Method of depositing an anti-wear coating by thermal spraying

Abstract

A method of depositing an anti-wear coating on a mechanical part by thermal spraying of the AC-HVAF type, said coating being made of a copper-based alloy containing 30% to 42% by weight of nickel and 4% to 6% by weight of indium.

Description

The invention relates to a method of depositing an anti-wear coating on a mechanical part by thermal spraying, and more particularly it relates to a gas turbine part made of titanium or titanium alloy such as a fan blade or a compressor blade of a turbomachine.

BACKGROUND OF THE INVENTION

Fan or compressor blades constitute good examples of parts that are subjected to wear while a turbine is in operation. Such blades are held by their roots in slots of appropriate shape that are formed in the peripheries of rotary disks, referred to below as compressor disks or fan disks.

While a turbojet is in operation, the blade roots move in said slots under the effects of centrifugal force and of vibration. The roots of blades are shaped in a manner that matches the shapes of the slots so as to make such relative displacements possible. The surfaces of blade roots that come to bear against the edges of said slots under the effect of centrifugal force are subjected to significant compression stresses (which are generally cyclical). These stresses in combination with vibratory movement damage and wear said surfaces. The wear that is observed is found to be even greater when the blade roots and the fan or compressor disks are made of titanium or titanium alloy. This is because the coefficient of friction of titanium on titanium is rather high.

To protect blade roots, it is known to make use of anti-wear coatings that are constituted by copper nickel alloys (CuNi), copper aluminum alloys (CuAl), or indeed copper nickel indium alloys (CuNiIn). It is generally preferred to use a copper nickel indium type alloy (CuNiIn) since it presents better mechanical characteristics at high temperatures.

In order to deposit these alloys on blade roots, it is common practice to use a thermal spraying technique known as plasma spraying. That technique can be implemented using a plasma gun such as that described in U.S. Pat. No. 3,145,287. Plasma spraying consists in bringing alloy powder to a plasma torch that is producing a jet of gas at very high temperature: greater than 2000° C. The speed at which the particles are sprayed lies in the range 100 meters per second (m/s) to 400 m/s.

The microstructure of the coating deposited by plasma spraying nevertheless presents very high porosity and oxidation, thereby affecting the mechanical properties of the coating. In addition, the coating adheres poorly on titanium or titanium alloy. Thus, in practice, it is found that the coating flakes away quickly and is poor at withstanding the stresses to which it is subjected while the turbine is in operation.

A second type of thermal spraying is also used for depositing anti-wear coatings: this is known as high velocity oxy fuel (HVOF) spraying which consists in taking advantage of combustion between oxygen and a fuel gas such as propane, propylene, hydrogen, or propadiene methyl acetylene, in order to heat and propel molten grains of alloy powder at very high speed. The temperatures reached with that method lie in the range 1500° C. to 2000° C. and the spray speeds lie in the range 300 m/s to 700 m/s. An example of depositing a nickel-based alloy using HVOF spraying is described in U.S. Pat. No. 5,518,683.

Although the lifetime of a deposit obtained with an HVOF method is better than that of a deposit obtained by plasma spraying, it is nevertheless found that the coating flakes quickly under ordinary conditions of turbomachine operation.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to propose a novel method of deposition that makes it possible to deposit anti-wear coatings that are better at withstanding the stresses to which they are subjected than are the coatings obtained by existing methods.

To achieve this object, the invention provides a method of depositing an alloy of copper, nickel, and indium by thermal spraying to constitute an anti-wear coating on a mechanical part, wherein said coating is deposited by thermal spraying of the activated combustion high velocity air fuel (AC-HVAF) type.

Thermal spraying of the AC-HVAF type is a known technique that differs from the above-mentioned HVOF spraying mainly by using a mixture of air and a fuel gas such as propane (instead of a mixture of oxygen and gas) that is burnt in order to heat and propel an alloy powder at very high speed. With AC-HVAF spraying, the molten alloy particles are sprayed at a speed lying substantially in the range 600 m/s to 800 m/s, and the temperatures reached lie in the range 800° C. to 1500° C.

The temperatures reached during spraying of the AC-HVAF type are lower than those reached during spraying of the HVOF or plasma type. This serves to limit oxidation of the sprayed particles.

In addition, the spray speeds that can be obtained with the AC-HVAF method are higher than the speeds obtained by plasma or HVOF spraying. Thus, the lapse of time between the moment when the particles are sprayed and the moment when they reach the part to be coated, during which time lapse the particles are particularly sensitive to oxidizing, is itself shortened. This also contributes to reducing the extent to which the coating is oxidized.

In addition, the high kinetic energy of the particles sprayed onto the part for coating makes it possible firstly to achieve better bonding of the particles on the part, and secondly to obtain a coating that is more compact, presenting porosity that is less than that obtained with the methods that have been used in the past. In particular, the structure of the resulting coating is unitary and not lamellar.

Decreasing the porosity and the quantity of oxide in the coating leads specifically to a reduction in the number of incipient cracks in the microstructure of the coating. This leads to greater ability to withstand stresses and more particularly the compression stresses to which the coating is subjected. Since the coating is also more compact and adheres better to the part on which it is coated, it is found in practice that problems of flaking occur less quickly during operation of the gas turbine, and that the lifetime of the coating of the invention is considerably better than that of known coatings.

Finally, by its very nature, AC-HVAF thermal spraying is less expensive than plasma spraying.

Advantageously, said coating is constituted by a copper-based alloy containing 30% to 42% by weight of nickel and 2% to 8% by weight of indium.

More advantageously, it is possible for said coating to comprise a copper-based alloy comprising 34% to 38% by weight of nickel and 4% to 6% by weight of indium.

As already emphasized, CuNiIn coatings are advantageous since they are mechanically very strong at high temperatures.

While undertaking research to improve the lifetime of anti-wear coatings of this type, the Applicant company has found that the melting temperatures of CuNiIn alloys are much lower than the temperatures reached during plasma spraying, and lower than those reached during a HVOF type spraying. In contrast, the temperatures reached during AC-HVAF spraying turn out to be of the same order as the melting temperatures of said CuNiIn alloy. It has thus been found that by using the AC-HVAF method, it is possible to melt a CuNiIn alloy while avoiding any useless oxidation associated with temperatures that are too high. The AC-HVAF method thus turns out to be particularly well suited to depositing CuNiIn coatings.

Advantageously, once the CuNiIn anti-wear coating has been deposited, a lubricating varnish layer is deposited thereon, e.g. comprising molybdenum disulfide (MoS₂) and an organic resin. CuNiIn coatings present high roughness and it is advisable to cover them in a layer of varnish having a low coefficient of friction in order to encourage sliding and limit wear. The combined coating of CuNiIn and a layer of lubricant gives results that are entirely satisfactory in terms of protecting the part and in terms of the lifetime of the coating.

Although the only embodiment of a part described in the present description is a titanium blade for a turbomachine compressor or fan, it is clear that the method of the invention can be used for coating any type of part, regardless of whether it is made of titanium or a titanium alloy. For example, the method can be used for coating at least one part taken from any two gas turbine parts of any kind that are liable to come into contact with each other.

BRIEF DESCRIPTION OF THE DRAWING

The invention and its advantages can be better understood on reading the following detailed description of embodiments of the invention that are given as non-limiting examples. The description refers to the accompanying figures, in which:

FIG. 1 is a comparative plot;

FIG. 2 is a micrograph of a CuNiIn coating deposited by AC-HVAF spraying in accordance with the method of the invention;

FIG. 3 is a micrograph of a CuNiIn coating deposited by plasma spraying;

FIG. 4 is a diagram of a device enabling the stresses exerted on a fan blade root in operation to be simulated; and

FIG. 5 is a graph showing a cycle in the variation of the traction force exerted on a fan blade root in operation, as a function of time.

MORE DETAILED DESCRIPTION

The plot of FIG. 1 has spray speed in m/s plotted along the abscissa and spray temperature in ° C. plotted up the ordinate, as obtained when using various thermal spraying methods. In this plot, there can be seen outlines for temperature and spray speed ranges for plasma, HVAF, and AC-HVAF spraying. Furthermore, the range of temperatures over which a copper-based alloy such as the CuNiIn alloy melts is also shown.

In this diagram, and as described above, it can be seen that the temperatures reached in AC-HVAF spraying are adapted to the melting range of the CuNiIn alloy used in the invention, thus making it possible to melt these alloys without useless overheating that would encourage oxidation. Furthermore, it can also be seen that higher spraying speeds can be obtained by using AC-HVAF spraying.

An implementation of the method of the invention is described below by way of example, in which a CuNiIn alloy was deposited on a part made of a titanium alloy of the TA6V type. Operating conditions were as follows:

Device Used:

An SB-500 model AC-HVAF torch sold by the supplier Uniquecoat Technologies.

Powder Used:

Composition: CuNiIn alloy comprising 36% by weight Ni, 5% by weight In, with the balance being Cu;

Particle size: 11 micrometers (μm) to 45 μm;

Torch feed rate: 8 kilograms per hour (kg/h);

Carrier gas: nitrogen.

Operating Parameters of the Torch:

Gas: propane;

Air pressure: 85 pounds per square inch (psi);

Pressure 1, propane; 74 psi;

Pressure 2 (0) of propane: 38 psi;

Pressure of carrier gas: 41 psi;

Distance: 150 millimeters (mm) to 165 mm;

Coating deposition rate: 45 μm per pass.

Information Concerning the Coated Part:

Preparation: sandblasting with aluminum oxide particles having a mean size of 300 μm;

Initial temperature: 29° C.;

Temperature variation: 50° C. to 95° C.

The thickness of the deposited coating was 165 μm, but greater thicknesses could have been obtained without any particular difficulty. The measured porosity of the coating was less than 1%.

The micrograph of FIG. 2 was taken of the CuNiIn coating deposited using AC-HVAF in accordance with the invention, while the micrograph of FIG. 3 was taken using a CuNiIn coating obtained by plasma spraying.

The oxides and the pores appear in the form of black spots in the layer of coating 2 deposited on the substrate 1.

It can clearly be seen that the presence of oxides and pores in the coating of FIG. 2 is less than in the coating of FIG. 3. Furthermore, it can be seen that the coating of FIG. 2 presents a microstructure that is compact and unitary, whereas that of the coating of FIG. 3 is lamellar. Consequently, the coating deposited with the method of the invention is less subject to becoming delaminated (and thus to flaking) than is the coating obtained by plasma spraying. To sum up, the microstructure of the coating in FIG. 2 is mechanically stronger.

In order to simulate the mechanical stresses to which a fan blade is subjected in operation, a device was used similar to that shown in FIG. 4 in which a mechanical part 10 representing the blade was mounted via its root 14 in a slot 15 defined between two uprights 16a and 16b, and it was held in position between two jaws 18. The assembly made in this way is analogous to a dovetail assembly. In this case the uprights 16a and 16b represented the fan disk. The root 14 of the part 10 had two surfaces 14a and 14b that were in contact with the uprights 16a and 16b. A cyclical traction force F was exerted on the part 10. The way the force F varied as a function of time is shown in FIG. 5.

The behavior of a CuNiIn coating deposited using AC-HVAF spraying in accordance with the invention was tested for 30,000 traction cycles. After 30,000 cycles, no flaking and no wear were observed. With a CuNiIn coating deposited by plasma spraying, flaking appeared in the range 15,000 cycles to 19,000 cycles.

This test demonstrates the significant improvement in terms of coating lifetime that the invention makes it possible to obtain.

Claims

1. A method of depositing an alloy of copper, nickel, and indium by thermal spraying to constitute an anti-wear coating on a mechanical part, wherein said coating is deposited by thermal spraying of the AC-HVAF type.

2. A method according to claim 1, wherein said coating is a copper-based alloy containing 30% to 42% by weight of nickel and 2% to 8% by weight of indium.

3. A method according to claim 2, wherein said coating is a copper-based alloy containing 34% to 38% by weight of nickel and 4% to 6% by weight of indium.

4. A method according to claim 3, wherein said coating is a copper-based alloy containing 36% by weight of nickel and 5% by weight of indium.

5. A method according to claim 1, wherein said mechanical part for coating is a part made of titanium or titanium alloy.

6. A method according to claim 1, wherein said coating is deposited on at least one part taken from two parts of a gas turbine that are liable to come into contact with each other.

7. A method according to claim 1, wherein said coating is deposited on a fan or compressor blade root of a turbomachine, and/or the fan or compressor disk in which said blade root is engaged.