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Aluminium magnesium boride

Aluminum magnesium boride or BAM is achemical

compound of aluminium, magnesium and boron.

Whereas its nominal formula is AlMgB14, the chemical

composition is closer to Al.Mg.B14. It is aceramic

alloy that is highly resistive to wear and has a low

coefficient of sliding friction, reaching a record value

of 0.02 in lubricated AlMgB14TiB2 composites. First

reported in 1970, BAM has anorthorhombic structure

with four icosahedral B12 units per unit cell.[1] This

ultrahard material has acoefficient of thermal expansion

comparable to that of other widely used materials suchas steel and concrete.

1 Synthesis

BAM powders are produced by heating a nearly

stoichiometric mixture of boron, aluminium and mag-

nesium for a few hours at a temperature in the range

9001500 C. Spurious phases are then dissolved in hot

hydrochloric acid.[1][2] To ease the reaction and make the

product more homogeneous, the starting mixture can be

processed in a high-energy ball mill. All pretreatmentsare carried out in a dry, inert atmosphere to avoid oxida-

tion of the metal powders.[3][4]

BAM films can be coated on silicon or metals bypulsed

laser deposition, using AlMgB14 powder as a target,[5]

whereas bulk samples are obtained by sintering the

powder.[6]

BAM usually contains small amounts of impurity ele-

ments (e.g., oxygen and iron) that enter the material dur-

ing preparation. It is thought that the presence of iron

(most often introduced as wear debris from mill vials

and media) serves as a sintering aid. BAM can be al-

loyed withsilicon,phosphorus,carbon,titanium diboride

(TiB2),aluminium nitride(AlN),titanium carbide(TiC)

orboron nitride(BN).[4][6]

2 Properties

2.1 Structure

Most superhard materials have simple, high-symmetry

crystal structures, e.g., diamond cubic or zinc blende.

However, BAM has a complex, low-symmetry crystalstructure with 64 atoms per unit cell. The unit cell is

orthorhombicand its most salient feature is four boron-

Crystal structure of BAM viewed along the a crystal axis. Blue:

Al, green: Mg, red: B.

containing icosahedra. Each icosahedron contains 12

boron atoms. Eight more boron atoms connect the icosa-

hedra to the other elements in the unit cell. The oc-

cupancy of metal sites in the lattice is lower than one,

and thus while the material is usually identified with theformula AlMgB14, its chemical composition is closer

to Al.Mg.B14.[3][4] Such non-stoichiometry is com-

mon for borides (seecrystal structure of boron-rich metal

boridesand boron carbide). The unit cell parameters of

BAM are a = 1.0313 nm, b = 0.8115 nm, c= 0.5848

nm,Z= 4 (four structure units per unit cell),space group

Imam,Pearson symboloI68, density 2.59 g/cm3.[1] The

melting point is roughly estimated as 2000 C.[7]

2.2 Optoelectronic

BAM has a bandgap of about ~1.5 eV. Significant ab-

sorption is observed at sub-bandgap energies and at-

tributed to metal atoms. Electrical resistivity depends

on the sample purity and is about 104 Ohmcm. The

Seebeck coefficientis relatively high, between 5.4 and

8.0 mV/K. This property originates from electron trans-

fer from metal atoms to the boron icosahedra and is fa-

vorable for thermoelectric applications.[7]

2.3 Hardness

The microhardness of BAM powders is 3235 GPa. Itcan be increased to 4550 GPa upon alloying BAM

with of TiB2[4] or by depositing a quasi-amorphous

1

https://en.wikipedia.org/wiki/Microhardnesshttps://en.wikipedia.org/wiki/Seebeck_coefficienthttps://en.wikipedia.org/wiki/Pearson_symbolhttps://en.wikipedia.org/wiki/Space_grouphttps://en.wikipedia.org/wiki/Boron_carbidehttps://en.wikipedia.org/wiki/Crystal_structure_of_boron-rich_metal_borideshttps://en.wikipedia.org/wiki/Crystal_structure_of_boron-rich_metal_borideshttps://en.wikipedia.org/wiki/Icosahedrahttps://en.wikipedia.org/wiki/Orthorhombichttps://en.wikipedia.org/wiki/Unit_cellhttps://en.wikipedia.org/wiki/Zincblende_(crystal_structure)https://en.wikipedia.org/wiki/Diamond_cubichttps://en.wikipedia.org/wiki/Superhard_materialhttps://en.wikipedia.org/wiki/Boron_nitridehttps://en.wikipedia.org/wiki/Titanium_carbidehttps://en.wikipedia.org/wiki/Aluminium_nitridehttps://en.wikipedia.org/wiki/Titanium_diboridehttps://en.wikipedia.org/wiki/Carbonhttps://en.wikipedia.org/wiki/Phosphorushttps://en.wikipedia.org/wiki/Siliconhttps://en.wikipedia.org/wiki/Sinteringhttps://en.wikipedia.org/wiki/Sinteringhttps://en.wikipedia.org/wiki/Pulsed_laser_depositionhttps://en.wikipedia.org/wiki/Pulsed_laser_depositionhttps://en.wikipedia.org/wiki/Ball_millhttps://en.wikipedia.org/wiki/Hydrochloric_acidhttps://en.wikipedia.org/wiki/Stoichiometrichttps://en.wikipedia.org/wiki/Coefficient_of_thermal_expansionhttps://en.wikipedia.org/wiki/Icosahedronhttps://en.wikipedia.org/wiki/Orthorhombic_crystal_systemhttps://en.wikipedia.org/wiki/Alloyhttps://en.wikipedia.org/wiki/Ceramichttps://en.wikipedia.org/wiki/Boronhttps://en.wikipedia.org/wiki/Magnesiumhttps://en.wikipedia.org/wiki/Aluminiumhttps://en.wikipedia.org/wiki/Chemical_compoundhttps://en.wikipedia.org/wiki/Chemical_compound

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2 5 EXTERNAL LINKS

BAM film.[5] Addition of AlN or TiC to BAM re-

duces its hardness.[6] By definition, a hardness value ex-

ceeding 40 GPa makes BAM a superhard material. In

the BAMTiB2 composite, the maximum hardness and

toughness are achieved at ~60 vol.% of TiB2.[6] The wear

rate is improved by increasing the TiB2 content to 70

80% at the expense of ~10% hardness loss.[8] The TiB2additive is a wear-resistant material itself with a hardness

of 2835 GPa.[6]

2.4 Thermal expansion

Thethermal expansion coefficient (TEC) for AlMgB14was measured as 9106 K1 by dilatometry and by high

temperatureX-ray diffraction using synchrotron radia-

tion. 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 AlMgB14and the mate-

rials themselves being used as wear resistant coatings, the

COTE of AlMgB14 could be used in determining coat-

ing application methods and the performance of the parts

once in service.[3][4]

2.5 Friction

A composite of BAM and TiB2(70 volume percent) has

one of the lowest values of friction coefficients, which

amounts to 0.040.05 in dry scratching by a diamond

tip[5] (cf. 0.04 for Teflon) and decreases to 0.02 in water-

glycol-based lubricants.[9][10]

3 Applications

BAM is commercially available and is being studied for

potential applications. 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 allow in-creased cutting speeds. Coatings only 23 micrometers

thick have been found to improve efficiency and reduce

wear in cutting tools.[11]

4 References

[1] V. I. Matkovich and J. Economy (1970). Structure of

MgAlB14 and a brief critique of structural relationships

in higher borides. Acta Cryst. B 26 (5): 616621.

doi:10.1107/S0567740870002868.

[2] Higashi, I; Ito, T (1983). Refinement of the structure ofMgAlB14. Journal of the Less Common Metals92 (2):

239.doi:10.1016/0022-5088(83)90490-3.

[3] Russell, A. M., B. A. Cook, J. L. Harringaand T. L. Lewis

(2002). Coefficient of thermal expansion of AlMgB14.

Scripta Materialia 46 (9): 62933. doi:10.1016/S1359-

6462(02)00034-9.

[4] Cook, B. A., J. L. Harringa, T. L. Lewis and A. M.

Russell (2000). A new class of ultra-hard materials

based on AlMgB14. Scripta Materalia42 (6): 597602.

doi:10.1016/S1359-6462(99)00400-5.

[5] Tian, Y.; Bastawros, A. F.; Lo, C. C. H.; Constant, A.

P.; Russell, A. M.; Cook, B. A. (2003). Superhard

self-lubricating AlMgB14 films for microelectromechan-

ical devices. Applied Physics Letters 83 (14): 2781.

doi:10.1063/1.1615677.

[6] Ahmed, A; Bahadur, S; Cook, B; Peters, J (2006). Me-

chanical properties and scratch test studies of new ultra-

hard AlMgB14 modified by TiB2. Tribology Interna-

tional39 (2): 129.doi:10.1016/j.triboint.2005.04.012.

[7] Werhcit, Helmut; Kuhlmann, Udo; Krach, Gunnar; Hi-gashi, Iwami; Lundstrm, Torsten; Yu, Yang (1993).

Optical and electronic properties of the orthorhombic

MgAIB14-type borides. Journal of Alloys and Com-

pounds202: 269.doi:10.1016/0925-8388(93)90549-3.

[8] Cook, B.A.; Peters, J.S.; Harringa, J.L.; Rus-

sell, A.M. (2011). Enhanced wear resistance in

AlMgB14TiB2 composites. Wear 271 (56): 640.

doi:10.1016/j.wear.2010.11.013.

[9] Kurt Kleiner (2008-11-21).Material slicker than Teflon

discovered by accident. New Scientist. Archived from the

original on 20 December 2008. Retrieved 2008-12-25.

[10] Higdon, C.; Cook, B.; Harringa, J.; Russell, A.; Gold-

smith, J.; Qu, J.; Blau, P. (2011). Friction and wear

mechanisms in AlMgB14-TiB2 nanocoatings. Wear271

(910): 2111.doi:10.1016/j.wear.2010.11.044.

[11] Tough nanocoatins boost industrial energy efficiency.

Ames Laboratory. Press release. Department of Energy.

18 Nov. 2008.

5 External links

Material slicker than Teflon New Scientist Article onBAM.

News on AlMgB14 Press Release with photos.

http://www.ameslab.gov/final/News/2008rel/Nanocoatings.htmlhttp://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.htmlhttp://www.ameslab.gov/node/5245https://dx.doi.org/10.1016%252Fj.wear.2010.11.044https://en.wikipedia.org/wiki/Digital_object_identifierhttp://web.archive.org/web/20081220162702/http://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.htmlhttp://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.htmlhttp://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.htmlhttps://dx.doi.org/10.1016%252Fj.wear.2010.11.013https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1016%252F0925-8388%252893%252990549-3https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1016%252Fj.triboint.2005.04.012https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1063%252F1.1615677https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1016%252FS1359-6462%252899%252900400-5https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1016%252FS1359-6462%252802%252900034-9https://dx.doi.org/10.1016%252FS1359-6462%252802%252900034-9https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1016%252F0022-5088%252883%252990490-3https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1107%252FS0567740870002868https://en.wikipedia.org/wiki/Digital_object_identifierhttps://en.wikipedia.org/wiki/X-ray_diffractionhttps://en.wikipedia.org/wiki/Thermal_expansionhttps://en.wikipedia.org/wiki/Superhard_material