Monolithic ceramics, ceramic matrix composites
(CMC) and intermetallics are among the best available materials
to withstand the high temperatures generated in rocket motors and
high-powered aero engines, and at the surfaces of re-entering space
vehicles. But only with fibre reinforcement ceramics can one achieve
the damage-tolerant fracture behaviour necessary for reliable performance
under these demanding conditions.
At the time of the IOPCMC project launch in September 1994, the
application of such materials was limited by the lack of suitable
protection systems capable of preventing oxidation damage, while
not cracking under thermal/mechanical loadings.
The consortium, led by DASA Dornier, therefore embarked on an extensive
theoretical and practical study to develop integrated oxygen barrier
systems for CMCs, especially carbon fibre-reinforced silicon carbide
(C/SiC). The approach adopted was to combine internal sealing of
inevitable cracks and pores in the substrate matrix with single-
and multi-layer external protective coatings . Because the aim was
to meet both re-entry and turbine application requirements, separate
systems were explored for medium-term (1600°C/>100 h) and
long-term (1200°C/>5000h) stability.
Theory guides practice
In order to guide the research and limit the
experimental workload, The University of Manchester's Institute
of Science and Technology carried out theoretical modelling to assess
the chemical stability of potential coating materials and to simulate
their exposure to oxygen. A specially developed methodology permitting
the simulation of variations in coatings , oxidising gas composition,
temperature and pressure enabled the most promising candidates to
As coated CMCs cool down from the processing temperature, some stress-induced
cracks form in the coating layer, because its coefficient of thermal
expansion is different from that of the substrate. A further theoretical
model calculated the stresses and enabled the cracking behaviour
to be determined. It was then possible to tailor the mechanical
properties of the coatings in order to minimise the phenomenon.
For the practical trials, Dornier produced a large number of C/SiC
samples by means of its established polymer infiltration process.
This involves filament winding of ceramic-precursor slurry impregnated
fibres, lay-up, curing and subsequent pyrolisis at 1100°-1600°C.
When the heat treatment converts the matrix into SiC, the ceramic
yield of the precursor is typically only 55-80% - so shrinkage produces
a large number of pores and cracks. Substrates with 25% and 40%
residual porosity were used for the IOPCMC experiments.
As a first step, Dornier examined a number of
infiltration methods using different silicon precursors to seal
the pores and cracks. Treatment under isostatic pressure proved
particularly successful in protecting the SiC from oxidation and
reducing the rate of fibre burn-off.
Various coating systems were then investigated - including the use
of glass and enamel interlayers to minimise the expansion mismatch
between the bond layers and the erosion protection layers. The partners
examined a broad range of functional coating layers, sealing cracks
by melting at high temperatures, based on silica, silicon/boron
and silicon/yttrium oxide mixtures, aluminium nitride, doped SiC
and tantalum pentoxide. The application mechanisms were sol-gel,
chemical vapour deposition and slurry coating. A hard SiC erosion
protection layer was normally added to protect against gas erosion
and particle impact.
Based on the theoretical modelling and
experimental data, several multilayer protection systems were proposed,
comprising bond, functional and erosion protection layers - each
of which could itself consist of one or more layers. With this choice,
protection could be optimised for temperatures from 400° to1600°C.
Thermal shock resistance had improved dramatically, while measured
temperature resistance ranged from over 10000h at 1000°C to
130 h at 1600°C. This performance was deemed to be ample for
re-entry applications, although questions remained about the suitability
of coated C/SiC for long-life service in turbines.
Following completion of the project in June 1997, Dornier went on
to conclude contracts with the USA and Japan for the supply of heat
shingles and other space mission components. Having already performed
successful trials, coated C/SiC rocket nozzles may be a candidate
for future Ariane launches.
Dr Walter D Vogel
DaimlerChrysler Research and Technology
Tel: +49 7545 84981
Fax: +49 7545 82140