| There has been increasing interest in refractory materials such as carbides, nitrides, borides and silicides owing to their outstanding hardness and stability at high temperatures. However, the attention has now turned to composite refractory materials, since monolithic refractory materials have reached their optimum stages. On the other hand, nanocrystalline refractory materials including nanocomposites have recently received great attention due to their improved functional and mechanical properties. A variety of techniques for preparing nanocrystalline materials have been proposed over a decade. Among them, high-energy ball milling has been recognized as an effective way of producing nanocryatalline, amorphous, supersaturated solid solution and other non-equilibrium structured materials. High energy ball milling accompanying chemical reactions is considered to be very effective for preparing nanocomposite powders with more than two phases. In the thesis, a planetary ball mill is applied to synthesize some kinds of carbides, nitrides, borides, silicides and their composites. Some as-milled samples were heat treated in Ar atmosphere. The structural evolutions and morphologies of the milled or annealed samples were investigated by using x-ray diffractometry(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM) and Fourier Transform Infrared Spectroscopy(FTIR). The mechanisms of reactions induced by ball milling for different blended powders were also discussed. The major conclusions are listed below:1. Micrometer sized Zr and h-BN powders are used as the raw materials. At the initial stage of ball milling, h-BN powder is cleaved and fragmented easily due to its layered structure. Computer simulation on the XRD patterns of h-BN indicates that the intensities of XRD peaks decrease obviously with decreasing crystallite size. BN reacts with Zr to form a Zr(B,N) solid solution during ball milling. The impact energy introduced by the planetary ball mill is not high enough to ignite the powders. So both the B and N atoms diffuse into Zr due to the formation of clean surfaces, grainboundaries and sub-grain boundaries, which are induced by repeated ball milling. The solubility of boron in fcc-ZrN is remarkably higher than that of nitrogen in h-ZrB2, so only the nanocrystalline fcc-ZrNi.xBy is obtained finally. The reaction progresses mainly as a diffusion-controlled process, but localized mechanically induced self-sustaining reaction (MSR) is still observed and the heat generated by the reaction leads to the particles grow. These results are reported by us for the first time.2. The as-received TiH2 and B4C used as raw materials are micrometer-sized powders. The grain size of T1H2 decreases quickly and decomposes into active Ti and H2 during ball milling when hard B4C powder is used. Ti powder begins to react with B4C soon. The difftisivity of C in Ti is significantly greater than that of B in Ti, therefore fcc-TiC is easier to form. And B atoms react with Ti to form an amorphous Ti(B) phase at first. Finally, the amorphous phase vanishes and it is suggested that B has diffused into fcc-TiC. A nanocrystalline TiCi-xBy here is obtained with an FCC structure. The reaction is a diffusion-controlled process during ball milling. When the as-milled sample is annealed at 1000°C, TiB2-TiC ceramic nanocomposite is formed. Such results are not reported elsewhere to our knowledge.3. Micrometer sized Ti, Si and graphite powders are used as the raw materials to synthesize TiC-SiC ceramic composite by MA for the first time. The milling parameters, such as milling rate (300r/min), mass of the powders (lOg) as well as ball to powder ratio (25:1), are fixed. Both the binary SisoCsoand TisoCso systems progress a diffusion-controlled process in a planetary ball mill. Si reacts with graphite to form nanocrystalline fcc-SiC after milling for a long time. When the TisoCso mixture is milled for longer than about 120 hours and then exposed to air, a large amount of heat is generated by the reaction between nano-Ti and O2 in air. The powders are ignited and become self-sustaining. The ternary Ti2sSi25Cso mixture undergoes an MSR process, which is different from the Ti-C and Si-C systems, even it is milled with the same milling parameters. Since the ignition temperature decreases with the decreasing particle size of the reactants, and much more energy is accumulated during ball milling by the addition of brittle Si. So the heat generated by the reaction between Tiand Si or the impact of the milling balls results in a rapid temperature rise in some localized regions to ignite the mixture easily, and a SiC-TiC composite is obtained.4. The solid-gas reaction is comparatively investigated by ball milling Ti with with air in closed vials and in open vials with the same milling rate and ball to powder ratio for the first time. It is suggested that the chemical adsorption between the clean metal surfaces and the mixed gas is a kinetic process at the initial stage of ball milling. So the amount of the adsorbed N atoms is higher than that of O in when Ti is milled in air and a solid solution of hcp-Ti(O,N)x forms at first. And hcp-Ti(O,N)x transforms into fcc-Ti(O,N)y gradually with the same lattice structure with TiN. It is found that the milling efficiency is higher when Ti powder is milled in a closed vial. When the milling time increases, a nanocrystalline fcc-Ti4(Oo.2No.s)3 phase is obtained when Ti powder is milled in a closed vial with certain amount of N2 and O2. However, when Ti is milled in a vial that is connected with ambient air, more O is adsorbed and TiC>2 is obtained. The formation of TiCh hinders the reaction of Ti with N2 and/or O2 subsequently. So hcp-Ti(O,N)x, fcc-Ti (O,N)y and rutile-TiC^ with small grain sizes coexist in the final product. |
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