Research On Processing Parameters Of MgO/AlN Composite By Directed Metal Nitridation

With the development of continuous casting technology, slide plate (especially for the steel treated with calcium alloy) demands more excellent properties. Magnesia, which has superior qualities such as a high melting point and better erosion resistance to basic slag, can perform well in this condition. However, magnesia products can not endure repetitious using, due to the poor thermal shock resistance. Aluminium nitride, which has high thermal conductivity coefficient and low thermal expansion coefficient, can compensate the disadvantages of magnesia material, and it also has bad wettability to molten metal, excellent erosion resistance to molten steel and slag. Combining AlN with MgO will be a promising way to create a new kind of refractory which has both excellent thermal shock resistance and erosion resistance to slag.The thesis studies the fabrication of MgO/AlN composite and discusses the formation mechanism of the composite. Aluminum block infiltrates upwards into the preform, which is packed loosely to column with mixture of Mg powder and fused magnesia grains, at high temperature in the nitrogen atmosphere. And phase composition and microstructure of the products are analyzed respectively by X-ray diffractometer, optical microscope, scanning electron microscope, energy dispersive and electron probe microanalyser.The research work is summarized as follows:First, the thesis studies the influence of Mg content, temperature and processing time on the fabrication of MgO/AlN composite, and discusses the formation mechanism, using pure aluminum block and magnesia grains. The results show that, Mg initiates molten aluminium infiltration and nitridation throughout the process, and lowers the oxygen partial pressure. When there is no Mg powder in the preform, both infiltration and nitridation of molten aluminium are hard to commence. The increase of temperature not only reduces the viscosity of molten aluminum, but also accelerates the infiltration and nitridation. With the increase of Mg content or processing temperature, the infiltration capacity and nitridation rate of molten aluminium both increase. However, the microstructure is loose when the processing temperature and Mg content are high. During the initial stage of the experiment, the molten aluminum penetrates and nitridates acutely, and the main matrix in the product is ceramic. With the increase of processing time, the quantity of infiltrated aluminum increase acutely and much aluminum remained in the composite. Further prolonging the process time, the aluminum infiltrated completely nitridates.Second, the thesis comparatively analyzes the role of alloy element Si via using Al-Si alloy. The results show that Si markedly lowers the viscosity of molten aluminum and promotes the infiltration of molten aluminum. However, at low temperature, Si playes a role of obstacles in the process of nitridation while it converts to promote the nitridation with the increase of processing temperature.Third, the thesis studies the effect of interface reaction between filler and molten aluminum on the fabrication process of composite, respectively using fused corundum and magnesium and aluminum spinel as fillers, compared with magnesia. The results show that the interfacial reaction, which occurs at the interface between the fillers and the molten aluminum, influences the microstructure of the composite.