Boron nitride
Uniquely versatile in technical ceramics
Boron nitride/ "white graphite" as a lubricant and release agent
Modern casting and high-temperature processes – e.g. for light metals – are subject to increasingly optimised requirements in series production. The core criteria are high output, increased vertical range of manufacture and chemical and thermal resistance of the production systems. In particular, it’s important to reliably prevent light metal melts from sticking to the walls of moulds and to melting devices. Because hot metal melts, such as aluminium melts, are very aggressive: they can adhere, attack mould walls and react. Only a few materials can meet these requirements, with modern high-performance ceramics such as boron nitride at the top of the list.
Capabilities of white graphite
The graphite-like modification of boron nitride (so-called α-BN, also known as “inorganic” or “white graphite”) is often used as a release agent and lubricant. Like the black carbon original, it has a hexagonal layer lattice; as a powder it’s therefore soft and supple and thereby adds the desired lubricating and separating effect. However, in contrast to classic graphite, the friction behavior of boron nitride remains unchanged at the temperatures of up to 1,000 °C/1,832 °F that prevail in light metal foundries. That’s why it’s ideal as a high-temperature solid lubricant. Graphite would oxidise under these circumstances, but release agents and lubricants containing boron nitride would not. They are characterised by excellent separation properties, especially compared to aluminium melts. This prevents molten metal from sticking to the walls of moulds and facilitates sliding of the molten metal on the linings of melt. The material in particular protects the walls of the mould from corrosion and improves the overall service life of all components that come into contact with molten metal.
Use of boron nitride as a lubricant and release agent
Depending on the area of application, boron nitride is widely used in the form of a powder, suspension, size, spray or paste as a lubricant and release agent for applications in high-temperature areas:
BN powder, for example, as a release agent for coating the running surfaces of pressing tools in aluminium extrusion. Without an effective release agent, the aluminium bolt to be extruded could otherwise easily stick to the press ram; in extreme cases it would even be pulled out again when the press is moved back. However, regular oil-containing lubricants are quick to ignite. Using a spray gun, BN can be applied electrostatically in the form of a powder as a release agent between the aluminium block and the press ram and lasts for a good five to six press cycles. Another advantage is that the surface quality increases because the release agent is not drawn into the profile. In addition, extrusion tools coated with boron nitride in the tool store are reliably protected against corrosion and contamination.
In aluminium metallurgy, boron nitride is mostly used in the form of a suspension. This means that BN can for example be used to finish transport channels for liquid aluminium that are provided with a fireproof lining. Conventional linings, on the other hand, are based on bone ash or graphite and have a much smaller range of efficacy.
Further applications of BN coatings:
- Refractory linings
- Coatings of crucibles, launders, sinks, slides, floats, casting ladles, skimmers, measuring probes, sieves, hot-top rings etc.
- Extrusion, hot pressing or super- or fast plastic forging
- Protective coating for systems in high-vacuum metalisation with aluminium
- As a release agent/protective coating in galvanising plants
- Linings offer reliable protection against unwanted thermite reactions in sand casting
- In titanium forming
- In the glass industry as a release agent to prevent adherence when hot glass comes into contact with the forming or processing tools
- As a separating or lubricant additive for greases and oils
BN linings can easily be applied by brushing on, spraying or dipping. Damaged coatings and cracks in the substrates can also be repaired in this way.
Pro tip: When performing several coating procedures using rollers or brushes, the object to be coated should always be preheated. This prevents the first coat from loosening and the protective function of the coatings will then be effective even at a thickness of just a few micrometres.
White graphite is also widely used as a spray with an ethanol basis as a solvent. This ensures quick and easy application for effective non-wettability with good lubricity. However, due to the solvent used, these are extremely flammable and should therefore not be used on components above a maximum of 70 °C/158 °F.
Boron nitride powder or granulate as an additive for thermally conductive plastics
Although they offer the advantage of, among other things, a flexible design for a wide range of technical products, plastics do not have particularly good thermal conductivity. However, a wide variety of electronic components require materials that can dissipate heat quickly and effectively in a confined space – from automotive to entertainment electronics. Up to now, primarily metallic aluminium or copper heat sinks have been used for this. The installation cost is always extremely high. Because they also conduct electricity, insulation layers must be used. In this situation, it’s better to produce cooling elements from thermally conductive plastic instead of metal. This reduces tool costs and avoids the need for an electrical insulation layer.
Capabilities of boron nitride as an additive for thermally conductive plastics
In the form of powder or granules, hexagonal boron nitride makes very good heat conductors in plastics. In addition to a high degree of thermal conductivity, boron nitride powder offers other useful properties, such as good temperature and abrasion resistance, a low dielectric constant and excellent capacity for electrical insulation. In combination with thermoplastics, a thermal conductivity of over 10 W/m*K is achieved. Boron nitride thus turns a plastic with inadequate thermal conductivity properties into a good heat conductor that at the same time offers electrical insulation, thereby providing the electronics sector with considerable freedom for design. This is because plastics containing boron nitride are easy to process through means of extrusion and injection molding, which means that even very complex cooling fin structures are simple to implement.
Advantages of plastic heat sinks optimised for heat conducting:
- Good thermal conductivity
- Great freedom for design
- Lower weight compared to metal components
- Low development times and costs
- Reduced production and assembly costs without additional costs for insulation
Fields of application of boron nitride as an additive
Thermally conductive plastics with boron nitride can be used in numerous fields of technology:
- In automotive electronics for sensors, LED lighting systems, heating systems or components of electric motors
- In the field of consumer electronics, from smartphones to LED TVs and tablets
- 3M engineers have already implemented the prototype of a new type of LED flashlight using thermally conductive plastic. It consists of only two components: a printed circuit board with a light-emitting diode and an electronic control unit with an integrated plastic body, which is melted directly around the board by means of injection molding. The heat sink of the demo LED flashlight consists of two-thirds PET plastic and one-third 3M boron nitride cooling filler. No further assembly steps are necessary for heat sinks. The operating temperatures in degrees Celsius have been halved.
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Boron nitride powder in cosmetics
As an inorganic filler, superfine boron nitride powder is a prominent quality enhancer for cosmetic products. In independent laboratory studies, for example, creams with 3% micronised boron nitride powder added to them are certified to show a significant improvement in skin moisture, elasticity and skin thickness, while the “average roughness of the skin” has been reduced by 21%. In the field of colour cosmetics, boron nitride offers e.g. an improved colour effect and a slight skin-lightening effect.
Skin care
- Improved feel to the touch
- High capacity for coverage
- Increased skin suppleness
- Visible wrinkle reduction
- Reduced blemishes
Colour cosmetics
- Improved colour effects
- Velvety feel on the skin
- Good dispersion
- Excellent adhesion to the skin
- Lasting effect
- Slight skin-lightening effect
- Stabilising effect in emulsions
Capabilities of boron nitride sintered bodies
Moulded parts made of boron nitride (in hexagonal form) are produced at high temperatures and under high pressure using the hot press process (sintering). With its excellent properties of inertness, using sintered boron nitride ceramic as a lining in furnace construction ensures that due to its good thermal and chemical stability coupled with extremely high resistance to thermal shock, no foreign material passes into the melt. This means that in high-temperature furnaces, for example, it’s possible to achieve defect-free detachment of solidified metal melts.
Application of BN sintered bodies
- BN sintered ceramic components are used in foundries or steelworks
- As side dams for thin-strip casting of metal melts (steel and non-ferrous alloys). These ensure that the rollers are sealed laterally to allow the production of flat metal strips directly from a melt
- For crucibles for high-purity molten metals
- As release rings for horizontal continuous casting with steel or non-ferrous alloys
- As thermocouple protection tubes or insulation bushings for current feed-throughs
- As mixed ceramics e.g. in evaporator boats for metalisation systems
- Boron nitride sintered body for atomising metal melts
- Nozzles for powder metal spraying
- Insulators in high-temperature furnaces
- As support elements for graphite heaters
- Sealing gaskets in lambda sensors
With sintered boron nitride components, a wide variety of physical and chemical properties can also be established by adding carbides, oxides and nitrides. For example, BN-ZrO2 composite material, which has a higher mechanical strength than conventional boron nitride ceramics, is used for refractory components and pouring nozzles in nickel- and cobalt-based alloys as well as in the aluminium industry.
Applications for pyrolytic boron nitride
The so-called pyrolytic boron nitride (PBN for short) is closely related to β-boron nitride. It’s usually obtained by chemical vapour deposition (CVD), in which a solid component is separated from the gas phase on the heated surface of a substrate. The advantage here is that components made of pyrolytic boron nitride are flexible and nevertheless dimensionally stable; these include:
- Refractory crucibles, plates or dishes
- Production of active layers for the production of photovoltaic cells
- Manufacture of OLED products
- Cleaning of rare earths or other metals
- Evaporation of pure metals in an ultra-high vacuum
- Crystal growing
Cubic boron nitride (CBN) as abrasive and cutting material
Machining of for example carbide metals is these days often subject to stringent requirements in terms of process speed (for example high-speed grinding), product quality, dimensional accuracy, costs and energy consumption. This requires very special, heavy-duty processing materials.
Advantages of CBN
Cubic boron nitride (CBN, also known as β-boron nitride) is one of the hardest materials known to man and is the second-hardest cutting material after diamond. This material therefore has a high degree of abrasion resistance combined with very good thermal conductivity and chemical resistance. Machining tools made from CBN wear out much more slowly than ordinary cutting materials made from corundum or silicon carbide. The result is a much higher degree of accuracy with regard to shape and dimension. In addition, reliable processing of extremely hard materials is always ensured. Microcrystalline CBN grades do not wear out as a whole, but form new sharp cutting edges under increasing pressure.
Fields of application of CBN
Of great technological importance, CBN is mainly used as a cutting material and an abrasive for indexable inserts in steel processing. The big advantage of this is that unlike diamond, there’s no carbon in the steel even when exposed to temperature. With CBN grinding wheels, even tough and hard steels such as high-speed steel, hot- and cold-work steel can be ground well.
CBN wheels are used for grinding:
- High-alloy and case-hardened tool steels (min. 55 HRC)
- Nickel-based superalloys
- Chilled casting
- Powder coatings with iron material
- Hard alloys based on cobalt-chrome (Stellite)
Sometimes the grains of CBN grinding wheels are also provided with a metal layer made of copper or nickel. This allows the grain to be held perfectly in its bond and the heat generated during the grinding process is dissipated into the bond.
Polycrystalline cubic BN (PCBN/ PKB in German)
Typical components that are processed with PCBN are:
- Engine blocks
- Brake discs and drums
- Valve seats and guides, machine parts
- Cylinder liners
- Transmission
- Flywheels
- Pressed and stamped parts
PCBN are CBN microgranules from the cubic boron nitride form with a ceramic binder phase. PCBN is chemically inert up to high temperatures and does not react with iron. PCBN is used as a cutting material for machining hard or abrasive steel workpiece materials, such as
- High-speed steel
- Case-hardened steel
- Hot-/cold-work tool steel
- Grey cast iron
- Hard-facing alloys
- Sintered iron
Binderless nanocrystalline cubic boron nitride (BNNC)
BNNC is a newly developed cutting material for materials that are difficult to machine. Compared to the established CBN cutting materials, it has an even higher degree of hardness and even better temperature resistance.
Other applications of boron nitride
- Hexagonal boron nitride is also a possible material for (UV) light-emitting diodes, mainly because of its interesting properties as an III-V compound semiconductor.
- Porous sponges made of boron nitride can absorb about 33 times their own weight in oil and organic solvents. After the absorbed liquids have burned away and evaporated, the filter sponge can be reliably recovered. This property is e.g. usable for water treatment and purification.
Production
- The (white) α-modification, hexagonal boron nitride BN is formed at high temperatures from boron oxide and elemental nitrogen N2 using calcium phosphate as a catalyst.
- β-boron nitride (CBN) can be produced from the hexagonal α-modification at high temperatures (from 1,500 °C/2,732 °F) and at the same time under high pressure (between 50 and 90 kbar) – very similar to the way in which graphite becomes diamond.
Forms
- In terms of structure, α-boron nitride is comparable to graphite (therefore also called white graphite), and consists of a honeycomb-shaped, planar, hexagonal crystal structure in layers.
- Cubic β-boron nitride (CBN) as a modification of a boron-nitrogen compound has a cubic, diamond-like structure.
Properties
- Colour: White
- Specific weight: 2.26 g/cm³
- Application temperature: Up to 900 °C/1,652 °F in air, in an inert atmosphere even up to 2,000 °C/3,632 °F
- Resistance to oxidation: up to 1,000 °C/1,832 °F
- Melting point: 2,700 °C/4,892 °F
- Good electrical insulator
- Thermal conductivity: up to 400 W/m*K
- Good slip even at high temperatures
Cubic β-boron nitride (CBN) is one of the hardest materials known to man: the Knoop hardness is around 48 GPa.
Conclusion: High-performance ceramics such as boron nitride offer a great deal of potential for new, innovative fields of application in industry. However, this requires a competent provider to develop bespoke solutions for the specific application together with the user.