Theoretical study of aluminum-ceramic interfaces.

다국어 초록 (Multilingual Abstract)

Aluminum is one of the most widely used metals by man. Its low cost, low density, corrosion resistance, high ductility and other properties make it applicable in vehicles, buildings, food containers, and other uses. However, it has poor surface properties and its softness allows it to wear and abrade easily. It adheres to most metals and its low melting point makes it susceptible to friction welding through localized melting. Many Al forming and machining processes such as drilling, rolling, casting, and extrusion require the use of either coatings or lubricants to prevent Al from sticking to the tools. This dissertation examines the adhesion between Al and three materials typically used as coatings or lubricants: graphite, hexagonal boron nitride (h-BN), and cubic boron nitride (c-BN). Atomic scale calculations using the Hohenberg-Kohn-Sham formulism of density functional theory were performed to examine the atomic and electronic structure, and the work of separation between interfaces of Al and each of the three materials listed above. The simulations performed are explained in detail, along with the reasons behind each one.
Minimal adhesion on the order of 0.11 J/m2 was found for the Al (111)-graphite (0001) interface. Adhesion was slightly higher, 0.14--0.16 J/m2, at the Al (111)-h-BN (0001) interface. This slight increase was due to attraction of B and N atoms in the BN (0001) to Al atoms on the opposing surface. Results for both interfaces suggest that either graphite or h-BN would be useful as lubricants for reducing Al adhesion to other materials. The Al (110)-c-BN (110) had a work of separation of 2.25 J/m2, much higher than the adhesion at the Al-graphite and Al-h-BN interfaces. This high adhesion argues against the use of c-BN as a tool coating for reducing adhesion of Al during aluminum machining.
To complement these quantum mechanical calculations, a simple thermodynamic study was carried out to determine the reaction enthalpies between specific metal-ceramic interfaces, where the ceramics are those commonly used as coatings in metallurgical applications. From this study, it is proposed that the reaction enthalpy between a metal and a ceramic should be considered as a criterion when determining whether that ceramic should be used in contact with that metal in metallurgical applications such as cutting, drilling or sawing.