Stephen W. Freiman

Effects of water and other dielectrics on crack growth

Effects of water and a variety of organic liquids on crack-growth rates in soda-lime-silica glass was investigated. When water is present in organic liquids, it is usually the principal agent that promotes subcritical crack growth in glass. In region I, subcritical crack growth is controlled primarily by the chemical potential of the water in the liquid; whereas in region II, crack growth is controlled by the concentration of water and the viscosity of the solution formed by the water and the organic liquid. In region III, where water does not affect crack growth, the slope of the crack-growth curves can be correlated with the dielectric constant of the liquid. It is suggested that these latter results can be explained by electrostatic interactions between the environment and charges that form during the rupture of Si-O bonds.

Fracture of ferroelectric ceramics

Abstract This paper surveys the temperature, microstructural and environmental variations of the fracture properties of ferroelectric ceramics. Earlier work shows that fracture toughness decreases on heating through the Curie temperature. There is also anomalous behavior in the strength at small crack sizes, indicative of a grain size effect. Further, the strength properties are known to be adversely affected by the presence of water in the atmosphere. Data from recent indentation studies on barium titanate are used to investigate these phenomena.

Intelligent Ceramic Materials: Issues of Brittle Fracture

In this paper we review the brittle fracture behavior of piezoelectric and electrostrictive ceramics of interest for intelligent material systems. Properties of importance, such as critical fracture toughness, strength, and susceptibility to moisture enhanced crack growth, are addressed, especially as they are affected by the materials microstructure and chemistry. Local strains and ferroelectric domains produced by phase transformations in these materials are shown to have a direct influence on both fracture toughness and strength. The cyclic stressing of a piezoelectric ceramic is discussed in terms of the structural damage produced by this mode of loading. Finally, tradeoffs in improved reliability, which may be achievable through suitable variations in composition or microstructure, are compared against possible losses in the piezoelectric properties of interest.

Fracture processes in ferroic materials

Abstract This paper analyzes recent studies on the fracture behavior of BaTiO3 and Pb(ZrTi)O3. The analysis shows that the measured fracture parameters (strength, fracture energy, flaw size) in the ferroic state can be described by standard fracture mechanics equations which are modified to take into account internal stresses. Thus, the analysis suggests that the effects of ferroic mechanisms (ferroelastic twinning, phase transformations) on fracture are important only prior to catastrophic crack propagation.

Fracture behavior of electronic ceramics

Abstract This paper reviews the fracture behavior, e.g., the strength, fracture toughness and susceptibility to environmentally enhanced crack growth, in electronic materials such as barium titanate, lead zirconate titanate (PZT), capacitor ceramics, and the new, high temperature superconductors. All of these materials are known to exhibit critical fracture toughness, KIC , which depends on grain size, chemical composition and crystal structure. The microstructure of these electronic ceramics has a direct effect on both their strength and fracture toughness. The paraelectric to ferroelectric phase transformation in barium titanate, PZT, and related ceramics induces internal stresses into the materials which can provide an additional driving force for flaw extension. The magnitude of such stresses calculated from strength data is in good agreement with that predicted by dielectric measurements. Crack-domain interactions in the ferroelectric structure as well as crack deflection, etc. give rise to increased...

High Tc superconducting Bi-Sr-Ca-Cu-O glass ceramics: A review

Abstract Glass-ceramic technology provides a convenient method for forming materials into a variety of near-net shapes. This paper reviews the current status of this method for preparing bulk Bi-Sr (Pb)-Ca-Cu-O superconducting ceramics. We also include discussion of the precursor glasses, processing techniques and reported formation mechanisms of the high Tc phases.

Effects of phase separation on crack growth in borosilicate glass

Abstract The effect of phase separation on crack propagation was determined in a soda-borosilicate glass. Crack velocity - K I curves in the velocity range of 10 −9 to 10 −4 m/s were shifted to lower K I with increasing heat-treatment time. A velocity plateau of 10 −4 m/s occurred, independent of heat treatment conditions. It is shown that the compositional changes in the SiO 2 rich phase formed during heat treatment controls crack growth rates. Results are discussed in terms of glass corrosion mechanisms.

The Fracture of Brittle Materials: Testing and Analysis

Preface ix Acknowledgments xi 1. Introduction 1 2. Fracture Mechanics Background 6 3. Environmentally Enhanced Crack Growth 19 4. Fracture Mechanics Tests 32 5. Strength Testing 68 6. Quantitative Fractography 99 7. Microstructural Effects 131 8. Predicting Reliability 145 9. Concluding Remarks 158 References 161 Name Index 175 Subject Index 179

Calculated potential for water enhanced crack growth in silicon

Abstract Ab initio molecular orbital calculations have been performed on the model molecules Si 8 H 17 OH (taking into consideration of the crack wall species -OH and -H) for the crack-tip strain simulation study of silicon and its reaction with water. The calculated equilibrium structure of silicon using a 6-31G ∗∗ basis set agreed well with experimental data. It was found that, regardless of the manner strain was applied, the values of net charge on the two crack-tip Si atoms are similar and that both become slightly more positive as strain increases. In addition to this absence of polarization of the crack-tip SiSi bond under strain, steric hindrance also inhibited the approach of the water molecule to the SiSi crack tip region. These calculations explain why moisture does not enhance crack-growth via chemical reaction in Si, which is different from that observed in silica.

Effect of microstructure on reliability predictions for glass ceramics

Abstract A study is made of the fracture properties of a range of three Li 2 OSiO 2 glass ceramics with different grain sizes. Both equilibrium and kinetic crack propagation parameters are evaluated using the controlled flaw indentation technique with a view to making reliability predictions. Fracture toughness in the well-behaved large-contact-flaw range is found to increase with increasing grain size. At low contact loads the strength and apparent toughness are found to fall below the predictions of the well-behaved region, this tendency increasing with increasing grain size. At the fixed contact load chosen for fatigue testing, the susceptibilities to slow crack growth for the three materials are found to be similar. A theory is developed to allow the variations in apparent toughness to be taken into account for lifetime prediction. The results suggest that caution must be exercised when reliability predictions are made for materials showing the effects of crack-microstructure interaction, especially in extrapolations to low contact loads.