[Sintering]. Introduction. The word "sinter" comes from the German Sinter , a cognate of English “cinder”, which according to Concise Dictionary means, “the refuse of burned coals” In plain English “solid piece of matter remaining after having been subjected to combustion”. Introduction.
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The ISO definition of the term ‘sintering’ reads:-
“The thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles”.
In other words
The bonding of powders by solid-state diffusion, resulting in the absence of a separate bonding phase.
Sintering is a method for making objects from powder, by heating the material (below its melting point) until its particles adhere to each other.
‘Sintering is a thermally activated mass transport process which leads to strengthening of particle contacts and/or a change in porosity and pore geometry accompanied by a reduction of the free energy. A liquid phase can take part in the process.’
Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.
Almost all ceramic bodies and metal powder compacts must be sintered to produce a microstructure with the required properties. The widespread use of the sintering process has led to a variety of approaches to the subject.
In metals as well as in ceramics, we are concerned with two types of structure, both of which have a profound effect on properties.
Only solid phases are present at the sintering temperature.
Small amounts of liquid phase are present during sintering.
Particles react with each other to form new product phases.
These parameters can be divided into four broad categories:
1. Powder preparation:
-- Particle size
-- Size distribution
2. Distribution of:
-- Second phases
3. Powder Consolidation:
-- Green density
-- Pore size distribution
-- Heating rate
-- Applied pressure
Metals consist of crystallites
As with all processes, sintering is accompanied by an increase in the free energy of the system. The sources that give rise to the amount of free energy are commonly referred to as the driving forces for sintering. The main possible driving forces are:
Schematically it can be shown as
A MODEL SKETCH
Three stages are distinguished in sintering
After burn out of any organic additives, two things happen to the powder particles when the mobility of the surface atoms has become high enough; initially rough surface of the particles is smoothed and neck formation occurs.
Densification and pore shrinkage. If grain boundaries are formed after the first stage, these are new source of atoms for filling up the concave areas which diminishes the outer surface of the particle.
Grain growth takes place, the pores break up and form closed spherical bubbles.
The three stages in the dry sintering can be shown as
Six distinct mechanisms can contribute to the sintering of a consolidated mass of crystalline particles:
LiquidLiquid Phase Sintering
Grain boundary ‘wetting’ breaks the polycrystalline particle into single crystal particles in the initial stages of liquid phase sintering. These single crystal particles then spheroidize and coarsen.
LiquidLiquid Phase Sintering
γlvLiquid Phase Sintering
Wetting is a very important phenomena which is happening during LPS.
Figure represents the surface tensions of a multi-phase junction as vectors drawn parallel to the respective surfaces.
The surface energies for different interfaces is given by
γsl = solid/liquid
γsv = solid/vapor
γlv = liquid/vapor
The vectors representing these surface energies must balance at the three phase triple junction. The equation representing this balance is known as “Youngs’ equation”;
γslLiquid Phase Sintering
Large γsv, wetting θ small
Large γsl, non-wetting θ Large
Grain ALiquid Phase Sintering
Grain boundary wetting during LPS occurs when θA and θB approach zero
-- Copper/Tin alloys
-- Iron/Copper structural parts
--Tungsten Carbide/Cobalt cemented carbides
-- Silicon Nitride with a glassy liquid phase (2wt% alumina + 6wt% yttria)
-- SiC with Silicon liquid phase
Example of reaction sintering is
3TiO2 + 4AlN → 2Al2O3 + 2TiN + N2
Ancient sintering techniques for the making of pottery and ceramic art objects remain in wide use to this day but research has also led to more advanced techniques which work for a wider array of ceramics and metals.
In a typical sintering procedure
-- Most ceramic materials have a lower affinity for water and a lower plasticity index than clay, requiring organic additives in the stages before sintering.
-- A mixture of binder, water and ceramic powder is pressed into a mold to form a green body (un sintered item).
-- The green compact is placed on a mesh belt and moved slowly through the sintering furnace.
-- In the preheat zone, the lubricant volatilizes, leaves the part as a vapor, and is carried away by the dynamic atmosphere flow.
-- The temperature within the furnace rises slowly in the preheat zone until reaching the actual sintering temperature.
-- It remains essentially constant during the time at that temperature, and proceeds into the cooling zone where the drop in part temperature is controlled.
The most widely used atmospheres primarily because of their lower cost, are produced by partial combustion of hydro-carbons.
Even with the best control that is feasible in practice, there will inevitably be some variation in the dimensions of parts produced from a given material in a given die set
Typically, it is possible for parts 'as-sintered' to be accurate to a tolerance of -0.0508mm per mm, in the direction at right angles to the pressing -direction, and 0.1016mm per mm parallel to the pressing direction
Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called “sizing”
2) Hot Re-Press
Hot Repressing will give even greater densification, with consequent greater improvement in the mechanical properties, but less accurate control of the final dimensions is to be expected
3) Hot Isostatic Pressing
HIP is used as a post-sintering operation to eliminate flaws and micro-porosity in cemented carbides
Forging is a comparatively recent technique in which a blank is hot re-pressed in a closed die which significantly changes the shape of the part, and at the same time can give almost complete density and hence mechanical properties approaching or even surpassing those of traditional wrought parts
An alternative method of improving the strength of inherently porous sintered parts is to fill the surface connected pores with a liquid metal having a lower melting point. Pressure is not required, capillary action is sufficient
This term is used for a process analogous to infiltration except that the pores are filled with an organic as opposed to a metallic material
7) Heat Treatment
Although many, perhaps the bulk of sintered structural parts are used in the as-sintered or sintered and sized condition, large quantities of iron-based parts, are supplied in the hardened and tempered conditions. Heating should be in a gas atmosphere followed by oil-quenching
Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated
9) Steam Treatment
A process peculiar to PM parts is steam-treatment which involves exposing the part at a temperature around 500°C to high pressure steam. This leads to the formation of a layer of magnetite.
Heating in air at a lower temperature (200-250°C) can also be used to provide a thin magnetite layer that gives some increase in corrosion resistance, but it is much less effective than steam treatment
Sintered parts may be plated in much the same way as wrought or cast metals, and copper, nickel, cadmium, zinc, and chromium plating are all used.
A large percentage of hard metal cutting tool inserts are now coated using chemical vapor deposition (CVD) or physical vapor deposition (PVD)
13) Mechanical Treatments
Although a major attraction of PM parts is that they can be produced accurately to the required dimensions, there are limitations to the geometry that can be pressed in rigid dies, and subsequent machining, for example of transverse holes or re-entrants at an angle to the pressing direction is not uncommon
De-burring is done with sintered parts, and is used to remove any 'rag' on edges, resulting from the compacting operation or a machining step
Particular advantages of this powder technology include: