Powder sintering theory revolves around two basic questions: First, the driving force of sintering or thermodynamics; second, sintering mechanism or dynamics problem.
The powder sintering theory began in the early 1820s. In 1942, scientists using physical and chemical research methods determined the effects of sintering temperature to the sintered body’ electromotive force, density, microstructure, mechanical properties, found that sintering is a very complex process. Chinese Powder Metallurgy scientists in 1961 come up with the combined effect sintering theory, think that the sintered powder is the result of spreading, flow and physical and chemical reactions combined effects.
The decrease of sintering thermodynamics sintering system free energy is the driving force of the sintering process, including: the increases binding surface (sintering neck) and the straightness of particle surface, the powder body’s total surface area and total surface free energy can be reduced; the total volume of sintered body’s internal pores and the total surface area decreases; within powder particles, the lattice distortion eliminated.
According to metal powder supplier, sintered motivity’s quantitative analysis can proceed from three aspects: considering the effects of surface tension from two goals model, deduced in the surface of the sintering neck exist tensile stress, make the sintering neck growing. The tensile stress is inversely proportional with the curvature of the sintering neck radius. This shows that with the sintering neck grew up, the driving force decreased. The sintering neck surface exist tensile stress, so that this area’s vacancy concentration C will be higher than the no-stress area’s vacancy concentration a. So that a small area under the surface of the sintering neck appears vacancy concentration gradient, cause the vacancies within the small region of the sintering neck’s surface diffuse to the sintered body, while the atoms remove to the sintering neck, make the sintering neck growing. Consider the equilibrium vapor pressure of the substance’s surface is related to its surface curvature radius, deduced that the sintered powder particles’ spherical surface (convex surface) and the surface of the sintering neck (concave) exist saturated vapor pressure, make the atoms evaporated from the sphere while the cohesion in the sintering neck concave down caused the sintering neck grew up.
Sintering kinetics of sintering theory is mainly conducted around this problem, through the model to determine the materials’ migration patterns and the rate of sintering process. Be summarized in the following: flow mechanism, think that the bonded and sintered neck grew up occurs in the powder sintering is due to the results of under surface tension caused viscous flow or plastic flow. Evaporating-cohesion mechanism, think that the substance migration in the powder sintering process is mainly due to the surface of the powder particles (convex surface) and the surface of the sintering neck (concave) exists the saturated vapor pressure differential, make the atoms evaporated from the particle surface and cohered in the sintering neck concave, make the sintering neck grew up. The relationship is the sintering neck radius z 3 power proportional to time. Diffusion mechanism, think that the substances migration in the powder sintering process is mainly the diffusion take effect.
The combined effect’s sintering theory. The relationship between sintering neck grow and time derived from ideal sintering model, although be verified under certain experimental conditions, but under some other conditions there are exist big differences with the actual sintering process. Reflect on the mathematical relationship is in the same kind of sintering mechanism, in the relationship of xm- t, the m value is inconsistent. This reflects the complexity of the actual sintering process. The combined effect’s sintering theory deduced that the dm-d0/dm-d value’s double logarithmic lnln (dm-d0/dm-d) and the sintering temperature’s reciprocal 1/T show linear relationship. Dm is the theoretical density of the powder, d0 is the density before sintering compacts, d is the density of the sintered body. Use metallic nickel powder, cobalt powder, copper powder, molybdenum powder, tantalum powder to conduct the powder sintering experiments, use tungsten powder to conduct the activated sintering experiments, use copper powder, beryllium oxide powder to conduct heat pressing test and other data experiments, are consistent very well.
Source:http://www.mhcmp.com
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