High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi₂–Al₂O₃ composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ～2 μm thick Mo₅Si₃ diffusion layer, a ～65 μm thick MoSi₂ inner layer and a ～36 μm thick outer layer of mixture of MoSi₂ and Al₂O₃. After oxidation at 1600 °C for 5 h, all MoSi₂ phases were completely converted to intermediate silicide Mo₅Si₃ by solid-state diffusion, and the formed Mo₅Si₃ phase would be transformed into Mo₃Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO₂-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi₂–Al₂O₃ coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi₂–Al₂O₃ coating is due to the formation of multi-layer structure containing a SiO₂-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.     

Dielectric Properties of Ultra‐Low Sintering Temperature Al2O3–BBSZ Glass Composite

Ceramic composites of B₂O₃–Bi₂O₃–SiO₂–ZnO (BBSZ) glass mixed with Al₂O₃ (10–50 vol%) were sintered at 450°C, and their microstructural and dielectric properties investigated. Dense structures were obtained when the Al₂O₃ content was lower than 30 vol%. Raman, XRD, and FESEM showed the existence of a secondary phase, Bi₂₄Si₂O₄₀, in all samples. The dielectric properties of the composite with 30 vol% addition of Al₂O₃ showed good dielectric properties with ε_r of 14.8 and 20.8 and 32.5 at 100 kHz and 100 MHz and 1 GHz, respectively. The tanδ values at the same frequencies were 0.004 and 0.006 and 0.016. The results show that BBSZ glass with different amounts of Al₂O₃ exhibit widely applicable relative permittivity values and affordable loss and are thus promising candidates for ultra‐low sintering temperature applications.     

In situ reactive spark plasma sintering of WSi2/MoSi2 composites

MoSi₂ based materials have the potential for use in high temperature structural parts. In this work, WSi₂ reinforced MoSi₂ composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi₂ at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi₂, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi₂/MoSi₂ was as high as 16.47 GPa.     

Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi₂–Al₂O₃ and W₅Si₃–Al₂O₃ composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO₃+2Al and 0.6WO₃+0.6SiO₂+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al₂O₃. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi₂/Al₂O₃ from 0.8 to 4.0 were achieved for the production of the WSi₂–Al₂O₃ composites. Due to the lower formation exothermicity, the W₅Si₃–Al₂O₃ composites were produced with a narrower range of W₅Si₃/Al₂O₃ from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi₂ and unreacted W in the as-synthesized W₅Si₃–Al₂O₃ composites.     

Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

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.     

Improved Dielectric and Nonlinear Electrical Properties of Fine‐Grained CaCu3Ti4O12 Ceramics Prepared by a Glycine‐Nitrate Process

Pure CaCu₃Ti₄O₁₂ was successfully prepared by a glycine‐nitrate process using a relatively low calcination temperature and short reaction time of 760°C for 4 h. Fine‐grained CaCu₃Ti₄O₁₂ ceramics with dense microstructure and small grain size were obtained after sintering for 1 h. The nonlinear coefficient of a fine‐grained CaCu₃Ti₄O₁₂ ceramic calculated in the range 1–10 mA/cm² was found to be very high of ~16.39 with high breakdown electric field strength of 1.46 × 10⁴ V/cm. This fine‐grained CaCu₃Ti₄O₁₂ ceramic also exhibited a very low loss tangent of 0.017 at 20°C with temperature stability over the range ?55°C to 85°C. The grain growth rate of the CaCu₃Ti₄O₁₂ ceramics was found to be very fast after increasing the sintering time from 1.5 to 3 h, leading to formation of a coarse‐grained CaCu₃Ti₄O₁₂ ceramic with grain size of about 100–200 μm. The dielectric permittivity of this coarse‐grained ceramic was found to be as high as 1.03 × 10⁵ with a low loss tangent of 0.054.     

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.     

Synthesis of Nanofiber‐like Mesoporous γ‐Al2O3 toward Its Adsorption Efficiency for Congo Red

Nanofiber‐like mesoporous γ‐Al₂O₃ was synthesized using freshly prepared boehmite sol in the presence of triblock copolymer, P123 following evaporation‐induced self‐assembly (EISA) process followed by calcinations at 400°C–1000°C. The samples were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X‐ray diffraction (XRD), N₂ adsorption–desorption, and transmission electron microscopy (TEM). The adsorption efficiency of the samples with Congo red (CR) was studied by UV – vis spectroscopy. XRD results showed boehmite phase in the as‐prepared sample while γ‐Al₂O₃ phase obtained at 400°C was stable up to 900°C, a little transformation of θ‐Al₂O₃ resulted at 1000°C. The Brunauer‐Emmett‐Teller surface area of the 400°C‐treated sample was found to be 175.5 m²g ^{? 1}. The TEM micrograph showed nanofiber‐like morphology of γ‐Al₂O_3. The 400°C‐treated sample showed about 100% CR adsorption within 60 min.     

Strength improvement and purification of Yb2Si2O7‐SiC nanocomposites by surface oxidation treatment

A two‐step processing was developed to prepare Yb₂Si₂O₇‐SiC nanocomposites. Yb₂Si₂O₇‐Yb₂SiO₅‐SiC composites were first fabricated by a solid‐state reaction/hot‐pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb₂SiO₅ into Yb₂Si₂O₇. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail.     

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.     

Crack Healing in Ti2Al0.5Sn0.5C–Al2O3 Composites

Oxidation induced crack healing of Al₂O₃ composites loaded with a MAX phase based repair filler (Ti₂Al_0.5Sn_0.5C) was examined. The fracture strength of 20 vol% repair filler loaded composites containing artificial indent cracks recovered fully to the level of the virgin material upon isothermal annealing in air atmosphere after 48 h at 700°C and 0.5 h at 900°C. SEM‐EBSD analysis of crack microstructure indicates two different oxidation reaction regimes to govern the crack filling: near the surface SnO₂, TiO₂, and Al₂O₃ were formed whereas deeply inside the cracks Al₂O₃ and TiO₂ and metallic Sn were detected. The presence of elemental Sn was attributed to partial oxidation of aluminum and titanium which lowered the local oxygen concentration below a threshold value required for Sn oxidation to SnO₂. Thus, Ti₂Al_0.5Sn_0.5C may represent an efficient repair filler system to trigger oxidation induced crack healing in ceramic composites at temperatures below 1000°C.     

Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying

MoSi₂, MoSi₂–10 vol.% Al₂O₃, MoSi₂–30 vol.% Al₂O₃ (denoted as MA0, MA1, MA3, respectively) coatings were fabricated by vacuum plasma spraying (VPS), and their oxidation behavior was examined at low temperature (500 °C) and high temperature (1500 °C). The test at 500 °C showed that the addition of Al₂O₃ effectively restrained the pest oxidation of MoSi₂. The MA1 coating had satisfactory fluid surface and presented good oxidation resistance at 1500 °C. However, the MA3 coating showed worse oxidation resistant behavior compared with the MA0 coating because of mullite formation.     

Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties

Al₂O₃/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al₂O₃ and TiH₂ powders. During sintering, TiH₂ decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al₂O₃ matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al₂O₃. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm^1/2, which was 37% greater than that of monolithic Al₂O₃. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10^?1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al₂O₃ increased the fracture toughness and improved the conductivity of Al₂O₃.     

Perfect High‐Temperature Plasticity Realized in Multiwalled Carbon Nanotube‐Concentrated α‐Al2O3 Hybrid

We investigate the high‐temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α‐Al₂O₃ matrix hybrid, which has a strong room‐temperature interfacial shear resistance (ISR) and a unique MWCNT‐concentrated grain‐boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10^?4 s^?1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine‐grain monolithic Al₂O₃. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high‐temperature and shear/compressive forces, and strongly preserves the nanostructure of Al₂O₃ matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy‐absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C‐ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room‐temperature ISR), as evidenced by the formation of some submicrometer‐scale MWCNT aggregates in GB area, the equiaxed grains and dislocation‐free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy‐absorbing nanostructures.     

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.     

Improved mechanical properties of Al2O3 ceramic by in-suit generated Ti3SiC2 and TiC via hot pressing sintering

Al₂O₃ multi-phase composites with different volume fractions of SiC varying from 0 vol% to 30.0 vol% were fabricated by vacuum hot pressing sintering at 1600 °C under the pressure of 30 MPa for 2.0 h. The aim of this work was to investigate the effect of SiC content on the morphology and mechanical properties of the Al₂O₃ multi-phase composite. The results show that the addition of SiC and Ti can produce new strengthening and reinforcing phases include Ti₃SiC₂, TiC, Ti₅Si₃, which would hamper the migration of grain boundaries and promote sintering. The mechanical performances could reach the comprehensive optimal values for 20.0 vol% SiC, delamination and transgranular fracture being the major crack propagation energy dissipation mechanisms.     

Comparison of Two‐Phase Thermal Conductivity Models with Experiments on Dilute Ceramic Composites

Thermal shock resistance of cubic 8 mol% yttria‐stabilized zirconia (YSZ) can be increased by the addition of dilute second phases. This study addresses how these dilute second phases affect the thermal conductivity for two‐phase ceramic composites of 8 mol% YSZ with 10–20 vol% alumina (Al₂O₃) or 10–20 vol% mullite (3Al₂O₃·2SiO₂). Thermal conductivity measurements from 310 K (37°C) to 475 K (202°C) were made using the 3ω method and compared with results from 3D analytical models and a 2D computational microstructure‐based model (Object‐Oriented Finite Element Analysis, OOF2). The linear Rule of Mixtures was the least accurate and significantly overestimated the measured thermal conductivity at low temperatures, with errors in some cases exceeding 100%. Calculations using the Bruggeman and OOF2 models were both much better, and the deviation of less than ±2.5% across all compositions and temperatures is within the range of experimental and modeling uncertainty. The Maxwell Garnett equation was a close third in accuracy (±8%). A sensitivity analysis for each model quantifies how small perturbations in the thermal conductivity of the dispersed second phase influence the effective thermal conductivity of the composite, and reveals that the linear Rule of Mixtures model is physically unrealistic and oversensitive to the thermal conductivity of the dispersed phase.     

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.