Stability in Air at Ambient Temperature of BN Coatings Processed
from BCl$_3$-NH$_3$-H$_2$ Gas Mixtures
High Temperature Ceramic Matrix Composites, (2006)DOI: 10.1002/3527605622.ch30
Zusammenfassung
Generally, BN coatings processed from BCl3-NH3-H2 gas mixtures display
a poor degree of crystallisation under commonly used CVD/CVI conditions
(low pressure and temperature). The crystallisation of these coatings
requires a post-deposition heat treatment. Some authors 1,2 have
noticed the poor stability of as-processed coatings, at ambient temperature.
The BN stability must be improved to limit the ageing of the interphase
in CMCs at ambient temperature and therefore to maintain the properties
of the composites at high temperatures 3. Furthermore, the presence
of oxygen within the as-processed coatings influences the crystallisation
behaviour of BN during heat treatment 2.
The stability in air at ambient temperature of the coatings before
and after heat treatment was studied through: (i) the weight gain
as a function of time (from few days to 2 months), (ii) the quantity
of oxygen or moisture absorbed (as measured by AES) and (iii) the
characterisation of new compounds formed at the surface of the coatings
by XRD. The oxidation resistance and the crystallisation degree of
the aged coatings before and after heat treatment were related to
the quantity of oxygen absorbed.