Sintering refers to the heating of the compacted powder perform to a specific temperature (below the melting temperature of the principle powder particles while well above the temperature that would allow diffusion between the neighbouring particles).
Sintering facilitates the bonding action between the individual powder particles and increase in the strength of the final part. The heating process must be carried out in a controlled, inert or reducing atmosphere or in vacuum for very critical parts to prevent oxidation.
Prior to the sintering process, the compacted powder perform is brittle and confirm to very low green strength. The nature and strength of the bond between the particles depends on the mechanism of diffusion and plastic flow of the powder particles, and evaporation of volatile material from the in the compacted preform.
Bonding among the powder particles takes places in three ways: (1) melting of minor constituents in the powder particles, (2) diffusion between the powder particles, and (3) mechanical bonding. The time, temperature and the furnace atmosphere are the three critical factors that control the sintering process. Sintering process enhances the density of the final part by filling up the incipient holes and increasing the area of contact among the powder particles in the compact perform.
• It is the process of consolidating either loose aggregate of powder or a green compact of the desired composition under controlled conditions of temperature and time.
• Types of sintering: a) solid state sintering – This is the commonly occurring consolidation of metal and alloy powders. In this, densification occurs mainly because of atomic diffusion in solid state.
b) Liquid phase sintering – The densification is improved by employing a small amount of liquid phase (1-10% vol). The liquid phase existing within the powders at the sintering temperature has some solubility for the solid. Sufficient amount of liquid is formed between the solid particles of the compact sample. During sintering, the liquid phase crystallizes at the grain boundaries binding the grains. During this stage, there is a rapid rearrangement of solid particles leading to density increase. In later stage, solid phase sintering occurs resulting in grain coarsening and densification rate slows down. Used for sintering of systems like tungsten-copper and copper-tin. Also covalent compounds like silicon nitride, silicon carbide can be made, that are difficult to sinter.
c) Activated sintering – IN this, an alloying element called ‘doping’ is added in small amount improves the densification by as much as 100 times than undoped compact samples. Example is the doping of nickel in tungsten compacts d) Reaction sintering – IN this process, high temperature materials resulting from chemical reaction between the individual constituents, giving very good bonding. Reaction sintering occurs when two or more components reacts chemically during sintering to create final part. A typical example is the reaction between alumina and titania to form aluminium titanate at 1553 K which then sinters to form a densified product.
Other than mentioned above, rate controlled sintering, microwave sintering, gas plasma sintering, spark plasma sintering are also developed and practiced.
Sintering theory
- Sintering may involve, 1) single component system – here self-diffusion is the major material transport mechanism and the driving force resulting from a chemical potential gradient due to surface tension and capillary forces between particles, 2) multi-component system (involve more than one phase) – inter-diffusion occurs with the concentration gradient being the major driving force for sintering in addition to self-diffusion caused by surface tension and capillary forces. IN this sintering, liquid phase formation and solid solution formation also occurs with densification.
- First theory was proposed by Sauerwald in 1922. This theory says that two stages are involved in sintering namely adhesion and recrystallisation. Adhesion occurs during heating due to atomic attraction and recrystallisation occurs at recrystallisation temperature (above 0.5 Tm). In recrystallisation, microstructure changes, phase changes, grain growth, shrinkage occurs.
Property changes during sintering
• Densification is proportional to the shrinkage or the amount of pores removed in the case of single component system
• IN multicomponent system, expansion rather than shrinkage will result in densification and hence densification can not be treated as equal to the amount of porosity removed.
• densification results in mechanical property change like hardness, strength, toughness, physical properties like electrical, thermal conductivity, magnetic properties etc. Also change in composition is expected due to the formation of solid solution.
Solid state sintering process
Condition for sintering: 1) densification occurs during sintering and solid state sintering is carried out at temperatures where material transport due to diffusion is appreciable. Surface diffusion is not sufficient, atomic diffusion is required.
2) This occurs by replacing high energy solid-vapour interfaces (with free energy SV) with the low energy solid-solid interface (particle-particle) of free energy SS. This reduction in surface energy causes densification.
3) Initially free energy of solid-solid interface must be lower than free energy of solid vapour interface. The process of sintering will stop if the overall change in free energy of the system (dE) becomes zero, i.e., dE = ϒSS dASS + ϒSV dASV< 0 Where dASS &dASV are the interfacial area of solid-solid and solid-vapour interfaces.
4) Initially, the surface area of compact represent the free surface area, since no grain boundaries have developed and hence ASV = ASV0 & ASS = 0. As sintering proceeds, ASV decreases and ASS increases. The sintering process will stop when dE = 0, i.e.,ϒSS dASS + ϒSV dASV = 0 => ϒSS / ϒSV = - dASV / dASS
5) Densification stops when - dASV / dASS is close to zero. To achieve densification without grain growth, the solid-solid interface must be maximized. Such conditions can be achieved by doping or by using suitable sintering conditions for surface free energy maximization.
Stages in solid state sintering
• In general, solid state sintering can be divided into three stages – 1st stage: Necks are formed at the contact points between the particles, which continue to grow. During this rapid neck growth takes place. Also the pores are interconnected and the pore shapes are irregular.
• 2nd stage: In this stage, with sufficient neck growth, the pore channels become more cylindrical in nature. The curvature gradient is high for small neck size leading to faster sintering. With sufficient time at the sintering temperature, the pore eventually becomes rounded. As the neck grows, the curvature gradient decreases and sintering also decreases. This means there is no change in pore volume but with change in pore shape => pores may become spherical and isolated. With continued sintering, a network of pores and a skeleton of solid particle is formed. The pores continue to form a connected phase throughout the compact.
• 3rd or final stage: In this stage, pore channel closure occurs and the pores become isolated and no longer interconnected. Porosity does not change and small pores remain even after long sintering times.
Driving force for sintering
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• The main driving force is excess surface free energy in solid state sintering. The surface energy can be reduced by transporting material from different areas by various material transport mechanisms so as to eliminate pores.
• material transport during solid state sintering occurs mainly by surface transport, grain boundary transportation. This surface transport can be through adhesion, surface diffusion. Many models available to describe sintering process – like viscous flow, plastic flow, grain boundary and volume diffusion models. These models will be briefly described here.
Mechanism in solid state sintering
As discussed earlier, material or atom transport forms the basic mechanism for sintering process. A number of mechanisms have been proposed for sintering operation.
These are,
1. Evaporation condensation, 2. diffusion (can be volume diffusion, grain boundary diffusion, surface diffusion), 3. plastic flow
1. Evaporation and condensation mechanism
The basic principle of the mechanism is that the equilibrium vapor pressure over a concave surface (like neck) is lower compared to a convex surface (like particle surface). This creates the vapor pressure gradient between the neck region and particle surface. Hence mass transport occurs because of vapor pressure gradient from neck (concave surface) to particle surface (convex surface).
2. Diffusion mechanism
- Diffusion occurs because of vacancy concentration gradient. In the case of two spheres in contact with each other, a vacancy gradient is generated between the two surfaces. This condition can be given by, μ-μ0 = RT ln(C/C0) = (-ϒ)(Ω)/r Where C and C0 are the vacancy concentration gradient around the curved and flat surface.
- Neck growth due to surface diffusion, lattice diffusion, vapour transport, grain boundary transport from GB source, lattice diffusion from sources on GB, lattice diffusion from dislocation sources
3. Plastic flow mechanism
Bulk flow of material by movement of dislocations has been proposed as possible mechanism for densification during sintering. Importance was given to identify dislocation sources during the sintering process. Even if frank read sources are present in the neck region, the stress available for dislocation generation is very small, indicating that the generation of dislocation must come from free surfaces.
Only if the surface is very small of the order of 40 nm, the stress required for dislocation generation will be sufficient. But experimental results have shown the absence of applied stress and plastic flow is expected to occur during early stages of sintering. Plastic flow mechanism is predominant during hot pressing.
LIQUID PHASE SINTERING
Mechanism during liquid phase sintering
In the sintering of multi-component systems, the material transport mechanisms involve self diffusion and interdiffusion of components to one another through vacancy movement. Sintering of such systems may also involve liquid phase formation, if the powder aggregate consists of a low melting component whose melting point is below the sintering temperature
Liquid phase sintering: In this, the liquid phase formed during sintering aids in densification of the compacts. Liquid phase sintering employs a small amount of a second constituent having relatively low melting point.
This liquid phase helps to bind the solid particles together and also aids in densification of the compact. This process is widely used for ceramics – porcelain, refractories.
Three main considerations are necessary for this process to occurs, 1. presence of appreciable amount of liquid phase, 2. appreciable solubility of solid in liquid, 3. complete wetting of the solid by liquid.
Three main stages are observed in liquid phase sintering:
1. Initial particle rearrangement occurs once the liquid phase is formed. The solid particles flow under the influence of surface tension forces
2. Solution & reprecipitation process:
in this stage, smaller particles dissolve from areas where they are in contact. This causes the particle centers to come closer causing densification. The dissolved material is carried away from the contact area and reprecipitate on larger particles 3. solid state sintering
This form of liquid phase sintering has been used for W-Ni-Fe, W-Mo-Ni-Fe, W-Cu systems. The three stage densification is schematically shown in figure.
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