Thin Precision Coating of Fine Powders

Abstract

Typical P/M Parts

P/M processes require fine powders to be coated with a lubricant to allow good flow and green strength.

In applying such coatings it is essential to minimize the amount of lubricants in order to minimize part shrinkage and porosity. Due to the large density difference between most metal powders and most lubricants, a coating that adds as little as 1 % weight to the powder can have major implications for shrinkage and porosity in the final part.

Niro has developed a process which allows very thin coatings (as little as a few hundred atoms thick) to be applied to fine powders. We have coated powders with a mean diameter of 15 microns up to 1000 microns (1 mm) or more. We have also coated very irregularly shaped particles.

The process works by suspending and tumbling the particles in a high velocity gas flow and applying the coating as a fine mist. Both liquid and solid powder coatings have been applied with the process.

Background

Niro HQ in Copenhagen

Niro is a leading provider of equipment for powder production and processing. The company was founded in the 1930s when a Danish Engineer Jan Nyrop developed a process for producing milk powder by spraying finely atomized milk into a heated air stream. Since that time Niro has applied the "spray drying" process to products ranging from cemented tungsten carbides and industrial ceramics to fertilizers, fine chemicals, foods and pharmaceuticals.

Niro's work in the pharmaceutical industry led to the development of the Niro Precision Coater. Pharmaceutical customers needed an economical process to apply coatings to powdered pharmaceutical preparations. In some cases the coating is the active drug which is applied to a substrate of starch or sugar. In other cases the coatings are applied to mask flavors or to control the release rate of the drug. The application of such coatings calls for very accurate control of coating quantity, thickness and uniformity.

Niro engineers undertook a detailed study of the gas, solid, and liquid flow patterns in traditional suspended gas coaters and developed a new process that we call Precision Coating. Patent applications for the process have been filed in the US and Europe.

How the Process works

A high velocity jet (30 to 60 ft/sec) is established by accelerating a stream of air or inert gas with our proprietary Swirl Accelerator. By careful control of the accelerator geometry we are able to establish a laminar flow pattern at Reynolds numbers where turbulent flow would normally occur. The gas is directed to a "coating tube".

A reservoir of the powder to be coated surrounds the coating tube and is kept lightly aerated by a low velocity gas stream that enters the powder bed from the bottom. A gap between the inlet fluidizing plate and the bottom of the coating tube allows powder to be exposed to the high velocity gas stream. Particles of powder are picked up at this interface and accelerated by the gas stream.

A fine spray (of the coating) is introduced into the bottom of the high velocity gas stream. The coating spray is moving faster than the solid particles so contact occurs and a coating is deposited.

Schematic of Precison Coater

The boundary layer effect causes a velocity gradient from high gas velocity at the center of the tube to zero at the wall. This gradient causes the powder to be tumbled by the gas stream so that all particle surfaces are exposed to the coating spray.

Once the coating is applied the coated particle travels on up the coating tube. The particle velocity is always lower than the gas velocity so there is always a movement of gas across the particle surface. This gas movement evaporates the solvent and dries the coating. The particle is substantially dry by the time it reaches the end of the coating tube.

At the end of the tube, the particles disengage from the high velocity stream and fall back to the holding area. In a typical process all particles make a number of trips through the coating tube and the coating is applied in layers.

The coating can be virtually any liquid material that will pass through the atomizing nozzle. Solutions of polymers have been sprayed, as have suspensions of carbon, metals and other substances. The "solvent" or carrier can be water, alcohol, chlorinated solvents, or almost any other solvent used industrially.

Rapid Coating and drying

As shown in the following figure, the actual coating is extremely short, on the order of .01 seconds. This means only a thin layer of coating is applied during each cycle. This thin layering allows extremely close control of the coating thickness and allows very thin coating layers to be applied. In one application we applied a coating on the order of 300 to 400 ATOMS thick. (Or perhaps we should say 300 to 400 atoms thin).

Typical conveying velocity=20 to 40 meter/sec (Equals 20,000 to 40,000 millimeters/sec)
If tube height is 200 millimeters (about 8 inches)
Then gas transit time is 200mm/20,000mm/sec=.01 seconds!

Coating Time

The drying phase is similarly very rapid. This limits agglomeration (wet particles sticking together to form large agglomerates). It is important to state though that agglomeration is a function of the operating conditions in the coating tube. We have in fact successfully applied this technology to other applications where agglomeration is the goal.

Although each coating layer is thin, the process has been successfully used to apply thick coatings- (up to several hundred microns).

Stability of the Process

Because the actual wetting and drying process is very short, at any given time most of the powder in the system is dry. This makes the process relatively easy to control. We can stop the process without disrupting it. We can for instance easily stop the process to examine the coated powder. Plugged nozzles can be replaced while the coating process is operating. Power interruptions or equipment shutdowns do not affect the coating process.

Uncoated Zinc Powder

Coated Zinc Powder

The rapid coating and drying cycle also means that the powder is only wetted by the solvent for a very short time. The solvent has little opportunity to penetrate the interior of the particles. This means that solvent/powder reactions are generally not a problem, and it is often possible to use solvents that would normally be considered incompatible with the powder.

We have also found that the process is relatively gentle to the powder and coating. Although it may not be evident from the process description, we have found that even relatively fragile coatings can be applied without unacceptable coating attrition.

What are some applications of this technology to the PM/MIM industries?

• Coating powders with a lubricant/binder for PIM applications
• Coating particles with a barrier film to prevent oxidation or reaction
• Coating particles with a wetting agent to enhance mixing
• Producing hybrid particles consisting of more than one material

How thin can we go? We have produced coatings a few hundred atoms thick.

How thick can we go? We have done several hundred microns but the process took several hours.

How fine can the particles be? We have processed particles with a D 50 of 15 microns.