Structural characterization of boron-doped alumina for smart lubricants*

We have investigated material properties of ceramic alumina (Al₂O₃) doped with boron oxide (B₂O₃) using density functional theory (DFT). Alumina exhibits favorable physical properties, including a high melting point, hardness, and corrosion resistance. This material can also possess good tribological properties i.e., friction, lubrication, and heat transfer, which are enhanced with the addition of solid lubricants such as boron oxide. In this study we have carried out atomic modeling of alumina and the stable aluminum borate phase. Our models revealed that the commonly accepted 9Al₂O₃ 2B₂O₃ polymorph contains ten aluminum oxide molecules within its structure. Defect formation energy calculations for several possible scenarios revealed that it is very energetically unfavorable to create vacancies in the 10Al₂O₃ 2B₂O₃ cell, and theoretically simulated x-ray diffraction patterns of the optimized 10Al₂O₃ 2B₂O₃ cell match well with experimental results. Through phonon analysis of both alumina and aluminum borate we produced theoretical infrared spectra for comparison to experimentally obtained spectra. The phonon calculations carried out can also be used to analyze tribological properties based on active modes.