Involving alumina sol-gel coating in the protection of carbon against oxidation
at high temperature was highly deserved because of its exemplar thermal
stability and the considerably reduced cost. However, the high temperature
required for getting the most stable crystalline a-Al2O3
(usually 1100oC) induces a considerable degradation of the carbon
substrate that limits its use. This limitation was hurdled using an anhydrous
preparation of alumina sol that led to a considerable lowering of the
crystallization temperature (850-950oC). The utilization of this sol
was also favored by its high stability in a non-controlled atmosphere (over 20
months) and by its easy viscosity adjustment. Attained a-Al2O3 thin
films using dip-blaze coating were uniform and crack-free on various types of
substrates (glass, silicon and SiO2 coated graphite).
 |
SEM cross-section
micrograph of the a- Al2O3
thin films resulting from 15 layers by dip-blaze coating followed by
annealing at 950oC that led to a pure crystalline phase.
This micrograph corresponds to Si/a-Al2O3. |
The
suitability of this a-Al2O3
coating as high temperature protective layer was demonstrated by a thermal oxidation test
in anisothermal and isothermal conditions. As inner layer, silica thin film with a
thickness of 9 µm was deposited using a home made
APCVD reactor (Atmospheric pressure chemical vapor deposition) and TEOS precursor. The
silica layer was deposited with a pronounced granular texture, that holds up thermal
stress resulting from the inherent expansion coefficient mismatch. The alumina thin films
were prepared by a single dip-blaze coating in the alumina sol.
 |
| Anisothermal
oxidation test (5oC/min) in air atmosphere.
|
The
performed oxidation tests demonstrate clearly the high potential of a-Al2O3 as outer layer for
preserving the body of carbon substrate up to 100 hours at 1450oC (where less
that 0.25% of weight loss was registered). Results demonstrate also the efficiency of
dip-blaze coating for performing a uniform and total covering single layer of a-Al2O3
 |
| 100h
isothermal oxidation test in air atmosphere.
|
Considering
the exceptional stability of the sol and the high structural uniformity, these a-Al2O3 thin films could find a
wide application area (such as optics) hopefully seeking an industrial partner.
More about the Alumina film
For more information
contact:
N. Bahlawane
Fukuoka University Institute 'FIRECS'
10 Koyo machi, Wakamatsu, Kitakyushu, Fukuoka-ken,
808-0002 Japan.
Tel: 81-93-751-9975
Fax:
81-93-751-9976
E-Mail : bahlawan@bronze.ocn.ne.jp
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