Boron nitride (BN) is also called white graphite because it is white and has characteristics in common with graphite.
It is mainly divided into two forms, one is a hexagonal structure, and the other is a cubic structure. The hexagonal structure can be converted into a cubic structure under high temperature and high pressure environment conditions.
The production of boron nitride ceramics mainly uses two methods, one is the cold pressing sintering, and the other is the hot pressing sintering. The hot pressing sintering method is briefly introduced below.
- Mixing the boron nitride fine powder with a certain amount of additives. There are strict requirements for the size of the boron nitride fine powder particles, which must be within 5 microns.
- Predicting the pressure of the static press, the test pressure is between 100 and 150 MPa, and then pulverize the raw materials to less than 0.5 mm.
- Loading the raw material into the graphite grinding tool, and then performing high under temperature and high pressure in the hot pressing furnace. There are carbon tubes in the hot pressing furnace which are mainly used for heating. The heating temperature is increased to 1600-1700 ℃ and the pressure is controlled between 20-25 MPa. It can be kept warm for about an hour. After cooling, it can be completed by machining.
Notes:
- Diboron trioxide (B2O3) plays a vital role in the production of boron nitride ceramics, which affects the quality of boron nitride ceramics. If there is no boron trioxide, the density of the BN ceramics will be greatly reduced.
For boron nitride raw materials that do not contain B2O3, a binder should be appropriately added to increase the ease of thermal sintering. - If a relatively high content of diboron trioxide is contained, stabilizers need to be appropriately added. And the boron nitride ceramics after hot pressing needs to be in a vacuum environment or nitrogen environment. High temperature treatment makes the B2O3 volatilize, thereby increasing the quality of boron nitride ceramic products and maintaining their stable performance.
