WO2004104260A1

WO2004104260A1 - Powder mixture for thermal projection, method for the obtention of a coating and coating obtained.

Info

Publication number: WO2004104260A1
Application number: PCT/IB2004/001705
Authority: WO
Inventors: Francisco Javier Sanchez Molino; Josep Maria Guilemany Casademon; Javier Fernandez Gonzalez; Juan Ramón MIGUEL CAMPRECIÓS; Jesús NAVARRO HERENCIA; Jaume Nin Pros
Original assignee: Sociedad Española De Carburos Metalicos, S.A.; Fernandez Iglesias, Daniel
Priority date: 2003-05-26
Filing date: 2004-05-25
Publication date: 2004-12-02
Also published as: ES2220228A1; ES2220228B1

Abstract

The invention refers to a binary mixture of powder for thermal projection comprising a metallic matrix of Al-Si and a disperse phase of polymeric material comprising at least a polymer selected from a polyamide and a polyolefin. It also refers to a method which requires only a mechanical mixing of said binary mixture of powder, followed by the thermal projection of it, and also to the coating obtained.

Description

POWDER MIXTURE FOR THERMAL PROJECTION, METHOD FOR THE OBTENTION OF A COATING AND COATING OBTAINED

Field of the invention

The present invention refers to a binary mixture of powder for thermal projection, to the method to obtain a coating from such a mixture and the coating obtained.

In particular, the present invention refers to a family of powders designed to be used, by means of thermal projection techniques, in obtaining abradable coatings

(coatings of mechanical sacrifice) that also have lubricating properties.

Background of the invention

Abradable materials are those that allow obtaining formings or coatings that can be easily machined with application in certain sectors where the minimisation of the tolerances between the movable parts is of a great importance. These abradable coatings consist on layers that can be machined, the matrix of which is relatively soft and, all together, the coating must be little compact to make the drawing easier without an excessive effort or deformation of the movable part. That is the reason why this kind of coatings are usually obtained by means of thermal projection techniques, that allow us to cover, depending on the technique chosen, a wide range in the level of compactness of the layers.

These abradable coatings must have a series of requirements :

• To have a good adherence with the substrate (possible use of bilayer, multilayer or gradual structures) ; Good machinability;

To generate, during the cutting, non-harmful detritus for the system and possible movable parts located at other areas; • To allow a process of extraction of abradable material without plastic deformation of the coating that would generate an elevated surface roughness decreasing the sealing; and • Easy removal of chemical affinity between the abradable coating and the movable part, to avoid seizing or adhesive wearing, which are fatal for the system, and particularly, to avoid adhesion to the movable parts of the system so that there is a decrease in the efficiency of the movable system.

The use of abradable coatings obtained by means of thermal projection has previously been documented in patents as well as scientific articles published in international magazines and is subject to world-wide deep investigation due to its great applications in reducing the tolerance for turbines, among others. The construction of turbines, especially those used in aviation, has evolved towards the increase of output which implies an increase in the system working temperature. So, the abradable coatings applied in turbines are subject of higher thermal requirements, as well as resistance against oxidation at high temperatures and resistance against wearing by particles erosion.

Some of the proposed abradable systems in the state of the art are based upon materials with a cellular or porous structure (US 3689971, US 4063743, US 4526509, US 4652209), materials with low melting point where the increase of temperature produced by the friction itself melts the material (US 2742224 and US 3836156) and materials with a ceramic base such as Zr0₂ and MgO (US 4405284, US 4460311 and US 4669955) .

The next evolution of the abradable coatings consists in the mixture by means of different techniques, of materials such as fluorite with metallic or ceramic matrix (US 4867639) or mixtures of ceramics based on stabilised Zr0₂ with several polymeric thermoplastic materials, thermosetting and other polymeric materials (US 5530050) . Other kind of abradable systems are the mixtures of three components: one of a metallic or ceramic nature, another which is a polymer and a solid lubricant. This tertiary self-lubricated system covers many metallic materials (aluminium, titanium, copper, zinc, nickel, etc.), ceramic (aluminium, titanium, zircon, silicon oxides and its mixtures, perovskites, spinels, etc.), polymers and solid lubricants such as CaF₂, MgF₂, BaF₂, MoS₂. All of these combinations are described in EP 0487273 Al with US 615557 priority and in US 5434210 which is a continuation of US 615557. Due to the morphology of the coating and the kind of work in which it will be used, such ternary mixtures include in the structure components that, because of its characteristics, give the coating lubricating characteristics, as it happens with graphite or some polymers. The fact of including this kind of solid lubricating materials in the system, gives them the capacity to be used as self-lubricated materials. The lubricating solids offer lower friction coefficients and improve the resistance against wearing of the coatings. These tertiary systems can also work at high temperatures, conditions in which the conventional liquid lubricants would degrade.

On the other hand, there are composed systems of Al-Si mixed with mica with different proportions that have been used in turbines with working temperatures of up to 427°C (WO 01/44533A1) to improve the previous systems based on Al-Si plus high porosity graphite. However, the use of some light phases, such as graphite, offers stability problems in the projection beam and the microstructure obtained after the process of projection is not the desired one.

Description of the invention

A first aspect of the present invention is to provide a binary mixture of powder for thermal projection which allows obtaining abradable coatings with a low friction coefficient that also have self-lubricating properties .

A second aspect of the invention is to provide a method to obtain a coating from the mixture of powder according to the first aspect of the invention.

A third aspect of the present invention is to provide a coating obtained from the mixture of powder according to the first aspect of the invention.

Detailed description of the invention

According with the first aspect of the invention, a binary mixture of powder is provided, which comprises a metallic matrix of Al-Si and a polymeric disperse phase which comprises at least a polymer selected from a polyamide and a polyolefin.

Surprisingly, the authors of the present invention have found that the combination of a metallic matrix of Al-Si with a polymeric disperse phase which comprises at least one polymer selected from a polyamide and a polyolefin allows obtaining an abradable coating with good friction properties, at lower costs, which also has lubricating properties. Advantageously, the presence of at least one polymer selected from a polyamide and a polyolefin in a binary mixture of powder for thermal projection allows the suppression the presence of the ternary-lubricating-solid element, without the detriment of the properties of such abradable coatings, when it comes to the low friction coefficient.

Unexpectedly, the presence of a polyamide or polyolefin or mixtures of both also acts, in a binary mixture for thermal projection, as a sacrificial material as well as solid lubricating.

Preferably, the binary mixture of powder according to the first aspect of the invention comprises from 20 to 80% of Al-Si powder and from 20% to 80% of polymeric material powder, being such a polymeric material a polyamide, polyolefin, or mixture of both. Even more preferably, comprises from 60 to 80% of Al-Si powder and from 40% to 20% of polymeric material powder.

Also preferably, the size of the particle of the metallic matrix of Al-Si is lower than 80 micrometers, and the size of the particle of the polymeric material is lower than 200 micrometers. Even more preferably, the size of the particle of Al-Si is lower than 60 micrometers, and the size of the particle of the polymeric material is lower than 150 micrometers.

Preferably, the polyolefin from the mixture of powder according to the invention is selected from polyethylene and polypropylene.

According to a second aspect of the invention, a method is provided to obtain a coating from a binary mixture of powder for thermal projection comprising the following steps: i) mechanic mixing of a binary mixture of powder comprising a metallic matrix of Al-Si and a polymeric disperse phase which comprises at least a polymer selected from a polyamide and a polyolefin; ii) Thermal projection of an inert gas that includes the mixture of powder along with a fuel and a comburent or plasmogenic gas according to the projection technique used.

Advantageously, the method for obtaining a coating from a mixture of powder for thermal projection according to the first aspect of the invention only requires a mechanic mixture previous to the thermal projection, which with no doubt makes the method to obtain the coating easier, and therefore, it can be obtained at lower costs.

Preferably, the mechanical mixing is carried out with a ball mill, ring mill, turbula or high energy mixer. Also preferably and after the step of mechanic mixing an additional step of dehumidification of the mixture is carried out to make sure that the mixture of powder to be projected has the desired degree of humidity.

To obtain coatings from powder materials by means of techniques of thermal projection, it is necessary the use of a raw material with a regular and well controlled grading, which allows a correct plastic behaviour of the particles at the moment of impinge the substrate that we want to cover. Due to the variety of techniques for thermal projection, the way to obtain the raw material and its quality must be careful from the beginning.

To prepare the mixture of the powders and independently of the proportions of the components of it, the following steps can be followed: a) Control of the size of the particles prior to the mixing of the powder; b) Mechanical screening to adjust the desired grading distribution if considered to be necessary; c) Mechanical mixing of the raw materials in different proportions according to the desired properties in order to homogenize the final product. This is done by means of a ball mill, ring mill, turbula or high energy mixer among others; d) If necessary, dehumidification after the process of mixing in oven, for example, at 50 °C during a variable time according to the volume of powder to be used; and e) Projection of an inert gas including the mixture of powder to be laid, along with a fuel selected from propane, propylene, hydrogen, acetylene or similar and a comburent gas as oxygen in the case of projection by combustion or a plasmogenic gas as argon, hydrogen or helium in the case of projection by plasma.

On one hand, the control of the grading of the powder can be carried out by means of the laser light dispersion technique, which allows detecting the irregularities in the distribution of the size of the particles. The control of the grading permits to assure a particle size of the metallic matrix of Al-Si lower than 80 micrometers, preferably lower than 60 and a size of particle of the polymeric material lower than 200 micrometers, preferably lower than 150.

On the other hand, the thermal projection technique used can be any of the known ones in the state of the art, particularly Plasma, High Velocity HVOF, Flame, Electric Arc, Transferred Plasma, among the most well known.

According to the third aspect of the invention, a coating is provided which has been obtained from the above mentioned method. The coating obtained according to the method and the mixture from this invention has abradable properties, with a low friction coefficient, as well as lubricating properties . Preferably, the coating according to the third aspect of this invention has a wearing path lower than 2.4 mm, more preferably lower than 2 mm, even more preferably lower than 1.8 mm, maintaining at the same time friction coefficients comparable to the friction coefficient of the state of the art coatings.

Also advantageously, the use of these coatings allows a reduction on the environmental impact of its applications, due to the reduction of the use of liquid lubricants . Furthermore, the results of corrosion, carried out on the coatings which are subject in this invention, have shown that the corrosion resistance is improved in relation to the Al-Si alloy (with a corrosion potential in NaCl of -750 mV approximately) , so the use of this kind of coatings has an additional advantage when compared to the state of the art coatings.

Figures

Figure 1 shows in a graphic the corrosion potential (mV) against time(s) of a coating obtained from a binary mixture of powder according to an embodiment of the invention.

In said figure 1 it may be seen the corrosion resistance that the coating according to the first embodiment of the invention offers .

Figure 2 shows in a graphic the corrosion potential (mV) against time(s) of a coating obtained from a binary mixture of powder according to another embodiment of the invention. In said figure 2 it may be seen the corrosion resistance that the coating according to a second embodiment of the invention offers.

EXAMPLES

Example 1

A mixture is prepared with the following formulation:

AlSi (AISI-SAE 339.0) 60%, Polyamide 40%, considering the percentages in weight. The aluminium-silicon alloy used, for the example, has a composition from 13-16 % Si, 2.4- 2.8 % Cu and 0.5-0.9 %Mg. The raw materials are introduced inside the feed hopper, to proceed to the process of thermal projection with the following conditions:

The number of layers applied is 8 and the average thickness is around 620 microns.

The coatings obtained offer homogeneity in the structural distribution, are consistent and have the following properties:

• Friction coefficient (ASTM Norm G99-90, with iron ball, load of 5 N and temperature of 25 °C) : = 0.069

• Wideness of wearing path (ASTM Norm G99-90, with iron ball, load of 15 N and temperature of 25 °C) : = 1.5 mm

• Corrosion assay (open circuit) carried out in NaCl (3.4%). See figure 1, wherein the resistance that the coating offers against the process of corrosion can be seen. Example 2

A mixture is made with the following formulation: AlSi (AISI-SAE 339.0) 60%, Polyolefin (Polyethylene, PE) 540%, considering the percentages in weight. The composition of the Al-Si alloy is identical to the used in example 1. The raw materials are introduced in the feed hopper, to proceed to the process of thermal projection with the following conditions: 0

The coatings obtained offer homogeneity in the structural 5 distribution, are consistent and have the following properties:

• Friction coefficient (ASTM Norm G99-90, with iron ball, load of 5 N and temperature of 25 °C) : = 0.089.

• Wideness of wearing path (ASTM Norm G99-90, with iron 0 ball, load of 15 N and temperature of 25 °C) : = 1.4 mm.

• Corrosion assay (open circuit) carried out in NaCl (3.4%). See figure 2, wherein the resistance that the coating offers against the process of corrosion can be seen. 5

The results obtained show, when compared with other similar materials, a clear improvement of the lubrication properties, on both cases.

The wearing path of a AlSi-Polyester system gives 0 average values of wearing path of 2.4 mm and, for example, the Al-graphite, wearing paths higher than 3 mm, whereas the use of a binary mixture according to the invention offers values of wearing path of 1.4 mm and 1.5 mm, at the same time the friction coefficient of the coating according to the invention is of the same value of the coefficient from other systems already in use.

Claims

C L A I S

1. Binary mixture of powder for thermal projection comprising a metallic matrix of Al-Si and a disperse phase of polymeric material comprising at least a polymer selected from a polyamide and a polyolefin.

2. Binary mixture of powder according to claim 1, characterised in that it comprises from 20 to 80% of Al-Si powder and from 20% to 80% of a polymeric material powder, being such a polymeric material a polyamide, polyolefin or mixture of both of them.

3. Binary mixture of powder according to claim 2, characterised in that it comprises from 60 to 80% of Al-Si powder and from 40% to 20% of polymeric material powder.

4. Binary mixture of powder according to any one of the previous claims, characterised in that the size of the particle of the metallic matrix of Al-Si is lower than 80 micrometers.

5. Binary mixture of powder according to any one of the previous claims, characterised in that the size of the particle of polymeric material is lower than 200 micrometers.

6. Binary mixture of powder according to any one of the previous claims, characterised in that the size of the particle of Al-Si is lower than 60 micrometers, and the size of the particle of the polymeric material is lower than 150 micrometers.

7. Binary mixture of powder according to any one of the previous claims, characterised in that the polyolefin is selected from polyethylene and polypropylene .

8. Method for obtaining a coating from a mixture of powder for thermal projection according to any of claims 1 to 7, characterised in that it comprises the following steps: i) mechanic mixing of a binary mixture of powder comprising a metallic matrix of Al-Si and a polymeric disperse phase comprising at least a polymer selected from a polyamide and a polyolefin. ii) thermal projection of an inert gas that includes the mixture of powder along with a fuel and a comburent or plasmogenic gas according to the projection technique used.

9. Method according to claim 8, characterised in that it comprises an additional step of dehumidification of the mixture prior to the thermal projection.

10. Coating obtained from the method defined according to any one of claims 8 to 9.

11. Coating according to claim 8, characterised in that it has a wearing path lower than 2.4 mm.

12. Coating according to claim 8, characterised in that it has a wearing path lower than 2 mm.

13. Coating according to claim 8, characterised in that it has a wearing path lower than 1.8 mm.