Silica in humid air environment (I): Diffusion in the absence of stresses. Open Access at KIT

Diffusion of water into silica results in diffusion zones near the silica surface. Due to the chemical reaction between H2O and SiO2 the material expands, i.e. swelling takes place. Since the unaffected bulk material suppresses free expansion, biaxial compressive swelling stresses will occur near the surface. For constant concentrations of molecular water at the surface, we already outlined the problem in earlier papers

Silica in humid air environment: (II): Diffusion under moderate stresses

Water diffusion into silica glass results in a thin zone near the surface of the glass. In this zone molecular and hydroxyl water are present. The content of hydroxyl water increases its specific volume so that the silica expands and the volume near the surface is larger than it was before being penetrated by the water. Suppressed free expansion of the glass causes biaxial compressive swelling stresses. The effect of such swelling stresses and also of moderate externally applied stresses on the diffusion behaviour will be studied in this report for the case of a gaseous water environment at temperatures <500°C. In the Appendix also the effect of stress-enhanced equilibrium constant is addressed and the behavior at higher temperatures briefly considered

Swelling strains from density measurements

Water in contact with silica glass surfaces diffuses into the glass, and reacts with the silica network under hydroxyl generation. Clear evidence has been reported in the literature for a volume swelling due to the water uptake and reaction in silica. In the past, the authors showed in a couple of papers the principle effects of volume swelling by hydroxyl generation on mechanics and fracture mechanics properties. From literature data on density as a function of water content a linear dependence of the volume swelling strain with water concentration was established. This dependency will be discussed in the present report including data scatter

Strength measurement on silica soaked in hot water. Open Access am KIT

The effect of water soaking on the strength of silica glass is studied. When silica glass is immersed in warm water and held there for an extended period of time, the strength increases over that of freshly damaged glass. The increase in strength is a consequence of water diffusion into exposed surfaces of the test specimen, which results in swelling of the glass and shielding of cracks present in the surface of the glass. In our first paper on this subject (Report 19 of this series), we considered swelling effects on the inert strength. In the present report, the strength under subcritical crack growth conditions is studied. For tests carried out in humid environment at various loading rates, so-called dynamic strength tests, we could show theoretically that the swelling effect cuased by the reaction of water with silica must result in apparently increased crack-growth exponents. This prediction is in good agreement with results from literature. In our experiments we could show via an evaluation of the crack extension that even in silicone oil environment local subcritical crack growth accurs

Inert strength measurement on hot water soaked silica

The effect of water soaking on the strength of silica glass is studied. When silica glass is immersed in warm water and held there for an extended period of time, the strength increases over that of freshly damaged glass. The increase in strength is interpreted as the consequence of water diffusion into exposed surfaces of the test specimen, which results in swelling of the glass and shielding of cracks present in the surface of the glass. Experimental results are compared with theoretical predictions

Investigation of the Subcritical Crack Growth Process In Glass By Atomic Force Microscopy

International audienceTo investigate the possibility of cavity formation in silica glass during subcritical crack growth, the topography of fracture surfaces formed in water at a crack velocity of 8 x 10(-11) m/s was mapped using an atomic force microscope. The objective of the study was to determine how well the two halves of a crack in silica glass matched by establishing a three dimensional quantitative comparison of the shape of the two surfaces. This procedure uses a minimization routine to correct first order and quadratic distortions between the two AFM images of the crack surfaces. A three-dimensional image representing topographical differences between the corresponding fracture surfaces followed a Gaussian law centered on zero with a standard deviation, sigma, of 0.22 nm. Within the resolution of the technique, 6 nm within the fracture surface and approximate to 0.44 nm normal to the fracture surface, no evidence for cavitation was found in silica glass. It is suggested that subcritical crack growth in silica glass occurs in a brittle way by the successive snapping of bonds as the crack advances

The Use of Atomic Force Microscopy to Study Crack Tips in Glass

International Symposium on the Stress Corrosion Cracking in Structural Materials at Ambient Temperatures, Padua, ITALY, SEP, 2009International audienceThis article presents a review of the application of atomic force microscopy (AFM) to crack-tip corrosion during subcritical crack growth in glass. The two principal experimental techniques used in this type of study are (1) the direct observation of crack motion by scanning the tip of a crack during crack growth and (2) the examination of fracture surfaces once the specimen has been fractured in two. The first technique has been used to demonstrate and quantify water condensation at crack tips during subcritical crack growth and is particularly useful at low crack velocities. The second technique has been used to quantify the crack-tip corrosion process and the shape of the crack tip during crack growth. In this article, we discuss experimental results showing that the environment that develops at the tips of freshly fractured glass surfaces in soda lime glass can corrode the glass surfaces near the crack tip. Soda lime silicate glass contains mobile alkali ions that will exchange with hydronium ions in solution at the crack tip, forming a highly basic solution that is corrosive to glass. Experimental evidence for such corrosion has been obtained by the atomic force microscope, which demonstrates a displacement of the two fracture surfaces near the crack tip that can be as much as 20 nm, depending on how long the crack is held open at the fatigue limit. Despite the corrosion and displacement of the crack surfaces, the crack tip itself appears to remain sharp, suggesting that the fatigue limit in soda lime silicate glass is not due to crack-tip blunting. Most likely, the fatigue limit is a consequence of ion exchange at the crack tip, in which hydronium ions in the crack-tip solution exchange with sodium ions in the glass. As hydronium ions are larger than sodium ions, this exchange process leaves a compressive stress within the fresh fracture surface of the glass that resists crack motion and results in a stress-corrosion fatigue limit, as first proposed by Bunker and Michalske. In agreement with this mechanism, no fatigue limit is observed for silica glass, which also exhibits no ion exchange. As the crack-tip solution in silica glass is only mildly acidic, pH approximate to 5, corrosion does not occur at crack tips of this glass as supported by the observation that no crack-tip displacements are observed in silica glass by AFM. As the proposed ion exchange mechanism used to explain the stress corrosion limit in glass is at variance with the belief that the fatigue limit in glass is the result of crack-tip blunting, we discuss the possibility of plastic deformation at crack tips in glass and conclude that the available experimental data does not support such a model. At the present time, chemical reaction based crack growth theories are most consistent with the body of crack growth data that is available on glass and are probably the best explanation for the phenomenon

Processing and Mechanical Properties of Al2O3/Ni3Al Composites with Interpenetrating Network Microstructure

Composites with microstructures of interpenetrating networks were manufactured by gas pressure infiltration of Ni3Al into porous preforms of aluminum oxide. Composites with Ni-3,Al contents of between 15% and 30% by volume were made and evaluated mechanically at temperatures between room temperature and 1000 degreesC. The fracture strength, the fracture toughness, Young's modulus, and the thermal expansion coefficient were measured for each composite and test condition and were correlated with the microstructures of the composites. Composites with low Ni3Al contents had strengths below 400 Wa, presumably due to microcracking along the interface between the Ni3Al and the Al2O3. The composite with the highest content of Ni3Al, 30 vol%, had a mean fracture strength of 675 +/- 16 MPa, a Weibull modulus of 23.9, and a room-temperature toughness of 9.2 +/- 0.5 MPa(.)m(1/2)