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Glass strength and fatigue is limited by surface cracks. As subcritical crack growth (SCCG) is governed by ambient humidity, stress corrosion at the crack tip is widely accepted to be the underlying mechanism. However, as water is known to have decisive effect on glass properties and can rapidly enter the crack tip near glass region, SCCG could be affected by such water related phenomena. We tried to mimic these effects studying water dissolution and speciation, mechanical properties, and SCCG in water-bearing glasses. For this purpose, glasses up to 8 wt% water have been prepared by means of high-pressure melting of glass powder - water mixtures.
As part of this effort, SCCG in dry and hydrous commercial micros¬cope slide glass (CW = 6 wt%) was studied in double cantilever beam (DCB) geometry and sub-Tg relaxation was measured by Dynamic Mechanical Analysis (DMA).
For SCCG in ambient air (24% r.h.), SCCG was promoted by the presence of 6wt% bulk water with respect to the dry glass. On the other hand, stress intensity values, KI, required to cause slow crack growth (v < 10-6 ms-1) resemble literature findings for float glass of similar composition in liquid water, which might represent the maximum possible promoting effect of ambient water on SCCG.
For SCCG in vacuum (10-3 mbar), dissolved bulk water causes even more pronounced effects. Most strikingly, it strongly decreases the slope of the log v(KI)-curve, which is a measure of dissipated energy during fracture. A strong increase of sub-Tg relaxation with increasing water content was confirmed by DMA. As a consequence, slow crack growth occurs at KI values as measured in the dry glass whereas fast crack growth occurs at much larger KI than that of the dry glass. Kinks and shoulders shown by the inert log v(KI)-curve indicate that bulk water does not simply affect bulk mechanical properties.
As subcritical crack growth (SCCG) can reduce tensile strength of glasses by many orders of magnitude, the potential for improvement of fatigue behaviour is most intriguing in developing ultra-strong glasses. An essential bottleneck is the basic understanding of the numerous interplaying pressure-, temperature- and water-affected relaxation phenomena at the crack tip and related toughening strategies. Therefore, the present project aims to advance the basic understanding of structural relaxation effects and local properties caused by increased water concentration and tensile stresses at the crack tip as they are a key for structural toughening designs to develop SCCG-free glasses and glass surfaces.
Our first studies give clear evidence that glass structure and dynamics is strongly modified upon hydration of glasses. These changes are highly related to the nature of network formers but are affected as well by the counter ions (network modifier). Results of the 1st project part suggest that structural relaxation below glass transition temperature, i.e. overlapping of short-range (beta) and long-range (alpha) interactions can contribute to SCCG in water-free environments and that structurally dissolved water in the glasses can have decisive impact on this effect.
In the 2nd project stage specific glasses compositions will be investigated to gain an improved understanding on the relation of sub-Tg relaxation and inert SCCG as well as to shed light to the related effects of dissolved water and its speciation. These glasses cover a broader range of different glass topologies and binding partners, whereby the coupling of alpha and beta relaxations is varied systematically by alkali-, alkaline earth ions and water species concentrations. Preparation of hydrous glasses (up to 8 wt% water) will be performed by high pressure syntheses. Structure will be resolved by NMR, Raman and IR spectroscopy while structural relaxation is accessed in the temperature and frequency domain using dynamic mechanical spectroscopy and ultrasonic damping. We will focus on measurements of inert SCCG (region III) conducting experiments in vacuum and dry gas atmospheres using indentation techniques and stressing of glass specimens in DCB geometry.
Experimental data on SCCG will be provided to SPP groups, which deals with fatigue in metallic glasses and vice versa we will test theoretical predictions of ab-initio simulations of partner within SPP 1594 in order to quantify the effect of water on the crack tip. In summa topological factors controlling the subcritical crack growth with respect to water will be identified from which structural toughening designs for highly fatigue resistant-glasses can be derived.
Advanced methods for 3D green density characterization like computed tomography and 3D FE sinter modeling can be utilized for increasing the reliability of sintered components. The experimental in situ observation of sintering, however, is currently restricted to silhouette methods, i.e. heating microscopy. For complex shaped samples, in situ shape screening during shrinkage would allow much better validation of 3D sinter simulation models. Further, by revealing temporary sinter warpage, 3D high-temperature shape screening allows to locate potential defects of complex sintered components. Against this background, BAM developed a testing device for in situ 3D high-temperature shape screening for ceramic and glass-ceramic tapes up to 1000°C [1-3]. Current work is focused on dropping this restriction in sample shape and temperature. The poster illustrates the current state of this work and possible applications of the method e.g. in detecting sinter warpage of metallized glass-ceramic LTCC tapes.
Das Hochtemperatur-Laserprofilometer der BAM4 ermöglicht eine 3D-Formerkennung an keramischen Folien oder anderen flachen Objekten während thermischer Behandlungen wie zum Beispiel bei laufenden Entbinderungs- oder Sinterprozessen. Mit dem Lasertriangulationssensor werden das Höhenprofil, die lateralen und axialen Schwindungen und andere charakteristische Maße der Probe wie die Folienbreite und -dicke, Abstände zwischen spezifischen Merkmalen oder Krümmungen bestimmt. Somit können insbesondere auch Deformationen und Verwerfungen der Objekte sowie Schwindungsinhomogenitäten an strukturierten Objekten erkannt und mit dem Herstellungsprozess verknüpft werden.