To control friction and wear between moving parts, lubricants are used widely. If lubricants are not used, then the moving parts in the machinery will wear out and become useless. There are different types of lubricants, including solid/dry, semi-solid, liquid, and gaseous.
One of the solid lubricants is Hexagonal Boron Nitride (hBN), which can provide low friction and reduce wear in dry lubricating conditions. hBN can also be mixed with liquid lubricants and greases to improve lubricating properties.
hBN has a low coefficient of friction due to its lamellar crystalline structure..Within each layer, the Boron and Nitrogen atoms are arranged in a hexagonal lattice. The atoms within the layers are connected using covalent bonds, while the layers are held together by weak Van der Waals forces. This leads to easy shearing of the layers of hBN.
The layers easily slide past each other when a shear force is applied, leading to a low coefficient of friction. The layers align themselves parallel to the direction of motion and shear at the contact interfaces to provide a low coefficient of friction. Also, the hBN particles interact with the surfaces of moving parts to form boundary films. The films protect the moving parts from the damage caused due to friction and wear.
hBN can be used in powder form or as an additive in oils and greases. hBN is chemically inert, non-toxic and biodegradable, and is highly stable at elevated temperatures. It is referred to as a ‘Clean’ lubricant. It is a preferred lubricant where the cleanliness of the working environment is a must. It does not get wetted by molten materials or slags and exhibits excellent solid lubricant properties.
Properties of hBN
Applications of hBN
Synthesis of hBN
Some of the methods for producing freestanding hBN crystals are:
Low-temperature growth process
This method involves a fast reaction and produces high yields; therefore, it is used for producing commercial hBN powders. hBN crystals are produced through nitridation of Boron oxide.
The main advantages of this process are ease of preparation and low temperature. It produces large quantities of micrometre to nanometer-sized hBN particles.
High-pressure high-temperature process
In this process, a hydraulic press is used to provide high mechanical pressure. A solvent, a closed BN crucible with a BN source is used.
Atmospheric pressure, high-temperature process
In this method, a horizontal tube furnace under gas flow is used, and N2 is used as a source of Nitrogen. Pure Born powder is used as a source of Boron. It produces high-quality hBN crystals.
Polymer-derived ceramic process
In this method, reactive molecular precursors containing Boron and Nitrogen are used. The precursors have a chemical activity and degrade to form hBN at low temperatures.
