||Ceramics for high Temperature Structural Applications
J.I. Peña, A. Larrea, R.I. Merino, I. de Francisco,
P.B. Oliete and V.M. Orera
Excellent chemical stability in oxidizing atmospheres makes ceramic oxides suitable
for high temperature structural applications. However, the poor creep resistance
and the presence of low melting point intergranular phases in conventional ceramics
deteriorates the mechanical behaviour of conventional oxide ceramics above 1000
ºC . This limitations can be successfully eliminated in the Al2O3
based eutectic ceramics grown from melt. This is the case for the Al2O3-ZrO2(Y2O3)
and Al2O3-YAG systems we have grown using laser assisted
directional solidification. The systems consist of YSZ or YAG phases, in the
form of rods or small platelets of micron or nanometer size dispersed into the
continuous sapphire phase. Combination of the outstanding creep behaviour of
Al2O3 along its c-axis, together with the huge amount
of clean and strong grain boundaries characteristic of the eutectic solids results
in a new set of ceramic oxides with excellent creep behaviour and a good retention
of their mechanical properties up to temperatures above 1700 ºC even in
Figure 1: SEM micrograph of Al2O3-ZrO2(Y2O3)
eutectic grown at 10 mm/h. Bright phase is c-ZrO2 and dark
phase a -Al2O3 . Scale bar = 20
Directionally-solidified eutectic rods or plates have been
produced by the laser-heated floating zone method using either CO2
or diode stack lasers.[4,5] The mechanical properties tests were done at the
E.T.S. de Ingenieros de Caminos in Madrid (Prof. J. Llorca group). Flexural
strength was measured using a three-point bend test. The fracture toughness
was determined by the notch technique and also by the indentation fracture method
using a Vickers indentor.
Several strategies have been used to improve the mechanical
properties of the composites. On the one side we modified the characteristic
size and shape of the microstructure by changing the growth parameters. Additionally,
we also changed the yttria content in the Al2O3-ZrO2
composites. The presence of the different ZrO2 phases, cubic, tetragonal
or monoclinic, was quantified using Raman and XRD techniques. By far the
best mechanical properties were achieved for the samples with the finest interpenetrating
microstructure in both compounds (Fig. 1 and 2) and about 1 mol % Y2O3
, which corresponds to the tetragonal ZrO2 phase in the Al2O3-YSZ
Multiphase composites may develop thermoelastic residual stresses
due to the differences between the thermal expansion coefficients of the component
phases. Residual stresses play an important role in the fracture mechanisms.
We have studied these residual stresses using piezo-spectroscopic and Raman
techniques.[7,8] Residual stresses in the Al2O3-YAG eutectics
are very small ( s h = -0.1 GPa) owing to the light mismatch in thermal expansion
Figure 2: SEM micrograph of transverse section
of Al2O3 -YAG eutectic grown at 350 mm/h. Bright
phase is YAG and dark phase a -Al2O3
On the contrary large tensile residual stresses in the sapphire
phase develop in the Al2O3-ZrO2 eutectic
when the yttria content is low and the zirconia is in the monoclinic phase.
These large residual stresses produce microcracking and a severe deterioration
of the mechanical properties of the composite. On the contrary, compression
residual stresses ( s h = -0.35 GPa) are established in the alumina phase when
the zirconia is either in the tetragonal or cubic phase. In this case the eutectic
shows an excellent mechanical behaviour.
In Figure 3 we plot the flexural strength measured in air in
rods of about 1 mm diameter showing a fine microstructure (< 1 m m) at RT
and 1800 K compared with those measured for the best SiC and Si3N4
composites. Our materials are much stronger than any other ceramic composite
reported up to date and their mechanical properties retention at high temperatures
in air is outstanding.
The Vickers hardness is also high H v = 16 ± 0.4 GPa
in both compounds. The high strength of the samples is accompanied by a high
toughness. In fact, the fracture toughness measured by the notch technique in
Al2O3-YSZ is 7.8 MPa.m 1/2 at RT.
It is interesting to point out here that the Vickers hardness,
fracture toughness and residual stresses measured in directionally solidified
eutectic plates at ambient temperature coincide with the values obtained in
rods of the same composition and microstructure. Consequently an excellent mechanical
behaviour at high temperature is anticipated. These directionally solidified
eutectic surfaces can be used in wear, impact or thermal resistant coatings,
thermal barrier coatings or anticorrosion barriers.
Figure 3: Flexural strength of several materials measured
by the three points technique in air at different temperatures. Black
squares, Al2O3-YSZ (3 mol%) eutectic with interpenetrating
microstructure. Red circles, Al2O3-YAG grown at
Summarising, using the laser assisted directional solidification
technique and optimized conditions we have prepared some eutectic composites
with exceptional mechanical properties. With the experimental equipments available
in our laboratory can be prepare materials in the form of thin rods of less
than 2 mm diameter or solidified plates up to 500 m m thick.
The flexural strength and fracture measurements have been done
by J.Y. Pastor and J. Llorca in the Universidad Politécnica de Madrid.
We acknowledge finantial support from the Spanish Ministry of Science and Technology
through projects MAT2000-1533-C03-02 and MAT2000-1495.
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Engineering & Science Proceedings, ed. By Hua-Tay Lin and Mrityunjay Singh,
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