MgO/AlN Composite Fabricated By Directed Metal Nitridation

As the foundation material of high temperature industries, refractories are required to have a high melting point, high strength, excellent erosion and abrasion resistance to hot melts in iron and steel industry. Besides, at alternate temperature conditions, refractories with good thermal shock resistance are especially demanded. After decades of development, refractories have changed from single oxide systems into composite ceramic materials with oxides and non-oxides, especially nitrides in order to improve the erosion and thermal shock resistance.Aluminum nitride (AlN), as a representative of non-oxide ceramics, has an excellent thermal conductivity coefficient including high melting point and high strength. Combining AlN with MgO matrix composite will be a promising way to resolve the intrinsic poor thermal shock resistance of tranditional basic refractories. Nowadays, the limitations of AlN as a refractory raw material mainly are the high cost and apt to hydration of AlN powder. Therefore, it is urgent to seek a suitable method with low product cost to manufacture MgO/AlN composite. Directed metal nitridation process is considered as a simple process with low processing temperature, which combines the synthesis of raw materials, shaping and sintering of products into one step and avoids the hydration of AlN during synthesis process. The present article attempts to investigate the growth mechanism and the influence of processing parameters on the MgO/AlN composites fabricated by directed metal nitridation process.Firstly, the thermodynamic software Factsage is used to plot the predominance region phase diagrams and ternary phase diagrams among Al-Mg-O-N system, and to simulate the phase composition change of product during the experiment. Furthermore, the influence of Mg, processing temperature and various atmosphere statuses on the fabrication of AlN/Al composite is discussed in terms of thermodynamics. The results show that the presence of gaseous Mg, elevated processing temperature and static atmosphere benefit the nitridation of Al melt. Based on the thermodynamics, an appropriate special experimental apparatus and an experiment scheme are designed.Secondly, the influence of external dopant Mg powder, processing temperature, heating time and various crucibles on nitridation and infiltration of Al, phase composition and microstructure of AlN/Al composites, is investigated by X ray diffractometer, optical microscope, scanning electron microscope linked with energy dispersive spectrometer and electron probe microanalyser. The phase transformation of Mg vapor is simulated according to thermodynamic calculation, and a thickness model of Mg vapor layer is built using the principle of gas flow diffusion dynamics. Additionally, based on the microstructure characteristics, a gas-liquid-solid mechanism model, including the expression of the reaction rate and mass transfer resistances is built. The results show that Mg initiates the infiltration by eliminating the inert alumina layer on the top of Al melt and forms a gaseous blanket to act as gettering agents to remove residual oxygen. The thickness of Mg gaseous blanket increases with the elevated original Mg content, and an AlN-AlN/Al-MgAl2O4/AlN/Al sandwich structure is formed due to continuous depletion of gaseous Mg. The apparent activation energy of Al melt nitridation is 196.21kJ/mol, and chemical absorption will control the nitridation rate of Al melt. Furthermore, capillary radii between AlN crystal columns become thinner due to the continuous precipitation of AlN, which lower the infiltration rate and decrease the supply of Al melt at the growth frontier. Eventually, Al nitridation terminates prematurely. Thirdly, the influence of external dopant Mg powder, processing temperature, heating time and Al-Si alloy on the Al nitridation, infiltration and interface reaction in the MgOp/AlN composite is investigated by X ray diffractometer, optical microscope, scanning electron microscope linked with energy dispersive spectrometer and electron probe microanalyser, where 1-0.5mm MgO particles (MgOp) preform mixed with Mg powder is loosely packed on the surface of Al block. Then the support vector machine is used to optimize the Mg powder content, processing temperature, heating time and Si content in the formation of MgOp/AlN composite. The results show that the infiltration of Al into MgOp preform first happens, and then the nitridation and infiltration of Al simultaneously happen during the formation of MgO/AlN composite by directed metal nitridation method. The presence of Mg promotes the nitridation and infiltration of Al melt. Si alloyed in Al melt decreases the viscosity of melt and elevates the activity of N in the Al melt, which also improves the nitridation and infiltration of Al. When processing temperature is 900oC-1000oC, composite growth is governed by capillarity and metal predominates the matrix of composite. On the other hand, at 1100oC -1200oC composite growth is governed by capillarity and nitridation of the Al melt, and AlN is the main phase at the matrix of composite. Also, Selecting appropriate processing conditions can alleviate the interface reaction between Al melt and MgO particles. Finally, an accurate model is established between the processing parameters above and the mass change of MgOp/AlN composites, which provides a new data minning method for the fabrication of MgO/AlN composite.