Javier Pascual

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.

Fracture of Monoliths Fabricated by Stacking Water Processed Green Ceramic Tapes

A lamination technique for joining ceramic green tapes fabricated from aqueous ceramic slurries via tape casting process is investigated. Monolithic pieces with varying compositions within the Al2O3 / t-ZrO2 system were fabricated by gluing the constituent tapes and pressing at room temperature. The quality of the interfaces between the different tapes in sintered specimens was evaluated by means of the fracture behavior of the pieces during the ball on three balls test. Different fracture patterns were found as a function of pressure and the characteristics of the tapes. Large scale interface defects led to delamination during fracture whereas no critical defects could be observed in the smooth fracture surfaces. Small scale interface defects were revealed by fracture.

Influence of an apparent R-curve on strength and fracture statistics of ceramic laminates

Abstract Laminates with strong interfaces and having compressive residual stresses in the outer layer are designed to have high strength and high reliability. Bending strength tests on such laminate specimens were performed and compared with tests made on monolithic specimens made from the outer layer material. In both types of specimens fracture started from similar flaws, but the laminate specimens showed a higher strength and Weibull modulus. In the case of the monolithic material the toughness is constant but in the case of the laminates it increases – due to the action of the residual stresses – with crack extension (i. e. it is an apparent toughness). The influence of the apparent toughness curve on the fracture statistics is discussed. It is also shown that the statistics of the laminate can be determined using the statistics of the monolithic material, if the influence of the residual stresses on the toughness and strength is properly taken into account.