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Graphic element Research > Growth > Research projects > Previous projects > New Products and Materials > Coated composites take the heat out of space travel
Graphic element Coated composites take the heat out of space travel
     
 
For the aerospace industry to meet the challenges of hypersonic travel and the development of more efficient, less polluting future aircraft engines, the development of new high-temperature-resistant materials is essential. Fibre-reinforced ceramics offer excellent potential as heat shields and component liners. However, to achieve adequate lifetimes under the extreme operating conditions, they require added oxidation protection. This Brite-Euram project explored a range of coatings and application techniques in the search for an effective answer to this need.
 

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 be identified.
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.

Multilayer systems

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.

Wide-ranging protection

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.

Contact:

Dr Walter D Vogel
Dornier GmbH
DaimlerChrysler Research and Technology
Dept. FT4/WK
88039 Friedrichshafen
Germany
Tel: +49 7545 84981
Fax: +49 7545 82140
E-mail: walter.d.vogel@daimlerchrysler.com

Cordis RCN: 23157
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