Sintering surface diffusion

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

Sintering consists of heating a mixture of fine materials to an elevated temperature without complete fusion. Surface diffusion and some incipient fusion cause the soHd particles in contact with one another to adhere and form larger aggregates. In the processing of hematite, Fe202, or magnetite,... 

Ashby pointed out diat die sintering studies of copper particles of radius 3-15 microns showed clearly the effects of surface diffusion, and die activation energy for surface diffusion is close to the activation energy for volume diffusion, and hence it is not necessarily the volume diffusion process which predominates as a sintering mechanism at temperatures less than 800°C. 

The mechanisms above allow rapid diffusional transport of atoms along the surface. We discuss the role of surface diffusion in the morphological evolution of surfaces and pores during sintering in Chapters 14 and 16, respectively. 

J.M. Dynys. Sintering Mechanisms and Surface Diffusion for Aluminum Oxide. PhD thesis, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 1982. 

If sintering occurs by diffusion through the bulk crystal (mechanism BS-XL), all the ratios will be the same as for surface diffusion except for the cross-sectional area Ab/As, which will scale as A2. Therefore,... 

If sintering occurs by grain-boundary diffusion, the ratio of rates will be the same as for the surface-diffusion case, A-4. A A-1 scaling law can be derived for viscous flow and a A-2 law applies for vapor transport [32],... 

As discussed above, a thermodynamically unstable surface will reduce its total surface energy by forming facets. From the point of view of kinetics, gradients in the chemical potential on a nonequilibrium surface will drive the movement of surface materials toward equilibrium. The transport mechanisms are the same as those that can operate during sintering (47) (a) surface diffusion, (b) bulk diffusion, (c) evaporation-condensation, and (d) plastic or viscous flow. 

Note y = surface energy, D = volume diffusivity, Ds = surface diffusivity, Db = grain boundary diffusivity, ij = viscosity, b = Burgers vector, k = Boltzman s constant, p = density, S = width of grain boundary diffusion path, P = pressure, M = molecular weight, and 2 = atomic volume. Source From R. M. German, Sintering Theory and Practice (New York Wiley, 1996). Reprinted with permission of John Wiley Sons, Inc. 

Of course, the time required for sintering is decreased as the particle size decreases. Figure 14.7 shows that if surface diffusion is the controlling mechanism, the time is inversely proportional to DA. 

Time needed to reach a neck size of x/D = 0.1 in the sintering of various sizes of ice particles. The controlling mechanism is surface diffusion. Data from W. D. Kingery, J. Appl. Phys. 31 (1960) 833-8. 

Stainless steel powder with a mean particle diameter of 50 mm has been compacted to a green density of 58% and sintered in pure H2. The resulting shrinkage measurements are given below. Published diffusion data for this stainless steel show that the activation energies are 225 kJ/mol for surface diffusion, 200 kJ/mol for grain boundary diffusion, and 290 kJ/mol for volume diffusion. Use the data below to determine the mechanism. 

Fig. 5. Calibration measurement of solid hydrogen film layer thickness via energy loss of a particles [27,28]. Americium a source is embedded on the surface of the gold-plated copper substrate, onto which hydrogen thin film is deposited by releasing the gas through porous sintered metal (diffuser). Silicon detector, mounted on the vertically movable diffuser, measured the a particle energy loss in the film, which is converted the thickness using the stopping power...

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