| 其他摘要 | Metal matrix composite (MMC) is one kind of newly developed structural and functional materialse. As MMC shows superior performance than conventional metallic materials in terms of specific strength, specific stiffness, fatigue, high-temperature creep, wear resistance and thermal expansion etc., they always can be employed to meet those applications in space, electric device and ground transportation etc. Nanocrystalline (NC) materials, with ultrafine grains and hence extremely high fraction of interface volume, have been thought to have many particular mechanical, physical and chemical properties in comparison with their coarse-grained polycrystalline counterparts. In this thesis, by combining the structural controllability of MMC and the special features of NC materials, aluminum nitride (AlN) reinforced Al based nanostructural composites were designed and fabricated. Employment of the system of AlN/Al were motivated mainly by the considerations that AlN is a ceramic with high hardness, high thermal conductivity, low thermal expansion, and especially it does not react with common metals. Based on the fabrication and detailed characterizations, mechanical and thermal properties, as well as the friction and wear behaviors were investigated on these nanocomposites.
A technique of arc-discharge plasma evaporating of pure Al in nitrogen containing atmosphere was employed to in-situ synthesize AlN/Al nanocomposite powders with homogenous structure. It is found that, the fraction of AlN particles within those powders increases with an increase in the partial pressure of nitrogen, and the mean particle size of Al decrease as the fraction of AlN increases. In-situ bonding between AlN and Al was frequently found in as-deposited powders. Besides, the nanopowders are easy to be consolidated with high relative density after cold-pressing. Because the phase composition of the powders can be controlled by tuning the partial pressure of nitrogen, a series of nanopowders were synthesized with the AlN fraction varying in 0~43wt.%. Particulate agglomeration was avoid by using this technique. Bulk nanocomposites were fabricated by hot-pressing consolidation of the nanopowders. Densification of the composites was investigated as a function of hot-pressing temperature. As a result, an optimized temperature range, 450℃~500℃, was obtained, at which composites formed are fully dense and structurally stable. In particular, atomically bonded AlN-Al interface was formed during hot pressing.
AlN/Al nanocomposites reveals remarkably improved hardness, which linearly increases with the volume fraction of AlN (Vp). Also, elastic modulus of the nanocomposites was notably improved. For instance, by changing the content of AlN particles from 0 to 39%, the hardness increases from 1.06 GPa to 3.48 GPa, and the elastic modulus was enhanced by 57%. Hardness of 39%AlN/Al is more than 20 times higher than that of annealed pure Al. The improved hardness was attributed to the combination of AlN induced grain-refinement strengthening, load-sharing at interface and Orowan strengthening. Whereas, the most important strengthening comes from interface load-sharing. The elastic modulus can be well predicted by using the Hashin-Shtrikman model.
The composites shows both lowered friction coefficient and improved wear resistance in comparison with non-reinforced NC Al. For instance, wear rate of 39%AlN/Al is only 30% of that of NC Al (normal load of 5 N and sliding speed of 0.01 m/s). With an increase in normal load, wear of NC Al is distinctly enhanced, while that of 39%AlN/Al seems non-insensitive to load within 5~25 N. However, severe friction and wear was promoted for the nanocomposite when further elevate the load. The wear rate of the composite reduces with an increase in sliding speed, in a hyperbolic trend. In other words, this composite has better wear resistance at higher sliding speed.
Particularly, it was found that tribo-oxidation plays important role in friction and wear of the nanocomposites. With the promotion of oxidation, a semi-continuous O-rich tribolayer was formed on the worn surface. This layer was thought to protect the composite from severe wear by lessening the direct contact between the composite and frictional pair. Wear of the composite was much more severe at the load of 35 N than that at 5~25 N, since the tribolayer was seldom formed and wear-induced damage mainly happens beneath the tribolayer at this load. In contrast, higher sliding speed facilitates the formation of O-rich tribolayer, so that the wear rate is lower at higher speed. In a word, oxidation and tribolayer dominate the tribological behavior at a large extent at the load range of 5~25 N and sliding speed range of 0.01~0.08 m/s. The improved wear resistance of the composites compared with NC Al was mainly attributed to enhanced hardness and the protection given by tribolayer.
Thermal conductivity (TC) of AlN/Al reduces with an increases in Vp, i.e. it varies from 160 Wm-1K-1 to 50 Wm-1K-1 with changing Vp from 0 to 39%. The intrinsic contributions of high-conduction AlN particles to TC of the composite was evaluated by comparing with electrical conductance. TC of the nanocomposites containing less than 23% AlN approaches that of Al matrix containing non-conducting inclusions, while the TC are 2 times of the latter when Vp is higher than 30%. Besides, percolation behavior was found for both thermal and electrical conduction, with the threshold at 23%~30%. No significant reduction was found for the TC of AlN/Al measured at elevated temperatures (up to 500℃). The coefficient of thermal expansion is lowered by 50% by introducing 39% AlN nanoparticles. In addition, by comparing with reported data, it was found that nano-sized reinforcing particles seem much effective to reduce thermal expansion of Al based composites than those reported micro-sized particles.
AlN/Al nanocomposite exhibits enhanced hardness and wear resistance, reduced thermal expansion and an acceptable thermal conductivities, so that is promising to meet some applications in microelectronic device and high strength-weight ratio and wear-resistant components. |
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