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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 corrosive environments.[1-3]

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 m m.

 

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.[6] 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.[7] 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 composites.

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 coefficients.

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.[9] 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.[1]

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 750 mm/h.

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.

Acknowledgements

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.

Principal publication

R.I. Merino, J.I. Peña, N.R. Harlan, G. F. de la Fuente , A. Larrea, J.A. Pardo, V.M. Orera, J.Y. Pastor, P. Poza and J. Llorca, Ceramic Engineering & Science Proceedings, ed. By Hua-Tay Lin and Mrityunjay Singh, The American Ceramic Society, OH. Vol. 23, pp 663-670 (2002)

References

  1. J.Y. Pastor, P. Poza, J. Llorca, J.I. Peña, R.I. Merino and V.M. Orera, Mater. Sci. Engng., A308, 241-249 (2001)
  2. A. Sayir and S.C. Farmer, Acta mater ., 48, 4691-4697 (2000)
  3. N. Bahlawane, T. Watanabe, Y. Waku, A. Mitani and N. Nakagawa, J. Am. Ceram. Soc ., 83, 3077-81 (2000)
  4. J.I. Peña, R.I. Merino, G. F. de la Fuente and V.M. Orera, Adv. Mater., 8, 909-912 (1996)
  5. A. Larrea, G. F. de la Fuente , R.I. Merino and V.M. Orera, J. Eur. Ceram. Soc., 22, 191-198 (2002)
  6. J.I. Peña, R.I. Merino, N.R. Harlan, A. Larrea, G. F. de la Fuente and V.M. Orera, J. Eur. Ceram. Soc., 22, 2595-2602 (2002)
  7. N.R. Harlan, R.I. Merino, J.I. Peña, A. Larrea, V.M. Orera, C. Gonzalez, P. Poza and J. Llorca, J. Am. Ceram. Soc., 85, 2025-2032 (2002)
  8. V.M. Orera, R. Cemborain, R.I. Merino, J.I. Peña and A. Larrea, Acta mater., 50, 4677-4686 (2002)
  9. A. Larrea, V.M. Orera, R.I. Merino and J.I. Peña, J. Eur. Ceram . Soc. (in the press)

 

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