translucency of all three ceramics showed a significant decrease with increasing thickness. A translucency gradient was present within a ceramic block of 5Y-TZP, whose translucency was higher than that of both 3Y-TZP products. Aging at 134 °C for 20 h did not reduce the TPs of the three ceramics. The XRD analysis showed a high monoclinic phase content in both 3Y-TZP surfaces after aging, while no monoclinic phase was detected in 5Y-TZP. Hydrothermal aging had no significant effect on the flexural strength; however, the flexural strength of 5Y-TZP was significantly lower than that of 3Y-TZP. No statistical difference in surface hardness was observed among the three products. In summary, 5Y-TZP presented higher LTD resistance than 3Y-TZP.     

Characterization of 3Y-TZP/TiO2 hybrid experimental dental ceramics

The addition of titania to zirconia dental implants has been considered a promising choice to improve its bioactivity. This study aimed to evaluate the effect of different sintering conditions on the microstructure, density, optical properties and flexural strength of a 3Y-TZP/TiO₂ dental ceramic based on zirconia with two different titania contents (7.5 mol% and 12.5 mol%). 3Y-TZP/TiO₂ ceramic powders were synthesized by coprecipitation, uniaxially pressed and sintered at six different sintering conditions. Microstructural analysis of the sintered samples was performed by scanning electron microscopy and X-ray diffraction. Optical properties were measured using a spectrophotometer. The density was determined by Archimedes principle. Flexural strength was estimated by the biaxial flexure device. The microstructure and flexural strength of the 3Y-TZP/TiO₂ dental ceramic with 7.5% and 12.5 mol% were affected by the sintering conditions. Sintering the specimens at 1460 °C for 2 h increased the grain size and significantly decreased the flexural strength of 3Y-TZP/TiO₂ dental ceramic. The interaction (titania content x sintering conditions) affected the relative density and optical properties. A relative density greater than 98% was achieved for the T7.5 groups (sintered at 1260 °C/1 h, 1300 °C/1 h and 1300 °C/2 h) and for the T12.5 groups (sintered at 1260 °C/1 h, 1260 °C/4 h, 1300 °C/1 h and 1300 °C/2 h). The highest values of L*, a* and b* were respectively 87.2 (T7.5 group sintered at 1460 °C/2hs), 4.3 (T12.5 group sintered at 1300 °C/2hs) and 15.8 (T12.5 group sintered at 1300 °C/1 h). The material developed with 12.5 mol% of titania and sintered at 1300 °C/2 h showed high densification, flexural strength of 670 MPa and has good potential to be used in dentistry.     

Improving low temperature degradation of 3Y-TZP ceramics via high temperature carburizing

3Y-TZP ceramics are prepared by solid state method and surface carburization process, and the effect of surface carburization on its the low temperature degradation is studied. The conventional sintered samples completely lost its mechanical properties after aging for 15 h, while the failure time of the surface carburized samples are 300 h. In addition, the nuclear growth rate of the surface carburized samples (αd) and the nucleation rate (Nr) is lower than that of sintered samples, αd plays a dominant role in the degradation process at low temperature and is the key factor determining the aging rate. At the same time, it is found that carbon is dissolved in zirconia lattice in the form of electrically neutral atoms, which will not destroy the original charge balance and produce new oxygen vacancies when entering the interstitial site. More importantly, the precipitation rate of Y³⁺ from zirconia lattice is the key factor to determine the low-temperature phase transition of tetragonal-monoclinic(T-M). The treatment method of surface carburization has significantly improved the low-temperature degradation performance of 3Y-TZP ceramics, which provides a basis for the application of zirconia ceramics in low-temperature and humid environment.     

The effects of dry and wet grinding on the strength of dental zirconia

The purpose of this study was to evaluate the effect of different dry and wet surface finishing on the mechanical strength and surface characters of a dental yttria-stablized zirconia ceramic (Y-TZP). Surface grinding treatments with a dental air turbine handpiece were performed with: coarse diamond (DC) and fine diamond (DF), tungsten carbide (Tc) and fine tungsten carbide (TcF) burs with or without water coolant. Air particle abrasion with 50?µm alumina (APA), combination of burs treatments or burs-abrasion, i.e. DC-TcF and DC-APA, were also performed with non-treatment group as control (C). Statistical analyses (α?=?0.05) on results revealed that all surface treatments significantly increased the surface roughness (Ra) than control (p?<?0.05), whilst decreased breaking force (BFN) and biaxial flexural strength (BFS). Tungsten carbide surface treatment could significantly lower (p?<?0.05) BFS and BFN, but DC only significantly lowered BFN. DC and tungsten carbide treatments exhibited significantly lower BFS values in wet than dry. A positive correlation was found between the BFS and BFN with the number of fragments. Only tetragonal phase of ZrO2 was presented by XRD. Synchrotron XRD revealed the (101) peak exhibits a broadening effect in the tungsten carbide treated specimens (38?nm for Tc and 30?nm for TcF), i.e. grain sizes in these specimens were smaller than the control (60?nm for C). This study outcome suggested that tungsten carbide burs should be avoided for grinding Y-TZP because of significant reduction in the BFS. Water cooling during grinding did not consistently reduce the potential heat damaging effects expected with dry grinding.     

Relationships between fracture toughness,Y2O3 fraction and phases content in modern dental Yttria-doped zirconias

The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y₂O₃-stabilized zirconia (YSZ) materials currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y₂O₃ was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4−5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y₂O₃ concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (K_Ic) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between K_Ic and bulk mol% Y₂O₃ content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between K_Ic and the fraction of the transformable t-phase was established within the range of 30–70 vol%.     

Microwave sintering of dense and lattice 3Y-TZP samples shaped by digital light processing

Nowadays it is possible to produce ceramic parts with solid and complex shapes with rapid and efficient shaping and sintering techniques. In this paper, 3mol% yttria stabilized zirconia (3Y-TZP) dense and lattice parts were shaped by Digital light processing method (DLP) and densified by conventional (CV) and microwave (MW) sintering. 3Y-TZP samples were MW sintered up to 1550 °C with different heating rates (10, 30, and 50 °C/min) for the dense samples and 30 °C/min for the lattice samples. Controlled thermal cycles with a homogenous heating and no thermal runaway was reached. CV sintering was carried out at 10 °C/min up to 1550 °C. No inter-layer delamination was detected after sintering by the two methods. Both dense and lattice MW-sintered samples reached high final densities (equivalent to obtained values with CV-sintered samples, i.e., ≥98% T.D.), but exhibited a lower average grain size than CV-sintered materials. The different architectures between dense and lattice samples resulted in a different specific absorbed power: the power absorbed by the dense sample is lower than that absorbed by the lattice one meaning that this sample architecture heats up easily.     

CHARACTERIZATION OF ULTRAFINE ZIRCONIA PARTICLES PRODUCED BY A GAS-SOLID REACTION

The microstructure of a zirconia powder prepared from a gas-solid reaction was determined by SEM, EDXS, DRX and TEM analysis. At low temperatures (773 K) and up to 0.5 of reaction degree two types of particles were formed: a melaslable tetragonal ZrO₂ phase fixed on the Fe₂O₃ grain surfaces and isolated grains of monoclinic ZrO₂ phase.     

Effect of focused ion beam sample preparation on the phase composition of zirconia

The investigation of phase transformations in metastable ceramic systems such as zirconia often requires local phase analysis within the areas of interest. Electron backscatter diffraction is a suitable method in combination with focused ion beam sample preparation. The interaction between ion beam and sample has to be carefully considered. In case of metastable Y-PSZ and Mg-PSZ, phase transformations were observed after FIB preparation with 30?kV, 30?nA and 5° incidence angle. Damage was the dominating effect for angles of 72°. The expected local temperature increase due to the ion bombardment with 30?kV and 30?nA is 700?K for ZrO₂. Thus, the observed phase transformations can be explained on the basis of the temperature increase in the corresponding Y-PSZ phase diagram. In case of Mg-PSZ, the transition temperature is 1083?°C. The local temperature increase was obviously lower. The excitation energy for the observed phase transformation was smaller than expected from the phase diagrams of the thermodynamic equilibrium. Using 5?kV, 4.8?nA and 5° incidence angle, no phase transformations and no damage were observed. Thus, these conditions are well suited for the FIB preparation of metastable zirconia.     

Fracture resistance of dental glass-ceramics under sliding contact

Aesthetic glass-ceramics are widely employed dental materials, both in bulk form and as veneers. As they are highly brittle, the durability of these materials is limited by fracture and wear processes originated from contacts with opposing dentition and/or third-body particles during mastication and bruxism. This work investigates the resistance to fracture of commercial dental glass-ceramics under sliding contact, simulated by means of scratch tests. It finds that materials with relatively larger crystals (feldspathic and leucite) require a lower stress to fracture upon sliding than lithium silicates containing smaller (but more elongated) crystals, due to their larger defects and lower toughness. Results are analyzed as a function of material microstructure within the framework of Weibull theory and fracture mechanics. Implications for materials selection and development in prosthetic dentistry are briefly discussed.     

Influence of CAD-CAM milling on the flexural strength of Y-TZP dental ceramics

In this work, the influence of milling strategy and cutter wear on the flexural strength of sintered Y-TZP ceramics machined in a CAD-CAM system, was investigated. Pre-sintered Y-TZP blocks were machined with tungsten carbide (WC) milling cutters and divided into three groups: a) samples machined with New Cutting Tools (NCT); b) samples machined with End-of-life cutting tools (ELCT); and c) samples machined and subsequently polished. The samples were machined parallel and perpendicularly to the diameter of pre-sintered Y-TZP blocks aiming to evaluate the effect of the machining orientation on roughness. After cutting, the specimens were sintered at 1530?°C for 2?h and characterized. After sintering, dense Y-TZP samples presenting grain size average of 0.81?±?0.23?μm, hardness of 1205?±?12 HV, K_C of 7.7?±?0.4 MPam^1/2 and flexural strength of 1207?±?199?MPa were obtained. Furthermore, the control roughness (polished surfaces) presented R_a?=?0.058?±?0.011?μm. The surface roughness of the samples was influenced by the machining methodology using NCT mills, with R_a?=?0.386?±?0.149?μm and R_a?=?0.292?±?0.105?μm for samples machined parallel and perpendicularly to the pre-sintered Y-TZP blocks, respectively. The use of ELCT led to a roughness increasing (R_a?=?0.582?±?0.183?μm for samples milled perpendicularly, and R_a?=?0.919?±?0.164?μm for samples cut parallel to the pre-sintered blocks. The flexural strength of the sintered Y-TZP decreases with use of ELCT as consequence of surface roughness increasing, resulting from wear of WC mills, which presented chippings with sizes ranging from 30 to 340?μm in the cutting edge.     

Determining specimen thickness limit in fracture toughness tests applied to dental ceramics and metal alloy interfaces

The aims of the current study are to present computational simulations based on the finite element method in order to determine the lowest viable thickness of materials used to make ceramic samples based on the interface of groups of materials (Ni–Cr/ceramics, alumina/ceramics), and samples made of each of these materials, alone; as well as to report the effect of variation in the stress intensity factor of these interfaces (β). Test conditions were simulated in Abaqus CAE software and mesh features focused the crack tip. Fracture toughness analysis was carried out by simulating the three-point bending test (SEVNB specimen of the Fracture Mechanic standards). Based on the present results, the lowest material thickness necessary to meet the stress intensity requirement at the crack tip was 8 mm of each material, under symmetry condition, for sandwich composition. Such a thickness included fracture process zone (FPZ) size and plastic zone (PZ). Sandwich thickness was enough to maintain the test conditions established for the standards. Samples produced in the present study met the requirements of the fracture toughness test, based on the ASTM E?399 standard - specimens with the lowest possible sandwich thickness were obtained and they met the stress intensity requirements of the crack tip. This finding pointed towards a sample based on using a reduced amount of test materials - such as those used for dental restoration. The lowest material thickness in the sandwich was 8 mm, for all groups of materials, under symmetry conditions. This thickness includes fracture process zone size and plastic zone size. The herein proposed sandwich thickness was enough to maintain the test conditions established by fracture mechanic standards.     

Highly transparent polycrystalline Gd2O3 ceramic attained via ZrO2 stabilization effect

The transition from the cubic to monoclinic phase of Gd₂O₃ at high temperatures poses a significant challenge to the preparation of transparent Gd₂O₃ materials. In this work, we presented a straightforward yet effective method for fabricating transparent Gd₂O₃ ceramics. Via ZrO₂ stabilization effect for phase structure, highly transparent Gd₂O₃ ceramics were successfully fabricated by vacuum sintering at 1850 °C for 8 h. The effect of different Zr (0 ∼ 13 at%) concentrations on phase transition, grain growth, fracture mode and optical properties of Gd₂O₃ transparent ceramics was investigated. As the Zr content increases, the transition from the cubic (C) to monoclinic (M) phase is effectively suppressed, which is crucial for achieving Gd₂O₃ transparent ceramics. Moreover, the results indicate that the addition of ZrO₂ has a significant effect on grain growth by not only impeding the migration of grain boundaries but also affecting the phase composition. In addition, the 11 at% Zr-doped Gd₂O₃ ceramic exhibits the best optical properties, of which transmittance is about 76% at 850 nm and about 80% in the 2.5 µm ∼ 6 µm mid-infrared range. This work provides an illustrative example for the development of other ceramics with phase transition. The obtained Zr-doped Gd₂O₃ transparent ceramics with high optical quality are potential candidates for optical window, scintillator host and mid-infrared transmission materials.     

Effect of Dy2O3 content on microstructure and mechanical properties of zirconia-toughened alumina

In this study, the effect of adding Dy₂O₃ on the microstructure and mechanical properties of zirconia-toughened alumina (ZTA) stabilised by yttrium oxide was investigated. ZTA-Dy₂O₃ composites with different Dy₂O₃ contents (0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt%) were prepared by sintering at 1600 °C for 4 h. The phases and structures of the samples were characterised through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Dy₂O₃ formed a solid solution c-DYZ with YSZ, and an appropriate amount of Dy₂O₃ could refine the grains and contribute to densification. The densities, hardness, flexural strength and toughness all increased and then decreased with increasing Dy₂O₃ content, reaching maximum values of 99.2%, 1741 ± 19 HV, 449 ± 10 MPa and 5.87 ± 0.42 MPa?m^1/2, respectively, at 3 wt% Dy₂O₃ content.