Papers by Author: Luca Ceseracciu

Abstract: Layered ceramics are foreseen as possible substitutes for monolithic ceramics due to their attractive mechanical properties in terms of strength reliability and toughness. The different loading conditions to which ceramic materials may be subjected in service encourage the design of tailored layered structures as function of their application. The use of residual stresses generated during cooling due to the different thermal strain of adjacent layers has been the keystone for the improvement of the fracture response of many layered ceramic systems, e.g. alumina-zirconia, alumina-mullite, silicon nitride-titanium nitride, etc. In this work, the fracture features of layered ceramics are addressed analysing two multilayered structures, based on the alumina-zirconia system, designed with tailored compressive residual stresses either in the external or internal layers. Contact strength and indentation strength tests have been performed to investigate the response of both designs to crack propagation. The experimental findings show a different response in terms of strength and crack growth resistance of both designs. While layered structures with compressive stresses at the surface provide a better response against contact damage compared to monoliths, a flaw tolerant design in terms of strength and an improved toughness through energy release mechanisms is achieved with internal compressive stresses. The use of layered architectures for automotive or biomedical applications as substitutes for alumina-based ceramics should be regarded in the near future, where reliable ceramic designs are needed.

Abstract: The presence of surface compressive residual stress in a laminated material enhance the resistance of the component by reducing the stress intensity factors acting on the cracks -either natural or artificial- existing in the surface. Fissures in the form of cone crack are often generated by blunt contact in service, that can affect the functionality as well as the strength of the material. In this work, a two-steps analysis of the effect of residual stresses on the geometry of cone crack and how this change in geometry influences the far-field strength of the material was performed by means of a Finite Elements model and of experimental observations. In the first part, an automatic incremental model was formulated, which allowed to establish the crack shapes that were used in the second part for simple four-points test models. It was observed that residual stresses change considerably the crack shape, with important implications in the design of contact-damage tolerance, and that this reflects on corresponding changes in the strength.

Abstract: The mechanical properties of a quasi-symmetrical planar functionally graded material (FGM) of Al2O3 / Al2O3+20%ZrO2 / Al2O3+5%ZrO2 prepared by electrophoretic deposition and pressureless sintering has been studied. Hardness and fracture toughness were measured using indentation methods on cross sections of samples. From the difference between lengths of cracks parallel and perpendicular to layers the residual stresses (arisen due to the thermal expansion coefficient mismatch) were calculated, their profile along the cross section has been estimated and compared with values obtained using a finite element model.

Abstract: In this work, the study of the contact fatigue of an alumina-based laminated composite with compressive residual stress in the surface is presented, together with monolithic alumina as a reference material. Hertzian indentation techniques were employed, which better represent the inservice actual contact loading. Different tests were performed under static and cyclic loading. The evolution of damage and the different types of fracture produced were examined, and an improvement in contact resistance of the composite, due to compressive residual stress, was highlighted. A better resistance of the laminated composite was observed for every test in comparison with the monolithic reference material.