Sintering Stages
The kinetics of the process can be broken down into three sintering stages:
1. Initial Neck Growth
Sintering initially causes the particles that are in contact to form grain boundaries at the point of contact through diffusion. This is the point contact stage and does not result in any dimensional changes. The greater the initial density of compaction (increased particle contact and potential grain boundary area), the higher the degree of coherency in the material
In this initial stage of sintering, necks begin to form at the contact points between adjacent particles see Figure 3-5. This stage is therefore referred to as the "neck growth" stage .No change in the dimensions is observed nor does porosity decrease .
Neck formation is driven by the energy gradient resulting from the different curvatures of the particles and the neck, Surface diffusion is usually the dominant mass-transport mechanism during the early stages of neck growth, as the compact is heated to the sintering temperature
Figure (3-5) neck formation
2. Intermediate Stage sintering
Intermediate stage sintering begins when adjacent necks begin to impinge upon eachother. This occurs when the quantity D/2R= 3.0, see Figure 3.5. Densification and grain growth occur during this stage.
The packing density and coordination number of the green packing are important during this stage. A high green packing density produces rapid sintering with relatively few pores in the final object .
The intermediate stage is pore channel closure where interconnected pore charnels are closed off isolating porosity. One of the causes of pore channel closure is neck growth. Another cause is the creation of new contact points by pore shrinkage within the pore itself .
Very low green packing densities (around 40%), which are also associated with low coordination numbers, can lead to coarsening (increase in mean grain size) without densification (decrease in porosity). In extreme cases, this may lead to open-pore structures lacking in structural integrity, See Figure 3-6 .
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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