Papers by Author: Luis Llanes

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: In this work, the thermal shock behavior of an Al2O3-5%tZrO2/Al2O3-30%mZrO2 multilayer ceramic is studied. On these materials, a tetragonal to monoclinic phase transformation within the Al2O3-30%mZrO2 layers takes place when cooling down from sintering. The latter induces an increase in volume and therefore compressive residual stresses arise in these layers. The residual stress distribution profile in the laminate influences the thermal shock response of the material. A finite element model has been developed to estimate both the thermal strain effects during the sintering process as well as the temperature distribution and stress profile within the laminate during thermal shock testing. Experimental tests on the monoliths and laminates were carried out and compared to the model. It is observed that the presence of the compressive layers within the laminate inhibits the penetration of thermal shock cracks into the body at even more severe conditions than in the monolithic material.

Abstract: The fracture and fatigue behaviour of a mullite/molybdenum composite is investigated. The attention is focused on the measurement of fracture toughness, KIc, on long through the thickness cracks by using SENB specimens, and on the growth of indentation cracks under static, monotonic and cyclic loads. Molybdenum was chosen to reinforce the mullite matrix because of the similar thermal expansion coefficients for both phases. It is essential to know and take into account the shape of the initial indentation cracks as well as the eccentricity change after extension under monotonic and cyclic stress. This study shows that, in mullite/molybdenum composites, static fatigue effects are negligible, but these composites are susceptible to mechanical degradation under cyclic loads. It is shown that the fatigue crack growth rate exhibits a high dependence on Kmax and that the fatigue sensitivity, defined as the ratio between fatigue crack growth rate threshold and KIc, is much lower than for other materials processed by powder metallurgy.