Anisotropic shrinkage during sintering of glass-powder compacts under uniaxial stresses: Qualitative assessment of experimental evidence

The shrinkage anisotropy that occurs during sintering of glass-powder compacts under uniaxial compressive stresses is considered. Available experimental data from the literature are used to calculate a shrinkage anisotropy factor (k) and to study its dependence on the applied stress and progress of densification. The factor k is defined by the ratio between axial and radial strains. The experimental data are interpreted in terms of qualitative trends proposed for the variation of k with the progress of sintering for a constant applied load. It is shown that when sintering densification predominates over viscous deformation induced by an external load at the onset of the process, k increases with the sintering time. The existence of a stage at which the shrinkage anisotropy factor becomes negative is confirmed by the analysis of available experimental data. The trends for the variation of k are also given for the case of the externally induced deformation predominating over sintering densification. However, no sufficient experimental data were found to validate the proposed trends in this case. The prediction of the theoretical model of Scherer for viscous sintering under uniaxial stresses is shown to agree qualitatively with the experimental trends. In connection with the use of dilatometry in sintering kinetic studies, the effect of small stresses on shrinkage anisotropy is highlighted.