Key Engineering Materials Vols. 368-372

Abstract: The present study focuses on the dynamic oxidation resistance of five representative ZrB2-SiC based ultra-high temperature ceramics (UHTCs): ZrB2-SiC, ZrB2-SiC-Si3N4, ZrB2-SiC-TiB2, ZrB2-SiCHfB2 and ZrB2-SiC-ZrC using oxyacetylene torch and arc jet testing. The effects of second phase incorporation (Si3N4, TiB2, HfB2, ZrC) on oxidation resistance were compared and analyzed. The mechanism of oxidation based on experimental results and thermodynamic calculations were explored. Some approaches to improvement of oxidation resistance and future directions of UHTC are also presented.

Abstract: Two kinds of ultra high temperature ceramics (UHTC), ZrB2-20Vol.%SiC and ZrB2-15Vol.% SiC-5Vol.% SiCn (nano-size) were prepared by spark plasma sintering (SPS). The residual strength was used to characterize thermal shock resistance of ZrB2-SiC ceramics by quenching tests in the water. The mechanism of thermal shock damage of the ceramics was examined by SEM analysis. The results showed that the formed glassy phase has significant effect on thermal shock resistance of ZrB2-SiC ceramics. When the thermal shock temperature rises, the residual strength of ZrB2-SiC ceramics after thermal shock will decrease gradually. The formed glassy phase when quenched from the temperature of 1400°C can repair or heal the cracks and induce the higher residual strength. However, when the temperature increases to 1600°C, the residual strength of ZrB2-SiC ceramics decreases significantly due to the volatilization of the glassy phase. The residual strength of ZrB2-15Vol.% SiC-5Vol.% SiCn ceramic is higher than that of ZrB2-20Vol.%SiC ceramic, because nano SiC is easy to be oxidized and to induce the formation of more glassy phases and to improve the thermal shock resistance of the sample.

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.

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: ZrB2-based ultra-high temperature ceramics (UHTCs) were prepared from a mixture powder of Zr/B4C/Si with different ratio via reactive hot pressing. The experimental results showed that the sintering temperature above 1800°C was necessary for enhancing the activity of the powders and thus improving the densification of the product. The sinterability and densification properties of ZrB2-based UHTCs meliorated with the amount of Si increasing. However, many large ZrB2 agglomerates formed when the amount of synthesized SiC in the product reached 25vol%, which led to decrease the mechanical property. The composite had highest mechanical properties when the volume ratio of ZrB2: SiC: ZrC was 73.86:20:6.14, and its flexual strength and the fracture toughness were 645.8MPa and 5.66MPa·m1/2 respectively. The microstructure investigation showed the in-situ formed SiC and ZrC were located in the triple point of ZrB2 grains with a size less than 3μm.

Abstract: In this study, two rare earth oxides, Y2O3 and La2O3, are used as the additives in the sintering of ZrB2-SiC composites to improve the sinterability and control development of microstructure during densification. The results show that the use of rare earth oxides (5vol.%) improves the powder sinterability, hindered excessive growth of matrix particles and increase fracture toughness of ZrB2-SiC composites, in comparison to ZrB2-SiC with additions free. Nearly full dense materials are obtained by hot pressing at 1900°C. XRD analyses indicate that lanthanum-containing phases were formed in the composite with La2O3. Microstructure observations by SEM reveal that the grain size of ZrB2-SiC with Y2O3 and La2O3 composites are less than the sample without additives, which indicates Y2O3 and La2O3 may restrain the grain growth and increase the fracture toughness. The fracture toughness of ZrB2-SiC composites with Y2O3 and La2O3 reached 5.0MPa·m1/2 and 5.5MPa·m1/2 respectively. Therefore, the additive Y2O3 and La2O3 are very effective as sintering aids for the ZrB2-SiC composite.

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: ZrB2-SiC ultra-high temperature ceramics (UHTCs) were pressureless sintered with Y2O3-Al2O3 as the sintering additives. The effects of sintering additive and crystallization annealing on the microstructure and properties of ZrB2-SiC UHTCs were investigated. Sintering was activated by producing liquid phase of Y2O3 and Al2O3. The relative density of sintered ZrB2-20wt%SiC ceramic could reach 96% when the content of sintering additive was 6% and the sintering temperature was 1750°C and its bending strength, Vickers hardness, and fracture toughness were 412 MPa, 13 GPa, and 6.0 MPa•m1/2, respectively. The crystallization annealing can result in YAG phase from grain boundary and enhance the high temperature properties of the UHTCs. The UHTCs have excellent ablation resistance at ultra-high temperatures, and a very low ablation rate of 0.0006 mm/s after ablation for 900s at 2800°C.

Abstract: An oxidation kinetics model of hot-pressed ZrB2-SiC composites was established and then proved by experiments. The relationship between oxidation rate and temperature was attained by reaction activation energy and frequency factor at the different oxidation stages. Results showed that the relations between the oxidation gain of unit area and the oxidation time are linear, conic and parabolic, respectively, during the stage of the oxidation prophase, midterm and anaphase.

Abstract: The oxidation behavior of ZrB2-20 vol.% SiC (ZS) and ZrB2-20 vol.% SiC containing 20 vol.% short carbon fiber (ZSC) was studied using thermal gravimetric analysis and oxyacetylene torch test. It was shown that weight gains changed from 3.71 wt.% for ZS to 4.57 wt.% for ZSC after heating 10°C /min to 1450°C in air. A thin layer of Si-rich glass and then a depletion layer of SiC was found on the cross section of both materials and carbon fiber of ZSC exposed in air was oxidized. Under oxyacetylene, an average mass loss of 0.8 wt.% for ZS and 0.9 wt.% for ZSC was measured after 180 seconds. After exposure, an oxidized layer with the formation of ZrO2 and SiO2 was found on the surface of both materials. Meanwhile, fiber in the surface of ZSC appeared oxidized and removed.