Activated Sintering

Activated sintering refers to any of several techniques that lower the activation energy for sintering. Several techniques have been developed to achieve this goal, including chemical additions to the powder and the use of special atmospheres. In this respect, the sintering treatments described previously in this article that achieve stabilization of the body-centered cubic crystal structure in iron are considered a form of activated sintering.

Many of the detailed investigations of activated sintering have been conducted on tungsten. Tungsten without special treatment exhibits a high level of porosity and is quite weak. Treatment with 0.4 wt% Pd promotes activated sintering. Diminished porosity produced with enhanced sintering increases strength significantly.

Fine tungsten powder coated with a uniform layer of certain transition metals undergoes rapid densification at unusually low temperatures. The amount of additive required to promote low-temperature sintering is equivalent to one atomic layer on the powder surface. Additive contents above this amount produce minimal further enhancement in sintering and tend to lessen the degree of activation. Densification occurs in two stages. The second, slower stage begins with the onset of grain growth. Thus, the importance of grain boundaries is well demonstrated in activated sintering studies.

The effect of palladium as an activator in the sintering of tungsten on the compressive strength of tungsten compacts is shown in Fig. 15. The strength of the compacts is plotted as a function of palladium content and sintering temperature. In addition to the amount of activator and sintering temperatures, particle size of the powder is an important parameter. In Fig. 16, the density of sintered compacts of two types of tungsten powder with 0.5 and 5 t-' m particle size doped with nickel are plotted as a function of nickel content.

Fig. 15 Compressive strength of compacts from 0.8 tungsten powder. Treated with varying amounts of palladium as a function of sintering temperature

Fig. 16 Density of compacts of two tungsten powders treated with nickel with 0.5- and 5-/^m particle size. Sintered 1 h at 1400 °C (2550 °F) as a function of the amount of nickel

The type of additive that proves successful as an activator must meet several criteria. First, it must form a phase that has a lower melting temperature than the base metal being sintered. Second, the activator must have a high solubility for the base metal, while the base metal should have a low solubility for the activator. The function of the activator is to remain segregated to the interparticle interfaces during sintering. Such a segregated layer provides a high diffusivity path for rapid sintering. A lower melting temperature ensures a lower activation energy for diffusion, while the solubility ensures that the activator is not dissolved into the base metal during sintering. Typically, an activator that decreases the liquidus and solidus of the base metal remains segregated to the interface between particles. Figure 17 shows an ideal phase diagram for activated as well as liquid-phase sintering systems. Note that at temperatures slightly above the activated sintering range, a liquid forms. The formation of a liquid phase is another means of enhancing sintering.

Fig. 17 Idealized phase diagram showing favorable conditions for enhanced sintering by activated and liquidphase sintering

The kinetics of activated sintering are dependent on the rate of diffusion of the base metal through the thin activator layer. It is necessary to form sufficiently thick layers to provide significant diffusion fluxes by sintering activation. Concentrations above this level frequently do not prove beneficial. The measured activation energies for shrinkage in activated systems closely approximate those for self-diffusion in the activator. Because the process has a low activation energy for diffusion, temperature is the most sensitive process control. The mechanism resembles grain-boundary diffusion-controlled sintering, thus shrinkage initially depends on the cube root of time. During the later stages of sintering, the rate of grain growth appears accelerated because of the low porosity and the high grain-boundary motion. Consequently, rapid grain growth during long-term sintering actually degrades the sintering rate and sintered properties.

Activated sintering also refers to sintering processes in which activation is produced through control of the sintering atmosphere. For example, the addition of a halide to the sintering atmosphere aids transport during sintering by the formation of high-vapor-pressure molecules. Consequently, major changes in pore shape are possible. In such cases, the sintered product has greater strength and significantly higher ductility.

Table 2 compares the strengths and the ductilities for iron compacts sintered in hydrogen. The addition of 1% hydrogen chloride to a hydrogen atmosphere results in improved properties. Metallographic examination indicates that the main effect of the hydrogen chloride is in promoting more rapid vapor phase transport by iron chloride molecules. Improvement in mechanical properties has been demonstrated in other systems. It is thus possible to achieve unique benefits from the sintering atmosphere in addition to control of oxide or carbon content.

Table 2 Effect of hydrogen chloride on iron sintered in hydrogen

Temperature

Time,

Atmosphere,

Density,

Strength

Elongation,

°C

°F

min

% hydrogen chloride

g/cm3

MPa

ksi

%

950

1740

30

0

6.20

131

19

6

30

1

6.30

159

23

10

120

0

6.30

138

20

6

120

1

6.30

159

23

10

1375

2505

30

0

7.00

193

28

11

30

1

7.20

234

34

20

120

0

7.50

234

34

17

Chemical additions are the most successful means of effecting activated sintering. Other processes, such as radiation treatments, have been successful in promoting rapid sintering. However, there has been little interest in this process. Sintering activation treatments can be classified as means of altering either the kinetics or the driving force of sintering. In radiation bombardment, sintering kinetics are altered by the creation of a vacancy excess. This eliminates the vacancy formation energy from the activation energy for diffusion. Alternatively, treatments such as cyclic heating of a material such as iron through the polymorphic phase transformation represent a change in the driving force. In the latter case, cyclic heating generates an internal stress, which effectively raises the driving force. Such a treatment is analogous in some respects to external stresses with hot pressing.

0 0

Post a comment