Reactive Hot-pressing Process Research Articles

Tribological Property of Al3BC3 Ceramic: A Lightweight Material

Lightweight materials with a density less than 3 g/cm3 as potential tribo-materials for tribological applications (e.g., space tribology) are always desired. Al3BC3 ceramic, a kind of ternary material, is one of the lightweight materials. In this study, dense Al3BC3 ceramic is prepared via a reactive hot-pressing process in a vacuum furnace. Its tribological properties are investigated in two unlubricated conditions (one is at elevated temperature up to 700 °C in air, and another is in a vacuum chamber of back pressures from 105 Pa to 10−2 Pa at room temperature) and lubricated conditions (i.e., water and ethanol as low-viscosity fluids). At 400 °C and lower temperatures in air, as well as in vacuum, the tribological property of Al3BC3 ceramic is poor due to the fracture of grains and formation of a mechanically mixed layer. The beneficial influence of adsorbed gas species on reducing friction is very limited. Due to the formation of lubricious tribo-oxide at 600 °C and 700 °C, the friction coefficient is reduced from ca. 0.9 at room temperature and 400 °C to ca. 0.4. In the presence of low-viscosity fluids, a high friction coefficient and wear but a polished surface are observed in water, while a low friction coefficient and wear occur in ethanol. A lubricious carbide-derived carbon (CDC) coating on top of Al3BC3 ceramic through high-temperature chlorination can be fabricated and the wear resistance of CDC can be improved by adjusting the chlorination parameters. The above results suggest that Al3BC3 ceramic is a potential lubricating material for some tribological applications.

Reactive hot pressing of SiC-ZrC composites at low temperature

SiC-ZrC composites with relative density in excess of 99% were prepared by reactive hot pressing (RHP) of SiC and ZrH2 at 1800°C for 1h. The reaction between SiC and ZrH2 resulted in the formation of ZrC1-x. The formation process and densification behavior during RHP process were investigated. Low temperature densification of SiC-ZrC composites is attributed to the formed nonstoichiometric ZrC1-x and the removal of SiO2 impurity on the surface of SiC particles. As reinforced phase, ZrC1-x has inhibiting effect on the abnormal grain growth of SiC, resulting in homogeneous microstructure of fine SiC grains. Adding 10wt% ZrH2 to SiC, the formed SiC-4.62vol% ZrC composite exhibited better mechanical properties (Vickers hardness of 27.6±0.7GPa, flexure strength of 448±38MPa, fracture toughness of 6.0± 0.3MPa·m1/2, respectively) than monolithic SiC ceramic.

Microstructure and mechanical properties of TiB2–SiC ceramic composites by Reactive Hot Pressing

TiB2–SiC ceramic composites with different contents of Ni as additive were prepared by the Reactive Hot Pressing (RHP) process at 1700°C under a pressure of 32MPa for 30min. For comparison, a monolithic TiB2 ceramic and TiB2–SiC ceramic composite were also fabricated under the identical temperature, pressure and holding time by the Hot Pressing (HP) process. The effects of the fabrication process and Ni on the microstructure and mechanical properties of the composites were investigated. About 8vol.% of elongated TiB2 grains with an aspect ratio of 3–6 and a diameter of 0.5–1μm were produced in the composite prepared by the RHP process. The improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC particles and the elongated TiB2 grains such as crack deflection. The TiB2–SiC–5wt.% Ni ceramic composite had the optimum mechanical properties with a flexural strength of 858±87MPa, fracture toughness of 8.6±0.54MPa·m1/2 and hardness of 20.2±0.94GPa. The good mechanical properties were ascribed to the relatively fine and homogeneous microstructure and the strengthening effect of Ni. Ni inhibited the anisotropic growth of TiB2.

Microstructure and mechanical properties of ZrB2–SiC–BN composites fabricated by reactive hot pressing and reactive spark plasma sintering

ZrB2–SiC–BN composites were fabricated from a powder mixture of ZrH2, Si3N4 and B4C via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS). The phase composition, density, microstructure and mechanical properties of the final products were studied and compared. The sample fabricated by the R-SPS process showed higher density, flexural strength and fracture toughness and had a more homogeneous microstructure compared to the one fabricated by the RHP process.

Friction and wear properties of reactive hot-pressed TiB2–TiN composites in sliding against Al2O3 ball at elevated temperatures

TiB2–TiN composites with different TiB2:TiN molar ratios of 2:1 and 1:1 were synthesized by the reactive hot-pressing process using Ti, B and BN powders as raw materials. Friction and wear properties of reactive hot-pressed TiB2–TiN composites were evaluated in sliding against Al2O3 ball from room temperature to 700 °C in air. For TiB2–TiN composites, typical break-in stage in evolution of friction coefficients is observed at room temperature. At 700 °C, the friction coefficient of TiB2–TiN composites is relatively low and steady, as contrasted with the data obtained at room temperature and 400 °C. Under all testing conditions, TiB2–TiN composites exhibit small wear rates, which range in the order of 10−6–10−7 mm3/Nm. The specific wear rates of TiB2–TiN composites and the coupled ball decrease with testing temperature. Wear mechanisms of TiB2–TiN composites depend mainly upon testing temperature. At room temperature, mild abrasion is the dominated wear mechanism of TiB2–TiN composites. However, tribo-oxidation as well as mild abrasion plays an important role during wear test at 400 °C. At 700 °C, continuous oxidized films comprised of rutile form on the surface by thermal oxidation, and provide excellent lubrication and protection.

Synthesis, mechanical and physical properties of bulk Zr 2Al 4C 5 ceramic

Bulk Zr 2Al 4C 5 ceramics was synthesized by an in situ reactive hot-pressing process using ZrC, Al and carbon black as staring materials. The reaction path of synthesis was discussed based on differential thermal analysis, Raman spectra and X-ray diffraction results. Compared with the reported Zr 2Al 3C 4, Zr 2Al 4C 5 has a relatively high hardness (Vickers hardness of 12.2 GPa), superior strength (flexural strength of 420 MPa) and lower electrical resistivity (1.05 μΩ m). In addition, the room-temperature thermal conductivity value, 14.6 W (m K) −1 for Zr 2Al 4C 5, was smaller than the value for Zr 2A 3C 4 ceramics (15.5 W (m K) −1), and the heat capacity of Zr 2Al 4C 5 is much higher than that of Zr 2A 3C 4 in the overall temperature range. The results here indicated that Zr 2Al 4C 5 is a potential high-temperature structural ceramic.

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.

Nano-Al<sub>2</sub>O<sub>3</sub> Reinforced Fe-Cr-Ni Composites Fabricated by Reactive Hot Pressing

Based on low-temperature hot-press sintering and rapid thermit reaction, heat-resistant metal matrix composites with nano-ceramic reinforcement were prepared via reactive hot pressing. According to XRD, the composites comprised predominantly of (fcc) Cr0.19Fe0.7Ni0.11, (fcc) Fe-Cr and alumina at 700°C through the highly-exothermic thermit reaction between the starting powders. Three-point bending strength, fracture toughness, Vickers hardness and relative density increased with the increase of hot-press sintering temperature and holding time. The improving mechanical properties may be explained by increasing of content of (fcc) Cr0.19Fe0.7Ni0.11. SEM analysis showed a microstructure consisting of equiaxial granules at 700°C for 1 h and a uniformly dispersed network of very fine grains at 700°C for 2 h. It is considered that, in the reactive hot-pressing process, Al atoms diffused into the metal matrix (Fe2O3, Cr, Ni) sites and formed Al2O3 and Fe-Cr-Ni matrix. Such a technique offers the possibility of synthesizing heat-resistant metal matrix composites with nano-ceramic reinforcement materials at considerably lower temperature.

High-resolution electron microscopy study of interfacial structure in TiB2[sbnd]Ti0·9W0·1C[sbnd]SiCin-situcomposite

Abstract The ultrafine structure of multiphase ceramics TiB2Ti0·W0·C[sbnd]Sic formed by the in-situ reactive hot-pressing process has been characterized using high-resolution electron microscopy (HREM) and analytical electron microscopy. The results reveal that TiB2 crystallized into plate-like shapes, containing Tic plate precipitates. Three different orientation relationships between Tic precipitates and the TiB2 matrix were determined and the formation mechanism was proposed. Crystalline intergranular phase containing Fe, Si and Ti elements were formed at grain edge intersections. Along the TiB2 plates, the intergrowth of 6H-SiC bands were often found. An orientation relationship between the 6H-SiC and TiB2, in which [1120]TiB2//[1120]Sic and (0001)TiB2//(0001)SiC, was found and the atomic structures at the TiB2/6H[sbnd]SiC interface have been investigated by HREM combined with image simulations of several proposed models. Comparison of simulated images and the experimentally observed image resulted into an optimal structure model with Ti[sbnd]B[sbnd]C[sbnd]Si stacking sequence at the interface.