How Boron Nitride Acts as a Solid Lubricant

Boron nitride may exist in two forms of crystal lattice: cubic and hexagonal.

Due to its tight diamond-like structure cubic boron nitride is extremely hard. It has poor lubrication properties and is used in cutting and abrasive tools as a diamond substitute.

Hexagonal boron nitride (HBN) is a solid lubricant relating to the class of Inorganic lubricants with lamellar structure, which also includes molybdenum disulphide, graphite and some other sulphides, selenides and tellurides (chalcogenides) of molybdenum, tungsten, niobium, tantalum and titanium.
The crystal lattice of hexagonal boron nitride consists of hexagonal rings forming thin parallel planes. Atoms of boron (B) and nitrogen (N) are covalently bonded to other atoms in the plane with the angle 120 ° between two bonds (each boron atom is bonded to three nitrogen atoms and each nitrogen atom is bonded to three boron atoms).
The planes are bonded to each other by weak Van der Waals forces.


 

The layered structure allows sliding movement of the parallel planes. Weak bonding between the planes provides low shear strength in the direction of the sliding movement but high compression strength in the direction perpendicular to the sliding movement.

Friction forces cause the particles of boron nitride to orient in the direction, in which the planes are parallel to the sliding movement. The anisotropy of the mechanical properties imparts the combination of low coefficient of friction and high carrying load capacity to boron nitride.

Boron nitride forms a lubrication film strongly adhered to the substrate surface. The lubrication film provides good wear resistance and seizure resistance (compatibility).

Similar to molibdenum disulfide moist atmosphere is not required for lubrication by boron nitride. It demonstrates low friction in dry atmosphere and in vacuum.

Coefficient of friction of boron nitride is within the range 0.1-0.7, which is similar to that of graphite and molybdenum disulfide. Impurities (eg. boron oxide) exert adverse effect on the lubrication properties of boron nitride.

Boron nitride is chemically inert substance. It is non-reactive to most acids, alkalis, solvents and non-wetted by molten aluminum, magnesium, molten salts and glass.

The main advantage of boron nitride as compared to graphite and molybdenum disulfide is its thermal stability. Hexagonal boron nitride retains its lubrication properties up to 5000°F (2760°C) in inert or reducing environment and up to 1600°F (870°C) in oxidizing atmosphere.

Boron nitride has high thermal conductivity.

Some applications of hexagonal boron nitride:

• Additives in lubricating oils
• Components of polymer based composite anti-friction coatings
• Second phase particles of metal based composite anti-friction coatings
• Solid lubricant in metal forming
• Release coatings and non-sticking refractory linings in foundry
• Sintered ceramic parts for high temperature application

Boron Nitride Break Rings for Continuous Casting

Continuous casting is one of the most common metal manufacturing technologies. The liquid metal flows from a tundish into the mold, in which the strand shell is formed. The metal strand is then taken off continuously or intermittently, at a speed and solidification rate appropriate to the particular alloy.
The ceramic break ring or the nozzle comes into contact with both the melt and the solidified alloy, and must satisfy correspondingly strict requirements:

• Chemical resistance to the solidified metal, i.e. no contamination of the alloy
• Good sliding properties, i.e. does not stick to the alloy
• High thermal shock resistance
• Defined thermal conductivity
• High wear resistance
• Easy to machine

Boron Nitride Rings are extensively used in the Industry.  Momentive offers a group of different boron nitride materials and boron nitride composites that are matched to different metal alloys. From these materials,  manufacture nozzles or break rings that increase the lifetimes for the following metals:

• Steels of all kinds
• Copper alloys
• Nickel alloys
• Superalloys
• Precious metals, particularly gold alloys

Spinning Boron Nitride Tubes into Yarns

Researchers have long been able to make nanotubes out of carbon -- super-tough carbon nanotube fibers are suitable for weaving into electronic cloth, are four times tougher than spider silk, and 17 times tougher than the Kevlar used in bullet-proof vests.

Creating such fibers from boron nitride has proved elusive. Carbon and boron nitride are about the same strength, but boron nitride nano tubes(BNNTs) can survive temperatures that are twice as high as carbon nanotubes can survive –- 800°C and higher. Up until now, researchers have only been able to create high-quality BNNTs a micron long. Larger versions have been perforated with defects in the crystalline structure.

These problems now appear to be largely resolved. In a recent paper published in Nanotechnology, A team of materials scientists at the NASA Langley Creativity and Innovation Program, the NASA Subsonic Fixed Wing program, DOE's Jefferson Lab and the Commonwealth of Virginia, describe the ability to create high-quality, uniformly crystalline BNNTs in large quantities. "Other labs can make really good nanotubes that are short or really crummy ones that are long. We've developed a technique that makes really good ones that are really long," said Mike Smith, a staff scientist at NASA's Langley Research Center.*
A cotton-like mass of nanotubes is finger-twisted into a yarn about one millimeter wide. "They're big and fluffy, textile-like," said Kevin Jordan, a staff electrical engineer at Jefferson Lab. "This means that you can use commercial textile manufacturing and handling techniques to blend them into things like body armor and solar cells and other applications."

The “spinning” process involves a laser aimed at a cake of boron inside a chamber filled with nitrogen. This forms a plume of boron gas that shoots upward. A cooled metal wire is then inserted into the gas, causing the gas to cool and form liquid droplets. The droplets combine with the nitrogen to self-assemble into BNNTs. "It's like fuel-air-spark in an engine," says NASA aerospace scientist Michael Smith. "The reaction advances violently, creating the superlong tubes in just milliseconds."

Why boron nitride rather than carbon? Building large amounts of inexpensive boron nitride nanotubes opens the door for lighter, faster car frames; affordable space vehicles and ultralightweight armor. Because of their excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. Boron nitride has great potential for nanotechnology applications –- BNNTs are more thermally and chemically stable than carbon nanotubes. And BNNTs can be produced with a structure similar to that of carbon nanotubes. However, their properties are very different –- carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, whereas BNNT is an electrical insulator with a wide band gap of ~5.5 eV (the same as diamond). Chemical resistance is better for BNNTs, which are able to survive in air up to much higher temperatures. According toScienceNOW, BNNTs also offer the potential for “pinpoint precision to attack cancer cells by sticking to tumors, absorbing neutrons from a targeted beam, and generating localized alpha radiation to kill the cancer.”

Building large amounts of inexpensive boron nitride nanotubes opens the door for lighter, faster car frames; affordable space vehicles and ultra lightweight armor.

"This is the start of a revolution in materials," says Dennis Bushnell, a NASA engineer who has hopes of using BNNTs for space vehicles. "Just about everything can be made lighter, and hopefully, cheaper. You're talking about energy savings all over the place."

Boron Nitride Coatings Improve Metal Stamping Operation and Die Life

Metal Stamping operations are typical in the Industry. In many cases the manufacturer experience typical problems of metal sticking and poor die life. In such cases many times Boron Nitride Coatings are quite effective. For example, a metal stamping operation was experiencing sticking and poor die life on a particularly difficult-to-form shape; several remedies were tried unsuccessfully. A thin layer of boron nitride was sprayed on the die surfaces. Not only did the stamping operation see a marked improvement in release of the part, but also because of the inherent lubricity of the coating, the die life was significantly increased. 
So Boron Nitride Coatings can also be tried in metal stamping operations.

Boron Nitride Coating Ideal on Cast Iron Stalk Tube in Low Pressure Die Casting

Low Pressure Die Casting (LPDC) is a process that is being widely used in the automobile component industry. Stalk tube is a critical component of the casting machine. Various types of Stalk Tubes are being used in the Low Pressure Die Casting process by automotive component manufacturers. Due to cost advantages and strength over the ceramic tubes, Cast Iron Stalk tubes are being preferred by many casters.  However, the following are the major issues faced with these tubes:

• These tubes get corroded very fast due to oxidation as molten aluminium is quite aggressive
• The ferrous particles mix with the molten aluminium and impact the quality of the casting
• It is very difficult to remove the deposited aluminium metal from the stalk tube

Boron Nitride Coatings have been tried and have been found to be quite effective on Cast Iron Stalk Tubes. The tube can be coated with EPC/GPC grade coating of Momentive(formerly GE Advanced Materials) Boron Nitride. The tube has to be coated from both inside as well outside with a thin layer of Boron Nitride. Spray coating method can be used as otherwise it is difficult to cover the inside surface of the tube. After the coating, it should be dried thoroughly either at room temperature or by blowing hot air at 80 deg C. The second coat is to be done once the first coat is completely dry. Wipe the tube with a soft cloth so that excess boron nitride is removed. Use the tube.

Boron Nitride coatings can withstand temperature  of 850 deg C in oxidising environment and up to 1850 deg C in reducing environment. So the layer of these coatings protect the tube from aggressive molten aluminium, not allow traces of ferrous metal to mix with aluminium melt as well as deposited aluminium metal can be removed easily as these coatings also act as release agent.

Boron Nitride Coatings GPC/EPC now come as Momentive

Aluminium Die Casting: Coating Ladles and Dies with Boron Nitride Coatings

In the die casting and pressure die casting foundries ladles from cast irons or from cast steel are usually used. Due to the large mass these ladles extract large amounts of heat from the melt. In order to avoid this disadvantage, the metal casting working group of the FH Aalen uses cylindrical ladles made out of formed metal sheets. The sheet metal has a wall thickness of 0.8 mm. These ladles have only a small mass and to extract therefore only small amounts of heat the melt.

Usually ladles in the Aluminum foundry industry are protected from an attack of the melt with Boron Nitride Coatings. The more thin-walled however the ladle is, the more important is an appropriate protection by a coating. So consumption of ladles is to be reduced among other things. Still more important is however the avoidance of the iron accommodation of the aluminium melts from the ladles. The metal casting working group of the FH Aalen used ladles coated with the Boron Nitride coating by spraying. It was shown that the coating remains practically unwetted by the aluminium melts, which are poured in the test foundry of the metal working group. Also after numerous castings the non-wetting behaviour of the coating is preserved. This is in as much noteworthy as the non-wetting of the ladle surface prevents a skin formation in the ladle. These skins bring numerous problems with itself. They oxidize at the surface and when reconducting the ladle into the holding furnace then they are usually brought in. Gradually so the oxides in the melt are enriched. Simultaneous their accumulation in the cast parts is to be determined.

Since the Boron Nitride Coating is relatively soft due to the small hardness of the boron nitride, a damage of the coating layer can occur by a mechanical effect. A damage of the coating became apparent with the attempts at the top margin of the ladles. This is caused by hitting the ladles upon outside parts of the holding furnace in order to remove possible remains of the melts from the ladle.

If such a mechanical damage of the coating is avoided, can be operated here with satisfying service lives of the coating.

Boron Nitride Spray Acts as Slumping Release Agent in Glass Moulding

Boron Nitride Spray is used for coating steel moulds in glass industry. There is no pre heating required. Here is experience of an actual user:

I use the **GE (Momentive) BN, and have been having much better luck with it as a slumping release than a casting release (which is what I wanted it for). I think there must be many different formulations out there, and that probably accounts for some of the variations in max temp.

What works best for me is multiple thin applications, prefired. That is, I spray a thin coat of GE BN across the piece as evenly as possible, fire it to about 1400, let it cool and rub it down gently to smooth it out. Then I respray and refire and--if I'm really paranoid, respray and refire again. Done that way, it releases very well at slumping temps, has fewer problems at higher temps and lasts through several firings if I'm careful. I doubt it would last 60 firings though--it has a tendency to flake off after about the fifth firing.
The big question for me is why that's better than kilnwash for slumping. For full fuse and casting, BN doesn't clog up fine detail in the mold and the finish is a little glossier with BN than with kilnwash...if your mold is a relatively smooth, even shape. If not, I'll probably be grinding the mold off the glass in a few places, and I'm getting so I can predict where that'll be...

I figure by the time I really figure it out I'll be at the end of the spray can, and I'll decide then if I really want to buy another can. I've also tried colloidal alumina in solution for a casting release as recommended by a refractory mfg...but so far that's not been very successful.

*GE Boron Nitride Spray is now Momentive Boron Nitride Spray II