Aluminium magnesium boride
Encyclopedia
Aluminium magnesium boride (AlMgB14), also known as BAM, is a chemical compound
of aluminium
, magnesium
and boron
, and is a ceramic
alloy
that is highly resistive to wear with a low coefficient of sliding friction. First reported in 1970, it has an orthorhombic structure with four icosahedral B12 units per unit cell.
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.
, 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.
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.
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.
, 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.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, and is a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
that is highly resistive to wear with a low coefficient of sliding friction. First reported in 1970, it has an orthorhombic structure with four icosahedral B12 units per unit cell.
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
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
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 siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, titanium diboride (TiB2), aluminium nitride
Aluminium nitride
Aluminium nitride is a nitride of aluminium. Its wurtzite phase is a wide band gap semiconductor material, giving it potential application for deep ultraviolet optoelectronics.-History:...
(AlN), and boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
(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
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
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 graphiteGraphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
die
Die (manufacturing)
A die is a specialized tool used in manufacturing industries to cut or shape material using a press. Like molds, dies are generally customized to the item they are used to create...
at temperatures near 1670 K and pressures near 100 MPa
MPA
-Academic degrees:* Master of Professional Accountancy* Master of Public Administration* Master of Public Affairs* Master of Physician's Assistant-Chemicals:* Medroxyprogesterone acetate, also known by the brand name Depo-Provera* Morpholide of pelargonic acid...
. The powder is loaded into a graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
die in an inert atmosphere to minimize oxygen pick-up by the fine powder particles. The die interior is coated with hexagonal boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
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 cubicDiamond cubic
The diamond cubic crystal structure is a repeating pattern of 8 atoms that certain materials may adopt as they solidify. While the first known example was diamond, other elements in group IV also adopt this structure, including tin, the semiconductors silicon and germanium, and silicon/germanium...
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 LaboratoryAmes Laboratory
Ames Laboratory is a United States Department of Energy national laboratory located in Ames, Iowa. The Laboratory conducts research into various areas of national concern, including the synthesis and study of new materials, energy resources, high-speed computer design, and environmental cleanup...
, 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.
External links
- Material slicker than Teflon New Scientist Article on BAM.
- News on AlMgB14 Press Release with photos.
- Newtech Ceramics