Advanced Materials Research Vols. 105-106

Abstract: High strain rate superplastic forming is the key issue of the industrial application of superplastic ceramic and is also an effective approach to achieve “near-net shaping” of ceramic parts with complicated shape. The sol-gel method was adapted to synthesize the nano-sized composite powders. A three-phase nanocomposite ceramic Al2O3-30mol%YSZ-30mol%MgAl2O4 (AZ30S30) was prepared by using hot-pressing sintering in vacuum. Then the superplastic forming of a ceramic part with relatively complicated shape was accomplished and some new characters in superplastic forming were studied. The results demonstrate that as-sintered ceramic is a typical inter/intra granular nano-structured composite. Because the dispersed second phase inhibited the growth of the alumina matrix grains in deformation as well as in sintering, the material behaves excellent superplastic formability. The extrusion forming at a high strain rate was achieved and the part in cup shape was obtained. The yttrium element segregated obviously at the grain boundary after deformation, which demonstrated that the yttrium element directionally diffused during the deformation. The mechanical properties of the ceramic did not change much after extrusion. The maximum hardness of as-deformed composite ceramic is 32GPa, a litter lower than that of 35GPa before deformation.

Abstract: Cr2AlC ceramics (Cr:Al:C =1:1.2:1 mol.%) were synthesized by powder metallurgical method in argon in the temperature range of 700 to1250°C using Cr, Al and graphite powders as the initial materials. The phase relationships during reaction in the ternary system Cr-Al-C were investigated. The products were characterized by X-ray diffraction (XRD) and differential thermal analysis (DTA). It was found that Cr9Al17, Al8Cr5 and Cr2Al intermediate phases were formed in turn with increase of temperature. Up to 1050°C, with consumption of Cr9Al17 and Al8Cr5 phases completely, the amount of Cr2Al increased significantly. Single phase Cr2AlC with small amount of Cr7C3 was produced until 1250°C. Combined with the results of differential thermal analysis (DTA) and X-ray diffraction (XRD), it is revealed that Cr2AlC phase is formed by the reaction of Cr–Al intermetallic compounds, Cr, Al and graphite. In addition, the reaction equations of the process from 660 to1250°C were given.

Abstract: Ti5Si3 matrix composites reinforced by carbon nanotubes were fabricated by vacuum hot pressing sintering. X-ray diffraction and scanning electron microcopy were carried out to analyze the phase and microstructure of the composites. The effects of carbon nanotubes on mechanical properties were investigated. Experimental results showed that the nanotubes partly reacted with Ti and Si powders to obtain Ti5Si3 and Ti3SiC2, TiSi2 when the sintering temperature is about 1380oC. The mechanical properties of composites can be affected by carbon nanotubes. Meanwhile, the maximal increments of Vickers hardness, bending strength and fracture toughness of the composites, compared with the Ti5Si3 matrix, were 62.9%, 160.9% and 159.3%, respectively. Both of transgranular and intergranular fracture in the composites were the main fracture mode. The fracture manners of composites mainly include “bridging” of CNTs, “deflection” of minor phases and the evolution of grain.

Abstract: ZrB2-SiC and ZrB2-SiC-C ultra-high temperature ceramics (UHTCs) were fabricated by pressureless sintering under an argon atmosphere. The mass and linear ablation rates were tested in an oxyacetylene flame with high velocity. The microstructure and phase transformation of the ZrB2-based UHTCs were characterized by scanning electron microscopy along with energy dispersive spectrometry. Results show that the UHTCs have excellent properties of ablation resistance at ultra-high temperature. The values of mass and linear ablation rates were lower in the ZrB2-SiC UHTCs than those measured for ZrB2-SiC-C. The effect of C addition on the ablation resistant was not obvious but it influenced the microstructure of the ZrB2-SiC UHTCs. And the ablation resistant mechanisms of ZrB2-based UHTCs were discussed according to microstructure analysis.

Abstract: ZrB2 powder has been prepared through carbothermal reduction boronization of zirconia/boron carbide/carbon mixtures heating assisted by microwave. The powder characteristics were investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen absorption (BET model) and scanning electron microscope (SEM). The experiments indicated that excessive B4C is necessary and the carbothermic reaction reacts severely at a higher temperature and complete at 1600oC. The crystallite size has ranged from 50-100 nm, according to the calculated surface area. Highest purity of ZrB2 powder, which was synthesized at 1600oC, is 99.67 wt%. The surface area of ZrB2 powder synthesis at 1600oC is 18.33 m2/g. Vibration of temperature should affect the purity of ZrB2, as the sub reaction acted.

Abstract: Fully dense and single-phase Ti3AlC2 bulk ceramic was successfully fabricated by a high energy milling and hot-pressing with Ti, C and Al as starting materials. The details of reaction and phase evolution in high energy milling and hot pressing sintering were investigated. The results show that most of the starting materials transform to Ti-Al intermetallics and TiC in high-energy milling. The as milled powders react into Ti2AlC firstly in hot pressing sintering. When the temperature raise high enough, the Ti2AlC transform to Ti3AlC2. The content of aluminum is higher than that in the theoretical composition, which is attributing to the volatilization of Al at high temperature.

Abstract: The parameters, such as thermal expansion coefficient and Young's modulus, are considered as a function of temperature and incorporated into ANSYS code to calculate the stress field of UHTCs under high temperature condition in the present work. The stress fields of two kinds of heating cases are calculated and compared. By establishing the relation between the temperature and the mechanical properties of the UHTCs, it is found that the mechanical behavior of UHTCs is strongly affected by the oxide film thickness, initial temperature and the heating rate.

Abstract: Ultra-fine ZrB2 powders and ZrB2-SiC composite powders were synthesized by solution-based method, using zirconium oxynitrate (ZrO(NO3)2•2H2O), boric acid (H3BO3), phenolic resin, and tetraethyl orthosilicate (TEOS) as raw materials, respectively. The zirconia sol with other precursor was formed via adjustment of the pH value. After drying, grinding and sieving, the precursor powders were obtained. Exposed to carbothermal reduction heat treatments (1300°C~1500°C, 0.5h, flowing argon), pure ZrB2 powders and ZrB2-SiC composite powders with fine particle size (~1μm) were synthesized.

Abstract: ZrB2-SiC-B4C is sintered at 1700°C by spark plasma sintering process. The effect of B4C content on the mechanical properties and microstructure of ZrB2-SiC based ceramics is studied. The results show that, with the content of B4C increases, the relative density and room-temperature strength decrease in the ZrB2-SiC-B4C composite. The fracture toughness rises at first and then falls down. The high temperature strength increases.

Abstract: Fully dense Ti3SiC2-64vol.%SiC(0.5m) composite materials are successfully fabricated by in situ synthesis under hot isostatic pressing. The high temperature flexure, tension and creep of Ti3SiC2-SiC composites were studied, and the results indicated that the composite had a high tensile strength under high temperature. Above 1000°C the composite were displayed a good high temperature plastic. The creep behavior is characterized by two regimes: an initial transient regime, where έ(dε/dt) decreases with t (time); a secondary creep regime in which έ is more or less constant with time.