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Abstract
The instrumented indentation response of polycrystalline alumina and alumina-based ceramic multilayers with well-known residual stresses is investigated. Young's modulus and hardness were evaluated as a function of penetration depth with Berkovich and spherical indenters by continuous stiffness measurements. A size effect was observed due to the length scale of the microstructure. Values of hardness and Young's modulus were in good agreement with previous works. No significant differences in the P–h curves were observed for materials with and without residual stress, making it difficult to employ the various methodologies in the literature for estimation of residual stress or crack length evaluation. Crack length had, therefore, to be measured by direct observation. Thus, cube corner indentations did not produce well-developed radial cracks but chipping of the material, whereas indentations with Berkovich indenters produced radial cracks suitable for apparent fracture toughness estimation and, consequently, residual stress.
Acknowledgements
This work is supported in part by the European Community's Human Potential Programme under contract HPRN-CT-2002-00203 [SICMAC] and by the Spanish Ministry of Science and Technology, through grant MAT-2002-00368. LC, YG and MB acknowledge the financial support provided through the European Community's Human Potential Programme under contract HPRN-CT-2002-00203 [SICMAC].