[Sintering]

1. [Sintering]

2. 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”

3. Introduction 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”.

4. 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.

5. 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.’

6. 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.

7. 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).

8. 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)

9. Types of Sintering • Solid state sintering Only solid phases are present at the sintering temperature. • Liquid phase sintering Small amounts of liquid phase are present during sintering. • Reactive sintering Particles react with each other to form new product phases.

10. Important Parameters in Sintering These parameters can be divided into four broad categories: 1. Powder preparation: -- Particle size -- Shape -- Size distribution

11. Important Parameters in Sintering 2. Distribution of: -- Second phases 3. Powder Consolidation: -- Green density -- Pore size distribution

12. Important Parameters in Sintering 4. Firing/Sintering: -- Heating rate -- Temperature -- Time -- Applied pressure -- Atmosphere

13. 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.

14. What Happens During 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.

15. 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).

16. 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).

17. 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.

18. Driving Force for 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 surfaces • An externally applied pressure • A chemical reaction

19. Driving Force for Sintering Schematically it can be shown as

20. SINTERING A MODEL SKETCH

21. Stages of Sintering 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.

22. INITIAL STAGE OF BONDING

23. Stages of Sintering • 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.

24. Stages of Sintering The three stages in the dry sintering can be shown as

26. Mechanisms of Sintering Six distinct mechanisms can contribute to the sintering of a consolidated mass of crystalline particles: • Surface diffusion • Lattice diffusion from the surface • Vapor transport • Grain boundary diffusion • Lattice diffusion from the grain boundary • Plastic flow

27. Mechanisms of Sintering

28. Liquid Phase 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.

29. 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.

30. 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.

31. Polycrystalline particle Liquid 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.

32. Polycrystalline particle Liquid Liquid Phase Sintering

33. γsv θ γsl γlv Liquid 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.

34. Liquid Phase Sintering 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”;

35. γlv θ γsv γsl γlv γsv θ γsl Liquid Phase Sintering Large γsv, wetting θ small Large γsl, non-wetting θ Large

36. Grain B γBL θB Liquid γAB θA γAL Grain A Liquid Phase Sintering Grain boundary wetting during LPS occurs when θA and θB approach zero

37. Examples of LPS • 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

38. Disadvantages 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.

39. Reactive Sintering • 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.

40. 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

41. Sintering Procedure 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.

42. 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).

43. 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.

44. 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.

45. Sintering Procedure Schematically

46. Sintering Procedure • 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.

47. Sintering Atmospheres • 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.

48. 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.

49. 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.

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