Journal of the European Ceramic Society 19(1999)255-262 C 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved PII:S0955-2219(98)00189-7 0955-2219/98/S--see front matter The role of Residual Stresses in Layered Composites of Y-ZrO2 and AlO3 Henryk Tomaszewski, Jan Strzeszewski" and Wojciech Gebicki "nstitute of Electronic Materials Technology, Wolczyriska 133, 01-919 Warsaw, Poland Institute of Physics, Warsaw University of Technology, Koszykowa 75. 00-662 Warsaw, Poland (Received 6 March 1998; accepted 31 July 1998) abstract knowledge of the residual stresses is important for a number of reasons. One is that the stresses across Laminar composites, containing layers of Y-TZP grain boundaries can be sufficiently large that grain and either Al2O3 or a mixture of Al203 and Y-ZrO2 boundary microcracking can occur. The impor technique of water solutions containing suspended of work analyzing the mps s led to a large body have been fabricated using a sequential centrifuging tance of this phenomenon h particles. Controlled crack growth experiments with ing -A second, related, reason is that the local notched beams of composites were done and showed residual stress affects the path of a crack as it pro- the significant effect of barrier layer thickness and agates through the microstructure and is believed composition on crack propagation path during fre by many to lead to R-curve behaviour. b ,/In a ture. Distinct crack deflection in alumina layers was result, crack deflection and the increase in tough observed. The increase of crack deflection angle with ness of ceramics can be observed.8.9 The aim of this the alumina layer thickness was also found In the work was to investigate laminar composites con case of the barrier layer made of a mixture, crack taining layers of Y-ZrO2 and either Al2O3 or a deflection did not occur independently on layer mixture of AlO3 and Y-ZrO2 fabricated by thickness. The observed changes have been corre- sequential centrifuging of aqueous particle suspen lated with the radial distribution of residual stresses sions. The source of distinct stresses found here in barrier layers created during cooling of sintered were the difference in thermal expansion and composites from fabrication temperature. The stres- shrinkage between zirconia and alumina and the ses found were the result of the difference in the crystallographically anisotropic thermal expansion thermal expansion and sintering shrinkage of alu- of the Al2O3 phase. The distribution of compressive and zirconia and the crystallographically ani- stresses in barrier layers of composites occured to be sotropic thermal expansion of the alumina. The dependent on layer thickness and composition residual stress distribution has been measured by (alumina only or a mixture)and to be regarded as a piezo-spectroscopy based on the optical fuorescence factor responsible for observed toughness increase of Crt3 dopants in alumina. C 1998 Elsevier Sci- ence Limited. All rights reserved 2 Experimental Procedure Keywords: composite, thermal expansion, tough- ness, Zro2, AlO3. Laminar composites of Y-TZP and Al2O3 with layers with thicknesses of 10 to 60 um (equal for both type of materials) were fabricated by the 1 Introduction sequential centrifuging(Z382 Hermle) of powder suspensions. Aqueous slurries containing 5 to In cooling sintered polycrystalline ceramics from 10 wt% zirconia powder (ZrO2+ 3. 4 mol%Y2O3 their fabrication temperature residual stresses are 0-6 um median particle size obtained from Unitec created as a result of the difference in thermal Ceramics) or alumina powder (AKP-53 type, expansion between the phases present. The 0.29 um median particle size obtained from Sumitomo) were prepared by ultrasonicating the *To whom correspondence should be addressed. Fax: +48 powders in deionized water at pH 4. Cast samples 22-834-9003: e-mail: tomasz h(@sp itme. edu. pl were dried, additionally isostatically pressed atThe Role of Residual Stresses in Layered Composites of Y±ZrO2 and Al2O3 Henryk Tomaszewski,a * Jan Strzeszewskib and Wojciech Ge,bickib a Institute of Electronic Materials Technology, WoÂlczynÄska 133, 01-919 Warsaw, Poland b Institute of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland (Received 6 March 1998; accepted 31 July 1998) Abstract Laminar composites, containing layers of Y±TZP and either Al2O3 or a mixture of Al2O3 and Y±ZrO2 have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the signi®cant e€ect of barrier layer thickness and composition on crack propagation path during frac￾ture. Distinct crack de¯ection in alumina layers was observed. The increase of crack de¯ection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack de¯ection did not occur independently on layer thickness. The observed changes have been corre￾lated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stres￾ses found were the result of the di€erence in the thermal expansion and sintering shrinkage of alu￾mina and zirconia and the crystallographically ani￾sotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical ¯uorescence of Cr+3 dopants in alumina. # 1998 Elsevier Sci￾ence Limited. All rights reserved Keywords: composite, thermal expansion, tough￾ness, ZrO2, Al2O3. 1 Introduction In cooling sintered polycrystalline ceramics from their fabrication temperature residual stresses are created as a result of the di€erence in thermal expansion between the phases present. The knowledge of the residual stresses is important for a number of reasons. One is that the stresses across grain boundaries can be suciently large that grain boundary microcracking can occur. The impor￾tance of this phenomenon has led to a large body of work analyzing the mechanics of microcrack￾ing.1±5 A second, related, reason is that the local residual stress a€ects the path of a crack as it pro￾pagates through the microstructure and is believed by many to lead to R-curve behaviour.6,7 In a result, crack de¯ection and the increase in tough￾ness of ceramics can be observed.8,9 The aim of this work was to investigate laminar composites con￾taining layers of Y-ZrO2 and either Al2O3 or a mixture of Al2O3 and Y±ZrO2 fabricated by sequential centrifuging of aqueous particle suspen￾sions. The source of distinct stresses found here were the di€erence in thermal expansion and shrinkage between zirconia and alumina and the crystallographically anisotropic thermal expansion of the Al2O3 phase. The distribution of compressive stresses in barrier layers of composites occured to be dependent on layer thickness and composition (alumina only or a mixture) and to be regarded as a factor responsible for observed toughness increase. 2 Experimental Procedure Laminar composites of Y±TZP and Al2O3 with layers with thicknesses of 10 to 60m (equal for both type of materials) were fabricated by the sequential centrifuging (Z382 Hermle) of powder suspensions. Aqueous slurries containing 5 to 10 wt% zirconia powder (ZrO2+3.4 mol%Y2O3, 0.6m median particle size obtained from Unitec Ceramics) or alumina powder (AKP-53 type, 0.29m median particle size obtained from Sumitomo) were prepared by ultrasonicating the powders in deionized water at pH 4. Cast samples were dried, additionally isostatically pressed at Journal of the European Ceramic Society 19 (1999) 255±262 # 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved PII: S0955-2219(98)00189-7 0955-2219/98/$Ðsee front matter 255 *To whom correspondence should be addressed. Fax: +48- 22-834-9003; e-mail: tomasz_h@sp.itme.edu.pl