Anisotropic shrinkage during sintering of glass-powder compacts under uniaxial stresses: Qualitative assessment of exper
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I.
INTRODUCTION
THE sintering of glass-powder compacts under the effect of uniaxial loads has been studied in the past, both experimentally[1–4] and theoretically.[5] The technological interest in studying the effect that an applied uniaxial stress will have on the sintering kinetics of a densifying glasspowder compact arises from the increasing use of the sinter-forging technique to fabricate dense glass-ceramic products. In this particular technique, the material is densified under pressure (typically .50 kPa) in its viscous state (as a glass), and subsequently the dense compact is subject to a heat-treatment process to yield the desired crystalline structure. When the densification stage is conducted under load without using a die,* the dimensions of the samples *The term sinter forging is usually reserved for sintering under uniaxial stress without using a die, whereas hot pressing refers to the process in which densification of a porous body takes place in a die.
in both the longitudinal and transversal directions will vary, and therefore it is important to be able to predict the changes of sample shape that may occur. Other situations where sintering under load becomes important is when using dilatometry to study the sintering behavior of powder compacts. In this case, the weight of the dilatometer push-rod will produce a compression stress (of the order of 5 kPa) that may have a non-negligible effect on the shrinkage and shrinkage anisotropy being recorded. The reported experimental studies on the sintering of glass-powder compacts under stresses were conducted using loading dilatometry,[1–4] and dealt mainly with the determination of
ALDO R. BOCCACCINI and EUGENE A. OLEVSKY, Postdoctoral Fellows, are with the Institute for Mechanics and Materials, University of California, San Diego, La Jolla, CA 92093-0404. Manuscript submitted May 5, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
sintering parameters such as sintering pressure, viscosity, and densification and creep rates. No specific attention was paid to the effect of the applied stresses on the anisotropy of shrinkage in those studies. It is intuitive to realize that the application of an external axial compressive stress to a cylindrical body undergoing sintering will tend to accelerate the axial shrinkage, but will produce the opposite effect to the radial contraction, as the sample will be squashed by the axial compression load. Thus, it is rather straightforward to visualize that there will be a significant effect of the applied stress on the relative radial and axial shrinkage strains that, in turn, will lead to a more or less pronounced degree of shrinkage anisotropy. The relative effect of the applied load on shrinkage anisotropy will depend on the result of the competition between the two mechanisms involved, i.e., viscous deformation induced by sintering and viscous deformation induced by the external uniaxial stress. Shrinkage anisotropy can be quantified by the shrinkage anisotropy factor (k), which has been defined as follows:[6] k5
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