The physical and economic differences between the two types of powders result in clearly defined applications for each. Fine powders are used in both consolidated and unconsolidated forms. Markets include metal injection molding (MIM), electronic pastes and inks, thermal spray, additives, radar absorption coatings, and filtration.

Metal Injection Molding. The growth forecasts for MIM encouraged development of fine powder production processes. The first MIM parts were made from available powders generated by carbonyl (iron or nickel) processes. On a tonnage basis, carbonyl iron still accounts for a large percentage of the powders used by MIM. Nonetheless, both waterand gas-atomized powders are used extensively in MIM. Typically water-atomized powders are used to improve green strength and lower costs, while gas-atomized powders are preferred for moldability, lower oxide content, and tolerance control.

Electronics. Precious and nonprecious metals are used in the electronics industry for inks and pastes, conductive path, and capacitor terminations. Due to the extreme space limitations that these applications impose, finer distributions, (<10 m) are required (Ref 10). Because atomization yields for all but a few elements in this range are extremely low, precipitation is a more economical production method, especially for precious metal formulations. Because copper has a low viscosity and atomizes easily, it is an exception, and it is available as either a precipitated or atomized product in these size ranges. The powders are blended with a polymer and applied to a substrate via precise silk screening techniques. Continued miniaturization is driving these applications into the submicron power range.

Thermal Spray. Thermal spray equipment has evolved to produce extremely dense metallic coatings. Gas-atomized powders have always been preferred over water-atomized powders for both fine and coarser distributions. The spray process favors spherical, free-flowing particles over irregular water-atomized particles.

Two types of equipment have been designed that require fine powders. Plasma equipment requires powders in the 5 to 45 m range, and high velocity oxygen fuel (HVOF) equipment requires an even tighter range of 15 to 45 m, with controlled intermediate ranges. Although HVOF has been enthusiastically embraced for carbide coatings (carbides had the most room for density improvement offered by HVOF), metals are a significant part of the HVOF supply mix. The market for fine powder from dedicated production processes is primarily an economic market. Metal alloys have always been available from fractions of coarse as-atomized distributions. The opportunity for dedicated fine powder systems is to supply these distributions in higher yields and lower overall costs.

Additives. Many industrial products are formulations of many particulate constituents. The most cost effective production method for these powders is normally grinding, but grinding has drawbacks. Grinding can lead to explosions, can introduce contaminants such as dirt and oxygen, and can generate a wider than desired particle size distribution. For these reasons, fine powders produced by the methods discussed in this chapter have found a niche in specialized applications where purity, size control, and distribution consistency are necessary.

Metal filled polymers require a consistent size distribution and shape to achieve desired properties. Conductive polymers need high-conductivity powders with low or controlled oxygen levels. In some cases, applications of fine powders are required to ensure an even loading in the polymer structure.

Grinding wheel formulations use fine-atomized powders because they disperse more uniformly than coarse distributions. The lower oxygen levels of gas-atomized products allows sintering at lower temperatures.

Radar Absorption. Although most of this work remains classified, fine carbonyl and prealloyed powders have been used in radar absorption coatings for stealth fighter planes. Soft magnetic compositions are used in fine sizes, which offer the best combination of absorption and weight reduction. Below the Curie point, carbonyl iron is cost effective. Above the Curie point, prealloyed materials are necessary to maintain performance. Table 3 lists two grades of microwave absorbing carbonyl powders.

Filtration. Metal filter media is used in very demanding and corrosive applications. New applications require removal of finer and finer particles. Clean room applications are the most demanding, and this places tremendous demands on filter-reduced pore size and control. Smaller particles can create finer pore sizes. Typical compositions are 316L and 304 stainless and Hastelloys. The preferred morphology is an irregular shape that provides green strength during the fabrication process, which favors water-atomized particles. However, in extremely corrosive environments, the purity of gas-atomized particles makes them a preferred material.

Magnetic Power Cores. Fine carbonyl iron powders with suitable insulator coatings improve the performance of electrical power cores. It is possible to achieve a substantial reduction in eddy current losses compared with cores produced from coarser powder. This reduction in eddy current losses is achieved when the insulation breaks conduction between the individual, primary powder particles.

Figure 8 shows circuit quality with two types of carbonyl iron powder as a function of the frequency and the degree of insulation. The insulation was carried out with a 60% phosphoric acid. The powders, insulated and bound with 5% Epikote, were compressed with a compaction pressure of 0.6 GPa to rod cores having a diameter of 9.2 mm and a length of 20 mm.

Fig. 8 Circuit quality with different levels of insulation on the primary carbonyl iron powders. See Table 3 for powder designations. Courtesy BASF

With an increasing degree of insulation, the quality at first increases, and then reaches a saturation value typical for the particular powder. The thickness of the insulating layer depends on the proportion of phosphoric acid. This depresses the content of the pure iron, and so the permeability falls (Fig. 9). This means that the gain in quality is detrimental to the permeability.

Fig. 9 Effect of powder insulation on permeability. Courtesy BASF

It is therefore necessary to find a compromise between sufficient additions of an insulating material for high quality without excessive reduction of permeability.

The aim of the insulation is to cover the powder particles with a layer that is as thin as possible, but completely surrounds the powder particle. The layer should also be thick enough to prevent damage during compaction under high pressure. Finally, the binding agent content also has an effect on quality and permeability. A higher binding agent content improves quality within certain limits, but also reduces the permeability.

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