User:Mate413/Aluminum Magnesium Boride

Aluminum magnesium boride (AlMgB14), also known as BAM, is a ceramic alloy that is highly resistive to wear with a low coefficient of sliding friction. This ultra-hard material is also characterized by its high hardness and coefficient of thermal expansion comparable to other widely used materials. BAM alone is among the hardest non-diamond materials. However, the addition of Titanium diboride (TiB2) produces one of the hardest known bulk materials with microhardness values ranging from 35-45 GPa.

Components
AlMgB14 (BAM) without additives is sometimes called baseline material to distinguish it from BAM containing second phase or solid solution additives such as silicon, phosphorus, carbon, titanium diboride (TiB2), aluminum nitride (AlN), and boron nitride (BN). Baseline BAM contains elemental aluminum, magnesium, and boron, but it also contains small amounts of impurity elements (e.g., oxygen and iron) that enter the material during preparation. It is thought that the presence of iron (most often introduced as wear debris from mill vials and media) actually serves as the sintering aid.

Processing
The baseline material is produced by comminuting the elemental materials in high-energy mills. This produces a fine powder (often submicron average particle diameter). The finer the milled powder, the better it will sinter during the subsequent hot pressing step and the more completely the elements will react with each other to produce product that is AlMgB14. The quality of baseline samples depends on the milling time, mill medium, milling container geometry, atmosphere, and the type of milling (e.g., SPEX, planetary, Zoz) that is performed. The powder is consolidated in a graphite die at temperatures near 1670 K and pressures near 100 MPa. The powder is loaded into a graphite die in an inert atmosphere to minimize oxygen pick-up by the fine powder particles. The die interior is coated with hexagonal boron nitride to minimize adhesion to the die, and this coating can produce minor impurities into the surface of the sintered BAM specimen. This procedure typically produces a 98% dense body. AlMgB14 has a theoretical density of 2.6 g/cm3.

Hardness
Most superhard materials have simple, high-symmetry crystal structures (e.g., diamond cubic or zinc blende structure). However, BAM has a complex, low-symmetry crystal structre (OL64) with 64 atoms per unit cell. The unit cell is orthorhombic in its most salient feature is four boron-containing icosahedra. Each icosahedron contains 12 boron atoms. Eight more boron atoms connect the icosahedra to the other elements in the unit cell. BAM also differs from ultra-hard materials because it becomes harder when certain elements or compounds are added to the baseline material. For instance, baseline BAM typically displays microhardness of 28-32 GPa, but additions such as TiB2 have been reported to increase the microhardness to 45 GPa.

Coefficient of Thermal Expansion
The coefficient of thermal expansion (COTE) for AlMgB14 was measured to be 9 x 10-6 K-1 by dilatometry and by high temperature X-ray diffraction using synchroton radiation. This value is fairly close to the COTE of widely used materials such as steel, titanium, and concrete. Based on the hardness values reported for AlMgB14 and the materials themselves being used as wear resistant coatings, the COTE of AlMgB14 could be used in determining coating application methods and the performance of the parts once in service.

Applications
BAM is commercially available from Newtech Ceramics. Research on BAM properties and potential uses is ongoing at Ames Laboratory, and several applications have been proposed for the material. For example, pistons, seals, and blades on pumps could be coated with BAM or BAM + TiB2 to reduce friction between parts and to increase wear resistance. The reduction in friction would reduce energy use. BAM could also be coated onto cutting tools. The reduced friction would lessen the force necessary to cut an object, extend tool life, and possibly permit increased cutting speeds. Coatings only 2-3 microns thick have been found to improve efficiency and reduce wear in cutting tools.