Preheated particles make superior coatings

 

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The application of molten-sprayed ceramic powders has been limited to the use of relatively low quality of coatings due to an incomplete in-flight melting of the coating powders. A new process using preheating of powders dramatically improves the quality of spraying ceramic coatings in general but especially that of thick coatings deposited from particles with diameters greater than 50 µm. This process also achieves a much greater quality of coatings made from metal powders.

 The quality improvement is especially remarkable in achieving a coating porosity that is lower by a factor of 2.5 to 8.

Powerful plasma or conventional flame torches are used to deposit molten ceramics or metals on substrate surfaces. This process is called thermal projection.

The materials to be projected onto the substrate have in most cases a very limited grain size and are fed at a controlled rate into the plasma or flame jets from a powder dispenser. The powder is transported by a gas carrier, typically at 5 to 10 liters per minute, in a flexible hose to an injector on the projection torch.

The powder is injected into the jet that has a very high temperature (up to 14 kK for electric plasma jets) and high speed. The powder is heated up to its partial or total fusion temperature and accelerated in the jet. Upon impact on the substrate the particles collapse from their spherical shape and solidify into layer to form the coating. The quality of the coatings depends in part on good contact between the grains in the coating. This contact is determined by the state of particles impacting on the substrate. Good contact between particles is reached when all particles are fully melted before the impact.

·        The state of a particle at impact depends on its initial injection temperature and its size. This initial temperature is ambient in all currently used processes. The size of the injected particles is also limited to give them opportunity to melt in the available flight-time and heat of the jet.

·        The state of the particles also depend on their trajectories in the jet. The maximum temperature is found at the jet's central axis and is considerably lower in the radial direction of the fringes of the jet. This is why a small particle trajectory brings it to the outer part of the jet center. A too large particle anywhere in the jet is insufficiently heated and impacts on the substrate or partially formed coating while still being solid or only partially melted.

To avoid this dilemma we propose a small electric device to preheat powders to high temperatures just before their injection into the jet. Thus, the heating of the particles to beyond their melting temperature becomes much less dependent on their trajectory and lifetime in the jet, and their melting is now not entirely influenced by the temperature gradients in the jet. Due to this preheating the fraction of molten particles impacting on the substrate or partially formed coating can be increased to virtually 100%. This drastically improves coating properties. In addition, the size of the particles to be projected by the jet can be increased; hence the intensification of the thermal projection process can be achieved.

Two different types of reactors can preheat large particle powders before they are injected into the thermal spray equipment:

·                    a gliding arc (GlidArc) device heats the particles up to 500°C or

·                    a tubular Joule-based device can heat them up to 900°C (see below).

Both reactors can easily be connected to conventional powder-feeding thermal spray equipment.

 

Example 1

Calculated power to preheat a flow of Al2O3 powder at 50 g/min, carried in nitrogen at 5 L(n)/min, from 25 to 725°C is 0.74 kW. Our 1 kW Joule-based device is capable to provide that power. The same device can also continuously preheat other powders in other gas carriers such as air, O2, Ar, H2, CO2 or CH4. Some fruitful tests of such a controlled preheating of different powders of 22 µm to 1 mm diameter have been achieved with an alumino-silicate (0.16 mm), nickel (0.21 mm), Cr2O3 ("Amperite 740.1", 22 to 45 µm), SiO2 (0.2 to 1 mm) and SiC (0.16 to 0.20 mm). The preheat temperature can reach 880°C, which is 1/2 or 1/3 of the final melting point temperatures of these powders.
 

Example 2

The same preheater as in Example 1 has been installed between a powder dispenser and a METCO plasma torch. The use of a CASTOLIN-EUTECTIC powder feeder achieved the transport of the same powder Cr2O3 (see Example 1) to the preheater. The exit of the preheater has been closely connected to the usual torch injector in a particular way that permits good electric current passage. The heat leaks between the preheater and the cold body of the torch are however reduced. Indeed, one of difficulties of our invention's realization rested in a risk of preheated gas/powder mixture cooling if exposed to the cold torch injectors.

We also improved the electric safety of the system by putting the preheater/torch attachment point to the ground potential, the same as the plasma torch body.

The projection has been done with the following parameters:

A layer of Cr2O3 has been projected onto a steel roller of 30 mm diameter on 4 tracks of about 10 cm length each. The roller surface was sanded to remove all scale and to make it free of grease. During the projection the roller rotated at 300 rpm whereas the speed of the torch translation in relation to the roller was 0.36 m/min. The temperature of the preheated powder/ gas mixture has been measured at the exit of the preheater with a thermocouple: it was 600°C. Data pertaining to these tests and some resulting coating properties parameters are shown below:

 

Reference mark

Thickness (µm) 

Number of passages

Porosity (%)

Hardness

(HV)

Preheat

1

130

6

2.5

1260

without

2

150

6

0.9

1390

 

3

520

15

4.8

1740

without

4

550

15

0.6

1200

 

 

* * * * *

Results of tests show the advantages of the preheater. It permits the necessary temperature elevation for the fusion of particles that gives:


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