PhD
Thesis – Abstract
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Oxide
Ceramic Matrix Composites for Gas Turbine Applications
by
Magnus
Holmquist
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The appealing
properties of ceramics include retention of strength and
hardness at high temperatures, chemical inertness and low
density. However, monolithic ceramics fail in a brittle,
unpredictable manner making them unsuitable for many
applications where reliability is a key requirement. It has
been recognized that continuous fibre reinforced ceramic
matrix composites (CMCs) can show pseudo-plastic, non-brittle
fracture behaviour, whilst maintaining the attractive high
temperature properties of monolithic ceramic materials. Such
materials offer the prospect of improved thrust-to-weight
ratios, greater efficiencies and reduction of hazardous
emissions in gas turbine aero-engine applications. Currently
available non-oxide based CMCs usually have an interphase
between the fibre reinforcement and the surrounding matrix of
graphitic carbon or boron nitride which is prone to oxidation.
This effect is particularly severe when matrix cracks are
present and under cyclic conditions. In contrast, an all-oxide
based CMC would be insensitive to damage by oxidation, even at
high temperatures and after matrix cracking.
The main objectives of the work
presented in this paper have been to identify concepts for
CMCs stable at high temperatures (>1400°C)
for long lifetimes (>10 000 hours) in oxidising
environments and to develop and characterise such materials.
Single crystal continuous a-Al2O3
sapphire fibres were used to reinforce an Al2O3
matrix. A fibre/matrix interphase promoting crack deflection,
giving the composite a non-brittle fracture behaviour was
developed. Zirconia (ZrO2) is known to be
thermochemically stable with alumina (Al2O3)
and was chosen as an interphase material. However, in order
for the zirconia interphase to behave as a debond layer, it is
necessary to reduce the strength. Two solutions were explored:
a fugitive and a porous interphase. The fugitive interphase
was made by depositing a double carbon and zirconia layer on
the fibre. Once the composite was processed, the carbon
interphase was oxidised, leaving a gap between the fibre and
the zirconia interphase. The porous interphase was based on a
mixture of the carbon and zirconia phases. After burn-out, the
carbon phase left porosity within the zirconia interphase. A
process based on prepreg technique was developed to make the
composite. The fibres were passed through powder slurries
containing zirconia and/or carbon powders. After drying, the
coated fibre was wound around an alumina powder tape placed on
top of a large diameter spool. A second alumina layer was then
tape-cast directly onto the spool. Prepregs were cut and
stacked to form unidirectional or cross-ply green composites.
Final sintering was done by hot pressing or hot isostatic
pressing (HIP).
Mechanical behaviour of the hot
pressed composites was studied in flexure and tensile testing.
The porous interphase was chosen for further studies. Ultimate
tensile strength (UTS) at room temperature (RT) was 100-130
MPa with a strain to failure of 0.45% (cross-ply material,
fibre volume fraction, ~30%). The strength was reduced with
40% at 800°C
and 50% at 1200°C
and 1400°C.
This is primarily due to the decrease in fibre strength at
high temperatures. It is suggested that the fibre/matrix load
transfer mechanism is based on a wear effect of the porous
zirconia interphase. Non-brittle composite fracture behaviour
was retained even after thermal exposure of up to 1000 hours
at 1400°C.
Although a coarsening of the porous zirconia interphase had
taken place, the fracture behaviour was still non-brittle. The
material also showed good resistance to thermal cycling; the
as fabricated properties were retained even after >1300
cycles between RT and 1200°C.
The HIPed composites showed similar properties in flexure.
Flat
composite tiles were fabricated and testing in a combustor rig
operating at conditions realistic of a gas turbine combustor.
Cracking was predicted to occur locally due to thermally
induced strains. Maximum temperatures of 1260°C
were measured in the rig tests, which was very close to the
predicted value (1210°C).
A tile was successfully tested up to 46 cycles and exhibited
limited non-catastrophic matrix cracking after the tests. |
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Keywords: ceramic matrix composite, oxide fibre, single
crystal oxide fibre, sapphire, alumina, interphase, zirconia,
tape casting, hot pressing, hot isostatic pressing, mechanical
properties
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