B

Fig. 16 Freezing time and relative density of powders as a function of melting point, (a) Freezing time of 100 tl m particles versus melting point. (b) Relative apparent density of water-atomized powders versus melt points. Source: Ref 10

Oxidation. During water atomization, most metals react with water or steam to form oxides, following a chemical reaction of the form: xMe + yH2O = MexOy + yH2. Oxidation generally increases with increasing superheat of the liquid metal and depends on particle size. Some materials exhibit a minimum if oxygen content is plotted versus particle size. Such a minimum may result from two opposing factors. With increasing particle size, cooling time, and exposure to oxidation increase, oxide layers become thicker. Conversely, with decreasing particle size, the specific surface area of a powder increases rapidly.

In addition to the amount of oxidation, the nature and composition of oxides and their spatial distribution within a particle can have a great effect on the usefulness and subsequent processing requirements of a powder. Surface analytical techniques have shown that the surfaces of water atomized alloys are typically highly enriched in the oxides of elements that have high negative free energies of oxide formation and high diffusivities at elevated temperature. Oxide films have thicknesses ranging from tens to thousands of nanometers.

This phenomenon is also true for gas atomized powders in which some oxidation has occurred during atomization. Typical oxygen contents of water atomized metal powder are:

Metal

Oxygen

content, ppm

Silver

100

Gold-nickel

<100

Copper

<800

Copper-tin

<400

Nickel

<500

Iron

1,000-3,000

Cobalt

2,000-3,000

Iron-carbon

500

Aluminum

2,000

Fe-13Cr

1,000-3,000

Fe-45Si

2,000

Ni-Cr-B-Si

250-1,000

Fe-80Mn

Within the group of alloys that contain elements with intermediate oxygen affinity (chromium, manganese, and silicon), silicon-containing containing alloys generally produce low-oxygen-content powders, while manganese-containing alloys become heavily oxidized during water atomization. This tendency is attributed to the more protective nature of silicon oxide films compared to manganese oxide films. Figure 17 shows this effect for 304L stainless steel.

8000

7000

E 0000 a

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