ZrB2 SiC ZrC Research Articles

In Situ Synthesis of a ZrB2‐Based Composite Powder Using a Mechanochemical Reaction for the Zircon/Magnesium/Boron Oxide/Graphite System

A ZrSiO4/B2O3/Mg/C system was used to synthesize a ZrB2‐based composite through a high‐energy ball milling process. As a result of the milling process, a mechanically induced self‐sustaining reaction (MSR) was achieved in this system. A composite powder of ZrB2–SiC–ZrC was prepared in situ by a magnesiothermic reduction with an ignition time of approximately 6 min. The mechanism for the formation of the product was investigated by studying the relevant subreactions, the stoichiometric amount of B2O3, and thermal analysis.

Mechanochemical synthesis of ZrB2–SiC–ZrC nanocomposite powder by metallothermic reduction of zircon

Aluminium and magnesium were used in the M/ZrSiO4/B2O3/C (M=Al, Mg) system to induce a mechanically induced self-sustaining reaction (MSR). Aluminium was not able to reduce the system to the desired products, and the system became amorphous after 10h milling. However, nanocomposite powder of ZrB2–SiC–ZrC was in situ synthesized by the magnesiothermic reduction with an ignition time of approximately 6min. The mechanism for the formation of the product in this system was determined by studying the relevant sub-reactions.

Effect of ZrO2 impurity on promoting reactive sintering of ZrB2–SiC–ZrC composites

ZrO2 impurity phase was examined by means of X-ray diffraction and scanning electron microscopy in reactive-sintered ZrB2–SiC–ZrC composites hot-pressed at 1600°C and 1800°C. An analytical electron microscopy study further revealed solid-solution of both Zr and O in the β-SiC phase: Zr:Si ratios were measured to be 1.83 ± 0.17 × 10−2 and 1.91 ± 0.16 × 10−2 respectively for each sample, while O:Si ratios were in the range of 2.3 ∼ 2.8 × 10−2. Common co-existence of ZrO2 grains and SiC grains containing Zr–O solutes indicates that this oxide plays a significant role in the sintering process, which substantially modifies the reactive phase relations. Observation of an intergranular Si–Zr–C–O glassy phase suggests the presence of a similar sintering liquid as a fast or even dominant diffusion path to promote the reactive sintering.

S042011 2000K以上での使用を目的としたZrB_2-SiC-ZrCの耐酸化性評価([S042]セラミックスおよびセラミックス系複合材料)

ZrB_2-SiC-ZrC(ZSZ) with four different compositions containing 16 vol% SiC were sintered by hot pressing and spark plasma sintering. In this study, ZSZs were oxidized in the ultra-high temperature range by two different tests. Firstly, ZSZs were oxidized at 1973 K in an airflow by an IR image furnace for several minutes. The result showed that ZSZs have oxidation resistance in this temperature range due to formation of oxide scale which acts as a barrier against oxygen diffusion into the unoxidized area. Moreover, ZSZ containing more ZrC formed ZrO_2 rich scale, whereas ZSZ containing more ZrB_2 formed SiO_2 rich scale. These results suggested that the highest oxidation resistance would berealized if the silica scale is formed with ZrO_2 rigid skelton. In the next step, ZSZ, ZrB_2-SiC and monolithic SiC were oxidized continuously up to 2073 K in air by a ZrO_2 furnace to compare their oxidation resistance. The results revealed that oxide scale disappeared from monolithic SiC surface, whereas ZSZ and ZrB_2-SiC maintained their oxide scaleduring the oxidation test. Comparing ZSZ with ZrB_2-SiC, ZSZ formed silica scale with ZrO_2 skelton, whereas no ZrO_2 skelton was formed in the silica scale of ZrB_2-SiC. The IR image furnace test implied that it is important to have not only high oxidation resistant SiO_2 layer but also coexistence rigid oxide scale. Therefore, it is suggested that ZSZ is more attractive candidate for the advanced aerospace heat-resistant material than SiC and ZrB_2-SiC.

Effect of the surface oxidation on the flexural strength of the ZrB2–SiC–ZrC ceramic

► Discussing effect of surface oxidation on strength of ZrB 2 -based ceramics. ► Analyzing mechanism of increase in strength of ZrB 2 -based ceramics. ► Proposing a promising method for improving strength of ZrB 2 -based ceramics. The isothermal oxidation of the ZrB 2 –SiC–ZrC ceramic was carried out at a constant temperature of 1000 ± 2 °C, 1200 ± 5 °C and 1400 ± 5 °C, respectively, for different times in static air. The surface oxidation was favorable to increase the flexural strength of the ZrB 2 –SiC–ZrC ceramic because the flaws on the surface of the specimen were healed by the glass layer. The distribution of strength significantly decreased as the oxidation time increased in spite of the oxidation temperature. The analysis on the mechanism of the strength increase indicated that the strength increased as the reaction rate increased, and the increase in strength was related to change rate in volume induced by reaction, the initial crack geometry, elastic modulus and the surface free energy and so on.

S042011 超高温域におけるZrB_2-SiC-ZrCの酸化メカニズムの解明

ZrB_2-SiC-ZrC (ZSZ) ceramics with four different compositions containing 16 vol% SiC were sintered by hot press and spark plasma sintering. The relative density of all the ZSZ ceramics made by both methods reached around 95%. These ZSZs oxidized continuously up to 1500℃ using thermo gravimetric analysis. The ZSZ containing the highest ZrB_2 indicated the lowest weight gain, and that containing the highest ZrC indicated the highest weight gain. After the oxidization, the microstructure of the samples' surface and cross section were studied by X-ray diffraction analysis, optical microscope and field emission scanning electron microscope along with X-ray dispersive spectroscopy. Results of these analyses for the samples revealed that ZSZs have oxidation resistance due to formation of the oxide scale, which act as the barrier for oxygen diffusion into the unoxidized substrate. In the next study, ZSZ ceramics was heated continuously up to 1700℃ in air using zirconia ultra high temperature furnace. Oxidation test was earned out at 1700℃ by IR image furnace. The surface of the oxidized specimen had a number of bubbles and appeared to be covered with zirconia and glassy compounds. These results suggest that the active oxidation of SiC was occurred at the interface between the oxide scale and the substrate. In addition, because the spallation between the oxide scale and the substrate was not occurred, it was implied that ZSZs showed the resistance to the active oxidation of SiC.

Repeated thermal shock behavior of the ZrB2–SiC–ZrC ultrahigh-temperature ceramic

The one thermal cycle of the heating was carried out at maximal temperature of 1800±15°C in 5s using a resistance heating method with a large current and low voltage, and then the power supply was shut off and the specimen was naturally cooled to room temperature. According to the energy dispersive spectroscopy (EDS) analysis and scanning electron microscopy (SEM) observations, the surface microstructure of the specimen after the repeated thermal shock composed of: (1) a layer of SiO2-rich+ZrO2, (2) a layer of ZrO2+ZrB2 and (3) unoxidized substrate. The flexural strength of the specimen after the thermal shock of 10, 20 and 30cycles were 650MPa, 684MPa and 675MPa, respectively, which were obviously higher than the flexural strength of 580MPa before the thermal shock due to the formation of the SiO2-rich+ZrO2 layer and the compressive stress in the oxide layer. The flexural strength of the specimen after the thermal shock of 50cycles decreased to be 427MPa that was significantly lower than 580MPa before the thermal shock because of the presence of a large numbers of pores in oxide layer and through holes in the surface.

Reactive Hot Pressing of ZrB2–SiC–ZrC Composites at 1600°C

A ZrB2–SiC–ZrC ceramic was produced by reactive hot pressing using Zr, Si, and B4C as raw materials. The kinetics of the reaction process was studied. The reduction of powders by ball milling is of crucial importance for the sintering. The self‐propagating high‐temperature synthesis reaction between the raw powders can be ignited by controlling the sintering conditions, which leads to a sintering temperature as low as 1600°C, the lowest sintering temperature reported thus far. The relative density is 97.3%, with an open porosity of 0.6%, and the mechanical properties are comparable to the composites that sintered at higher temperatures. The depletion of oxygen impurities during the sintering was discussed.

Synthesis and microstructural features of ZrB 2–SiC-based composites by reactive spark plasma sintering and reactive hot pressing

Four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2–SiC–AlN, were synthesized in situ via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), using Zr, Si, B4C, BN and Al as raw materials. The synthesis process plays a critical role in the microstructural features of the composites obtained. The R-SPS process can lead to a more homogeneous and finer microstructure due to its high heating rates and short holding time, while the RHP process is likely to result in coarse microstructures due to a long enough holding time for grains growth.

Reactive Hot Pressing of ZrB2–SiC–ZrC Ultra High‐Temperature Ceramics at 1800°C

A ZrB2–SiC–ZrC composite was prepared from a mixture of zirconium, silicon, and B4C via reactive hot pressing at a relatively low temperature (1800°C) for 60 min under 20 MPa in an argon atmosphere. The relative density was 96.8%, the micro‐hardness (Hv10) was 16.7 GPa, and the fracture toughness was 5.1 MPa·m1/2. The presence of ZrC was helpful for the densification process and improved the mechanical properties of the composite. A model of the microstructure development of the composite was proposed to explain the phase distribution.