The toughening mechanism and mechanical properties of graphene-reinforced zirconia ceramics by microwave sintering

ABSTRACT

The graphene/ZrO₂ composites were fabricated by impregnating graphene dispersion into the ZrO₂ ceramic matrix and sintered by microwave, and the microstructure and mechanical properties were investigated. The results showed that the graphene was well dispersed in the ceramic matrix and refined the grain size. The fracture toughness reached 8.62?MPa?m^1/2, confirmed by single-edge notched beam, which was 42% higher than that of the pure ZrO₂. Also, the toughening mechanisms were investigated by micro-hardness testing and showed that a combination of crack deflection, micro-crack and crack bridging increased the fracture toughness.     

Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia

In this research, the comparison between microwave sintering and conventional sintering on the mechanical properties and microstructural evolution of 3?mol% yttria-stabilised zirconia were studied. Green bodies were compacted and sintered at various temperatures ranging from 1200?°C to 1500?°C. The results showed that microwave assisted sintering was beneficial in enhancing the densification and mechanical properties of zirconia, particularly when sintered at 1200?°C. It was revealed that as the sintering temperature was increased to 1400?°C and beyond, the grain size and mechanical properties for both microwave- and conventional-sintered ceramics were comparable thus suggesting that the sintering temperature where densification mechanism was activated, grain size was strongly influenced by the sintering temperature and not the sintering mode.     

Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

Effect of microwave sintering on the properties of copper oxide doped Y-TZP ceramics

The effect of various amounts of copper oxide (CuO) up to 1?wt% on the densification behaviour and mechanical properties of 3?mol% yttria-tetragonal zirconia polycrystal (Y-TZP) were studied by using microwave (MW) sintering method. The MW sintering was performed at temperatures between 1100?°C and 1400?°C, with a heating rate of 30?°C/min. and holding time of 5?min. The beneficial effect of MW in enhancing densification was also compared for the undoped and 0.2?wt% CuO-doped Y-TZP when subjected to conventional sintering (CS) method. The results showed that significant enhancement in the relative density and Vickers hardness were observed for the undoped Y-TZP when MW-sintered between 1100?°C and 1250?°C. It was revealed that the 0.2?wt% CuO-doped Y-TZP and MW sintered at 1250–1300?°C could attain ≥?99.8% of theoretical density, Vickers hardness of about 14.4?GPa, fracture toughness of 7.8 MPam^1/2 and exhibited fine equiaxed tetragonal grain size of below 0.25?µm. In contrast, the addition of 1?wt% CuO was detrimental and the samples exhibited about 50% monoclinic phase upon sintering coupled with poor bulk density and mechanical properties. The study also revealed that the addition of 0.2?wt% CuO and subjected to conventional sintering produced similar densification as that obtained for microwave sintering, thus indicating that the dopant played a more significant role than the sintering method.     

Rapid microwave sintering of centimetric zirconia: Scalability and electromagnetic-thermal-fluid-dynamic simulation

Microwave sintering is a method presenting the following advantages for flash sintering: contactless/volumetric heating, and the possibility to control the heating cycle of the microwave power. In this study, the transition from a typical 100 K/min to an ultra-rapid heating rate of 500 K/min is studied. The heating homogeneity of the typical hybrid configuration using silicon carbide susceptors is tested up to the stability limit of the system. We show that zirconia specimens as thick as 10 mm can be heated and sintered up to 500 K/min heating rate at which thermal cracks appear. However, the centimetric size of the specimens seems to favor coarsening implying an important remaining porosity in the end. A comprehensive simulation including microwave heating and convection has allowed the determination of the heating regime transition during the flash process and the quantification of each specimen's cooling fluxes.     

Cold sintering and microwave dielectric properties of dense HBO2-II ceramics

HBO₂-II ceramics were prepared by cold sintering with 10wt% dehydrated ethanol as the transient liquid phase. When the processing temperature is 30°C, the relative density of the mechanically robust HBO₂-II ceramics increases from 77.5% to 84.5% with increasing the uniaxial pressure from 200 to 500 MPa. It changes less than 0.2% for higher pressure up to 700 MPa. Under a constant uniaxial pressure of 500 MPa, the relative density further increases to 94.7% for the processing temperature of 120°C. HBO₂-I is observed as the secondary phase when the processing temperature is 150°C. In comparison, the compacts prepared in the absence of ethanol are fragile, and the relative densities are 78.5%-84.5% for the processing temperatures of 30-120°C and uniaxial pressure of 500 MPa. It is indicated that ethanol promotes the densification significantly through the dissolution-precipitation mechanism. The permittivity increases with increasing the processing temperature, while the Qf value decreases. The optimal properties with the relative density of 94.7%, ε_r = 4.21, Qf = 47 500 GHz, and τ_f = −70.0 ppm/°C were obtained in the single-phase HBO₂-II ceramics cold sintered at 120°C under 500 MPa for 10 minutes. The relative density and Qf value are significantly higher than those of the HBO₂-II ceramic prepared by sintering the H₃BO₃ compact at 180°C for 2 hours (70.3% and 32 700 GHz, respectively). The results indicate that the nonaqueous solvent can also be used as the transient liquid phase for cold sintering, so that more materials that are unstable or insoluble in water can be densified by this method.     

Transition metals increase hydrothermal stability of yttria-tetragonal zirconia polycrystals (3Y-TZP)

In this paper, the influence of transition metals on phase stability of zirconia in 3?mol% Y₂O₃ doped tetragonal zirconia polycrystals (3Y-TZP) in hydrothermal environments was reported. 3Y-TZP with and without stainless-steel or CoCr metal stains on the sample surface were subjected to different isothermal treatments in water vapor, and their respective monoclinic fractions were quantified by confocal Raman spectroscopy. The outputs of these spectroscopic experiments revealed transition metals conspicuously could stabilize the tetragonal zirconia polymorph in the monolithic zirconia, possibly due to the occurrence of off-stoichiometric chemistry in the presence of metal stains.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Plastic deformation and effects of water in room‐temperature cold sintering of NaCl microwave dielectric ceramics

NaCl ceramics were prepared by room‐temperature cold sintering using moistened NaCl powder with 4 wt% water and dry pressing using dehydrated powder. When the applied uniaxial pressure is low, the relative density of dry‐pressed NaCl ceramic is significantly lower than that of cold‐sintered ceramic, while the former is 98.5%‐99.3% and much higher than the latter (94.3%‐94.6%) for high applied pressure of 200‐300 MPa. The uniaxial pressure‐induced plastic deformation dominates the densification of dry‐pressed NaCl ceramic, and also plays a role during cold sintering as well as the dissolution‐precipitation process. The lower density of cold‐sintered NaCl ceramic under high applied pressure is attributed to the trapped water in ceramic body during cold sintering. Besides, the presence of water always promotes the microstructural homogeneity, which is responsible for the much higher Qf value of cold‐sintered NaCl ceramic. The optimal microwave dielectric properties with ε_r = 5.55, Qf = 49 600 GHz, and τ_f = ?173 ppm/°C are obtained in cold‐sintered NaCl ceramic under the applied pressure of 300 MPa, indicating that it is a promising candidate as a microwave dielectric material.     

Phase evolution and microwave dielectric properties of BaTi4O9 ceramics prepared by reaction sintering method

BaTi₄O₉ microwave dielectric ceramics were prepared by reaction sintering method using BaCO₃ and TiO₂ as raw materials. The phase evolution and the chemical reactions were proposed based on the X-ray diffraction results with sintering temperature. The microstructure characteristics were observed using scanning electron microscopy and energy dispersive spectrometer. The compact ceramics with a single phase of BaTi₄O₉ could be prepared successfully by reaction sintering method, exhibiting optimum microwave dielectric properties: a dielectric constant of 36.9, a high quality factor of 52 735 (at 7.5GHz), and a near zero temperature coefficient of resonant frequency of 5.8 ppm/°C, after sintering at 1200°C for 6 hours.     

Fabrication and characterization of Nb2O5-doped 3Y-TZP materials sintered by microwave technology

The purpose of this research is focused on the manufacture and characterization of a partially stabilized zirconia ceramic with 3 mol% of Yttria and doped with .5 and 1.5 mol% of Nb₂O₅ to analyze the influence of doping, with the purpose of improving the properties before hydrothermal degradation. In the first instance, the microwave sintering process was used for the consolidation of this material, then the physical and mechanical properties were characterized. Together, the results obtained by the conventional sintering process were compared. A low hydrothermal degradation study (LTD) is presented at low temperatures in which possible changes in the mechanical properties of the ceramic materials are analyzed and its influence on the phase transformation that zirconia may present is observed. The mechanical properties were evaluated through hardness, fracture toughness, and Young's modulus tests. Likewise, their density was analyzed, and microstructure was characterized by FESEM. It was found that the microwave-sintered samples at 1200°C exhibited superior properties of toughness than even samples sintered by conventional methods at higher temperatures (1400°C). The sample of 3Y-TZP with 1.5 mol% Nb₂O₅ sintered by microwave with <.2% of porosity achieved a maximum fracture toughness value around 40% higher than the dense monolithic 3Y-TZP material.     

An oscillatory pressure sintering of zirconia powder: Densification trajectories and mechanical properties

The densification trajectories and mechanical properties of zirconia ceramics obtained by oscillatory pressure sintering (OPS) process were investigated, during the sintering process an oscillatory pressure was applied at three stages. Current results indicated that at intermediate stage the oscillatory pressure revealed a favorable improvement of mechanical properties compared with conventional hot pressing (HP) and pressureless sintering (PS) procedures, while the enhancement was not obvious at initial stage. When the oscillatory pressure was applied at final stage, the OPS specimens exhibited the highest bending strength and hardness of 1455 ± 99MPa and 16.6 ± 0.31GPa compared with the PS and HP specimens. Considering the high elastic modulus and Moiré patterns observed in the OPS specimen, the oscillatory pressure applied at intermediate and final stages was detected to facilitate the sliding of grain boundary, plastic deformation of monolithic grains, the removal of pores and the strengthening of atomic bonds.     

Flash microwave pressing of zirconia

Microwave Pressing is a promising way to reduce microwave sintering temperatures and stabilize microwave powder materials processing. A multiphysics simulation was conducted of the regulated pressure-assisted microwave cavity. This simulation took into consideration resonance phenomena and the nonlinear temperature-dependent material parameters of zirconia. The intrinsic behaviors of microwave systems and zirconia make the regulation of the microwave pressing difficult. However, the same phenomena can be used to activate flash sintering. Flash microwave sintering uses high electric fields of the resonant microwave profile, the Negative Temperature Behavior (NTC) of zirconia resistivity, and the mechanical pressure applied to the powder via a die compaction configuration. The resulting flash microwave pressing still needs improvement in terms of the processed material structure homogeneity, but it has the capacity to become the fastest sintering treatment as it allows room temperature activation where the total process time only takes a few seconds. In addition, this 10-20 seconds processing technique has shown good potential for improving the transparency of alumina presintered specimens.     

Dielectric properties of alumino-silicate ceramics synthesized by plasma sintering

The polycrystalline ceramic specimens of three different alumino-silicate solid solutions (Al_0.70Si_0.30O, Al_0.73Si_0.27O and Al_0.75Si_0.25O) consisting of different alumina and silica concentrations have been synthesized by thermal plasma sintering technique. From structural analysis carried out by X-ray diffraction, the ceramics are mostly found to consist of two different phases of mullite and sillimanite. SEM images of these ceramics reveal a high dense and less porous microstructure with homogeneous distribution of grains throughout their surface. These materials exhibit high dielectric constant value (>10³) with low dissipation factor. The AC conductivity analysis reveals that Al_0.70Si_0.30O and Al_0.75Si_0.25O ceramics possess room temperature conductivity values of the order of 10^?5, whereas Al_0.73Si_0.27O has conductivity of 10^?7 order that increases with rise in temperature. From the Nyquist plots, the grain and grain boundary conductivities are distinguished and negative temperature coefficient of resistance behavior is identified in these ceramics with small positive temperature coefficient of resistance effect.     

Rutile TiO2 microwave dielectric ceramics prepared via cold sintering assisted two step sintering

In this work, a sintering route named cold sintering assisted two step sintering process (CSP-TS) is presented to prepare rutile TiO₂ ceramics with submicron grain sizes. Cold sintering process at 300 °C with tetrabutyl titanate and water as the liquid phase yields a ‘green body’ with a relatively high density of ～80 %, and finally dense (98.5–99.8 %) rutile TiO₂ ceramics with grain sizes of ～600 nm can be obtained in the second sintering process at 950?1000 °C. The microstructural analysis with SEM and TEM indicates that the CSP-TS samples sintered at 950 °C have an obvious phenomenon of recrystallization, accompanying by a decrease of amorphous phases and a formation of clear grain boundaries. Besides, the rutile TiO₂ ceramics prepared by CSP-TS possess excellent microwave dielectric properties with relative permittivity of 92.0–98.4 and Q × f values of 27,800?31,900 GHz. Therefore, it is feasible to utilize CSP-TS to prepare ceramics with small grain sizes at low sintering temperatures.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Speed sintering translucent zirconia for chairside one-visit dental restorations: Optical,mechanical, and wear characteristics

The fabrication of zirconia dental restorations is a time-consuming process due to traditional slow sintering schemes; zirconia (Y-TZP) produced by these conventional routes are predominantly opaque. Novel speed sintering protocols have been developed to meet the demand for time and cost effective chairside CAD/CAM-produced restorations, as well as to control ceramic microstructures for better translucency. Although the speed sintering protocols have already been used to densify dental Y-TZP, the wear properties of these restorations remain elusive. Fast heating and cooling rates, as well as shorter sintering dwell times are known to affect the microstructure and properties of zirconia. Thus, we hypothesize that speed sintered zirconia dental restorations possess distinct wear and physical characteristics relative to their conventionally sintered counterparts. Glazed monolithic molar crowns of translucent Y-TZP (inCoris TZI, Sirona) were fabricated using three distinct sintering profiles: Super-speed (SS, 1580 °C, dwell time 10 min), Speed (S, 1510 °C, dwell time 25 min), and Long-term (LT, 1510 °C, dwell time 120 min). Microstructural, optical and wear properties were investigated. Crowns that were super-speed sintered possessed higher translucency. Areas of mild and severe wear were observed on the zirconia surface in all groups. Micropits in the wear crater were less frequent for the LT group. Groups S and SS exhibited more surface pits, which caused a scratched steatite surface that is associated with a greater volume loss. Tetragonal to monoclinic phase transformation, resulting from the sliding wear process, was present in all three groups. Although all test groups had withstood thermo-mechanical challenges, the presence of hairline cracks emanating from the occlusal wear facets and extending deep into the restoration indicates their susceptibility to fatigue sliding contact fracture.     

In situ reactive synthesis and plasma-activated sintering of binderless WC ceramics

Binderless micrometre tungsten carbide ceramics (～1.1?μm) were in situ synthesised and densified by plasma-activated sintering (PAS) from mixed powders of tungsten trioxide and carbon black. The influence of sintering process and powders’ composition ratio on the phase composition, microstructure and mechanical properties of as-prepared samples was clarified in detail. The phase evolution was ascertained by X-ray diffraction to be WO₃→WO_2.72→WO₂→W₂C→WC, with the formation of CO and CO₂ gases. The sample with nearly single WC phase, dense structure and excellent mechanical properties was fabricated under the optimised process with the proper composition ratio. Owing to the micrograin size, the fracture toughness (8.88?MPa?m^1/2) was enhanced while high hardness (2159 HV10) was maintained, in the absence of any ceramic toughening phase.     

Low-temperature sintering of single-phase,high-entropy carbide ceramics

Dense (Hf, Zr, Ti, Ta, Nb)C high-entropy ceramics were produced by hot pressing (HP) of carbide powders synthesized by carbothermal reduction (CTR). The relative density increased from 95% to 99.3% as the HP temperature increased from 1750°C to 1900°C. Nominally phase pure ceramics with the rock salt structure had grain sizes ranging from 0.6 µm to 1.2 µm. The mixed carbide powders were synthesized by high-energy ball milling (HEBM) followed by CTR at 1600°C, which resulted in an average particle size of ~100 nm and an oxygen content of 0.8 wt%. Low sintering temperature, high relative densities, and fine grain sizes were achieved through the use of synthesized powders. These are the first reported results for low-temperature densification and fine microstructure of high-entropy carbide ceramics.