Sintering Furnaces
The choice of a suitable furnace atmosphere, which is governed by the characteristics of the materials and to some extent by the properties -1 desired in the sintered product, is one of the most important factors in practically all sintering processes. Sintering furnaces are either fuel fired or electrically heated. In the first one, heating is relatively cheaper involving low temperature range) since the products of combustion
provide the protective atmosphere. Electrically heated furnaces provide extremely stable temperature conditions by means of proportionate current controllers [38].
There are some difficulties associated with conventional fast firing. Differential sintering that causes differential densification (non¬uniform microstructure, low density or specimen cracking) is one of the problems most often encountered when energy is delivered from the surface to the bulk of the sample by thermal conduction. At sufficiently high heating rates, large temperature gradients can be developed since the sample s thermal time constant is larger than the inverse heating rate. In contrast, microwave sintering, which is characterized by volumetric heating, has the potential to overcome this difficulty [39].
Microwave processing of ceramics is an alternate to conventional heating. It offers many advantages over conventional heating. In which, the surface of the material is directly heated, while the heating of the interior is induced by conduction. This will cause large temperature
Infiltration
Post Sintering process
The infiltration process involves preparing porous metallic body of skelton metal with high melting point and subsequently filling the pore of the skelton with a molten metal (infiltranted) either by immersing the porous skelton metal in the molten infiltrant metal or by placing the skelton compact into contact with the solid infiltrant metal in the form of powder, powder compact of sheet on top of the skelton and then heating in a sintering furnace to a suitable temperature between the melting point of the two metals.
The method is used either to prepare single phase structure or duplex ( two phase) structure, but single phase material is rarely made because of the insig – nifi cant role played by infiltration.
The duplex materials are of two types. The first types ar W – cu, W – Ag, MO – cu etc. where metals shows insolubility in each other at room temperature as well as at infiltration temperature. The second type consists of those exhibiting partial solubility of of the skelton metal in the infiltrant metal at infiltraion temperature.
These include CU- infiltrated Fe and steel products, CO infiltrated tungsten carbide etc.
There is no widely accepted theory of infiltration However it has been noted that very small or very large size of the pore is detrimental.
Thuse, fine powder and high compacting pressure may produce alarge a mount of closed porosity preventing infiltration.
( schwartzkopf) has suggested that two stages occur in the process of infiltration.
The first stage called infiltration proper, involves filling of the process of the skelton with the molten infiltrant metal by capillary action.
The second stage of infiltration process structural changes occure during the soaking period ( i . e the period in which the system is placed at the temperature of infiltration after filling all pore Volumes) because, with a dihedral angle equal to Zero – the infiltrant will penetrate along the
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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