Effects of Pr6O11 on the microstructure and mechanical properties of Ti/Al2O3 composites prepared by pressureless sintering

Ti/Al₂O₃ composites with different volume percentages of Pr₆O₁₁ added (0–12.0  vol.%) were prepared by pressureless sintering at 1600 °C for 1.5 h. The influences of Pr₆O₁₁ on the composition, microstructure and mechanical properties of the composites were characterized and investigated. The results showed that Pr₆O₁₁ could promote the sintering of the composites by generating some new interfacial reaction products, such as AlTiO₂, Pr₂Ti₂O₇ and PrAlO₃. Pr₆O₁₁ could also inhibit the production of TiAl and Ti₃Al by the same mechanism. Additionally, Pr₆O₁₁ changed hexagonal alumina to tetragonal alumina. The latter could improve the mechanical properties of the composites by the effects of crack deflection and particle pullout when it was present in proper amounts. Composites showed satisfactory comprehensive properties when the content of Pr₆O₁₁ was no more than 3.0 vol.%.     

Synthesis of transparent YIG ceramics by pressureless sintering

Transparent YIG (Y₃Fe₅O₁₂) ceramics are successfully synthesized by reactive sintering at normal pressure using γ-Fe₂O₃ and Y₂O₃ as starting materials. The grain size of the sintered YIG ceramics is ca. 10–15 µm. Residual pores are not observed on the surface of sample, but numerous residual pores are observed by infrared transmission microscopy. In-line transmittance of a commercially available high-quality YIG single crystal (thickness 1 mm) fabricated by the floating zone method is 75 % in the near to mid-infrared region, whereas the sample produced in this study shows an in-line transmittance of 71 % in the wavelength range above 1.5 µm.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Influence of vanadium content on the microstructural evolution and mechanical properties of (TiZrHfVNbTa)C high-entropy carbides processed by pressureless sintering

A series of (TiZrHfVNbTa)C high-entropy ceramics with different vanadium contents have been fabricated by pressureless sintering at 2300 °C–2500 °C for 1 h, utilizing self-synthesized carbide powders obtained by carbothermal reduction. The addition of vanadium is beneficial to promote densification process and refine grain, as well as facilitate the homogeneous distribution of metal elements. The distribution of pores is also modified, almost entirely existing at grain boundary, and the integral mechanical properties achieve optimization. However, excess adding vanadium does not favor forming a single-phase (TiZrHfVNbTa)C high-entropy ceramic. The optimal (TiZrHfVNbTa)C high-entropy ceramic sintered at 2300 °C possesses a high relative density of 97.5 % and homogeneous microstructure with small grain size of 1.2 μm. The flexural strength and Vickers hardness reach 473 MPa and 24.9 GPa, respectively. This work has established a cost-effective and convenient preparation of novel (TiZrHfVNbTa)C high-entropy carbide ceramics.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Physical and mechanical properties of hexagonal boron nitride ceramic fabricated by pressureless sintering without additive

Hexagonal boron nitride hBN ceramic was successfully fabricated by pressureless sintering at 2100C using submicrometre hBN powders without any sintering additive. The as-prepared hBN ceramic showed a room temperature flexural strength of 30.7MPa. Its flexural strength increased with the increment of temperature in N₂ atmosphere, and it retained a strength of 57.2MPa nearly two times of the room temperature strength at 1600C due to clean grain boundaries with no glassy phase. Additionally, the as-prepared hBN ceramic showed a high thermal conductivity of 31.76Wm¹k¹ and a good thermal shock resistance, which retained a relatively high residual flexural strength of 22.6MPa 73.5 of the original flexural strength at T800C. The as-prepared hBN ceramic presents a good application prospect at high temperature.     

Highly-transparent AlON ceramic fabricated by tape-casting and pressureless sintering method

Non-aqueous tape-casting forming technology was used in the fabrication of highly-transparent aluminum oxynitride (AlON) ceramics with different sizes and shapes. The tape-casting process utilizes the warm-pressing and the cold isostatic pressing methods, which can optimize the microstructure and increase the density of the AlON green tapes. In this work, highly-transparent AlON ceramics were fabricated by the pressureless sintering method at 1880 °C for 8 h. The transmittance of final polished AlON ceramic reaches 84 % at the wavelength of 2000 nm.     

Effect of zirconia addition on pressureless sintering of boron carbide

This paper presents the results of experiments on pressureless sintering of boron carbide with varying addition of zirconia (ZrO₂: 0–30 wt.%). Green pellets were densified by sintering at 2275 °C in vacuum for 60 min and characterized by measurement of density, hardness, thermal conductivity and microstructure. Samples prepared with the addition of ≥5 wt.% ZrO₂ showed higher densities in the range of 93–96% ρth, compared to 86.63% ρth for boron carbide only. Addition of ZrO₂ was found to increase the hardness of sintered samples and regardless of ZrO₂ content, the hardness values ranged between 30 and 31.5 GPa. XRD of the sintered pellets showed the presence of ZrB₂. Optical microscope as well as electron probe microanalysis (EPMA) showed the presence of two phases, grey matrix with white precipitates. EPMA analysis of second phase revealed the presence of Zirconium in this phase. Fractography of boron carbide with 25% ZrO₂ showed the failure to be by mixed fracture (transgranular and intergranular). Thermal conductivity values of the samples measured in the temperature range of 400–1000 °C were marginally higher with the addition of ZrO₂.     

Sintering of zirconia ceramics by intense high-energy electron beam

A comparative analysis of the efficiency of zirconia ceramics sintering by thermal method and high-energy electron beam sintering was performed for compacts prepared from commercial TZ-3Y-E grade powder. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1200–1400 °C. Sintering of zirconia ceramics by high-energy accelerated electron beam is shown to reduce the firing temperature by about 200 °C compared to that in conventional heating technique. Ceramics sintered by accelerated electron beam at 1200 °C is of high density, microhardness and smaller grain size compared to that produced by thermal firing at 1400 °C. Electron beam sintering at higher temperature causes deterioration of ceramics properties due to radiation-induced acceleration of high-temperature recrystallization at higher temperatures.     

Towards fabrication of high-performance Al2O3 ceramics by indirect selective laser sintering based on particle packing optimization

A novel method to fabricate high-performance Al₂O₃ Ceramics by indirect Selective Laser Sintering (idSLS) based on particle packing optimization was reported. Al₂O₃ ceramics with the particle size distribution (PSD) being adjusted based on a particle packing model was prepared with the idSLS method, and the effects of PSD on the macro-performance and microstructure of the idSLSed ceramics part was investigated. Results show that an appropriate PSD of raw material is not only essential to the good SLS formability of powder bed but also contribute to the favorable sintering characteristics of the SLSed green body through an ideal particle packing. The improvement of overall properties including mechanic strength, dimensional accuracy, and surface flatness of the SLSed green body and the final parts were achieved with a fine-tuned PSD exponent. When the raw material has a PSD exponent of 0.5–0.6, the final ceramic parts showed relatively balanced comprehensive properties of bulk density >2.0 g cm⁻³, linear change <1.4%, fracture strength >3.97 MPa, compressive strength >5.25 MPa and surface altitude difference <148 μm.     

Flash sintering behaviour of 8YSZ-NiO composites

Flash sintering is an electric field/current assisted sintering technique, which is reported to lower the furnace temperature and to reduce sintering time significantly. In this work, we have studied the processing of 8YSZ/NiO composites by flash sintering, for the first time. Two composites, with different amount of NiO (one below the percolation limit and another one above it) were processed in two different sintering atmospheres. Constant heating rate experiments were performed to know the minimum furnace temperature required to flash sinter the samples for a given applied electric field. Subsequently, isothermal flash sintering experiments were performed at different current densities. The flash onset temperature of the composites was lower in the reducing atmosphere compared to in air. The power dissipated in stage III of the flash was strongly influenced by the composite composition and the sintering atmosphere. The extent of densification in the composites was controlled by the current density. The composites were densified up to a relative density of ～90% in 30 s when flash sintered in air. In reducing atmosphere, there was in-situ reduction of NiO to Ni. As a result, for composites containing NiO above the percolation limit, the current preferentially flew through the in-situ formed metallic phase and there was no densification in the composite in reducing atmosphere. Phase and microstructure evolution in the composites was studied through XRD, SEM and EDS. With proper control of the electrical parameters (electric field and current density), composites with controlled porosity can be processed through flash sintering which may have applications for solid oxide fuel cells.     

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y₂O₃–ZrO₂) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y₂O₃–ZrO₂ during sintering were studied, and the effect of sintering temperature on the properties of Y₂O₃–ZrO₂ was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.     

Ultra-high temperature ceramics based on ZrB2 obtained by pressureless sintering with addition of Cr3C2, Mo2C,and WC

ZrB₂-MeC and ZrB₂-19 vol% SiC-Me_xC_y where Me=Cr, Mo, W were obtained by pressureless sintering. The capability to promote densification of ZrB₂ and ZrB₂-SiC matrices is the highest for WC and lowest for Cr₃C₂. The interaction between the components results in the formation of new phases, such as MeB (MoB, CrB, WB), a solid solution based on ZrC, and a solid solution based on ZrB₂. The addition of Cr₃C₂ decreases the mechanical properties. On the other hand, the addition of Mo₂C or WC to ZrB₂-19 vol% SiC composite ceramics leads increased mechanical properties. Long-term oxidation of ceramics at 1500 °C for 50 h showed that, in binary ZrB₂-Me_xC_y, a protective oxide scale does not form on the surface thus leading to the destruction of the composite. On the contrary, triple composites showed high oxidation resistance, due to the formation of dense oxide scale on the surface, with ZrB₂-SiC-Mo₂C displaying the best performance.     

Fabrication and mechanical properties of large-scale SiC impeller via vacuum gelcasting and pressureless sintering

Impeller is the key part of centrifugal pump to convey liquid. And combined with the engineering practice, we fabricated two types of open and semi-open SiC ceramic impellers with diameter of 100 mm and 160 mm via gelcasting and pressureless sintering. B4C and C were used as the sintering additives and sprayed drying granulation method was adopted for the starting SiC powder processing. Given the size and shape complexity of impeller, the optimized pH value, dispersant content, and solid-loading content for the fabrication of the SiC impellers were determined to be 10-11, 0.8 wt%, and 40 vol%, respectively. For the open impeller, blade height, cover board thickness, drying shrinkage, and sintering shrinkage are 10.5 mm, 4 mm, 10.3%, and 21.3%, respectively. For the semi-open ones, the corresponding parameters are 41 mm, 10 mm, 10.3%, and 21.3%, respectively. The average density of the two types of impellers is 3.15 g/cm3, and the mechanical properties of both impellers, including average hardness, flexural strength, compressive strength, and fracture toughness are 2478 HV, 436 MPa, 2093 MPa, and 3.17 MPa·m1/2, respectively. All variation coefficient values are smaller than 5%, which indicates a good uniformity in densities and mechanical properties of both the impellers.     

Crystal structure and morphology of CeO2 doped stabilized zirconia ceramics under high-frequency microwave field sintering

Under high-frequency microwave irradiation, zirconia ceramics were prepared by sintering nano-CeO₂ (Ce = 7 mol%) doped zirconia powder. The different effects of temperature environment on the phase structure transformation, surface functional groups, microstructure, growth process, and density of doped zirconia were analyzed, and the optimized microwave sintering process for zirconia was determined. The experimental results reveal that the tetragonal phase of zirconia is positively correlated with the temperature when the temperature reaches about 1100 °C in the studied range. The reason is that the grain grows with the increase of sintering temperature, and the surface energy of grain decreases, which leads to the fluctuation of tetragonal phase content. The density of zirconia reaches 98.03% at 1300 °C, and the growth activation energy is 27.40 kJ/mol. There is no abnormal growth of zirconia particles, and the phase transition temperature decreases, which is attributed to the efficient heating of microwave and the incorporation of nano-ceria stabilizer.     

Graphene nanoplatelets for electrically conductive 3YTZP composites densified by pressureless sintering

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 2.5, 5 and 10 vol% graphene nanoplatelets (GNP) were pressureless sintered in argon atmosphere between 1350 and 1450 °C. The effects of the GNP content and the sintering temperature on the densification, microstructure and electrical properties of the composites were investigated. An isotropic distribution of GNP surrounding ceramic regions was exhibited regardless the GNP content and sintering temperature used. Electrical conductivity values comparable to the ones of fully dense composites prepared by more complex techniques were obtained, even though full densification was not achieved. While the composite with 5 vol% GNP exhibited electrical anisotropy with a semiconductor-type behaviour, the composite with 10 vol% GNP showed an electrically isotropic metallic-type behaviour.     

Pressureless sintering of high-entropy boride ceramics

High-entropy boride ceramics were densified by pressureless sintering. Green densities of the ceramics varied by composition with the highest green density of 53.6 % for (Hf, Nb, Ta, Ti, Zr)B₂. After pressureless sintering, relative densities up to ∼100 % were obtained for (Cr, Hf, Ta, Ti, Zr)B₂ and (Hf, Ta, Ti, V, Zr)B₂. Two compositions, (Hf, Ta, Ti, W, Zr)B₂ and (Hf, Mo, Ti, W, Zr)B₂ contained secondary phases and did not reach full density. All compositions had average grain sizes less than 10 µm and less than 2 vol % of residual B₄C. This is the first report of conventional pressureless sintering of high-entropy boride ceramics powder compacts without evidence of liquid phase formation.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Effect of Ni and C additives on pressureless sintering and mechanical properties of ZrB2

High density bulk zirconium diboride was successfully produced by pressureless sintering with the Ni additive as sintering aid. Microstructure and phase formation were investigated by using scanning electron microscopy and X-ray diffractometry respectively. Microhardness, fracture toughness and compressive strength of the sintered samples were measured. Results showed that the higher Ni content led to higher densification, fracture toughness and hardness and lower compressive strength. A significant grain growth was observed during sintering at 2000°C, which may be due to the oxygen impurity in the surface of particles. The addition of 1 wt-%C led to oxygen removing and smaller grains after sintering.     

Preparation of carbon fiber/Mg-doped nano-hydroxyapatite composites under low temperature by pressureless sintering

In order to protect carbon fibers (CF) from oxidation damage during sintering process, rod-like Mg-doped nano-hydroxyapatite (Mg-nHA) with an increased thermal decomposition temperature and reduced sintering temperature was synthesized by hydrothermal method. The synthesized bone-like Mg-nHA with similar composition and morphology to bone apatite was used as the matrix to prepare CF reinforced Mg-nHA composites (CF/Mg-nHA) at a low temperature of 700 °C by pressureless sintering. The increase of temperature slightly influenced the growth of Mg-nHA prepared by hydrothermal method from 160 °C to 200 °C. The Mg-nHA were short and rod-like in structure with a length of approximate 100 nm. When doping 1% magnesium, the decomposition temperature of Mg-nHA increased by 100 °C compared with that of nHA. This can protect CF from oxidation damage which is often encountered when sintering CF reinforced hydroxyapatite composites at high temperature and enhance reinforcing effects of CF. The bending strength of CF/Mg-nHA with 1 wt% CF was 8.51 MPa, which increased by 19.5% compared with Mg-nHA. Alternatively, the rod-like Mg-nHA was prepared on the surface of CF by electrochemical deposition and Mg-nHA coated CF was used to reinforce Mg-nHA, the coefficient of thermal expansion mismatch between CF and HA matrix could be mitigated. The compressive strength of Mg-nHA coated CF reinforced Mg-nHA (CF/Mg-nHA/Mg-nHA) composites with 0.5% CF sintered at 800 °C were 41.3 ± 1.56 MPa, which was attributed to the improved strengthening effect of CF and the good interface between CF and Mg-nHA matrix.