Aluminium magnesium boride

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), aluminium nitride (AlN), and boron nitride (BN). Baseline BAM contains elemental aluminium, 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 submicrometre 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.

Properties

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 structure (oI64) with 64 atoms per unit cell. The unit cell is orthorhombic and 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 32-35 GPa, but additions such as TiB2 have been reported to increase the microhardness to 45 GPa.

Material

Hardness (GPa)

C (diamond)

70

BN (cubic boron nitride)

45-50

AlMgB14 + TiB2

40-46

AlMgB14

32-35

TiB2

30-33

WC

23-30

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.

Material

Coefficient of Thermal Expansion (K-1)

AlMgB14

9 x 10-6

Steel

11.7 x 10-6

Ti

8.6 x 10-6

Concrete

10-13 x 10-6

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 micrometres thick have been found to improve efficiency and reduce wear in cutting tools.

See also

BAM (material)

References

^ Structure of MgAlB14 and a brief critique of structural relationships in higher borides, V. I. Matkovich and J. Economy, Acta Cryst. (1970). B26, 616-621, doi:10.1107/S0567740870002868

^ Russell, A. M., B. A. Cook, J. L. Harringa, and T. L. Lewis. “Coefficient of thermal expansion of AlMgB14.” Scripta Materialia 46 (2002): 629-33.

^ Cook, B. A., J. L. Harringa, T. L. Lewis, and A. M. Russell. “A new class of ultra-hard materials based on AlMgB14.” Scripta Materalia 42 (2000): 597-602.

^ Department of Energy. Ames Laboratory. “TOUGH NANOCOATINGS BOOST INDUSTRIAL ENERGY EFFICIENCY.” Press release. 18 Nov. 2008. Public Affairs Office. 18 Nov. 2008 <http://www.external.ameslab.gov/final/news/2008rel/nanocoatings.html>.

External links

Material slicker than Teflon New Scientist Article on BAM.

News on AlMgB14 Press Release with photos.

Newtech Ceramics

Categories: Ceramic materials | Borides | Aluminium compounds | Magnesium compounds

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