Boron nitride is an electric insulator with a low coefficient thermal expansion and strong corrosion resistance. Due to its low hardness, it is frequently used as a machinable material. Pyrolytic boron nitride is a thin boron nitride material (1-3mm) produced using CVD processes.
Pyrolytic boron nitride has a lot enhancing properties due to its unique structure. As the material is “grown” from chemicals around in gas phase, PBN by nature has a layered structure. The strong, thin and paralleled slices of PBN give this material flexibility, which is abnormal for most of ceramics, including hot pressed boron nitride. Although PBN is thin and semi-transparent, PBN thick discs are very durable.
*It takes a large force to take this 3mm PBN disc apart, and it’s significantly layered before broken.
PBN material is used to make crucible for grow single crystals in most of cases. For this application, it would be a serious issue if the slices tore apart from each other. As the crystal grow process require accurate temperate control, the lower thermal conductivity caused by the layered structure will be harmful. During the manufacturing, it is common to build up some internal force inside the PBN material, especially on the tips, and this force is the major reason for layered defects for PBN products. For this reason, VGF crucible is harder to make and cost considerably more than simple shape crucibles.
It is quite simple to examine if a PBN part is layered. As the material is usually quite thin, it is semitransparent. Under strong lights, there will be shadow at the layered area, or spot which was formed due to small piece of impurity. Though it looks normal from outside, these flaw may have negative affects on crystal growth. Qualified PBN supplier SAM is treating both of these flaws are unqualified products and will not pass the QC process.
*Layered crucible tip. It can be detected with the LED on a cell phone