Papers by Author: Goffredo de Portu

Abstract: To improve mechanical properties of mullite, a mullite-Al2O3\mullite laminate composite was prepared. Lamination generates residual stresses within the structure, measured by piezospectroscopy. A preliminary and complete piezo-spectroscopic characterization of the Al2O3\mullite system was carried out. A method to determine the concentration of Al2O3 in the composite by Raman spectrum was proposed and used to assess the composition of the laminated structure along the cross section. The experimental results evidenced a gradual change of composition and residual stress state between the two layer.

Abstract: In asymmetrical 3-layer laminates with constant overall and inner layer thickness, the residual compressive stresses in the two outer layers are not longer the same: compression is higher in the thinner layer.Therefore, it can be expected that the strength also depends on the outer layer thickness. Experimental evidence of this behavior was found by measuring the bending strength of asymmetrical tri-layers. A value of the thermal expansion mismatch was determined by fitting the theoretical expression to the experimental data.

Abstract: In this paper a method to produce laminated ceramic composites containing residual stresses is described. The method consist in superimposing thin layers obtained by tape casting, their worm-pressing and sintering. Detailed information on the process and on the slurry compositions are reported. The reasons why laminated structure can exhibit improved performances are also illustrated. The model on which a multilayer composite, containing residual stresses, can be designed is briefly illustrated. The relationship among the physical, chemical and microstructural properties of the different layers, necessary to stimulate the residual stresses outlined.

Abstract: This study presents an investigation about the damage characteristics and mechanical properties of Al2O3/ZrO2 laminate composites under low velocity biaxial and transient loading (BLTL). These multilayer laminates have different layer thickness therefore residual stresses and are compared to a 13-layer alumina specimen. Impact tests were conducted using a drop-weight test facility with impact energies ranging from 0.033-0.160 kgm. Optical microscopy was used to observe the damage characteristics of the impacted specimens. The load-time and energy-time histories produced by the impact tests in conjunction with the observation of the optical photographs give a good view of the behaviour that characterize the multilayers of Al2O3 (A) and Al2O3/ZrO2 (AZ) ceramics under the selected experimental conditions.

Abstract: Multilayered composite specimens consisting of Al2O3 / Al2O3+ 3Y-TZP (A/AZ) layers with different compositions and thicknesses were prepared starting from ceramic sheets obtained by tape casting. Residual stresses arisen from mismatch in thermal expansion coefficient during sintering were evaluated using luminescence piezo-spectroscopy. The stress in the superficial A layer was found to be compressive, and its value depended on the ratio between thickness of A and AZ layer. The influence of the superficial compressive stress on the abrasive wear resistance was investigated using microscale ball cratering test; results were correlated with the superficial compressive stress and compared with a specimen of pure unstressed Al2O3 prepared both by lamination and by cold isostatic pressing. Experiments show an improvement of performances in the samples containing compressive residual stress in the surface.

Abstract: In this work the strength of ceramic laminates is analyzed. Two different architectures with the same surface are studied. One of them, a Al2O3 / Zr(3Y)O2 laminate, contains compressive residual stresses, the other one, a monolithic Al2O3 laminate, has no residual stresses. Residual stresses are estimated analytically on the basis of the materials properties and by an indentation technique. The influence of the residual stress on the strength distribution is investigated. Strength distributions for laminates with compressive stresses at the surface follow a Weibull distribution only under certain conditions.

Abstract: Micro- and macro-hardness of the individual layers of an Al2O3 / Al2O3 + ZrO2 layered composite have been investigated. The indentation load size effect has been studied in a range of indentation load of 25 mN – 100 N using depth sensing indentation technique and conventional Vickers hardness testers. Fracture toughness values of the layers have been measured using indentation fracture method. The effect of residual stresses on the KIC value has been analyzed.

Abstract: The aim of the paper is to present an analytical model of thermal stresses in a laminar system, and consequently to verify the model validity by comparing calculated thermal stresses with measured ones by indentation method. Analytical models of thermal stresses acting in anisotropic and isotropic laminar plane systems are presented, and consequently applied to the thermal-stress-strengthened Al2O3-ZrO2 laminar ceramics.

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.

Abstract: In order to better understand the fracture behavior of Al2O3-Al2O3/ZrO2 laminates, controlled crack growth experiments were carried out. The method proposed consists of a threepoint bending test of a sandwich beam in which a notched specimen is located in between two metallic bars. Under certain conditions that will be commented, stable crack growth can be obtained. The fracture pattern is related to the residual stresses present in the laminate. R-curves for the laminate were also calculated using the weight function method, including the influence of the free surface. Toughness values are presented in the R-curve after friction considerations.