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Powder metallurgy

At IEK-1, a variety of highly innovative production processes are used in the area of powder metallurgy.

Applying ceramic manufacturing technologies to powder metallurgy

In the area of process engineering, there is often a strict separation between ceramic manufacturing technologies and powder metallurgy. At present, it is not well-known that many of the methods established in the ceramics industry can also be transferred to powder metallurgy with only minor modifications. Wet powder spraying, screen printing, roll coating, and tape casting in particular have a great potential for producing metallic components with functional properties. The institute has the know-how for using the above-mentioned methods for metallic powders. In addition, it offers the possibility to produce metallic-ceramic composite structures by appropriately combining the technologies. Examples of this include metal-supported fuel cells and metal-supported gas separation membranes.
The following methods can be transferred from ceramic manufacturing technologies to powder metallurgy:

Comparing ceramic and metallic powders with respect to processing aspects

The properties of the starting powders have a major impact on powder technology processing. Table 1 compares the major properties of ceramic and metallic powders with respect to processing-related aspects. The major differences between metallic powders and ceramic powders include the coarser size of the particles, ductile properties, in some cases, a different particle form and a higher affinity for contamination such as carbon, oxygen, nitrogens.

Coarser powder fractions with a particle size > 25 µm are disadvantageous for processing methods based on low-viscose suspensions or slips. As particle size increases, so does the tendency of the powder particles to cause sedimentation both in the storage vessel during powder delivery, as well as during the actual shaping process. Under unfavourable circumstances, during the coating process tiny cross-sections in the coating unit (e.g. in the spray gun) may become clogged. Furthermore, a change in particle size distribution in the suspension may occur during the coating process if the coarser particles settle with the duration of the coating. While stabilization of coarser particles in a suspension can in principle be achieved by significantly increasing viscosity, in most cases the coating process can no longer be implemented without problems. For this reason, one solution consists of distributing the powder particles in the suspension homogeneously by using a stirring device or a pump.

Table: Comparison of ceramic and metallic powders with respect to processing aspects
Ceramic powders Metallic powders
Particle sizeusually < 1 µm

usually in the range of 5 µm to 500 µm

Danger of fire for fine powders due to spontaneous oxidation in air

Particle shapeUsually fractured edgesDepending on manufacture, spherical or irregular
Powder delivery

Production of spherical

agglomerates by means of spray drying

Good flowability of spherical powders
 

Compressibility

Green densities 50 % to 60 % th.d.

Pressing pressures 40 MPa to 200 MPa

Risk of cracking in

compacted pellets due to elastic recovery

Tool wear due to abrasion

Green densities up to 80% th.d.

due to plastic deformation

of the powder particles

Pressing pressures up to 600 MPa

Tool wear due to cold welding

Green strengthUse of binders generally required

Angular particles: possibly do not use

binders if powders 

cling during shaping

Spherical particles generally require binders

DebinderingUsually in air, low susceptibility to contamination

Under inert gas

Susceptible to contamination such as

carbon, oxygen, nitrogen

Sintering

Usually in air

Sintering temperatures:

starting from 500 °C (nanopowder,

sol-gel)³ 1200 °C standard powder

Usually under inert gas or vacuum

Sintering temperatures:

Typical range 900 °C to 1300 °C

Refractory metals require higher temperatures


While the higher ductility of metallic powders offers advantages through compression, especially for shaping, it does not have a substantial influence on shaping through wet chemical methods. The possibility of using gas-atomized, spherical metallic powders is advantageous. They have very good flowability, thus facilitating powder delivery during the coating process. Furthermore, spherical powders allow the adjustment of high packing densities. This supports homogeneous compression of the components during the sintering process and is advantageous, especially when low residual porosity after sintering is desired.
An important difference between ceramic and metallic powders with respect to process engineering is the higher affinity of metallic powders to the uptake of carbon, oxygen, or nitrogen. This is demonstrated by the high risk of fire during handling, especially with fine powders. For ceramic powders, in most cases, debindering and sintering can be performed in air, which means that a complete burn-up of the organic material is unproblematic. As a result, the component properties are hardly influenced by residues of binders or solvents. In contrast, metallic powders require debindering and sintering under inert gas atmosphere. The reducing conditions hamper the binder burn-up and as a result, increase the risk of organic contamination in the debindered component. During the final sintering process, this can lead to embrittlement of the component caused by the formation of carbides, oxides, or nitrides.


Table: Comparison of wet chemical processing of ceramic and metallic powders with respect to feasible layer thicknesses and permissible particle sizes
Ceramic powders Metallic powders

Feasible
layer thicknesses

[µm]

Permissible particle size

[µm]

Feasible
layer thicknesses

[µm]

Permissible particle size

[µm]

Wet powder spraying*10 – 40Particle size generally smaller than 1 µm50 – 400< 25
Dip coating 5 – 40Particle size generally smaller than 1 µm100 – 300< 10
Screen printing*5 – 20Particle size generally smaller than 1 µm50 – 100< 50
Tape casting*100 – 500Particle size generally smaller than 1 µm200 – 800< 100
Roll coating 5 – 40Particle size generally smaller than 1 µm< 50< 10
* production of unsupported layers and components also possible



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