Powder Manufacturing Methods

There are four basic methods of producing powders for thermal spraying. These are "crushing", "agglomeration", "chemical" and "atomisation". Each method can be further sub-divided and powders can also be made by various combinations from each group. As a result there is a wide range of production routes available each producing powders with their own particular characteristics.

Crushing

Crushing, milling and grinding are generally used in the production of ceramic powders. However, it is also used to manufacture some metals and metal alloys.

The most common use is in the manufacture of alumina and zirconia powders. The feed oxide is melted using graphite electrodes which then is allowed to solidify into a large block. The block is then broken up and sized. This route is also know as the "Bake and Bash" method! The powders produced are very dense but they also have a course, irregular, and blocky morphology.

For metal alloys such as CuNiIn, rolling is sometimes used to produce a flake product This is then broken up to produce a powder.

Cryogenic milling is used to break down some metals which become brittle at low temperatures. When used in the manufacture of molybdenum powder this method is known as the Coldstream process. Here bulk material is cooled with liquid nitrogen and projected in two opposing streams. In the crossover point of these two streams the material is broken down and the process is continued until the desired size distribution is obtained.

Metal alloy powders can also be made by the hydride-dehydride method. In this process, metal alloys (usually in ingot form) are hydrided to produce brittle phases and the broken down in, for example, a ball mill. When the material is of a suitable size range the material is de-hydrided in a vacuum furnace to leave a metal or metal alloy powder. This method is used in the manufacture of titanium and titanium alloy powders. The powder is generally blocky in form and can contain high levels of hydrogen and oxygen.

Agglomeration

The most common method of agglomeration is where the constituents are physically mixed together with an organic binder. The solvent is then driven off and the resultant material sized. The binder should be burnt off during spraying. This process is used in the manufacture of NiAl, AlSi-polyester powders.

The use of spray drying has become another common method for the agglomeration of powders. Here, a slurry is formed with the constituents and this is then fed into a rotary spray head. Here, the slurry forms an atomised cloud which is solidified by an opposing warm air stream to produce a powder. This method is used for ceramics such as zirconia and cermets such as WC-cobalt. The powder is largely spherical but in the as spray dried state can be porous and friable. The material is often densified and stabilised by sintering and/or spray densification.

There are also methods of mechanical agglomeration (eg the Hosakawa method) where for example a hard constituent is mechanically driven into a softer matrix particle to form a composite powder. Indeed, simple ball grinding can be used to mechanically alloy two or more constituents together.

Although sintering can be used as part of the spray drying process it can also be used alone as a method to manufacture powders. The constituents are mixed together and heated to get some solid state diffusion going and then the resultant product is crushed. A number of repeated cycles can be used to promote further alloying in which case the powder is called a "reacted" powder.

Chemical

There are numerous chemical methods for producing powders. Generally, chemical methods result in very fine powder particle sizes. Such methods include Sol Gel, Chemical Precipitation, Reaction, CVD, Reduction (hydrogen in an autoclave to reduce metal salts to the metal), Decomposition (eg metal carbonyls) and Electrolytic.

In CVD heating in a hydrogen atmosphere enables the following reaction to take place to make TiC powder

TiCl4(g) + CH4(g) ---> TiC (s) + 4HCl (g))

Sol Gel was originally developed to make ceramic materials for the nuclear industry in order to avoid the dust generated in grinding and sieving processes. While generally fine (<20 mm) such powders have excellent flow characteristics.

Another interesting chemical method is INCO's unique carbonyl process which is used to produce clad powders such as nickel - graphite.

Atomisation

There are a number of atomisation techniques which all rely on the production of a molten pool as the source. Atomisation methods include Rotating Electrode, Vibrating Electrode (arc), Centrifugal (from a melt) and Rapid Solidification (eg aluminium ribbon). However, by far the most commonly used methods are either water or gas atomisation.

Here, molten metals are poured from a melt through an atomising gas or water nozzle to produce a finely atomised stream. This stream is then allowed to fall down a tower where it solidifies and is collected. In this process, the cheapest method is that of water atomisation whereas the most expensive is argon atomisation. The advantage of the latter process is the better control of oxygen levels in oxygen sensitive materials such as MCrAlY's.

Main Characteristics of Powders

Morphology

The morphology of powders can be critical in determining both flow characteristics as well as playing an important role in the heat and momentum transfer to the powder during spraying. Generally, the morphology of powders can be described as irregular, blocky or spherical. Irregular powders are characterised by the presence of a wide range of shapes from cubic like structures through to needles. Blocky powders tend to have shapes where the largest and smallest dimensions of the powder particles are quite close. Spherical powders are broadly spherical in shape.

The morphology to be expected from each of the manufacturing routes is as shown below.

Powder Size Range and Distribution

In general, powders are designated in terms of a simple size range. Historically these are often quoted in terms of the mesh size used in traditional sieving classification. The mesh number actually defines the number of wires used per inch in a "standard" sieve. The symbols + and - are also used to denote, respectively, powder which is either retained by or passes through a particular sieve. The following table lists the US and UK standard sieves with their micron equivalent.

US Standard Mesh British Standard Mesh Particle Size (Micron)

While sieves may still be used for classification of powders, measurement of size is frequently carried out by instruments using laser light scattering. These plot the size of the powder particles in terms of an "equivalent" diameter of a projected sphere. For spherical powder this is quite accurate but for irregular shapes (ie most material) the sizes given will always be larger than the "actual" sizes. The plot is usually drawn in terms of percentage finer than.

Since in most thermal spray processes spray parameters are optimised so that the majority of powder particles reach the appropriate state it is clear that any significant shift in size distribution will have important consequences. Finer than usual powders will become overheated and vaporise and oxidise while courser material will be insufficiently melted or accelerated. As a result, more and more end users are paying particular attention to the distribution of powder on a lot by lot basis.

Powder Flowability

The flowability of powders is very important in all thermal spray processes. Poor flow leads at the very least to inconsistencies in the powder feed rate and thus coating build-up rates. Generally, Hall Flow is used to characterise powders. The Hall flow is measured by timing the flow of a sample of powder through a standard orifice. Hall flow is affected by both powder morphology and size range. Generally the more irregular shaped and the finer the powder the worse is the flow.

Chemical composition

Obviously although the general chemical specification can be set, there can be wide variations in chemistry from lot to lot of powder. These variations can often be blended out by mixing different lots together. However, there can also be problems in that while the average chemistry is correct eg less than 0.05% Fe this impurity can be formed by clumps of iron particles present due to a mechanical treatment during sizing.

Sprayability

Regardless of how and what is specified in the resultant powder product, the sprayability of the powder is often the only sure way of ensuring that the product is consistent. There is strong evidence that for sensitive materials, the only way to ensure that the power is suitable is to carry out a spray trial prior to use. Here, coating characteristics are used to check the performance of each lot prior to release.

Author: Peter Chandler

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