MEANING:

Actually sintering is defined as the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.

The sinter word is taken from the Middle High German sinter, a cognate of English cinder.

Solid state material transport
Fig. 1 Solid state material transport

It is also called as frittage.

With metals, ceramics, plastics, and other materials this process takes place naturally in mineral deposits or as a manufacturing process.

By fusing the particles together and creating one solid piece, the atoms in the materials diffuse across the boundaries of the particles.

Temperature does not need to reach the melting point of the material due to sintering.

With extremely high melting points such as tungsten and molybdenum, this process is often chosen as the shaping process for materials.

Vapor phase material transport
Fig. 2 Vapor phase material transport

Study of sintering in metallurgy powder-related processes is known as powder metallurgy.

For making bonds between solid particles sintering consumes surface energy.

VARIOUS TYPES OF SINTERING

GENERAL SINTERING

When strength, electrical conductivity, translucency and thermal conductivity, reduces it’s porosity and enhances the properties, then this process is much more effective.

The bond area in relation to the particle size is the determining factor, for properties such as strength and conductivity.

As the vapor pressure depends upon temperature, the variables such as the temperature and initial grain size can be controlled at any given material.

CERAMIC SINTERING

For the manufacturing of pottery and other ceramic objects, sintering is a important part of the firing process.

The ceramic sintering are made from substances such as glass, alumina, zirconia, silica, magnesia, lime, beryllium oxide, and ferric oxide.

Lower affinity for water and a lower plasticity index than clay, are the some ceramic raw material.

The simply process of creating ceramic objects through sintering of powders includes

  1. Mixing water, binder, deflocculant, and unfired ceramic powder to form a slurry.
  2. Spray-drying the slurry.
  3. By keeping the spray dried powder in the mold and pressing it to form a green body.
  4. Heating the green body at low temperature to burn off the binder.
  5. Sintering at a high temperature to fuse the ceramic particles together.

PLASTICS SINTERING

For the control of fluid and gas flows Sintered plastic porous components are used in the filtration.

Such as the nibs in whiteboard markers, inhaler filters, and vents for caps and liners on packaging materials sintered plastics are used in applications requiring caustic fluid separation processes.

For the purpose of ski and snowboard base materials, sintered ultra high molecular weight polyethylene materials are used.

LIQUID PHASE SINTERING

The material which are difficult to sinter, that process is called as liquid phase sintering process .

Si3N4, WC, SiC, these are the common material which is used for the liquid phase sintering method.

By addition of an additive to the powder which will melt before the matrix phase, this process belongs to the liquid phase sintering.

This process is divided in three stages

REARRANGEMENT

In this process as the liquid get melted the capillary action will pull the liquid into pores and also cause grains to rearrange into a more favorable packing arrangement.

SOLUTION-PRECIPITATION

In areas where capillary pressures are high atoms will preferentially go into solution and then precipitate in areas of lower chemical potential where particles are not close or in contact.

This is also called contact flattening.

Densification of solid skeletal network, liquid movement from efficiently packed regions into pores.

For improving grain growth of thin semiconductor layers from nanoparticle precursor films liquid phase sintering plays important role.

MICROWAVE SINTERING

Sometimes heat is generated internally within the material in microwave sintering, rather than via surface radiative heat transfer from an external heat source.

It is restricted to be useful due to some materials, which fail to couple and others exhibit run-away behavior.

Advantage of this process is faster heating for small loads, meaning less time is needed to reach the sintering temperature, less heating energy is required and there are improvements in the product properties.

At the same temperature this process and side-reactions run number of times faster during microwave sintering.

For the purpose of good maintaining fine grains/nano sized grains in sintered bioceramics, the microwave sintering plays important role.

The well known example of this process are Magnesium phosphates and calcium phosphates.

ADVANTAGES

It’s allows making complex geometries .

High precision .

Green technology.

Products are always ready for assembly.

Reduces energy consumption.

DISADVANTAGES

Large material quantity are required .

High initial capital cost relative to small alternative molding processes .

On the basis of size of chamber of machine, the size limitation is fixed.

High temperature leads to high energy loss.

Price of machine and materials are expensive.

Some materials are brittle.

APPLICATION

For making strong reliable and high-performance shapes, like as ceramic, medical implants, sintering is used .

When atoms of material around boundaries of particles get diffuse and form one single solid particle then the sintering take place .

For the purposes of decreasing or removing porosity and increase in conductivity this process gets much helpful.

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