Key Engineering Materials Vols. 368-372

Abstract: Stable ZrB2-based ultra-high temperature ceramic slurries were prepared and characterized with a solid content 40 vol% by aqueous gelcasting which was suitable to form high quality and complexshaped ceramic parts. In the present work, Ammonium Citrate Tribasic (ACT) was used as the dispersant. The properties of ZrB2 slurry, and the influencing factors were investigated by conventional techniques such as sedimentation tests, particle size distribution measurements, electrokinetic measurements and rheological analysis. High solids loading and low viscosity slurry was obtained by controlling the optimal conditions for the ZrB2-based ceramic powders.

Abstract: ZrB2-SiC based composites with 0,5 and 15 vol% addition of ZrC were synthesized via reactive hot pressing at 1800°C using Zr, Si and B4C as raw materials. The mechanical properties of the composites were investigated. The composite of ZSC15 that contained 15 vol% of ZrC has the highest hardness. ZSC5 with 5 vol% of ZrC owns a most homogenous microstructure and the highest fracture toughness and flexural strength.

Abstract: HfB2-HfC-SiC ultrahigh temperature ceramics (UHTCs) were prepared and characterized in this paper. It is showed that the densities of the HfB2-HfC-SiC reach 98.5% of the theory density. The room temperature compressive properties of the HfB2-HfC-SiC are good, while those at high temperature decrease rapidly. The volume expansion ratio monotonously increases (up to 2.35% at 2300°C) with increasing temperature. Furthermore, with increasing temperature, the average linear expansion coefficient hardly changes, while the instant linear expansion coefficient decreases first, and followed by an increase. The minimum value of the instant linear expansion coefficient is 5.65×10-6/K at 900°C and that of the mean linear expansion coefficient is 7.39×10-6/K at 1340°C. HfB2-HfC-SiC were burned with the plasma arc heater. After 8-second ablation, part of the SiC particles melted and spurted from the composites, and holes appeared.

Abstract: The ZrC-W composites with iron as sintering additive were fabricated by hot-press sintering. The densification, microstructure and mechanical properties of the composites were investigated. The incorporation of Fe beneficially promotes the densification of ZrC-W composites. The relative density of the composite sintered at 1900°C can attain 95.3%. W2C phase is also found in the ZrC-W composite sintered at 1700°C. The content of W2C decreases with the increase of sintering temperature. However, W2C phase is not identified in the composite sintered at 1900°C. The flexural strength and fracture toughness of the composites are strongly dependent on sintering temperature. The flexural strength and fracture toughness of ZrC-W composite sintered at optimized temperature of 1800°C are 438 MPa and 3.99 MPa·m1/2, respectively.

Abstract: Combustion synthesis (CS) of Si3N4 was accomplished by using as-milled Si/NH4Cl as reactants at low nitrogen pressure. The additive of NH4Cl decreased the combustion temperature and promoted the Si nitridation. Full nitridation of Si was achieved by burning Si in pressurized nitrogen with 10 ~ 25 wt. % NH4Cl as additives while no Si3N4 diluent added. The maximum combustion temperature (Tc), the combustion velocity (u) together with the α-Si3N4 content and mean particle size (d50) of the powder products were found to be great dependent on the NH4Cl content added in the reactants. Fine Si3N4 powder products with α-phase content up to 85 wt. % were obtained via steady combustion mode. A mathematical approach named combustion wave velocity methods for the analysis of temperature profiles in CS was proposed and the reaction kinetics was discussed. The apparent activation energy calculated according to the temperature profile analysis method is 29.7 kJ/mol, which agrees well with the corresponding low temperature nitriding combustion of Si.

Abstract: Ultra high temperature ceramic matrix composites (UHTCC) are being considered as the most promising materials for leading edge and nose cap of hypersonic spacecrafts, reusable space vehicles and so on. In the paper, 2D carbon fiber cloth reinforced silicon carbide-tantalum carbide (2D SiC-TaC) UHTCC was fabricated by slurry-pasting and precursor infiltration pyrolysis process (PIP). Influences of the volume ratio (10, 20, 30, 60, 80 and 100%) of TaC powder on mechanical properties and ablative resistance of 2D C/SiC-TaC composites were studied. The results showed that the relative density of composites with 60vol% TaC powder was the highest, the flexural strength of the composites reached 356MPa and the mass loss rate and recession rate were 0.0116g/s and 0.026mm/s respectively, while those of C/SiC composites were 0.0166g/s and 0.062mm/s respectively. Moreover, the higher TaC powder content, the smaller the fracture toughness of the composites was. The fracture toughness of the 2D C/SiC-TaC composites with 100vol% TaC powder was only 8.69 MPa-m1/2, while that of C/SiC composites was over 15.0 MPa-m1/2.

Abstract: Ultra-high-temperature resistant Si-Al-C fibers derived from polyaluminocarbosilane were prepared by the procedures of melt-spinning, air curing and heated at 1800°C. It was found that oxygen content in the cured fibers has great influences on the chemical composition of the Si-Al-C fibers and its crystalline behavior during sintering. When oxygen content of cured fibers was too high, crystallite grains in the fiber would grow up. Oxygen evolvement in the fiber during the preparation was studied in detail by chemical analysis. It was found that oxygen is liberated mainly as CO gas from 1300°C to1600°C.

Abstract: A combined technique comprising electrophoretic deposition (EPD) and low-pressure infiltration was used for the fabrication of multi-layer woven mullite ceramic fabric reinforced alumina ceramic matrix composites (CMCs) for high temperature applications. Two different interface materials, NdPO4 and ZrO2 were synthesised and used for coating the woven ceramic fibres by EPD. The manufactured CMC components with suitable interface material are targeted for use at 1300-1400 oC in an oxidising atmosphere and have shown very good mechanical properties in multi-layer plate forms. Damage mechanisms, such as debonding, fibre fracture, delamination and matrix cracking within the composite plates subjected to flexural loading are analysed. It is shown that the composites with NdPO4 interface and 40 vol.% fibre loading have better mechanical properties in terms of strength and damage-tolerant behaviour. The final components produced are considered to be suitable for use as shroud seals and insulating layers for combustor chambers in aircraft engines.

Abstract: Thermal shock resistance of Ultra-High Temperature Ceramics is one of the most important parameters in UHTCs characterization since it determines their performance in many applications. In order to reflect practical cases, the temperature-dependent thermal shock resistance parameter of UHTCS was measured since the material parameters of UHTCs are very sensitive to the changes of temperature. The influence of some important thermal environment parameters and the size of the material on the thermal shock resistance and critical temperature difference of rupture of UHTCs at different stages in the thermal-shock process were investigated. The results show that thermal shock behaviour of the UHTCs is strongly affected by the size of the material and the thermal environments parameters, such as the surface heat transfer coefficient, heat transfer condition and initial temperature of the thermal shock.

Abstract: This paper presents a theoretical model to predict the fracture strength of ultra-high temperature ceramics (UHTCs). According to different mechanisms, the environmental temperature is divided into four ranges. Effects of temperature and oxidation on the fracture strength of UHTCs are investigated in each temperature range. The results show that oxidation plays an important role in enhancing the fracture strength of UHTCs at high temperatures.