cBN-Al2O3 composites in NaCl environment under high pressure and high temperature conditions

A series of experiments of cubic boron nitride (cBN)-Al₂O₃ composites was conducted in NaCl environment under a pressure of 5 GPa at 1200–1650 °C using a Chinese multi-anvil high-pressure apparatus. The oxidation resistance of cBN-Al₂O₃ composites reached 1300 °C, which was 200 °C higher than that of raw cBN powder. The porosity was estimated by the content of NaCl impurities in cBN-Al₂O₃ composites. The content of NaCl impurity increases with increasing temperature and decreases with increasing Al₂O₃ level under high pressure and high temperature conditions. cBN+30 vol% Al₂O₃ sintered at 5 GPa and 1200 °C shows no NaCl impurity, and the Vickers hardness of the sample is 21.6 GPa which is half of cBN+10 vol% Al.     

Tailored pore structures and mechanical properties of porous alumina ceramics prepared with corn cob pore-forming agent

In this work, the effects of porosity and different particle sizes of pore-forming agent on the mechanical properties of porous alumina ceramics have been reported. Different grades of porous alumina ceramics were developed using corn cob (CC) of different weight contents (5, 10, 15, and 20 wt%) and particle sizes (<63 µm, 63-125 µm and 125-250 µm) as the pore-forming agent. Experimental results showed that total porosity and pore cavity size of the porous alumina ceramics increased with rising addition of CC pore former. Total porosity increased with increasing particle size of CC with the Al₂O₃-^<63CC₅ sample exhibiting the lowest total porosity of 41.3 vol% while the highest total porosity of 68.1 vol% was exhibited by the Al₂O₃-^125-250CC₂₀. The particle size effect of CC on the mechanical properties revealed that diametral tensile strength and hardness of the porous alumina ceramics deteriorated with increasing particle size of CC pore former. The Al₂O₃-^<63CC₅ sample exhibited the highest diametral tensile strength and hardness of 25.1 MPa and 768.2 HV, respectively, while Al₂O₃-^125-250CC₂₀ exhibited the lowest values of 1.1 MPa and 35.9 HV. Overall, porous alumina ceramics with the smallest pore sizes under each particle size category exhibited superior mechanical properties in their respective categories.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Pressureless Sintering of Al2O3/Ni Nanocomposites

In the present study, an Al₂O₃/Ni nanocomposite containing 5 vol% Ni is prepared by pressureless sintering at 1400°C for 2 h. Most nickel inclusions, around 70% in the sintered nanocomposite, locate at the intergranular sites, the triple junctions and Al₂O₃/Al₂O₃ grain boundaries. The average size of the nickel inclusions at the triple junctions, grain boundaries and intragranular locations is 145, 131 and 73 nm, respectively. The average size of all nickel inclusions is 118 nm. The presence of nickel inclusions can prohibit the grain growth of matrix grains. The size of Al₂O₃ grains in the sintered nanocomposite is only 490 nm. The strength of the nanocomposite is thus high for the refined microstructure. The matrix Al₂O₃ grains and Ni inclusions at triple junctions underwent considerable coarsening during a post-annealing treatment at 1300°C for 2 h. The strength of the annealed composites is thus reduced significantly after annealing.     

Microstructure and mechanical properties of Al2O3–Ti(C,N)–cBN composites prepared by a novel hot-forging process

Al₂O₃–cBN has received considerable attention in the field of ceramic cutting tools due to its high hardness, high wear resistance, and low cost, but poor interfacial bonding affects the performance of the composite. In this study, a novel hot-forging process was used to prepare high-performance Al₂O₃–cBN composites using Ti(C,N) as a binder. The evolution of the morphology, phase, and microstructure of the hot-forged Al₂O₃–Ti(C,N)–cBN composites was determined, and the mechanical properties were measured. The relative density of the composites increases significantly after hot forging, and the deformation of the composites increases with the hot-forging temperature. The highest performing Al₂O₃–Ti(C,N)–cBN composite was prepared by hot forging at 1600°C and has a hardness of 20 GPa, a bending strength of 647 MPa and a fracture toughness of 5.37 MPa m^1/2, which are superior to those of a directly hot-pressed sintered composite. However, at hot-forging temperatures higher than 1700°C, Al₅O₆N and TiB₂ are formed in the composite. In the composite hot forged at 1800°C, serrated grain boundaries promote the strength and toughness of the composite to 877 MPa and 6.76 MPa m^1/2, respectively. Therefore, the novel hot-forging process is expected to enhance material properties.     

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.     

Preparation and properties of porous mullite-based ceramics fabricated by solid state reaction

In this study, the effect of the alumina particle size on the formation of mullite using a silica gel powder and micro- and nano-scale Al₂O₃ powders as raw materials was investigated. The optimized Al₂O₃ source was then reacted with the silica gel to prepare porous mullite-based ceramics. The results revealed that the highly reactive nano-Al₂O₃ powder could form mullite at a relatively low firing temperature. Therefore, the nano-Al₂O₃ powder was used to prepare porous mullite-based ceramics by firing at 1600 °C, 1650 °C and 1700 °C. The pore size of the prepared porous mullite-based ceramics ranges from tens to hundreds of micrometres, with the apparent porosity being 42.8–58.0%. Further, the mullite content in the samples increased with increasing firing temperature, and a higher firing temperature promoted sintering, resulting in improved strength of the sample. After calcination at 1700 °C, the mullite content in the sample reached 81.8%, and the sample showed excellent thermal shock resistance. The strengths of the samples before and after thermal shock were found to be 23.6 and 15.58 MPa, with the residual strength ratio being 66%.     

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.     

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi₂ and commercial powders of Al₂O₃ and spinel (MgAl₂O₄). Careful oxidation of pulverised waste MoSi₂ rendered a precursor mixture of MoO₃ and amorphous SiO₂, which served as pore forming agent and SiO₂ source, respectively. Evaporation of MoO₃ at ～750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10^?6 K^?1) and elastic modulus (2.27 GPa).     

A high strength alumina-silicon carbide-boron carbide triplex ceramic

A ceramic particulate composite composed of oxide, and carbide ceramics was found to have high strength, hardness, and fracture toughness values. A composition consisting of Al₂O₃ with 15 vol% SiC and 15 vol% B₄C additions was produced by hot-pressing at 1650 °C for 30 min, with full density reached after ~5 min at temperature. Both WB and WB₂ were observed, with the W source presumably an impurity from WC milling media, and Al₁₈B₄O₃₃ was also detected following densification. Strength was ~880 MPa, which is greater than values reported for comparable composites of Al₂O₃ containing 30 vol% SiC or B₄C. Vickers hardness was ~21 GPa, and fracture toughness was ~4.5 MPa m^½, comparable to values reported for the binary mixtures. The calculated critical flaw size of the material was similar to the size of the SiC/B₄C clusters and microcracking at grain boundaries. The latter resulting from thermal expansion mismatch between the Al₂O₃ matrix and SiC/B₄C reinforcing phases.     

Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor

A CaO‐B₂O₃‐SiO₂ (CBS) glass/40 wt% Al₂O₃ composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al₂O₃ and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al₂O₃ composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.     

Fabrication and characterization of alumina and zirconia-toughened alumina porous structures

In the present study, alumina (Al₂O₃) and zirconia-toughened alumina (ZTA) porous structures (foams) were manufactured using the space holder technique. Al₂O₃ and ZTA foams with varying porosities from 20% to 69% were fabricated by adding different sizes (10, 20, and 40 μm) and different volume % of polystyrene beads (space holders) to Al₂O₃ and ZTA powders. All the fabricated foams were investigated under static conditions to assess the compressive behavior. It is observed that the compressive strength of these foams strongly depends on porosity, pore size, pore size distribution and pore wall thickness. Among all fabricated foams, Al₂O₃ foams with 20 vol% beads of 10 μm size showed a higher compressive strength of 700 MPa with low porosity (21%) and a higher pore wall thickness (2.8 μm). It is also observed that the pore wall thickness decreased with the increase in beads size and the volume % of the beads, resulting in a low compressive strength value of 8 MPa with a lower pore wall thickness of 1.75 μm at 80 vol% of 40 μm beads. All the foams, irrespective of pore size, showed a typical ceramic failure phenomenon up to 70 vol% of beads; after that, the failure behavior changed to complete open-cell fracture.     

Effect of YAG content on creep resistance and mechanical properties of Al2O3-YAG composite

Comprehensive study on effect of YAG amount on densification, creep resistance and room-temperature mechanical properties of Al₂O₃-YAG composite pressureless sintered at 1600 °C was conducted. The main goal was to optimize the amount of YAG in order to fabricate a composite with improved creep resistance and sufficiently good room-temperature mechanical properties. The composite was made by mixing a commercially available Al₂O₃ powder with fine YAG powder obtained by glycine-nitrate combustion synthesis starting from aluminum nitrate and yttrium nitrate. Increased driving force for sintering of fine YAG powder allowed fabrication of dense Al₂O₃-YAG composite with up to 30 vol% YAG. The presence of YAG was found to be very effective in improving creep resistance of Al₂O₃-YAG composite. Large Y³⁺ ions blocked diffusion along Al₂O₃ grain boundaries, reduced diffusivity and therefore enhanced creep resistance of Al₂O₃-YAG composite which continuously increased as the YAG amount increased. Тhe presence of YAG was also found to improve mechanical properties such as hardness and elastic modulus. The improvement of these properties was ascribed to increased density of Al₂O₃-YAG composites owing to high sintering activity of YAG powder. While fracture strength of the composite can be as high as that of monolithic Al₂O₃, fracture toughness of composite decreased continuously as the YAG content increased. The decrease was ascribed to transgranular fracture of both YAG and Al₂O₃ grains in samples containing larger amounts of YAG. The proper balance between fracture toughness and creep resistance was found in composite containing 18 vol% YAG which had considerably improved creep resistance accompanied by a relatively small decrease in fracture toughness.     

Mechanical Properties and Microstructure of Nano-SiC–Al2O3 Composites Densified by Spark Plasma Sintering

Heterogeneous precipitation method has been used to produce 5 vol% SiC–Al₂O₃ powder, from aqueous suspension of nano-SiC, aqueous solution of aluminium chloride and ammonia. The resulting gel was calcined at 700°C. Nano-SiC–Al₂O₃ composites were densified using spark plasma sintering (SPS) process by heating to a sintering temperature at 1350, 1400, 1450, 1500 and 1550°C, at a heating rate of 600 °/min, with no holding time, and then fast cooling to 600°C within 2–3 min. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 vol% SiC–Al₂O₃ densified by SPS at 1450°C reached as high as 1000 MPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al₂O₃ grains and the fracture mode of the nanocomposites was mainly transgranular fracture.     

Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The recently developed technique of cold sintering process (CSP) enables densification of ceramics at low temperatures, i.e., <300°C. CSP employs a transient aqueous solvent to enable liquid phase‐assisted densification through mediating the dissolution‐precipitation process under a uniaxial applied pressure. Using CSP in this study, 80% dense Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were obtained at 120°C in 20 minutes. After a 5 minute belt furnace treatment at 650°C, 50°C above the crystallization onset, Li‐ion conductivity was 5.4 × 10^?5 S/cm at 25°C. Another route to high ionic conductivities ~10^?4 S/cm at 25°C is through a composite LAGP ‐ (PVDF‐HFP) co‐sintered system that was soaked in a liquid electrolyte. After soaking 95, 90, 80, 70, and 60 vol% LAGP in 1 M LiPF₆ EC‐DMC (50:50 vol%) at 25°C, Li‐ion conductivities were 1.0 × 10^?4 S/cm at 25°C with 5 to 10 wt% liquid electrolyte. This paper focuses on the microstructural development and impedance contributions within solid electrolytes processed by (i) Crystallization of bulk glasses, (ii) CSP of ceramics, and (iii) CSP of ceramic‐polymer composites. CSP may offer a new route to enable multilayer battery technology by avoiding the detrimental effects of high temperature heat treatments.     

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.     

Porous ceramics based on high-thermal-stability Al2O3–ZrO2 nanofibers for thermal insulation and sound absorption applications

Al₂O₃ fibers are promising candidates for porous ceramics, but the sudden growth of grains in the fibers above 1200 °C will limit their applications for high temperature. Herein, we reported the successful fabrication of the Al₂O₃–ZrO₂ nanofibers by electrospinning and the nanofiber-based porous ceramics by a combination of gel-casting, freeze-drying and high-temperature sintering. Results show that the addition of Zr could greatly improve the thermal stability (up to 1400 °C) of the Al₂O₃-based nanofibers, owing to the inhibition of the sudden growth of the grains in the fibers at high temperature. The Al₂O₃–ZrO₂ nanofiber-based porous ceramics after sintering at 1100–1400 °C possessed a multi-level pore structure and exhibited high thermal stability, ultra-high porosity (97.79–98.04%), ultra-low density (0.075–0.091 g/cm³) and thermal conductivity (0.0474–0.0554 W/mK), and excellent sound absorption performance with the average sound absorption coefficient of 0.598–0.770. These porous ceramics are expected to be employed in the fields of high-temperature thermal insulation and sound absorption.     

Stability and electrical properties of MoSi2‐ and WSi2‐oxide electroconductive composites

MoSi₂‐ and WSi₂‐based electroconductive ceramic composites were fabricated using 40‐80 vol% fine‐ and coarse‐Al₂O₃, and ZrO₂ particles (refractory oxides) after sintering in argon. Their chemical and thermal stability was tested between 1400°C‐1600°C for up to 48 hours. X‐ray diffraction analysis showed the formation of secondary 5‐3 metal silicide (Mo₅Si₃, W₅Si₃) and silica phases on the grain boundaries and surface. The fraction of the W₅Si₃ (11.4‐38.8 vol%) was significantly higher than that of the Mo₅Si₃ (3.3‐7.3 vol%) in the composites after annealing at 1400°C for 48 hours. The rates of grain growth in the composites (0.013‐0.023 μm/h) were highly decreased by a grain‐boundary pinning effect. This effect was relatively better with the addition of the coarse‐grained oxides due to their more homogeneous distribution throughout the microstructure. The 20–80 vol% MoSi₂‐Al₂O₃ (fine‐grained) composite exhibited an electrical conductivity of 8.8 S/cm at 900°C. At the 60 vol% silicide content, MoSi₂–Al₂O₃ (coarse‐grained) and WSi₂–Al₂O₃ (fine‐grained) showed higher electrical conductivity (126‐128 S/cm) at 900°C. The density, porosity level, particle distribution, intrinsic conductivity of silicide phase, particle size, and fraction of the secondary 5‐3 silicide phase highly influenced their electrical properties.     

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.