WO2005114038A2 - Method of coating a surface with a dry lubricating film - Google Patents

Abstract

The invention relates to a method of coating a surface, such as the skirt (7a, 7b) of a piston (1) of an internal combustion engine, with a dry lubricating film (3) which has a high mechanical strength and a low friction coefficient and which is intended to reduce friction between the skirt (7a, 7b) of the piston (1) and the cylinder. The inventive method comprises the following steps consisting in: coating the surface (7a, 7b) with a paste which is disposed in an aqueous solvent and which comprises a polymer having a low friction coefficient and, optionally, non-organic compounds; and heating the film (3) to a temperature T for a time t, such that said film (3) assumes an amorphous structure.

Description

The present invention relates to a method for coating a surface, in particular a piston skirt of an internal combustion engine, with a dry lubricating film with high mechanical resistance and low coefficient of friction. The object of the invention is mainly to reduce friction between the skirt of the piston and the cylinder. It is in fact known that the main function of the piston skirts is to guide the piston in the cylinder while avoiding any risk of seizure, the upper part of the piston being composed of ring holders which seal between the chamber and the lower part of the engine. The friction between the skirt and the resulting cylinder is therefore a source of noise, loss of energy and damage to the surfaces. In order to remedy these drawbacks, several solutions have been considered. The introduction of a play of several microns between the skirt and the cylinder accompanied by a lubricant such as oil indeed makes it possible to avoid the problems of seizing up but creates a significant consumption of oil. Other solutions consisting in coating the skirt with a coating resistant to seizing and provided with a low coefficient of friction seem to have been the preferred track of the automobile manufacturers and the equipment manufacturers. Thus, patent application EP0380415 describes a composite coating, based on a resin from the family of sulfonic polymers, which is fixed to the skirt of the piston by screwing, shrinking or gluing. This invention has the disadvantage of adding an assembly step between the piston skirt and the very restrictive coating in terms of manufacture and mechanical strength. Similarly, patent FR2808461 proposes a process for depositing pure polyaryietherketone by pure thermal spraying on the piston skirt, but it has the drawback of being restrictive in terms of equipment, since it requires the use of projection torches. Finally, patent EP0614416 proposes a solution which consists in coating by transfer pad the piston skirt with a paste comprising a dry moistened lubricant then in drying the paste. The dry lubricant is composed of a fluoropolymer or graphite or molybdenum disulphide mixed with a binder and a very volatile organic solvent such as acetone. This method has the advantage of being simple to implement, since it is essentially based on the step of coating with a transfer buffer, but has the disadvantage of using a very volatile organic solvent, which requires the use of heavy pollution control accessories to manage in a production chain. In addition, if the fluoropolymers, graphite and molybdenum disulphide give the coating a good coefficient of friction, they are not satisfactory in terms of mechanical strength. This is why, the object of the present invention is to propose a method for coating a surface, in particular a piston skirt of an internal combustion engine, with a dry lubricating film with high mechanical resistance and low coefficient of friction. overcoming all or part of the drawbacks noted in the prior art detailed above. This object is achieved by the fact that said process consists in: - coating the surface of a paste produced in an aqueous solvent and comprising a polymer with low coefficient of friction, - heating said film at a temperature T for a period t, of so that said film has an amorphous structure,

According to other characteristics: The step of coating the surface with the paste can be followed by a step of drying said paste so as to obtain a dry lubricating film. The step of heating said film can be followed by a step of cooling said film so that said film largely retains its amorphous structure. Said low friction polymer is preferably a polyaryiether ketone. According to a preferred embodiment of the invention, said paste can be produced from a mixture of water, a binder in the proportions of 2 to 5% by mass, a dispersant in the proportions of 1 to 3% by mass, of an anti-foaming agent in the proportions of 5 to 30 ppm, to which is added the polymer with a low coefficient of friction in the proportions of 60 to 80% by mass. Said paste can also contain non-organic compounds with a lubricating property, and preferably in proportions of less than 10% by mass. Said inorganic compounds are one or more of the following compounds: molybdenum disulphides, carbon, carbon nanotubes, silicon carbides or even graphite.

According to other characteristics: The step of coating the surface with said paste can be preceded by a step of preparing the surface which comprises at least a first step of cleaning said surface using a solvent or 'A detergent. The temperature T is preferably between 350 ° C and 450 ° C. The duration t is preferably between 5 s and 15 s. Another object of the invention is to provide two piston skirts of an internal combustion engine coated with a dry lubricating film according to the method described above. The invention will be better understood on reading the description which follows and which is drawn up by way of nonlimiting example, and in the single figure annexed which represents the upper part of a piston 1 of an internal combustion engine comprising two skirts 7a, 7b coated with a dry lubricating film 3 in accordance with the invention. Before depositing the film 3 on the skirts 7a, 7b, the external surface of the piston 1 is cleaned using a detergent or a solvent in order to remove the impurities as well as any protective coating applied to protect the piston 1 during transport. In order to increase the roughness of the skirts 7a, 7b, the cleaning can be followed for example by a sandblasting operation which consists in spraying silica grains at high speed on the skirts 7a, 7b so as to create small impacts which favor the attachment of the film 3. However, this operation is optional insofar as the skirts 7a, 7b admit a base roughness of approximately 20 microns which allows sufficient attachment of the film 3 to said skirts. According to a first embodiment, the preparation of the dough, not shown in Figure 1, is carried out from a mixture of water, a binder in the proportions of 2 to 5% by mass, a dispersant in the proportions of 1 to 3% by mass, of an anti-foaming agent in the proportions of 5 to 30 ppm, to which is added a polymer with a low coefficient of friction in the proportions of 60 to 80% by mass. The proportions by percentage by mass refer to the total mass of the paste consisting of the mixture of water, binder, dispersant, anti-foaming agent, polymer with a low coefficient of friction. The polymer with a low coefficient of friction is preferably a polyaryiether ketone sold under the name of Poly Ether Ether

Ketone (PEEK ™). It is a semi-crystalline polymer, the basic formula of which is as follows: oxy-1, 4-phenylene-oxy-1, 4-phenylene-carbonyl-

1, 4-phenylene, shown in the figure below:

Le polyaryiethercetone est un polymère à faible coefficient de frottement qui confère les propriétés tribologiques au film 3 et qui permet par conséquent de diminuer de manière significative les frottements observés entre les jupes de piston et le cylindre. Plus précisément, par rapport aux résultats obtenus avec des films lubrifiants secs comportant des polymères fluorés, du graphite ou du bisulfure de molybdène, on observe une réduction jusqu'à 20 % des frottements en régime de lubrification hydrodynamique, ce qui autorise un gain de consommation en carburant d'au moins 1%. Il a également été choisi pour sa grande inertie chimique et ses bonnes performances mécaniques notamment à hautes températures. En régime hydrodynamique, il admet en effet une énergie de surface au moins deux fois plus faible que les polymères fluorés, le graphite et le bisulfure de molybdène. Pour l'invention, il est avantageux d'utiliser deux poudres ultra-fines de faible viscosité, soit de granulométrie moyenne de 25μm, soit de granulométrie moyenne 10μm. Concernant le liant, il est avantageux d'employer le RHODOVIOL 25/140 ™ qui est un alcool polyvinylique obtenu par hydrolyse de l'acétate de polyvinyle. Il permet de favoriser l'adhérence de la pâte sur la surface à revêtir. Concernant l'agent anti-moussage, il est avantageux d'employer le

Polyaryietherketone is a polymer with a low coefficient of friction which gives tribological properties to film 3 and which therefore makes it possible to significantly reduce the friction observed between the piston skirts and the cylinder. More precisely, compared with the results obtained with dry lubricating films comprising fluorinated polymers, graphite or molybdenum disulphide, a reduction of up to 20% in friction is observed in hydrodynamic lubrication regime, which allows a gain in consumption. in fuel of at least 1%. It was also chosen for its high chemical inertness and good mechanical performance, especially at high temperatures. In hydrodynamic regime, it indeed admits a surface energy at least twice lower than fluoropolymers, graphite and molybdenum disulfide. For the invention, it is advantageous to use two ultra-fine powders of low viscosity, either with an average particle size of 25 μm, or with an average particle size of 10 μm. Regarding the binder, it is advantageous to use RHODOVIOL 25/140 ™ which is a polyvinyl alcohol obtained by hydrolysis of polyvinyl acetate. It helps to promote the adhesion of the paste on the surface to be coated. Regarding the anti-foaming agent, it is advantageous to use the

BEVALOID 581 B ™. It is a mixture of aliphatic hydrocarbons and nonionic surfactants. It avoids the foaming phenomena that appear when mixing PEEK ™ powder in water. Regarding the dispersant, it is advantageous to use SINNOPAL OP11 ™. It is a non-anionic surfactant (Octylphenol ethoxyl), which avoids the settling of the PEEK ™ powder in solution. According to a second embodiment, the preparation of the dough is carried out according to the same recommendations as those described in the first embodiment, except that one or more non-organic compounds with lubricating property are added in proportions less than 10% by mass. The percentages by mass refer to the total mass of the paste made up of the water, binder, dispersant, anti-foaming agent, polymer with low friction coefficient, non-organic compounds with lubricating properties. With regard to non-organic compounds with lubricating properties, it is advantageous to use molybdenum disulphides, carbon nanotubes, carbon, silicon carbides or even graphite in proportions of less than 10% by mass. These inorganic compounds are in the form of powder admitting an average particle size, G, in accordance with that specified in the table below:

The presence in the paste of one or more of these non-organic compounds is particularly desirable, insofar as they optimize the mechanical properties in terms of friction and of the wear resistance of the film 3. In fact, the addition of molybdenum disulfide or carbon nanotubes in proportions of 10% by mass, makes it possible to reduce by around 15% the coefficient of friction corresponding to a film obtained according to the first embodiment of the paste, that is to say with a paste produced from a mixture of water, a binder in the proportions of 2 to 5% by mass, a dispersant in the proportions of 1 to 3% by mass, an anti-foaming agent in the proportions of 5 to 30 ppm, and of the polyaryiether ketone in the proportions of 60 to 80% by mass. Likewise, the addition of molybdenum disulphides, carbon nanotubes, carbon, silicon carbides or even graphite in proportions of 10% by mass, makes it possible to increase resistance to wear corresponding to a film obtained according to the first embodiment of the paste. The process for depositing the dough on the skirt 7a, 7b can be carried out by a process of the screen printing type, which consists in rotating the piston 1 around its axis 2 and in bringing the skirt 7a, 7b into contact with a sieve, not shown in Figure 1, through which flows said dough. Advantageously, and unlike a thermal spraying process, it is not necessary to mask the area of the grooves 5 of the piston 1 provided for the segments, to prevent the dough from being deposited there. Once the skirt 7a, 7b thus coated, it is advantageous to heat the piston 1 to a temperature of approximately 200 ° C. in order on the one hand to dry the dough so as to obtain the dry lubricating film 3 and in order on the other part of avoiding the thermal shocks which result from the heat treatment which will follow and which is located at the level of said film 3 and not over the entire thickness of the skirt 7a, 7b. In fact, in order for said film 3 to have an amorphous structure, it is necessary to sinter it, that is to say to heat it to a temperature T for a duration t. The heating device can use YAG laser technology or C02 lasers with a power between 800 and

5000 Watt. Advantageously, the temperature T is between 350 ° C and 450 ° C. Advantageously, the duration t is between 5 s and 15 s. So that the film 3 largely retains its amorphous structure, it must then be cooled. Using the YAG laser heater as described above, simple cooling with ambient air results in a semi-crystalline structure. According to the method described above, the thickness of the film 3 is from 20 to 100 μm. According to an alternative sintering method, the film can be sintered by means of a heating device other than the laser, such as an induction heating device. The temperature T is then between 380 ° C and 450 ° C and the duration t is reduced to an interval of 4 to 8 s. The cooling advantageously comprises a first air quenching intended to cool the surface of the skirt 7a, 7b to a temperature of approximately 260 ° C. then a second quenching with water, intended to bring the film 3 to the room temperature, while retaining an amorphous structure.

Claims

1. Process for coating a surface (7a, 7b) with a dry lubricating film (3), characterized in that it consists in: a) coating the surface (7a, 7b) with a paste produced in an aqueous solvent and comprising a low friction polymer, b) heating said film (3) at a temperature T for a period t, to induce an amorphous structure, 2. Method for coating with a dry lubricating film (3) a surface (7a, 7b), according to claim 1, characterized in that the step of coating the surface (7a, 7b) with the paste is followed by a step of drying said paste so as to obtain a dry lubricating film (3). 3. Method for coating with a dry lubricating film (3) a surface (7a, 7b), according to any one of the preceding claims, characterized in that the step of heating the film is followed by a step of cooling said film (3) so that said film (3) largely retains its amorphous structure. Method for coating a surface (7a, 7b) with a dry lubricating film (3) according to any one of the preceding claims, characterized in that the low-friction polymer is a polyaryietherketone. 5. Method for coating with a dry lubricating film (3) a surface (7a, 7b), according to any one of the preceding claims, characterized in that said paste is produced from a mixture composed of water, of a binder in the proportions of 2 to 5% by mass, of a dispersant in the proportions of 1 to 3% by mass, of an anti-foaming agent in the proportions of 5 to 30 ppm, to which the polymer with low coefficient of friction in the proportions of 60 to 80% by mass. 6. Method for coating with a dry lubricating film (3) a surface (7a, 7b), according to any one of the preceding claims, characterized in that the paste also comprises non-organic compounds with lubricating properties. 7. Method for coating with a dry lubricating film (3) a surface (7a, 7b), according to claim 6, characterized in that the paste comprises non-organic compounds in proportions of less than 10% by mass. 8. Method for coating a surface (7a, 7b) with a dry lubricating film (3), according to any one of claims 6 to 7, characterized in that the inorganic compounds are chosen from the group of compounds consisting of molybdenum disulphides, carbon, carbon nanotubes, silicon carbides, graphite, and the mixture of said compounds. 9. Method for coating a surface (7a, 7b) with a dry lubricating film (3), according to any one of the preceding claims, characterized in that the step of coating the surface with said paste is preceded by 'a step of preparing the surface (7a, 7b) which comprises at least a first step of cleaning said surface (7a, 7b) using a solvent or a detergent, 10. Method for coating a dry lubricating film (3) on a surface (7a, 7b), according to the preceding claim, characterized in that the step of preparing the surface (7a, 7b) comprises a second step allowing d 'increase the roughness of said surface, 11. Method for coating a surface (7a, 7b) with a dry lubricating film (3), according to any one of the preceding claims, characterized in that the temperature T is between 350 ° C and 450 ° C, 12. Method for coating a surface (7a, 7b) with a dry lubricating film (3), according to any one of the preceding claims, characterized in that the duration t is between 5 s and 15 s, 13. Skirt (7a, 7b) of piston (1) of internal combustion engine, characterized in that they are coated with a dry lubricating film (3) according to the method according to one of the preceding claims.