Wetting And Adhesion At AI/MgAl₂O₄Interfaces And The Effect Of Substrate Crystallographic Orientation

A great interest has been triggered in Al–Mg matrix composites reinforced by Al2O3orSiC due to their outstanding properties such as low density, high specific strength andstiffness. MgAl2O4formed in the interface due to the reaction between Al2O3or pre-oxidizedSiC and Al–Mg matrix in the preparation of such composite materials. The properties ofAl/MgAl2O4interfaces have significant technological applications in the preparation ofAl–Mg matrix composites and the processability and the properties of the final product.In this paper, wettability testing experiments combined with first-principles calculationsare applied to the research of the effect of crystal orientation on the interfacial bonding toAl/MgAl2O4system. The contact angles was used to measure the wettability of systems inthe wetting experiments, and the interfacial bonding strength was evaluated by the work ofseparation and the interfacial electronic structure was used to analyze the nature ofinterfacial bonding. The major results obtained in this paper are as follows:(1) Wetting of MgAl2O4single crystals with different crystallographic orientations,(100),(110) and (111), by molten Al at1073–1273K was investigated using a dispensedsessile drop method in vacuum to determine the effect of the MgAl2O4surface orientation onthe wettability and reaction. The initial contact angles are generally in the range of96–106°,slightly decreasing with increasing temperature but without a significant dependence on theMgAl2O4surface orientation. The wetting behavior was governed by the formation of thealumina interfacial layer. The interfacial reaction is sensitive to the MgAl2O4surfaceorientation and the thickness of the Al2O3reaction layer is in order of (100)>(110)>(111).(2) During the wetting process which is coupled with the reaction, significantreconstruction and “whisker growth” are observed at the periphery of the triple line due tothe stress created in the substrate as a result of volume change between the reactant and product, and the morphology of the whiskers was closely related to the MgAl2O4crystalorientation. The formation of the whiskers not only affects the wetting behavior but alsoweakens the effect of the MgAl2O4surface orientation on the final wettability.(3) DFT calculations show that the most stable surfaces of MgAl2O4(100),(110) and(111) are Mg–, MgAlO–and O–terminated surface structure, which the surface energy are1.63,2.1and2.66J/m2。The interfacial bonding strength of Al/MgAl2O4systems are relatedto the different surface energies of the MgAl2O4(100),(110) and (111) as well as atomicconfiguration at interface. The strength of the interface of metal atoms on top of O atoms isthe strongest, and the interfacial dominant bonding mechanism is the Al–O ionic bonds and asmall amount of covalent bonds, while the dominant bonding mechanism that metal Alatoms on top of Al and Mg atoms is the metallic and covalent interaction.In conclusion, this study not only enriches our understanding of metal–ceramicinterfacial bonding but also provides guidance for research and design of ceramic reinforcedmetal matrix composites.