Combusion Synthesis Process And Performance Characterization For MgAl₂O₄-matrix Nanocomposite Ceramics

MgAl₂O₄ is widely used in many fields due to its excellent physical and chemical properties.In this paper,MgO·nAl₂O₃ spinel solid solution was prepared by combustion synthesis and spray water atomization method.MgAl₂O₄-matrix nanocomposite ceramics were prepared on the basis of studying the solid solution precipitation of the metastable powders.MgAl₂O₄-matrix nanocomposite ceramics were also prepared by combustion synthesis casting method.The properties and microstructure of ceramics were characterized.In this paper,MgO·nAl2O3 metastable powders with different components were prepared by combustion synthesis and spray water atomization method.During the preparation process,the melting solidification rate was about 7.7?10⁵K/s.Phase analysis and morphology characterization of the prepared powders showed that particles were spherical.The particle diameter was about 2-5μm in monodisperse state,and the particle size was distributed between 8-30μm in the natural state.There should be agglomeration between the powders.The magnesium-rich systems consist of spinel solid solution and MgO,and MgO is dissolved in MgAl₂O₄ to form gap-type solid solution,but the lattice constant of the spinel is not changed.The aluminum-rich system consist only of spinel solid solution,and Al₂O₃ is dissolved in MgAl₂O₄to formvacancy solid solution,which makes the crystal lattice constant of spinel decrease linearly and conforms to Vegard’s law.The Vegard constant is-3.3?10^-2（nm/（Al₂O₃/（Al₂O₃+MgO）））.The law of solid solution precipitation of metastable powders was studied by heat treatment and hot pressing sintering.MgO·nAl₂O₃ spinel solid solution with different components has different precipitation behaviors.The equilibrium precipitation phase of MgO·0.563Al₂O₃ spinel solid solution powder is MgO.When the heat treatment temperature reaches 1200℃,MgO precipitates rapidly and the amount of precipitation at this time is the largest.Partial dissolution of MgO is caused by temperature rise and time extension.MgO·0.818Al₂O₃ and MgO·1.857Al₂O₃ did not precipitate in solid solution within the temperature range of heat treatment,but they did precipitate in the process of hot pressing sintering.The equilibrium phase of MgO·0.818Al₂O₃ is MgO and the precipitation temperature is 1200℃.The equilibrium phase of MgO·1.857Al₂O₃ is Al₂O₃ and the precipitation temperature is1400℃.MgO·3.762Al₂O₃ and MgO·9Al₂O₃ have similar solid solution precipitation behavior.With the increase of temperature,intermediate phase is precipitated.At1300℃,mesophase is dissolved and equilibrium phaseα-Al₂O₃ is precipitated.MgAl₂O₄-matrix composite ceramics were prepared by hot pressing sintering of metastable micro-powders.The sintered samples had a hardness of 23.47GPa,a bending strength of 316.71MPa,and a fracture toughness of 3.88 MPa·m^1/2.The fracture mode was transgranular and intergranular mixed fracture.Spherical Al₂O₃particles with a size of about 150nm were precipitated inside the grain,which plays a role in strengthening and toughening and improves the material performance.MgAl₂O₄-matrix composite ceramics were prepared by combustion synthesis melting casting method.The growth of the molten casting parallel to and perpendicular to the solidification direction was oriented and eutectic was formed.MgAl₂O₄ was the matrix,and MgO distributed uniformly in the matrix with discontinuous sheet and bar shape.The formation of eutectic microstructure improved the mechanical properties of ceramics.