Dimension and Damage Control in Sintering of Multilayerpowder Composites
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Dimension and Damage Control in Sintering of Multilayer Powder Composites Eugene A. Olevsky1, Andrey L. Maximenko1, John H. Arterberry1, and Veena Tikare2 1 Mechanical Engineering Department San Diego State University 5500 Campanile Drive, San Diego, CA 92182-1323 2 Sandia National Laboratories Albuquerque, NM 87185
ABSTRACT Sintering distortions of two and three-layer composite powder structures are determined based upon the continuum theory of sintering implemented in a finite element computer code. The macroscopic constitutive parameters of the powder material are obtained on the basis of the meso-scale simulations of a realistic grain-pore structure. The model follows both the densification and the damage development during sintering using a new fracture criterion for the prediction of macroscopic strength in sintering.
INTRODUCTION At first glance, one could expect, that the shrinkage of a porous body under conditions of the uniform hydrostatic compression, which is characteristic for free sintering is self-similar, changing only the volume without changing the shape. This assumption, however, turns out to be justified only in the ideal case of homogeneous article properties and uniformity of the external load (for sintering under pressure). In practice, for most cases, these conditions are not met, as a result of which the shrinkage becomes variable in different parts of the porous object’s volume. Therefore, it is important to identify the crucial factors, having an influence on shrinkage inhomogeneity. Control of these factors can enable the production of near- net shape components. For sintering, major factors causing shrinkage anisotropy are: Anisotropy of pore-particle structure [1], Density and chemical composition nonuniformity (artificially introduced or caused (for density) by previous treatment by pressure) [2]; Nonuniform temperature distribution; Applied external loads [2]; Kinematic constraints (adhesion or friction of external surfaces) [2]; Gravity influence [3]. It appears that for the production of multilayer ceramics used for wireless applications, the second group of factors is the most important one. Indeed, a particular example of density or of chemical composition non-uniformity can be given as a bi-layer structure.
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MODEL BASIS In the following work, several assumptions are made: that each layer of porous material is homogeneous and isotropic, and that each layer obeys the Skorohod-Olevsky [2] linear viscous model under the action of surface tension in sintering. σij - PL*δij = 2η0 *[ ϕ *dεij/dt + (ψ-ϕ/3)(dε11/dt + dε22/dt + dε33/dt)*δij ]
(1)
σij - externally applied stress; PL - sintering stress: PL (θ) = PL 0 (1 − θ) 0.26
(2)
An analytical expression for the normalized effective sintering stress is obtained by fitting the me
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