User:Tamoghna.c/Reaction sintering

Reaction sintering is a type of sintering where reaction of the starting powder and sintering (i.e. densification) of the whole mass is done in a single heat treatment step.

It has advantages and disadvantages over the conventional sintering process.

Advantages

1. Reduction in processing steps as the reaction of starting powder and the sintering of the whole mass is done is same heating step.
2. Free energy change of the reaction can help in densification.

Disadvantages

1. Very difficult to control the microstructure
2. Reaction may be incomplete
3. Significantly reduced densification rate due to the effect of reaction on microstructure

In chemical reaction typical driving force is in order of 1000 J/mole where as in sintering which is driven by reduction in surface energy of the material the driving force in ~1 J/mole for micron sized starting particle. If the driving force for the reaction can be harnessed for densification then the result could yield fully densified product at much lower temperature. Another advantage is elimination of a heat treatment step required for reacting the starting material as reaction and densification here is done in the same heat treatment step. This will also eliminate subsequent milling and grinding step. Problems lies in reaction in reaction sintering process. It has been observed that in many cases recation causes disruption in microstructure such that full densification remain impossible to achieve. Consequently controlling the kinetics of reaction and densification is of paramount importance. In reaction where there is considerable volume change after reaction densification is difficult. In such cases it is desirable to densify the material prior to any reaction. Controlling the process variable enable us to do that.

Effect of process variable

Control of Densification and Reaction is the most tricky part of reaction sintering. Relative rate of reaction and densification can be influenced by the following parameters

1. Particle size
2. Temperature
3. Externally applied pressure
4. Heating rate

Increase in particle size, temperature, applied pressure or heating rate increases the densification more than the reaction.

Recation sintering can be successfully applied where the process involve satisfy the following criteria

1. Chemical reaction should not involve significant volume change
2. Densification should occur prior to reaction
3. Micro-structural in-homogeneity - a must

Types

Reaction sintering can also be sub divided into two category depending on the reaction involved i.e. single phase when the product is single phase and multi phase when the reaction product is multi phase.

1. A(powder) + B(powder) = C(solid)
2. D(powder) + E(powder) = F(solid) + G(solid)