Effects of Surface Diffusion and Heating Rate on First‐Stage Sintering That Densifies by Grain‐Boundary Diffusion

Abstract

This paper presents a computational study of the role played by surface diffusion on first stage sintering of powders that densify by grain-boundary diffusion.Coupled grain-boundary and surface diffusion is considered as the mechanism for matter redistribution. By using several novel approaches of presentation of the numerical data, it is shown that the assumption of fast surface diffusion is invalid for typical sintering conditions and materials in first stage of sintering. The study reveals a simple explanation for the role played by surface diffusion in matter redistribution of combined grain-boundary and surface diffusion -surface diffusion changes direction from moving atoms away from a contact neck to depositing atoms onto it as the rate of surface diffusion increases. The reverse surface diffusion blunts the neck and retards densification. It is shown that this mechanism is significant not only for free sintering but also for pressure assisted sintering. It is further confirmed that the widely observed beneficial effect of spark plasma sintering on densification can be, at least partially, attributed to its fast heating rate, which quickly passes through sintering at low temperatures where surface diffusion dominates.

Sintering forces acting among particles during sintering by grain‐boundary/surface diffusion

“…Computer simulations were carried out to study the initial stage of solid‐state sintering of 2 particles for several decades. Many aspects of the results of each of these investigations lie in quantitative agreement with one another and with analytical models . For example, all of the simulation models yield a value of 6 for the exponent m of Equation , and they find that the shrinkage takes on a power‐law form of Equation .…”

“…Bross and Exner and Swinkels and Ashby included the coupling of grain‐boundary diffusion and surface diffusion. In recent years, several techniques for the computer simulation of solid‐state sintering have been developed, including finite difference method (FDM), finite element method (FEM), Monte Carlo Potts models (MC), phase field approaches (PH), discrete element method (DEM) and Surface Evolver program …”

Ultralow Temperature Densification of a Titanium Alloy by Spark Plasma Sintering

“…During this regime, necking and further plastic deformation of particles are presumed to occur with minimal impact in the densification of the sample . The increased packing density attained during the first regime by the specimens sintered at lower temperatures allows the diffusion processes to occur at a much faster rate, specifically, surface diffusion, where the mass transport between particles in contact is responsible for the evolution of the neck . Surface diffusion events are completed at temperatures 100 °C less than high temperature‐sintered specimens.…”

Finite‐difference calculation on an uncertainty evaluation of the kinetic parameters from initial sintering of UO2

“…Luo and Pan pointed out that the Bouvard and McMeeking method does not rigidly define the curvature at the edge of the neck and proposed the introduction of the Pan and Cocks method . In the present study, for simplification of the calculation, the Bouvard and McMeeking method was adopted in order to determine the curvature at the edge of the neck because the principal goal of the present study is to clarify the effect of particle configuration on the shrinkage behavior.…”

“…Computational methods to understand sintering behavior have been developed using various models in step with the progress of computers. Various models, such as the Monte Carlo (MC) model, the discrete element model (DEM), the phase‐field method (PFM), the finite element model (FEM), and the finite‐difference model, have been developed and used. However, the available models have insufficiently simulated the entire sintering process.…”