GB2393452A

GB2393452A - Superfine powder and spraying

Info

Publication number: GB2393452A
Application number: GB0220008A
Authority: GB
Inventors: P E Chandler; Michael Bernard Coupla Quigley
Original assignee: C A TECHNOLOGY Ltd; QUIGLEY ASSOCIATES
Current assignee: C A TECHNOLOGY Ltd; QUIGLEY ASSOCIATES
Priority date: 2002-08-28
Filing date: 2002-08-28
Publication date: 2004-03-31
Anticipated expiration: 2022-08-28
Also published as: GB2393452B; GB0220008D0

Abstract

A method of manufacturing powder and coatings uses superfine powder particles in which the superfine powder is first agglomerated into particles of a size suitable for flowing through a powder feeder but in such a way that the particles self atomise during the spraying process. Thus superfine particles can be sprayed without the normal problems of the powder jamming up the powder feeder. The agglomeratron is carried out by first mixing the powder with a binder and then spray dried at controlled furnace temperatures, below the sintering temperature of the constituents, so that it is both robust and easily breaks up into the component superfine powders when heated inside the spray torch. The superfine powder has a size of less than 5 microns. The agglomerated powder is fed together with fuel to a spray torch where it is heated and accelerated.

Description

1 2393452

i Improvements to powder production and spraying This invention describes a method of manufacturing self-atomising agglomerated powder for use in powder spraying processes such as flame, high velocity oxy-fuel, detonation or plasma spraying.

In these spraying processes powder is injected into a plasma or flame or fast flowing gas which then heats and accelerates the particles. The combination of the heating and acceleration ensures that at impact the temperature of the particles has been raised to a level that is such that the particles can bond to the substrate to be coated.

Ultimately the objective is to produce a coating that imparts specific properties to the coating such as corrosion or wear resistance. Such coatings are widely used in engineering plant for a very wide range of applications but aerospace is the best known application whereby such coatings are used in gas turbines to enable the turbines to operate at high temperatures thus improving efficiency and offer environmental advantages.

In all of the above processes powders are typically classified, e.g. by sieving, into an appropriate size range. Typically powder particles in the size range +45 microns -

106 microns (i.e. between 45 and 106 microns) are defined here as coarse powder.

Typically powder particles in the size range +5 microns - 45 microns (i.e. between 5 and 45 microns) are defined here as fine powder. Those powder particles less than 5 microns are generally removed as they tend to inhibit the flow of powder in powder feeders, feed lines, injectors etc. Here we define these powders, i.e. less than 5 microns, as superfine powders. This is for all types of powder including those agglomerated from very fine material including nano-sized (less than 1 micron) particles. Existing thermal spray technology has not yet been sufficiently developed to be able to spray superfine powders. There are two main reasons for this. First, spray guns are designed such that powder is injected into turbulent zones which, while providing good heat transfer, makes injection of very fine particles extremely difficult. Second, powder feed systems whether mechanical or gas (e.g. fluidised bed) based tend to clog when feeding superfine powders. While these problems are well understood there has yet to be a satisfactory solution.

There are a number of applications where a higher density and reduced porosity coatings would be advantageous. Such improvements could be achieved by the spraying of superfine powders. Furthermore the use of superfine powders in processes such as HVOF could even reduce the need for grit blasting and provide very smooth coating surfaces. Indeed, there has been a significant amount of work carried out in this area over the last 10 years. Recently attempts have been made to use themmally sprayed coatings to replace hazardous electro-plated chromium (e.g. "hard-chrome") . Unfortunately, the feeding and spraying of superfine powders has raised some, so far, insurmountable problems. In particular, poor flow characteristics lead to clogging of conventional feeders and injection systems within the spray guns.

The ability to spray superfine powders not only impacts on density and coating thickness. Use of finer powder automatically leads to a reduced surface roughness on the coating. Improving density would lead to a reduction in coating thickness required. The reduction in surface roughness would have a major impact by the elimination of expensive/diffficult post coating processes such as grinding and polishing. For HVOF, there is also evidence that the use of superfine powders would also remove the need for grit blasting. All these aspects would provide significant cost benefits.

The superfine powder technology developed in this patent for thermal spray applications will also find application in many other areas where the use of superfine powders is required.

One method of powder manufacture is that of agglomeration. There are a number of techniques that can be used to agglomerate powders. One example is to mix the superfine powder particles with an organic or inorganic binder and a liquid (water or alcohol) to produce a slurry. This slurry can then be dried in an oven, crushed and classified to produce powder. Alternatively, the slurry can be delivered to a spray dryer to be atomised to form the agglomerated powders. In the case of the spray dried technique, the powder is collected and typically is subjected to a number of other treatments such as sistering, plasma densification etc. These post-spray drying treatments are used to ensure that the agglomerated powder becomes sufficiently robust to pass through the powder feeder and, more importantly, to be sprayed without significant break-up.

In this patent we describe an altemative, innovative solution to the problem of feeding and spraying superfine powders. Instead of attempting to feed superfine powders it is proposed that agglomerated powders are produced which will 'self-atomise' (ie break-up into it's constituent particles) within the gas jet of the thermal spray system.

This removes the need to feed superfine powder but at the same time delivers a source of superfine powder particles within the spray stream.

This invention refers to the manufacturing method and technique for making self atomising powders.

Superfine powders (including nano-sized particles) are mixed with a range of constituents. These include, but are not limited to, binders (organic (sugars) and inorganic), oxidisers, fuel, liquid (water or alcohol). This slurry is then used to produce an agglomerated powder e.g. by spray drying as shown in figure 1.

Unlike conventional treatments this spray dried powder is then given a carefully controlled furnace treatment at temperatures below the sintering temperature of the constituents. This treatment is used to ensure that the agglomerated powder is robust enough to pass through the powder feeder, feed lines, injector etc without significant break up.

The principle of this invention is that the superfine powders to be sprayed are first agglomerated in such a way that they easily break up into the component superfine powders when they are heated inside the spray torch. The original material will be superfine particles of diameter less than 5 microns and including nano-sized particles. These will then be agglomerated with a binder to fomm particles of diameter greater than 5 microns.

The important feature is that the agglomerated powder breaks up inside the gas or plasma stream of the spray process. It will be necessary to arrange the proportions of binder to the superfine powders to assist selfatomisation at the appropriate time.

Further assistance for self-atomisation may be provided by the incorporation of oxidisers and fuel within the agglomerated powder. This additional heat source may also provide further benefits such as binder removal. The exact proportions and type of constituents will need to be determined by experiment for each powder type to be sprayed.

A further advantage of the proposed invention is that a greater range of materials can now be sprayed as essentially any superfine material that can be agglomerated into a powder can now be sprayed in this manner.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings (Figure 1,2,3 and 4).

In Figure 1, the superfine powder particles 1 are first mixed with the binder material as a slurry and then spray dried in the nommal manner to produce the agglomerated particles 2. An example of these is shown in Figure 2. In Figure 3, the agglomerated powder is then passed through the powder feeder 3 into the nozzle 4 and then into the barrel 5 of the spray torch 6. inside the spray torch 6 the agglomerated particles seH-atomise into a spray stream 7 consisting once again of the initial superfine powder particles 1. These particles 1 then exit the spray torch 7 and are propelled towards the component surface 8 and impact to form the coating 9. In Figure 4, agglomerated powder particles 2 are shown self-atomising into the original superfine powder particles 10.

Claims

What we claim is

1. A method for the production of powders and coatings using superfine powders.

2. A method as claimed in claim 1 in which the superfine powders to be sprayed are first agglomerated in such a way that they easily break up into the component superfine powders when they are heated inside the spray torch.

3. A method as claimed in claim 1 and 2 where the particles to be sprayed are first mixed together and then mixed in a slurry with a binder after which they are spray dried to produce the agglomerated particles.

4. A method as claimed in claim 1, 2 and 3 where the particles to be sprayed are first mixed together and then mixed in a slurry with a binder and oxidants and fuel after which they are spray dried to produce the agglomerated particles.

5. A method as claimed in claims 1, 2, 3 and 4 whereby the agglomerated particles are introduced into the spray torch through a powder feeder and are then heated and accelerated in that torch. The binder and other constituents are chosen such that the particles self-atomise in the conditions inside the torch.

6. A method as claimed in claims 1,2,3,4 and 5 whereby the self atomised superfine powder particles are heated and accelerated towards the surface to be coated and there fomm the coating.

7. A method as claimed in claims 1,2,3,4 and 5 whereby the self atomised superfine powder particles are assisted in their self-atomisation by the the presence of the oxidants and fuel during heating and acceleration towards the surface to be coated and there fomm the coating.