EP0954619B1 - Method and device for producing a coating on a substrate - Google Patents

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

The present invention relates to a method and a device for production of a coating on a substrate and, more particularly, the making thermal deposits on a substrate using particles in fusion which are animated by kinetic energy.

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

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

After preparation of the substrate by any suitable technique such as than 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 adhesion, the state of stress.

Such a technique is described in the documents (DE 42 20 063 - FR 2 681 538 - J-03050169A, J-01139749A).

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 powder particles projected towards the air. It is therefore necessary to define in a very specifies the conditions of the deposit while constantly maintaining a balance thermal by modifying the relative displacements of moving parts with respect to each other and using cooling means.

However, such methods of producing a metal deposit, ceramic, ceramometallic or organometallic, have many disadvantages. Among these, we can cite a large amplitude of variation of the temperature of the substrate at the point or zone of impact of particles during deposition, an inhomogeneous temperature of the layer of particles deposited, which can induce internal tensions in said layer, the temperature variation being due to an imperfect control of the heating conditions; similarly, the substrate / particle surface energies in fusion can be totally unsuitable especially when the energy at the surface of the substrate is low, which leads to poor adhesion of molten particles to the substrate and / or poor cohesion of the deposit.

A major drawback is that current techniques for substrate preheating are totally unsuitable for substrates made up of large parts. Indeed, it is difficult to preheat a large room in its entirety before thermal spraying, because we rarely have suitable means for perform such preheating.

Another disadvantage is that when the cooling temperature is not satisfactorily controlled, results in a non-homogeneous coating or deposit layer.

A solution to remedy this drawback and applicable on particularly large parts is to treat them mechanically before thermal spraying then build the layer of coating with many 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 particles and stabilize them at acceptable values.

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

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

We know from JP-A-63218272 a method of depositing on a substrate and a device for implementing this method comprising preheating means, thermal spraying means of particles and cooling means. This document does not specify the temperatures used according to the process supposed to improve the adhesion of the substrate and its resistance and decrease porosity. However the deposit obtained is not homogeneous.

The object of the present invention is to remedy the drawbacks mentioned above and to propose a method and a device for coating, by thermal spraying, parts of different structure and dimensions without the need to adapt them to the type of substrate or part to be treated.

The subject of the present invention is a method for producing a coating on a substrate, as defined in claim 1.

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

The method according to the invention thus makes it possible to obtain a deposit adherent, cohesive and with a high residual stress level 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 as defined in claim 5.

An advantage of the present invention lies in the fact that in limiting the heated area of the substrate and keeping the rest of the substrate at the room temperature, oxidation is greatly avoided or reduced or nitriding the substrate 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 superficial energy. Indeed, depending on the nature and the structure of the substrate, it is possible to select a heating temperature appropriate, so the difference between the particle temperatures projected and substrate can be adjusted to a value which improves the adhesion of the particles on the substrate.

Likewise, internal constraints likely to develop in the substrate, as well as porosity and microcracks, are strongly decreased due to subsequent cooling which is carried out immediately after depositing the particles on the coated area.

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

Furthermore, the relative movement between the substrate and the means heating, particle projection and cooling, is determined for example depending on the dimensions of said substrate, although others considerations can be taken into account. In a first mode of operation, we can keep the substrate fixed and move each or all the means if they are mounted in tandem in front of the fixed substrate, zone after area, until the entire surface of the substrate is coated with the deposit wish. But according to a second operating mode, we can move the substrate, always zone after zone, in front of the various aforementioned means.

• la figure 1 est une vue schématique d'un dispositif selon la présente invention pour un substrat en rotation.
• la figure 2 est une vue de dessus et schématique du dispositif selon la présente invention pour un substrat fixe,

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:

• Figure 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 an area of determined surface of a substrate to be coated with a deposit, then to be projected immediately after a powder or mixture of powders on said area to using thermal spraying means and then to cool as well immediately after said coated area using means of cooling known per se. This sequence of operations can be performed either by holding the substrate fixed and moving in one piece said heating, spraying and cooling means which constitute a so-called covering assembly, either by maintaining said coating set and moving the substrate either by moving the substrate and coating set, in directions and with velocities of displacement (rotation or translation) which are defined so that an area is treated, as specified above, before the area will not be processed in turn, until the total recovery 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 according in particular to the dimensions of the substrate, its shape or any other parameter linked directly or not to the substrate.

The device for implementing the method according to the invention includes a set 1 consisting of 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 of thermal projection, heating means 5, mounted upstream of the projection means 4, and cooling means 6 arranged downstream of 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 arranged 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 high energy and strong 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, of 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 centimeter square.

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

We could, if necessary or preferable, determine a different relative movement between the device and the part 7 so as to deposit on a circumferential strip 11 and treat said part 7, strip after strip, but keeping the same sequence of operations heating-projection-cooling as previously indicated about 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 that the temperature of projection is around 1100 ° C, we decrease the temperature difference between the molten powder and the part, 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 CO ₂ 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. The cooling temperature is lower than the oxidation and / or deformation temperature of the substrate, thus preventing 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 artwork. In another mode, you can only make a pass of a few micrometers on each zone and 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 constituting then an undercoat for the next layer.

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

In the same way as above with regard to Figure 1 the heating means 5 heat a given zone 16 to bring it to a predetermined temperature before it passes the means of projection 4 to receive a deposit. While part of 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 repository and which organizes the sequence of operations with the appropriate relative speeds between device 2 and the part to be treated. At each end of the substrate 12, the system is either inverted be brought back to the starting side by an external path.

Thanks to the present invention, coated parts are obtained which exhibit very good corrosion resistance, as shown by the figures of 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 H ₂ SO ₄ 2N medium with a potential sweep speed of 1 mVsec ^-1 . 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 results in microhardness of this same deposit obtained under 300 g of filler. The method according to the invention allows to obtain results superior to those of a solid alloy, which does not not allow conventional methods.

torche de projection HVOF :

• méthane   220 l/minute
• O₂   440 l/minute
• gaz poudre   15 l/minute
• poudre de FeNiCrBSi   75 g/minute

brûleur :

• O₂/C₂ H₂   1000 l/minute
• largeur de flamme   50 mm
• température de chauffage   300°C

buse :

• CO₂   50 kg/heure
• diamètre   25 mm
• température de refroidissement   ambiante
• vitesse de rotation du cylindre   200 t/minute
• déplacement torche projection   1 m/minute

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 l / minute
• O ₂ 440 l / minute
• gas powder 15 l / minute
• FeNiCrBSi powder 75 g / minute

burner :

• O ₂ / C ₂ H ₂ 1000 l / minute
• flame width 50 mm
• heating temperature 300 ° C

nozzle :

• CO ₂ 50 kg / hour
• diameter 25 mm
• ambient cooling temperature
• cylinder rotation speed 200 rpm
• projection torch movement 1 m / minute

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

The device according to the invention is compact and can be transported to carry out a repair on site if necessary, in particular for very heavy and large parts. Reference state I passive (µA.cm ^-2 ) NiCrBSi coating classic coating 3,500 to 7,500 NiCrBSi coating new process 40 Austenitic stainless steel type 316 L solid 54 Reference state Hv300 hardness FeNiCrBSiC deposited by conventional thermal process 650 ± 150 FeNiCrBSiC deposited according to the process of the invention 870 ± 75 FeNiCrBSiC in solid or remelted alloy state 800 ± 50

Claims (7)

1. Method for producing a coating on a substrate, of the type in which molten particles moved by kinetic energy are sprayed onto a surface of a substrate, of the type consisting of heating a zone of the substrate to be coated to a very shallow depth just before impact of the particles, and of cooling the coated zone to a cooling temperature lower than the temperature of oxidation and of deformation of the substrate, characterised in that it consists of keeping the rest of the substrate at ambient temperature and in that the cooling is carried out immediately after deposition of the coating, the heated zone being substantially equal to the impact surface of the particles, then of sequentially recommencing the preceding operation zone by zone until the said surface is completely coated.
2. Method according to claim 1, characterised in that the heating temperature depends on the nature of the substrate.
3. Method according to claim 1, characterised in that the cooling means consist of a jet of a cryogenic fluid such as liquid CO₂.
4. Method according to claim 1, characterised in that the heating and particle-spraying means perform a relative movement with respect to the substrate, the pitch of the relative movement being equal to the surface of the zone to be coated.
5. Device for implementing the method according to claims 1 to 4, of the type comprising heating means(5), thermal particle-spraying means (4) and cooling means (6), the said heating and spraying means performing a relative movement with respect to the substrate (3), the said heating means (5) being disposed upstream of the spraying means (4) so that each heated zone (9) is immediately in advance of its passage under the spraying means, characterised in that the zone of the substrate located under the spraying means has a cross-section which is equal to the heated zone and in that the coated zone is subjected immediately afterwards, without transition, to cooling by a cryogenic fluid, the cooling temperature being lower than the temperature of oxidation and of deformation of the substrate (3).
6. Device according to claim 5, characterised in that the cooling means perform the same said relative movement with respect to the substrate.
7. Device according to claims 5 or 6, characterised in that the heating and cooling temperature are selected depending on the nature of the substrate and of the sprayed particles.

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