| 其他摘要 | Binary transition metal carbides, ZrC and HfC are characterized by high melting point, good thermal shock resistance, good high-temperature strength and chemical inertness. Thus, thery are ideal candidates for ultrahigh-temperaure application. However, intrinsic brittleness and poor oxidation resistance restrict their extensive application as high-temperature structural materials. Ti2AlC and Ti3AlC2 ceramics have been developed by incorporating Al into TiC, which show excellent toughness and high-temperature oxidation resistance, but poor high-temperature stiffness and strength above 1000oC. In this dissertation, ternary Zr-Al-C and Hf-Al-C ceramics were synthesized by adding Al into ZrC and HfC, respectively, and their micostructure, mechanical and thermal properties as well as oxidation resistance were systematically characterized. The goal of this dissertation is to obtain some insights into understanding the relationship of “processing-microstructure-property” for Zr-Al-C and Hf-Al-C ceramics.
Predominatly single-phase Zr3Al3C5 powders were synthesized at 1500oC using Zr-Al intermetallics and graphite powders. Zr3Al3C5 powders have better oxidation resistance than ZrC powders due to higher starting oxidation temperature. Based on liquid-phase-sintering theory, highly pure and dense Zr3Al3C5 ceramic was successfully synthesized at 1750oC using Zr, Al and graphite powders as starting materials as well as Si and Y2O3 as sintering additive. The microstrucure of as-synthesized Zr3Al3C5 ceramic is composed of fine and elongated grains, which endow it with higher strength and toughness than ZrC at room temperature. Zr3Al3C5 ceramic exhibits excellent high-temperautre stiffness and the Young’s modulus at 1600oC is 78% of that at room temperature. In addition, the molar heat capacity of Zr3Al3C5 ceramic is about 5-6 times that of ZrC. However, due to many electron and phonon scattering sources, the electrical and thermal conductivities are lower than ZrC.
Zr2Al3C4 ceramic was synthesized by hot-pressing Zr, Al and graphite powders at 1900oC and thereafter by annealing at 1600oC in vacuum. The as-synthesized Zr2Al3C4 ceramic has similar microstructure, mechanical and thermal properties to Zr3Al3C5. The stiffness of Zr2Al3C4 at 1600oC and the strength at 1400oC is close to 80% of those at room temperature. The oxidation kinetics follows the parabolic law at 600-800oC, which gradually transforms into linear law at higher temperatures. The high-temperature oxidation resistance of Zr2Al3C4 was greatly improved by forming a ZrSi2/SiC coating using Si pack cementation. Rapid induction heating technique was used to characterize the ultrahigh-temperautre oxidation behavior of Zr2Al3C4 ceramic. Due to porous ZrO2/Al2O3 scales, the oxidation kinetics at 1600 and 1750oC follow linear law.
Zr2[Al(Si)]4C5 and Zr3[Al(Si)]4C6 solid solutions were synthesized by hot-pressing Zr, Al, Si and graphite powders. The space group of both Zr2[Al(Si)]4C5 and Zr3[Al(Si)]4C6 was determined to be R3m; and the atomic-scale microstructures of these two compounds were presented. Zr-Al-Si-C and Zr-Al-C ceramics are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked Zr-C layers and Al3C2/[Al(Si)]4C3 slabs. However, their properties are influenced more or less by the layer thickness of Zr-C and Al-C. Thicker layer of Zr-C and/or thinner layer of Al-C are in favor of stiffness, hardness, thermal and electrical conductivities, but go against specific stiffness, Debye temperature, and coefficient of thermal expansion. Zr-Al-Si-C ceramics also have excellent high-temperautre stiffness and strength. The oxidation behavior of Zr-Al-Si-C ceramics at 900-1300oC were investigated and compared with Zr2Al3C4 ceramic. The oxidation kinetics of all three carbides change from a parabolic law at a very short initial stage to a linear law for a long period as a result of graudation interconnection of defects in scales. Compared with Zr2Al3C4, Zr-Al-Si-C ceramics have higher activation energy and lower oxidation rates, therefore better oxidation resistance. In addition, Zr2[Al(Si)]4C5-30 vol% SiC composite was synthesized, which shows superior mechanical and thermal properties to Zr2[Al(Si)]4C5.
Two new compounds, Hf3Al4C6 and Hf2Al4C5 were discovered in Hf-Al-C system, and their crystal structures were characterized by X-ray diffraction and high resolution Z-contrast image. The microstructure, mechanical and thermal properties as well as oxidation resistance of a Hf-Al-C composite composed of Hf3Al3C5、Hf3Al4C6 and Hf2Al4C5 were chacterized. The composite has high stiffness and hardness, superior strength and toughness to HfC at room temperature. The excellent stiffness of the composite at high temperature makes Hf-Al-C compounds good high-temperautre materials. The specific heat capacity of the Hf-Al-C composite is about twice that of HfC, while the thermal conductivity is much lower than HfC. The oxide scales are non-protective and the oxidation kinetics of the Hf-Al-C composite generally follow linear law at 900-1300oC.
A new approach to synthesize ZrO2-Al2O3 composite was developed based on the non-selective oxidation of Zr and Al in Zr-Al-C ceramics. ZrO2-Al2O3 nanocrystalline powders were synthesized by oxidizing Zr2Al3C4 powders, and then bulk nano- and submicro- composites were fabricated by hot-pressing the as-oxidized nanocrystalline powders at 1100-1500oC. The evolution of composition, microstructue, density and hardness during sintering were characterized. |
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