WO1998026104A1

WO1998026104A1 - Method and device for producing a coating on a substrate

Info

Publication number: WO1998026104A1
Application number: PCT/FR1997/002117
Authority: WO
Inventors: Christian Coddet; Han Lin Liao; Bernard Hansz
Original assignee: Institut Polytechnique De Sevenans
Priority date: 1996-12-09
Filing date: 1997-11-24
Publication date: 1998-06-18
Also published as: DE69710495D1; EP0954619B1; FR2756756B1; EP0954619A1; FR2756756A1; ATE213281T1

Abstract

The invention concerns a method and a device for producing a coating on a substrate whereby molten particles driven by kinetic energy are sprayed on a previously heated surface of a substrate and it is characterised in that it consists in heating each zone of the substrate to be coated immediately before the impact of the particles, the rest of the substrate being at room temperature, and in cooling each said coated zone immediately after the coating has been applied then in repeating sequentially the previous operation on each successive zone until said surface is completely coated.

Description

Method and device for producing a coating on a substrate

The present invention relates to a method and a device for producing a coating on a substrate and, more particularly, producing thermal deposits on a substrate by means of molten particles which are animated with kinetic energy.

The coatings are generally obtained by spraying a powder or a mixture of powders onto the substrate to be coated, so as to give the said substrate properties of hardness, resistance to wear and / or to corrosion, good lubrication, or make them better conductors of electricity.

The powder or mixture of powders is sprayed onto the substrate by a technique known under the generic name of thermal spraying which is generally carried out in air or under low pressure by means of special devices known for example under the acronyms VPS (Vaccum Plasma Spray) , LPPS (Low Pressure Plasma Spray), CAPS (Controlled Atmosphere Plasma Spray ing), APS (Atmospheric plasma spraying), IPS (inert gas plasma spray ing) or HVOF (High Velocity oxyfuel spraying).

After preparation of the substrate by any suitable technique such as sandblasting and before the actual projection, it is usual to preheat the part or substrate to be treated, in order to improve the characteristics of the layer deposited on said substrate such as for example the adhesion, the state of constraint.

Such a technique is described in the documents (DE 42 20 063 - FR 2 681 538 - J-03050169A, J-01 139749A).

However, the prior heating of the substrate to be treated to the ambient atmosphere induces a risk of oxidation of the deposit and / or of the substrate due to the reactivity of the substrate and / or of the powder particles projected with respect to the air. It is therefore necessary to very precisely define the conditions of the deposition by constantly maintaining a thermal equilibrium by modifying the relative displacements of the moving members with respect to each other and by using cooling means. However, such methods of producing a metallic, ceramic, ceramometallic or organometallic deposit have many drawbacks. Among these, there may be mentioned a large amplitude of variation of the temperature of the substrate at the point or zone of impact of the particles during the deposition, a nonhomogeneous temperature of the layer of deposited particles, which can induce internal tensions in said layer, the temperature variation being due to an imperfect control of the heating conditions; similarly, the surface energies of the substrate / molten particles may be totally unsuitable, in particular when the energy at the surface of the substrate is low, which leads to poor adhesion of the molten particles to the substrate and / or poor cohesion of the deposit. .

A major drawback is that the current techniques for preheating the substrate are totally unsuitable for substrates made up of large parts. In fact, it is difficult to preheat a large part in its entirety before thermal spraying, since there are rarely means suitable for carrying out such preheating.

Another drawback lies in the fact that when the cooling temperature is not satisfactorily controlled, the result is a non-homogeneous coating or deposit layer.

One solution to remedy this drawback, which can be applied to particularly large parts, is to treat them mechanically before thermal spraying and then to build the coating layer by numerous successive passes, each pass producing a thin part of the coating layer, such a solution making it possible to reduce the internal tensions developing during the projection of the particles and to stabilize them at acceptable values.

Cooling techniques are also known either by air jet, or by spraying cryogenic fluid, or by spraying water, as described for example in EP-A-0 546 359 or DE-A-2 615 022 .

All these techniques consist in keeping the entire part to be treated at low temperature, so as to avoid deformation or decomposition of the substrate, but do not increase the adhesion of the coating layer or its density.

The aim of the present invention is to remedy the aforementioned drawbacks and to propose a method and a device for coating, by thermal spraying, parts of different structure and dimensions without it being necessary to adapt them to the type of substrate or part to be treat.

The subject of the present invention is a production method according to which particles of animated kinetic energy are projected onto a surface of a substrate which is previously heated, the method being characterized in that it consists in heating each zone of the substrate to be coated just before the impact of the particles, the rest of the substrate remaining at room temperature, and in cooling each said coated area immediately after the coating has been deposited, then in sequentially starting the previous operation, area after area, until that said surface is fully coated.

The local heating of the substrate is carried out with high energy means (flame, induction, laser, electron beam, plasma), immediately before the arrival of the particles so as to benefit from a high surface temperature at the time of l impact of the particles while minimizing the heat transfer in the substrate. Likewise, the cooling of the surface is carried out by energetic means, such as pulverized liquefied gases, immediately after the deposition of the particles in order to always minimize the heat transfer in the substrate.

The method according to the invention thus makes it possible to obtain an adherent, cohesive deposit, the residual stress level of which is greatly reduced compared to what is achieved with the methods of the prior art.

Another object of the present invention is a device allowing the implementation of the method, the device being characterized in that at least said heating and projection means are driven in a relative movement relative to the substrate, said means heating being arranged upstream of the particle projection means, so that each zone of the substrate to be coated is heated immediately before the deposition of the particles on said zone.

An advantage of the present invention is that by limiting the heated area of the substrate and keeping the rest of the substrate at the ambient temperature, the oxidation or nitriding of the substrate is avoided or greatly reduced during the projection of the particles.

Another advantage is better adhesion of the deposit due to the choice of the temperature of the impact zone on the substrate in relation to its surface energy. Indeed, depending on the nature and structure of the substrate, it is possible to select an appropriate heating temperature, so that the difference between the temperatures of the projected particles and of the substrate can be adjusted to a value which improves the adhesion of the particles on the substrate. Likewise, the internal stresses liable to develop in the substrate, as well as the porosity and microcracks, are greatly reduced due to the subsequent cooling which is carried out immediately after the deposition of the particles on the coated area.

The combined actions of heating the area before deposition and cooling the same area immediately after deposition allow better control of the deposition itself.

Furthermore, the relative movement between the substrate and the heating, particle projection and cooling means, is determined for example as a function of the dimensions of said substrate, although other considerations may be taken into account. In a first operating mode, it is possible to keep the substrate fixed and to move each or all of the means if they are mounted in tandem in front of the fixed substrate, zone after zone, until the entire surface of the substrate is coated with the deposit desired. However, according to a second mode of operation, the substrate, always zone after zone, can be moved in front of the various aforementioned means.

Other advantages and characteristics will emerge more clearly on reading the description of a preferred embodiment of the invention, as well as the appended drawings in which:

FIG. 1 is a schematic view of a device according to the present invention for a rotating substrate,

FIG. 2 is a top schematic view of the device according to the present invention for a fixed substrate,

The method according to the invention consists in heating a determined surface area of a substrate to be coated with a deposit, then in spraying immediately after a powder or a mixture of powders onto said area. using thermal spraying means, then also cooling immediately after said coated area using cooling means known per se. This sequence of operations can be carried out either by keeping the substrate fixed and by moving in a single block said heating, projection and cooling means which constitute a so-called coating assembly, or by holding said coating assembly fixed and by moving the substrate, either by moving the substrate and the coating assembly, in directions and with displacement speeds (rotation or translation) which are defined so that an area is treated, as specified above, before that the consecutive area is not treated in turn, until the total covering of the surface of the substrate to be coated with the desired thickness of the deposit.

Obviously, the displacements can be vertical and / or horizontal or even in rotation, the combination of displacements being chosen as a function in particular of the dimensions of the substrate, of its shape or of any other parameter linked directly or not to the substrate.

The device for implementing the method according to the invention comprises an assembly 1 constituted by at least one support 2 which moves relatively with respect to a substrate 3 to be coated with a deposit, and on which are mounted at least means 4 for thermal projection, heating means 5, mounted upstream from the projection means 4, and cooling means 6 arranged downstream from the projection means 4.

In the embodiment of FIG. 1, the substrate 3 is constituted by a part 7 of large dimensions and of cylindrical shape; the external face 8 must be coated with a metallic deposit, the components of which are placed in the reserve of a torch, for example of the CDS type sold by the company PLASMA TECHNIK, the torch 4 projecting the components in the form of powders with great energy. and under a high temperature on said face 8. The three elements 4 to 6 of the assembly are located in the same horizontal plane or at least their ends, so that the area of the external face 8 is treated successively by each of the three elements.

The dimensions of each zone 9 to be treated are defined by the impact surface of the projection means 4, which impact surface is on the order of a few square centimeters and rather on the order of a square centimeter.

In the device of FIG. 1 and due to the dimensions of the part 7 which moves with a determined rotational speed and in the direction of the arrow 10, the assembly 1 moves vertically so as to sweep successive zones 9 and located at different levels, the various successive zones being located on fictitious sinusoids as and when the passes made by the device.

One could, if this proved necessary or preferable, determine a different relative movement between the device and the part 7 so as to deposit it on a circumferential strip 11 and to treat said part 7, strip after strip, but keeping the same sequence of heating-spraying-cooling operations as indicated above in connection with the process. In an embodiment according to FIG. 1, the area to be treated 9 is heated to a temperature of the order of 400 ° C.

Immediately after heating and as indicated above, the heated zone is brought opposite the projection torch 4. As the zone is already brought to a given temperature and the projection temperature is of the order of 1100 ° C. , the temperature difference between the molten powder and the part is reduced, thus allowing better spreading of the deposit.

In another step, and immediately after deposition, the zone 9 is brought opposite at least one nozzle through which liquid CO2 or another cryogenic fluid is sprayed. This rapid cooling, immediately after the projection, in fact constitutes a quench which solidifies the deposit and makes it adhere better to the part 3, while preserving its structure. Preferably, the cooling temperature is lower than the oxidation and / or deformation temperature of the substrate, thus avoiding oxidation of said part and / or of the metallic components of the powder.

The sequence of operations which has just been described is carried out continuously from heating to cooling.

Various ways of coating the part 3 can be implemented. In another mode, it is possible to make only a pass of a few microns over each zone and to coat the external face 8 with a first layer, then repeat the same operations as many times as necessary until the final thickness of the deposit is obtained, each layer then constituting an under-layer for the next layer.

In the device of FIG. 2, the substrate consists for example of a fixed plate 12. The support 2 is, in this case mobile in translation along the arrow 13.

In the same way as previously with respect to FIG. 1, the heating means 5 heat a given zone 16 to bring it to a predetermined temperature before it passes in front of the projection means 4 to receive a deposit. While part of the zone 15 is coated, the cooling means 6 cool the zone 14 which has already been coated. This is automatically programmed by a program which takes into account all the parameters of the deposit and which organizes the sequence of operations with the appropriate relative speeds between the device 2 and the part to be treated. At each end of the substrate 12, the system is either inverted or brought back to the starting side by an external path.

Thanks to the present invention, coated parts are obtained which have a very good corrosion resistance, as evidenced by the figures in Table 3 and a very high hardness (Table 4). Table 3 presents by way of example the results obtained for a coating of the NiCrBSi type deposited on an aluminum substrate using on the one hand a conventional method and on the other hand the method according to the invention. These are corrosion test results carried out in normally aerated H2SO42N medium with a potential sweep speed of 1 mVsec ^~ l. It is clearly seen, in this case, that the intensity of the passive current with the conventional coating does not allow protection against corrosion whereas with the process according to the invention the coating behaves as well or even better than stainless steel .

Table 4 presents the microhardness results of this same deposit obtained under 300 g of filler. The method according to the invention makes it possible to obtain results which are superior to those of a solid alloy, which conventional methods do not allow.

By way of example and for a substrate constituted by a cylinder with a diameter of 250 mm and one meter in length, the operating conditions are as follows: HVOF projection torch:

- methane 220 1 / minute - 0 ₂ 440 1 / minute

- gas powder 15 1 / minute

- FeNiCrBSi powder 75 g / minute burner

0 ₂ / C ₂ H ₂ 1000 1 / minute flame width 50 mm heating temperature 300 ° C nozzle

- C0 ₂ 50 kg / hour

- diameter 25 mm

- ambient cooling temperature

- cylinder rotation speed 200 t / minute - projection torch movement 1 m / minute

With the above deposition parameters, a coating 0.6 mm thick and with a hardness greater than 850 Hv (Vickers hardness) was obtained, the porosity of the coating being invisible with a 1000 magnification of a microscope. optical and which is, in any event, less than 1%.

The device according to the invention is space-saving and can be transported to make a repair on site if necessary, in particular for very heavy and large parts.

TABLE 3

TABLE 4

Claims

1. Method for producing a coating on a substrate, of the type according to which molten particles animated with kinetic energy are projected onto a surface of a substrate which is previously heated, characterized in that it consists of heating each zone of the substrate to be coated to a very reduced thickness, just before the impact of the particles, the rest of the substrate being at room temperature, and to cool each said zone coated immediately after the coating is deposited, the cooling temperature being below the oxidation and deformation temperature of the substrate, then to repeat the previous operation sequentially, zone after zone, until said surface is completely coated.

2. Method according to claim 1, characterized in that the zone which is heated is substantially equal to the impact surface of the particles.

3. Method according to claims 1 or 2, characterized in that the heating temperature depends on the nature of the substrate. .

4. Method according to claim 1, characterized in that the cooling means consist of a projection of a cryogenic fluid such as liquid C0 ₂ .

5. Method according to claim 1, characterized in that the heating means and of projection of particles are animated by a relative movement relative to the substrate, the pitch of the relative movement being equal to the surface of the area to be coated.

6. Device for implementing the method according to claims 1 to 5, of the type comprising heating means (5), means for thermal projection of particles (4) and cooling means (6), said means for heating and projection being moved relative to the substrate (3), characterized in that said heating means (5) are arranged upstream of the projection means (4), so that each zone (9) heated is disposed immediately before its passage under the projection means, said heated and coated area after the projection of particles passing immediately under the cooling means, the cooling temperature being lower than the oxidation and deformation temperature of the substrate (3).

7. Device according to claim 6, characterized in that the cooling means are driven by the same said relative movement relative to the substrate.

8. Device according to claims 6 or 7, characterized in that the heating and cooling temperatures are chosen according to the nature of the substrate and of the projected particles.