Key Engineering Materials Vols. 512-515

Abstract: In this paper, titanium silicon carbide (Ti₃SiC₂) powders were synthesized from TiH₂ as Ti source by pressureless sintering in flowing argon atmosphere without preliminary dehydrogenation. Starting materials are powder mixtures with the mole ratio of 3TiH₂/Si/2C or 3TiH₂/SiC/C. Both kinds of starting materials were sintered in a tube furnace at the temperature range from 1300°C to 1500°C for 10~180min in flowing argon atmosphere. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase compositions and morphology of the products after different treatments. It was showed that almost single phase Ti₃SiC₂ powder (94.7 wt.%) can be synthesized by pressureless sintering from 3TiH₂/Si/2C powders at 1400~1425°C for about 180min or from 3TiH₂/SiC/C powders at 1425~1500°C for about 180min. From SEM micrographs, as-synthesized samples were porous. Most plate-like grains were about 5~10 μm in diameter and 1~2 μm in thickness. The speed of temperature increasing is an important factor to affect the purity of as-synthesized Ti₃SiC₂.

Abstract: Because of its combined characteristics of metals and ceramics, such as low density, high Young’s modulus, thermal and chemical resistance with low hardness, high electrical and thermal conductivity, it was expected that the introduction of Ti3SiC2 to fiber reinforced ceramic matrix can make the composite own some unique properties. In the present research, Ti3SiC2 powders used as inert fillers were fabricated by the in-situ reaction between Ti and polycarbosilane mixtures. The purity of Ti3SiC2 powders analyzed by XRD was determined by RIR method, which is a semi-quantitative XRD analysis. The results showed that the purity of Ti3SiC2 powders is about 96%. Cf/Ti3SiC 2-SiC composites are obtained by polymer infiltration and pyrolysis process using Ti3SiC2 powders as the inert fillers. The bending strength of Cf/Ti3SiC2-SiC composites was about 160 MPa.

Abstract: By adjusting the mole ratio of C and B elements in combustion system, solidified TiC-TiB₂ composites with different TiB₂ mole fraction were achieved by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that with increasing TiB₂ content, the matrix of TiC-TiB₂ composite ceramics transformed a number of fine TiB₂ platelets from the TiC spherical grains, and fine-grained even ultrafine-grained microstructures were achieved in solidified TiC-50mol% TiB₂ due to eutectic growth under rapid solidification of the ceramic. Properties showed that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB₂ simultaneously reached the maximum values of 21.5 ± 1.5 GPa and 860 ± 35 MPa , whereas TiC-66.7mol%TiB₂ presented the maximum fracture toughness of 13.5 ± 1.5 MPa • m^0.5. FESEM fractography analyses of the ceramics exhibited a mixed mode of transcrystalline fracture of TiC spherical grains and intercrystalline fracture of TiB₂ platelets, and the tendency of intercrystalline fracture was obviously enhanced with increasing TiB₂ content to be 66.7 mol%, resulting in enhanced toughening mechanisms of crack deflection, crack-bridging and pull-out by fine TiB₂ platelets, thus, the highest flexural strength was achieved in TiC-50mol%TiB₂ due to the achievements of both fine-grained microstructures and high fracture toughness in the full-density solidified ceramics.

Abstract: Micro-nanocrystalline microstructures characterized by TiB₂ platelets of the average thickness close to or smaller than 1 μm, were achieved in full-density solidified TiC-TiB₂ composites by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that a large number of fine TiB₂ platelets were uniformly embedded in irregular TiC grains, a few Cr-Al metallic phases or in between those phases. The achievement of micro-nanocrystalline microstructures results from low-velocity faceting growth of TiB₂ crystal, high-velocity non-faceting growth of TiC solid and high diffusion rate of C relative to B in liquid TiC-TiB₂. The relative density, Vickers hardness, flexural strength and fracture toughness of TiC-TiB₂ composites measured 99.3%, 21.5 ± 1.5 GPa, 845 ± 35 GPa and 16.8 ± 1.5 MP • m^0.5, respectively. High flexural strength of the materials benefits mainly from the achievement of micro-nanocrystalline microstructure in the full-density solidified ceramic and high fracture toughness contributed from intensive coupled toughening mechanisms by a large number of fine TiB₂ platelets and a few Cr-Al metallic phases.

Abstract: The two kinds of the Ti-rich (Ti0.8Zr0.2)B2 and Zr-rich (Zr0.8Ti0.2)B2 permutation solid solutions were formed when hot pressed ZrB2 and TiB2 ceramics. On the base of the empirical electron theory (EET) of solids and molecules, the crystal bond energy of the two solid-solutions was calculated by use of Average Atom Model, Average Cell Model and Real Cell Model. In Real Cell Model, the crystal cell parameters were assumed to be unchanged and changed. The calculation results were compared among the three models. The results showed that the general trend about the deviation electrons of B element is consistent except Real Cell Model when the lattice constant is unchanged. Namely, the number of the deviation electrons in TiB2 is more than that in ZrB2.The maximum error of crystal bond energy calculated by the three models is 38KJ/mol and the relative error is less than 2%.The general trend of the crystal bond energy calculated by the three models is monotonically increased similar to the trend of melting point of these materials. Relatively speaking, the calculation of crystal bond energy calculated by Average Atom Cell Model is simpler and more reasonable.

Abstract: Through strong magnetic field alignment (SMFA) method followed by spark plasma sintering (SPS), dense tailored ZrB₂ ceramic could be prepared using the plate-like grains as initial material. The measured Lotgering orientation factor f_(00l) on the textured top surface (TTS) was high as 0.62 and f_(hk0) on the textured side surface (TSS) was 0.65. Vickers hardness measurement showed the anisotropic mechanical property of textured sample. On the TTS, the hardness was 13.4 ± 0.9 GPa, while on the TSS the hardness was 12.0 ± 0.9 GPa.

Abstract: ZrB₂-20%volSiC ceramic composites with different volume of BN short fiber were fabricated by the hot-pressing sintering under 2000°C. The content of BN short fiber changed from 0 to 15vol%. The density, flexural strength, fracture toughness and thermal expansions coefficient were studied. The microstructures of the samples were observed by scanning electron microscopy. The results show that the introducing of BN short fiber into the ZrB₂-20%volSiC lead to a serious of change to the mechanical properties of the ceramic. When the content of the BN short fiber is 10vol%, the flexural strength and fracture toughness reach 422.1MPa and 6.15 MPa•m ^1/2 respectively. And the mechanism of the increasing toughness was studied.

Abstract: Ablation behavior of ZrB₂-SiC sharp leading edges with five different curvature radiuses was investigated using an oxy-acetylene torch. During the test, the curvature radiuses were 0.15 mm, 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm, respectively. Under the same ablation condition, the smaller was the radius, the severer ablation underwent. The sharp leading edge with a curvature radius of 0.15 mm had the highest surface temperature and maximum surface temperature rising rate, exceeded 2100°C in less than 30 s. However, the surface of sharp leading edge with a curvature radius of 2.0 mm achieved only 1900°C in more than 60 s. After 5 min ablation, the mass and linear ablation rates were measured. All the five sharp leading edges evolved to nearly a same radius after ablation. The microstructure of the oxidation layers was also investigated. A ZrO₂-SiO₂ layer generated from oxidation of ZrB₂-SiC acts as a thermal barrier and reduces the diffusion of oxygen.

Abstract: ZrC-SiC powders were fabricated by means of liquid precursor conversion method, using Zr containing polymer precursor and polycarbosilane. The effects of staring reagents and the pyrolysis temperature on the fabrication of ZrC-SiC powders were studied. Results show that ZrC-SiC powders with different ZrC/SiC ratio could be formed when the staring reagents were different. Pyrolysis temperature affects the pyrolysed product. When temperature was lower, less amount of ZrC was formed in the powder. The size of crystallite and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.

Abstract: With the background of thermal protection applications of anti-oxidation carbon fiber reinforced composites, carbon fiber reinforced ultra-high temperatureceramics with homogeneous disperse complex matrix of C-ZrB2-SiC (C/C-ZrB2-SiC) was prepared. Carbon fiber performs were deposited with pyrolytic carbon by chemical vapor infiltration method. Subsequently, the composite precursors were prepared by completely mutually dissolving of ZrB2 polymeric precursor and polycarbosilane dimethylbenzene solution. Then the nano-dispersed ZrB2-SiC composite ceramic was introduced into the C/C preforms by polymer impregnant and pyrolysis process. The C/C-ZrB2-SiC composite shows excellent ablation behavior with the ablating rate of 8*10-4mm/s. The microstructural and compositional characterizations of the C/C-ZrB2-SiC composites indicates that ZrB2 nanoparticle is distributed homogeneously in the continuous SiC phase, which is beneficial to enhance ultra-high temperature ablation resistance of the composites.