Frequently Asked Questions
HVOF spraying is used for the preparation of metallic coatings to provide corrosion protection and, in some applications, wear protection. Compared with many other conventional thermal spraying processes, metal coatings deposited with the HVOF process offer low levels of porosity, lower amounts of oxide and greater adhesion to the substrate. Types of metallic coatings deposited using the HVOF process include:
- Iron based alloys, including some stainless steels
- Nickel based alloys
- Cobalt based alloys
- MCrAlY alloys
- Self-fluxing alloys such as NiCrBSi & CoCrWBSi
- Aluminium bronze
The HVOF process requires finer sized powder particles, less than about 60µm in diameter, compared with metallic powders used with more conventional coating processes such as flame or plasma spraying or plasma transferred arc (PTA). The characteristics of the powder, resulting from its method of manufacture, will influence the deposited coating properties. Powder manufacture is an important consideration in the preparation of HVOF sprayed metallic coatings.
There are three main methods of producing metallic powders suitable for HVOF spraying:
- water atomisation
- gas atomisation
- agglomeration and sintering
Most metal powders are prepared using the atomisation processes. In this method, molten metal is injected through an atomising nozzle at the top of a chamber to form spherical particles. These particles solidify within the chamber and are collected at the bottom. Two types of atomisation are possible: water atomisation or gas atomisation.
In water atomisation, the molten metal droplets solidify on impact with a water tank inside the chamber. In gas atomisation, the particles solidify in an inert gas atmosphere while travelling towards the chamber wall. The gases used are usually nitrogen, argon and, occasionally, helium.
The powders produced by water and gas atomisation are quite different. In general, water atomisation gives particles with irregular shape which can lead to poor powder size classification and powder feed difficulties. These powders may also contain higher oxide levels and small pockets of water or gas. However, water atomised powders are considerably cheaper than gas atomised powders. The main advantage of gas atomised powders is their spherical shape and uniformity in size distribution, along with good chemical purity and homogeneity. Their uniformity and shape provide good flowability.
Agglomeration and sintering
Agglomeration and sintering is widely used in the preparation of carbide powders for HVOF spraying. It is less important for the production of metallic powders, but is used for preparing powders of refractory metals such as molybdenum and its alloys.
In this method, homogenised starting materials are dispersed in water along with a suitable polymer binder (PVA). The dispersion is atomised to a powder by a technique known as spray drying. In this technique, a nozzle or centrifugal atomizer is used to produce droplets of the dispersion inside a heated chamber. The water is rapidly removed and solid particles are formed held together by the organic binder.
These powder particles are collected as they fall to the bottom of the chamber. Sintering is carried out in a furnace, with an initial lower temperature step to first remove the organic binder. Like gas atomised powders, these powders can have the advantage of being spherical in shape.
Particle size distribution
Particle size distribution plays an important role in coating preparation and in determining the properties of a coating. For example, excessively large particles will bounce away from the substrate leading to poor deposition efficiency, while particles that are too small will melt within the spray gun leading to blockage. Also, very small particles tend to oxidize more readily, leading to higher oxide levels in the coating. The most appropriate particle size distribution will depend on the type of HVOF system. Values given by the equipment manufacturers should be used or expert advice sought. Typical powder size is in the range 15 to 60µm, and often quite narrow distributions are used such as 15 to 45µm. Classification of the powder to the correct size fraction or distribution is done by sieving for sizes above 25µm, and by air sifting below this size.
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