Investigation On Strengthening-toughening Behavior And Thermal Stability In MgAl₂O₄-based Nanomultilayers

Recently,the design of materials in accordance with a need and precise control of service performance are the inevitable development trendency of advanced materials.Materials selection plays an important part in the design of multilayers that can lead to higher strength and toughness.To find candidates of multilayer systems that exhibit not only improved mechanical properties but also other properties,such as thermal stability and wear resistance compared to individual layers is the primary task of studying multilayers.With the deepening research on nanomultilayers,however,not only the hardness but also the relations between structure and mechanical properties at elevated temperatures should be noticed.In the past few years,MgAl₂O₄ as a bulk material has proved its efficacy on several applications,such as fusion reactor power core insulation,electronic humidity sensors,refractories,catalysts,catalyst supports,on account of its best combination of several important properties including high hardness,high mechanical strength both at room and elevated temperatures,high resistance against chemical attack,etc.The structure of MgAl₂O₄ is similar to Al₂O₃ at room temperature.The main difference is that there are 24 cations fill in the sites of oxygen ions in the structure of MgAl₂O₄ while only1/3 sites of oxygen ions in Al₂O₃ are occupied by Al³⁺.Such a saturated structure of MgAl₂O₄ can remain stable effectively in structure and performance over a wide range of temperature and pressure.However,there are few investigations on MgAl₂O₄as a candidate of multilayer film systems.Whether the MgAl₂O₄ as a film has excellent properties that similar to the bulk materials has still not be confirmed yet.Some other oxides have already been applied to multilayer systems maturely and showed both improved mechanical properties and good high-temperature stability.And we are full of interest to explore the hardness and toughness in MgAl₂O₄-based nanomultilayers as well as the relations between structure and mechanical properties at elevated temperatures.Inspired by the experiment results mentioned above,in this paper we planned to synthesize TiAlN/MgAl₂O₄ and VN/MgAl₂O₄ nano-multilayer films with different layer thickness of MgAl₂O₄(l_MgAl2O4)and explore the effects of l_MgAl2O4gAl2O4 on the mechanical properties and the relations between structure and mechanical properties at elevated temperatures.The investigation results of this paper indicate that:1.Under the template effect of TiAlN and VN layers driven by interfacial energy minimization,amorphous MgAl₂O₄ crystallized and grew epitaxially with TiAlN and VN layers,forming a high-quality layered structure and coherent interface at low l_MgAl2O4.Specially,a high-quality layered structure and coherent interface accured at l_MgAl2O4=0.7 nm with a maximum hardness enhancement of⁴6 GPa in TiAlN/MgAl₂O₄ nano-multilayer films and at l_MgAl2O4=0.6 nm with a maximum hardness enhancement of³6 GPa in VN/MgAl₂O₄ nano-multilayer films.Nevertheless,the hardness enhancement was obviously weakened by the partial amorphization of MgAl₂O₄ layer at larger l_MgAl2O4.2.The formation of layered-structure in the TiAlN/MgAl₂O₄ and VN/MgAl₂O₄nanomultilayers remarkably improved the fracture toughness by deflecting and blunting cracks through layer interfaces.3.Thethermalstability andoxidation resistanceof TiAlN/MgAl₂O₄nanomultilayers were positively affected by MgAl₂O₄ layer.After annealing at600?and 800?in vacuum,the number of internal defects in the TiAlN/MgAl₂O₄nanomultilayers were decreased and internal stress was released.So the TiAlN/MgAl₂O₄ nanomultilayers maintained stable phase structure,good periodic structure and excellent mechanical properties.After annealing in air,the crystallinity and hardness decreased with the increasing temperature.But the hardness of TiAlN/MgAl₂O₄ nanomultilayers was still higher than monolayer film.After annealing at 600?in vacuum,the thickness of MgAl₂O₄ layer was conducive to maintain the periodic structure of the multilayer films.When the annealing temperature arrived to 800?,all the periodic layers structure of nanomultilayers were disappeared.Since the Gibbs free energy of MgO is higher than V₂O₅ and V₂O₃,V can be oxidized preferentially than Mg.So the interface between MgAl₂O₄ layer and VN layers may be miscible after annealing.