Papers by Author: Long Zhang

Abstract: This article presents the results for the prestress of ceramics lateral confined by aluminum alloy. Prestressed ceramics composite is produced by casting molten aluminum alloy around a hot ceramic tile and then cooling it down with blowing air. Different technical parameters of the casting are investigated. Al2O3/ZrO2(4Y) composite ceramics alumina tiles prepared by combustion synthesis under high gravity are used in this study to investigate the effect of confinement and prestress. The samples of ceramics are lateral confined by aluminum alloy, which lead to monaxial compressive stress in the tile and tensile stress in the metal. Thin and thick target configurations are used to understand the effect on pre-stressing performance, which simply and approximately allows high monaxial compressive pre-stresses. The residual stress in the metal should be lower than the yield strength of the metal, and this requirement can be approximately realized by Vickers indentation crack length.

Abstract: By introducing self-pressure processing into combustion synthesis under high gravity, Al2O3-ZrO2(4Y) eutectic composite ceramics without shrinkage cavities and holes were achieved, and the self-pressure processing also made the macro-crack be controlled and the ceramic densification be promoted evidently. XRD results showed the eutectic ceramics were composed of α-Al2O3, t-ZrO2 and a few m-ZrO2 phases, and the volume fraction of the transformable t-ZrO2 increased with self-pressure force increasing. SEM images showed that with increasing self-pressure force, the rod-shaped eutectic colonies were refined and the volume fraction of the colonies increased. Meanwhile, the irregular ZrO2 crystals around eutectic colonies transformed to be fine t-ZrO2 spherical crystals gradually, and thickness of eutectic colonies decreased. With the changes in microstructures of the ceramics, the mechanical properties of the ceramics were improved greatly.

Abstract: Large bulk solidified TiC-TiB2 eutectic composite ceramics were prepared by combustion synthesis under high gravity, and the WO3 was introduced into the combustion synthesis as one of oxidants in thermit to obtain the Ti-W-Cr-C-B liquid, so near-full-density TiB2-(Ti,W)C eutectic composite ceramics without the macrocracks were achieved. The ceramic matrix was mainly composed of TiB2-(Ti,W)C eutectic microstructures, and a few of Al2O3 and Al2O3-ZrO2 eutectic structures were also detected in between Ti-W carbides. Due to the introduction of the high gravity field, Stocks immigration of the immiscible liquids took place due to their density differences, resulting in float-up of oxide liquid and settle-down of Ti-W-Cr-C-B liquid, and the layered melt consisting of oxide liquid and Ti-W-Cr-C-B liquid was formed, finally, TiB2-(Ti,W)C eutectic composite grown from the melt. Due to the mutual solubility of W-Ti, the W atom diffused into the TiC, leading to the formation of (Ti,W)C solid solution as same as crystal lattice structure of TiC. The relative density, Vickers hardness and fracture toughness of the composite ceramics measured 98.4%, 26.4 GPa and 7.6±0.5 MPa•m1/2, respectively.

Abstract: (Ti, W)C matrix metal ceramics were prepared through combustion synthesis under high gravity, structure formation mechanism of the ceramics was discussed and the properties of the ceramics were also measured. XRD and FESEM images show the matrix of metal ceramics was mainly composed of TiC and (Ti, W)C1-x. The formation mechanism mainly involves two stages: firstly, combustion reaction is advanced rapidly under high gravity, and the layered melt consisting of Ti-W-Fe-C-B liquid at the bottom and oxide liquid at the top is formed due to rapid liquid-liquid separation under high gravity, subsequently, TiC solids as the primary phase precipitates from Ti-W-Fe-C-B liquid due to the higher concentration and faster diffusion of C relative to B in the alloy liquid, whereas (Ti, W)C1-x solid nucleates and grows on the surface of TiC solids. The Vickers hardness, flexural strength and fracture toughness of the (Ti, W)C matrix Metal ceramics measured 25.6 GPa, 1060 MPa and 8.5 MPa•m1/2, respectively.

Abstract: Based on preparing the large-bulk solidified Al2O3-ZrO2 (Y2O3) ceramic composites by combustion synthesis under high gravity, the solidification behavior and microstructure transformation of the ceramics as well as their effects on the ceramic properties were investigated. XRD, SEM and EDS analyses showed the ceramic microstructures were composed of the irregular eutectics at the surface and the tetragonal ZrO2 micrometer spherical crystals in the center of the ceramics respectively. By combining with irreversible nucleation theory, it is considered that the formation of irregular eutectics with micro-nanocrystalline microstructures is a result of the leading nucleation of Al2O3 high-fusion-entropy phases followed by the coupled growth of Al2O3-ZrO2 phases, whereas the presence of the tetragonal ZrO2 spherical microstructures in the center of the ceramics results from the leading nucleation of ZrO2 cubic phases followed by the independent growth of Al2O3-ZrO2 phases. As a result, it is just the unique microstructure transformation during the solidification process that the highest hardness at the surface and the highest fracture toughness in the center of the ceramics are achieved respectively.

Abstract: The Al2O3/YSZ eutectic composite ceramics-lined pipes have been prepared by combustion synthesis. By adding ZrO2 (4Y) to the thermit, the influences of centrifugal force and eutectic composition on the microstructure, mechanical properties, strengthening and toughening of ceramics lining are researched. The growth of melt is based on eutectic transformation under high degree of under-cooling at centrifugal force above 200G. The linings are comprised of rod-shaped Al2O3 matrix eutectic grains. The diameters of tetragonal lattice ZrO2 fiber in rod-shaped grains are on nano-micron scale. Causing the strengthening and toughening of ceramics to be promoted, and the Al2O3/YSZ eutectic composite ceramics-lined pipes have high comprehensive mechanical properties.

Abstract: Based on preparing large bulk Al2O3/ZrO2 (4Y) eutectics, SiO2 additive is used for controlling the microstructures, densification and mechanical properties. XRD pattern showed SiO2 additive in the ceramics was in the form of glass phase. SEM images showed that the microstructure morphologies transformed from the cellular eutectics to the rod-shaped colonies with increasing content of SiO2 additive. With increasing content of SiO2 additive, the relative density of the ceramics increased whereas the hardness of the ceramics decreased. As the content of SiO2 additive reached 4%, fracture toughness of the ceramics had the maximum value due to the coupled toughening mechanisms of crack-pinning, crack- bridging and crack-deflection by rod-shaped colonies. As content of SiO2 additive reached 6%, the highest flexural strength of the ceramics was achieved due to high fracture toughness and small-size critical defect.

Abstract: Based on preparation of Al2O3/ZrO2 (4Y) by combustion synthesis in high-gravity field, the microstructure transformation and properties of the materials are investigated through adjusting the ZrO2 (4Y) content in the composites. As the content of ZrO2 changed from 37% to 40%, the microstructures of the ceramics transformed the sphere-like tetragonal ZrO2 crystals from the rod-shaped colonies with nanocrystalline structures. Al2O3/33%ZrO2 (4Y) had the maximum relative density, hardness and flexural strength due to the low solidification temperature, the highest volume fraction of the colonies, small-size defect and high fracture toughness, whereas Al2O3/44%ZrO2 (4Y) was somewhat weakened in strength in despite of its highest fracture toughness.

Abstract: The large-bulk solidified TiC-TiB2 composites were prepared by combustion synthesis in high-gravity field. XRD, FESEM, SEM and EDS results showed that TiC-TiB2 composites were mainly composed of TiC matrix in which a number of fine TiB2 platelets were embedded, surrounded by the boundary regions consisting of (Cr, Ti) C0.63 carbides. The hardness, flexural strength and fracture toughness of TiC-TiB2 composites measured 28.5GPa, 750±25MPa and 6.2±0.5MPa•m1/2. High hardness of the composites benefits from the absence of the intermediate borides and the achievement of stoichiometric TiC phases due to rapid solidification, whereas the achievement of high strength benefits the refinement and homogeneity of the microstructures due to rapid separation of liquid oxides in high-gravity field and rapid coupled growth of TiC and TiB2 phases.

Abstract: The large bulk Al2O3/ZrO2 (Y2O3) eutectics were achieved by combustion synthesis in high-gravity field. With increasing high-gravity level, the matrix of eutectics transformed the rod-shaped colonies from the cellular ones, and the nanocrystalline microstructures came into existence as the high-gravity level was larger than 200g. The relative density, hardness, flexural strength and fracture toughness increased simultaneously with increasing high-gravity level, and reached the maximum values of 98.6%, 18.6GPa, 1248MPa and 15.6MPa•m1/2 respectively as the high-gravity level was 250g.