Search

Phase stability of Ti3SiC2 was studied under high pressure and high temperature using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. From the results obtained, the decomposition temperature of Ti3SiC2 decreases quickly against pressure, and the low temperature limits of phase segregation of the sample Ti3SiC2 lie between 1100°C and 1000°C, 1000°C and 900°C, 900°C and 800°C, under high pressures of 3, 4 and 5GPa, respectively. Ti3SiC2 decomposes to generate TiC, SiC, and TiSi x . On the basis of the experimental results, we suggest two decomposition models to explain the phase decomposition of Ti3SiC2 at high pressure and high temperature. [ABSTRACT FROM AUTHOR]

The cold compression behavior of alumina samples, with average grain sizes of 350 nm, 7.9 μm, and 18.2 μm, has been investigated under non-hydrostatic conditions by using a diamond anvil cell. After treatment with various pressures, the in situ axial X-ray diffraction patterns, transparency variations, and microstructures after decompression of the samples were also analyzed. The results showed that the submicron grains became easily translucent. The larger the grain size, the higher the pressure needed to compress the sample to transparency. On the other hand, for a similar grain size, the various geometrical shapes of the grains did not affect the pressure dependency on transparency. The elastoplastic characteristics of the alumina grains of different sizes under cold compression were found to be consistent with the transparency variation. As a conclusion, alumina particles under cold compression were found to be applicable in the pressure-assisted preparation of alumina transparent ceramics. [ABSTRACT FROM AUTHOR]

Herein, we report transparent LaGdZr 2 O 7 nano-grained ceramics with an average grain size of 10 nm prepared using the high-pressure sintering method at 5.0 GPa/450 °C. These ceramics showed improved plastic deformation as compared to transparent micro-grained LaGdZr 2 O 7 ceramics. Moreover, the fracture toughness of the nano-grained ceramics reached up to 3.4 MPam1/2, which is 79 % higher than that of the micro-grained samples. This improvement in the fracture toughness of the nano-grained ceramics was associated with the presence of a large number of weak grain boundaries in them. [ABSTRACT FROM AUTHOR]

High-hardness TaN compacts with a diameter and height of 6 mm were synthesized through the phase transformation of CoSn-type into WC-type TaN under experimental conditions of high temperature and high pressure. The Vickers hardness of WC-type TaN compacts obtained at the pressure of 5 GPa and the temperature of 1873 K is found to be ~21.5 ± 1.5 GPa with a load of 3 Kg, which is comparable to that of the pure tungsten carbide compact (~20 GPa) and higher than those of ultrahigh temperature ceramics ZrB 2 and HfB 2 under the same load. The high hardness of WC-type TaN compacts presented in this work is attributed to the deviatoric strain and well-bonded nanograins. The synthesized WC-type TaN compacts also possess a high oxidation temperature (1072 K). These results suggest that the WC-type TaN with high hardness and high thermal stability holds great promise for industrial applications. [ABSTRACT FROM AUTHOR]

The in situ axial X-ray diffraction patterns of four ceramic powder samples (MgO, Al 2 O 3 , AlN, and cBN) that were compressed in a diamond anvil cell under uniaxial non-hydrostatic conditions were recorded. The microscopic deviatoric stress as a function of the pressure was determined from the X-ray diffraction peak broadening analysis: the curves increased approximately linearly with the pressure at the initial compression stage and then levelled off under further compression. Pressure-induced transparency was observed in all of the samples under compression, and the pressure at the turning point on the curves of the microscopic deviatoric stress versus pressure corresponded to the pressure at which the samples became transparent. Analysis of the microstructural features of the pressure-induced transparent samples indicated that the compression caused the grains to fracture, and the broken grains bonded with each other. We demonstrated that the ceramics' pressure-induced transparency was a process during which the grains were squeezed and broken, the pores were close between the grains, and the broken grains were re-bonded under compression. [ABSTRACT FROM AUTHOR]

We report a method for strengthening microcrystalline alumina compaction by work hardening under conditions of high pressure and temperature, in which the hardness significantly improved. Micro-sized spherical alumina powders were treated without additives under 5.5 GPa–14 GPa and 600 °C–1200 °C. A polycrystalline sample sintered at 12 GPa and 900 °C yielded a Vickers hardness of 30.0 GPa, which was 1.5 times that of single-crystal alumina and the highest value reported to date for alumina. By analysing the density, hardness, and microstructure of the sintered samples, the results demonstrated that under ultra-high pressure and below the recrystallisation temperature of alumina, plastic deformation of the grains produced many substructures such as stacking faults and lattice distortions showing obvious work hardening effects. Combined with well-bonded crystalline and amorphous grain boundaries, the hardness of the samples was unexpectedly enhanced. [ABSTRACT FROM AUTHOR]

Creep behavior at room temperature was firstly observed in nanocrystalline Gd 2 Zr 2 O 7 ceramic (NC-GZO) having an average grain size about 7.8 nm. The creep strain rate sensitivity of NC-GZO was calculated, and many dislocations can be observed in the strained region. The mechanism causing this room temperature creep behavior was associated with the dislocations slipping which most likely occurs though grain boundaries sliding. [ABSTRACT FROM AUTHOR]

Studying the mechanical and thermal properties as well as the relationship between microstructure evolution and strengthening mechanisms is crucial for obtaining superior high-temperature refractory molybdenum disilicide-silicon carbide composites. In this study, molybdenum disilicide and silicon carbide powders with different volume ratios were used to fabricate molybdenum disilicide-silicon carbide composites via high pressure and high temperature at 5.5 GPa/1773 K. The introduction of the second phase improved the materials' mechanical properties. Transmission electron microscopy demonstrated that severe plastic deformation generated numerous micro-defects and grain refinement during the high pressure sintering process. The second phase enhancement also improved the samples' performances. Under high pressure and high temperature sintering process, the thermal and mechanical properties of the materials increased dramatically, including a well Vickers hardness (~20.6 GPa), fracture toughness (~10.3 MPa·m1/2) and thermal stability (>1673 K). [ABSTRACT FROM AUTHOR]

Cubic boron nitride (cBN), a typical superhard material, has garnered significant interest for both fundamental scientific research and technical applications. Here, nanostructured cBN with high-density nanotwins and stacking faults was synthesized by rapid quenching using hexagonal boron nitride (hBN) as the starting material at a moderate temperature (2200 K) and low pressure (13 GPa). This synthetic nanostructured cBN exhibits high Vickers hardness (78 GPa), high fracture toughness (11.8 MPa·m1/2), and extremely high oxidization temperature (1565 K). We propose that the refined grain structure and excellent mechanical properties of the cBN synthesized in this study is attributed mainly to dislocation slips and (111) mechanical nanotwins that dominate this phase transformation. [ABSTRACT FROM AUTHOR]

High-performance polycrystalline cubic boron nitride (PcBN) was sintered without binders at 1500 °C in a pressure range from 11 to 15 GPa using commercial micrometre cubic boron nitride (cBN) with a diameter of approximately 2–4 μm. The results demonstrated that the sample sintered at 12 GPa and 1500 °C had the best mechanical properties and thermal stability. Its average Vickers hardness, fracture toughness, and thermal stability was 63 GPa, 15 MPa m1/2, and 1315 °C, respectively. The considerable improvement in the mechanical properties was mostly attributed to the high compactness, close bonding between grains, and the sample's internal defect structures. The relatively small specific surface area of the micron grains provides an advantage due to its high thermal stability. The amorphous regions observed in the sample's local areas may provide a new strengthening mechanism under high pressure and temperature. [ABSTRACT FROM AUTHOR]

Translucent nanocrystalline Gadolinium Zirconate (Gd 2 Zr 2 O 7) ceramic with grain size below 10 nm was successfully fabricated using high pressure sintering technology at 5 GPa and low temperature of 550 °C. The bilinear compressive stress-strain relationship of this sample was firstly extracted from nanoindentation procedure based on the partial unloading method. It indicated that the plastic deformation did not originate from dislocation sliding and was controlled by the grain boundary sliding. Moreover, the compressive yield strength, σ y = 7.8 GPa of the translucent nanocrystalline Gd 2 Zr 2 O 7 ceramic was also determined by the intersection of the theoretical elastic line and the indentation compressive stress-strain line. [ABSTRACT FROM AUTHOR]

High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al 2 O 3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure. [ABSTRACT FROM AUTHOR]

Abstract High-pressure strengthening is reported to be an effective approach to strengthen materials in addition to traditional nanocrystalline strengthening. Numerous studies have proven that high pressure is an important factor for improving the mechanical properties of materials. Here, we sintered high-purity sub-micron tungsten carbide (sm-WC) ceramics without any additives under a high pressure of 10 GPa and temperatures ranging from 1000 °C to 1500 °C. Numerous microdefects (stacking faults and twins) were introduced inside the grains, and the Vickers hardness of the well-sintered WC bulks reached a high value of 33 GPa, which could be compared with that of single crystal and nanocrystalline counterparts. Thus, the high-pressure strengthening can be applied to the WC system and improves the mechanical properties. The strengthening mechanism is that the high pressure can make the grains take place bulk yield, promote a severe plastic deformation, and introduce numerous microdefects. These defects act as obstacles against dislocations glide during deformation, and thus improve the mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract A series of pure HfB 2 ceramics have been prepared by sintering nano-grained powder using high-energy ball milling at 700–1600 °C and 5.5 GPa. The HfB 2 ceramics are characterized via various techniques for their residual stress, grain size, density, microstructures and defects, hardness, fracture toughness, thermal stability, and oxidation resistance. All properties strongly depend on the treatment temperature, but the exact manner of dependence for each property varies. The results identified that the HfB 2 ceramic sintered at a relatively low temperature of 1000 °C and 5.5 GPa – a bulk pure nano-grained composite for the first time – has the best overall performance. It has a relative density of 99.6%, a Vickers hardness of 26 GPa, a fracture toughness of 5.2 MPa m1/2, and excellent thermal stability and oxidation resistance at high temperatures. Additional strengthening and stabilizing effects are provided by microstructures and defects such as large-angle grain boundaries, stacking faults and twinning. Simultaneous high temperature and high pressure is an effective sintering route for HfB 2 ceramics with grain-size ranging from nanometer to micron. [ABSTRACT FROM AUTHOR]