More than 100 years ago, a material that was very similar to graphite in terms of structure and performance was launched, that is, hexagonal boron nitride (h-BN). It is a crystal composed of 56.4% nitrogen nuclei and 43.6% boron atoms. Because it is white, it is also called "white graphite". Its structure is shown in the figure below.

Comparison of the lattice structures of graphite (a) and hexagonal boron nitride (b)

In each atomic layer, boron atoms and nitrogen atoms are connected by strong sp² hybrid covalent bonds, with a bond length of a=b=0.2504nm; each two adjacent atomic layers can rely on relatively strong They are connected by weak interatomic force - van der Waals force, and the bond length is c=0.6661lnm. Therefore, h-BN has a very small binding force in the c-axis direction and a large distance between atoms. It is easy to slide between layers and has good lubricity even at high temperatures.

So what are the advantages of slippery h-BN in applications?

Generally speaking, lubricating oil additives improve the lubricity and wear resistance of the surface by changing the form of the lubricating oil film adsorbed on the surface of the moving body to prevent the contact of the rough surface asperities when the friction pair surfaces move with each other. At present, the use of nano-lubricating materials to prepare lubricating additives is the main means to improve the friction-reducing and anti-wear properties of lubricating oil.

These nanomaterials mainly play two roles in lubricating oil, one is the "miniature ball bearing effect"; the other is the "self-healing effect". They will enter the surface of the friction pair when the friction pair is working, changing the friction mode between the friction pairs - from pure sliding friction to sliding and rolling compound friction. Since the friction coefficient of rolling friction is an order of magnitude smaller than sliding friction, so Friction and wear are reduced.

Compared with more reported metal nanoparticles (such as copper and iron), h-BN actually has more application advantages in lubricants. This is because although soft nano-additives have excellent performance, they are consumed quickly during the use of lubricating oil, and their blocking effect on the friction pairs will gradually decrease, thereby reducing the service life of the lubricating oil; on the other hand, these metals Nanoparticles are mostly polyhedral in shape, with surface atoms distributed in a ladder-like manner and highly active. The improvement of lubricating oil is mainly accomplished through the self-healing effect on the surface of the friction pair, and its "miniature ball bearing" effect is not obvious.

As an excellent solid lubricant, h-BN with a layered structure has the advantages of high load-bearing capacity, good stability at high temperatures, and stable physical properties compared to liquid or semi-solid lubricants. It can be dispersed in heat-resistant lubricating grease, water or solvent, sprayed on the friction surface, and wait for the solvent to evaporate to form a dry film; it can also be filled in the surface of resin, ceramics, metal, etc. as a heat-resistant and high-temperature self-lubricating material.