Sintering in Global Material Perspective

In honor of 50th anniversary of publication of Science of Sintering, we bring you a special review article on the development and current state of the sintering theory. Development of sintering theory in the 20th century is outlined and placed into context of modern materials science and recent technological developments. The author looks to converge different approaches to sintering stemming from different fields of materials science and show the potential and the necessity for a single theoretical approach to sintering.

shape and size of the particle do get affected, which leaves its signature in the final sintered part.In such a scenario, even the subtle response of cold sintering is to be included.
In bringing a multi-component powder system, the situation is more challenging.The technical aspect of blending etc.Is not to be ignored and the tribological aspect between dissimilar powder surfaces is to be tackled.Here comes the role of a small quantity of lubricant, which after its job done needs to be removed by heating (delubrication) the green part [4].Such addition permits higher compaction speed while maintaining the excellent surface finish of the green part.

Sintering Processes
Solid state sintering has stood the hallmark historically.Later with establishing the binary and ternary phase diagrams, the significance of the presence of melt in enhancing sintering kinetics was realized.The best examples were sintered bronzes and Al-alloys.This was the beginning of the role of physical metallurgy in elucidating sintering science.The solid-state sintering and the reactions in the system govern the initial state of liquid phase sintering in real systems.In solid-state sintering of complex systems, the role of diffusion in its various forms was postulated.During hot pressing, the science of creep in materials came handy.The concept of transient liquid phase sintering also was exploited to our advantage.
The selection of sintering temperature is modified, if alloying additions can be selected on the basis of their ability to modify diffusion rates, interfacial energies or the stability of surface layers.These additions may be introduced as sintering aids, in which there is a limited influence in final properties other than density, or as bonafide alloying elements where improvements in the mechanical properties are achieved.

Sintering of Premixed Powders:
In case of powder metallurgy alloy developments, apart from using prealloyed powders, a much handy and economic way is to select premix elemental powders.Such premixes may encounter the following situations [5]: • A mixture of a powder of the same composition but differing sizes.It is recognized that the appropriate mixture of coarse and fine particles can enhance densification.• A mixture of powders of differing compositions, which are prone to alloying, for example, Fe-C, Cu-Sn, Al-Cu, W-Cu, Wc-Co etc. • Mixtures of powders, where one component is inert and does not sinter, eg.
Oxide dispersion strengthened materials.
In category II above, the formation of solid solution can lead to densification during sintering by: • Enhancing diffusion coefficient for the controlling species in the lattice, or parallel to the grain boundaries, by affecting the point defect concentration to boundary or lattice

Role of Mater Alloys in Low Alloy
Sintered Steels: Since Cr and Mn are cheap alloying elements in sintered steels, their introduction as master alloys in form of complex carbides has been studied in great details [6,7].Such master alloys fulfilled the following conditions: • They contain carbon in the combined form.
• They contain at least two of the elements Mn, Cr and or V.
• They are stable during the heating up time in sintering temperature.
• They dissociate under sintering conditions.
• They minimize production cost.
After a lull of a few decades, a recent impetus in developing new types of master alloys is underway.Boron is a valuable alloying addition to facilitating enhanced liquid phase sintering.A detailed description is given in the treatise by Upadhyaya [5].Of late, liquid phase sintering of PM steels by adding gas atomised Ni (46 %)-Mn (40 %) -B (8 %) master alloy has been reported for enhancing the density levels of Fe-and Mo-prealloyed steel powder compacts [8].When boron was introduced in the master alloy, it provides the flexibility of generating a liquid phase from the melting of master alloy and the eutectic reaction between iron and boron.Liquid phase formation occurred in two stages around 1000 o C, first from the matrix alloy melting and second from the eutectic formation 1240 o C.

Sintering of Some Real Systems
Tungsten and its alloys: Sintering of tungsten powder dates back to the beginning of the 20 th Century when Dr. Wiliam D. Coolidge of General Electric, USA developed the process for producing ductile tungsten incandescent lamp filament to replace Edison's carbon filaments, which were brittle and short-lived.Sintering of tungsten powder mass can be achieved by two routes: direct sintering and indirect sintering.
The significant reduction in sintering temperature for reaching densification by the minor additions of transition metals like palladium or nickel in tungsten has been reported since 1946 when Kurtz [9] first reported and was followed by Agte [10] in 1953.A few tenths of nickel lowered the sintering temperature of tungsten from 2000 o C to ~ 1300 o C [11].Other metals such as iron and cobalt have a similar but smaller influence.Nickel diffuses on to W-particles and enters the grain boundaries formed during sintering resulting in an increase in grain boundary self-diffusion of tungsten by a factor of up to 5000 at 1300 o C. It is important that the activating element remains concentrated at the grain boundaries during sintering.German and Munir [12] extensively reviewed the activated sintering of refractory metals by transition metal additions.Samsonov and Yakovlev [13] schematically showed the relative degree of activation during sintering of tungsten with respect to the position of addition element in the Periodic Table .Such an approach was the first attempt to unravel activated sintering from the view point of the electronic structure of additive element [14].A cursory look at bringing phase diagrams of W-Fe/Co/Ni already indicates the substantial positive role of nickel, where the solubility of tungsten in the binder (ie nickel) is maximum.The mechanical properties of activated sintered tungsten are a function of density, grain size, grain shape and grain boundary cohesion [4].The cooling rate also influences the segregation level at the tungsten grain boundary.
It is reported [15] that while sintering of W-Ni alloy at 1000 o C in association with copper, nickel imparts solid solution sintering prior to liquid phase formation.However, once copper melts, nickel gets dissolved into the melt and no longer remains segregated at the W-W interparticle interfaces.Such a deliberate attempt to combine two mechanisms is not as per expectation.During the liquid phase sintering stage, the wettability aspect of the intermetallic layer found on the tungsten particle surface is also to be taken into consideration.In the case of W-25 Cu system without any activator, the wetting angle was noticed to be 22 degrees, but nickel addition reduces this value [16].In case of W-Co-Cu system, the improvement in the wettability with cobalt was caused not by the modification of the tungsten particle surface, but by the diffusion of cobalt in the copper melt to form intermetallic compounds.It is envisaged that iron also has similar densification mechanism for W-Cu system.
Cheynet [17] found that Al and Ti additions in W-Ni-Fe alloys lead to in situ precipitation of coherent intermetallic phases Ni 3 Al and Ti 3 Al in Ni-Fe matrix, thereby enhancing the binder phase strength and consequently the mechanical properties of W-Ni-Fe alloys.However, it was difficult to control the size distribution and homogenization of the intermetallics formed in situ through precipitation.Griffo et al. [18] investigated the processing of a range of intermetallic bonded tungsten composites consisting of 93 % tungsten, therein the Ni-Fe matrix was partially substituted by various fractions of Ni 3 Al or Fe 3 Al.It was noticed that aluminide addition reduced the coarsening of tungsten grains, which was attributed to the reduced solubility of tungsten in aluminides as compared to either Ni-or Fe-matrices.Debata [15] noticed that the densification correlation with respect to the melting point is not simple, since it did not consider the possibility of the mutual phase interactions on the matrix liquidus temperature.The distortion in sintered alloys was noticed in case (a) the density difference between the tungsten and binder phase was large, and (b) in cases where the volume fraction of liquid phases were high [19].
Sintered Metal-Ceramic Composites: Metal-ceramic composites are gaining significances as high-performance materials.Particle-reinforced and fiber-reinforced sintered composites are very common.The first International Conference on Sintered Metal-Ceramic Composites was held in 1983 [20].The conventional pressing and sintering route for metal-ceramic composites have limitations in the availability of property level.To obtain full density the simultaneous application of stress and temperature is required so as to close the pores.Various densification methods are used such that the rate of densification is aided by stress, high diffusivities, smaller grain size and larger processing periods.Thus changes which can simultaneously alter one or more of these properties will aid the attainment of full density.However, it must not be forgotten that the success of full density processing is not the attained full density, but rather in the attained properties [21].Aluminum-Ceramic composites, historically, have major commercial applications in the automotive sector.Other applications are for electronic /thermal management including contact materials [22].
Sintered Functionally Gradient Materials: Functionally graded materials (FGM) have deliberately or naturally created gradients in their composition/structure which result in properties superior to those of homogeneous or multilayered materials.Sintering is the most suited route, particularly in metal-ceramic gradient materials.Watanabe [23] has done extensive research in this area.FGMs have developed into a new field of artificial structures that utilize non-conventional properties resulting from the synergistic integration of different materials in one component.FGMs are essentially 'new structures' of 'old' materials and not themselves 'new materials'.FGM cemented carbides, where the bulk is tough and the surface hard is a good example of cutting picks used in the mining industry [24].

Sintered nanocrystalline Materials:
The main aim of nanopowder consolidation is the retention of the initial nanocrystalline structure while achieving full density.The fine grain size may induce effects like alternate structure, extended solubilities or changes in physical properties.The conventional consolidation process, usually performed at high temperatures, will tend to destroy the initial metastable condition of the nanopowders, Thus during sintering, it is important to define the conditions under which metastability is lost.There are a number of advantages associated with lower sintering temperatures: faster densification, a small grain size, the avoidance of undesirable phase transformations or interfacial reactions, elimination of sintering aids, and less expensive sintering equipment [25].

Sintering Science and Technology: A conjugate relation
From all the discourse, it is evident how sintering, which historically began as technology, flourished taking the subsequent help of science.Sintering scientists were not shy in welcoming new technologies into their fold, for example, injection molding and additive manufacturing.In either case, the origin was the type and nature of starting powder and how to manipulate the processing sequence accordingly.It is no wonder that no academic institute anywhere in the field of materials science exists without having a specialist dealing with powders and their efficient tailoring for subsequent consolidation.
Slurries, polymeric binders or higher compaction pressures provide possible means to overcome the poor handling and packing characteristics of extremely fine powders.With finer powders, special care is needed during heating stages to remove contaminants such as binder components, oxygen, and metallic impurities since the pore closes off at rather lower temperatures.
Techno-economic considerations invariably play role in various processing permutations.As an example, in order to make AM (additive manufacturing) economical, cold gas dynamic spraying (CGDS) has been introduced, where powder characteristics have shown a similar effect as that of conventional pressing/sintering route.The use of Armstrog titanium powder (coral type morphology), with much more cost-effectiveness, applying a low temperature and a low pressure nitrogen as propellant gas [26] is an example.A post-deposition heat treatment (sintering) reduced porosity and internal stresses and improved mechanical properties, The basic core to fully understand sintering, in any system, is the synthesisstructure-properties-processing quadrangle.One need not be expert in all the above sectors, but the cooperative spirit among the specialists has contributed the maximum.Tikkanen [27] was vocal in highlighting the universal controversy between application and sintering theory.He wrote "On the one hand there are people to whom theory begins where their own knowledge ends, and who, consequently cannot understand or accept which they might give.On the other hand, there are many research workers who do not understand or want to understand the requirements of the practical applicability of the problem they are studying.As a result, their theories remain as plain theories, which cannot be applied because of their impracticability or because they are too difficult to understand." Sintering practice is in no way less significant as compared to theory.German [28] highlighted this is his latest book, where aspects such as dimensional control, composition control, atmosphere control, and defect avoidance are described.He rightly terms sintering as 'messenger' and not the 'cause' for the defects in sintered products.The recent impetus in sintering like field assisted sintering is related to the novel developments in powder materials at the nanoscale.Pressure-assisted sintering techniques such as HIP, SPS and of late microwave sintering may increase the size of the processing windows for producing extremely fine-grained materials.
In conclusion, it is worth quoting Jones [29], who wrote as back as in 1960: "The development of the theories of sintering is a fascinating story of a phenomenon which at first appeared mysterious, then temporarily simple and eventually unexpectedly complex".After a lapse of ten years, Samsonov tried to simplify the complexity as reported by Upadhyaya [30].He wrote "Today there are a few sintering theories, but the principles on which they are built up are not uniform.They give predominance to the diffusion process, surface tension etc. Evidently, the development of sintering in each of these and many other trends is useful and necessary.Soon the time will come when the need for unification of sintering principles will appear.The need is of dielectric nature.Connected with the transition of quantity into a new quality".However, one thing is evident.In each alloy system, whether metallic or non-metallic, the prediction of sinterability can be approximately found by knowing the basic parameters pertaining to that system under the selected sintering regime.