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Open Access Research Article Just Accepted
Sintering-free fabrication of dense SiBCN monolith at a lower temperature and its high-temperature performance
Journal of Advanced Ceramics
Available online: 12 June 2024
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In this study, a crack-free pyrolysis process of partially cured precursor powder compacts was developed to prepare dense SiBCN monoliths at much lower temperatures (1300℃), thereby circumventing the challenges of sintering densification (>1800℃). Unlike the elastic fracture in over-cured precursors or the viscoelastic deformation in under-cured ones, the partially cured precursor, exhibiting elastic-plastic deformation behavior, facilitates limited nanoscale pore formation in a dense structure, achieving a balance between crack-free pyrolysis and densification. Compared to SiBCN derived from the over-cured precursor (σ=~159 MPa, KIC=1.9 MPa·m1/2, HV=7.8 GPa, E=122 GPa), the resulting SiBCN monolith exhibits significantly improved mechanical properties (σ=~304 MPa, KIC=3.7 MPa·m1/2, HV=10.6 GPa, E=161 GPa) and oxidation resistance. Besides, this study delves into the high-temperature performance of the SiBCN monolith including crystallization and oxidation and determines the oxidation kinetics law transition induced by the pore structure healing and the different oxidation mechanisms of Si-C-N and B-C-N clusters in the amorphous structure. Due to its unique composition and structure, the oxide layer of SiBCN ceramic exhibits exceptional self-healing effects repairing the nanoporous system in the initial stage and shows outstanding high-temperature stability during prolonged oxidation, mitigating adverse effects from bubble formation and crystallization. Due to the nanoporous structure, the oxidation rate is initially controlled by gas diffusion following a linear law before transitioning to oxide layer diffusion characterized by a parabolic law. Finally, due to different valence bond configurations, Si-C-N transforms into an amorphous SiCNO structure after phase separation, unlike the nucleation and growth of residual B-N-C.

Open Access Research Article Issue
Novel Csf/SiBCN composites prepared by densifying Csf/MA-SiBCN with the PIP process: Oxidation behavior and damage mechanism
Journal of Advanced Ceramics 2024, 13 (5): 666-678
Published: 21 May 2024
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To improve the oxidation resistance of short carbon fiber (Csf)-reinforced mechanically alloyed SiBCN (MA-SiBCN) (Csf/MA-SiBCN) composites, dense amorphous Csf/SiBCN composites containing both MA-SiBCN and polymer-derived ceramics SiBCN (PDCs-SiBCN) were prepared by repeated polymer infiltration and pyrolysis (PIP) of layered Csf/MA-SiBCN composites at 1100 °C, and the oxidation behavior and damage mechanism of the as-prepared Csf/SiBCN at 1300–1600 °C were compared and discussed with those of Csf/MA-SiBCN. The Csf/MA-SiBCN composites resist oxidation attack up to 1400 °C but fail at 1500 °C due to the collapse of the porous framework, while the PIP-densified Csf/SiBCN composites are resistant to static air up to 1600 °C. During oxidation, oxygen diffuses through preexisting pores and the pores left by oxidation of carbon fibers and pyrolytic carbon (PyC) to the interior of the matrix. Owing to the oxidative coupling effect of the MA-SiBCN and PDCs-SiBCN matrices, a relatively continuous and dense oxide layer is formed on the sample surface, and the interfacial region between the oxide layer and the matrix of the as-prepared composite contains an amorphous glassy structure mainly consisting of Si and O and an incompletely oxidized but partially crystallized matrix, which is primarily responsible for improving the oxidation resistance.

Open Access Research Article Issue
Composition-dependent structural characteristics and mechanical properties of amorphous SiBCN ceramics by ab-initio calculations
Journal of Advanced Ceramics 2023, 12 (5): 984-1000
Published: 04 May 2023
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The atomic structural features and the mechanical properties of amorphous silicoboron carbonitride ceramics with 13 different compositions in the Si–BN–C phase diagram are investigated employing ab-initio calculations. Both chemical bonds and local structures within the amorphous network relate to the elemental composition. The distribution of nine types of chemical bonds is composition-dependent, where the B–C, Si–N, Si–C, and B–N bonds hold a large proportion for all compositions. Si prefers to be tetrahedrally coordinated, while B and N prefer sp2-like trigonal coordination. In the case of C, the tetrahedral coordination is predominant at relatively low C contents, while the trigonal coordination is found to be the main feature with the increasing C content. Such local structural characteristics greatly influence the mechanical properties of SiBCN ceramics. Among the studied amorphous ceramics, SiB2C3N2 and SiB3C2N3 with low Si contents and moderate C and/or BN contents have high elastic moduli, high tensile/shear strengths, and good debonding capability. The increment of Si, C, and BN contents on this basis results in the decrease of mechanical properties. The increasing Si content leads to the increment of Si-contained bonds that reduce the bond strength of SiBCN ceramics, while the latter two cases are attributed to the raise of sp2-like trigonal configuration of C and BN. These discoveries are expected to guide the composition-tailored optimization of SiBCN ceramics.

Open Access Research Article Issue
Texture and anisotropy of hot-pressed h-BN matrix composite ceramics with in situ formed YAG
Journal of Advanced Ceramics 2022, 11 (4): 532-544
Published: 17 March 2022
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Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot- pressing using different contents of 3Y2O3-5Al2O3 (molar ratio of 3:5) as the sintering additive. During hot-pressing, the liquid Y3Al5O12 (YAG) phase showing good wettability to h-BN grains was in situ formed through the reaction between Y2O3 and Al2O3, and a coherent relationship between h-BN and YAG was observed with [010]h-BN// [1¯11]YAG and (002)h-BN//(321)YAG. In the YAG liquid phase environment formed during hot-pressing, plate-like h-BN grains were rotated under the uniaxial sintering pressure and preferentially oriented with their basal surfaces perpendicular to the sintering pressure direction, forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure direction, which give these composite ceramics anisotropy in their mechanical and thermal properties. The highest texture degree was found in the specimen with 30 wt% YAG, which also possesses the highest anisotropy degree in thermal conductivity. The aggregation of YAG phase was observed in the specimen with 40 wt% YAG, which resulted in the buckling of h-BN plates and significantly reduced the texture degree.

Open Access Research Article Issue
Microstructural evolution of h-BN matrix composite ceramics with La–Al–Si–O glass phase during hot-pressed sintering
Journal of Advanced Ceramics 2021, 10 (3): 493-501
Published: 24 March 2021
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BN/La–Al–Si–O composite ceramics were fabricated by hot-pressed sintering using hexagonal boron nitride (h-BN), lanthanum oxide (La2O3), aluminia (Al2O3), and amorphous silica (SiO2) as the raw materials. The effects of sintering temperature on microstructural evolution, bulk density, apparent porosity, and mechanical properties of the h-BN composite ceramics were investigated. The results indicated that La–Al–Si–O liquid phase was formed during sintering process, which provided an environment for the growth of h-BN grains. With increasing sintering temperature, the cristobalite phase precipitation and h-BN grain growth occurred at the same time, which had a significant influence on the densification and mechanical properties of h-BN composite ceramics. The best mechanical properties of BN/La–Al–Si–O composite ceramics were obtained under the sintering temperature of 1700 ℃. The elastic modulus, flexural strength, and fracture toughness were 80.5 GPa, 266.4 MPa, and 3.25 MPa·m1/2, respectively.

Open Access Research Article Issue
Microstructural evolution and mechanical properties of in situ nano Ta4HfC5 reinforced SiBCN composite ceramics
Journal of Advanced Ceramics 2020, 9 (6): 739-748
Published: 27 November 2020
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The in situ nano Ta4HfC5 reinforced SiBCN-Ta4HfC5 composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering. The microstructural evolution and mechanical properties of the resulting SiBCN-Ta4HfC5 were studied. After the first-step milling of 30 h, the raw materials of TaC and HfC underwent crushing, cold sintering, and short-range interdiffusion to finally obtain the high pure nano Ta4HfC5. A hybrid structure of amorphous SiBCN and nano Ta4HfC5 was obtained by adopting a second-step ball-milling. After reactive hot-pressing sintering, amorphous SiBCN has crystallized to 3C-SiC, 6H-SiC, and turbostratic BN(C) phases and Ta4HfC5 retained the form of the nanostructure. With the in situ generations of 2.5 wt% Ta4HfC5, Ta4HfC5 is preferentially distributed within the turbostratic BN(C); however, as Ta4HfC5 content further raised to 10 wt%, it mainly distributed in the grain-boundary of BN(C) and SiC. The introduction of Ta4HfC5 nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics, reaching to 344.1 MPa and 4.52 MPa·m1/2, respectively. This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix, which is beneficial to the real applications of SiBCN ceramics.

Open Access Review Issue
Preparation and anisotropic properties of textured structural ceramics: A review
Journal of Advanced Ceramics 2019, 8 (3): 289-332
Published: 02 August 2019
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Ceramics are usually composed of randomly oriented grains and intergranular phases, so their properties are the statistical average along each direction and show isotropy corresponding to the uniform microstructures. Some methods have been developed to achieve directional grain arrangement and preferred orientation growth during ceramic preparation, and then textured ceramics with anisotropic properties are obtained. Texture microstructures give particular properties to ceramics along specific directions, which can effectively expand their application fields. In this review, typical texturing techniques suitable for ceramic materials, such as hot working, magnetic alignment, and templated grain growth (TGG), are discussed. Several typical textured structural ceramics including α-Al2O3 and related nacre bioinspired ceramics, Si3N4 and SiAlON, h-BN, MB2 matrix ultra-high temperature ceramics, MAX phases and their anisotropic properties are presented.

Open Access Research Article Issue
Enhanced mechanical properties and thermal shock resistance of Si2BC3N ceramics with SiC coated MWCNTs
Journal of Advanced Ceramics 2019, 8 (1): 121-132
Published: 13 March 2019
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Bulk Si2BC3N ceramics were reinforced with SiC coated multi-walled carbon nanotubes (MWCNTs). The phase compositions, mechanical properties, and thermal shock resistance, as well as the oxidation resistance of the designed Si2BC3N ceramics were comparatively investigated. The results show that nano SiC coating can be formed on MWCNTs through pyrolyzing polysilazane, which improves the oxidation resistance of MWCNTs. A stronger chemical bonding is formed between the SiC coated MWCNTs and SiC particles, contributing to improved flexural strength (532.1 MPa) and fracture toughness (6.66 MPa·m1/2). Besides, the 2 vol% SiC coated MWCNTs reinforced Si2BC3N ceramics maintains much higher residual strength (193.0 MPa) after thermal shock test at 1000 ℃. The enhanced properties should be attributed to: (1) the breaking of MWCNTs and the debonding between MWCNTs and SiC interfaces, which leads to more energy dissipation; (2) the rough surfaces of SiC coated MWCNTs increase the adhesion strength during the "pull out" of MWCNTs.

Open Access Research Article Issue
Enhanced thermal shock and oxidation resistance of Si2BC3N ceramics through MWCNTs incorporation
Journal of Advanced Ceramics 2018, 7 (3): 276-288
Published: 10 October 2018
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Multi-walled carbon nanotubes (MWCNTs) reinforced Si2BC3N ceramics were prepared through mechanical alloying (MA) and following spark plasma sintering (SPS). The thermal shock resistance of Si2BC3N ceramics was evaluated comparatively through ice water quenching test and theoretical prediction. Furthermore, the oxidation resistance of MWCNTs incorporated Si2BC3N ceramics was evaluated under high temperature. The results show that the calculated parameters such as the critical thermal shock temperature (R) and the thermal stresses resistance (Rst), as well as the toughness (R′′′′) are improved with addition of 1 vol% MWCNTs. In addition, the crack propagation resistance of 1 vol% MWCNTs incorporated Si2BC3N ceramics is obviously improved through generating more tortuous crack propagation paths attributing to the “crack bridging”, “pull-out”, and “crack deflection” mechanisms of MWCNTs. Therefore, the residual strengths of 1 vol% MWCNTs containing specimens remained the highest after the thermal shock tests. Besides, the present work also reveals that the oxidation resistance is more sensitive to relative density than MWCNTs addition.

Open Access Research Article Issue
Microstructure and mechanical properties of SiCf/SiBCN ceramic matrix composites
Journal of Advanced Ceramics 2015, 4 (1): 31-38
Published: 31 January 2015
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SiC fiber reinforced SiBCN ceramic matrix composites (CMCs) have been prepared by mechanical alloying and consolidated by hot pressing. During the sintering process, amorphous SiC fibers crystallized seriously and transformed into β-SiC. Meanwhile, the interfacial carbothermal reactions caused the strong bonding between the matrix and fibers. As a result, SiCf/SiBCN fractured in a typical catastrophic manner. Room-temperature mechanical properties reached the maximums for the CMC samples sintered at 1900 ℃/60 MPa/30 min. The density, flexural strength, Young’s modulus and fracture toughness are 2.56±0.02 g/cm3, 284.3±17.9 MPa, 183.5±11.1 GPa and 2.78±0.14 MPa·m1/2, respectively.

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