Objective. 3Y-TZP ceramics with reduced alumina content have improved translucency and are used in monolithic dental restorations without porcelain-based veneers. The workflow can be further streamlined with rapid sintering. This study was designed to assess how these approaches affect ageing when the materials are exposed to the oral environment in vivo. Methods. 43 discs were fabricated from 3Y-TZP powder with 0.05% Al2O3 and sintered with conventional or rapid regimens (1450 degrees C 2 h, 1530 degrees C 2 h, or 1530 degrees C 25 min). Their surfaces were polished or airborne-particle abraded with 50 mu m Al2O3. The discs were incorporated in complete dentures of 16 volunteers and worn continuously for up to 48 months. Ageing changes on disc surfaces were monitored every 6 months by X-ray diffraction, scanning electron microscopy and atomic force microscopy. Data was statistically analysed with linear models. Results. The amount of monoclinic phase on polished surfaces increased linearly, reaching up to 40% after 48 months in vivo. The ageing process observed for rapid sintering was 1.6 times faster compared to conventional sintering. A nano-scale increase in roughness with microcracking was also detected on polished surfaces. Airborne-particle abraded surfaces did not exhibit clear signs of ageing during the course of the study. Significance. Highly-translucent 3Y-TZP ceramics are more susceptible to ageing than classic 3Y-TZP. After 4 years in vivo, the extent of degradation did not yet constitute grounds for clinical concern, but was more pronounced in materials prepared with rapid sintering. (C) 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

The breaking load and the breaking moment during transversal and longitudinal loading of fibre-cement profiled (corrugated) sheets were measured on products manufactured on an industrial Hatschek machine, and the corresponding bending strengths were calculated. The influence of two processing parameters, i.e., the pressure applied to the green sheet directly during the manufacture and the content of the reinforcing organic fibres within the portland cement matrix, on the fracture strength of the products was studied and statistically analyzed assuming Weibull statistics. The Weibull parameters were compared for different fabrication conditions. While the fibres content influences considerably the Weibull modulus for both transversal and longitudinal loading, there is no correlation between the shaping pressure and Weibull modulus.

OBJECTIVE:In vivo aging of biomedical grade 3Y-TZP ceramics in the oral environment was assessed and compared to artificially accelerated in vitro hydrothermal aging extrapolations at 37°C. METHODS:88 discs were pressed and sintered (1450-1500°C) from two commercial 3Y-TZP compositions containing 0.25% Al2O3 to generate finer- and coarser-grained specimens. As-sintered (AS) and airborne-particle abraded (APA; 50μm Al2O3) surfaces were investigated. In vivo aging was performed by incorporating specimens in lingual flanges of complete dentures of 12 edentulous volunteers who wore them continuously for up to 24 months. For comparison, in vitro hydrothermal aging at 134°C was also performed and analysed by XRD and (FIB)-SEM. Data was statistically analysed with linear regression models. RESULTS:Finer and coarser-grained specimens exhibited statistically insignificant differences in aging in vivo. The monoclinic fraction (Xm) on AS surfaces abruptly increased to ∼8% after 6 months. The aging process then proceeded with slower linear kinetics (∼0.24%/month). After 24 months, Xm reached ∼12%. The calculated maximum transformed layer was 0.385μm representing one layer of transformed grains. APA surfaces were highly aging resistant. The initial Xm of ∼4.0% linearly increased by 0.03%/month in vivo. In vitro aging exhibited an initial induction period, followed by linear aging kinetics. Coarser-grained AS surfaces aged significantly faster than fine-grained (2.41%/h compared to 2.16%/h). APA discs aged at a rate of 0.3%/h in vitro. Microcracking within a single grain and pull-out of grain clusters were observed on aged AS surfaces. SIGNIFICANCE:Biomedical grade 3Y-TZP was susceptible to in vivo aging. After 2 years in vivo, the aging kinetics were almost 3-times faster than the generally accepted in vitro-in vivo extrapolation.

This study presents a fabrication route for an electrically conductive ZrO2–TiN ceramic nanocomposite with a nanoscale TiN phase occupying ≤30 vol% to improve the mechanical reinforcement of the zirconia matrix, and at the same time provide electrical conductivity to facilitate electro-discharge machining (EDM). The TiN nanoparticles were incorporated into a 3 mol% yttria-stabilized tetragonal zirconia (Y-TZP) powder, either by admixing a TiN nanopowder (MCP) or by using in-situ synthesis (ISS) via the forced hydrolysis of a titanyl sulphate aqueous solution and the direct nitriding of as-synthesized titania nanoparticles, followed by consolidation and rapid sintering in a spark plasma sintering (SPS) system. The initial phase composition and crystal structure of the as-synthesized powders and the sintered samples were characterized by transmission electron microscopy (TEM) and X-ray difraction (XRD). The influence of the different fabrication routes on the microstructural evolution, electrical and mechanical properties, and affinity for EDM were assessed using TEM, focused ion beam scanning electron microscopy (FIB-SEM, Vickers indentation, electrical conductivity measurements, and profilometry. The MCP synthesis route resulted in finer microstructures that are less prone to microstructural inhomogeneities; however, using the ISS route, it was possible to fabricate electrically conductive Y-TZP nanocomposites containing only 15 vol% of the TiN nanoparticulate phase. Both synthesis routes resulted in an increase of the fracture toughness with an increase of the TiN phase due to the nanoparticulate TiN reinforcement of the Y-TZP ceramic matrix via crack-bridging toughening mechanisms. As both synthesis routes yielded Y-TZP nanocomposites capable of successful EDM machining at a TiN content of ≥30 vol% for the MCP and ≥ 15 vol% TiN for the ISS, a possible mechanism was developed based on the microstructure evolution and grain growth.

beta-tricalcium phosphate (beta-TCP) coatings were synthesized on tetragonal zirconia (Y-TZP) discs by heating the apatite coating between 800 degrees C and 1200 degrees C. The study results suggest that heating temperature has a strong influence on the coating bond strength and microstructure of the substrate/coating interface. The beta-TCP coatings fired at 800 degrees C and 900 degrees C exhibited excellent tensile bond strength (similar to 50 MPa) while heating at 1100 degrees C and 1200 degrees C led to decreased bond strength (similar to 30 MPa) as the result of substantial structural and microstructural changes: diffusion of Y3+ from the zirconia substrate in the coating resulting in partial crystal transformation (t-m) of zirconia, formation of surface uplifts and nanoporosity in zirconia, as well as generation of large residual thermal stresses leading to microcracking of the beta-TCP coatings. However, these structural changes did not have any measurable effect on the flexural strength of the bulk zirconia substrates.

The fracture resistance of glazed four-unit posterior dental bridges after mechanical fatigue testing and artificial ageing was evaluated. Thirty identical monolithic bridges were fabricated from “translucent” zirconia and divided into three groups of ten. The first group was monotonically loaded to fracture; the second group was dynamically loaded in water (0–300N, 106 cycles) prior to fracture, while the third group was first subjected to accelerated ageing (in a diluted acetic acid solution at 134°C for 12h), then to mechanical fatigue and finally monotonically loaded to fracture. Two tested bridges did not survive 106 cycles of dynamic loading, one in Group 2 and one in Group 3. Mean monotonic fracture loads (N) were: Group 1: 547.3±66.3, Group 2 (n=9): 465.2±118.0, and Group 3 (n=9): 408.8±58.9. According to the fractographic analysis the glaze embrittlement during artificial ageing and the stress corrosion during fatigue loading are proposed to be responsible for the reduced fracture resistance.

The yttrium-segregation-dependent phase partitioning and residual stress development that influence both the aging and the fracture behaviour in 3Y-TZP bioceramics were studied by sintering alumina free 3Y-TZP, varying the sintering temperature and the time, to yield ceramics with identical grain size distributions, but with different phase compositions. The structure and stability of the resulting tetragonal phases, in the form of transformable, yttria-lean t-ZrO2 (YLZ) and non-transformable, yttria-rich t"-ZrO2 and/or t'-ZrO2 (YRZ), were studied by X-ray diffraction (XRD) and focused ion beam scanning electron microscopy (FIB-SEM). The accelerated aging kinetics was fitted to the Mehl-Avrami-Johnson equation. The specimen sintered at the lowest sintering temperature but with the longest dwell time contained the smallest and the largest concentrations of yttria in the YLZ and YRZ phases, respectively, as well as the largest amount of YRZ. As a consequence, it exhibited the fastest linear aging kinetics accompanied by more extensive micro-cracking of the transformed layer, as well as largest amount of intergranular fracture and the greatest resistance to fracture. These properties were ascribed to the increased transformability of the YLZ phase and the greatest propensity of the YRZ phase to relax the accumulated residual stresses during transformation (tetragonal to monoclinic, t-m) manifested as a 2.4% unit-cell volume increase. The observed relaxation provides additional understanding of the t-m transformation mechanism, which governs both the aging and fracture behaviour of 3Y-TZP. Statement of Significance A novel approach to understanding the effect of yttrium segregation on t-m transformation of 3Y-TZP zirconia bioceramics is presented. Carefully designed sintering strategy facilitated fabrication of ceramics with identical grain size distributions but with different yttrium concentrations. The influence of phase partitioning on stability and structure of transformable yttria-lean tetragonal phase (YLZ) and non transformable yttria-rich phases (YRZ; t"- and t'-prime) and on the formation of residual stresses in YRZ were investigated. It is shown that YRZ phases are under compressive stresses in YLZ matrix, since a systematic relaxation after ageing was observed and explained for the first time. It puts additional perspective on the understanding of the t-m transformation mechanism ultimately governing both the ageing and fracture behaviour of 3Y-TZP. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Translucent, high‐performance, mullite ceramics with anisotropic grains were prepared by the spark plasma sintering (SPS) of a powder mixture consisting of commercial mullite powder, which already contained small amounts of alumina (θ and α) and silica (cristobalite) (≤3 wt% in total), to which 2 and 1 wt% of yttria and amorphous silica was admixed, respectively. The combination of low‐viscosity Y2O3–Al2O3–SiO2 transient liquid formation and SPS sintering provided enhanced densification, also provoking anisotropic grain growth (which became exaggerated after 20 min of SPS dwell time), at a relatively low sintering temperature of 1370°C. In this way, it was possible to meet the conflicting demands for obtaining a dense mullite ceramic with anisotropic grains, ensuring good mechanical properties, while preserving a noticeable light transmittance. In terms of mechanical and optical properties, the best results were obtained when SPS dwell times of 5 and 10 min were employed. The as‐sintered samples possessed densities in the range 3.16–3.18 g/cm3, anisotropic grains with an aspect ratio (AR) of 7 and a grain thickness of approximately 0.45 μm, a flexural strength between 350 and 420 MPa, a Vickers indentation toughness and a hardness of approximately 2.45 MPa·m1/2 and 15 GPa, respectively, and an optical transmittance of between 30% and almost 50% in the IR range.

OBJECTIVES:This study was designed to explore the complex relationships between the sintering-temperature-dependent grain size, airborne-particle abrasion, ageing and strength of 3Y-TZP ceramics. METHODS:Biomedical grade 3Y-TZP powder was used to fabricate 180 discs. Half of them were sintered at 1400°C for 2h and half at 1500°C for 2h. A total of 18 groups of 10 were formed and subjected to the fully crossed experimental protocol of airborne-particle abrasion with Al2O3 at 2.5bar (no abrasion, 50μm, 110μm) and accelerated ageing at 134°C (no ageing, 12h, 48h). The relative amount of monoclinic phase was determined with XRD. The biaxial flexural strength was measured and statistically analyzed using the three-way ANOVA followed by predetermined contrasts and Tukey's HSD test (α=0.05). RESULTS:The low-temperature-sintered, fine-grained ceramic exhibited an excellent ageing resistance, while the high-temperature-sintered, coarse-grained ceramic experienced a higher surface strengthening and a substantially improved ageing resistance with respect to the airborne-particle abrasion. The overall performance of this material was superior. SIGNIFICANCE:Our results show that the sintering temperature has a minor effect on the flexural strength, but it plays a crucial role in the surface strengthening and the ageing behaviour of 3Y-TZP dental ceramics.

Objectives. To examine the effects of glass infiltration (GI) and alumina coating (AC) on the indentation flexural load and four-point bending strength of monolithic zirconia.Methods. Plate-shaped (12 mm x 12 mm x 1.0 mm or 1.5 or 2.0 mm) and bar-shaped (4 mm x 3 mm x 25 mm) monolithic zirconia specimens were fabricated. In addition to monolithic zirconia (group Z), zirconia monoliths were glass-infiltrated or alumina-coated on their tensile surfaces to form groups ZGI and ZAC, respectively. They were also glass- infiltrated on their upper surfaces, and glass-infiltrated or alumina-coated on their lower (tensile) surfaces to make groups ZGI2 and ZAC2, respectively. For comparison, porcelain-veneered zirconia (group PVZ) and monolithic lithium disilicate glass-ceramic (group LiDi) specimens were also fabricated. The plate-shaped specimens were cemented onto a restorative composite base for Hertzian indentation using a tungsten carbide spherical indenter with a radius of 3.2 mm. Critical loads for indentation flexural fracture at the zirconia cementation surface were measured. Strengths of bar-shaped specimens were evaluated in four-point bending.Results. Glass infiltration on zirconia tensile surfaces increased indentation flexural loads by 32% in Hertzian contact and flexural strength by 24% in four-point bending. Alumina coating showed no significant effect on resistance to flexural damage of zirconia. Monolithic zirconia outperformed porcelain-veneered zirconia and monolithic lithium disilicate glass-ceramics in terms of both indentation flexural load and flexural strength.Signcance. While both alumina coating and glass infiltration can be used to effectively modify the cementation surface of zirconia, glass infiltration can further increase the flexural fracture resistance of zirconia. (C) 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

The influence of higher sintering temperatures on the microstructure and equilibration of alumina-doped and alumina–silica-co-doped 3-mol%-yttria-stabilized tetragonal zirconia ceramics was investigated. The temperature-related (⩾1550°C) penetration of the silica phase from the grain junctions into the grain boundaries was accompanied by the emergence of a fraction of larger, heterogeneous grains. The overall partitioning process was slower in the co-doped material, increasing the resistance to ageing, while upon cooling the partitioned yttria-rich phase was found to be tetragonal (t′-ZrO2).

Background: A group of inorganic non-metal biomaterials, that are commonly used in clinical medicine to replace or repair tissues, can be classified as a bioceramics. This group includes bioactive glasses, glass-ceramics, hydroxy-apatite and some other calcium phosphates. In addition, some bio-inert engineering ceramics materials have become increasingly utilised, aluminum oxide, zirconium oxide and their composites being the most popular. With the developement of yttria stabilized tetragonal zirconium oxide ceramics (Y-TZP) medical community received a high strength biomaterial that is currently a material of choice for the manufacturing of medical devices. Y-TZP ceramics is becoming also increasingly used in dental medicine, where frameworks are manufactured by the use of computer-assisted technology.Conclusions: The article describes the basic properties of zirconia oxide ceramics important for the use in clinical medicine; high strength and fracture toughness, biocompatibility and negligible radiation. The ageing issue of this particular material, which is attributable to the thermo-dynamical instability of tetragonal zirconium oxide in hydrothermal conditions, is also discussed. When exposed to an aqueous environment over long periods of time, the surface of the Y-TZP ceramic will start transforming spontaneously into the monoclinic structure. The mechanism leading to the t-m transformation is temperature-dependent and is accompanied by extensive micro-cracking, which ultimately leads to strength degradation. The degradation might influence the clinical success rate of medical devices and therefore Y-TZP femoral heads are no longer made of pure zirconium oxide. Composites of zirconium and aluminium oxides are used instead, that are currently the strongest ceramic materials used in clinical medicine. In this work the clinical application of zirconia oxide ceramics in dental medicine is also presented. Conventional porcelain fused to metal technique is successfully replaced with Y-TZP ceramics in some clinical situations that are described in detail. It is important that computer design of the zirconia frameworks shortens and simplifies laboratory procedures and contributes to a precise final product.

The combined effect of alumina and silica co-doping on the ageing resistance of 3Y-TZP bioceramics was investigated. In order to differentiate between the distinct contributions of two dopants to the overall resistance to low-temperature degradation (LTD), specimens were prepared by infiltration of silica sol into pre-sintered 3Y-TZP pellets, produced from commercially available powders, which were alumina-free or contained 0.05 and 0.25 wt.%. After sintering, specimens were exposed to accelerated ageing in distilled water at 134 °C for 6–48 h. X-ray diffraction was applied to quantify the tetragonal-to-monoclinic (t–m) phase transformation associated with the LTD, while a focused ion beam–scanning electron microscopy technique was employed to study the microstructural features in the transformed layer. The results showed that the minor alumina and/or silica additions did not drastically change the densities, grain sizes or mechanical properties of 3Y-TZP, but they did significantly reduce LTD. The addition of either alumina or silica has the potential to influence both the nucleation and the propagation of moisture-induced transformation, but in different ways and to different extents. The co-doped ceramics exhibited predominantly transgranular fracture, reflecting strong grain boundaries (limiting microcracking of the transformed layer), for alumina doping, and rounded grains with a glassy phase at multiple grain junctions (reducing internal stresses) for silica-doped material. These two additives evidently have different dominant mechanisms associated with the deceleration of LTD of 3Y-TZP, but their combination increases resistance to ageing, importantly, without reducing the fracture toughness of this popular biomaterial.

The growth of implant and fixed prosthodontics practices in dentistry has created a rapidly increasing demand for advanced ceramics and ceramic processes. Innovations in ceramics and ceramic processes are vital to ensure reliable and affordable dental-restoration solutions with aesthetically pleasing outcomes. The work aims to engage the bioceramics and engineering communities to meet the challenges of modern dental restoration using advanced ceramics. Incorporating fundamental science, advanced engineering concepts, and clinical outcomes, the work is suitable for bioceramicists, ceramics manufacturers, dental clinicians and biologists. * State-of-the-art-coverage encompasses bioresorbable ceramics for bone regeneration and bioactivating surfaces of inert, high-strength ceramics for implantation, keeping research knowledge appropriately updated* Discusses transition from the baseline stable and physically stiff ceramics research into engineering of highly coherent laminate composites for prosthetic crowns and bridges* Showcases current feasible techniques for producing, in cost-effective and materials-saving ways, long-lasting individualized ceramic components with biocompatibility, complexity and high precision

This chapter reviews the structure, mechanical properties, and biocompatibility of load-bearing ceramics used in dentistry. The development of this class of ceramic biomaterials is traced from the late sixties when alumina was introduced in dentistry. The literature on both polycrystalline and single crystal alumina dental implants is reviewed. The use of alumina declined when zirconia-toughened ceramics were introduced in orthopedics in the eighties. The use of yttria partially-stabilized tetragonal zirconia (Y-TZP) in dentistry allowed the production not only of dental implants and abutments, but also a broad range of load-bearing fixed partial dentures, such as multi-unit bridges and crowns, thanks to the development of CAD/CAM technology. Today, the trend is to use alumina and zirconia ceramics for making more aesthetic parts by improving their optical translucency.

The role of surface preparation, hydrothermal ageing exposure and subsequent cyclic fatigue testing on the biaxial strength of a dental Y-TZP material are investigated. The initial strength and survival rate of a dental Y-TZP ceramic material to fatigue testing was found to be highly dependent upon surface preparation more so than exposure to various hydrothermal exposure conditions. The results suggest that the monoclinic phase generated by either surface damage (especially sandblasting) and to a lesser extent hydrothermal exposure does appear to mitigate strength and fatigue degradation. The results are discussed in terms of the size of defects generated following various surface treatments and the role of cyclic fatigue induced crack growth. A critical ratio is established between the monotonic strength and fatigue stress survival. From the specimens that failed and exhibited reduced strength after cycling a plot of averaged crack growth rate versus max cyclic stress intensity factor was established which closely matched existing results for Y-TZP ceramics.

High-performance bioceramics, such as zirconia, alumina, and their composites, are attractive materials for the fabrication of load-bearing bone implants because of their outstanding mechanical properties, biocompatibility, corrosion resistance, and aesthetic quality. However, a lot of additional work is still needed on these ceramics before their full potential as implant materials can be exploited, especially in the area of surface optimization. The two most important issues relating to the surface of ceramic implants that need to be addressed are surface chemistry and topography. They both have an influence on protein adsorption and cell behavior and play a key role in providing sufficient biomechanical stability for the long-term success of implants. Therefore, extensive studies have been performed that are aimed at a better understanding of how specific surface modifications affect the biological response. In this chapter, various surface-modification techniques are described and their potential for improving the osseointegration of ceramic implants is discussed.

Ceramic materials are currently applied to two categories of restorative dentistry, as all-ceramic fixed-partial dentures and as implantable components. While the former demands mainly integrated and balanced properties of mechanical and aesthetic origins, the latter also relies strongly on the material's bio-oriented properties. This chapter discusses the material demands for solving the problems encountered in current practice that indicate the direction for future developments. This is done by bearing in mind both process restrictions and compatibilities. Focus is placed on developing materials that have the potential for improving aesthetics, for preserving a healthy situation to secure a prolonged treatment survival, and for improving the durability and reliability of the restorations while also simplifying the procedures of materials manufacture and clinical operation. Biomimetic materials and processes related to them are topics of general importance from a long perspective.

The objective of this work was to develop a synthesis procedure for the deposition of β-TCP coatings with tailored physico-chemical properties on zirconia bioceramics. The synthesis procedure involved two steps: (i) a rapid wet-chemical deposition of a biomimetic CaP coating and (ii) a subsequent post-deposition processing of the biomimetic CaP coating, which included a heat treatment between 800 and 1200 °C, followed by a short sonication in a water bath. By regulating the heating temperature the topography of the β-TCP coatings could be controlled. The average surface roughness (Ra) ranged from 42 nm for the coating that was heated at 900 °C (TCP-900) to 630 nm for the TCP-1200 coating. Moreover, the heating temperature also affected the dissolution rate of the coatings in a physiological solution, their protein-adsorption capacity and their bioactivity in a simulated body fluid.

We have investigated the possibility of merging the wick-debinding and sintering of injection-molded ceramic parts into a single-step operation. The basic idea is to use an embedment that serves as a wicking agent in the first stage of the procedure, but then burns out completely when the temperature exceeds approximately 500°C, leaving the samples free of any organic binder or contamination so that they are ready for sintering. Carbon black was used as the wicking agent for two reasons. First, it has great wicking-agent properties, due to its high porosity with a small pore size, while exhibiting good wetting with the molten paraffin binder. Second, it burns in air at high temperature, at which point the wick-debinding has already been completed. The whole process from molded parts to fully sintered parts can be completed in a single furnace. Besides making the process shorter, the tedious and delicate operations of cleaning and transporting the fragile debinded parts after wick-debinding are avoided.