Mechanical properties of 2.45 GHz microwave sintered Si3N4–Y2O3–MgO–ZrO2 system

Mechanical properties of 2.45 GHz microwave sintered Si₃N₄–Y₂O₃–MgO–ZrO₂ system have been investigated. Microwave sintered samples exhibited higher hardness compared to conventionally sintered samples. SEM microstructures of microwave sintered samples revealed lower average grain length and width than those of the conventionally sintered samples. Fracture toughness increased with increasing sintering temperature in the case of conventionally sintered samples whereas microwave sintered samples exhibited no variation despite differences in microstructure. The results of present study demonstrated that microwave sintering could influence the microstructure and thereby improve the mechanical properties.     

Phase stability and mechanical properties of TZP with a low mixed Nd2O3/Y2O3 stabiliser content

The phase assembly of 1.0–5.0 mol% Nd₂O₃-doped ZrO₂ sintered at 1400 °C revealed that the tetragonal ZrO₂ phase could not be completely stabilised. Co-stabilising of 0.5–2.5 mol% Nd₂O₃ with 0.5–1.0 mol% Y₂O₃, however, allowed the preparation of fully dense (Nd,Y)-TZP ceramics by pressureless sintering in air at 1450 °C. The mixed stabiliser monoclinic zirconia nanopowder starting material was synthesized from a suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder in an alcohol/water mixture. A HV₃₀ hardness of 10 GPa combined with an excellent indentation toughness of 13 MPa m^1/2 could be achieved for the (1.0Nd,1.0Y)- and (1.5Nd,1.0Y)-TZP ceramics. The influence of the mixed stabiliser content on the phase stability and mechanical properties are investigated and discussed.     

Development of ZrO2–WC composites by pulsed electric current sintering

ZrO₂–WC ceramic composites with 40 vol% WC were consolidated by pulsed electric current sintering (PECS) for 4 min at 1450 °C under a pressure of 60 MPa. The effect of ZrO₂ stabilizers and the source of WC powder on the densification, phase constitution, microstructure and mechanical properties of the ZrO₂–WC composites were investigated and analyzed. The experimental results revealed that the amount and type of ZrO₂ stabilizers played a primary role on the phase constitution and mechanical properties of the composites in comparison to the morphology and size of the WC powder. The 2 mol% Y₂O₃-stabilized composites exhibited much better mechanical properties than that of 1.75 mol% Y₂O₃-stabilized or 1 mol% Y₂O₃ + 6 or 8 mol% CeO₂ co-stabilized composites. A Vickers hardness of 16.2 GPa, fracture toughness of 6.9 MPa m^1/2, and flexural strength of 1982 MPa were obtained for the composites PECS from a mixture of nanometer sized WC and 2 mol% Y₂O₃-stabilized ZrO₂ powder.     

Preparation and properties of an Al2O3/Ti(C,N) micro-nano-composite ceramic tool material by microwave sintering

One kind of Al₂O₃/Ti(C,N) micro-nano-composite ceramic tool material with acceptable properties was prepared by microwave sintering. Effects of sintering temperature and holding time on densification, mechanical properties and microstructure were studied. The optimal relative density, fracture toughness and Vickers hardness were 98.4±0.30, 6.72±0.28 MPa m^1/2 and 18.42±0.59 GPa, respectively, which were obtained at 1550 °C for 10 min. Compared to the conventional sintering, the sintering temperature and holding time of microwave sintering were reduced by 14% and 89%, respectively. The microwave sintering made the sizes of some particles keep in nano-scale, which leaded to the formation of intragranular structures. The residual stress in the intragranular structures increased the ratio of grain boundary toughness to grain toughness of matrix (K_cb/K_cg), and thus the micro-Al₂O₃ grains were more inclined to transgranular fracture.     

Low temperature sintering of the Ba2Ti9O20 ceramics using B2O3/CuO and BaCu(B2O5) additives

The effects of B₂O₃/CuO and BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of Ba₂Ti₉O₂₀ ceramics were investigated. The B₂O₃ added Ba₂Ti₉O₂₀ ceramics were not able to be sintered below 1000 °C. However, when both CuO and B₂O₃ were added, they were sintered below 900 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the Ba₂Ti₉O₂₀ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the Ba₂Ti₉O₂₀ ceramics. Good microwave dielectric properties of Qxf = 16,000 GHz, ɛ_r = 36.0 and τ_f = 9.11 ppm/°C were obtained for the Ba₂Ti₉O₂₀ ceramics containing 10.0 mol% BaCu(B₂O₅) sintered at 875 °C for 2 h.     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Mechanical properties and cytotoxicity of 3Y-TZP bioceramics reinforced with Al2O3 particles

The influence of Al₂O₃ addition and sintering parameters on the mechanical properties and cytotoxicity of tetragonal ZrO₂–3 mol% Y₂O₃ ceramics was evaluated. Samples containing 0, 10, 20 and 30 wt.% of Al₂O₃ particles were prepared by cold uniaxial pressing (80 MPa) and sintered in air at 1500, 1550 and 1600 °C for 120 min. The effects of the sintering conditions on the microstructure were analyzed by X-ray diffraction analysis and scanning electron microscopy. Hardness and fracture toughness were determined by the Vickers indentation method and the mechanical resistance by four-point bending tests. As a preliminary biological evaluation, “in vitro” cytotoxicity tests were realized to determine the cytotoxic level of the ZrO₂–Al₂O₃ composites, using the neutral red uptake method with NCTC clones L929 from the American Type Culture Collection (ATCC) bank. Fully dense ceramic materials were obtained with a hardness ranging between 1340 HV and 1585 HV, depending on the amount of Al₂O₃ in the ZrO₂ matrix. On the other hand, no significant influence of the Al₂O₃ addition on fracture toughness was observed, exhibiting values near 8 MPa m^1/2 for all compositions and sintering conditions studied. The non-cytotoxic behavior, the elevated fracture toughness, the good bending strength (σ_f = 690 MPa) and the elevated Weibull's modulus (m = 11) exhibited by the material, show that these ceramic composites are highly suitable biomaterials for dental implant applications.     

Transparent Lu3NbO7 bodies prepared by reactive spark plasma sintering and their optical and mechanical properties

Transparent lutetium niobate (Lu₃NbO₇) bodies were prepared by reactive spark plasma sintering using Lu₂O₃ and Nb₂O₅ powders. Fully dense Lu₃NbO₇ bodies with density greater than 99.5% of the theoretical were obtained at 1300–1650 °C. The grains steadily grew from 0.1 to 0.6 μm with increasing sintering temperature from 1200 to 1450 °C and significant grain growth from 2.2 to 9.2 μm occurred at 1550–1650 °C. The Lu₃NbO₇ body sintered at 1450 °C showed the highest transmittance of 63% at 550 nm after heat treatment at 850 °C in air for 6 h. Fully dense, submicron-size transparent Lu₃NbO₇ exhibited Vickers hardness of ～13.0 GPa and indentation fracture toughness of 1.0 MPa m^1/2.     

Influence of cerium doping on mechanical properties of tetragonal scandium-stabilized zirconia

Mechanical properties of tetragonal scandium-stabilized zirconium oxide (ScSZ) co-doped with cerium oxide are studied before and after a complete reduction of Ce⁴⁺ to Ce³⁺. Samples of 6Sc1CeSZ (i.e., 6 mol% Sc₂O₃, 1 mol% CeO₂), 6Sc2CeSZ, and 4Sc1CeSZ are produced by pressure-less sintering in air at 1450 °C. Test pieces are annealed at 1200 °C in an atmosphere of forming gas (5% H₂, 95 % N₂). Vickers hardness, Young’s modulus, 4-point bending strength, and fracture toughness are measured at room temperature. Values of hardness and Young’s modulus are similar for different compositions of ScSZ:Ce and stay practically unaffected by the reduction. Both the fracture toughness and strength significantly decrease and the largest drop is measured for the 6Sc2CeSZ, having the highest concentration of cerium. The observed degradation of mechanical properties suggests that cerium-free electrolytes should be preferred for the fabrication of the electrolyte-supported planar SOFCs.     

Characterisation of SnO2–CoO–MnO–Nb2O5 ceramics

Varistors based on SnO₂ have attracted increasing interest in recent years. However, the combined effect of CoO–MnO on SnO₂ ceramics is still unclear. In this study, the non-Ohmic behaviour of the 98.95 mol%SnO₂–0.5 mol%CoO–0.5 mol%MnO–0.05 mol%Nb₂O₅ system, the microstructures and the influence of sintering temperature were investigated. The samples were prepared by the mixed oxide route, and were sintered at temperatures in the range 1250–1450 °C. SEM observation and EDS analysis revealed that the ceramics have a two-phase microstructure comprising SnO₂ primary grains and a Mn, Co rich secondary phase of small particles. The sintered density of the samples increased with the increase in sintering temperature. The maximum non-linear coefficient (α = 10) was obtained at a sintering temperature of 1350 °C.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Mechanical and optical properties of spark plasma sintered transparent Y2O3 ceramics

Commercial Y₂O₃ nanopowder was used to fabricate transparent Y₂O₃ ceramics by spark plasma sintering under the pressure of 100 MPa for 20 min with the heating rate of 100 °C/min. The microstructures, mechanical and optical properties of the Y₂O₃ ceramics sintered at different temperatures were investigated in detail. Densification occurred up to a sintering temperature of 1500 °C, and above 1500 °C, rapid grain growth and pore growth occurred. The highest relative density of 99.58% and the minimum average grain size of 0.58±0.11 µm were obtained at 1500 °C. The flexural strength, hardness and fracture toughness of the optimal spark plasma sintered Y₂O₃ ceramic were 122 MPa, 7.60 GPa and 2.06 MPa.m^1/2, respectively. The Y₂O₃ ceramic sintered at 1500 °C had the in-line transmission of about 11–54% and 80% in the wavelength range of 400–800 nm and 3–5 µm, respectively.     

Impact of microwave processing on porcelain microstructure

Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by ~ 75 °C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 °C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (~ 32 ± 3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having ~ 63 mol% alumina content compared to ~ 60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al⁺³ diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (~ 900–1000 °C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.     

Microwave sintering of fine grained HAP and HAP/TCP bioceramics

The effect of microwave sintering conditions on the microstructure, phase composition and mechanical properties of materials based on hydroxyapatite (HAP) and tricalcium phosphate (TCP) was investigated. Fine grained monophase HAP and biphasic HAP/TCP biomaterials were processed starting from stoichiometric and calcium deficient nanosized HAP powders. The HAP samples microwave (MW) sintered for 15 min at 900 °C, with average grain size of 130 nm, showed better densification, higher density and certainly higher hardness and fracture toughness than samples conventionally sintered for 2 h at the same temperature. By comparing MW sintered HAP and HAP/TCP samples, it was concluded that pure HAP ceramics have superior mechanical properties. For monophase MW sintered HAP samples, the decrease in the grain size from 1.59 μm to 130 nm led to an increase in the fracture toughness from 0.85 MPa m^1/2 to 1.3 MPa m^1/2.     

Effect of graphite nanoplatelets on the mechanical properties of alumina-based composites

Al₂O₃-based composites using exfoliated graphite nanoplatelets (xGnPs) have been developed by powder metallurgy (PM) route using both conventional as well as spark plasma sintering (SPS) processes. Al₂O₃-0.2, 0.5, 0.8, 3 and 5 vol% xGnP composites have been developed, and the effect of the addition of xGnP on the density, hardness, fracture toughness and wear behaviour of the various Al₂O₃-xGnP composites have been analyzed. Conventional sintering was done at a temperature of 1650 °C for 2, 3 and 4 h in inert atmosphere, whereas SPS was carried out at 1450 °C under 50 MPa pressure for 5 min. A uniform dispersion of the xGnP in the Al₂O₃ matrix was observed in the composites upto the addition of 3 vol% xGnP. Results indicate that a significant improvement in hardness, wear resistance and fracture toughness of the composites could be achieved by using xGnP as nanofiller. The hardness and fracture toughness of the composites developed by both conventional sintering and SPS show an increase upto the addition of 3 and 0.8 vol% xGnP respectively. The wear resistance of the composites also shows significant improvement upto the addition of 3 vol% xGnP. The composites developed by SPS have been found to possess superior mechanical properties as compared to the composites developed by conventional sintering. The improvement in the mechanical properties can be attributed to the strong interaction between the xGnP and the Al₂O₃ matrix at the interfaces and to the toughening mechanisms such as crack bridging and crack deflection.     

Microstructure development,hardness, toughness and creep behaviour of pressureless sintered alumina/SiC micro–nanocomposites obtained by slip-casting

Al₂O₃–SiC micro–nanocomposites are much more resistant materials than monolithic alumina regarding some mechanical properties. In order to study the possibility of obtaining creep resistant alumina/SiC micro–nanocomposites using inexpensive forming methods, alumina 1 and 5 vol% SiC materials were produced by slip-casting and pressureless sintering. Well-densified alumina–SiC pressureless sintered materials were obtained at 1700 °C for 2 h and attained 97–99% of the theoretical density. The microstructure, hardness and toughness were examined and 4-point flexure creep tests were performed at 1200 °C and 100 MPa in air. Compared with pure alumina materials, the creep resistance, toughness and hardness were enhanced drastically in materials containing 5 vol% of SiC.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.