Residual stress and strain in pyrolytic boron nitride resulting from thermal anisotropy

Residual stresses in chemically deposited pyrolytic boron nitride (PBN) crucibles caused by thermal expansion anisotropy during cooling immediately following deposition are analysed. The calculations reveal that radial tension and combined tangential tension and compression exist in the crucible. The maximum stresses increase with an increase in the thickness of the crucible. Furthermore, while the outer wall of the crucible always shrinks upon cooling, the inner wall may expand, due to the residual stress states, resulting in a negative effective thermal expansion coefficient in the tangential direction. The influence of the PBN attachment to the mandrel on which it is deposited is also considered. Specifically, the radial tensile stress in the crucible is shown to increase due to this attachment, which in turn, enhances the delamination of the crucible.