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'''Aluminium magnesium boride''' or '''Al<sub>3</sub>Mg<sub>3</sub>B<sub>56</sub>''',<ref>{{Cite web|url=https://kundoc.com/pdf-structural-and-mechanical-properties-of-almgb-films-experimental-study-and-first.html|title=Structural and mechanical properties of Al―Mg―B films: Experimental study and first-principles calculations - PDF Free Download}}</ref><ref>{{Cite web|url=https://www.thegeneticatlas.com/Kenneth-Nordtvedt-Molecular-Hardness.htm|title=The Genetic Atlas}}</ref><ref>{{Cite journal|url=https://www.osti.gov/pages/servlets/purl/1458625|doi=10.1063/1.4952391|title=First-principles study of crystalline and amorphous AlMgB14-based materials|year=2016|last1=Ivashchenko|first1=V. I.|last2=Turchi|first2=P. E. A.|last3=Veprek|first3=S.|last4=Shevchenko|first4=V. I.|last5=Leszczynski|first5=Jerzy|last6=Gorb|first6=Leonid|last7=Hill|first7=Frances|journal=Journal of Applied Physics|volume=119|issue=20|page=205105|bibcode=2016JAP...119t5105I|osti=1458625}}</ref> colloquially known as '''BAM''', is a [[chemical compound]] of [[aluminium]], [[magnesium]] and [[boron]]. Whereas its nominal formula is AlMgB<sub>14</sub>, the chemical composition is closer to Al<sub>0.75</sub>Mg<sub>0.75</sub>B<sub>14</sub>. It is a [[ceramic]] [[alloy]] that is highly resistive to wear and has an extremely low [[coefficient of sliding friction]], reaching a record value of 0.04 in unlubricated<ref name= ULFC2>{{cite journal
| doi=10.1063/1.1615677
| title=Superhard self-lubricating AlMgB[sub 14] films for microelectromechanical devices
| year=2003
| last1=Tian|first1=Y.|last2=Bastawros|first2=A. F.|last3=Lo|first3=C. C. H.|last4=Constant|first4=A. P.|last5=Russell|first5=A.M.
| last6=Cook|first6=B. A.
| journal=Applied Physics Letters|volume=83|issue=14|pages=2781|bibcode = 2003ApPhL..83.2781T | url=http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1004&context=mse_pubs
}}</ref> and 0.02 in lubricated AlMgB<sub>14</sub>−TiB<sub>2</sub> composites. First reported in 1970, BAM has an [[Orthorhombic crystal system|orthorhombic structure]] with four [[icosahedron|icosahedral]] B<sub>12</sub> units per unit cell.<ref name=s1/> This ultrahard material has a [[coefficient of thermal expansion]] comparable to that of other widely used materials such as steel and concrete.

==Synthesis==
BAM powders are commercially produced by heating a nearly [[stoichiometric]] mixture of elemental boron (low grade because it contains magnesium) and aluminium for a few hours at a temperature in the range 900&nbsp;°C to 1500&nbsp;°C. Spurious phases are then dissolved in hot [[hydrochloric acid]].<ref name=s1/><ref name=s2/> To ease the reaction and make the product more homogeneous, the starting mixture can be processed in a high-energy [[ball mill]]. All pretreatments are carried out in a dry, inert atmosphere to avoid oxidation of the metal powders.<ref name=e1/><ref name=e2/>

BAM films can be coated on silicon or metals by [[pulsed laser deposition]], using AlMgB<sub>14</sub> powder as a target,<ref name=f1/> whereas bulk samples are obtained by [[sintering]] the powder.<ref name=h1/>

BAM usually 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) serves as a [[sintering]] aid. BAM can be alloyed with [[silicon]], [[phosphorus]], [[carbon]], [[titanium diboride]] (TiB<sub>2</sub>), [[aluminium nitride]] (AlN), [[titanium carbide]] (TiC) or [[boron nitride]] (BN).<ref name=e2/><ref name=h1/>

==Properties==
BAM has the lowest known unlubricated coefficient of friction (0.04) possibly due to self-lubrication.<ref name= ULFC2/>

===Structure===
[[File:AlMgB14s.png|thumb|300px|Crystal structure of BAM viewed along the ''a'' crystal axis. Blue: Al, green: Mg, red: B.]]
Most [[superhard material]]s have simple, high-symmetry crystal structures, e.g., [[diamond cubic]] or [[Zincblende (crystal structure)|zinc blende]]. However, BAM has a complex, low-symmetry crystal structure 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. The occupancy of metal sites in the lattice is lower than one, and thus, while the material is usually identified with the formula AlMgB<sub>14</sub>, its chemical composition is closer to Al<sub>0.75</sub>Mg<sub>0.75</sub>B<sub>14</sub>.<ref name=e1/><ref name=e2/> Such non-[[stoichiometry]] is common for borides (see [[crystal structure of boron-rich metal borides]] and [[boron carbide]]). The unit cell parameters of BAM are ''a'' = 1.0313&nbsp;nm, ''b'' = 0.8115&nbsp;nm, ''c'' = 0.5848&nbsp;nm, ''Z'' = 4 (four structure units per unit cell), [[space group]] ''Imma'', [[Pearson symbol]] oI68, density 2.59 g/cm<sup>3</sup>.<ref name=s1/> The melting point is roughly estimated as 2000&nbsp;°C.<ref name=prop1/>

===Optoelectronic===
BAM has a bandgap of about ~1.5 eV. Significant absorption is observed at sub-bandgap energies and attributed to metal atoms. Electrical resistivity depends on the sample purity and is about 10<sup>4</sup> Ohm·cm. The [[Seebeck coefficient]] is relatively high, between −5.4 and −8.0 mV/K. This property originates from electron transfer from metal atoms to the boron icosahedra and is favorable for thermoelectric applications.<ref name=prop1/>

===Hardness & Fracture toughness===
The [[microhardness]] of BAM powders is 32–35 GPa. It can be increased to 45 GPa by alloying with Boron rich Titanium Boride, Fracture toughness can be increased with TiB<sub>2</sub><ref name=e2/> or by depositing a quasi-amorphous BAM film.<ref name=f1/> Addition of AlN or TiC to BAM reduces its hardness.<ref name=h1/> By definition, a hardness value exceeding 40 GPa makes BAM a [[superhard material]]. In the BAM−TiB<sub>2</sub> composite, the maximum hardness and toughness are achieved at ~60 vol.% of TiB<sub>2</sub>.<ref name=h1/> The wear rate is improved by increasing the TiB<sub>2</sub> content to 70–80% at the expense of ~10% hardness loss.<ref name=h2/> The TiB<sub>2</sub> additive is a wear-resistant material itself with a hardness of 28–35 GPa.<ref name=h1/>

===Thermal expansion===
The [[thermal expansion]] coefficient (TEC, also known as Coefficient Of Thermal Expansion, COTE) for AlMgB<sub>14</sub> was measured as 9{{e|-6}} K<sup>−1</sup> by dilatometry and by high temperature [[X-ray diffraction]] using synchrotron 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 AlMgB<sub>14</sub> and the materials themselves being used as wear resistant coatings, the COTE of AlMgB<sub>14</sub> could be used in determining coating application methods and the performance of the parts once in service.<ref name=e1/><ref name=e2/>
{| class="wikitable sortable" border="1"
|-
! Material
! TEC (10<sup>−6</sup> K<sup>−1</sup>)<ref name=e1/>
|-
| AlMgB<sub>14</sub>
|align="center" |9
|-
| Steel
|align="center" |11.7 
|-
| Ti
|align="center" |8.6 
|-
| Concrete
|align="center" |10–13 
|}

===Friction===
A composite of BAM and [[Titanium diboride|TiB<sub>2</sub>]] (70 volume percent of TiB<sub>2</sub>) has one of the lowest values of [[coefficient of friction|friction coefficient]]s, which amounts to 0.04–0.05 in dry scratching by a diamond tip<ref name=f1/> (cf. 0.04 for Teflon) and decreases to 0.02 in water-glycol-based lubricants.<ref name=f3/><ref name=f2/>

==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 + TiB<sub>2</sub> 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 increased cutting speeds. Coatings only 2–3 micrometers thick have been found to improve efficiency and reduce wear in cutting tools.<ref>[http://www.ameslab.gov/node/5245 Tough nanocoatins boost industrial energy efficiency] {{Webarchive|url=https://web.archive.org/web/20120524102003/http://www.ameslab.gov/node/5245 |date=2012-05-24 }}. Ames Laboratory. Press release. Department of Energy. 18 Nov. 2008.</ref>

==See also==
* [[Non-stick surface]]

==References==
{{reflist|35em|refs=
<ref name=s1>{{cite journal|title=Structure of MgAlB<sub>14</sub> and a brief critique of structural relationships in higher borides|author1=V. I. Matkovich |author2=J. Economy |journal= Acta Crystallogr. B|year=1970| volume=26|pages= 616–621 |doi=10.1107/S0567740870002868|issue=5|bibcode=1970AcCrB..26..616M }}</ref>

<ref name=s2>{{cite journal|title=Refinement of the structure of MgAlB14|doi=10.1016/0022-5088(83)90490-3|year=1983|last1=Higashi|first1=I|last2=Ito|first2=T|journal=Journal of the Less Common Metals|volume=92|issue=2|pages=239 }}</ref>

<ref name=f1>{{cite journal|doi=10.1063/1.1615677|title=Superhard self-lubricating AlMgB14 films for microelectromechanical devices|year=2003|last1=Tian|first1=Y.|last2=Bastawros|first2=A. F.|last3=Lo|first3=C. C. H.|last4=Constant|first4=A. P.|last5=Russell|first5=A. M.|last6=Cook|first6=B. A.|journal=Applied Physics Letters|volume=83|issue=14|pages=2781|bibcode=2003ApPhL..83.2781T|url=https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1004&context=mse_pubs}}</ref>

<ref name=f3>{{cite web
| url = https://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.html
| title = Material slicker than Teflon discovered by accident
| author = Kurt Kleiner 
| date = 2008-11-21
| work = New Scientist
| accessdate = 2008-12-25| archiveurl= https://web.archive.org/web/20081220162702/http://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.html| archivedate= 20 December 2008 <!--DASHBot-->| url-status=live}}</ref>

<ref name=f2>{{cite journal|doi=10.1016/j.wear.2010.11.044|title=Friction and wear mechanisms in AlMgB14-TiB2 nanocoatings|year=2011|last1=Higdon|first1=C.|last2=Cook|first2=B.|last3=Harringa|first3=J.|last4=Russell|first4=A.|last5=Goldsmith|first5=J.|last6=Qu|first6=J.|last7=Blau|first7=P.|journal=Wear|volume=271|issue=9–10|pages=2111}}</ref>

<ref name=e1>{{cite journal |author1=Russell, A. M. |author2=B. A. Cook |author3=J. L. Harringa |author4=T. L. Lewis|title=Coefficient of thermal expansion of AlMgB14|doi=10.1016/S1359-6462(02)00034-9 |journal= Scripta Materialia |volume=46|year=2002|pages= 629–33|issue=9}}</ref>

<ref name=e2>{{cite journal |author1=Cook, B. A. |author2=J. L. Harringa |author3=T. L. Lewis |author4=A. M. Russell |title=A new class of ultra-hard materials based on AlMgB14|journal= Scripta Materialia |volume=42|year=2000|pages= 597–602|doi=10.1016/S1359-6462(99)00400-5|issue=6|url=https://zenodo.org/record/1260234}}</ref>

<ref name=h1>{{cite journal|doi=10.1016/j.triboint.2005.04.012|title=Mechanical properties and scratch test studies of new ultra-hard AlMgB14 modified by TiB2|year=2006|last1=Ahmed|first1=A|last2=Bahadur|first2=S|last3=Cook|first3=B|last4=Peters|first4=J|journal=Tribology International|volume=39|issue=2|pages=129}}</ref>

<ref name=h2>{{cite journal|doi=10.1016/j.wear.2010.11.013 |title=Enhanced wear resistance in AlMgB14–TiB2 composites|year=2011|last1=Cook|first1=B.A.|last2=Peters|first2=J.S.|last3=Harringa|first3=J.L.|last4=Russell|first4=A.M.|journal=Wear|volume=271|issue=5–6|pages=640|url=https://zenodo.org/record/1259455}}</ref>

<ref name=prop1>{{cite journal|doi=10.1016/0925-8388(93)90549-3|title=Optical and electronic properties of the orthorhombic MgAIB14-type borides|year=1993|last1=Werhcit|first1=Helmut|last2=Kuhlmann|first2=Udo|last3=Krach|first3=Gunnar|last4=Higashi|first4=Iwami|last5=Lundström|first5=Torsten|last6=Yu|first6=Yang|journal=Journal of Alloys and Compounds|volume=202|issue=1–2|pages=269–281}}</ref>
}}

==External links==
*[https://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.html Material slicker than Teflon] New Scientist Article on BAM.
*[https://web.archive.org/web/20081217173930/http://www.ameslab.gov/final/News/2008rel/Nanocoatings.html News on AlMgB<sub>14</sub>] Press Release with photos.

{{aluminium compounds}}
{{magnesium compounds}}

[[Category:Ceramic materials]]
[[Category:Superhard materials]]
[[Category:Dry lubricants]]
[[Category:Borides]]
[[Category:Aluminium compounds]]
[[Category:Magnesium compounds]]