[Sintering][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”

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”.

Introduction

In other words

The bonding of powders by solid-state diffusion, resulting in the absence of a separate bonding phase.

OR

Sintering is a method for making objects from powder, by heating the material (below its melting point) until its particles adhere to each other.

Introduction

The definition by Thummler from the point of view of physical chemistry is:

‘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.’

Introduction

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.

Introduction

Background

In metals as well as in ceramics, we are concerned with two types of structure, both of which have a profound effect on properties.

The first type of structure is at the atomic scale: the type of bonding and the crystal structure (for a crystalline material) or the amorphous structure (if it is glassy).

Background

The second type of structure is at larger scale: the microstructure, which refers to the nature, quantity, and distribution of the structural elements or phases in the metals/alloys or ceramics (e.g., crystals, glass and porosity)

1. Solid state sinteringOnly solid phases are present at the

sintering temperature.2. Liquid phase sintering

Small amounts of liquid phase are present during sintering.3. Reactive sintering

Particles react with each other to form new product phases.

Types of Sintering

These parameters can be divided into four broad categories:

1. Powder preparation:

-- Particle size

-- Shape

-- Size distribution

Important Parameters in Sintering

2. Distribution of:

-- Second phases

3. Powder Consolidation:

-- Green density

-- Pore size distribution

Important Parameters in Sintering

4. Firing/Sintering:

-- Heating rate

-- Temperature

-- Time

-- Applied pressure

-- Atmosphere

Important Parameters in Sintering

Some parameters, such as the sintering temperature, sintering time, applied pressure, average particle size and atmosphere can be controlled with sufficient accuracy.

Others, such as the powder characteristics and particle packing are more difficult to control but have a significant effect on sintering.

Important Parameters in Sintering

Atomic diffusion takes place and the welded areas formed during compaction grow until eventually they may be lost completely.

Recrystallisation and grain growth may follow, and the pores tend to become rounded and the total porosity, as a percentage of the whole volume tends to decrease.

What Happens During Sintering?

In the pressing operation the powder particles are brought together and deformed at the points of contact.

At elevated temperature - the sintering temperature - the atoms can move more easily and quickly migrate along the particle surfaces (the technical term is Diffusion).

What Happens During Sintering?

Metals consist of crystallites

At the sintering temperature new crystallites form at the points of contact so that the original inter-particle boundaries disappear, or become recognizable merely as grain boundaries (This process is called Recrystallisation).

What Happens During Sintering?

The total internal surface area of the pressed body is reduced by sintering.

Neck-like junctions are formed between adjacent particles as can be seen on the adjoining scanning electron micrograph.

What Happens During Sintering?

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:

The curvature of the particle surfacesAn externally applied pressureA chemical reaction

Driving Force for Sintering

Schematically it can be shown as

Driving Force for Sintering

SINTERING

A MODEL SKETCH

Three stages are distinguished in sintering

First Stage

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.

Stages of Sintering

INITIAL STAGE OF BONDING

Second Stage

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.

Third Stage

Grain growth takes place, the pores break up and form closed spherical bubbles.

Stages of Sintering

The three stages in the dry sintering can be shown as

Stages of Sintering

Six distinct mechanisms can contribute to the sintering of a consolidated mass of crystalline particles:

1. Surface diffusion2. Lattice diffusion from the surface3. Vapor transport4. Grain boundary diffusion5. Lattice diffusion from the grain boundary6. Plastic flow

Mechanisms of Sintering

Mechanisms of Sintering

During liquid phase sintering a liquid phase coexists with a particulate solid at the sintering temperature

The wetting liquid provides a capillary force that pulls the solid particles together and induces particle re-arrangement.

Liquid Phase Sintering

The second phase chosen has lower melting temperature than the main constituent.

The sintering temperature is set just above the melting point of the added phase so that during sintering it forms a liquid phase that wets the solid particles.

Liquid Phase Sintering

The pores in the compact are largely surrounded by the liquid phase and the driving force for sintering is liquid surface energy.

With high liquid fractions, full density can be achieved almost entirely by rearrangement.

Liquid 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.

Liquid Phase Sintering

Liquid

Polycrystalline particle

Liquid Phase Sintering

Liquid

Polycrystalline particle

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.

Liquid Phase Sintering

γlv

γsl

γsv θ

The surface energies for different interfaces is given by

γsl = solid/liquid

γsv = solid/vapor

γlv = liquid/vaporThe vectors representing these surface energies must balance at the three phase triple junction. The equation representing this balance is known as “Youngs’ equation”;

Liquid Phase Sintering

lvslsv )cos(

Liquid Phase Sintering

γsl

γsv

γlv

θ

γsl

γsv

γlvθ

Large γsl, non-wetting θ Large

Large γsv, wetting θ small

Grain boundary wetting during LPS occurs when θA and θB approach zero

Liquid Phase Sintering

Grain A

Grain B

LiquidγBL

γALγAB

θB

θA

)cos()cos( AB ALBLAB

Powder Metallurgy parts-- Copper/Tin alloys -- Iron/Copper structural parts--Tungsten Carbide/Cobalt cemented carbides

Ceramics-- Silicon Nitride with a glassy liquid phase

(2wt% alumina + 6wt% yttria)-- SiC with Silicon liquid phase

Examples of LPS

Compact slumping (shape distortion) which occurs when too much liquid is formed during sintering.

The same parameters which control the sintered microstructure often control the final properties.

Useful application temperature of the material is sometimes limited by the presence of too much low melting point material.

Disadvantages of LPS

Two or more constituents in a compact react during sintering to form a new phase or phases.

The reaction is normally exothermic and can contribute to an enhancement of sintering.

In some cases the reaction is so exothermic that it can generate sufficient heat to cause self-sintering without external heating except that required for initiating the reaction.

Reactive Sintering

This is the basis of combustion synthesis which if properly controlled can produce a relatively dense compact of the synthesized reaction product.

Example of reaction sintering is

3TiO2 + 4AlN → 2Al2O3 + 2TiN + N2

Reactive Sintering

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.

Sintering Procedure

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).

Sintering Procedure

-- 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.

Sintering Procedure

-- 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.

Sintering Procedure

Sintering Procedure

Schematically

As the parts travel through the furnace, the temperature cycle results in changes in composition and microstructure.

In the hot zone metallurgical bonds develop between particles and solid state alloying takes place.

The microstructure developed during sintering determines the properties of the part.

Sintering Procedure

The operation is almost invariably carried out under a protective atmosphere, because of the large surface areas involved, and at temperatures between 60 and 90% of the melting-point.

These are essential for almost all sintering processes, to prevent oxidation and to promote the reduction of surface oxides.

Sintering Atmospheres

In practice dry hydrogen, cracked ammonia, and partially combusted hydrocarbons are mainly used.

Although the first named is often precluded because of cost. It is however, used for sintering carbides and magnetic materials of the Alnico type.

Sintering Atmospheres

It can replace pure hydrogen for many applications at approximately one-third the cost, with the obvious exceptions where reaction with nitrogen cannot be tolerated.

It is particularly useful for sintering iron, steel, stainless steel, and copper-base components.

Sintering Atmospheres

The most widely used atmospheres primarily because of their lower cost, are produced by partial combustion of hydro-carbons.

Sintering Atmospheres

1)Re-Pressing

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

Post Sintering Operations

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

Post Sintering Operations

Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called “sizing”

Post Sintering Operations

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

Post Sintering Operations

3) Hot Isostatic Pressing

HIP is used as a post-sintering operation to eliminate flaws and micro-porosity in cemented carbides

Post Sintering Operations

4) Forging

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

Post Sintering Operations

5) Infiltration

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

Post Sintering Operations

6) Impregnation

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

Post Sintering Operations

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

Post Sintering Operations

8) Surface-Hardening

Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated

Post Sintering Operations

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.

Post Sintering Operations

10) Blueing

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

Post Sintering Operations

11) Plating

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.

Post Sintering Operations

12) Coatings

A large percentage of hard metal cutting tool inserts are now coated using chemical vapor deposition (CVD) or physical vapor deposition (PVD)

Post Sintering Operations

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

Post Sintering Operations

14) De-burring

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

Post Sintering Operations

Particular advantages of this powder technology include:

1. the possibility of very high purity for the starting materials and their great uniformity

2. preservation of purity due to the restricted nature of subsequent fabrication steps

Advantages of Sintering

3. stabilization of the details of repetitive operations by control of grain size in the input stages

4. absence of stringering of segregated particles and inclusions (as often occurs in melt processes)

5. no requirement for deformation to produce directional elongation of grains

Advantages of Sintering

Ceramic Processing and Sintering by M.N. Rahaman

Ceramic Matrix Composites by Richard Warren

Ceramic Fabrication Technology by Roy. W. Rice

The Inorganic Chemistry of Materials- How to Make Things Out of Elements by Paul J. Van Der Put

References

Liquid Phase Sintering by Randell M. German-Technology

Blake Concise Dictionary Wikepedia-the Free Encyclopedia www.azom.com www.mse.mtu.edu www.epma.com

References