Papers by Keyword: Ultra-High Temperature Ceramics (UHTC)

Abstract: The paper presents the results of obtaining silicon carbide SiC ceramic from organic raw materials by the reaction spark plasma sintering (SPS). Annealed rice husks are used as an organic raw material. A feature of the batch preparation for sintering is annealing in an inert atmosphere. Thus, the carbon source C for further reaction with silicon dioxide SiO2 to form SiC is the organic part of the rice husk. This way let to achieve the most uniform reacting components mixing directly at the stage of annealing the husks and to exclude the mixing stage from the technological process. The reaction SPS method was used for the first time for the production of ceramic material from such a raw material. The results of a comprehensive study of the obtained material by various methods are presented in the article: scanning electron microscopy, Raman spectroscopy, X-ray phase analysis, strength analysis, and others.

Abstract: Ultra-high temperature ceramics (UHTCs) have attracted increasing attention of material scientists and engineers due to their promising application in reactive atmospheres such as monatomic oxygen at high temperatures over 1600°C. However, the mechanical properties of this kind of ceramics at ultra-high temperature are difficult to measure because of the temperature limits of the furnace and fixture. In this work, an innovative measuring approach, trace analysis method, is presented for estimating the ultra-high temperature mechanical properties. Various material parameters, including elastic modulus, hardness, recovery deformation and energy dissipation capacity at ultra-high temperature could be determined via a residual indent trace that was induced by a fast impact of a ceramic conical indenter on the local area at high temperature.

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.

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: 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: 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: 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: ZrB2-SiC composite is a promising candidate for ultra-high temperature ceramics, which is difficult to be sintered due to strong covalent bonding of ZrB2 and SiC. ZrB2-30Vol.%SiC composite was prepared by spark plasma sintering technique (SPS) at the sintering temperature of 1850°C, sintering pressure of 50MPa, heating rate of 200°C/min and holding time of 3 minutes. The phase components and microstructure were examined by X-ray diffraction, scanning electron microscopy and transmitting electron microscopy. The results show that the product is composed of ZrB2 phase, SiC phase and ZrO2 phase. A rationalization for the presence of ZrO2 phase is based on the impurity of raw material and oxidation of ZrB2 during SPS. The consolidated product is very dense and no apparent pores exist in the microstructure. ZrO2 phase with irregular shape is found among some particles as a binder phase. It is shown that the presence of ZrO2 phase may be beneficial to the densification of ZrB2-SiC composite.

Abstract: Two processing routes, both starting from powders of Zr, B4C and Si, which take advantage of the Spark Plasma Sintering (SPS) apparatus, are proposed in this work for the preparation of fully dense 2ZrB2-SiC composite. The first method consists of the in-situ reaction synthesis and densification of the product while, in the second one, reactants are first converted by SHS (Self-propagating Hightemperature Synthesis) into the desired composite and the obtained powders are then sintered by SPS. Based on the results reported in this work, both routes are particularly convenient as compared to the techniques available in the literature for the preparation of analogous materials.

Abstract: SiC whisker-reinforced ZrB2 matrix ultra-high temperature ceramic were prepared at 2000°C for 1 h under 30MPa by hot pressing and the effects of whisker on flexural strength and fracture toughness of the composites was examined. The flexural strength and fracture toughness are 510±25MPa and 4.05±0.20MPa⋅m1/2 at room temperature, respectively. Comparing with the SiC particles-reinforced ZrB2 ceramic, no significant increase in both strength and toughness was observed. The microstructure of the composite showed that the SiC whisker was destroyed because the SiC whisker degraded due to rapid atom diffusivity at high temperature. The results suggested that some related parameters such as the lower hot-pressing temperature, a short sintering time should be controlled in order to obtain SiC whiskerreinforced ZrB2 composite with high properties.