Mechanical properties and microstructure for 3 mol% yttria doped zirconia/silicon carbide nanocomposites

3 Mol% yttria stabilized tetragonal zirconia polycrystalline (3Y-TZP) is well known as a transformation toughening material with excellent mechanical properties at ambient temperature. However, the properties of 3Y-TZP drop down with increasing temperature. In this study, nanocomposite techniques were applied in order to improve mechanical properties of 3Y-TZP. 3Y-TZP/SiC nanocomposites were fabricated by hot-pressing, and effects of SiC particles on microstructure, transformation from tetragonal zirconia (t-ZrO₂) to monoclinic ZrO₂ (m-ZrO₂) and its mechanical properties were investigated. Fracture toughness of the nanocomposite was improved without decrease of strength. This should be due to not only crack deflection by dispersed SiC particles with high Young's modulus, but also the phase transformation of t-ZrO₂ accelerated by the residual stresses from coefficient of thermal expansion mismatch between 3Y-TZP and SiC.     

Effect of Fe2O3 doping on colour and mechanical properties of Y-TZP ceramics

Yttria-stabilized zirconia (Y-TZP) samples with different Fe concentrations were prepared aiming to study the effects of Fe₂O₃ doping on colour and mechanical properties. Since colour is an important optical property for jewellery and watchmaking, the investigation of colour in zirconia ceramics has a great scientific and technological interest. An investigation of the mechanical and optical properties, specifically the colour, was developed starting from commercial partially yttria-stabilized zirconia (Y-TZP) powders produced by Emulsion Detonation Synthesis (EDS). Within the strategies to get colours, the use of colouring oxides such as iron oxide (Fe₂O₃) was the chosen approach. The addition of specific ions into the ZrO₂ matrix can be used to tune zirconia colour without compromising its outstanding mechanical properties. Doping with iron oxide has proved to be a suitable, reproducible and irreversible colouring mechanism, allowing the development of a chromatically beige stable material with respect to its use in different processing conditions such as different atmospheres and temperature ranges. XRD results suggested that iron ions dissolved into tetragonal zirconia phase are at interstitial positions since the unit-cell volume of the tetragonal zirconia increases with increasing iron content. The effect of dopant addition on the mechanical properties of Y-TZP ceramics was also assessed. Compared to the undoped samples, doped ones exhibit a similar Vickers hardness (>1200?MPa) and biaxial flexural strength (>1000?MPa). However, it was observed that Fe₂O₃ doping slightly decreased the fracture toughness of Y-TZP ceramics.     

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.     

One-step hydrothermal synthesis of yttria-stabilized tetragonal zirconia polycrystalline nanopowders for blue-colored zirconia-cobalt aluminate spinel composite ceramics

A novel approach for the preparation of blue-color giving zirconia nanopowders by doping of 3?mol% Y₂O₃ through simple one-step hydrothermal process is proposed. A blue-color giving yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powders were prepared by urea-based solution, zirconium acetate, CoCl₂·6H₂O and AlCl₃ precursors in hydrothermal vessel at 24?h, 150?°C and 20 bars. Based on the results, the synthesized blue-color giving Y-TZP nanopowders have entirely tetragonal structure as mono phase with 3.8?±?0.2?nm average grain size and (Y, Co, Al)_xZrO₂; x?≤?0.03?at. with chemical composition. Thermal treatment was also applied to synthesized Y-TZP powders at 1200?°C and 1450?°C to observe the color evolution. Only sharp blue was obtained in Y-TZP powders resulting the development of zirconia-cobalt aluminate spinel (ZrO₂-CoAl₂O₄) composite ceramic structure for both temperatures after heat-treatment. Herein, not only formation of CoAl₂O₄ but also incorporation of cobalt (Co) and aluminum (Al) into the Y-TZP grains plays a critical role on evolving of blue color. This synthesized Y-TZP nanopowders can be a good candidate for one-step production of blue-color sintered ZrO₂-CoAl₂O₄ spinel composite ceramics in numerous ceramic applications due to their superior structural and functional properties.     

Magnetic field oriented tetragonal zirconia with anisotropic toughness

(0 0 1)-oriented 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) has been developed by reactive synthesis of undoped pure monoclinic zirconia and co-precipitated 8 mol% yttria-stabilized zirconia (8Y-ZrO₂). The dispersed pure monoclinic ZrO₂ powder, having magnetic anisotropy, was first aligned in a strong magnetic field and co-sintered in a randomly distributed cubic 8Y-ZrO₂ fine matrix powder. The reactive sintering resulted in a 3Y-TZP ceramic with a (0 0 1) orientation. The (0 0 1)-oriented 3Y-TZP showed a substantial toughness anisotropy, i.e. the toughness along the [0 0 1] direction is 54% higher than that of its perpendicular direction. Moreover, the toughness along the [0 0 1] direction is 49% higher than that of a non-textured isotropic reactively synthesized 3Y-TZP and 110% higher than that of an isotropic co-precipitated powder based 3Y-TZP. The substantially enhanced toughness was interpreted in terms of the tetragonal to monoclinic martensitic phase transformability.     

Mechanical and electrical properties of 3Y-TZP/TiSi2 composites sintered at different temperatures

Various amounts of TiSi₂ (30, 40, and 50 wt.%) were added to 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) to fabricate 3Y-TZP/TiSi₂ composites by vacuum sintering. The effects of the TiSi₂ added amount, as well as the sintering temperature on the microstructure, mechanical, and electrical characteristics of the 3Y-TZP/TiSi₂ composites were examined. The sintered samples consisted of three phases: tetragonal (t-ZrO₂), TiSi₂, and reaction product Ti₅Si₃. The maximum bending strength and relative density of the composites, reaching 501.20 MPa and 98.59% respectively, were achieved at a TiSi₂ content of 30 wt.% and sintering temperature of 1500°C. The resistivity of 3Y-TZP/TiSi₂ composites showed a nonlinear decrease with increasing TiSi₂ content. These results indicated that 3Y-TZP/TiSi₂ composites had a typical percolation threshold phenomenon due to the different TiSi₂ content and a conductivity model of 3Y-TZP/TiSi₂ composites at room temperature was founded on the generalized effective medium equation. The resistivity of the composites could optionally adjust between 10² and 10⁻⁴Ω·cm with 30–50 wt.% TiSi₂ under room temperature. Overall, the 3Y-TZP/TiSi₂ composites show great potential for applications in the heat-not-burn tobacco field.     

Influence of WC particles on the microstructural and mechanical properties of 3 mol% Y2O3 stabilized ZrO2 matrix composites produced by hot pressing

Yttria stabilized polycrystalline tetragonal zirconia (Y-TZP)-tungsten carbide (WC) composites were fabricated by hot pressing. Yttria (Y₂O₃) stabilizer content was kept at 3 mol% to ensure the phase structure of the Y-TZP composites to be tetragonal. To increase the moderate hardness of the 3 mol% Y₂O₃ added TZP structure, hard WC particles were added with various proportions up to 40 vol%. The TZP/WC composites were sintered at different sintering temperatures between 1450 and 1550 °C.The mechanical and microstructural properties of the resulting composites as well as the phase compositions were investigated. Reciprocating pin-on-disk tests were carried out to determine the wear behavior of the Y-TZP/WC composites. Using bi-modal WC reinforcement, the performance of the composite against wear was improved. Using dry wear sliding conditions under 55 N normal load and 45 km sliding distance, the worn volume of the 75 vol% nanosized - WC distributed 3Y-TZP/40WC composite was about 0.003 mm³.     

Sintering kinetics of ZrO2 nanopowders modified by group IV elements

The sintering behavior of tetragonal zirconia nanopowders modified by the group IV elements at the initial sintering stage was investigated. It was found that different additives SiO₂, SnO₂, and GeO₂ have a significant influence on the densification kinetics of 3Y-TZP nanopowders obtained by coprecipitation during sintering as it depends on the amount of additives (0-5 wt%). The shrinkage of zirconia-based specimens during the nonisothermal sintering was analyzed using the dilatometric data. The constant rate of heating technique was applied in order to determine the dominant mass transfer mechanism at the initial stage of sintering in modified zirconia nanopowders. It was found that there was a change in the mass transfer mechanism and diffusion activation energy in 3Y-TZP as a result of the additives. The dominant sintering mechanism in 3Y-TZP changed from the volume diffusion to the grain boundary diffusion due to the addition of SiO₂ and SnO₂ and the sintering activation energy increased in these cases. However, GeO₂ additive activated the viscous flow mechanism in sintering process of 3Y-TZP nanopowders which led to acceleration of the densification due to the decrease in the diffusion activation energy.     

Analysis of reactions during sintering of CuO-doped 3Y-TZP nano-powder composites

3Y-TZP (yttria-doped tetragonal zirconia) and CuO nano powders were prepared by co-precipitation and copper oxalate complexation–precipitation techniques, respectively. During sintering of powder compacts (8 mol% CuO-doped 3Y-TZP) of this two-phase system several solid-state reactions clearly influence densification behaviour. These reactions were analysed by several techniques like XPS, DSC/TGA and high-temperature XRD. A strong dissolution of CuO in the 3Y-TZP matrix occurs below 600 °C, resulting in significant enrichment of CuO in a 3Y-TZP grain-boundary layer with a thickness of several nanometres. This “transient” liquid phase strongly enhances densification. Around 860 °C a solid-state reaction between CuO and yttria as segregated to the 3Y-TZP grain boundaries occurs, forming Y₂Cu₂O₅. This solid-state reaction induces the formation of the thermodynamic stable monoclinic zirconia phase. The formation of this solid phase also retards densification. Using this knowledge of microstructural development during sintering it was possible to obtain a dense nano–nano composite with a grain size of only 120 nm after sintering at 960 °C.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

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.     

Sintering behaviour and microstructure of 3Y-TZP + 8 mol% CuO nano-powder composite

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate complexation–precipitation, respectively. A significant enhancement in sintering activity of 3Y-TZP nano-powders, without presence of liquid phase, was achieved by addition of 8 mol% CuO nano-powder, resulting in an extremely fast densification between 750 and 900 °C. This enhancement in sintering activity was explained by an increase in grain-boundary mobility as caused by dissolution of CuO in the 3Y-TZP matrix. The nano-powder composite was densified to 96% by pressureless sintering at 1130 °C for 1 h. Considerable tetragonal to monoclinic phase transformation of the zirconia phase was observed by high temperature XRD analysis. This zirconia phase transformation is discussed in terms of reactions between CuO and yttria as segregated to the 3Y-TZP grain boundaries.     

Terahertz probing of low-temperature degradation in zirconia bioceramics

ZrO₂-based ceramics are widely used in biomedical applications due to its color, biocompatibility, and excellent mechanical properties. However, low-temperature degradation (LTD) introduces a potential risk for long-term reliability of these materials. The development of innovative nondestructive techniques, which can explore LTD in zirconia-derived compounds, is strongly required. Yttria stabilized zirconia, 3Y-TZP, is one of the well-developed ZrO₂-based ceramics with improved resistance to LTD for dental crown and implant applications. Here, 3Y-TZP ceramic powders were pressed and sintered to study the LTD phenomenon by phase transition behavior. The LTD-driven tetragonal-to-monoclinic phase transition was confirmed by XRD. XPS analysis demonstrated that induced LTD reduced the oxygen vacancies which supports these findings. It is proved that after the degradation, the 3Y-TZP ceramics show the decreased dielectric permittivity at terahertz frequencies due to the crystallographic phase transformation. Terahertz nondestructive probe is a promising method to investigate LTD in zirconia ceramics.     

Fracture resistance of yttria stabilized zirconia manufactured from stabilizer-coated nanopowder by micro cantilever bending tests

Yttria stabilized tetragonal zirconia polycrystal (Y-TZP) owes its high toughness to transformation toughening, a mechanism that requires the development of a process zone. It is important to measure if and to what extent the size of components can be reduced. In this study, tests were carried out using focused ion beam or picosecond milled pre-notched, 3 mol percent Y₂O₃ (3Y-) TZP micro-cantilever (10–250 μm) beams. The tests show clearly that the maximum fracture resistance is size dependent and the plateau toughness was not reached in any of the small-scale samples. A correlation between transformability of the tetragonal phase and the measured fracture resistance became visible only for the largest micro cantilever but did not reach the values measured in macroscopic samples. Based on these results, it is not advantageous to use very tough zirconia materials in components with dimensions smaller than ～0.25 mm, as the high toughness is not fully realized.     

Transformation-induced damping behaviour of Y-TZP zirconia ceramics

Stiffness and internal friction of yttria-stabilised tetragonal zirconia ceramics (Y-TZP) with varying yttria content (2–3 mol%) were measured between room temperature and 1000 K with the use of the impulse excitation technique (IET). The contribution of transformation-related damping events to internal friction is recognised and separated from damping due to elastic dipole relaxation. Damping effects associated with tetragonal-to-monoclinic and reversed ZrO₂-phase transformation and low temperature degradation (LTD), are identified with the help of dilatometry. The experimental results indicate that 2Y-TZP with an inhomogeneous yttria distribution shows controlled transformation induced damping behaviour and less susceptibility to LTD during thermal cycling compared to a co-precipitated 2Y-TZP material with homogeneous Y-distribution.     

Influence of zirconia surface treatments on resin cement bonding and phase transformation

It evaluated the effects of different zirconia surface treatments on the bond strength of a resin cement to Y-TZP (yttria-stabilized tetragonal zirconia) ceramics, as well as their phase-transformations. 75 blocks (5 mm × 5 mm × 4 mm) of Y-TZP were assigned into five groups (n = 15): (tribochemical silica coating - TBS) zirconia surface was abraded by silica coated alumina particles followed by silanization; (GLZ₁) zirconia surface received the application of a thin layer of low-fusing porcelain glaze, followed by hydrofluoric acid (HF) etching for 1 min; (GLZ₅) glaze application + HF etching by 5 min; (GLZ₁₀) glaze application + HF etching by 10 min; (GLZ₁₅) glaze application + HF etching by 15 min. After etching, all the specimens were washed, dried and silanized. Cylinders of composites (diameter: 3.25 mm; height: 3 mm) were cemented to the Y-TZP blocks using a resin cement. All the specimens were subjected to aging (10,000 thermal cycles and 90 days storage), tested under shear conditions, and finally analyzed by a stereomicroscope (failure analysis). In addition, we also performed topographical and phase transformation analyses of the treated zirconia surfaces. The TBS group presented the highest bond strength value (23.34 MPa). The glazed groups presented low bond values and high prevalences of pretest failures. X-ray diffraction analysis showed a phase transformation for the TBS group (13.14%); however, there was no clear phase change observed for the GLZ groups. From our results, we concluded that tribochemical silica coating is the main Y-TZP surface conditioning for resin bond improvements.     

Subeutectoid Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystal and Ceria-Doped Yttria-Stabilized Tetragonal Zirconia Polycrystal Ceramics

Yttria-doped tetragonal zirconia containing 2 and 3 mol% Y₂O₃ (Y-TZP), and CeO₂-doped Y-TZP containing 0 to 12 mol% CeO₂ were sintered at 1350°C in a tetragonal single-phase field for 2 h in air, and the degradation behavior at low temperature in air and in hot water was observed. X-ray photoelectron spectroscopy studies on the surface of hydrothermally treated samples show evidence for the formation of a YO(OH) species, along with the simultaneous formation of purely tetragonal zirconia nuclei that retain their coherence in the Y-TZP matrix. Above a critical size, the tetragonal nuclei spontaneously transform to a monoclinic structure, giving rise to macro- and microcracking. The strong tetragonal grains degrade to produce a spalling phenomenon that facilitates further degradation. Y-TZP ceramics alloyed with adequate amounts of CeO₂ show no tetragonal-to-monoclinic transformation after hydrothermal treatment.     

Aging behavior of Y-TZP with bioglass addition and its impact on the flexural strength and osteoblastic cell response

Y-TZP containing Ca₂P₂O₇ are promising bioceramics with potential applications in dental implants and dentistry. These ceramics were developed by the introduction of a refractory sol-gel derived CaO-P₂O₅-SiO₂ bioglass into Y-TZP; Ca₂P₂O₇, and ZrSiO₄ phases were formed in situ after sintering. The aging process of Y-TZP with different glass additions was studied. The effect of glass addition on the flexural strength and osteoblastic cell response of non-aged and aged Y-TZP was investigated. Y-TZP exhibited the most pronounced tetragonal (t) to monoclinic (m) transformation of zirconia (ZrO₂) during aging; the addition of glass contents between 5 and 20 vol% improved the aging resistant of Y-TZP. Y-TZP flexural strength markedly decreased with increasing aging time; in contrast, the ceramics with glass did not alter their flexural strength upon aging. An increase in the Ca₂P₂O₇ content with increasing glass up to 10 vol%, promoted both the cell viability and the osteogenic differentiation of UMR-106 cells on non-aged and aged samples. The high micro-roughness of Y-TZP with 20 vol% glass after aging, limited the proliferation and the osteogenic potential of the cultures. Y-TZP with 10 vol% glass had the best combination of properties in terms of flexural strength and osteoblast cell response.     

Mechanical performance and microstructure of 3Y-TZP/Al2O3/GNPs medical ceramic surgical scalpel material prepared through SPS–HF dual sintering method

In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP)/Al₂O₃/graphene nanoplatelets (GNPs) medical ceramic materials for manufacturing surgical scalpels were sintered in vacuum in an SPS–625HF furnace. The mechanical performances and microstructures of the composites were investigated, and the influence mechanisms of the sintering temperature and amount of added GNPs were studied. During the sintering process at 1400°C and 30 MPa for 5 min, the added GNPs enhanced the mechanical properties of the 3Y-TZP/Al₂O₃ composites. The results showed that the composite with .1 wt.% GNPs had 6.4% (910 ± 11 MPa) higher flexural strength than 3Y-TZP/Al₂O₃. The composite with .4 wt.% GNPs had 38.7% (12.95 ± .22 MPa m^1/2) greater fracture toughness than 3Y-TZP/Al₂O₃. The main toughening mechanisms of 3Y-TZP/Al₂O₃/GNPs were crack bridging, crack deflection, crack branching, GNPs bridging, transgranular fracture structures, and phase transformation of t-ZrO_1.95. The two-stage densification displacement curve appeared at the optimal sintering temperature of the materials, and the 3Y-TZP/Al₂O₃/GNPs composites with a two-stage densification displacement curve had excellent mechanical properties. The added GNPs can inhibit the grain growth during the sintering process, thereby refining the zirconia grains. With the increase in GNPs content, the grain size and flexural strength of the composites decreased gradually. However, higher content of GNPs was beneficial to improve the relative density and thermal diffusivity of 3Y-TZP/Al₂O₃/GNPs composite material.     

Effect of in-situ formed whiskers diameter from tourmaline on microstructure and mechanical properties of 3Y-TZP ceramics

In this paper, in-situ whiskers reinforced 3 mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) ceramics with different diameters were prepared using pressureless sintering by introducing tourmaline with different particle sizes into 3Y-TZP powders. The purpose of this research was to investigate the influence of in-situ formed whisker diameters on the densification, microstructure and mechanical properties of 3Y-TZP ceramics. The prepared ceramics were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. Findings indicated that in-situ mullite whiskers formed by phase transformation of tourmaline particles can promote the densification of 3Y-TZP ceramics, and further improve the dispersion of mullite whiskers in the 3Y-TZP ceramics. More importantly, the average diameter of mullite whiskers can be controlled by altering the tourmaline particle size. When the average particle size of tourmaline is 500 nm, 3Y-TZP composites have a near-fully dense microstructure of 99.09%, with the ZrO₂ grain size of about 335 nm, the average diameter of mullite whiskers is 330 nm. Both the bending strength and fracture toughness reached optimal values of 836 ± 24 MPa and 10.6 ± 0.5 MPa m^0.5, respectively. This paper provides a new way to design of the microstructure and strength-toughness of zirconia composite ceramics.