The easy-going oxidation of silicon nitride (Si₃N₄) at high temperature greatly hampers its potential applications. Here, we explored the reaction mechanism between β-Si₃N₄ and O₂ via density functional theory (DFT) calculation, which discloses that O atoms are preferentially adsorbed on the top of Si atoms and N₂ starts to be generated as the dominant gas product at 2/3 monolayer (ML) O coverage. The vacancies formed by N₂ removal attract the O adatoms to transfer to the site of the N vacancy, which accelerates the adsorption of O and the formation of Si–O bonds toward the growth of SiO₂ product. The surface oxidation of β-Si₃N₄ (0001) has been clarified by the unambiguous evolution of [SiN_{4 -n}O_n] (n = 0-4) tetrahedrons going through from [SiN₄] tetrahedron to [SiO₄] tetrahedron, providing a deep insight into intrinsic oxidation process of Si₃N₄ ceramic.

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The ultra-high temperature ablation of a polycrystalline, fully dense, predominantly single phase MoAlB ceramic discs under an oxyacetylene flame is examined. The linear ablation rate decreases from 1.3 μm/s during the first 30 s to - 0.7 μm/s after 60 s when the surface temperature reached about 2050 °C (with a flame temperature around 3000 °C). Up to 60 s, the MoAlB is ablation resistant due to the formation of a protective and viscous surface Al2O3 layer. As the ablation time is prolonged, the protective Al2O3 scale becomes porous and is almost fully destroyed at the central ablation region after 120 s. This accelerates the formation of large amounts of volatile species (mainly B and Mo oxides), resulting in a reduction in the ablation resistance.@en

Almost pure (Ti_1-xZr_x)₃SiC₂ MAX phase solid solutions with x ranging up to 0.17 were synthesized at temperatures in the range of 1450–1750 °C with reactive Spark Plasma Sintering (SPS). The zirconium partially replaces the M-element titanium of the Ti₃SiC₂ MAX phase up to x equals 0.17. The lattice parameters of the hexagonal (Ti_1-xZr_x)₃SiC₂ MAX phase are determined with X-ray diffraction using Rietveld refinement and show an anisotropic lattice expansion upon Zr insertion into Ti₃SiC₂. This observation is in very good agreement with density functional theory calculations where the deviation between the measured and calculated lattice parameter is less than 1%. The predicted elastic modulus reduction is only 4%. This behavior can be rationalized by considering the electronic structure, where only minute changes are observable as Zr is incorporated into Ti₃SiC₂. The measured nanohardness of the synthesized (Ti_1-xZr_x)₃SiC₂ MAX phase increases from 12.7 ± 1 GPa for Ti₃SiC₂ to 16.3 ± 1.1 GPa when x is raised from 0 to 0.17 due to an increasing amount of accompanying Ti_1-yZr_yC. The elastic moduli of (Ti_1-xZr_x)₃SiC₂ solid solutions measured by an ultrasonic pulse-echo method (325–354 GPa) are in good agreement with the predicted elastic moduli (357–342 GPa).

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Aluminum nitride (AlN) with a combination of very high thermal conductivity and excellent electrical insulation properties exhibits wide applications. However, it is quite sensitive to a moist environment and hydrolyzes slowly in water. In this work, density functional theory was adopted to examine the atomistic reaction mechanism on the wurtzite AlN(0001) surface. The results indicate that water molecules are preferentially adsorbed at the top site of the AlN(0001) surface. The decomposition of adsorbed H ₂ O into OH and H on the AlN(0001) surface occurs spontaneously without any energy barrier. However, a further dissociation reaction of OH into O and H has an energy barrier of 22.046 kcal/mol. The dissociation of H ₂ O is strongly dependent on the H ₂ O coverage when more water molecules are adsorbed. Ammonia (NH ₃ ) is determined as the dominant gas product at 4/9 monolayer H ₂ O coverage, which will induce N vacancy (V _N ) formation in AlN ceramic. The V _N will be occupied by O ^2- after geometry optimization and plays an acceleration role in the degradation of AlN by water. H ₂ O adsorption and the formation of NH ₃ occur alternately with the H ₂ O coverage increasing. An OH-Al-O layer is formed as a precursor of AlOOH after the first AlN bilayer is fully degraded. This work can guide the manufacture and application of AlN from the theoretical viewpoint.

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The room temperature abrasive wear behavior of three selected MAX phases, Ti₃SiC₂, solution strengthened Ti_2.7Zr_0.3SiC₂ and Cr₂AlC, is investigated by low velocity scratch testing using a diamond conical indentor with a final radius of 100 μm and a cone angle of 120° and applied loads of up to 20 N. All three materials showed a relatively low wear resistance in comparison to most engineering ceramics such as Al₂O_3, Si₃N₄ and SiC. For all three materials, the wear rate scaled more or less linearly with the applied load. The softer Ti₃SiC₂ with a hardness of 2.8 GPa showed the lowest wear resistance with extensive ploughing and grain breakout damage, both within and outside the direct wear track, in particular at the highest load. The hardest material, Ti_2.7Zr_0.3SiC₂, with a hardness of 7.3 GPa, showed a 5 times better wear resistance. The Cr₂AlC with a hardness of 4.8 GPa showed a wear resistance equal to or even better than that of the Ti_2.7Zr_0.3SiC₂. The wear mechanism depends on the applied load and the microstructure of the MAX phase materials tested. For the Ti₃SiC₂ sample, a quasi-plastic deformation behavior occurs below a point load of 10 N, resulting in grain bending, kink band formation and delamination, grain de-cohesion, as well as trans-and intra-granular fracture near the scratch groove. At this load, the Ti_2.7Zr_0.3SiC₂ and Cr₂AlC MAX samples display plastic ploughing, grain boundary cracks and material dislodgments.

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The oxidation-induced crack healing of an Al₂O₃ composite loaded with various volume fractions of Ti₂Al_0.5Sn_0.5C repair filler particles was investigated by annealing in air at a relatively low temperature of 700°C. After annealing a composite with 20 vol.% repair fillers (with a particle size of ~5.6 µm) for 48 hours, artificial indentation cracks prepared on the surface, as well as pores near the surface, were completely healed by filling with condensed oxidation products. Additionally, minor fraction of metallic Sn was detected. A complementary study by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy revealed that nano-sized oxidation products (SnO₂, TiO₂, and α-Al₂O₃ phase) were formed as major crack-filling species. After healing, the strength recovery of the Al₂O₃ composites was significantly improved in the composites loaded with more than 10 vol.% repair fillers and achieved 107% at 700 for 48 hours.

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In this work, the oxidation-induced crack healing of Al₂O₃ containing 20 vol.% of Ti₂AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti₂AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti₂AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4-point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 ± 35 to 229 ± 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full-strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high-temperature applications.

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In this paper, the fabrication and thermal conductivity (TC) of water-based nanofluids using boron (B)-doped SiC as dispersions are reported. Doping B into the β-SiC phase leads to the shrinkage of the SiC lattice due to the substitution of Si atoms (0.134 nm radius) by smaller B atoms (0.095 nm radius). The presence of B in the SiC phase also promotes crystallization and grain growth of obtained particles. The tailored crystal structure and morphology of B-doped SiC nanoparticles are beneficial for the TC improvement of the nanofluids by using them as dispersions. Using B-doped SiC nanoparticles as dispersions for nanofluids, a remarkable improvement in stability was achieved in SiC-B6 nanofluid at pH 11 by means of the Zeta potential measurement. By dispersing B-doped SiC nanoparticles in water-based fluids, the TC of the as-prepared nanofluids containing only 0.3 vol.% SiC-B6 nanoparticles is remarkably raised to 39.3% at 30 °C compared to the base fluids, and is further enhanced with the increased temperature. The main reasons for the improvement in TC of SiC-B6 nanofluids are more stable dispersion and intensive charge ions vibration around the surface of nanoparticles as well as the enhanced TC of the SiC-B dispersions.

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Advanced engineering and functional ceramics are sensitive to damage cracks, which delay the wide applications of these materials in various fields. Ceramic composites with enhanced fracture toughness may trigger a paradigm for design and application of the brittle components. This paper reviews the toughening mechanisms for the nanolayered MAX phase ceramics. The main toughening mechanisms for these ternary compounds were controlled by particle toughening, phase-transformation toughening and fiber-reinforced toughening, as well as texture toughening. Based on the various toughening mechanisms in MAX phase, models of SiC particles and fibers toughening Ti₃SiC₂ are established to predict and explain the toughening mechanisms. The modeling work provides insights and guidance to fabricate MAX phase-related composites with optimized microstructures in order to achieve the desired mechanical properties required for harsh application environments.

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Highly pure Al₄SiC₄ powders were prepared by carbothermic reduction at 2173 K using Al₂O₃, SiO₂, and graphite as raw materials. The obtained Al₄SiC₄ powders owned hexagonal plate-like grains with a diameter of about 200–300 μm and a thickness of about 2–6 μm. Based on the experimental results, the reaction of Al₄SiC₄ formation and grain evolution mechanisms were determined from thermodynamic and first-principles calculations. The results indicated that the synthesis of Al₄SiC₄ by the carbothermic reduction consisted of two parts, i.e., solid–solid reactions initially followed by complex gas–solid and gas–gas reactions. The grain growth mechanism of Al₄SiC₄ featured a two-dimensional nucleation and growth mechanism. The gas phases formed during the sintering process favored the preferential grain growth of (0010) and (1 ī0) planes resulting in formation of hexagonal plate-like Al₄SiC₄ grains.

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Crack healing in MAX phase-based ceramics is mainly attributed to the reaction of M and A metal with oxygen penetrating into the crack. The intrinsic healing mechanisms of Al-contained MAX phases are due to formation of adhesive Al2O3 phase in the crack gaps, while Sn-contained MAX phases are ascribed to formation of major TiO2 and SnO2 in the bigger crack gaps and metallic Sn formation in the smaller secondary cracks. The oxides as well as metallic phase filling the crack may restore the integrity of MAX phase ceramic components. The mechanical strength and the electrical conductivity of the MAX phase ceramics can be fully restored after the healing process. The remarkable healing abilities of those MAX phases make them promising repair fillers for those ceramics with poor healing ability. Al2O3 mixed with Ti2Al0.5Sn0.5C MAX phase was selected as a model to demonstrate the healing efficiency. The fracture strength of 20 vol.% repair filler 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. The evolution of the crack-filling microstructure was explored by X-ray powder diffraction and transmission electron microscopy analyses.@en