| 其他摘要 | Layered ternary ceramics, Mn+1AXn phases, possess an unusual combination of the merits of metals and ceramics. Such unique properties make them of great interest for potential applications in many industrial fields. However, monolithic Mn+1AXn have certain shortcomings, and cannot meet the over-all properties required for the uses in high-technology industries. The most promising approaches to solve these problems may include composite strengthening and surface modification.
In this dissertation, Ti3AlC2 and Ti3SiC2, the two most attractive Mn+1AXn phases, are selected as investigated subjects. Compared with traditional binary carbides, their hardness and strength are unsatisfied. In order to enhance these properties, particulate strengthening and surface modification are carried out on Ti3AlC2 and Ti3SiC2, respectively. The following conclusions were drawn:
I. The Ti3AlC2/TiB2 composites have been successfully synthesized by means of in-situ reaction of Ti, Al, graphite and B4C powder mixtures at a relatively low temperature of 1500oC. The effects of TiB2 on the microstructure，mechanical properties and oxidation behavior of Ti3AlC2 were investigated.
（1） In the composites, most of the TiB2 exist as equiaxed grains with an average diameter of less than 200 nm, and penetrate into Ti3AlC2 grains to form interpenetration composites. There are no glass phases at the interface of TiB2 and Ti3AlC2. Besides, TiB2 lowers grain boundary mobility in Ti3AlC2, so the matrix grain size decreases with the increasing TiB2 content.
（2） The addition of TiB2 obviously increases the elastic modulus, hardness, compressive and flexural strength at room temperature. Grain refinement, inter-granular strengthening and anchoring of basal plane dislocations are proposed to be the main strengthening mechanisms.
（3） At high-temperatures, the addition of TiB2 phase still has a visible strengthening effect on the mechanical properties of Ti3AlC2, all of the Ti3AlC2/TiB2 composites have higher elastic modulus and flexural strength than Ti3AlC2 till 1200oC. The existence of TiB2 possibly increases the creation of defects in the Ti3AlC2 grains at high temperature, cause the internal friction peak of the composites to intensify and shift toward low temperature with increasing TiB2 content. Besides, the Ti3AlC2/TiB2 composites possess excellent high-temperature ductility because the sub-micro TiB2 particles can play a coordinated deformation role.
（4） The results of oxidation tests show the Ti3AlC2/20TiB2 composite has excellent oxidation resistance. During the oxidation at 1000-1400oC, because a continuous protective Al2O3 internal layer can form on the surface of Ti3AlC2/20TiB2, the composite possesses excellent high-temperature oxidation resistance. During the oxidation at 500-600oC, owing to the formation of glassy B2O3, the composite, unlike Ti3AlC2, avoids the destabilizing oxidation happening induced by the formation of a large number of bulky rutile grains.
II. In order to modify surface properties of Ti3SiC2, boride coating was prepared on its surface by boronizing treatment. The microstructure, forming mechanism and properties of the coating were investigated.
（1） After boronizing treatment, one mixture layer, composed of TiB2 (main phase) and β-SiC (dispersing at the grain boundary of TiB2 grains), forms on the surface of Ti3SiC2. The growth of the coating is processed by inward diffusion of boron and simultaneous outward diffusion of carbonaceous species, and obeys a linear rule.
（2） The surface hardness of Ti3SiC2 increases with increasing boronizing treatment temperature and time. And high level of surface hardness is beneficial to preventing substrate from being indented and ploughed, which was proposed to be the main reason for improvement of the wear resistance of Ti3SiC2. |
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