Microstructure and Fracture Toughness of Yttria-Daped Tetragonal Zirconia Polycrystal/Mullite Composites Prepared by an in Situ Method

Yttria-daped tetragonal zirconia polycrystal (Y-TZP)/ mullite composites were prepared by three methods: in situ whisker growth (IS), physical mixing (PM) of zirconia powder and mullite whiskers, and reaction sintering (RS). Microstructures and fracture toughness values were compared. All the composites with 15 vol% of mullite could be densified to more than 95% relative density by firing at 1500° to 1500°C for 10 h. The fracture toughness of the composites as measured by the indentation method showed a clear enhancement compared with that of pure Y-TZP; the ranking was Y-TZP ≦ RS composite < PM composite < IS composite. Enhancement of the fracture toughness in composites was found to relte strongly to the aspect ratio of mullite particles.     

High-toughness mullite ceramics fabricated by hot-press sintering of pyrophyllite and AlOOH at low temperatures

High-toughness mullite ceramics were fabricated through hot-press sintering (HPS) of pyrophyllite and AlOOH, which were wet-milled and well mixed using a planetary ball mill. The impacts of sintering temperatures and contents of AlOOH on mullite phase formation, densification, microstructure and mechanical properties in ceramic materials were investigated through XRD, SEM and mechanical properties determination. The results indicated that high-toughness mullite ceramics could be successfully prepared by HPS at temperatures higher than 1200°C for 120 min. Increasing the sintering temperature from 1000 to 1300°C significantly enhanced the flexural strength and fracture toughness of samples. The highest flexural strength of 297.97±25.32 MPa and fracture toughness of 4.64±0.11 MPa⋅m^1/2 were obtained for samples sintered at 1300°C. Further increase of temperature to 1400°C resulted in slight decrease of flexural strength and fracture toughness. Compared with the mullite ceramics prepared only using pyrophyllite as raw material, incorporation of AlOOH into raw material significantly increased the mechanical properties of final mullite ceramics. And stoichiometric AlOOH and pyrophyllite as starting material gave the best performance in fracture toughness. The high-toughness of mullite ceramics were ascribed to the high mullite phase content, fine mullite whiskers and in situ formed, intertwined three-dimensional network structure obtained through HPS at a low temperature of 1300°C.     

Mechanical properties of in situ synthesized mullite-based composite ceramics with three-dimensional network structure

Needle-like nanocrystalline mullite powders were prepared through the molten salt process at the temperature of 900°C using coal gangue as raw material. Then, mullite-based composite ceramics were prepared by a conventional solid-state reaction between in situ synthesized mullite and Al₂O₃ powders. Effects of Al₂O₃ content and sintering temperatures on phase compositions, microstructure, and mechanical properties of the mullite-based composite ceramics were also studied. The results show that mullite content productivity increase from 72% to 95%, as the sintering temperature increased from 1480°C to 1580°C, which led to the improvement in the bulk density and flexural strength of the samples. The three-dimensional interlocking structure for mullite-based composite ceramics was obtained by the in situ solid-state reaction process. The maximum bulk density, flexural strength, and fracture toughness for the sample with 15 wt% Al₂O₃ content are 2.48 g/cm³, 139.79 MPa, and 5.62 MPa··m^1/2, respectively, as it was sintered at the temperature of 1560°C for 3 h. The improved mechanical properties of mullite-based composite ceramics maybe ascribed to good densification and increased mullite phase content, as well as to the in situ three-dimensional network structure. Therefore, the results would provide new ideas for high-value utilization of coal gangue.     

Study on mechanical properties of hot pressing sintered mullite-ZrO2 composites with finite element method

In this study, mullite–zirconia (ZrO₂) composites were fabricated by hot pressing sintering method. The effects of sintering temperature and holding time on the microstructures, phase compositions and mechanical properties of the composites were investigated. The results indicated that the size of t-ZrO₂ grain varies with sintering temperature and holding time, and the maximum flexural strength of 674.05?MPa and fracture toughness of 12.08?MPam^1/2 are obtained when the sintering temperature is 1500?°C with holding times of 20 and 60?min, respectively. Finite element method was employed to analyze the relationship between grain size and mechanical properties of mullite–ZrO₂ composites for the first time. The results showed that the maximum stress on mullite–ZrO₂ interface increases with the growth of t-ZrO₂ grain size, which enhances the generation and propagation of cracks on grain boundaries significantly and degrades the flexural strength and fracture toughness of the mullite–ZrO₂ composite ceramics.     

Mullite–molybdenum composites fabricated by pulse electric current sintering technique

Pulse electric current sintering of monolithic mullite and mullite/0–100 vol.% Mo composites was performed in vacuum of 4.5×10⁻⁵ Torr at temperatures and pressures of 1500 °C and 20 MPa, respectively. No traces of additional phases were observed by SEM and XRD for these composites. Microstructural observations reveal that Mo (molybdenum) particles dispersed uniformly at lower Mo contents and exhibited flaky and elongated structure at higher content. Simultaneous increase of fracture strength and toughness occurred with increase in Mo content. It attained a maximum of 1.1 GPa and 9.2 MPa m^1/2, respectively for 90 vol.% Mo composites. The increase in flexural strength is due to smaller initial flaws in mullite/Mo composites for lower Mo contents and due to plastic deformation of Mo phase for higher Mo contents. Similarly, frontal process zone toughening and crack bridging are expected to be the responsible mechanisms for enhanced toughness in these composites. Partial debonding in the mullite–Mo interface, giving rise to plastic deformation of Mo phase also contributes in the increase of toughness values.     

Processing and mechanical properties of Al2O3–5 vol.% Cr nanocomposites

The use of chromium (III) acetylacetonate as a source of nanometre sized chromium particles for the production of Al₂O₃–5 vol.% Cr nanocomposites has been investigated. The details of the processing procedure are crucial in determining the mechanical properties of the composite. The highest strength and fracture toughness, 736±29 MPa and 4.0±0.2 MPa m^1/2, respectively, were obtained for the nanocomposite hot pressed at 1450 °C. It is shown that the strengthening in Al₂O₃–5% Cr nanocomposites mainly results from microstructure refinement in that the mean alumina matrix grain size in the optimum composite was 0.68 μm compared with a grain size of 3.6 μm in the monolithic alumina hot pressed under identical conditions. Crack bridging and crack deflection by the nano-sized Cr particles did not occur to any significant extent. The slight improvement in fracture toughness may result from the observed change in fracture mode from intergranular fracture for monolithic alumina to transgranular failure for the nanocomposites.     

Influence of starting powder choice on structure property relations in gadolinia stabilized zirconia 3Gd-TZP manufactured from co-milled starting powders

Two 3Gd-TZP materials were manufactured from powders produced by intense mixing and milling of unstabilized zirconia starting powders and gadolinia as stabilizer oxide by hot pressing at 1250 °C – 1400 °C. The materials show a combination of high toughness and moderate strength. In detail depending on starting powder the two TZP showed distinct differences concerning transformation characteristics, sintering temperature dependence of mechanical properties and the tendency to develop R-curve related deformation features such as non-linear stress strain curves and formation of transformation bands prior to fracture.     

Electrical Conductivity of Mullite Ceramics

The electrical conductivity of a lab‐produced homogeneous mullite ceramic sintered at 1625°C for 10 h with low porosity was measured by impedance spectroscopy in the 0.01 Hz to 1MHz frequency range at temperatures between 300°C and 1400°C in air. The electrical conductivity of the mullite ceramic is low at 300°C (≈0.5 × 10^?9 Scm^?1), typical for a ceramic insulator. Up to ≈ 800°C, the conductivity only slightly increases (≈0.5 × 10^?6 Scm^?1 at 800°C) corresponding to a relatively low activation energy (0.68eV) of the process. Above ≈ 800°C, the temperature‐dependent increase in the electrical conductivity is higher (≈10^?5 Scm^?1 at 1400°C), which goes along with a higher activation energy (1.14 eV). The electrical conductivity of the mullite ceramic and its temperature‐dependence are compared with prior studies. The conductivity of polycrystalline mullite is found to lie in‐between those of the strong insulator α‐alumina and the excellent ion conductor Y‐doped zirconia. The electrical conductivity of the mullite ceramic in the low‐temperature field (< ≈800°C) is approximately one order of magnitude higher than that of the mullite single crystals. This difference is essentially attributed to electronic grain‐boundary conductivity in the polycrystalline ceramic material. The electronic grain‐boundary conductivity may be triggered by defects at grain boundaries. At high temperatures, above ≈ 800°C, and up to 1400°C gradually increasing ionic oxygen conductivity dominates.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Toughening enhanced at elevated temperatures in an alumina/zirconia dual-phase matrix composite reinforced with silicon carbide whiskers

A unique temperature dependence of toughening is observed in an alumina/zirconia dual-phase matrix composite reinforced with silicon carbide whiskers. The work of fracture (WOF) of the composite is maximized at 400 °C to 130 J/m², which is about 6.5 times larger than that of monolithic alumina at room temperature. The WOF decreases sharply with an increase in temperature above 400 °C. The enhanced toughening at elevated temperatures is described by the stress-induced transformation toughening of tetragonal zirconia, which is affected by the internal thermal stress owing to thermoelastic mismatch between the matrix and the whiskers. The maximum WOF is not given only by the stress-induced transformation but also by the crack-face bridging of the whiskers. The WOF was optimized at a specific zirconia volume fraction of 0.7 in the matrix, which was essentially due to the maximized tensile internal stress on zirconia in the dual-phase matrix.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Zirconium diboride-mullite composite form mineral: Combustion synthesis,consolidation, characterizations and properties

Zirconium diboride-mullite composite powder was synthesized in-situ by combustion in argon of a zircon sand/B₂O₃/Al reactant system in a 3 : 3: 10 M ratio. Zircon sand with a particle size less than 45 μm was activated by high-energy milling for 360 min. The optimum reactant system included the addition of 0.01 mol of Si. The product of the synthesis of this system contained 34 wt% ZrB₂ and 50 wt% mullite. The obtained zirconium diboride-mullite powder was consolidated by hot pressing at 25 MPa in an argon environment, ramping at 10 °C/min to 1,450, 1550 and 1650 °C and holding for 60 min. The sintered composite hot-pressed at 1650 °C had a density of 3.39 g/cm³, flexural strength of 153.25 ± 1.19 MPa, hardness of 10.66 GPa and fracture toughness of 4.23 MPa.m^1/2. The flexural strength and hardness of the composite was significantly influenced by the grain size of the reinforced ZrB₂. The predominantly intergranular fracture observed in surface micrographs confirmed the high toughness of the composite. The coefficient of thermal expansion of the product hot-pressed at 1650 °C was 6.53 × 10⁻⁶/°C: much lower than reported coefficients of existing Al₂O₃, ZrO₂ ZrB₂, and ZrB₂–SiC refractory ceramics.     

SiC–AlN Particulate Composite

A SiC–AlN composite was fabricated by mechanical mixing of SiC and AlN powders, hot pressed under 40 MPa at 1950°C in Ar atmosphere. The object of this attempt was to achieve full density and a little solid solution formation. Fine microstructure and crack deflection behaviour are to improve the mechanical properties of the SiC–AlN composite. The bending strength and fracture toughness were achieved 800 MPa and 7·6 MPa m^1/2 at room temperature, respectively. The fracture toughness of the SiC–AlN composite shows minimal change between room temperature and 1400°C. Post-HIP improves the surface densification of the SiC–AlN composite resulting in an increase of the strength and the ability to resist oxidization. The bending strength of SiC–AlN composite increases from 800 to 1170 MPa after HIP treatment for 1 h under 187 MPa at 1700°C in N₂ atmosphere.     

High temperature mechanical properties of Al2O3/TiC micro–nano-composite ceramic tool materials

A type of Al₂O₃-based composite ceramic tool material simultaneously reinforced with micro-scale and nano-scale TiC particles was fabricated by the hot-pressing technology with different contents of cobalt additive. The effects of cobalt on the ambient temperature mechanical properties and high temperature flexural strength were investigated. The flexural strength and fracture toughness of the composite with 3 vol% cobalt as a function of temperature were investigated. Cobalt greatly enhanced the ambient temperature flexural strength and fracture toughness, while further increasing the content of cobalt led to a dramatic strength degradation, especially at high temperature. The flexural strength of the composite containing 3 vol% cobalt decreased as the temperature increased from 20 to 1200 °C, and the fracture toughness decreased as a function of the temperature up to 1000 °C but increased at 1200 °C. The degradation of high temperature flexural strength was ascribed to the change of the fracture mode, the grain and grain boundary oxidation, the decrease of elastic modulus and the grain boundary sliding.     

Characterization and analysis of high-entropy boride ceramics sintered at low temperature

Multicomponent transition metal boride composite–sintered bodies were prepared by spark plasma sintering, and the composite sintered bodies prepared at different sintering temperatures (1500–1900°C) were characterized. The experimental results showed that several other compounds diffused into the TiB_x phase at lower sintering temperatures under the combined effect of temperature and pressure due to the nonstoichiometric ratio of TiB_1.5 vacancies. When the temperature reached 1900°C, only the hexagonal phase remained. With the continuous increase of sintering temperature, the Vickers hardness and fracture toughness of the sintered bodies had a trend of increasing first and then decreasing, due to the continuous reduction of the porosity of the cross section of the sintered bodies and the growth of the grain size. The Vickers hardness and fracture toughness of sintered body obtained at 1800°C are the best, which are 24.4 ± 1.8 GPa and 5.9 ± 0.2 MPa m^1/2. At 1900°C, the sintered body was a single-phase hexagonal high-entropy diboride. Its Vickers hardness and fracture toughness were 21.9 ± 1.5 GPa and 5.4 ± 0.2 MPa m^1/2, respectively; it showed a clear downward trend.     

Fabrication and properties of three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite

In this study, a three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite with excellent properties was prepared by the precursor impregnation and pyrolysis (PIP) method. The influence of the number of PIP cycles on the microstructure and mechanical properties of the composite was studied. The density and mechanical properties of the composite enhanced with increasing number of PIP cycles. After the 6th PIP cycle, the density of the composite increased by only 0.8%, and the flexural strength and fracture toughness of the composite reached 130.1 MPa and 4.91 MPa m^1/2, respectively. Moreover, the thermal shock resistance of the composite was investigated by the high-low temperature (from 1300 °C to room temperature) thermal treatment and water quenching method. The strength retention rate of the composite after 10 high-low temperature thermal cycles was 90.1%; the retention rate dropped to 79.5% after 15 thermal cycles. The composite with the critical thermal shock temperature difference 843.9 °C displayed excellent thermal shock resistance.     

Synthesis and mechanical properties of mullite ceramics with coal gangue and wastes refractory as raw materials

With coal gangue and high alumina refractory solid wastes as raw materials, needle-like mullite powder, with an average diameter of about 1 μm, was synthesized at 1300°C by using the conventional solid-state reaction method. Mullite ceramics were derived from the inexpensive needle-like powder. Phase composition was examined by using X-ray diffraction (XRD), while morphologies of the ceramics were observed by using scanning electron microscopy. The content and distribution of elements in the sintered samples were characterized with energy dispersive spectrometer and X-ray fluorescence spectroscopy. Mechanical properties of the mullite ceramics were studied by using the three-point bending method. The aspect ratio of the needle-like mullite particles was up to 6. The mullite sample sintered at 1500°C for 3 hours had a density of 2.515 g·cm⁻³, which was slightly lower than the theoretical density. Maximum fracture toughness and bending strength of the mullite ceramics were 1.82 MPa·m^1/2 and 71.76 MPa, respectively. This study realizes the resource utilization of gangue and high alumina refractory solid wastes, and the prepared mullite ceramics have good application prospect.     

Effect of manganese addition on sintering behavior and mechanical properties of alumina toughened zirconia

The effect of MnO₂ additives on the sintering behavior and mechanical properties of alumina-toughened zirconia (ATZ, with 10 vol% alumina) composites was investigated by incorporating different amounts of MnO₂ (0, 0.5, 1.0, and 1.5 wt%) and sintering at various temperatures ranging from 1300 to 1450 °C. The addition of MnO₂ up to 1.0 wt% improved the sintered density, hardness, flexural strength, and fracture toughness of the composite. However, the addition of 1.5 wt% MnO₂ degraded the relative density, hardness, and flexural strength of the composite due to the transformation of the ZrO₂ phase from tetragonal to monoclinic and grain coarsening. Optimal results were obtained with 1.0 wt% MnO₂ and sintering at 1450 °C, which improved the mechanical properties (hardness: 13.5 GPa, flexural strength: 1.2 GPa, fracture toughness: 8.5 MPa m^1/2) and lowered the sintering temperature compared to the conventional sintering temperature of ATZ composites (1550 °C). Thus, the ATZ composite doped with MnO₂ is a promising material for structural engineering ceramics owing to its improved mechanical properties and lower sintering temperature.     

Effect of the volume ratio of zirconia and alumina on the mechanical properties of fibrous zirconia/alumina bi-phase composites prepared by co-extrusion

Fibrous zirconia/alumina composites with different composition were fabricated by piston co-extrusion. After a 3rd extrusion step and sintering at 1600 °C, crack-free composites with a fibre width of 50 μm were obtained for all compositions. The effect of the volume ratio of secondary phase on the mechanical properties was investigated. The Young's modulus of the composites decreased linearly with increasing the zirconia content. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPa m^1/2 was attained in the 3rd co-extruded 47/53 vol% zirconia/alumina composite. The improvement in toughness was attributed to both “stress-induced” transformation of zirconia and a crack deflection mechanism due to thermal expansion mismatch between the two phases. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.     

Temperature-dependent mechanical and oxidation behavior of in situ formed ZrN/ZrO2-containing Si3N4-based composite

In this work, Si₃N₄ and Zr(NO₃)₄ were used as raw materials to prepare ZrN/ZrO₂-containing Si₃N₄-based ceramic composite. The processing, phase composition, and microstructure of the composite were investigated. Hardness and fracture toughness of the ceramics were evaluated via Vickers indentation in Ar at 25°C, 300°C, 600°C, and 900°C. During spark plasma sintering, Zr(NO₃)₄ was transformed into tetragonal ZrO₂, which further reacted with Si₃N₄, resulting in the formation of ZrN. The introduction of ZrN enhanced the high-temperature mechanical properties of the composite, and its hardness and fracture toughness reached 13.4 GPa and 6.1 MPa·m^1/2 at 900°C, respectively. The oxidation experiment was carried out in air at 1000°C, 1300°C, and 1500°C for 5 h. It was shown that high-temperature oxidation promoted the formation and growth of porous oxide layers. The microstructure and phase composition of the formed oxide layers were investigated in detail. Finally, it was identified that the obtained composite exhibited a higher thermal diffusivity than that of monolithic Si₃N₄ in the temperature range of 100°C–1000°C.