Cyclic thermal shock resistance of h-BN composite ceramics with La2O3–Al2O3–SiO2 addition

The effects of La₂O₃–Al₂O₃–SiO₂ addition on the thermal conductivity, coefficient of thermal expansion (CTE), Young's modulus and cyclic thermal shock resistance of hot-pressed h-BN composite ceramics were investigated. The samples were heated to 1000 °C and then quenched to room temperature with 1–50 cycles, and the residual flexural strength was used to evaluate cyclic thermal shock resistance. h-BN composite ceramics containing 10 vol% La₂O₃–Al₂O₃ and 20 vol% SiO₂ addition exhibited the highest flexural strength, thermal conductivity and relatively low CTE, which were beneficial to the excellent thermal shock resistance. In addition, the viscous amorphous phase of ternary La₂O₃–Al₂O₃–SiO₂ system could accommodate and relax thermal stress contributing to the high thermal shock resistance. Therefore, the residual flexural strength still maintained the value of 234.3 MPa (86.9% of initial strength) after 50 cycles of thermal shock.     

Thermal shock behavior of co-continuous TiCx-Cu cermets in air and anaerobic environment

The present work investigated the microstructure and mechanical properties of TiC_x-Cu cermets before and after thermal shock tests. Thermal shock temperature was from 800 °C to 1200 °C and number of cycles was from 1 to 20. The results indicated that TiC_x-Cu cermets with co-continuous structure exhibited a stable and excellent thermal shock resistance. When quenched at 1000 °C for 1 cycle, the residual flexural strength of the cermet increased to 882 MPa, which was 10.1% higher than that without thermal shock. After 20 cycles of thermal shock, the residual flexural strength still maintain 760 MPa. When quenched at 800 °C and 1200 °C, the strengths of cermet decreased correspondingly, which were caused by the thermal mismatch between metal and ceramics and effusion of Cu or collapses of oxide layer, respectively. Herein, the recrystallization and refinement of metal phase grains caused during the thermal shock process, resulting in the superior thermal shock performance of cermet.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

Joining of Ti–Al–C ceramics by oxidation at low oxygen partial pressure

The joining of titanium aluminum carbides has been successfully performed at high temperature and low oxygen partial pressure. The mechanism of the bonding is attributed to the preferential oxidation of Al atoms in the titanium aluminum carbides at low oxygen partial pressure, which leads to the formation of an Al₂O₃ layer through the joint interface. The specimens joined at 1400 °C exhibit a high flexural strength of 315 ± 19.1 MPa for Ti₂AlC and 332 ± 2.83 MPa for Ti₃AlC₂, which is about 95% and 88% of the substrates, respectively, and the high flexural strength can be retained up to 1000 °C. The high mechanical performance of the joints is attributed to the similar density and thermal expansion coefficient values of Al₂O₃ to those of the Ti₂AlC and Ti₃AlC₂ substrates. It indicates that bonding via preferential oxidation at low oxygen partial pressure is a practical and efficient method for Ti₂AlC and Ti₃AlC₂.     

Spark plasma sintering of TiB2-based ceramics with Ti3AlC2

A TiB₂–Ti₃AlC₂ ceramic was manufactured by spark plasma sintering at 1900 °C temperature for 7 min soaking time under 30 MPa biaxial pressure. The role of Ti₃AlC₂ additive on the microstructure development, densification behavior, phase evolution, and hardness of the ceramic composite were studied. The phase characterization and microstructural investigations unveiled that the Ti₃AlC₂ MAX phase decomposes at the initial stages of the sintering. The in-situ formed phases, induced by the decomposition of Ti₃AlC₂ additive, were identified and scrutinized by XRD and FESEM/EDS techniques as well as thermodynamics principles. The sintered TiB₂–Ti₃AlC₂ ceramic approached a near full density of ~99% and a hardness of ~28 GPa. The densification mechanism and sintering phenomena were discussed and graphically illustrated.     

Anisotropic microstructures and mechanical properties of textured Ti3AlC2/TiAl3/Al composite

In the present work, a textured Ti₃AlC₂/TiAl₃/Al composite with anisotropic microstructures and properties was successfully prepared by combining multistep ball-milling, flaky powder self-assembly, spark plasma sintering (SPS), and in-situ reaction. The effects of phase constitution and crystallographic orientation on anisotropic strengthening and fracture mechanisms of the composite were discussed. The results show that the preferred orientation of Ti₃AlC₂ flakes was achieved during densification with the Lotgering orientation factor of the textured top surface (TTS) of 0.58. The high surface energy of Ti₃AlC₂ submicron flakes provided the driving force for the low-temperature in-situ formation of TiAl₃. The in-situ formed TiAl₃ further improved the strength of the composite. The//c-axis samples achieved a high flexural strength of 565.9 MPa. The analysis of the flexural fracture surfaces shows that the fracture modes of Ti₃AlC₂ flakes include basal plane cleavage, delamination, kinking, particle pull-out, and prismatic plane fracture, which in turn affects the toughness of the samples with different loading directions. Particle pull-out of the Ti₃AlC₂ flakes is the primary mechanism to improve the toughness. This novel technical route provides a new idea for the design of metal matrix composites.     

Processing and thermal properties of SrTiO3 /Ti3AlC2 ceramic nanocomposites

Modulating the thermal conductivity has been a pragmatic approach for the development of high-performance thermoelectric material and thereby a step forward towards commercialization. Despite some efforts, the reduction in thermal conductivity of SrTiO₃ ceramic has not been fully realized. In this work, Ti₃AlC₂ in 3, and 7 vol% were uniformly incorporated in SrTiO₃ through nanostructured powder processing. The pristine SrTiO₃ and composites powders were consolidated by the spark plasma sintering at 1200 °C under uniaxial pressure of 50 MPa. Thermal properties of the bulk samples were evaluated from room temperature to 750 K through laser flash analysis. The thermal conductivity of SrTiO₃ based composites decreases substantially with the addition of nanostructured Ti₃AlC₂ from the pristine SrTiO₃ bulk sample. The reduction in thermal conductivity of 7 vol% composites is more than 30% at room temperature and even higher at elevated temperatures from the SrTiO₃. The interface thermal resistance was estimated which indicates a dominant role in diminishing the thermal conductivities of the composites. The results suggest that the addition of Ti₃AlC₂ as a second phase and nanostructuring through ball milling has significantly altered the phonon scattering mechanisms through multiple factors and thereby contributed to reducing effective thermal conductivities of the composites. This, work provide a scalable and economical route for the development of high-performance thermoelectric material.     

Effect of TiO2 on sinterability and physical properties of pressureless sintered Ti3AlC2 ceramics

TiO₂ was selected as effective sintering aid for pressureless sintering of Ti₃AlC₂ ceramics in this study. The addition of only 5?wt% TiO₂ largely promotes the densification and nearly dense Ti₃AlC₂ ceramic was obtained by pressureless sintering at 1500?°C. Significant strengthening and toughening effects were observed with the addition of TiO₂. High Vickers hardness, flexural strength and fracture toughness of 3.22?GPa, 298?MPa and 6.2?MPa?m^?1/2, respectively, were achieved in specimen pressureless sintered with 10?wt% TiO₂. Additionally, the addition of 5?wt% TiO₂ had no deleterious effect on the excellent oxidation resistance of Ti₃AlC₂ ceramic under 1200?°C water vapor atmosphere, while addition of 10?wt% TiO₂ accelerates the oxidation rate by two orders of degree.     

Thermal shock behavior of Ti2AlC from 200°C to 1400°C

The thermal shock behavior of Ti₂AlC synthesized by means of self‐propagating high‐temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti₂AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al₂O₃. Although the flexural strength decreases quickly in the temperature range of 300°C‐500°C, no discontinuous decrease in the retained strength is observed in Ti₂AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti₂AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared with the first quench. Quenching at 300°C and above, voids after the pullout of grains and cracks are present, which however are absent in the un‐quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock.     

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.     

Effect of MgO on thermal shock resistance of CaZrO3 ceramic

In order to improve the thermal shock resistance of CaZrO_3, CaZrO₃ was synthesized by a solid-state reaction method with analytical ZrO₂ and CaCO₃ as raw materials, and MgO as additive. The effects of MgO on Flexural strength at room temperature, thermal shock resistance, XRD and microstructure of CaZrO₃ were characterized. The results show that the grain growth of CaZrO₃ is inhibited and the thermal shock resistance of CaZrO₃ is improved by adding MgO. With the increasing of MgO, the flexural strength at room temperature of samples are improved due to the grain refinement. When the addition of MgO is 8%, the flexural strength at room temperature increases to 270.15?MPa. The thermal shock resistance of samples are improved by MgO deflecting and bridging cracks. When the addition of MgO is 4%, the residual flexural strength of samples is the maximum (26.94?MPa).     

Thermal shock damage assessment of a BNW/SiO2 ceramic: Insight from temperature-dependent material properties

The high-temperature service performance of nearly fully dense 20 wt% BN_W/SiO₂ ceramic was systematically investigated. The oxidation damage and strength degradation of the whiskers combined with the surface microstructures of the samples predominantly influence the flexural strength from RT to 1000 °C. In previous work, the temperature dependence of the material properties is invariably ignored when evaluating thermal stress crack initiation and propagation behaviour. In this work, modified thermal shock models that include temperature-dependent material properties were established based on thermal-shock fracture (TSF) theory and thermal-shock damage (TSD) theory. Then, the thermal shock resistance (TSR) of the BN_W/SiO₂ ceramic was evaluated by preforming a water quenching test. The modified models could better explain the TSR behaviour of the ceramic, indicating that considering the temperature-dependent material properties will reveal the thermal shock damage mechanism more precisely.     

Joining of sintered silicon carbide using ternary Ag–Cu–Ti active brazing alloy

Sintered silicon carbide was brazed to itself by Ag–35.25 wt%Cu–1.75 wt%Ti filler alloy at 860 °C, 900 °C and 940 °C for 10 min, 30 min and 60 min. Mechanical properties both at room temperature and high temperature were measured by flexural strength. The interfacial microstructure was investigated by electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results indicate that increased brazing temperature heightens the flexural strength and the maximal four-point flexural strength reaches 342 MPa at room temperature. In addition longer holding times result in thicker reaction layer, which increases mismatch of coefficients of thermal expansion (CTE) between SiC substrate and reaction layer and finally leads to poor mechanical properties due to high residual stresses. High temperature flexural strength decreases with an increase of test temperature due to softening of the filler alloy. A reaction layer composed of TiC and Ti₅Si₃ was observed at the interface of SiC/filler alloy and there is a representative microstructure: SiC/continuous fine TiC layer/discontinuous coarse Ti₅Si₃ layer/filler alloy.     

High-performance Al2O3 ceramics prepared by DCC-HVCI via microwave heating

A novel and rapid fabrication method for Al₂O₃ ceramics by the DCC-HVCI method via microwave heating was proposed. Effects of microwave heating temperature on coagulation time, micromorphology, as well as performance of the green body and ceramic sample were studied. As the microwave heating temperature rises, the coagulation time gradually reduced and compressive strength of green sample decreased while relative density and flexural strength of ceramics rose at the beginning and then dropped. The 50 vol.% Al₂O₃ suspension was coagulated and demolded after treating at 60°C for 800 s by microwave heating. The compressive strength of green samples reached 1.12 ± 0.13 MPa. The relative density of Al₂O₃ ceramic samples reached 99.39%. And the flexural strength of Al₂O₃ ceramics reached 334.55 ± 26.41 MPa. The Weibull modulus of Al₂O₃ ceramics reached 19. In contrast with the ceramic samples heated through water bath, the ceramic samples treated through microwave possessed uniform microstructures. Microwave heating could reduce the coagulation time by 77%. Meanwhile, it could significantly raise the compressive strength of green bodies by 65%. Additionally, it could increase the flexural strength of ceramics by 30%.     

Effect of Al2O3 Addition on the Flexural Strength and Light‐Transmission Properties of Bone China

To improve the flexural strength and light‐transmission properties of bone china, the effects of adding different amounts of alumina (0–3%) to bone china bodies were studied and the phase composition and microstructure of different samples were studied by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, physical properties, such as the bulk density, the thermal expansion coefficient (TEC) and thermal shock resistance, were studied. It was found that adding alumina increased the overall sintering temperature while reducing the sintering temperature range of bone china. Furthermore, addition of 1% Al₂O₃ improved the tree‐point flexural strength from 120 MPa to 150 MPa, the light transmittance (at 2 mm thickness) from 6.7% to 7.5%, the thermal expansion coefficient from 8.4 × 10^?6°C^?1 to 8.1 × 10^?6°C^?1 and the thermal shock resistance from 140°C to 180°C. Higher corundum content results in similar high flexural strength but lower light transmittance.     

Repeated thermal shock behavior of ZrB2-SiC-graphite composite under prestress

The thermal shock resistance of ZrB₂-SiC-graphite composite under nominal prestress of 0, 20, 30, 40 or 50 MPa after subjected to 10 and 30 cycles of thermal shock was evaluated by measuring the residual flexural strength of the tested specimen. In each test the applied prestress kept constant and in each cycle the specimen center was heated to 2000 °C within 5 s through electrical resistance heating method and cooled down naturally to room temperature. A lot of broken SiO₂ bubbles in the tested specimens were observed with a SEM. For the specimen subjected to 10 cycles of thermal shock, the residual flexural strength does not show big change under different levels of prestress, although the thickness of oxide layer increases at larger prestress, which is presumably attributed to the effect of the oxide layer that heals the cracks and the pores and enhances the strength. For the specimen subjected to 30 cycles of thermal shock, the residual strength decreases, in general, with the increase of prestress level. The thermal shock fatigue under different levels of prestress was also tested, and it was found that the increase of prestress may speed the failure of the specimen, indicating that the level of prestress may fatally affect the failure of the material.     

Improved mechanical properties and toughening mechanism of mullite ceramics reinforced by introducing Ti3AlC2 particles

Ti₃AlC₂, as a toughening phase, was introduced into mullite ceramics for the first time by the pressureless sintering process aiming at improving the mechanical properties. Significant enhancement in density and mechanical performance of mullite ceramics was achieved through the introduction of Ti₃AlC₂ particles. The density of as-prepared mullite–Ti₃AlC₂ composites was increased by 23% (from 2.86 g/cm³ to 3.51 g/cm³) with Ti₃AlC₂ increasing from 0 wt% to 20 wt%. The formation of the liquid phase and decomposed particles from Ti₃AlC₂ are supposed to be responsible for the densification of mullite–Ti₃AlC₂ composites. The optimal mechanical properties were obtained in the mullite–Ti₃AlC₂ composites with 15 wt% Ti₃AlC₂. The bending strength, fracture toughness as well as Vickers hardness were reached 214.36 MPa, 4.84 MPa·m^1/2, and 9.21 GPa, which are 40%, 74%, and 113% higher than pure mullite ceramics, respectively. The improved mechanical performance was mainly attributed to the synergetic action of crack deflection, crack branching and bridging, and strengthened grain boundary.     

Residual thermal stress of SiC/Ti3SiC2/SiC joints calculation and relaxed by postannealing

Residual thermal stresses in SiC/Ti₃SiC₂/SiC joining couples were calculated by Raman spectra and simulated by finite element analysis, and then relaxed successfully by postannealing. The results showed that the thermal residual stress between Ti₃SiC₂ and SiC was about on the order of 1 GPa when cooling from 1300°C to 25°C. The thermal residual stresses can be relaxed by the recovery of structure disorders during postannealing. When the SiC/Ti₃SiC₂/SiC joints postannealed at 900°C, the bending strength reached 156.9 ± 13.5 MPa, which was almost twice of the as‐obtained SiC/Ti₃SiC₂/SiC joints. Furthermore, the failure occurred at the SiC matrix suggested that both the flexural strength of joining layer and interface were higher than the SiC matrix.     

Mechanical properties and pre-oxidation behavior of spark plasma sintered B4C ceramics using (Ti3SiC2+CeO2/La2O3) as sintering aid

B₄C ceramic with the addition of 5 wt % (Ti₃SiC₂+ CeO₂/La₂O₃) as sintering aids was fabricated by spark plasma sintering at a relatively low temperature of 1650 °C for 5 min at 80 MPa. The phase composition, microstructures, and comprehensive mechanical properties of the ceramics were studied in detail. The existence of reinforced second phase particles, the refinement of the matrix grains, the formation of residual stress along the grain boundaries and the appearance of the mixed fracture mode had a synergetic strengthening effect on the mechanical properties. The flexural strength, fracture toughness and Vickers hardness of B₄C ceramics reached 565.2 ± 21.8/551.0 ± 25.2 MPa, 6.28 ± 0.01/6.41 ± 0.12 MPa·m^0.5, and 28.51 ± 0.86/27.23 ± 1.08 GPa, respectively. In addition, to reduce the crack sensitivity of the ceramic, the ceramics were pre-oxidized at 800 °C for different durations. The flexural strength was increased by approximately 13.4% after the ceramic was oxidized at 800 °C for 45 min due to the crack-healing effect induced by the oxide glass B₂O₃ on the ceramic surface.     

Preparation of Ti3AlC2 bulk ceramic via aqueous gelcasting followed by Al-rich pressureless sintering

Ti₃AlC₂ bulk ceramics were prepared via aqueous gelcasting followed by C-rich and Al-rich pressureless sintering. The optimized pH value, zeta potential, dispersant content, and solid loading content were determined to be 10, 72.6 mV, 1.6 wt%, and 52 vol%, respectively. Impurities at ppm level containing in the flowing argon could cause severe decomposition of gelcasted bulk Ti₃AlC₂, forming whiskers of Al₂OC and Al₄O₄C and floccule of AlN. C-rich pressureless sintering resulted in the delamination of a duplex layer of Ti(CO) and Ti₃(AlO)C_x-Ti(O,C). The channels formed after debinding facilitated the outward diffusion of Al and the inward diffusion of O and C, and thereby promoting the decomposition of C-rich sintered Ti₃AlC₂. The combined effect of the unclosed channels and the porous reaction Ti₃(AlO)C_x-Ti(O,C) layer brought a catastrophic reduction in the density and mechanical properties of the C-rich sintered Ti₃AlC₂ ceramic. While the Al-rich pressureless sintering system isolated C, CO and N₂ and supplied a closed Al-rich atmosphere, thereby suppressing the decomposition reactions and promoting the sintering densification and ultimately leading to the superior in mechanical properties. The density, hardness, flexural strength and fracture toughness of the Al-rich sintered ceramic reaches 4.13 g/cm³, 4.36 GPa, 345 MPa, 4.79 MPa m^1/2, respectively.