Francis Chalvet

Strength Distributions in Ceramic Laminates

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.

R-Curves in Al2O3 - Al2O3/ZrO2 Laminates

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.

Strain Mismatch in Ceramic Multilayers: Determination by Strength Measurements

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.

Residual Stresses in Al2O3-ZrO2 Laminar System

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.

Residual Stress Assessment in the Al2O3\Mullite Based Laminated System

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.

Indentation Testing of an Al2O3/Al2O3 + ZrO2 Layered Composite

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.

Contact Fatigue in Ceramic Laminated Composites

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.