George Stavnes, Pyrotek Global Product Manager and Coatings Expert
Generally speaking, since we’ll break it down more specifically in the following questions and answers, why is it critical for aluminium operations to use high-quality boron nitride (BN) coatings, even during times when value-add casting is reduced?
George Stavnes: The biggest reason to continue using a good coating is to continue to protect your investment – i.e., refractory, launders, etc. Using a protective coating that does not adequately protect your refractory will cost more money in the long run.
A coating increases the life of a substrate, providing a sacrificial and protective barrier. Can you explain how spending money on a coating actually saves you money?
GS: Choosing the correct coating and using it wisely will extend the life of a substrate, and, the better the coating, typically the more casts obtained between recoating activities.
Proper application of high-performance coatings is critical to maximizing performance. What could lead to challenges?
GS: The first is applying the coating too thick, causing it to spall. BN is most effective when applied to a dry film thickness of 50-150 microns. Spray application is the best way to control film thickness, and works great on hot refractory, as well as cold.
The second is the thixotropy or rheology of the coating. These coatings utilize ingredients where their viscosity is dependent on shear rate. Because they are shear-sensitive ingredients, they are pseudoplastic in nature, and their viscosity is dependent on several variables. It is always best to thoroughly mix any coating prior to use, and use wise dilution as a tool in making a customer’s application work best.
What part can protective coatings, like those offered by Pyrotek, play when it comes to aluminium scrap? What’s the potential for money savings related to scrap?
GS: BN is a good choice both before and after filtration. Being chemically inert to molten aluminium and having a lower density than most other coatings (like zirconium silicate, etc.), defects are less likely.
Better metal quality leads to greater product quality. How do coatings improve metal quality?
GS: A perfect example is in the foundry industry where the benefits derived from proper coating selection are:
- Control of heat transfer to obtain better directional solidification and ensure that the castings fill properly.
- Control of the metal flow to ensure that it reaches all parts of the die at a sufficient temperature to prevent the formation of seams, cold laps, etc.
- Easy release, since castings are extracted at just below the solidification temperature, easy release ensures that castings do not come out deformed.
- Good surfaces, which reduce finishing costs.
- Longer die life, which increases productivity and reduces maintenance.
In aluminium casthouses, the right coating prevents substrate degradation, allows for better flow with less oxide generation, and smooth scull release providing quicker casting turn around.
Maximizing productivity is on everyone’s mind as they try and stay competitive in this business environment. Can you tie productivity to the use of protective coatings? What about profit?
GS: When a customer protects their refractory, they protect their pocketbook via throughput consistency, optimized labor, less refractory capital, less downtime, etc.
All coatings are not created equal. What should someone look for when selecting a coating? What do they need to know when comparing one coating with another?
GS: Percent solids and the type of solids are critical measurements that drive the cost of a coating. Other things to look for:
- BN quality used – avoid chalky BN which may indicate poor reaction manufacturing.
- Dilution may not always be an economical advantage. Dilution is more of a methodology or tool to reach a desired film finish on the substrate. For example, the higher the dilution, the more wet paint is required to reach a minimal dry film thickness target.
Boron nitride, or just BN, is considered the industry standard best practice globally. What are some key properties that have put it in that position?
GS: As a best practice, hexagonal boron nitride (h-BN) thermal properties fit most aluminium process conditions and provide the best lubricity and non-wetting properties for those processes. Other best practices and key properties include:
- Use temperatures up to 1800° C (3300° F) are possible in inert and reducing atmospheres, up to 1400° C (2550° F) before sublimation in vacuum atmospheres, and approaches 1000° C (1832° F) in oxidizing atmospheres.
- Very high thermal conductivity — at room temperature, it is about 0.08 cal/(cm-sec-K), ideal for use in heat sinks and as an additive to increase the thermal conductivity of oils, coolants, potting compounds, friction plates and release agents.
- As a ceramic, BN is an excellent low-loss insulator material with dielectric properties superior to those of dense alumina. It has a dielectric constant of about 4 (half that of Al203) and a dielectric strength of nearly 1000 volts per mil, more than two times that of alumina. BN is also transparent to microwave energy.
- Completely inorganic and inert, and is totally non-wetted by molten aluminium, magnesium, and their drosses, and offers exceptional non-reaction by most molten metals and slags. It is commonly used as a release agent and as a container material for molten salts, cryolite, aluminium, magnesium, silicon, iron, steels, copper, non-lead glasses, germanium, indium, antimony, tin, cadmium, and metal oxides, as well as many other materials. Also used as braze stop-off and weld spatter release.
- Hexagonal boron nitride (h-BN) is hexagonal and platy like graphite and offers similar lubrication, but is usable to much greater temperatures. For this reason, boron nitride is often referred to as “white graphite.” However, unlike graphite, BN maintains lubricity in vacuum atmospheres, making it ideal as a high-temperature lubricant for hot-pressing and isothermal forming of superalloys.
Pyrotek offers BN coatings by ZYP Coatings Inc., of Oak Ridge, Tennessee, USA.
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