GB2415708A - High velocity oxy-fuel spraying system utilising superfine powder particles - Google Patents

Abstract

A method of manufacturing coatings using superfine powder particles of size less than 5 microns. The powder is made into a slurry and deposited onto a sacrificial wire so that the resultant wire can be fed through a HVOF spraying system. The wire coating breaks down during spraying and a coating of superfine particles is formed.

Description

Improvements to powder production and spraying This invention describes a

method of manufacturing special wire containing ultra-fine particles that cannot normally be sprayed for use in wire spraying processes using flame, high velocity oxy-fuel, detonation or plasma to melt and propel the melted material.

In thermal spraying processes powder or wire 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 miuons) 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.

In UK Patent application number GB 2393452 the applicants discussed the idea of agglomerating ultra-fine powders into larger particles size so that they could then effectively be sprayed.

In this application we propose an alternative arrangement whereby the ultra-fine particles are deposited onto a wire which is then fed into the flame, high velocity oxy- fuel, detonation or plasma jet, hereinafter called the jet. The wire is readily consumed in the jet and merely acts as a medium onto which to deposit the particles in a slurry or liquid form.

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 thermally sprayed coatings to replace hazardous electro-plated chromium (e.g. 4hard-chromen). 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/difficult 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 Cored wires are generally considered inadequate because they tend to split and fracture and in so doing can be a danger to the spray operators. It is also found that there is settling of the constituent powder inside the outer tube and air spots or blank spots are found. This all leads to great inconsistency in the spraying operation and therefore prevents this process being used for quality operations.

This invention refers to the manufacturing method and technique for making coated wires for spraying purposes.

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 coat the base wire as shown in figure 1.

The principle of this invention is that the superfine powders to be sprayed are first contained in a slurry or slurries 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.

The important feature is that the dried slurry breaks up inside the jet so that the powders are released and form a stream of super-fine powder being sprayed onto the workpiece.

The exact proportions and type of constituents required for the slurry or slurries will need to be determined by experiment for each powder type or combination of powders 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 made into a slurry and deposited onto the base wire 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 (Figures 1 and 2) In Figure 1, two superfine powders having been mixed into a slurry are contained in baths 1 8 2. Other slurries will contain further coating materials or powders. The base wire 3 from the reel 4 is driven by gear box 5 into a spray chamber 6. The superfine powder stream is then deposited onto the wire by a spray deposition process (or by Immersion). A final sealing layer 7 (such as gelatins) is then spray deposited onto the coated wire. The coated wire is then heated by radiant or other heating systems 8 before being guided through soft rollers 9 before being fed through a conical guide 10 that incorporates air or gas jets to further guide the coated wire into the spray torch 1 1.

In Figure 2 is shown a cross-section of the coated wire; in this case two layers of coating are shown, any number could be used if mixtures of powders are required.

Any number of slurries could be employed in a spray cell.

Claims (6)

1. What we claim is 1. A method for the production of coatings using superfine powders.
2. 2. A method as claimed in claim 1 in which the superfine powders to be sprayed are first made into a slurry or slurries so that they can be deposited onto base wires.
3. 3. A method as claimed in claims 1 and 2 whereby the coating breaks up into the constituent powders when passed through the hot gases from the jet.
4. 4. A method as claimed in claim 1, 2 and 3 where the particles to be sprayed are made into a slurry or slurries contained in a cell holding a number of coating baths which allow for a number of coating powders and/or mixtures of powders to be used.
5. 5. A method as claimed in claims 1, 2, 3 and 4 whereby the coated wire is introduced into the spray torch through a wire feeder and is then heated. The particles are released from the wire, which vaporises7 and heated and accelerated in the jet from the torch.
6. 6. A method as claimed in claims 1,2,3,4 and 5 whereby the released superfine powder particles are heated and accelerated towards the surface to be coated and there form the coating.