Experimental And First-principle Research On Processing And Mechanism Of Dip-coating Aluminum On Al₂O₃ And AlN

Techniques of joining Al with Al₂O₃ and AlN ceramics have broad applications in many industrial fields, such as the high-power circuit packaging, light weight composite armor and automobile production. However, due to the easiness of Al oxidation and the significant differences of physical and chemical properties between Al and ceramics, wettability of AI/Al₂O₃ and Al/AlN is generally regarded poor and the joining of Al with ceramics is hindered. Nevertheless, the novel dip-coating method invented by the group of Ning can form a smooth Al film with several microns in thickness on the surface of Al₂O₃ and AlN ceramics, indicating a huge improvement of wettability of AI/Al₂O₃ and Al/AlN systems, which is nominated as the "super-wetting" phenomenon. This work experimentally studied the influence of temperature, oxygen content and holding time on the dip-coating method and theoretically illustrated the mechanisms of "super-wetting" phenomenon of Al/Al₂O₃ and Al/AlN by using first-principle method.Experimental resultsof Al/Al₂O₃ dip-coating process show that the oxygen content in the atmosphere is of significant effects. At a certain temperature, an optimal range of oxygen content exists so as to obtain a smooth Al film with high coverage and this optimal range of oxygen content varies with the temperature. Oxygen content influences the spreading of Al on Al₂O₃ surface mainly by affecting the surface tension of liquid Al. High Resolution Transmission ElectronMicroscopy （HRTEM） image of the Al₂O₃ C-plane single crystal dip-coated sample shows no amorphous oxide layer at the interface and most of the interface is quite sharp and plane. However, epitaxial steps with height of 5 nm are formed at some parts of the interface. It is reckoned that the extremely oxygen-deficient atmosphere inside the liquid Al causes the "evaporation" of the outmost oxygen layers of the Al₂O₃ surface, and oxygen diffuses along the interface in the form of Al2O, nucleating, growing eptiaxially and forming steps. First-principle calculations reveals that the ((（31）^1/31)× (（31）^1/31))R ± 9°reconstructed Al₂O₃ （0001） surface formed by oxygen "evaporation" will increase the work of adhesion Wad of AI/Al₂O₃ interfacial system and decrease the contact angle 6. When surface tension of liquid Al decreases lower than 0.97 J/m2 as a result of surface oxidation, the contact angle of Al/Al₂O₃ will reach 0° and achieve "perfect wetting", which explains the mechanisms of "super-wetting" phenomenon of Al/Al₂O₃in the dip-coating process.Experimental resultsof Al/AlN dip-coating process show that the holding time of AlN ceramic plates inside the liquid Al affects significantly. No smooth and high covered Al film can be obtained on AlN surface unless the AlN ceramic is immersed in liquid Al for more than 10 min. This is because the fresh surface of AlN can contact with Al until the layer of AlOxNy on the surface of AlN is reacted completely with liquid Al. The extremely Al-rich atomosphere inside the liquid Al will cause the formation of ((3^1/3)×(3^1/3)) R30° LCM reconstructed surface of AlN （0001）. First-principle calculations show that the contact angle of Al on the reconstructed surface of AlN is 35-41°, while the contact angle can be further lowered to 0° when the surface tension of liquid Al is below 1.02 J/m2 as a result of surface oxidation. In this way, "perfect wetting" of Al/AlN can be achieved and the mechanisms of the "super-wetting" phenomenon in the dip-coating process can be explained.