Determination of shear viscosity of borosilicate glass + silica powder compacts by an optical system
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Determination of shear viscosity of borosilicate glass 1 silica powder compacts by an optical system Jaecheol Bang, Guo-Quan Lu, and Jesus Noel Calata Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received 9 February 1998; accepted 26 August 1998)
The shear viscosity of borosilicate glass (BSG) 1 silica powder compacts was determined during isothermal sintering at temperatures between 665 and 715 ±C. An optical system was used to measure in situ the densification profiles of the compacts in the form of thick films constrained on a rigid substrate and in-plane stresses generated during sintering. The shear viscosity as a function of relative density was calculated from these measurements by using Scherer’s viscous constitutive equations for a porous sintering body. The shear viscosity for samples with silica contents from 0 to 20 vol% generally showed a slow increase with density in the low-density regime followed by a rapid rise after some threshold density that decreased with either increasing silica powder content or decreasing sintering temperature. The dependence of shear viscosity on relative density is consistent with results from sinter-forging experiments. However, results obtained from pure BSG compacts also revealed strong dependence of shear viscosity on temperature and microstructure, which has been all but neglected in previous theoretical studies.
I. INTRODUCTION
Inhomogeneous packing of particles or inclusion of nonsintering particles in a powder compact can lead to differential shrinkage rates. Such conditions will generate shear stresses during sintering and produce shear deformation in addition to densification. Deformation during sintering may produce flaws in sintered parts, such as distortion or camber. To understand sinteringdefect formation, it is important to know how shear viscosity is affected by porosity (or relative density) in a porous sintering body. Several experimental works1–7 have been carried out to determine the shear viscosityporosity (or density) relationship, mostly by sinter forging-type experiments. The experiment is typically carried out by varying the uniaxial load on the sintering body and measuring the strain as a function of the applied load. Measured values of the axial and radial strains are resolved to give a shear strain rate component. The correspondence between the shear stress component of the applied stress and the shear strain rate yields the shear viscosity as a function of density. However, this type of experiment is very difficult to perform, and often, “barreling” of the sample due to the friction between the push rods and sample surfaces can make the dimensional measurement difficult.2 In this paper, an experimental method to determine shear viscosity-relative density relationship in a porous body is presented. It involves measurements of the densification profiles of powder compacts in the form of thick films constrained on r
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