Boron Nitride is a versatile substance that can be used for many things from lubrication of machine parts, to creating advanced ceramics (what we use it for), and even in the cosmetics industry. It’s available in both a powdered form and as a solid and brings with it a multitude of properties that enhance ceramic materials by introducing those unique properties.
While Boron Nitride is used for many things, it also comes in many grades, all of which are engineered specifically to give a distinct set of properties, which cater to specific applications in a wide range of industries. What do a makeup company, locksmith, and a kiln operator have in common? They all use varying grades of Boron Nitride for their own purposes. Let’s dive into the 6 grades and what they do.
There Are 6 Grades of Boron Nitride with Unique Properties
The manufacturing industry breaks down Boron Nitride into 6 distinct grades, which handle a different range of temperatures, have different densities, and differ in their physical characteristics drastically enough to be differentiated from one another.
The highest purity of Boron Nitride comes in the form of “AX05”, which is considered to be the “pinnacle” of the various Boron Nitride materials available in the ceramics and manufacturing industries. It has a purity of over 99.7% and can be formed into large slabs or machined into a variety of hollow shapes like crucibles or cylinders.
AX05 can withstand temperatures over 3,632F, which makes it an outstanding material for use inside of even the highest temperature kilns and furnaces. Moreover, it's a great electric insulator with an incredibly high dielectric strength; high temperatures and corrosive environments react very little to AX05 Boron Nitride.
The most unique characteristic of AX05 sets it apart from most other grades; it does not depend on binders, making it a non-wet material, which in turn means it is inert against almost all molten metals. Since it lacks binders, it can stand the most amount of stress from high-purity molten metals and has no thermal shock whatsoever.
A-Grade is the oldest form of machined Boron Nitride in our industry. It’s hot-pressed with Boric Anhydride, which brings it all together into a workable material. Since this grade of Boron Nitride is old, it finds itself most used in legacy applications such as the high-temperature insulators that are found in older semiconductors. To put it simply, this is the stuff used on the boards of our antique electronics, which still get a surprising amount of use today (for instance, much of our missile defense!). While there are few contemporary applications for A-Grade Boron Nitride, it still has plenty of relevance.
The primary reason this grade has found itself used less and less is due to the tendency for its Boric Anhydride bonder to corrode in even slightly moist environments. Keeping this grade of Boron Nitride in anything but a low-humidity, air-controlled room will ruin it.
While AX05 is mighty yet expensive and A is too antiquated for most of our modern needs, HP is the “goldilocks” grade of Boron Nitride that finds itself in most of the applications in ceramics, manufacturing, et al. It is a middle-of-the-road grade with a low rate of thermal expansion, high resistance to thermal shock, and strong electrical properties.
Think of HP as "Grade-A 2.0". It functions in much the same way and is just as easy to manufacture. The key difference between the two grades is that HP uses a new binder known as calcium borate glass. This bonding agent is minimally affected by moisture and humidity, making a low-cost and shelf-stable grade of Boron Nitride that is extremely functional due to its high degree of machinability. In a nutshell, HP grade Boron Nitride is easy to shape, functions well under high temperatures (1,832F) and reacts very little to the rigors of high-intensity applications.
As long as we are using point systems, you could take things a step further and think of HPL-Grade Boron Nitride as “Grade A 2.5”. It’s not a huge improvement over HP but it’s notable enough. HPL-Grade Boron Nitride can withstand temperatures of 3,632F and that is its primary difference.
The higher temperature range of HPL-Grade is due to a smaller amount of calcium borate glass binder. This lends itself to a softer material that can be machined just as easily yet benefits more from the thermal durability of pure Boron Nitride.
Of all the grades of Boron Nitride, M26/M is the only grade that uses a silica matrix as its binder at a ratio of about 60% Boron Nitride and 40% high-quality silica. Using silica as a bonding agent makes M26/M the go-to grade of Boron Nitride for semiconductor manufacturing.
While A-Grade Boron Nitride could unravel in a humid room, M26/M can be immersed in water for up to 48 hours and still function. The 40% ratio of silica means it’s durable, machinable, and has almost no reaction to moisture or temperature fluctuation. The only sacrifice is a lower degree of thermal conductivity. However, the electrical properties coupled with this grade’s moisture resistance makes it the primary grade of Boron Nitride used in modern military electronics.
The unique hardness of zirconia is mixed with a ratio of Boron Nitride and borosilicate glass to make ZBSN-Grade Boron Nitride. This unique mixture gives ZBSN the mechanical strength of hot-pressed Boron Nitride without the reactivity we often see with grades that use that method as a binder. Furthermore, the zirconia gives ZBSN an incredible hardness.
ZBSN is used often in molten metal applications where a mold needs to have a high degree of wear resistance and extremely low thermal conductivity.