Goffredo de Portu

Preparation and properties of electroconductive Al2O3-based composites

Abstract Al 2 O 3 -based composites with the addition of 20 and 30 vol.% of TiN, TiC, TiB 2 have been produced by hot pressing and by gas-pressure sintering. These electroconductive, toughened and reinforced composites proved to be suitable for the manufacture of complex shapes by electrical discharge machining. Mechanical, electrical and thermal properties have been correlated to the microstructure and to the composition. The mechanical properties of the hot-pressed samples reveal an increase in fracture toughness of 30–70% and in flexural strength of some 70% in comparison with the Al 2 O 3 matrix at room temperature. The oxidation, which takes place at the secondary phases, strongly affects the flexural strength at relatively high temperatures (≈700°C), limiting the potential operating temperature range for this type of composites.

Ceramic dielectric mirrors for the terahertz range.

We have produced a rigid dielectric terahertz mirror by tape casting and sintering 25 stacked ceramic layers of alumina and alumina-zirconia. Our structure shows a pronounced stop band between 0.3 and 0.38 THz. The experimental data agree well with transfer matrix calculations.

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.

Lamination Process to Obtain Structure with Tailored Residual Stress Distribution

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.

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.

Characterization of delamination cracks in Al2O3/Al2O3+3Y-TZP multilayered composite by raman and fluorescence piezo-spectroscopy.

Institute of Science and Technology for Ceramics, National Research Council, ISTEC-CNR, via Granarolo 64, 48018, Faenza, Italy (G.d.P., L.M.); Ceramic Physics Laboratory, Kyoto Institute of Tecnology, Sakyo-ku, Matsugasaki, 606-8585, Kyoto, Japan (G.P.); and Research Institute for Nanoscience, RIN, Kyoto Institute of Tecnology, Sakyo-ku, Matsugasaki, 6068585, Kyoto, Japan (G.d.P., L.M., G.P.)

2.14 – Wear of Hard Ceramics

A general introduction to the science of tribology and behavior of advanced ceramics is discussed in this chapter. The influences of chemical, physical, mechanical and microstructural properties along with the effect of test methods and experimental conditions on tribological characteristics are described. Wear mechanisms exhibited under different testing conditions are presented and the diverse wear behavior of various ceramics is documented. Oxides, nonoxides, ceramic-metal and ceramic-ceramic composites as well as laminated structures are discussed.

Effect of Superficial Residual Stresses on Abrasive Wear Resistance of Al2O3 / Al2O3+3Y-TZP Multilayers

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.

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.