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Electron beam-physical vapour deposited (EB-PVD) yttria-stabilized zirconia (YSZ) thermal barrier coatings have been investigated by nanoindentation. It is shown that the mechanical properties, and particularly the Young's modulus, of these coatings are closely related to their columnar microstructure. The transition from intracolumnar to intercolumnar deformation is characterized by a difference between the mechanical properties of the bulk YSZ and those of the coatings. The use of different indenter shape showed that the contact area was the parameter characteristic of this scale dependence. Deformation mechanisms studied by AFM have shown a good correlation with the evolution of the mechanical properties during the deformation. [ABSTRACT FROM AUTHOR]

This study investigates the incipient plasticity during a nanoindentation test with a spherical indenter in magnesium oxide single crystals. The load–displacement curves show multiple pop-ins, interspaced by elastic deformation, and reveal the transition from an initial elastic deformation regime to a continuous elastoplastic one. Nanoindentations have been stopped after the first pop-in and the dislocation structures have been three-dimensionally characterized by AFM observations of the surface deformation and by nanoetching. The dislocation pattern shows two activated slip systems. The resolved shear stress has been calculated just before the pop-in, for all of the activated slip systems. The final positions of the dislocations generated during the first pop-in reveal that they were all nucleated at the point of maximum resolved shear stress. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This letter is based on recent progress in the observation of dislocation distributions around nanoindentations by chemical etching. This so-called nanoetching technique is used to determine the dislocation mechanisms associated with the pop-in phenomenon in MgO and LiF. Successive stages of highly controlled chemomechanical polishing have revealed that these dislocations are half-loops lying in the classical slip systems of MgO or LiF. However, they do not extend on the surface in the classical rosette-arms pattern but stay concentrated around the imprint. A mechanism of dislocation interactions, enhanced by the fact that the dislocations are suddenly nucleated in a small volume, is proposed to explain this specific distribution. [ABSTRACT FROM AUTHOR]